Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/32—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
  • H01S5/327—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIBVI compounds, e.g. ZnCdSe-laser
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/1053—Comprising an active region having a varying composition or cross-section in a specific direction
  • H01S5/1067—Comprising an active region having a varying composition or cross-section in a specific direction comprising nanoparticles
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
  • H01S5/341—Structures having reduced dimensionality, e.g. quantum wires
  • H01S5/3412—Structures having reduced dimensionality, e.g. quantum wires quantum box or quantum dash
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
  • H01S5/041—Optical pumping

Definitions

• This technology relates generally to lasers.
• this technology relates colloidal quantum dot lasers.
• CQDs Colloidal quantum dots
• the methods and materials described herein contemplate the use films of colloidal quantum dots as a gain medium in a vertical-cavity surface-emitting laser.
• the present disclosure demonstrates a laser with single-exciton gain in the red, green, and blue wavelengths. Leveraging this nanocomposite gain, the results realize a significant step toward full-color single-material lasers.
• a laser includes a film of colloidal quantum dots as the optical gain medium.
• the colloidal quantum dots comprise hydrophobic surface ligands.
• the colloidal quantum dots comprise aromatic surface ligands.
• the colloidal quantum dots have a core-shell structure comprising a core and a shell.
• the core diameter is in the range of 2-10 nm.
• the shell thickness is in the range of 1-5 nm.
• the core-shell bandgap difference is about 1-2 eV as measured for bulk-equivalent materials.
• the colloidal quantum dots are CdSe/ZnCdS.
• the colloidal quantum dots are type-I quantum dots.
• red, green, and blue wavelength lasing is enabled.
• the gain medium is enabled by a single-exciton gain mechanism.
• the packing density of colloidal quantum dots in the film is between 50% and 100%.
• the film is a spin-cast self-assembled film.
• the amplified spontaneous emission threshold of the optical gain medium is at energy densities less than 90 ⁇ . ⁇ / ⁇ 2 .
• the laser's stimulated emission time exceeds the Auger time constant of the quantum dot film.
• the film supports more than 1 GHz frequency acoustic wave propagation.
• the film of colloidal quantum dots is positioned in a resonant laser cavity.
• the laser is a vertical-cavity surface-emitting laser.
• Figure 1 shows the structural characteristics of CQDs and their spin-cast closely packed films, according to one or more embodiments,
• TEM Transmission electron microscope
• image of a red CQD cluster displaying finite shape anisotropy.
• High resolution TEM of a single red CQD demonstrating a well-defined lattice of crystalline structure
• c Angled cross-sectional view of CQD film on fused silica under scanning electron microscopy. The small grains in the image are due to 15 nm thick metallic (Au-Pd) coating to reduce charging effects and image distortion.
• Au-Pd metallic
• Figure 3 shows the dynamics of CQD light emission from epitaxial-like thin films under various excitation conditions, according to one or more embodiments, (a) Spectral analysis of the edge emission in stripe excitation for red CQD film, integrating for two different time windows: 0 - 40 ps and 300-640 ps after impulsive pumping, (b) PL decay of the red CQD in small round "spot" excitation geometry at different preparations. The spot diameter is 22 urn, whose gain length is incapable of supporting ASE. The two exponential decay rate fitting is for CQD film at 100 ⁇ / ⁇ 2 excitation, (c) High temporal resolution of the fast PL and the ASE decays in the spot and stripe excitation geometries, respectively.
• FIG. 4 shows optically pumped CQD-VCSELs in the red and green, according to one or more embodiments,
• CQD gain media was placed inside a resonant cavity which was made by two distributed Bragg reflectors (DBRs) with reflectivity higher than 99%. Laser light is produced above threshold,
• (b) Photographic images of red and green CQD-VCSELs showing spatially well-defined output beams which are collinear with the pump beam (Fig. 1 1).
• Emergence of laser modes from spontaneous emission in a CQD-VCSEL when increasing pump power Emergence of laser modes from spontaneous emission in a CQD-VCSEL when increasing pump power.
• Figure 5 shows energy-dispersive X-ray spectroscopy analysis of the red CQD films showing all four elements of CdSe/Zn x Cdi_ x S core/shell structure, according to one or more embodiments.
• Figure 6 shows the results of pump-probe experiment used to study high frequency ultrasound propagation in the densely packed CQD thin films, according to one or more embodiments,
• (a) The transient reflected probe-beam intensity (indicated as signal on lock-in amplifier), as a function of probe beam delay time after impulsive pump at t 0 ps; inset, the material structure for experiment, where 90 nm thick Aluminum acousto-optic transceiver layers were deposited on 300 nm thick CQD films (spin casting the high concentration CQD solution on fused silica substrate),
• the coherent oscillations appear as the telltale signal for showing the films' ability to support > 1 GHz frequency acoustic wave propagation, thus demonstrating the epitaxial-like thin film material.
• Figure 7 shows spectral analysis of edge emission in stripe excitation configuration and absorbance of the blue CQD film, according to one or more embodiments.
• the ASE threshold is clearly demonstrated by abrupt intensity increase and spectral narrow.
• Figure 8 shows stripe width measurements, according to one or more embodiments, (a) A sketch of the experiment setup with a movable blade to scan along and across the focused excitation beam; a power meter was used to measure transmission intensity of the beam after the blade, (b) The measured intensity as function of blade position (z- axis) whereby its first derivative yields the beam profile, (c) The focused beam width as a function of blade position (x- axis).
• Figure 9 shows analysis of CQD film absorbance, according to one or more embodiments, (a) The raw absorbance of a 'green' CQD film with the visible lowest exciton state near 535nm and the superposed 'interpolating background curve' (in red), (b) After subtraction of the interpolating function, revealing several Gaussian absorption peaks.
• Figure 10 shows the edge emission intensities as functions of stripe length in green and red CQD films, according to one or more embodiments.
• Figure 11 shows red CQD-VCSEL in operation without a long pass filter to show the spontaneous emission and the laser beam as well as the coaxial unabsorbed, focused pump laser beam (white spot on the screen), according to one or more embodiments, (a) Below threshold and (b) Above threshold excitation.
• Figure 12 shows ASE of the red CQD film pumping by a compact 1 kHz repetition rate sub-nanosecond pulsed 532 nm laser, according to one or more embodiments, (a) Intensity of edge emission as a function of pump energy density per pulse, (b) Transient ASE at two different pumping energy density, time referenced to the pumping laser.
• CdSe-based CQDs are widely applied from fluorescent bio-labels to prototype light- emitting diodes.
• their use as gain media for lasers has been challenged by both practical and fundamental obstacles.
• One key challenge is realization of high-performance optical gain medium, wherein each individual CQD preserves its quantum size effect and high quantum efficiency and a dense assembly of CQDs offers a homogeneous optical continuum.
• requirement for light amplification is that the ensemble averaged number of exciton per CQD is greater than unity, ⁇ N> > 1, signifying an exact balance between absorption and stimulated emission for ensemble averaged single exciton occupancy per CQD. This condition is referred to as biexciton gain.
• the gain threshold When adding the electronic structure and inclusion of biexciton states", the gain threshold will be closer to ⁇ N> -1.5 whereupon a fundamental complication comes from enhanced non-radiative multiexciton (Auger) recombination in nanometer size systems.
• the Auger process for example in CdSe-based CQDs (xAu g er ⁇ 100 ps), is two orders faster than photoluminescence (PL) decay, severely hindering the dynamic buildup of population inversion.
• Quantum rod structures were investigated to reduce Auger processes but this also dilutes joint electronic density of states thereby reducing interband oscillator strength.
• the present disclosure provides a quantum yield (QY) exceeding 80% at
• Fig. 2a Emission from the film edge with increasing pump power exhibits a clear transition from PL to stimulated emission (here observed as ASE) by an abrupt increase in output intensity and spectral narrow (Fig. 2b-e).
• Fig. 2c displays the ASE thresholds for CQD films of 90 ⁇ . ⁇ / ⁇ 2 , 145 ⁇ . ⁇ / ⁇ 2 and 800 ⁇ ]/ ⁇ 2 in RGB colours, respectively.
• the values for red and green films represent an improvement of more than one order of magnitude from commonly reported values (> 1 mJ/cm 2 ) for similar colours in both type-I" and type-II CQDs.
• the ASE for deep blue CQD films is demonstrated here for the first time ( Fig.
• the transient spectra of the ASE in Fig. 3 a show that the spectrally narrow, red- shifted ASE only occurs at very fast time scales (0-40 ps) after impulsive excitation. In the subsequent time window (300-640 ps), the broader PL is the dominant emission. Under small diameter (22 ⁇ ) circular "spot" excitation geometry (Fig. 3b) where short optical path length prevents ASE, PL at high excitation level (>ASE threshold) shows two identifiable decay rates. The slower rate is similar to single-exciton decay in the low excitation regime, Ti ⁇ 14.2 ns, comparable to that of CQD solution (-15.8 ns).
• the number of excitons per CQD, n can be taken as a Poisson
• VCSELs are among the most challenging device configurations for any optical gain material.
• the present disclosure provides the first optically pumped CQD-VCSELs (Fig. 4a).
• Fig. 4b, c show two CQD-VCSELs lasing with well- defined spatial coherence (beams) in the red and green, respectively.
• the PL spectra in Fig 4d shows Fabry-Perot microcavity modes. Above threshold, sharp laser lines appeared in the CQD gain spectrum coincident with the cavity modes and emergence of a spatially coherent beam.
• the inset of Fig. 4d shows the single mode green CQD-VCSEL at shorter cavity length.
• the overall efficiency of the red CQD-VCSEL is 0.4% at excitation energy density of 140 ⁇ / ⁇ 2 , and lasting few minutes before fading out as commonly seen in other conventional VCSELs without any heat sinking. The performance can be vastly improved with device optimization.
• the effective refractive index of CQD films was measured by ellipsometry technique at a wavelength of 632.8 run (He-Ne laser). All the experiments in this work were performed at room temperature and under ambient conditions.
• the ultra-fast impulsive laser excitation source was a frequency-doubled amplified titanium-sapphire pulsed laser (Coherent Reg A Model 9000, 400 nm wavelength, 100 fs pulse width, 100 kHz repetition rate). While impractical in any real devices, such a source is advantageous for the investigative work which we report. As shown in Fig. 12, much longer pump laser pulses (sub-nanosecond) could likewise be used to elicit stimulated emission.
• a time correlated single photon counting system and streak camera for the transient spectroscopy experiments at two different time scales was employed.
• CQD solution was mildly squeezed and dried between these two DBRs which form a Fabry-Perot cavity.
• the density of self-assembled CQD wedge inside cavity can be improved to the level of quality of the spin-casted epitaxial-like CQD films for higher CQD-VCSEL performance.
• EDS Energy-dispersive X-ray spectroscopy
• Table SI Weight and atomic percentages of individual elements in CdSe/ Zn x Cdi_ x S core/shell structure.
• a nondestructive method using ultrafast optoacoustics to test for the material continuum of our laser films from their ability to support the propagation of sound waves was employed.
• the epitaxial- like CQD films (300 nm thick) were spin casted on fused silica substrate; then a 90 nm Aluminum layer was deposited atop the CQD films by electron-beam evaporation.
• the Al film absorbs the laser pulse and, through instantaneous thermal expansion (on ps time scale) acts as optoacoustic transducer to generate a high frequency (- 10 GHz) ultrasound impulse. This strain pulse then propagates into CQD film.
• the second ("probe") laser pulse arrives at the structure at normal incidence and its transient reflection (from strain induced index of refraction change) is monitored by a photodiode (plus lock-in amplifier electronics).
• Fig. 6a shows the modulated probe beam reflection as a function of delay time between pump and probe pulses.
• the very fast electronic cooling curve after absorbing pump pulse and very fast oscillation as a result of strain pulse propagating and reflecting inside aluminum transceiver layer were subtracted from raw data in Fig. 6a.
• Fig. 6b yields the useful subtracted signal which shows transient oscillations as a result of optical interference between a probe beam reflecting from the propagating strain pulse (in CQD films) and probe reflection beams from the Al surface.
• Such oscillations are telltale signs of the ability of a material to support high frequency (-10 GHz) acoustic waves and characterize the propagating ultrasonic wave fronts.
• the ASE behavior here is similar to the green and red CQD films, so that e.g. the spectra of edge emission in blue CQD films shows clear transition from PL to ASE when increasing pumping levels (Fig. 7).
• the abrupt intensity increase and spectral narrow occur when excitation level exceeds the threshold of -800 uJ/cm 2 .
• the value is high (by standards of this paper) and assigned in large part to the low absorbance of blue CQD films at our fixed 400 nm pump wavelength (in comparison with that of green and red CQD films).
• the average number of excitons per CQD in the epitaxial-like films under spot or stripe excitation condition can be accurately obtained from the following:
• the average volume of a single CQD (from TEM data) is denoted as V, and E p is excitation photon energy (3.1 eV).
• Fig. 8a sketches the setup where a sharp blade edge was aligned parallel to the stripe while the blade was moved across the stripe (z-direction), and the transmitted beam was captured by a calibrated average power meter. After measuring the power as a function of blade position (z-position), the first derivative of this quantity, representing the beam intensity profile, was fitted into a Gaussian function (Fig. 8b) to find the full-width at half maximum (FWHM) of the stripe. Then the blade was moved along the lens axis (X direction) to measure the waist and also depth of focus of the excitation (Fig. 8c).
• An 8 cm focal length lens used for spot excitation focuses the similar laser beam into a spot of 22 urn in diameter, the value which was measured by a very similar method.
• the measurements show the optical gain behaviors before reaching the saturation at the stripe length greater than 1.1 mm.
• the pumping levels are 120 ⁇ . ⁇ / ⁇ 2 and 155 ⁇ /cm 2 for red and green CQD films, respectively.
• Equation S3 is represented as a theoretical model line in Fig. 3d.
• the model agrees very well with experimental results for both green and red CQD films.
• Fig. 11 shows close-up photographic images in the experiment when the long pass filter was removed.
• the pumping level is below threshold, spontaneous emission from CQD-VCSEL illuminates the target screen weakly and almost uniformly (Fig. 11a).
• pump level exceeds the threshold (Fig. 1 lb)
• a red laser beam from CQD-VCSEL emerges as a spatially well defined and centered with respect to the transmitted pumping beam.
• the time resolved ASE (Fig. 8b) was measured by a fast photodiode (25 GHz) and a digital sampling oscilloscope (50 GHz).
• the pulse width of ASE output is seen to increase with pumping level so as to finally reach the duration of the pump laser pulse width which in turn is more than twice of Auger time constant for this CQD system.
• This result directly shows quasi-CW ASE from red CQD film without the inhibition by the Auger process and invites further device development.

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