WO2024259528A1 - Plateforme de capteur photoacoustique à double résonance optimisée par couplage pour la détection de gaz et leurs concentrations - Google Patents

Plateforme de capteur photoacoustique à double résonance optimisée par couplage pour la détection de gaz et leurs concentrations Download PDF

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WO2024259528A1
WO2024259528A1 PCT/CA2024/050827 CA2024050827W WO2024259528A1 WO 2024259528 A1 WO2024259528 A1 WO 2024259528A1 CA 2024050827 W CA2024050827 W CA 2024050827W WO 2024259528 A1 WO2024259528 A1 WO 2024259528A1
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resonant
resonant transducer
resonator cavity
transducer
acoustic
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Alaric BERGERON
Geoffroy BERGERON
Reuble MATHEW
Vahid TAYARI
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Nxtsens Microsystems Inc
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Nxtsens Microsystems Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/24—Probes
    • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
    • G01N29/2425—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics optoacoustic fluid cells therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/02—Analysing fluids
    • G01N29/036—Analysing fluids by measuring frequency or resonance of acoustic waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/24—Probes
    • G01N29/2437—Piezoelectric probes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/32—Arrangements for suppressing undesired influences, e.g. temperature or pressure variations, compensating for signal noise
    • G01N29/326—Arrangements for suppressing undesired influences, e.g. temperature or pressure variations, compensating for signal noise compensating for temperature variations
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/04—Wave modes and trajectories
    • G01N2291/042—Wave modes
    • G01N2291/0427—Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever

Definitions

  • the present disclosure relates to the field of photoacoustic spectroscopy.
  • DAS direct optical absorption spectroscopy
  • PAS photoacoustic spectroscopy
  • a typical photoacoustic spectroscopy system includes a source of electromagnetic radiation, which can be absorbed by a detection target, for example analyte gas molecules. Upon absorbing the electromagnetic radiation, the target heats up, and transfers heat energy to the surrounding medium (sample fluid), such as a gas matrix, which heats up that surrounding medium. The heating of the surrounding medium induces an increase in the pressure of that medium. When the interaction between the electromagnetic radiation and the target is modulated, the resulting pressure in the surrounding medium is also modulated, yielding an acoustic pressure wave.
  • sample fluid such as a gas matrix
  • the amplitude of the acoustic pressure wave is proportional to the energy absorbed by the target, which is proportional to the electromagnetic radiation power, and to the target absorption.
  • the absorption is proportional to the concentration of the analyte gas molecules in the gas volume exposed to the electromagnetic radiation, as is the acoustic pressure wave amplitude.
  • the acoustic pressure wave is typically measured by means of a mechanical transducer which transduces the acoustic pressure wave in the surrounding medium into a mechanical displacement of the transducer.
  • a readout system converts this mechanical displacement into an electrical signal that can be read by a conventional electronic measurement device.
  • the limit of detection of a PAS system is limited by the noise floor of the entire apparatus which sets the signal-to-noise ratio for a given concentration of measured gas.
  • the three main noise sources are (i) the acoustic thermal noise in the medium in which the target gas is to be detected, (ii) the thermal noise of the pressure (acoustical) to mechanical transducer and (iii) the noise in the readout system that translates the mechanical displacement of the transducer into an electrical signal of the desired final form, for instance a digital signal or an analog electrical signal.
  • the magnitude of the acoustic thermal noise of the system is set by the acoustical configuration of the device.
  • acoustic cells comprising a resonator cavity are often used in the implementation of the acoustical configuration.
  • Q quality-factor
  • a first approach attempts to limit the noise of the acoustical to mechanical transducer and utilizes a low-noise readout system. If the combined noise of these two noise sources is brought significantly below the level of the acoustic thermal noise, then the resultant signal-to-noise ratio is limited only by the acoustical configuration, see for example US 7797983. This implementation of that approach requires a very stable laser light source and interferometer system, and increases the complexity of the readout system design, see for example US 9170397 and US 8497996.
  • Another viable approach is to increase the signal measured from the acoustical domain, together with its intrinsic noise, beyond the combined noise of the acoustical to mechanical transducer and the readout system.
  • double resonant photoacoustic spectroscopy systems are discussed in the literature, for example on-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) and off-beam QEPAS (see for example Kosterev et al., Applications of quartz tuning forks in spectroscopic gas sensing, Review of Scientific Instruments, 76(4): 1-9 (2005), Liu et al., Off-beam quartz-enhanced photoacoustic spectroscopy, Opt. Lett. 34(10): 1594-1596 (2009), and US 10908129).
  • QEPAS on-beam quartz-enhanced photoacoustic spectroscopy
  • off-beam QEPAS see for example Kosterev et al., Applications of quartz tuning forks in spectroscopic gas sensing, Review of Scientific Instruments, 76(4): 1-9 (2005), Liu et al., Off-beam quartz-enhanced photoacoustic spect
  • on-beam QEPAS utilizes a resonator cavity made of two tubes aligned with the gap between the tines of a quartz tuning fork (QTF), in order to increase the typically weak acoustic-mechanical coupling of QTFs.
  • QTF quartz tuning fork
  • on-beam QEPAS is very sensitive to misalignment and vibration; a focused laser beam with high beam quality must pass through the typically sub-millimeter gap between the tines of the QTF without directly illuminating them, as this can increase noise significantly.
  • Off- beam QEPAS utilizes a resonator cavity made of a single tube with a side slit. The QTF is placed near the side slit to maximize coupling of the acoustic-mechanical domains.
  • off-beam QEPAS does not require the same degree of alignment as on-beam QEPAS, the acousticmechanical coupling tends to be weaker.
  • a double resonant photoacoustic spectroscopy system which is not sensitive to alignment or vibration, while still exhibiting high acoustic-mechanical coupling would be useful for many applications.
  • the work offers no clear approaches on tuning the coupling regime of the two oscillators to maximize the resonant transducer displacement (and thus readout system output signal amplitude), taking into account variations due to manufacturing tolerances of the resonator cavity, manufacturing tolerances of the resonant transducer, or environmental changes.
  • the detuning of the double-resonant photoacoustic system presented in Ruck was not designed to be controlled.
  • the fluid handling system does not allow the temperature of the sample fluid, resonator cavity and resonant transducer to equilibrate, a critical step in tuning the coupling regime of the two oscillators.
  • tunability may be possible via changing the gas composition, but this is not an optimal approach for trace gas sensing in a non-lab oratory environment.
  • This lack of tunability presents manufacturability concerns, as each resonator cavity would have to be individually tested and carefully matched with a suitable resonant transducer to achieve high acoustic-mechanical coupling. This is not feasible for high- volume production.
  • it would be ideal if a tunable double resonant photoacoustic spectroscopy system could be provided, as well as methods for adjusting the detuning between the resonator cavity and resonant transducer of double resonant photoacoustic systems after assembly and/or during operation.
  • quartz enhanced photoacoustic spectroscopy utilizes a quartz tuning fork (QTF) as a resonant transducer to measure pressure variations amplified by a resonator cavity.
  • QTFs have extremely high Q-factors, on the order of 10,000 in air and 100,000 in low vacuum.
  • the Q-factor is proportional to the resonant amplification factor and inversely proportional to the losses in the system (which are related to the noise sources via the fluctuation-dissipation theorem).
  • a resonant transducer which has (i) a Q-factor in the absence of coupling to the fluid domain as high as possible, (ii) a substantially lower Q-factor in a sample fluid (such as air), indicating a strong coupling with waves in the fluid domain and (iii) low coupling to dissipative vorticity waves and thermal waves in the fluid domain.
  • the resonant transducer When the resonant transducer is designed to display a much lower Q-factor in a sample fluid as compared to vacuum, its losses are dominated by those in the fluid domain.
  • a high Q-factor in vacuum as compared to sample fluid thus indicates that the energy lost by the resonant transducer via interaction with a sample fluid vastly dominates any other losses inherent to the pure mechanical oscillations of the resonant transducer.
  • low coupling to vorticity waves and thermal waves in the fluid domain ensures that most of the Q-factor losses observed when exposing the resonant transducer to the sample fluid as opposed to vacuum result from a high coupling with acoustic waves in the fluid domain.
  • the present disclosure demonstrates the significant gain in signal intensity, with concomitant increase in the signal-to-noise ratio of the output signal of a photoacoustic spectroscopy system, that can be obtained by (i) increasing the coupling between the resonator cavity and the acoustical to mechanical transducer and (ii) tuning the resonator cavity and resonant transducer closely to leverage the signal gains enabled by the increased coupling.
  • This signal enhancement is independent and in addition to the signal increase as obtained by the use of high Q-factor resonators such as in QEPAS.
  • the coupling strength and the Q-factor of the resonant transducer should be increased until the signal-to-noise ratio of the resulting output signal is dominated by the intrinsic thermal noise contribution of the resonator cavity.
  • This signal enhancement can also be realized without the use of complex or expensive low-noise readout systems by leveraging the large signal increase provided by the increased coupling between the resonator cavity and resonant transducer.
  • the present disclosure also provides methods and approaches to design a photoacoustic apparatus with a signal-to-noise ratio that is limited only by the fundamental limit of the thermal acoustic noise associated to the acoustic configuration of the photoacoustic spectroscopy system using easily manufactured and low-cost readout systems that are not themselves state-of-the-art in terms of their intrinsic noise floor.
  • double resonant photoacoustic spectroscopy systems which are configured to operate in such a fashion that they can exploit the potential signal gains enabled by the significant coupling regime are provided.
  • a photoacoustic spectroscopy system including: an acoustic cell comprising a resonator cavity, wherein the resonator cavity is configured to support a first resonance mode having a first resonant frequency, and wherein the acoustic cell is suppliable with a sample fluid; an electromagnetic radiation unit comprising an emitter and a control circuit, wherein the emitter is configured to transmit electromagnetic radiation through the resonator cavity; a resonant transducer unit comprising a resonant transducer and a readout system, wherein the resonant transducer is configured to support a second resonance mode having a second resonant frequency, and wherein the resonant transducer is operatively connected to the resonator cavity; and wherein the first resonance mode and the second resonance mode are configured to be coupled in a significant coupling regime.
  • a method for performing photoacoustic spectroscopy including: providing a photoacoustic spectroscopy system as described above; operating the emitter in a modulated mode, such that the target analyte molecules in the sample fluid of the resonator cavity experience periodic absorption and heating at an operating frequency; configuring the acoustic cell such that the periodic heating induces an acoustic pressure wave with an amplitude proportional to a concentration of target analyte molecules in the sample fluid of the resonator cavity thus exciting the coupled resonator cavity and resonant transducer modes; detecting the excitation of the coupled resonator cavity and resonant transducer modes by measuring the displacement of the resonant transducer in a frequency-dependent basis with the readout system; and outputting via the readout system a signal representative of the concentration of target analyte molecules.
  • the system includes an acoustic cell comprising a resonator cavity, wherein the resonator cavity is configured to support a first resonance mode having a first resonant frequency, and wherein the acoustic cell is suppliable with a sample fluid; an electromagnetic radiation unit comprising an emitter and a control circuit, wherein the emitter is configured to transmit electromagnetic radiation through the resonator cavity; a resonant transducer unit comprising a resonant transducer and a readout system, wherein the resonant transducer is configured to support a second resonance mode having a second resonant frequency, and wherein the resonant transducer is operatively connected to the resonator cavity; and wherein the first resonance mode and the second resonance mode are configured to be coupled in a significant coupling regime.
  • first resonant frequency of the resonator cavity, the second resonant frequency of the resonant transducer or both resonant frequencies may be configured to be tunable such that the system can be tuned to attain at least one of (i) a closely- tuned configuration and (ii) an optimally-tuned configuration.
  • the system may include control systems that are configured to at least stabilize one or a combination of (i) the temperature of the sample fluid, (ii) the pressure of the sample fluid, (iii) the humidity of the sample fluid, (iv) the mass flow of the sample fluid through the acoustic cell, (v) the temperature of the resonant transducer, and (vi) the temperature of the acoustic cell.
  • the first resonant frequency of the resonator cavity may be configured to be less than the second resonant frequency of the resonant transducer at a first temperature (Tl), wherein the first resonant frequency of the resonator cavity is configured to be greater than the second resonant frequency of the resonant transducer at a second temperature (T2), and wherein T l f T2, and wherein the control systems may be configured to at least stabilize the temperature of the acoustic cell at any chosen temperature between Tl and T2.
  • the resonant transducer may be selected from one or a combination of (i) an out-of-plane resonator, (ii) a tuning fork resonator, (iii) a cantilever resonator, and (iv) a diaphragm resonator.
  • the acoustic cell further may include at least one acoustic frequency filter operatively connected to the resonator cavity.
  • the acoustic cell may include at least one transducer enclosure, and wherein each of the at least one transducer enclosure is configured to encapsulate a volume of the sample fluid surrounding at least one active surface of the resonant transducer, separating the at least one active surface of the resonant transducer from an external environment.
  • the readout system may be at least configured to sense the resonant transducer using a piezoelectric material.
  • the readout system further may include a pre-amplifier proximal to the resonant transducer.
  • the resonant transducer unit may include a printed circuit board (PCB), wherein the resonant transducer is mounted on the PCB, wherein the PCB is mounted to the acoustic cell such that the resonant transducer is operatively connected to the resonator cavity, and wherein at least some of the readout system may be located on the PCB.
  • PCB printed circuit board
  • the acoustic cell may be constructed using a monolithic design and further comprises at least one acoustic frequency filter and an acoustic port which are machined from a single piece of material.
  • the acoustic cell constructed using the monolithic design further may include a heat exchanger machined into the acoustic cell that allows the sample fluid to reach thermal equilibrium with the acoustic cell at least before entering the resonator cavity.
  • a quality (Q-)factor of the second resonance mode of the resonant transducer in vacuum may be configured to be substantially larger than a free Q-factor of the second resonance mode of the resonant transducer in the sample fluid.
  • the Q-factor of the resonant transducer in vacuum may be configured to be greater than 1000.
  • the coupling strength (12) between the resonator cavity and the resonant transducer may be greater than or equal to the threshold value of m 0 .
  • the resonant transducer may have a Q-factor, Q2, which is 2 B 7 greater than or equal to the threshold value of — - - , wherein the readout system may have a noise amplitude spectral density of /?, and wherein both the sample fluid in the resonator cavity and the resonant transducer may have a temperature, T.
  • the acoustic cell further may include at least one acoustic port.
  • the acoustic cell may include at least one optical window.
  • the emitter may include one from a selection of (i) a quantum cascade laser (QCL), (ii) a continuous wave (CW) laser, (iii) a pulsed laser, (iv) an interband cascade laser (ICL), (v) a vertical-cavity surface-emitting laser (VCSEL), and (vi) a thermal emitter.
  • QCL quantum cascade laser
  • CW continuous wave
  • ICL interband cascade laser
  • VCSEL vertical-cavity surface-emitting laser
  • control circuit may be configured to at least modulate the output of the emitter, wherein modulating the output of the emitter comprises modulating one or a combination of (i) an emission wavelength of electromagnetic radiation (ii) an emission intensity of electromagnetic radiation (iii) a pulse repetition rate of electromagnetic radiation, and (iv) a patterned train of pulses of electromagnetic radiation.
  • the readout system may include a pre-amplifier, wherein the pre-amplifier may include one of (i) a differential charge amplifier, (ii) a differential transimpedance amplifier, (iii) a single-ended voltage amplifier, (iv) a single-ended charge amplifier, (v) a single-ended transimpedance amplifier, and (vi) an instrumentation amplifier.
  • the pre-amplifier may include one of (i) a differential charge amplifier, (ii) a differential transimpedance amplifier, (iii) a single-ended voltage amplifier, (iv) a single-ended charge amplifier, (v) a single-ended transimpedance amplifier, and (vi) an instrumentation amplifier.
  • the readout system may be configured to at least convert a displacement of the resonant transducer into one or more electrical signals and amplify and process the one or more electrical signals.
  • the acoustic cell may be configured to be sealed with the sample fluid contained within.
  • the acoustic cell further may include at least one port connected to a fluid handling system.
  • the fluid handling system may be at least partly composed of low adsorption tubing.
  • Another broad aspect is a method for performing photoacoustic spectroscopy.
  • the method includes providing a photoacoustic spectroscopy system as described herein; operating the emitter in a modulated mode, such that target analyte molecules in the sample fluid of the resonator cavity experience periodic absorption and heating at an operating frequency; configuring the acoustic cell such that the periodic heating induces an acoustic pressure wave with an amplitude proportional to a concentration of the target analyte molecules in the sample fluid of the resonator cavity, exciting the coupled resonator cavity and resonant transducer modes; detecting an excitation of the excited coupled resonator cavity and resonant transducer modes by measuring the displacement of the resonant transducer in a frequency-dependent basis with the readout system; and outputting via the readout system a signal representative of the concentration of target analyte molecules.
  • configuring the control systems in such a way as to stabilize the temperature of the sample fluid, the resonant transducer unit and the resonator cavity such that the detuning between the first resonance mode and the second resonance mode may be actively stabilized and maintained in either the closely-tuned or the optimally-tuned configuration.
  • the method may include increasing the amplitude of the signal transduced by the resonant transducer to improve the signal-to-noise ratio of the readout system by one or a combination of increasing the coupling strength by one or a combination of i) selecting a resonant transducer geometry with a large active surface area and ii) positioning the resonant transducer and the acoustic port at a pressure maximum in the resonator cavity; increasing the resonant transducer Q-factor via a reduction of the coupling to vorticity and thermal waves; and operating the photoacoustic spectroscopy system at the closely-tuned or optimally-tuned configuration.
  • the readout system may possess an input-referred noise amplitude spectral density ft and the sample fluid in the resonator cavity together with the resonant transducer are at a temperature of T , and wherein the method may include providing the photoacoustic spectroscopy system whereby the contribution of the intrinsic thermal acoustic noise of the resonator cavity to the total output noise of the resonant transducer unit is larger than the combination of the contribution of the intrinsic thermal mechanical noise of the resonant transducer and the contribution of the noise of the readout system to the total output noise of the resonant transducer unit, such that the noise of the electrical signal transduced by the resonant transducer unit is mostly comprised of the contribution of the intrinsic thermal acoustic noise of the resonator cavity by selecting the resonant transducer with a sufficiently high Q-factor which satisfies Q 2 > 64a>0 m2 ; selecting the resonant transducer
  • the method my include operating the control circuit in such a way as to modulate a wavelength of the emitter at half the operating frequency of the resonator cavity and the resonant transducer, while using a signal band selection system to measure the signal output of the resonant transducer unit at the operating frequency such that the undesired background photoacoustic signal is reduced.
  • Figure 1A depicts a double-resonant photoacoustic spectroscopy system including a resonant transducer and a resonator cavity described as a coupled oscillator system.
  • Figure IB depicts the theoretical MEMS displacement
  • Figure 1C depicts the position of local maxima of the resonant transducer displacement as a function of the detuning 8 between the resonant transducer and resonator cavity of the acoustic cell for a strongly-coupled system exhibiting frequency anti-crossing.
  • Figure ID depicts experimentally-measured frequency anti-crossing behavior of two hybridized-coupled modes in a double-resonant photoacoustic system.
  • Figure 2A depicts a double-resonant photoacoustic spectroscopy system.
  • Figure 2B depicts a double-resonant photoacoustic spectroscopy system having a fluid handling system and a control system.
  • Figure 3 depicts an example acoustic cell.
  • Figure 4 depicts an example of using temperature as a tuning parameter.
  • Figure 5 depicts a flowchart of an exemplary method for performing photoacoustic spectroscopy.
  • adjusting and controlling variable components within a circuit or network may adjust the performance of that circuit or network and that those adjustments may be described generally as tuning, adjusting, matching, correcting, and so forth.
  • tuning, adjusting, matching, correcting, and so forth A person of ordinary skill in the art will recognize that a particular topology discussed in this disclosure can be implemented in a variety of ways without departing from the present disclosure.
  • circuit and “circuitry” may include either a single component or a plurality of components, which are active and/or passive and are connected or otherwise coupled together to provide the described function.
  • PAS a technique based on the detection of photogenerated acoustic waves.
  • Some of the main advantages of PAS-based systems are the small system size, the absence of consumables and the ability to generate real-time data.
  • PAS-based systems are well suited for use in continuous monitoring applications, mobile applications, or large-scale applications which require high throughput of samples.
  • laser-based PAS systems can be easily adapted to detect virtually any molecule while retaining a high specificity. Broadband tunable lasers or arrays of lasers can also allow simultaneous detection of a large number of different molecules, tailored to a specific application.
  • PAS-based systems are easily customizable.
  • Typical PAS systems include an electromagnetic radiation source, an acoustic cell, which may include a resonator cavity, a means of modulating the electromagnetic radiation that reaches the acoustic cell, an acoustical to mechanical transducer operatively connected to the resonator cavity, and a readout system that measures the transducer displacement and converts it to a useable electronic signal.
  • an electromagnetic radiation source an acoustic cell, which may include a resonator cavity, a means of modulating the electromagnetic radiation that reaches the acoustic cell, an acoustical to mechanical transducer operatively connected to the resonator cavity, and a readout system that measures the transducer displacement and converts it to a useable electronic signal.
  • electromagnetic radiation of wavenumber u [cm’ 1 ] is absorbed by target analyte molecules in a sample fluid as it traverses at a rate proportional to the molar density p [mol cm’ 3 ] of the target analyte molecules, the absorption cross-section G(U) [cm 2 mol' 1 ] at wavenumber u, and the electromagnetic radiation source intensity I [W cm’ 2 ] at wavenumber u.
  • the subsequent relaxation to the molecular ground state can occur through a number of processes: radiation (stimulated or spontaneous emission of a photon), chemical reactions, non-radiative relaxation, or a combination of these processes.
  • Non-radiative relaxation can lead to an increase in the kinetic energy of the surrounding molecules, creating local heating and a corresponding local decrease in pressure.
  • the absorption process is periodically modulated either by the radiation source (intensity or wavelength), or by shifting the absorption lines, then the induced periodic heating will produce a pressure wave (synonymic for an acoustic wave) at the same frequency, or at a higher harmonic thereof.
  • the resulting pressure wave frequency co, coincides with a resonant frequency of a resonator cavity, it leads to an amplification of the acoustic wave generated within the resonator cavity.
  • the acoustic wave generated in the resonator cavity is not always driven to have the exact same frequency as the resonant frequency of the resonator cavity, and a different acoustic wave frequency can be chosen for a variety of reasons such as maximizing output signal amplitude or maximizing the signal-to-noise ratio.
  • the operating frequency is defined as the frequency of the pressure wave induced in the resonator cavity as a result of absorption of the modulated electromagnetic radiation by the target analyte molecules.
  • This periodic thermal excitation drives an acoustic resonance in the resonator cavity that is measured as pressure by a transducer.
  • the pressure p [Pa] at the transducer location is proportional to the density, absorption cross-section and radiation source intensity, such that: p oc p a(v) I
  • the transducer, and its associated readout system must be able to convert the acoustic pressure signal p [Pa] into a suitable signal for the application, usually a digital or analog electrical signal.
  • a pressure-to-mechanical displacement transducer wherein the acoustic pressure exerts a force on one or more surfaces of the transducer, herein and throughout this disclosure referred to as an active surface, causing a mechanical displacement of one or more parts of the transducer.
  • such a transducer includes a large active surface area, in order to increase the effective driving force applied by the acoustic pressure wave onto the transducer.
  • the transducer is constructed in such a way as to minimize the force exerted by the acoustic pressure wave onto undesired surfaces other than the chosen active surface, wherein exerting force onto those undesired surfaces would reduce the total or effective transducer displacement amplitude.
  • Examples of such resonant transducers can be found in US 63/480,069, which is incorporated by reference herein.
  • This mechanical displacement shall then be converted into a suitable electrical signal by means of a readout system, which includes one of a piezoelectric element, a capacitive sensor, an optical interferometry system, a strain gauge or piezoresistive element, or other means of converting a mechanical displacement to an electrical signal now known or later developed.
  • the readout system further includes a signal band selection system, which is configured to select and extract a specific frequency band centered at or around the operating frequency from the signal that is generated from the mechanical displacement of the pressure-to-mechanical displacement transducer, in order to increase the signal-to-noise ratio.
  • the signal band selection system may operate by the use of a fast Fourier transform signal processing, by the use of lock-in amplification techniques, by the use of frequency filters, or by other means of isolating a signal frequency band now known or later developed.
  • the readout system may include signal band selection systems.
  • Suitable resonant transducers may include resonant microelectromechanical system (MEMS) transducers and quartz tuning forks (QTFs). Additionally, non-resonant transducers such as broadband MEMS microphones or electret microphones may be used. It may be beneficial to utilize a resonant transducer, because in addition to the acoustic amplification granted by the use of the resonator cavity, the photoacoustic signal can also be amplified through the use of a resonant transducer with a high Q-factor. This is known as a double-resonant system. A physical interpretation of the Q-factor of an oscillator is the ratio of the accumulated energy to the dissipated energy during one period of oscillation.
  • Figure 1A depicts a double-resonant photoacoustic spectroscopy system including a resonant transducer and a resonator cavity described as a coupled oscillator system 100 with a simple coupling arrangement. More complex coupling schemes exist and it is understood that the application of more complex coupling mechanisms are covered in the scope of this disclosure.
  • the acoustic oscillation in the resonator cavity has an effective mass mi, an effective spring constant , an effective damping constant ai and an effective displacement xi, while the resonant transducer has a sprung mass m2, a spring constant fe, a damping constant a and a displacement 2.
  • the coupling is modeled with a spring of constant K joining the two masses.
  • the transfer function Fz describing the displacement of the resonant transducer X2 upon excitation of the acoustic oscillation in the resonator cavity by the force F is given by: throughout this disclosure Fl is referred to as the coupling strength, the physical quantities pertaining to the acoustic oscillation in the resonator cavity are identified with the subscript 1 and the physical quantities pertaining to the mechanical oscillation of the resonant transducer are identified with the subscript 2.
  • ‘uncoupled’ refers to the behavior of oscillators that do not exchange energy between themselves. This behavior is derived by modeling the evolution of the physical parameters of each oscillator as a function of the tuning parameter.
  • Figure IB depicts the behavior of the coupled oscillator system 100 in plot 104.
  • Coupled oscillator systems exchange energy between oscillators and dissipate energy to the surrounding environment; qualitatively distinct behavior emerges when energy exchange or energy dissipation dominates. These distinct behaviors are referred to as the strong coupling regime, indicated by bracket 504 of plot 104, and the weak coupling regime, indicated by bracket 505 of plot 104, respectively.
  • the transfer function H X2 can be simplified to:
  • the solid curve 500 of plot 104 depicts the normalized theoretical transducer displacement
  • the maximum value max n ⁇ H x is
  • Frequency anti-crossing behavior is a phenomenon observed in the response of a symmetrical coupled oscillator system, wherein two distinct local resonance maxima can be observed even when the detuning between both of the coupled oscillators is zero.
  • this notion is naturally extended by considering the splitting of the maxima of fitted Lorentzian curves, as is explained further, instead of the local maxima.
  • the significantly-coupled modes take on a hybridized nature when the resonance frequency of the first oscillator is sufficiently close to the second oscillator.
  • Both of the hybridized-coupled modes will exhibit a component from both oscillators.
  • the hybridized-coupled modes will exhibit both a substantial resonant transducer displacement and a substantial pressure-wave displacement, when the resonance frequency of the resonant transducer is sufficiently close to the resonance frequency of the resonator cavity (or vice versa).
  • the ‘ significant coupling regime’ refers to a regime which includes the strong coupling regime and the upper portion of the weak coupling regime, where the coupling strength 12 between the two coupled oscillators is considered to be a significant fraction of the coupling strength threshold for the strong coupling regime (I2 0 ). In some embodiments, this significant fraction may be at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80% of the coupling strength threshold for the strong coupling regime (I2 0 ).
  • Bracket 503 on Figure IB indicates the significant coupling regime for a significant coupling threshold corresponding to 10% of I2 0 .
  • the amplitude of the displacement of one oscillator that is part of a system of two coupled oscillators departs from the Q-factor product proportionality characteristic of the very weak coupling regime.
  • an optimally-tuned configuration is understood to be a system where the detuning between both oscillator uncoupled frequencies (here the resonator cavity and resonant transducer) is adjusted so that the resulting resonant transducer displacement, and thus signal strength, is at its global maximum.
  • This optimal detuning value 8 0 is close to zero, but in some highly-coupled systems, or in the presence of additional phenomena that are not taken into account in the model presented herein, the value of 8 0 may be nonzero, but is typically bounded by
  • a closely-tuned configuration is understood to be a system with a detuning value 8 which is within the range — ⁇ 6 ⁇ Fl.
  • Lorentzian functions L(m) res P onse P o wer peaks from the resonator cavity and resonant transducer, where a> is the central frequency of the peak, y is the dissipation factor, and corresponds to the peak full width at half maximum (FWHM) and A is the peak area. Note that it is necessary to measure a quantity which corresponds to signal power, which can be for example the square of the signal amplitude.
  • a resonant transducer which exhibits (i) a high Q-factor in vacuum and (ii) a lower free Q-factor in a sample fluid (for example, air) as compared to vacuum, where the free Q-factor is the Q-factor measured when the resonant transducer is not coupled to a resonator cavity.
  • the free Q-factor is the Q-factor measured when the resonant transducer is not coupled to a resonator cavity.
  • the Q-factor of the resonant transducer in vacuum is configured to be substantially larger than the free Q-factor of the resonant transducer in a sample fluid.
  • ‘substantially larger’ includes a ratio between the Q-factor of the resonant transducer in vacuum compared to the free Q-factor of the resonant transducer in a sample fluid that is larger than 2, or larger than 5 or larger than 20.
  • ‘substantially larger’ includes a ratio between a first Q-factor and a second Q-factor that is larger than 2, or larger than 5 or larger than 20.
  • the geometry and design of the resonant transducer can be optimized to increase the Q-factor in the sample fluid by i) increasing the Q-factor in vacuum by designing the support structure so that it minimizes coupling to the environment, ii) limiting the tangential component over all surfaces of the fluid velocity field that results from the resonant transducer mechanical oscillation in a sample fluid, iii) lowering the thermal conductivity and the thermal capacity of the resonant transducer by a careful choice of construction material and geometry and iv) avoiding the use of capacitive readout systems that require small gaps between moving surfaces with concomitant squeeze film damping.
  • the readout system will display a noise floor that depends on the principle of its operation, the quality of its construction, its temperature, etc.
  • the noise floor In order for the noise floor not to affect the signal -to-noise ratio of a PAS system, the maximization of the resonant transducer displacement resulting from a pressure signal from the acoustic cell as described in the present disclosure can be applied. This amplification of the resonant transducer displacement resulting from a pressure signal from the acoustic cell is concomitantly applied to the noise from the coupled system.
  • the efficient energy transfer between the significantly-coupled and closely -tuned, or ideally optimally-tuned, resonator cavity and resonant transducer through resonant coupling can allow for a much higher displacement amplitude of the one or more active surface of the resonant transducer, compared to a very-weakly coupled system for a given excitation of the resonator cavity.
  • the input-referred noise amplitude spectral density 0, associated with the readout system contributes less than 12% to the total output noise of the resonant transducer unit consisting of the noise from the readout system and the thermal noise, such that the total output noise is mostly comprised of the noise contribution from the coupled system, even for moderately noisy and potentially less expensive readout systems, such as those employing piezoelectric transduction, as opposed to ultra-low-noise readout systems.
  • m 2 is simply the sprung mass of the resonant transducer, which can be calculated from the geometry and the densities of the materials from which the resonant transducer is fabricated.
  • a PAS system that satisfies the above inequality will have that the displacement noise of the resonant transducer is mostly due to the contribution of the intrinsic thermal acoustic noise of the resonator cavity.
  • a PAS system that is limited in signal-to-noise ratio only by the acoustical thermal noise of the acoustic cell can be designed via the maximization of the resonant transducer displacement resulting from a pressure signal from the acoustic cell and the minimization of the noise contribution of the resonant transducer as described in the present disclosure.
  • selecting the operating frequency to be that of the resonator cavity with a detuning of zero it can be derived that selecting the resonant transducer such that the following inequalities are satisfied with Tr : being the temperature of both the sample fluid in the acoustic cell and the resonant transducer, allows one to obtain an acoustic thermal noise limited signal-to-noise ratio without demanding that the readout system be inherently low-noise which allows simpler and cheaper readout system transduction mechanisms such as piezoelectric transducers to be applied.
  • FIG. 2A depicts one embodiment of a double-resonant photoacoustic spectroscopy system 200.
  • the photoacoustic spectroscopy system 200 includes: an acoustic cell 202 including a resonator cavity 204, wherein the resonator cavity 204 supports a first resonance mode having a first resonant frequency, and wherein the acoustic cell 202 is suppliable with a sample fluid 206; an electromagnetic radiation unit 208 comprising an emitter 210 and a control circuit 212, wherein the emitter 210 transmits electromagnetic radiation through the resonator cavity 204; and a resonant transducer unit 214 comprising a resonant transducer 216 and a readout system 218, wherein the resonant transducer 216 supports a second resonance mode having a second resonant frequency, wherein the resonant transducer 216 is operatively connected to the resonator cavity 204, and ; wherein the first resonance
  • the acoustic cell 202 is configured to support a resonant acoustic wave within the resonator cavity 204.
  • the electromagnetic radiation unit 208 is configured to control the transmission of electromagnetic radiation through the resonator cavity 204.
  • the control circuit 212 is configured to controllably operate the emitter 210.
  • the resonant transducer unit 214 is configured to transduce a pressure signal into a mechanical displacement, and convert that mechanical displacement into an electrical signal.
  • the readout system 218 is configured to output a signal corresponding to the signal transduced by the resonant transducer 216 (i.e. by converting the mechanical displacement of the resonant transducer into an electrical signal).
  • the first resonance mode of the resonator cavity 204 is not necessarily referring to the first normal mode of the resonator cavity 204, rather a particular resonance mode of many possible resonant normal modes.
  • the second resonance mode of the resonant transducer 216 is not necessarily referring to the second normal mode of the resonant transducer 216, rather a particular resonance mode of many possible resonant normal modes.
  • Figure 2B depicts a further embodiment of a double-resonant photoacoustic spectroscopy system 250.
  • the system 250 further includes a fluid handling system 302 and control systems 232.
  • the fluid handling system 302 is configured to supply the acoustic cell 202 with sample fluid 206.
  • the fluid handling system further includes at least a heat exchanger 230 configured to allow the supplied sample fluid 206 to reach thermal equilibrium with the acoustic cell 202 before entering the resonator cavity 204.
  • the heat exchanger 230 is operatively connected to an inlet port 112a.
  • Control systems 232 may monitor and/or control one or a combination of (i) the temperature of the sample fluid 206, (ii) the pressure of the sample fluid 206, (iii) the humidity of the sample fluid 206, (iv) the mass flow of the sample fluid 206 through the acoustic cell 202, (v) the temperature of the resonant transducer 216, and (vi) the temperature of the acoustic cell 202.
  • one of a combination of (i) the temperature of the sample fluid 206, (ii) the pressure of the sample fluid 206, (iii) the humidity of the sample fluid 206, (iv) the mass flow of the sample fluid 206 through the acoustic cell 202 are measured at an outlet port 112b of the acoustic cell 202.
  • control systems 232 control at least a heating device 234 which can heat the acoustic cell 202, and the control systems 232 at least measure the temperature of the resonator cavity 204 by measuring the temperature of the acoustic cell 202 at one or more points 238 positioned near the resonator cavity 204 which have good thermal contact with the resonator cavity 204.
  • Figure 3 depicts an example interface between the acoustic cell 202 and the resonant transducer unit 214.
  • the acoustic cell 202 includes a resonator cavity 204.
  • the acoustic cell 202 is suppliable with a sample fluid 206.
  • the acoustic cell 202 further includes: at least one acoustic port 220, at least one transducer enclosure 222, at least one acoustic frequency filter 226, at least one optical window 224 and at least one port 112 for supplying the sample fluid 206.
  • the at least one acoustic port 220 extends from the resonator cavity 204 and is operatively connected to at least one active surface of the resonant transducer 216.
  • the at least one acoustic port 220 may take various forms including i) a hollow tube which extends from the resonator cavity 204, as depicted in Figure 3, ii) a hole in the wall of the resonator cavity 204 or iii) any other form which allows the active surface of the resonant transducer 216 to be operatively connected to an acoustic wave in the resonator cavity 204.
  • the at least one transducer enclosure 222 may serve several purposes including; i) sealing at least one encapsulation volume which covers at least one active surface of the resonant transducer 216 and isolates it from the external environment, ii) shielding the resonant transducer unit 214 from electromagnetic noise, iii) minimize vulnerability to ambient acoustical noise through mechanical shielding of the resonant transducer 216 from external pressure waves, iv) protecting the resonant transducer 216 from contamination and damage, and v) minimizing the steady gas flow around or through the structure of the resonant transducer 216 which would otherwise occur due to the possible pressure differential between the acoustic cell 202 and the ambient fluid.
  • the at least one acoustic frequency filter 226 in this example includes two buffer volumes. Buffer volumes reduce the background noise generated by laser absorption in the at least one optical window 224.
  • the at least one optical window 224 may be made of quartz, zinc selenide (ZnSe), UV-fused silica, sapphire, silicon, germanium or other materials which reduce the adsorption of energy from the electromagnetic radiation source, and may be coated in an anti-reflection optical coating.
  • the at least one optical window 224 serves to isolate the acoustic environment inside the acoustic cell 202 from that of the external environment. Where necessary, the acoustic cell 202 and specifically the transducer enclosure 222 may be made gas tight with the use of sealing gaskets.
  • the at least one port 112 includes the inlet port 112a and the outlet port 112b.
  • the resonant transducer unit 214 includes resonant transducer 216 mounted on a printed circuit board (PCB) 228.
  • the PCB 228 may be mounted to the acoustic cell 202 such that the resonant transducer 216 is positioned correctly to be operatively connected to the resonator cavity 204.
  • At least some of the readout system 218 may be located on the PCB 228.
  • resonant MEMS transducers are typically formed from silicon, which has a significantly higher temperature-induced frequency drift than other materials commonly used to manufacture resonators, such as AT-quartz (3750 ppm for silicon vs 20 ppm for AT-quartz over a temperature range of -40°C to 85°C).
  • AT-quartz 3750 ppm for silicon vs 20 ppm for AT-quartz over a temperature range of -40°C to 85°C.
  • the resonance frequency of the resonator cavity 204 will drift significantly due to the change in sound velocity in the sample fluid 206 contained within.
  • heating and maintaining the acoustic cell 202 and resonant transducer 216 at a precise temperature is necessary to enable reliable performance.
  • the temperature-induced frequency drifts of the resonator cavity’s 204 resonance frequency and of the resonant transducer’s 216 resonance frequency can also be exploited to correct for any frequency variations that may be introduced by manufacturing tolerances of those components, ensuring operation close to the optimal detuning 8 0 .
  • Heating the acoustic cell 202 also has the added benefit of reducing the adsorption of analytes on the acoustic cell 202 and resonator cavity 204 inner walls.
  • a heat exchanger 230 may be machined into the external structure of the acoustic cell 202.
  • Plot 400 in Figure 4 depicts an example of using temperature as a tuning parameter, plotting the effect of the acoustic cell 202, resonant transducer 216 and sample fluid 206 temperature (which are assumed to be equal in this example) on the measured photoacoustic system response as a function of frequency.
  • the curves shown correspond to increasing temperatures from the bottom to the top and are offset vertically for visibility.
  • the frequency of both the resonator cavity 204 and the silicon-based resonant transducer 216 shifts with temperature, although the resonant transducer 216 frequency shift is not only due to changes in the temperature-dependent elastic constants of silicon.
  • the resonant transducer 216 (in this example, a piezoelectric MEMS pressure transducer) is only expected to shift approximately -30 ppm/°C, whereas the observed shift is approximately -540 ppm/°C. This shift is mostly due to the change in acoustic properties of the volume of sample fluid 206 located behind the resonant transducer 216 in the encapsulation volume 222.
  • the second resonant frequency of the resonant transducer 216 or both resonant frequencies are configured to be tunable such that the system can be tuned to at least one of (i) the closely- tuned configuration and (ii) the optimally-tuned configuration.
  • tunable refers to a parameter which is deliberately controlled to produce a desired output. While the material properties of the sample fluid 206, acoustic cell 202, resonator cavity 204, and resonant transducer 216 may vary with environmentally-induced changes such as temperature, pressure or humidity, these parameters must be externally controlled in some way to be considered tunable.
  • the system further includes the control systems 232 that are configured to at least stabilize one or a combination of (i) the temperature of the sample fluid 206, (ii) the pressure of the sample fluid 206, (iii) the humidity of the sample fluid 206, (iv) the mass flow of the sample fluid 206 through the acoustic cell 202, (v) the temperature of the resonant transducer 216, and (vi) the temperature of the acoustic cell 202.
  • the control systems 232 are configured to at least stabilize one or a combination of (i) the temperature of the sample fluid 206, (ii) the pressure of the sample fluid 206, (iii) the humidity of the sample fluid 206, (iv) the mass flow of the sample fluid 206 through the acoustic cell 202, (v) the temperature of the resonant transducer 216, and (vi) the temperature of the acoustic cell 202.
  • the first resonant frequency of the resonator cavity is the first resonant frequency of the resonator cavity
  • Tl first temperature
  • T2 second temperature
  • stabilizing the temperature of the acoustic cell 202 may be understood to mean stabilizing the temperature of one or a combination of i) the resonant transducer 216, ii) the resonator cavity 204, iii) the sample fluid 206 within the resonator cavity 204 and iv) the entirety of the acoustic cell 202.
  • the resonant transducer 216 is selected from one or a combination of (i) an out-of-plane resonator, (ii) a tuning fork resonator, (iii) a cantilever resonator, and (iv) a diaphragm resonator.
  • the resonant transducer 216 is a MEMS device.
  • the acoustic cell 202 further includes at least one acoustic frequency filter 226 operatively connected to the resonator cavity 204.
  • an ‘acoustic frequency filter’ is any type of filter configured to reduce the transmission of noise to the resonator cavity 204.
  • the at least one acoustic frequency filter 226 is selected from i) a band-stop filter, ii) a low-pass filter or iii) an attenuation filter, which could be implemented as one or a combination of quarter-wave tubes, buffer volumes or acoustical attenuators.
  • the at least one acoustic frequency filter 226 may be connected to the at least one port 112 and/or the resonator cavity 204.
  • the acoustic cell 202 further includes at least one transducer enclosure 222, and wherein each of the at least one transducer enclosure 222 encapsulates a volume of sample fluid 206 surrounding at least one active surface of the resonant transducer 216, separating the at least one active surface of the resonant transducer 216from an external environment.
  • the readout system 218 is configured to sense the resonant transducer 216 using a piezoelectric material.
  • the readout system 218 may be configured to sense the resonant transducer 216 using a piezoelectric material.
  • the readout system 218 further includes a pre-amplifier proximal to the resonant transducer 216.
  • the resonant transducer 216 is mounted on a printed circuit board (PCB) 228, wherein the PCB 228 is mounted to the acoustic cell 202 such that the resonant transducer 216 is operatively connected to the resonator cavity 204, and wherein at least some of the readout system 218 is located on the PCB 228.
  • PCB printed circuit board
  • the acoustic cell 202 is constructed using a monolithic design including a resonator cavity 204, at least one acoustic frequency filter 226, and an acoustic port 220 which are machined from a single piece of material.
  • a monolithic design may be desirable for reducing assembly steps and ensuring fixed positioning and good thermal contact between components, as well as with the portion of the resonant transducer unit 214 comprising the resonant transducer 216 that is mounted on this monolithic acoustic cell.
  • the acoustic cell 202 constructed using the monolithic design further includes a heat exchanger 230 machined into the acoustic cell 202 that allows the sample fluid 206 to reach thermal equilibrium with the acoustic cell 202 at least before entering the resonator cavity 204.
  • the Q-factor of the resonant transducer 216 in vacuum is configured to be substantially larger than the Q-factor of the resonant transducer 216 in the sample fluid 206.
  • the Q-factor of the resonant transducer 216 in vacuum is configured to be greater than 1000.
  • the coupling strength 12 between the resonator cavity 204 is the coupling strength 12 between the resonator cavity 204
  • I 2 and the resonant transducer 216 is greater than or equal to the threshold value of /7r-T - m 0 .
  • the readout system 218 has an input-referred noise amplitude spectral density of 0, the sample fluid 206 in the resonator cavity 204 and the resonant transducer 216 have a temperature of T and the Q-factor Q 2 of the resonant transducer 216 is greater than or equal to the threshold value of 2a> ° m2 .
  • both the sample fluid 206 in the resonator cavity 204 and the resonant transducer 216 are at a temperature T
  • the operating frequency is configured to be the first resonant frequency of the resonator cavity 204
  • the detuning is configured to be zero
  • the coupling strength satisfies 12 4 > 4 7 -
  • the Q-factor of the resonant transducer 216 is configured to be greater than or equal to the threshold value such that the noise in the electrical signal transduced by the resonant transducer unit 214 is mostly due to the contribution of the intrinsic thermal acoustic noise of the resonator cavity 204.
  • the acoustic cell 202 further includes at least one acoustic port 220.
  • the acoustic cell 202 further includes at least one optical window 224.
  • the emitter 210 includes one from a selection of (i) a quantum cascade laser (QCL), (ii) a continuous wave (CW) laser, (iii) a pulsed laser, (iv) an interband cascade laser (ICL), (v) a vertical-cavity surface-emitting laser (VCSEL), and (vi) a thermal emitter.
  • QCL quantum cascade laser
  • CW continuous wave
  • ICL interband cascade laser
  • VCSEL vertical-cavity surface-emitting laser
  • VCSEL vertical-cavity surface-emitting laser
  • control circuit 212 is configured to at least modulate the output of the emitter 210, wherein modulating the output of the emitter 210 includes modulating one or a combination of (i) an emission wavelength of electromagnetic radiation (ii) an emission intensity of electromagnetic radiation (iii) a pulse repetition rate of electromagnetic radiation (iv) a patterned train of pulses of electromagnetic radiation.
  • the readout system 218 further includes a pre-amplifier, wherein the pre-amplifier consists of one from a group of (i) a differential charge amplifier, (ii) a differential transimpedance amplifier, (iii) a single-ended voltage amplifier, (iv) a single-ended charge amplifier or (v) a single-ended transimpedance amplifier, (vi) an instrumentation amplifier or (vii) another pre-amplifier architecture now known or later developed.
  • the pre-amplifier consists of one from a group of (i) a differential charge amplifier, (ii) a differential transimpedance amplifier, (iii) a single-ended voltage amplifier, (iv) a single-ended charge amplifier or (v) a single-ended transimpedance amplifier, (vi) an instrumentation amplifier or (vii) another pre-amplifier architecture now known or later developed.
  • the readout system 218 is configured to at least convert the displacement of the resonant transducer 216 into one or more electrical signals and amplify and process these one or more electrical signals.
  • the acoustic cell 202 is configured to be sealed with the sample fluid 206 contained within.
  • the acoustic cell 202 further includes at least one port 112 connected to a fluid handling system 302.
  • the fluid handling system 302 is at least partly composed of low adsorption materials, such as glass, ceramic, aluminum, stainless steel, fluoro-polymers (PTFE, PF A, etc.). In some embodiments, at least part of these components are coated with a passivation layer. In a preferred embodiment, the fluid handling system 302 is at least partly composed of metal components coated with a silicon-based passivation layer.
  • low adsorption materials such as glass, ceramic, aluminum, stainless steel, fluoro-polymers (PTFE, PF A, etc.
  • PTFE fluoro-polymers
  • the resonator cavity 204 has an absolute rate of change of the first resonant frequency as a function of temperature (
  • one or both of the first resonant frequency of the resonator cavity 204 and the second resonant frequency of the resonant transducer 216 are configured to be tunable with pressure.
  • one or both of the first resonant frequency of the resonator cavity 204 and the second resonant frequency of the resonant transducer 216 are configured to be tunable with humidity.
  • the Q-factor of the uncoupled resonant transducer 216 in the sample fluid 206 is configured to be substantially larger than the Q-factor of the resonator cavity 204.
  • the method further includes operating the system in an optimally tuned configuration.
  • the resonator cavity supporting a first resonance mode having a first resonant frequency further includes a plurality of resonator cavities supporting a first resonance mode having a first resonant frequency
  • the resonant transducer supporting a second resonance mode having a second resonant frequency further includes a plurality of resonant transducers supporting a second resonance mode having a second resonant frequency
  • the coupling of the first resonance mode of the plurality of resonator cavities and the second resonance mode of the plurality of resonant transducers is configured to be in the significant coupling regime and closely tuned
  • the emitter transmits electromagnetic radiation through the plurality of resonator cavities and wherein the gas handling system is configured to circulate the sample fluid throughout the plurality of resonant cavities.
  • the resonator cavity supporting a first resonance mode having a first resonant frequency further includes a plurality of resonator cavities supporting a first resonance mode having a first resonant frequency
  • the resonant transducer supporting a second resonance mode having a second resonant frequency further includes a plurality of resonant transducers supporting a second resonance mode having a second resonant frequency
  • the electromagnetic radiation unit comprising an emitter and a control circuit further includes a plurality of emitters; the coupling of the first resonance mode of the plurality of resonator cavities and the second resonance mode of the plurality of resonant transducers is configured to be in the significant coupling regime and closely tuned; wherein each emitter from the plurality of emitters transmits electromagnetic radiation through one resonator cavity from the plurality of resonator cavities and wherein the gas handling system is configured to circulate the sample fluid throughout the plurality of resonant cavities.
  • Step 512 includes providing a photoacoustic spectroscopy system 200 or 250 as described above.
  • Step 514 includes operating the emitter 210 in a modulated mode, such that the target analyte molecules in the sample fluid 206 of the resonator cavity 204 experience periodic absorption and heating at an operating frequency.
  • Step 516 includes configuring the acoustic cell 202 such that the periodic heating induces an acoustic pressure wave with an amplitude proportional to a concentration of target analyte molecules in the sample fluid 206 of the resonator cavity 204 thus exciting the coupled resonator cavity and resonant transducer modes (in other words exciting the coupled first resonance mode and second resonance mode).
  • Step 518 includes detecting the excitation of the coupled resonator cavity and resonant transducer modes by measuring the displacement of the resonant transducer 216 in a frequency-dependent basis with the readout system 218.
  • Step 520 includes outputting via the readout system 218 a signal representative of the concentration of target analyte molecules.
  • the method further includes configuring the control systems 232 in such a way as to stabilize the temperature of the sample fluid 206, the resonant transducer unit 214 and the resonator cavity 204 such that the detuning between the first resonance mode and the second resonance mode is actively stabilized and maintained in either the closely- tuned or the optimally-tuned configuration.
  • the method further includes increasing the amplitude of the signal transduced by the resonant transducer 216 to improve the signal-to-noise ratio of the readout system 218 by one or a combination of increasing the coupling strength by one or a combination of i) selecting a resonant transducer 216 geometry with a large active surface area and ii) positioning the resonant transducer 216 and the associated acoustic port 220 at a pressure maximum in the resonator cavity 204, and iii) increasing the resonant transducer 216 Q-factor via a reduction of the coupling to vorticity and thermal waves and operating the photoacoustic spectroscopy system at the closely-tuned or the optimally-tuned configuration.
  • the readout system 218 possesses an input-referred noise amplitude spectral density and the sample fluid 206 in the resonator cavity 204 together with the resonant transducer 216 are at a temperature of f, and wherein the method further includes providing the photoacoustic spectroscopy system whereby the contribution of the intrinsic thermal acoustic noise of the resonator cavity 204 to the total output noise of the resonant transducer unit
  • the noise of the electrical signal transduced by the resonant transducer unit 214 is mostly due to the contribution of the intrinsic thermal acoustic noise of the resonator cavity 204 by: selecting the resonant transducer 216 with a
  • the method further includes operating the control circuit

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Abstract

Système de spectroscopie photoacoustique comprenant une cellule acoustique, une unité de rayonnement électromagnétique et une unité de transducteur résonant. La cellule acoustique comprend une cavité de résonateur qui prend en charge un premier mode de résonance possédant une première fréquence de résonance, et peut être fournie avec un fluide échantillon. Le transducteur résonant prend en charge un second mode de résonance possédant une seconde fréquence de résonance, et est connecté de manière fonctionnelle à la cavité de résonateur. Le premier mode de résonance et le second mode de résonance sont configurés pour être couplés dans le régime de couplage significatif et l'accord entre le premier mode de résonance et le second mode de résonance est configuré pour produire une augmentation de l'intensité du signal ou du rapport signal sur bruit.
PCT/CA2024/050827 2023-06-20 2024-06-19 Plateforme de capteur photoacoustique à double résonance optimisée par couplage pour la détection de gaz et leurs concentrations Ceased WO2024259528A1 (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10908129B2 (en) * 2016-12-13 2021-02-02 Pendar Technologies, Llc Devices and methods for quartz enhanced photoacoustic spectroscopy

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10908129B2 (en) * 2016-12-13 2021-02-02 Pendar Technologies, Llc Devices and methods for quartz enhanced photoacoustic spectroscopy

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* Cited by examiner, † Cited by third party
Title
LEE TAEHWA, LI XIAOPENG, YU ZIQI, NOMURA TSUYOSHI, DEDE ERCAN M., IIZUKA HIDEO: "Coupled acoustic resonance for wave control and sensing", FRONTIERS IN PHYSICS, vol. 10, XP093258507, ISSN: 2296-424X, DOI: 10.3389/fphy.2022.998253 *
RÜCK THOMAS: "Development, characterization and miniaturization of a trace gas detection system for N02 in air based on photoacoustic spectroscopy", DOCTORAL DISSERTATION, UNIVERSITÄT REGENSBURG, 1 January 2017 (2017-01-01), XP093258509 *

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