WO2009070821A2 - Laser à semi-conducteurs comportant une couche d'absorption - Google Patents

Laser à semi-conducteurs comportant une couche d'absorption Download PDF

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Publication number
WO2009070821A2
WO2009070821A2 PCT/AT2008/000431 AT2008000431W WO2009070821A2 WO 2009070821 A2 WO2009070821 A2 WO 2009070821A2 AT 2008000431 W AT2008000431 W AT 2008000431W WO 2009070821 A2 WO2009070821 A2 WO 2009070821A2
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Prior art keywords
absorption layer
semiconductor laser
absorption
laser according
layer
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PCT/AT2008/000431
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German (de)
English (en)
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WO2009070821A3 (fr
WO2009070821A4 (fr
Inventor
Bernhard Basnar
Gottfried Strasser
Maximilian Austerer
Aaron Maxwell Andrews
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TU Wien
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TU Wien
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Priority claimed from AT19792007A external-priority patent/AT505915B1/de
Priority claimed from AT19802007A external-priority patent/AT505916B1/de
Application filed by TU Wien filed Critical TU Wien
Priority to EP08857009A priority Critical patent/EP2229715A2/fr
Publication of WO2009070821A2 publication Critical patent/WO2009070821A2/fr
Publication of WO2009070821A3 publication Critical patent/WO2009070821A3/fr
Publication of WO2009070821A4 publication Critical patent/WO2009070821A4/fr
Anticipated expiration legal-status Critical
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    • 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
    • H01S5/3401—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 having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
    • H01S5/3402—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 having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers intersubband lasers, e.g. transitions within the conduction or valence bands
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • 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
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2004—Confining in the direction perpendicular to the layer structure
    • H01S5/2018—Optical confinement, e.g. absorbing-, reflecting- or waveguide-layers
    • H01S5/2022—Absorbing region or layer parallel to the active layer, e.g. to influence transverse modes
    • 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
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
    • H01S5/2205—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure comprising special burying or current confinement layers
    • H01S5/2218—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure comprising special burying or current confinement layers having special optical properties
    • H01S5/2219—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure comprising special burying or current confinement layers having special optical properties absorbing
    • 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
    • H01S5/3211—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures characterised by special cladding layers, e.g. details on band-discontinuities

Definitions

  • the invention relates to a semiconductor laser with an emission wavelength changing absorption layer.
  • Spectroscopy methods especially in the mid-infrared (IR), terahertz and ultraviolet (UV) wavelength range, are in the fields of environmental analysis and organic and pharmaceutical synthesis of great importance and are also promising candidates for future use in security (eg airports).
  • semiconductor lasers diode lasers
  • QCLs quantum cascade lasers
  • Semiconductor lasers consist essentially of an inter-waveguide active region of semiconductor material, e.g. GaAs, through which electrical current passes, is usually pulsed ("pumped") to release photons due to the recombination of electrons from the conduction band with holes in the valence band of the semiconductor (interband junctions), which in turn stimulate the formation of further photons of the same wavelength.
  • radiation especially IR, i.e., heat radiation
  • Both types of lasers share the inherent disadvantages that the wavelength of the emitted light depends on the one hand on numerous factors such as the material of the active zone, the operating temperature, the dimensions of the active zone, the refractive index of the medium adjacent to the waveguide, etc., and on the other hand after the finished production of the laser, which takes place mainly by epitaxy of active layers on a substrate, is fixed. This means that a separate laser has to be produced for different wavelengths. For subsequent adjustment of the emission wavelengths, optical gratings can be used, whereby homogenization of the laser light is primarily caused by interference effects.
  • the grating can either be applied to the waveguide (eg by etching) ("distributed feedback", DFB) 1 etched through the active zone ("distributed Bragg reflector", DBR) or externally provided ("external cavity”).
  • DFB distributed feedback
  • DBR distributed Bragg reflector
  • external cavity the wavelength of a QCL can be shifted continuously (between 8.2 and 10.4 ⁇ m) (Appl Phys Lett., 89, 41115 (2006)).
  • the fabrication of such external cavity lasers is very cumbersome and, due to the external grid, can not be used for some applications, such as "lab-on-chip” (LOC) systems.
  • Displacement of the wavelength has also been achieved by applying certain media of higher refractive index than air to the waveguide to affect the effective refractive index of the waveguide.
  • QCLs were encapsulated in a microfluidic flow cell through which several liquids (various dipping oils, methanol) were passed.
  • the emission shift that could be achieved was only about 1 cm -1 (Optics Express 14 (24), 11660-11677 (2006)).
  • the waveguide was cladded with chalcogenide glass, eg As 2 S 3 , causing a shift in the laser light of (at best) 3.8 cm -1 .
  • the object of the invention was to find a way to be able to modulate the emission of a semiconductor laser after its production in an alternative, relatively simple manner and to a greater extent than in the prior art.
  • a semiconductor laser comprising: an active zone for generating electromagnetic radiation after electrical excitation, which consists of optionally doped semiconductor layers and is surrounded in two spatial axes four spatial directions of waveguide layers, wherein at least a portion of the the surface of a waveguide layer facing away from the active zone is optionally connected to a substrate; and at least one power supply and removal, e.g.
  • one or more of the waveguide layers are electrically non-conductive and form insulating layers between the active zone and current supply and removal; and which is characterized in that at least a part of at least one waveguide layer with an electromagnetic radiation generated by the absorption zone in the active zone, hereinafter referred to as absorption layer, of inorganic and / or organic material that in the emission wavelength range the laser shows nonhomogeneous absorbance, is in contact or consists of this material.
  • This absorption layer absorbs part of the photons formed in the active zone. In this way, depending on the absorbance of the material of the absorption layer, both shifts and enhancements of emission maxima of the original laser over the entire gain range - which usually extends over several dozen wavenumbers - as well as a narrowing ("sharpening") of its emission can be achieved.
  • Such an absorption layer is easy to produce and does not require complex process steps which significantly increase the production costs.
  • the material of the absorption layer has a lower effective refractive index than the active zone. This can lead to the formation of an evanescent field at the interface, which can increase the absorption and, as a result, cause a greater shift in the emission maximum of the laser, as will be explained in more detail later.
  • At least one absorption band of the material of the absorption layer which lies at least partially in the emission wavelength range of the laser, can be changed by at least one chromium effect acting on the layer.
  • the emission of the laser can be changed one or more times, i. E. With a single absorption layer, a plurality of different emission spectra of the semiconductor laser can be generated.
  • nonhomogeneous absorbance is meant that the absorptivity of the material of the absorption layer varies in the gain range of the laser in order to make a wavelength selection by absorption of photons of a certain wavelength.
  • the material does not need to have an absorption maximum in the emission range of the laser. Preferably, this is not the case, otherwise there may be too much attenuation of the power output. However, an edge or tail of an absorption maximum must reach into the gain range.
  • the absorption of the material of the absorption layer in the emission region changes greatly, i. Certain wavelengths are absorbed much more than others because this results in significant changes in the emission spectrum of the laser.
  • chroma effect is understood to mean not only the chromatic shift of absorption bands of a material or material mixture of the absorption layer ("color change"), but also a chromatic attenuation or enhancement of absorptions at the same wavelengths.
  • the change in the band due to chromium effects means both deliberately induced changes and changes in the environmental conditions that can not be influenced by the user.
  • a laser according to the invention in preferred embodiments, on the one hand selectively switched between at least two states and on the other hand can respond to external changes in the environment by emitting a modified light signal.
  • lasers according to the invention can be used, for example, as switching elements, signal transmitters or sensors.
  • Such a change is preferably a shift of one or more absorption maxima of the material to different wavelengths.
  • the effects of chromium can only cause an increase or decrease in absorption at a particular wavelength. In either case, the change must be at least sufficiently strong to significantly affect the emission spectrum of the semiconductor laser.
  • the usable for changing the absorption of the absorption layer Chromie are not particularly limited.
  • effects selected from halochromism, acid chromium, ionochromism, solvatochromism, thermochromism, photochromism, including heliochromism, electrochromism, piezochromism, and combinations thereof can be used to cause the material of the absorption layer to undergo a change that significantly alters its absorption characteristic.
  • Halochromism is generally understood to mean an absorption change due to changes in the charge state of molecules, especially (as the name implies) by salt formation. As special forms of these, acidochromism and ionochromism can be considered.
  • Solvatochromism includes absorption changes due to solvation, thermochromism, and piezochromism due to temperature and pressure changes, respectively.
  • Electrochromism is based on triggered by an external electric field electrochemical reactions, such as redox reactions.
  • photochromism involves light-induced changes in the absorption behavior, ie, state changes in the absorption layer due to changes in the incident light.
  • a special case is the field of heliochromism, which deals with changes in the spectrum of sunlight, ie in the UV / VIS range.
  • Halochromism, thermochromism, electrochromism and / or photochromism are preferably used in accordance with the present invention, since these effects are also applicable in miniaturized devices such as the subject semiconductor lasers without much effort and also can cause relatively rapid changes in state in the absorption layer, what the "switchability" of the laser improved since
  • the changes in the absorption characteristic of the absorption layer caused by chromium effects are at least partially, more preferably completely, reversible, since this enables a repeated switching of the laser.
  • the absorption layer if it is applied only to one or more waveguide layers and these do not consist thereof, not fixedly connected to these layers, but detachable or separable thereof.
  • the absorption layer merely rests on the waveguide and can be lifted off mechanically, for example, whereby the laser as a whole differs between at least two emission modes, namely the uncoated state by detachment of the absorption layer. layered state with overlying absorption layer, switchable.
  • the laser is even switchable between at least three emission modes, namely between the uncoated state, with the absorption layer lifted, and at least two chromatically different states in coated form.
  • the laser according to the invention can be used as a sensor, switching element or signal generator even without the use of any chromium effects of the absorption layer.
  • the removal of the absorption layer can be carried out in any desired manner, for example by lifting off a component coated with a film from the absorption layer material, for example platelets, by simple lever action or else by means of an electromagnet.
  • a component coated with a film from the absorption layer material for example platelets
  • an electromagnet Preferably, however, such a component is pressed against the waveguide by electrostriction and removed therefrom.
  • the component must be made of a material which shows reciprocal piezoelectric, ie electrostrictive, behavior, ie when the voltage applied undergoes deformation and returns to the ground state when the voltage ceases. Whether the component rests against the waveguide in the de-energized state or is only pressed against it under tension is not decisive.
  • the component can also consist exclusively of a film of the absorption material.
  • Such removal of the absorption layer can of course also take place if it forms a waveguide layer. In this case, after removal, an air layer would form the waveguide.
  • the absorption layer may consist of any material or material mixture capable of absorbing photons formed in the active zone, preferably of a material whose absorption characteristic is variable by one or more chromium effects. This may be solid, liquid or gaseous, preference being given to solid and liquid materials, inter alia for reasons of concentration.
  • the absorption occurs predominantly in the evanescent field present in the absorption layer of the photons guided at the interface between waveguide and absorption layer. These are thereby removed from the spectrum and can no longer induce emission of further photons, thus potentiating the effect of absorption of a photon in the sequence.
  • too few absorption-capable molecules remained in the evanescent field, so that too few absorption processes would be possible in order to have significant effects on the gain spectrum of the laser.
  • An increase of the gas concentration at the interface by higher gas pressure in a pressure-resistant embodiment of a laser according to the invention is possible, but not preferred, since such an embodiment is expensive to manufacture.
  • one or more waveguide layers can also be wholly or partly made of the material of the absorption layer and thus serve both as a waveguide and as an absorption layer. Whether waveguide layers are provided between the active zone and the absorption layer depends, inter alia, decisively on the design of the laser, on the material of the absorption layer and on the position of the respective waveguide in the laser device. When using solid materials at operating temperature as the absorption layer replacement of the waveguide by the absorption layer is sometimes easier than with liquids, especially if an inaccessible from the outside waveguide layer should be designed as an absorption layer or the absorption layer should be removable, which will be discussed later in more detail.
  • Liquid, but also solid absorption layers can be prepared in a simple manner, for example by brushing, spraying, spin coating or dip coating, e.g. Rotational coating (spin coating), vapor deposition or sputtering, be applied to the waveguide or as a waveguide, or the laser can be operated immersed in a liquid medium.
  • a liquid-filled chamber or flow cell described above, for each of which the adjacent waveguide layer or the surface of the active zone is the base area (or a part thereof), can serve as the absorption layer.
  • absorption layer materials are to be used whose absorption in the desired range is too strong - especially if this is not chromatically attenuatable, as should be the case in preferred embodiments - they can be diluted with substances which are nonexistent or weaker Show absorption in the gain range of the laser. This applies to all aggregate states of the absorption layer.
  • pure substances or mixtures thereof can be used as well as solutions (including solid solutions), suspensions, dispersions, slurries, pastes, gels, etc., which comprise these substances.
  • Solid materials as the absorption layer offer the advantage of a stable, high density of absorbent molecules at the interface and thus of permanently good performance.
  • the preferred, effected by the Chromie binen state changes in many cases faster.
  • the individual components should be capable of no or only predictable, desired reactions with each other in order to enable stable, reproducible conditions during laser operation.
  • the thickness of the absorption layer is not critical as long as it is sufficient to achieve the intended effect. It depends on the desired operating conditions of the laser and on the properties of the absorption material, above all on its absorbance and the chromium effect (s) which can be used in preferred embodiments, but also on the physical state and other physical and chemical properties (such as melting point, Boiling point, refractive index, thermal stability, any chemical reactivity with the environment, etc.) under the operating conditions.
  • the thickness of the absorption layer need not exceed the depth of the evanescent field, since no absorption occurs in this region can.
  • the person skilled in the art can easily determine the optimum thickness for his laser, wherein the cost-effectiveness of the application process must also be taken into account. The inventors have achieved good results with layer thicknesses of several hundred nanometers, but this range can be significantly exceeded or undercut if necessary.
  • any material suitable for the absorption layer which is capable of nonhomogeneous absorption in the wavelength range of the laser is suitable, but preferred is a material whose state is changeable by chromium effects.
  • the semiconductor laser according to the present invention preferably has an emission wavelength in the range of UV radiation, preferably near UV radiation, to terahertz radiation, ie from about 100 nm to about 1 mm. More preferably, the emission wavelength is in the infrared range, especially in the mid-infrared range, ie, between about 1 ⁇ m, preferably about 3 ⁇ m, and about 15 ⁇ m.
  • a quantum cascade laser conventional design can be used, which is known to emit mostly in the IR to terahertz range.
  • heat radiation emitting lasers a very wide range of inorganic and especially organic compounds is possible.
  • UV or visible range of the electromagnetic spectrum absorbing compounds are also available in large numbers. It is easily possible for the person skilled in the art, using a high-resolution IR or terahertz or UV spectrum, to select suitable compounds for the absorption layer which are applied to the respective laser.
  • the material of the absorption layer in preferred embodiments can also exhibit more than one chromium effect, eg thermochromism and photochemistry, which preferably causes it to have at least three absorption states, ie (for the above example) the ground state thermochromic and a photochromatically altered state.
  • more than one chromium effect is meant those cases in which the absorption of the material of the absorption layer can be chromatographed in two or more different ways - either because a single material of the absorption layer is capable of doing so, or because it is a mixture of materials, but also those in which the absorption of a single material by the same effect of chromium is more than a maximum deviating from the ground state maximum is displaceable.
  • the latter is comparatively rare in view of the narrow gain range of the lasers.
  • many compounds and classes of compounds that exhibit chromium effects there are also a number of compound classes, compounds, and combinations thereof that address more than one type of environmental change.
  • the invention is not particularly limited in this regard.
  • the average person skilled in the art by searching in corresponding spectral databases, is easily able to find potentially suitable materials for the absorption layer for the particular laser with a specific emission spectrum. Due to the very narrow emission range of semiconductor lasers, it is difficult to make general assignments of specific compounds or classes of compounds to defined wavelengths herein.
  • organic compounds which are known to exhibit at least one (mostly more than one) chromium effect: spiropyranes, spirooxazines, spirodihydroindolizines, azo compounds, polycyclic aromatic compounds, polycyclic quinones, chromenes, acetanilides, anils, aldehyde hydrazones, fulgides, fulgimides, Diarylethenes, stilbenes, rhodamines, triarylmethanes, viologens, retinals and phytochrome.
  • electrochromy is of great importance since, for example, metals which may be present in multiple charge states are easily changeable by redox reactions between two or more absorption states.
  • the following systems may be mentioned as examples: oxides of Co 1 In / Sn, Ir, Mo, Ni, W, V, Ce, Fe, Mn, Nb, Pa, Rh, Ru and Ti; Phthalocyanines of metals, eg Lu; Berlin Blue, alone or in a mixture with other inorganic (eg WO 3 ), organic (eg aniline) or mixed compounds (eg ytterbium bisphthalocyanine); Analogs of Berlin blue, such as ruthenium red, osmium red and hexacyanoferrate of V 1 Ni and Cu; and hexacyanometallates of various metals.
  • suitable organic compounds for electrochromism are viologens, ie bipyridinium systems which comprise, for example, i, i'-dimethyl-M'-bipyr
  • electroactive electroconductive polymers e.g. Electrochromes based on polyanilines, polypyrroles, polythiophenes and polycarbazoles; and other organic electrochromes, e.g. Methoxybiphenyls, quinones, diphenylamines and phenylenediamines and pyrazolines.
  • any compound colored in the normal state and / or after chromatic shift is suitable for the visible spectrum. Therefore, for all types of chroma, it is for a person skilled in the art to determine those compounds which show inhomogeneous absorption in the specific, narrow gain range of the respective laser and have at least two absorption states within or preferably in the vicinity of the gain range, between which Material by means of Chromieeffek- th at least once, but preferably repeated, "switched" back and forth. An example of such selection will be described in the later embodiments.
  • Particularly preferred materials for the absorption layer according to the present invention are pyrazine, poly (meth) acrylic acid, 7-acetoxycumarin, N- (chlorophenyl) -salicylaldimine (chlorosalicylideneaniline), dehydrodianthrone, o-nitrobenzylidene, indigo and thioindigo, combinations and derivatives thereof as described in US Pat Others is executed in the exemplary embodiments.
  • two opposing waveguide layers are made of electrically nonconducting material and therefore can simultaneously serve as insulating layers between the active zone and the current supply and removal.
  • the material of the absorption layer is electrically insulating, at least one of the insulating layers, only a part of it, but also all insulating layers, are also made of this material, which can enhance the effect of the invention, if more than one, the active zone surrounding waveguide layer serves as an absorption layer.
  • SiO 2 or SiN x is usually used as the insulating layer as a function of the wavelength range of the laser, but it is also possible for an air-filled space to serve as insulator.
  • At least one waveguide layer is entirely in contact with an absorption layer or consists entirely of the absorption material in order to achieve the strongest possible effect.
  • This is the method of choice especially for materials with not too strong absorbance in the gain range.
  • a plurality of waveguide layers may include or form an absorption layer.
  • only a part of one or more waveguide layers can be in contact with or consist of the absorption layer.
  • This variant can be selected for example in the case of particularly strongly absorbing compounds as the material of the absorption layer.
  • the absorption layer may also be formed in the form of a regular pattern. Combinations of full area and partial coverage of the active zone and / or multiple waveguide layers in different spatial directions are possible. For example, simultaneously serving as insulating layers waveguide layers consist entirely of the (non-conductive) absorption material, while another waveguide layer is covered with a grid-shaped absorption layer.
  • thermochromism the absorption layer does not necessarily have to be accessible from the outside, since temperature changes take effect in the entire laser device.
  • electric, piezo and photochromism as long as the respective absorption layer, if it is inside the device, is provided with electrical conductors or ter pressure or with an internal light source, such as a light emitting diode, or through a transparent "window" can be irradiated through.
  • Fig. 1 shows a schematic representation of an embodiment of a laser according to the invention.
  • Fig. 2 schematically illustrates the general operating principle of the invention.
  • Fig. 3 shows the emission spectrum of the laser produced in Example 1 with pyrazine as the material of the absorption layer.
  • Fig. 6 shows the emission spectra of the laser produced in Examples 1 to 4 in un coated state and with potassium polyacrylate as the material of the absorption layer.
  • Fig. 7 shows the emission spectra of the laser produced in Examples 1 to 4 in the uncoated state and with free polyacrylic acid as the material of the absorption layer.
  • Fig. 8 schematically illustrates the switchability of a polyacrylate system between salt and free acid.
  • Figure 10 shows sections of the IR spectrum of N- (4-chlorophenyl) salicylaldimine at four different temperatures.
  • FIG. 1 An embodiment of a semiconductor laser of the present invention in the form of a quantum cascade laser is schematically illustrated in FIG. 1 for illustrative purposes.
  • the reference numeral 1 designates the active zone in which the electromagnetic radiation is generated; 2a and 2b show two waveguide layers arranged above or below the active zone; 3 a substrate as a support for the active zone and for heat dissipation and to improve the current distribution in the active zone 1; 4 an inlet and outlet for electric current; 5a and 5b insulating layers between the active zone or the substrate and the Stromzuleitern 4; 6 an absorption layer according to the invention; and 7, a carrier plate for the laser, which simultaneously serves as a counter electrode.
  • an optionally pulsed direct current flows through the current conductors 4, which usually consist of gold or Ge / Au / Ni / Au layers, preferably with a titanium layer as adhesion promoter, into the laser at two points in the cross section of the laser Waveguide 2a enters, wherein the electric field can spread homogeneously.
  • the current conductors 4 usually consist of gold or Ge / Au / Ni / Au layers, preferably with a titanium layer as adhesion promoter
  • the primary photons formed by electrical excitation induce the jumps of excited electrons to lower energy levels, which is an amplifier effect.
  • the resulting photons are also identical in terms of energy, propagation direction and phase with the incoming stimulating photons, which is why the emitted radiation is coherent.
  • the insulating layers 5a and 5b serve not only to prevent current supply to the side surfaces of the active region 1, but also to guide the light, whereby the laser emission finally emerges from the laser in the plane of the active zone or at a small angle thereto.
  • the middle image of FIG. 2 schematically shows an emission spectrum of a quantum cascade laser without absorption layer.
  • the absorption layer according to the invention influences this spectrum, for which the following mechanism is assumed.
  • the radiation generated in the active zone 1 is also conducted at the interface between the waveguide 2a and the absorption layer 6, but in preferred embodiments, due to the lower refractive index of the absorption layer, an evanescent field is formed which is up to one half the wavelength depth corresponding to the radiation into the absorption layer 6, although - as the inventors were able to show in their experiments - only a few percent of the guided optical mode has a real probability of residence in the absorption layer.
  • a portion of these photons can thus be absorbed by the material of the layer which exhibits absorbance at these wavelengths, as shown schematically in the upper image of FIG.
  • the absorption spectrum of the material of the absorption layer at lower wavelengths ends at a further absorption maximum, which causes an idealized laser in contact with a material of this absorption characteristic to emit only wavelengths lying between the two absorption maxima, such as in the lower image of FIG. 2 is shown.
  • the absorption bands of the material of the absorption layer i. in the ground state and after the chromium effect applied thereto, from opposite sides into the emission wavelength range of the laser, whereby its emission maximum is effectively displaceable over a large part of the gain spectrum. If multiple chromium effects of one or more materials of the absorption layer are utilized, the emission must thereby be shifted to significantly different positions and / or significantly attenuated or enhanced so that the laser thus coated can emit distinct signals.
  • an embodiment of the invention also consists of a semiconductor laser coated with a material or material mixture, the absorption of which can be both chromatically shifted and chromatically attenuated or intensified.
  • the material can show both thermal and photochromism and its absorption can be photochromatically shifted and thermochromatically attenuated / reinforced or vice versa.
  • Another embodiment is a material of the absorption layer, which causes both a narrowing ("sharpening") of the emission spectrum of the laser and at least a shift thereof, of which (at least) an effect is induced chromatically.
  • the absorption layer 6 is shown in Fig. 1 as a film covering the entire surface of the laser, which is a possible embodiment of the invention.
  • this will be the variant of choice, as any rapid application method, e.g. Spin or spin coating with a liquid absorbent material or a solution of a solid and evaporation of the solvent can be used.
  • any rapid application method e.g. Spin or spin coating with a liquid absorbent material or a solution of a solid and evaporation of the solvent can be used.
  • the absorption layer is to be provided only on the waveguide or even only on parts thereof, or as a waveguide directly on the active zone, methods such as various variants of lithography may be used.
  • quantum cascade lasers were prepared by epitaxially growing active layer layers on a substrate, which were then provided with an absorptive layer as described in Examples 1-6.
  • Layers of AIGaAs, more specifically Al o, 45 Gao, 55 As, and GaAs were alternately grown on a GaAs substrate in a 3-well design, so that in total 40 cascades were received.
  • This active zone was transformed into a lightly doped (3 ⁇ 10 16 cm -3 ) GaAs core (below 2.7 ⁇ m, above 3.3 ⁇ m) with a bottom (ie towards the substrate) optical confinement of a 1 ⁇ m thick Alo .gGao.iAs layer.
  • the upper confinement was given a fair cladding by the difference in refractive index relative to the ambient air. ")
  • the uppermost, 100 nm thick, heavily doped (3 ⁇ 10 18 cm -3 ) GaAs layer was merely grown to achieve a homogeneous current distribution.
  • RIE reactive ion etching
  • 96 min 20 Ncm 3 SiCl 4
  • 20 mTorr base pressure 60 W high frequency
  • ICP inductively coupled plasma
  • PECVD plasma-enhanced chemical vapor deposition
  • Thickness ⁇ 300 nm (Oxford Systems PlasmaLab 80 plus);
  • Ti / Au sputtering Ti target 2 ⁇ 30 s, 25 W high frequency power;
  • Au target 15 x 40 s 25 W high frequency power (thickness: ⁇ 500 nm); - Gold removal with acetone in an ultrasonic bath, 1 min, 10% power.
  • Ti / Au sputtering Ti target 2 ⁇ 30 s, 25 W high frequency power;
  • the active region was produced by molecular beam epitaxy of alternate InGaAs / lnAIAs- layers in a lno, 5 3 Gao, 47As / lno, 52 Alo, 4 8 As-4-well design. Starting from a heavily doped (2 x 10 18 cm '3 ) InP substrate, a 2.2 ⁇ m thick, lightly doped (1 x 10 17 cm ' 3 ) InP layer was grown as a dielectric lower waveguide, followed by a 100 nm thick InGaAs buffer layer (1 x 10 17 cm "3).
  • the mixture was then made of 35 active layers, a 1, 84 microns thick active zone, followed by a further 600 nm thick InGaAs buffer layer (1 x 10 17 cm" 3) a 10 nm thick, heavily doped (7.2 ⁇ 10 18 cm -3 ) InGaAs layer for contact improvement, the upper part of the waveguide was designed as an air waveguide, to provide an interaction region with the absorption layer. Therefore, air windows were left above the laser bars and the laser devices were contacted via outer contact pads with 2 ⁇ m wide lateral contact strips on both sides along the bars.
  • the contact layers composed of Ti / Au stripes (10/500 nm), the x -lsolier Anlagen on a 300 nm thick SiN were datessput- tert. In this way, four laser structures with a length of 2.02 mm and variable width of 28, 38, 48 and 58 microns were produced. Fabry-Perot modes were emitted between 1285 and 1317 cm -1 .
  • This absorbent layer material was prepared according to SJ. Joglekar and S.D. Samant, J. Ind. Chem. Soc. 65, 110-111 (1988), according to the following reaction scheme of 4-chloroaniline and salicylaldehyde (both from Acros Organics) in ethanolic solution.
  • Example 1 QCL with an absorption layer of pyrazine
  • FIG. 3 shows a detail of the IR spectrum of pyrazine and the emission spectrum of the uncoated and the pyrazine-coated layers. sers (for comparison, in arbitrary units over the wavenumber in cm '1 ). Pyrazine has a slope of a broad absorption maximum falling from about 1150 cm “1 to about 1161 cm “ 1 . Since the laser 1150-1167 cm “1 emitted, has been attenuated by the absorption layer of pyrazine the largest part of the spectrum. The originally at about 1157 cm" emission maximum lies 1 was shifted by about 17 wavenumbers to about 1174 cm "1, where the The peak in the immediate vicinity of the original emission maximum had almost completely disappeared, except for the mid-pyrazin absorption maximum.
  • FIG. 4 shows a section of the IR spectra of potassium polyacrylate and of free polyacrylic acid.
  • Potassium polyacrylate has an absorption maximum of about 1345 cm -1 , which falls to lower wave numbers down to about 1280 cm -1 . Since the uncoated lasers emit at about 1285 to 1317 cm -1 (denoted as "laser emission” in FIG. 4), the absorption layer of the potassium salt should shift the emission maximum of the lasers to lower wavenumbers (redshift), whereas the free acid possesses a Absorption maximum at about 1260 cm '1 , so that the absorption layer should shift the emission maximum of the lasers towards higher wavenumbers (blue shift).
  • Fig. 5 shows from top to bottom the emission spectra of the QCLs prepared in Examples 2 to 5 with widths of 28, 38, 48 and 58 microns each without (dotted lines) absorption layer, with the absorption layer of potassium polyacrylate (solid lines) and with the Absorbent layer of free polyacrylic acid (dashed lines).
  • Fig. 6 for comparison and interpretation purposes, only the states “uncoated” (dotted) and “salt-coated” (continuous) are shown, and in Fig. 7 the states “uncoated” (dotted) and “acid-coated” (continuous).
  • the width of the laser also exerts a noticeable influence on its emission: the emission maxima of the uncoated QCLs differ from each other by several wavenumbers.
  • the absorption layers of potassium polyacrylate of Examples 2, 4 and 5 caused relatively slight displacements of the emission maxima compared with the uncoated QCLs.
  • the maximum was shifted by about 8 wavenumbers from about 1293 cm -1 to about 1301 cm -1
  • the emission range after application of the potassium polyacrylate absorption layer was between about 1290 cm -1 and about 1303 cm -1 .
  • the difference to the uncoated lasers in the transition to the free acid was even more pronounced due to HCI exposure , ie, due to halochromism, Figures 5 and 7 clearly show this fact.
  • the QCL of Example 2 with a width of 28 microns without coating had three strong emission maxima around 1292, 1298 and 1302 cm -1 , which were strong by the absorption layer of potassium polyacrylate, ie to a single maximum at about 1298 cm -1 . Due to the halochromatic shift of the absorption spectrum of the absorption layer upon release of the polyacrylic acid, these absorption bands were completely "extinguished.” Instead, the PAA coating For example, lasers stratified an extremely sharp emission peak at about 1309 cm -1 , where previously only minor emission had been observed.
  • the coated laser of the present invention emits sharp emission peaks in both chromatic states which are clearly distinguishable from each other.
  • the QCL with a width of 38 ⁇ m used in Example 3 without coating had only a strong emission maximum of approximately 1293 cm -1 and two weak maxima of approximately 1285 cm and approximately 1303 cm -1 .
  • the two weak maxima were completely "erased” from the spectrum by the absorption layer in both states; the strong maximum was considerably attenuated by the salt layer and also completely extinguished by the acid layer.
  • the coated laser had an emission maximum of about 1301 cm -1 (salt) or about 1307 cm -1 (acid), where in both cases hardly any emission had been observed.
  • the emission maximum was thus shifted by 8 wave numbers by the absorption layer of PAA-potassium salt - although it was blue-shifted contrary to expectations - and homogenized and blue-shifted by the chromium effect by a further 6, ie by a total of 14 wavenumbers.
  • the difference of 6 wavenumbers between the two chromatic states of the absorption layer is again clearly detectable and detectable.
  • the 48 ⁇ m wide QCL used in Example 4 had a very broad emission maximum of about 1301 cm -1 (from about 1288 to 1310 cm -1 ) without coating. This broad maximum was concentrated by the absorption layer from the potassium salt and redshifted by about 4 wavenumbers to about 1297 cm -1 . With chromatic shift of the absorption of the absorption layer, both maxima were again almost completely “extinguished” or at an emission maximum of about 1310 cm " 1 blue-shifted, where previously had hardly observed emission.
  • the emission maximum of the laser from Example 4 was determined by the Absorption layer of PAA thus by 9 wave numbers (from the ground state) or by 13 wave numbers halochromatically shifted and homogenized. The halochromatic effect is therefore even more pronounced in this example than in Examples 2 and 3.
  • the 58 ⁇ m wide QCL used in Example 5 had a fairly sharp emission peak at about 1297 cm -1 without coating, which was redshifted by the salt coating by about 3 wavenumbers to about 1294 cm -1 . These emissions were completely blue-halochromed to an emission maximum of about 1311 cm -1 (double peak at about 1310 and 1312 cm -1 ), where no emission had previously been observed. The emission maximum of the laser was thus blue-shifted by 14 wave numbers compared to the ground state by the absorption layer of PAA and halochromatically to the salt coating by 17 wavenumbers, thus representing the strongest chromium effect of all four examples of this group.
  • the quantum cascade lasers prepared in Examples 2 to 5 as well as similarly constructed are not only evidence of the modulability of the emission spectrum of such lasers by chromium effects, but also in practice as signal generators or transducers Switch or as sensors for detecting environmental changes used.
  • the discharge of ammonia gas in a chemical laboratory or from a tank by means of a semiconductor laser of the present invention mounted therein could be determined within seconds by means of the changed light signal.
  • 7-Acetoxycumarin (7-acetoxychrom-2-one) undergoes reversible dimerization upon irradiation with ultraviolet light, more specifically at 365 nm, to form a cyclobutane ring between the pyron rings of two coumarin molecules, such as this is shown in the formula below.
  • the back reaction takes place upon irradiation with a wavelength of 254 nm.
  • the 7-acetoxycoumarin system is outstandingly suitable for the chromatic modulation of the laser light. Since a wave- length of 1250 cm "1 corresponds to a wavelength of 8 ⁇ m, infrared lasers are again affected, but the principle can be applied analogously to all wavelength ranges.
  • Example 7 Selection of Suitable Absorbent Layer Materials; Semiconductor laser with N- (4-chlorophenyl) salicylaldimine (p-chlorosalicylideneaniline) as absorption layer
  • QCLs prepared according to Synthesis Example 1 or 2 can be coated with N- (4-chlorophenyl) salicylaldehyde (CSA), for example by means of a syringe on the waveguide of the CSA solution (eg about 10 mg CSA in about 200 ⁇ l acetone) Lasers applied and the solvent is then allowed to evaporate. Parts of the laser that are not to be coated are wiped clean with an acetone-moistened cloth.
  • CSA N- (4-chlorophenyl) salicylaldehyde
  • Comparative samples for measuring the absorption of CSA are prepared by applying a few milligrams of CSA to a silicon wafer (eg, about 1 micron square area) and heating it above the melting temperature of CSA, forming a melt film on the wafer which, upon cooling, becomes a Solidified layer with homogeneous thickness.
  • a QCL with an emission maximum in this range (eg between 840 and 850 cm -1 ) is provided with an absorption layer of N- (4-chlorophenyl) salicylaldimine, this causes a change in its emission wavelength, which is subsequently due to the thermochromic effect of CSA varies depending on the ambient temperature or can be specifically "switched".
  • Such an inventive semiconductor laser with absorption layer is therefore also suitable as a sensor or switch or signal generator.
  • the emissions of semiconductor lasers can be modified in a significant manner and preferably also chromatically modulated by the present invention.
  • the inventors have been able to effect both significant shifts in the emission maxima by up to 17 wave numbers, which is an order of magnitude above the achievable values of the prior art, as well as considerable improvements in the homogeneity of the emissions.
  • the emissions from semiconductor lasers can also be switched between multiple states, providing a new type of laser sensor.
  • semiconductor lasers provided with absorption layers according to the invention, in particular quantum cascade lasers can be produced rapidly, simply and inexpensively and accordingly represent a valuable extension of the state of the art.

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Abstract

L'invention concerne un laser à semi-conducteurs comportant une couche active (1) composée de couches à semi-conducteurs éventuellement dopées, pour la production d'un rayonnement électromagnétique après excitation électrique, la couche active étant entourée dans quatre directions spatiales par des couches de guides d'ondes (2a, 2b, 5a, 5b); éventuellement un substrat (3); et respectivement au moins une ligne d'entrée et de sortie de courant (4) pour l'excitation de la formation de photons dans les couches à semi-conducteurs, certaines couches de guides d'ondes (5a, 5b) formant éventuellement des couches isolantes entre la zone active (1) et l'entrée de courant (4). Le laser à semi-conducteurs est caractérisé en ce qu'au moins une partie d'au moins une couche de guides d'ondes (2a, 2b, 5a, 5b) est en contact avec une couche d'absorption (6) interagissant avec le rayonnement électromagnétique, composée de matériau anorganique et/ou organique présentant une absorbance non-homogène dans la gamme de longueur d'ondes d'émission du laser, et un indice de réfraction efficace de préférence plus faible que celui-de la zone active (1), ou ladite partie est composée de ce matériau.
PCT/AT2008/000431 2007-12-05 2008-12-05 Laser à semi-conducteurs comportant une couche d'absorption Ceased WO2009070821A2 (fr)

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US11031753B1 (en) * 2017-11-13 2021-06-08 The Government Of The United States Of America As Represented By The Secretary Of The Air Force Extracting the fundamental mode in broad area quantum cascade lasers

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FR2787246B1 (fr) * 1998-12-09 2001-02-23 Alpes Lasers Laser de type semi-conducteur
DE19963807A1 (de) * 1999-12-30 2001-07-19 Osram Opto Semiconductors Gmbh Streifenlaserdiodenelement
US6563852B1 (en) * 2000-05-08 2003-05-13 Lucent Technologies Inc. Self-mode-locking quantum cascade laser

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11031753B1 (en) * 2017-11-13 2021-06-08 The Government Of The United States Of America As Represented By The Secretary Of The Air Force Extracting the fundamental mode in broad area quantum cascade lasers

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