Optically pumped stepped multi-well laser

An apparatus and methods for an optically pumped laser that has a cascade of light-emitting interband transitions are disclosed. The apparatus disclosed contains multistep interband cascade regions able to generate a plurality of photons for a pump photon absorbed from an optical pump source. The methods disclosed teach how to produce a plurality of photons for a pump photon absorbed from an optical pump source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application U.S. application Ser. No. 11/090,453, filed on the same date as the present application, for “Optically Pumped Laser With An Integrated Optical Pump” by Daniel Yap, Dave Persechini and Authi Narayanan, the disclosure of which is incorporated herein by reference.

FIELD

This invention relates to an optically pumped laser that makes more efficient use of the absorbed optical pump energy since one absorbed optical pump photon may result in the emission of more than one output photon by the laser.

BACKGROUND AND PRIOR ART

Optically pumped semiconductor lasers that emit at a wavelength of 2 to 5 microns or longer have typically been preferably pumped by light that has a wavelength close to and slightly shorter than the wavelength emitted by the optically pumped laser. In a typical optically pumped laser (OPL) each photon of the pump light can generate at most one electron-hole pair, by exciting an electron (the electrical charge carrier) from the valence band into the conduction band. The excited electron then can make an optical transition back to the valence band (an interband transition that also can be described as having the electron recombine with a hole) to produce at most one photon of the light emitted by the OPL. If the energy of the pump photon is much higher than the energy of the photon emitted by the OPL, the excess pump energy typically is not used effectively. Instead, the excess pump energy, which is related to the difference in the wavelengths of the pump light and the light produced by the optically pumped laser (OPL), is typically converted to heat. This heat must be removed. Otherwise, the temperature of the optically pumped laser can increase and the non-radiative processes, such as Auger recombination, that occur in the optically pumped laser can reduce the efficiency of that laser or can prevent that laser from operating at high output powers.

The more efficient semiconductor pump lasers emit at shorter wavelengths such as 0.98, 1.48 and 1.55 microns. These wavelengths can be quite different from the wavelength of the OPL. Thus, those OPL that are pumped by these more efficient pump lasers could be subject to more heating. It is desirable to make more efficient use of the pump energy so that less heat is produced in the OPL that are pumped by shorter-wavelength light.

One way to improve the efficiency of the laser is to use an energy cascade that enables one electron injected into the cascade to undergo multiple transitions, thereby generating multiple photons. Although some prior lasers have a cascade of interband or intraband wells, these prior art lasers require application of an external electrical voltage to establish the cascade (that voltage being supplied with the electrical pumping).

Electrically pumped cascade lasers that employ interband transitions for emission of the light are described in U.S. Pat. Nos. 5,799,026 and 6,404,791, with an extensive review of those lasers given in IEEE Journal of Quantum Electronics, v.38, n.6, pp. 559-568 (2002). Electrically pumped cascade lasers that employ intraband cascade laser with wide emission spectrum is described in Nature, v.415, pp. 883-887 (2002). In these prior art lasers, the electrons and holes that recombine to produce the emitted light are supplied from an external source of electrical current, such as a battery or a power supply. These prior lasers have to be electrically pumped to achieve the cascade, since it is the externally applied voltage accompanying the externally supplied current that establishes the electric field across the structure that forms the cascade of light-emitting regions and carrier injection regions in these prior lasers. Unlike these prior art electrically pumped lasers, the present specification discloses optically pumped lasers that are not externally biased to achieve the cascade.

Optically pumped lasers can potentially be more efficient than electrically pumped lasers. Part of this greater efficiency is achieved because the OPL does not need to have any p-doped layers. The free-carrier absorption that especially occurs in the p-doped layers of electrically pumped lasers is an undesirable loss mechanism. The amount of free-carrier absorption can be much lower in the optically pumped lasers.

Optically pumped lasers with integrated pump-light absorbing layers are described in paper CMM4 of the Digest of 2000 Conference on Lasers and ElectroOptics (CLEO), pages 63-64 and in Journal of Applied Physics, v.92, n.10, pp. 5621-5626 (2002). Unlike these prior art optically pumped lasers, the present specification discloses OPL containing integrated pump-light absorbers that have tilted valence and conduction bands. Such tilted bands would have no benefit in these prior art lasers and would unnecessarily complicate their construction.

Antimonide-based lasers that have light-emitting regions comprising W-shaped potential well structures with staggered, Type-II band alignments are described in Applied Physics Letters, v.67, n.6, pp. 757-759 (1995) and in Journal of Applied Physics, v.89, n.6, pp. 32883-3289 (2001). To improve the lasing efficiency, it is preferred in these prior art lasers that the W-shaped wells in them all be the same. Some embodiments of the present specification likewise make use of W-shaped potential wells. In contrast to the prior art, however, the presently disclosed Type II antimonide lasers implement cascades comprising multiple W-shaped potential wells that are not the same. According to the present disclosure, at least two different kinds of W-wells are implemented in each cascade.

DETAILED DESCRIPTION

A semiconductor laser uses electrons that have been excited or injected into the conduction band states of a semiconductor material. These excited, higher-energy electrons can then make an electronic transition into a lower energy state while simultaneously emitting one or more photons of light. For an interband laser, the electronic transition is from a state in the conduction band to a state in the valence band of the semiconductor material. For an intraband laser, the electronic transition is from a higher-energy state in the conduction band to a lower-energy state in the conduction band (i.e., between states in the same band). For an optically pumped laser, the excited electron may be produced as a result of absorption of a photon of the pump light. This excites the electron from being in a valence-band state into a conduction-band state. For an electrically pumped laser, the excited electron may be injected into the conduction band of the material from an external electrical source.

In contrast to prior art electrically pumped cascade lasers (either interband or intraband), discussed above, the present specification discloses an interband cascade laser that is optically pumped as will be seen. The basic feature of the interband cascade optically pumped laser (IC-OPL) is a combination of optical absorbing barrier layers, multi-step cascades of interband light-emitting well regions and tunneling barriers between the steps of the cascade. This combination permits the efficient use of shorter (0.98, 1.48 and 1.55 microns) optical pump wavelengths (higher pump photon energies), at which more efficient and higher power pump lasers are available. The multi-step interband optical cascade makes more efficient use of the absorbed optical pump energy since one absorbed optical pump photon may result in the emission of more than one output photon by the OPL. The multi-step interband cascade regions are preferably constructed as a result of the epitaxial material designs, which is compatible with the optically pumped operation, rather than as a result of an externally applied voltage, as in the prior cascade lasers described above, which are electrically pumped.

Referring toFIG. 1, in one exemplary embodiment, the cross-sectional structure of the interband cascade IC-OPL10may comprise one or more substructures40that may be combined as a vertically arranged stack. The IC-OPL10may be fabricated on a semiconductor substrate20of a material such as GaSb. The IC-OPL10may also comprise optional cladding layers30and50. The combination of substructures40form a core region45of the optical waveguide and the cladding layers30and50form waveguide cladding that sandwiches the core region45. Other layers of cladding material60,70may further separate the adjacent substructures40from one another. The cladding layers30,50,60and70may comprise AlGaInAsSb material or subsets of this material such as AlAsSb or AlGaAsSb.

An external pump light may be supplied from the topside of the IC-OPL structure10, as illustrated by arrow A in theFIG. 1, from the bottom side, as illustrated by arrow B in theFIG. 1, or from the ends of the IC-OPL structure10, as illustrated by arrows C and D in theFIG. 1.

Although IC-OPL10has a ridge55waveguide, other types of laser structures that yield well defined optical emission patterns, for example distributed feedback structures whose gratings are tilted at an angle to the end faces, may also be used. These and other suitable structures are disclosed in Applied Physics Letters, v.78, n.11, pp. 1475-1477 (2001), incorporated herein in its entirety by this reference.

Referring toFIGS. 2,3A and3B, in multiple exemplary embodiment, each substructure40may comprise barrier layers110,130,150,170and multistep interband cascade regions120,140,160. A laser light may be emitted from the plurality of potential well layers210,220,230-250,270-290, for both the (conduction band) electrons and the (valence band) holes, within the multistep interband cascade regions120,140,160. Each photon of the light may be emitted as a result of an interband electronic transition between a conduction band energy level and a valence band energy level. Interband electronic transition between a conduction band energy level and a valence band energy level may occur between the lowest energy conduction-band level and the highest energy valence-band level. Although substructure40, shown inFIG. 2, is depicted as comprising three multistep cascade regions120,140,160and three barrier layers130,150170, substructure40is not to be limited to only three multistep cascade regions and three barrier layers. Substructure40may comprise one or more multistep cascade regions and one or more barrier layers.

Besides separating the multistep interband cascade regions120,140,160, one or more of the barrier layers110,130,150,170may also serve as absorbing regions for the optical pump energy and may split the holes and electrons that are generated from the absorbed pump light so that the holes may flow in one direction and the electrons may flow in the opposite direction. A schematic band structure diagram illustrating this exemplary embodiment is shown inFIG. 4A.

An exemplary embodiment of multistep cascade region120within the substructure40is depicted inFIG. 3A. Although the following discussion refers to multistep cascade region120, this is done for clarity reasons and the following discussion may equally apply to other multistep cascade regions140and160. In this exemplary embodiment, each step,180,190of the multistep interband cascade region120may comprise a plurality of Type I quantum well structures. For a Type I quantum well structure, a potential well (quantum well) for the electrons and a potential well (quantum well) for the holes may be obtained from the well layers210,220. The barrier layers110,200and130may serve as the potential barriers that sandwich the two Type I quantum well layers,210,220. Type I quantum wells typically may be suitable for lasers, when constructed from antimonide-based materials, that emit at wavelengths of 2.7 microns or shorter.

Referring toFIG. 3A, a first Type I quantum well region may comprise well layer210sandwiched between barrier layers110and200. Well layer210may comprise material such as GaInAsSb. Barrier layer110may serve as a pump light absorbing region with tilted energy bands and may be formed from a material such as GaInAlAsSb. Barrier layer200may serve as a tunneling barrier and may be formed from a material such as AlAsSb or GaAlAsSb. A second Type I quantum well region may comprise well layer220sandwiched between barrier layers200and130. Well layer220may be formed from a material such as GaInAs. Barrier layer130may serve as a pump light absorbing region with tilted energy bands and may be formed from a material such as GaInAlAsSb. The first and the second Type I quantum well regions may share a tunneling barrier200.

Although multistep cascade region120, shown inFIG. 3A, is depicted as comprising of two-steps,180and190, multistep interband cascade regions120,140,160may comprise more than two-steps. Each step180,190may contain a different Type I quantum well layer. For example, if the interband cascade regions120,140,160comprise five steps, there would be five different quantum well layers, one for each step of the cascade. A Type I quantum well may, for example, be constructed from three layers of material. A layer of well material210,220may be sandwiched between two layers of barrier materials110,200and200,130, respectively, that have a larger bandgap, with higher conduction band and valence band energies, than the well material210,220. Thus, potential wells may be produced in both the conduction band and the valence band and may result in quantum confinement for both electrons and holes. The quantum-confining barrier layers also may have other functions, such as being absorbers of the pump light, being regions that split the holes and electrons that are generated from the pump-light absorption, being tunneling barriers200, and being cladding regions for an optical waveguide. The two type-I quantum well regions in steps180and190may be separated by a tunneling barrier200that may, for example, be formed from materials such as InAlAsSb. A schematic band structure diagram illustrating a two-step cascade of Type I quantum wells is shown inFIG. 5A.

Another exemplary embodiment of multistep cascade region120within the substructure40is depicted inFIG. 3B. Although the following discussion refers to multistep cascade region120, this is done for clarity reasons and this discussion may equally apply to multistep cascade regions140and160. In this embodiment, each step,180,190of the multistep interband cascade region120may comprise a plurality of W-well structures wherein each W-well structure (hereinafter “Type-II quantum well structure”) may have a Type-II energy band alignment. Each Type-II quantum well structure may comprise hole well layers240and280sandwiched between a pair of electron well layers230,250and270,290, respectfully. The Type-II quantum well structures increase the overlap between the wave functions for the electron and the hole when Type II energy band alignments are used, thus improving the efficiency of the light-emitting transition while also reducing the non-radiative, Auger recombinations. Type II quantum wells often are desired for antimonide-based lasers that emit at wavelengths of 2.5 microns or longer.

Although multistep cascade region120, shown inFIG. 3B, is depicted as comprising two-steps,180and190, multistep interband cascade regions120,140,160may comprise more than two steps and more than two different type-II quantum well structures within a single multistep interband cascade region120. The type-II quantum well structure in step180may be formed of a hole well layer240sandwiched between electron well layers230,250. Electron well layers230,250may, for example, be formed from materials such as InAs or InAsSb. Hole well layer240may, for example, be formed from materials such as GaInSb. The type-II quantum well structure in step190may be formed of a hole well layer280sandwiched between electron well layers270,290. Electron well layers270and290may, for example, be formed from materials such as InAs. Hole well layer280may, for example, be formed from materials such as AlGaSb. The electron well layers230and250may have a thickness that is different from the thickness of electron well layers270and290. The two type-II quantum well regions in steps180and190may be separated by a tunneling barrier200that may, for example, be formed from materials such as InAlAsSb. A schematic band structure diagram illustrating a two-step cascade of Type II quantum wells is shown inFIGS. 5B,5C.

With an interband cascade, the optical (light emitting) transition occurs as a result of the recombination of holes and electrons (an interband electronic transition between a conduction-band energy state and a valence-band energy state). With the epitaxial-growth defined interband cascade, each conduction-band electron generated from the absorbed pump light and supplied from the barriers, such as110,130,150,170shown inFIG. 2, may produce a multiple of photons, with potentially one photon produced in each step180and190of each multistep cascade region120,140,160. Thus, the energy of the optical pump may be used more efficiently to generate photons in the IC-OPL structure10.

Referring toFIGS. 3A and 3B, the Type I or Type II quantum well regions in each step180,190of the multistep cascade regions120,140may be different in their material compositions and layer thicknesses. Different material compositions and layer thicknesses within multistep cascade regions120,140,160may produce band-energy cascades for both the holes and the electrons while also determining the emission wavelength for each step. Such exemplary energy band cascades are further illustrated schematically in energy band diagrams inFIGS. 4A,4B.

The barrier layers110,130,150,170abutting two adjacent multistep interband cascade regions120,140,160may be grown with energy slopes for both the valence band and the conduction band. The energy slopes of the barrier layers110,130,150,170may assist the flow of electrons and holes (in opposite directions) into the adjacent multistep interband cascade regions120,140,160. The exemplary sloped conduction band and the exemplary sloped valence band of a pump light absorbing barrier layer110,130,150,170are further illustrated schematically in energy band diagrams inFIGS. 4A,4B. A material such as AlGaInAsSb may, for example, be used to construct the absorber barrier layers110,130,150,170that have tilted or sloped conduction band and valence band. The percentages of the various constituent elements in this material are varied as the layer is grown, in such a way that the desired variations in the conduction and valence band energies are achieved with a minimum amount of accumulated strain (from mismatch of the lattice constant in the crystalline material).

Referring toFIG. 4A, in one exemplary embodiment, substructure40may comprise three multistep cascade regions,120,140,160each of which has two steps180and190. There are four barrier layers,110,130,150,170that may function as absorbing regions for the pump light and also may have tilted conduction and valence bands that split the electrons and holes generated as a result of the pump absorption. A photo-generated electron, represented by ⊖, in barrier layers110,130and150would be directed into cascade regions120,140and160, respectively. Likewise, a photo-generated hole, represented by ⊕, in barrier layers130,150and170would be directed into multistep cascade regions120,140and160, respectively. However, photo-generated holes in barrier110and photo-generated electrons in barrier170would be directed away from the multistep cascade regions120and160, respectfully and would not contribute to the generation of light by the cascade regions. An external electrical path (not shown) may be provided between barrier layers110and170so that those holes and electrons can recombine, non-radiatively. Assuming IC-OPL10comprises a single substructure40, cladding layer30may optionally be doped p-type and cladding layer50may optionally be doped n-type to facilitate the flow of the holes and electrons that may be directed away from the multistep cascade regions120and160. In addition, the material composition of cladding layer30may be graded near the interface of layer30with multistep cascade region120to achieve a smooth variation of the conduction band edge. Likewise, the material composition of cladding layer50may be graded near the interface of layer50with multistep cascade region160to achieve a smooth variation of the valence band edge.

In other exemplary embodiments, only some of the barrier layers110,130,150and170may be formed to absorb the optical pump energy and split the photogenerated holes and electrons. Referring toFIG. 2, in one exemplary embodiment, only the barrier layers130and150may be grown to function as absorbing regions. In this exemplary embodiment, an electrical path may be provided that may allow electrons that flow away from multistep cascade regions160to be replaced by electrons from barrier layer110that flow toward multistep cascade regions120. Such an exemplary embodiment of IC-OPL10may be suitable if the barrier layer110and cladding layers30,50,60,70, as shown inFIG. 1, are formed from n-type materials. Assuming IC-OPL10comprises a single substructure40, the barrier layer110and cladding layer30, as shown inFIGS. 1 and 2, may be the same layer. A schematic band structure diagram illustrating this embodiment is shown inFIG. 4B.

Referring toFIG. 4B, in an exemplary embodiment, substructure40may comprise two multistep cascade regions120,140each having two steps180and190. The barrier layers130,150may function to absorb the pump light and also to split the photo-generated electrons and holes. There also are n-doped cladding layers30and50. Cladding layer30is adjacent to cascade region120and cladding layer50is adjacent to barrier layer150. Photo-generated electrons in barrier layer150are directed into cladding layer50. An external electrical path (not shown) may be provided for the charge of those electrons to be compensated by injection of electrons into cladding layer30and then into multistep cascade region120. The material composition of the heterojunction interfaces between absorber barrier150and cladding layer50may be graded to facilitate the flow of the electrons. The material composition of cladding layer30also may be graded to reduce the bandgap near the interface of layer30with multistep cascade region120. Cladding layer30may comprise a region32with abrupt heavy n-type doping and another region34with abrupt p-type doping. The two regions32and34may serve to ensure that multistep cascade regions120and140and barrier layers130and150are normally depleted of free carriers. Electrons in the conduction band of cladding layer30may tunnel through the tunneling barrier36into a conduction band state in step180of multistep cascade region120.

Referring toFIGS. 3A and 5A, in one exemplary embodiment, an electron510and a hole520may be generated as a result of absorption of a photon570of wavelength hνpfrom pump light in absorber layer110. An electron530and a hole540may be generated as a result of absorption of a photon590of wavelength hνpfrom pump light in absorber layer130. Electron510may be injected into multistep interband cascade region120from the absorber layer110and hole540may be injected into multistep interband cascade region120from the absorber layer130. Hole520and electron530are directed away from multistep interband cascade region120as a result of the tilted conduction and valence bands in absorber layers110and130.

In this exemplary embodiment, electrons may be confined in the vicinity of first well layer210and second well layer220because of the potential wells in the conduction band. Likewise holes may be confined in the vicinity of well layers210and220because of potential wells in the valence band. The quantum well containing well layer210has a conduction band (CB) energy level610(represented by a horizontal dotted line) and a valence band (VB) energy level620(represented by a horizontal dotted line). The quantum well containing well layer220has a CB energy level630(represented by a horizontal dotted line) and a VB energy level640(represented by a horizontal dotted line). The VB energy level620may approximately coincide in energy with the CB energy level630so as to be coupled to each other as a result of enhanced tunneling through tunneling barrier200. The tunneling is enhanced due to the energy of VB level620being approximately equal to the energy of CB level630. For clarity, each of the quantum wells210,220is assumed to have only one CB energy level610,630, respectively, and one VB energy level620,640, respectively. In actuality, the quantum wells210,220may comprise multiple CB energy levels, and multiple VB energy levels.

Referring toFIG. 5A, in one exemplary embodiment, the electron510injected from barrier110occupies CB level610. An electron510then makes a first interband transition660into VB level620. A first photon670of wavelength hν1may be emitted as a result of that interband transition660. An electron in VB level620may then tunnel, for example via interband tunneling, through the tunneling barrier200into CB level630. An electron in CB level630may then make a second interband transition680into VB level640that may be occupied by hole540that is injected from barrier130. A second photon690of wavelength hν2may be emitted as a result of that second interband transition680. In essence two photons670and690may be produced as a result of the recombination of an electron510and a hole540that were injected into the multistep interband cascade region120. Photons670and690may be emitted, as a result of interband transitions660,680, at the same wavelength (hν1=hν2) or at different wavelength (hν1hν2) as to be described below.

Although the exemplary embodiment inFIG. 5Adiscusses a multistep interband cascade region120with only two steps180,190, it should be evident to anyone skilled in the art that a multistep interband cascade region having more than two steps may also be implemented thereby emitting more photons. More photons may be emitted per the electron510and the hole540that is injected into the multistep interband cascade region120if the multistep interband cascade region120had more steps. For example, if the multistep interband cascade region120were to have 3 steps, 3 photons may be emitted as a result of the injection of the electron510and the hole540.

Referring toFIGS. 3B and 5B, in another exemplary embodiment, an electron510and a hole520may be generated as a result of absorption of a photon570of wavelength hνpof pump light in absorber layer110. An electron530and a hole540may be generated as a result of absorption of a photon590of wavelength hνpfrom pump light in absorber layer130. Electron510may be injected into multistep interband cascade region120from the absorber layer110and hole540may be injected into multistep interband cascade region120from the absorber layer130. Hole520and electron530are directed away from the multistep interband cascade region120as a result of the tilted conduction and valence bands in absorber layers110and130.

Referring toFIG. 5B, in one exemplary embodiment, barrier layer110provides one of the conduction-band potential barriers for the electron well in layer230. Tunneling barrier200provides one of the potential barriers for the electron well in layer250. The other potential barrier for the electrons is provided by hole well layer240. The electron well layers230and250provide the valence-band potential barriers for the holes that are confined in the vicinity of hole well layer240. In a similar way, barrier layer130, tunneling barrier200and hole well layer280provide potential barriers for the electrons confined by the coupled quantum wells defined by electron well layers270and290. Again, in a similar way, electron well layers270and290provide the potential barriers for the holes that are confined in the vicinity of hole well layer280.

In this exemplary embodiment, electrons may be confined in the vicinity of well layers230,250and well layer270,290because of the potential wells in the conduction band. Likewise holes may be confined in the vicinity of well layers240and280because of potential wells in the valence band. The coupled quantum wells for electrons defined by electron well layers230and250have a CB energy level710(represented by a horizontal dotted line). The quantum well for holes defined by hole well layer240has a VB energy level720(represented by a horizontal dotted line). The coupled quantum wells for electrons defined by electron well layers270and290have a CB energy level730(represented by a horizontal dotted line). The quantum well for holes defined by hole well layer280has a VB energy level740(represented by a horizontal dotted line). The VB energy level720may approximately coincide in energy with the CB energy levels730so as to be coupled to each other as a result of enhanced tunneling through tunneling barrier200. The tunneling is enhanced because the energy of VB level720is approximately equal to the energy of CB level730. For clarity, each quantum well230,250and270,290is assumed to have only one CB energy level710,730, respectively, and each quantum well240,280is assumed to have only one VB energy levels720,740, respectively. The quantum wells230,250,270,290may actually comprise multiple CB energy levels, and the quantum wells240,280may actually comprise multiple VB energy levels.

Referring toFIG. 5B, in one exemplary embodiment, the electron510injected from barrier110occupies CB level710. An electron from level710then makes a first interband transition760into VB level720. A first photon770of wavelength hν1may be emitted as a result of that interband transition760. An electron in VB level720may then tunnel via interband tunneling or resonant interband tunneling through the tunneling barrier200into CB level730. An electron in CB level730may then make a second interband transition780into VB level740that may be occupied by hole540that is injected from barrier130. A second photon790of wavelength hν2may be emitted as a result of that second interband transition780. In essence two photons770and790may be produced as a result of the recombination of an electron510and a hole540that were injected into the multistep interband cascade region120. Photons770and790may be emitted, as a result of interband transitions760,780, at the same wavelength (hν1=hν2) or at different wavelength (hν1hν2) as to be described below.

Although the exemplary embodiment inFIG. 5Bdiscusses a multistep interband cascade region120with only two steps180,190, it should be evident to anyone skilled in the art that a multistep interband cascade region having more than two steps may also be implemented thereby emitting more photons. More photons may be emitted per the electron510and the hole540that are injected into the multistep interband cascade region120if the multistep interband cascade region120had more steps. For example, if the multistep interband cascade region120were to have 3 steps, 3 photons may be emitted as a result of the injection of the electron510and the hole540.

In another exemplary embodiment, the composition of the hole-well layer240or the hole-well layer280could be designed so as to achieve a compressive strain that can improve the light-emission characteristics of the corresponding Type-II quantum well structure. For example, increasing strain may be obtained when more Indium is used in the GaInSb material of hole-well layer240. The strain in a hole-well layer may result in greater confinement of the hole wavefunction and better overlap between the hole wave-function and the corresponding electron wave-function for the coupled electron wells. The effects of incorporating such strain in the hole well layer has been described in Journal of Applied Physics, v.92, n.10, pp. 5621-5626 (2002), incorporated herein in its entirety.

The tunneling barrier layer200has multiple functions. Besides some of the functions described above, the tunneling barrier200may also reduce the likelihood that electron510injected into the first step180of the multistep cascade region120will tunnel directly or make a thermionic transition into the electron well in the second step190of multistep cascade region120without first making an interband transition. Similarly, the tunneling barrier200may reduce the likelihood that hole540injected into the second step190of the multistep cascade region120would make an interband transition before tunneling into the hole well in the first step180. The height of the tunneling potential barriers for those electrons and holes and the width of the tunneling barrier layer200may be adjusted so that the light-emitting interband transition time may be shorter than the time for direct (intraband) tunneling of the carriers injected from the pump absorbing barriers110,130.

Referring toFIGS. 3B and 5C, in another exemplary embodiment, the extents of the electron and hole wavefunctions in a given quantum well structure may be mismatched so that the undesired direct (intraband) tunneling may be reduced. Although Type-II well structures are illustrated inFIGS. 3B and 5C, the principles discussed may also be applied to multistep interband cascade regions comprising Type-I quantum wells.

In this exemplary embodiment, an electron510and a hole520may be generated as a result of absorption of a photon570of wavelength hνpof pump light in absorber layer110. An electron530and a hole540may be generated as a result of absorption of a photon590of wavelength hνpfrom pump light in absorber layer130. Electron510may be injected into multistep interband cascade region120from the absorber layer110and hole540may be injected into multistep interband cascade region120from the absorber layer130. Hole520and electron530are directed away from multistep interband cascade region120as a result of the tilted conduction and valence bands in absorber layers110and130.

Referring toFIG. 5C, in one exemplary embodiment, barrier layer110provides one of the conduction-band potential barriers for the electron well in layer230. Tunneling barrier200provides one of the potential barriers for the electron well in layer250. The other potential barrier for the electrons is provided by hole well layer240. The electron well layers230and250provide the valence-band potential barriers for the holes that are confined in the vicinity of hole well layer240. In a similar way, barrier layer130, tunneling barrier200and hole well layer280provide potential barriers for the electrons confined by the coupled quantum wells defined by electron well layers270and290. Again, in a similar way, electron well layers270and290provide the potential barriers for the holes that are confined in the vicinity of hole well layer280.

Referring toFIG. 5C, in one exemplary embodiment electron well230may be slightly wider than electron well250. Thus, electron level810defined by well230may have a slightly lower energy than the electron level811defined by well250. Since the energies of the two electron levels810and811differ only slightly, a coupled-well electron wavefunction910still may be defined. The coupled-well electron wavefunction910may be confined more in the vicinity of electron well layer230than in the vicinity of electron well layer250, and thereby farther from tunneling barrier200. Thus, the coupled-well electron wavefunction910does not extend appreciably into the tunneling barrier200. This may reduce the direct tunneling from a CB state in step180to a CB state in step190. On the other hand, electron well270is slightly wider than electron well290. Thus, electron level830defined by well270has a slightly lower energy than the electron level831defined by well290. Since the energies of those two electron levels830and831differ only slightly, a coupled-well electron wavefunction930still may be defined. This coupled-well electron wavefunction930is confined more in the vicinity of electron well layer270than in the vicinity of electron well layer290, and thereby closer to tunneling barrier200. Thus, the coupled-well electron wavefunction930may extend appreciably into the tunneling barrier200. This increases the probability of the desired interband tunneling between a CB state in step190and a VB state, such as hole level820, in step180. Given that this increased interband tunneling probability may be higher than necessary, it might be possible in this exemplary embodiment to further increase the thickness of tunneling barrier200and still maintain sufficient interband tunneling. Having a thicker tunneling barrier200may reduce the direct tunneling of holes from a VB state in step190into a VB state in step180and, may also, further reduce the direct tunneling of electrons from the CB in step180into the CB in step190. For this exemplary embodiment, the material compositions of the hole well layers240,280may need to be adjusted so that the light emitted by the associated interband transitions have the desired wavelengths. Note that although electron wavefunctions910and930are not centered with respect to the W-shaped well structures, there still is significant spatial overlap between those electron wavefunctions910,930and their associated hole wavefunctions920and940. The hole wavefunctions920and940may be approximately centered with respect to the W-shaped well structures.

Referring toFIGS. 3C and 6, in another exemplary embodiment, the tunneling barrier200may be formed so as to assist in the interband tunneling between a hole wavefunction of one well and an electron wavefunction of an adjacent well.

Referring toFIGS. 3B,3C and6, in this exemplary embodiment, an electron510and a hole520may be generated as a result of absorption of a photon570of wavelength hνpof pump light in absorber layer110. An electron530and a hole540may be generated as a result of absorption of a photon590of wavelength hνpfrom pump light in absorber layer130. Electron510may be injected into multistep interband cascade region120from the absorber layer110and hole540may be injected into multistep interband cascade region120from the absorber layer130. Hole520and electron530are directed away from multistep interband cascade region120as a result of the tilted conduction and valence bands in absorber layers110and130.

Referring toFIG. 6, in one exemplary embodiment, barrier layer110provides one of the conduction-band potential barriers for the electron well in layer230. Tunneling barrier200provides one of the potential barriers for the electron well in layer250. The other potential barrier for the electrons is provided by hole well layer240. The electron well layers230and250provide the valence-band potential barriers for the holes that are confined in the vicinity of hole well layer240. In a similar way, barrier layer130, tunneling barrier200and hole well layer280provide potential barriers for the electrons confined by the coupled quantum wells defined by electron well layers270and290. Again, in a similar way, electron well layers270and290provide the potential barriers for the holes that are confined in the vicinity of hole well layer280.

In this exemplary embodiment, electrons may be confined in the vicinity of well layers230,250and well layer270,290because of the potential wells in the conduction band. Likewise holes may be confined in the vicinity of well layers240and280because of potential wells in the valence band. The coupled quantum wells for electrons defined by electron well layers230and250have a CB energy level810(represented by a horizontal dotted line). The quantum well for holes defined by hole well layer240has a VB energy level820(represented by a horizontal dotted line). The coupled quantum wells for electrons defined by electron well layers270and290have a CB energy level830(represented by a horizontal dotted line). The quantum well for holes defined by hole well layer280has a VB energy levels840(represented by a horizontal dotted line). The VB energy level820may approximately coincide in energy with the CB energy levels830so as to be coupled to each other as a result of enhanced tunneling through tunneling barrier200. The tunneling is enhanced because the energy of VB level820is approximately equal to the energy of CB level830. For clarity, each quantum well230,250and270,290is assumed to have only one CB energy level810,830, respectively, and each quantum well240,280is assumed to have only one VB energy levels820,840, respectively. The quantum wells230,250,270,290may actually comprise multiple CB energy levels, and the quantum wells240,280may actually comprise multiple VB energy levels.

Referring toFIGS. 3C and 6, the modified tunneling barrier200may comprise two, three or four layers—a primary tunneling barrier layer950, an optional electron-tunneling barrier enhancement layer960, an optional hole-tunneling barrier enhancement layer970and an optional secondary tunneling barrier layer980. The primary tunneling barrier layer950and the secondary tunneling barrier layer980may be formed from materials such as AlGaSb. The electron-tunneling barrier enhancement layer960may be formed from materials such as AlGaSb. The hole-tunneling barrier enhancement layer970may be formed from materials such as InAs or InGaAs.

The primary tunneling barrier layer950may serve as a barrier for electron well layer250and may prevent direct (intraband) tunneling of electrons from step180CB level810into a CB level in the second step190of the multistep cascade region120. The primary tunneling barrier950also serves as a barrier for direct (intraband) tunneling of holes from step190VB level840into a VB level in the first step180. The optional secondary tunneling barrier980may serve as a barrier for electron well layer270and as a tunneling barrier for higher-energy electrons defined by coupled electron well layers270and290. Secondary tunneling barrier980also can serve as a barrier to tunneling between some VB states in steps180and190. An optional electron-tunneling barrier enhancement layer960can be added to further reduce the direct (intraband) tunneling of electrons from step180CB level810into the second step190of the multistep cascade region120. However, the electron-tunneling barrier enhancement layer960may not prevent interband tunneling between certain VB states of step180and certain CB states of step190. Instead, barrier enhancement layer960may have one or more VB states915that may serve to resonantly enhance the interband tunneling between steps180and190. Likewise, an optional hole-tunneling barrier enhancement layer970may be added to further reduce the direct (intraband) tunneling of holes from step190VB level840into the first step180of the multistep cascade region120. However, the hole-tunneling barrier enhancement layer970may not prevent interband tunneling between certain VB states of step180and certain CB states of step190. Instead, barrier enhancement layer970may have one or more CB states916that serve to resonantly enhance the interband tunneling between steps180and190. The resonant enhancement may occur due to the energies of VB state or states915and CB state or states916being approximately equal to the energies of VB state820and CB state830. The thicknesses of electron-tunneling barrier enhancement layer960and hole-tunneling barrier enhancement layer970may be adjusted so as to reduce the direct (intraband) tunneling from CB level810and from VB level840. The thickness and composition of the primary tunneling barrier layer950and optional secondary tunneling barrier layer980may be adjusted so as not to restrict the interband tunneling between VB level820and CB level830.

Referring toFIG. 6, in one exemplary embodiment, the electron510injected from barrier110occupies CB level810. An electron from level810then makes a first interband transition860into VB level820. A first photon870of wavelength hν1may be emitted as a result of that interband transition860. An electron in VB level820may then tunnel, for example via resonant interband tunneling involving VB state or states915and CB state or states916, through the primary tunneling barrier layer950, the electron-tunneling barrier enhancement layer960, the hole-tunneling barrier enhancement layer970and the secondary tunneling barrier980into CB level830. An electron in CB level830may then make a second interband transition880into VB level840that may be occupied by hole540that is injected from barrier130. A second photon890of wavelength hν2may be emitted as a result of that second interband transition880. In essence two photons870and890may be produced as a result of the recombination of an electron510and a hole540that were injected into the multistep interband cascade region120. Photons870and890may be emitted, as a result of interband transitions860,880, at the same wavelength (hν1=hν2) or at different wavelength (hν1hν2) as to be described below.

Although the exemplary embodiment inFIG. 6discusses a multistep interband cascade region120with only two steps180,190, it should be evident to anyone skilled in the art that a multistep interband cascade region having more than two steps may also be implemented thereby emitting more photons. More photons may be emitted per the electron510and the hole540that is injected into the multistep interband cascade region120if the multistep interband cascade region120had more steps. For example, if the multistep interband cascade region120were to have 3 steps, 3 photons may be emitted as a result of the injection of the electron510and the hole540.

Referring toFIG. 2, each cascade of multiple Type-I or Type-II quantum well regions120,140,160may be separated from its adjacent cascade by barrier layers110,130,150,170. The absorber barrier layers110,130,150,170confine the holes and electrons into the Type-I and Type-II quantum well regions. The absorber barriers110,130,150,170may also absorb the pump wavelength but not the wavelengths emitted by the Type-I or Type-II quantum well regions. Layers composed of materials such as AlGaAsSb, GaInAsSb, GaAlSb, AlAsSb, AlSb or even AlGaInAsSb may serve as the absorber barriers110,130,150,170.

Referring toFIGS. 5A,5B,5C, the energy of the conduction-band edge in absorber barrier110,130may be higher than the CB energy level610,710or810in the Type-I or Type-II quantum well region, respectively. Likewise, the energy of the valence-band edge in absorber barrier130,150may be greater than the VB energy level640,740or840in the Type-I or Type-II quantum well region, respectively. The energy gap of the absorber barrier may be smaller or fairly close to the pump energy, so that the cascade regions120,140,160may have as many steps as possible.

The absorber barrier layers110,130,150may further form electrical-potential gradients in both the conduction band and the valence band that assist the desired flow of electrons and holes, respectively, into the Type-I or Type-II quantum well regions. Each photo-generated electron hole pair in the absorber barrier layer may be split with the electron directed in one direction and the hole directed to an opposite direction. One way to form the potential gradients may be through delta doping. Alternatively, the composition of the absorber barrier layers may be graded to achieve the potential gradients. It is to be understood that composition grade in the absorber barrier does not so much change the bandgap but rather creates a tilt in both the valence and conduction bands (with both bands being tilted in the same direction). A material having sufficient degrees of freedom to achieve the combination of lattice match, band-energy and bandgap constraints for the absorber barrier may be, for example, AlGaInAsSb. Further, an absorber barrier layer may be used that self-compensates for strain (with the strain in one portion of the absorber barrier layer compensating for the strain in another portion of that absorber barrier layer).

The IC-OPL structure10may comprise multiple interband cascade regions120,140,160within each substructure40. The different quantum well regions of a given interband cascade region120,140,160may have the same or different light-emission (or luminescence) spectra.

Each substructure40may be formed from an assembly of multiple interband cascade regions and absorber barriers. In one exemplary embodiment, all of the substructures40and the quantum well regions in them could have the same emission spectrum. In another exemplary embodiment, each substructure40may emit at its own emission spectrum. For example, region120would emit light of a first wavelength, region140would emit light of a second wavelength and region160would emit light of a third wavelength. In this way, the overall width of the luminescence spectrum of the laser can be increased.

In yet another exemplary embodiment, the larger luminescence spectrum may be achieved by having the different quantum wells in different steps of a given multistep interband cascade region emit light of different wavelengths. For example, Type I quantum well210could have an interband transition that emits light of a first wavelength hν1and Type I quantum well220could have an interband transition that emits light of a second wavelength hν2(where hν1is different from hν2). For either quantum well type, achievement of the differing emission wavelengths may be done by changing the thicknesses and compositions of the materials in the layers that comprise those quantum well structures.

In yet another exemplary embodiment, the cascade regions with the same overall emission spectrum may be grouped together in the same substructure with different groups of cascade regions, i.e., different substructures40, having different emission spectra. In yet another exemplary embodiment, the cascades with different emission spectra may be placed in an alternating configuration in the same substructure40.

The different multistep interband cascade regions compete for the optical pump light and the carriers generated from absorption of that pump light. Light not absorbed in the absorber barriers surrounding one multistep interband cascade region may be available for absorption by the absorber barriers surrounding another multistep interband cascade region. A given quantum well region may absorb the light generated in another quantum well region. This characteristic may used to design the number of wells emitting at each wavelength so that the desired shape of the emission spectrum is achieved. The longest-wavelength light may be absorbed by a smaller number of quantum well regions, since the wells that emit at shorter wavelengths will not absorb that longest-wavelength light.

There are a number of ways for enhancing the efficiency with which the optical pump light is absorbed by the OPL. In one embodiment, the efficiency with which the optical pump light may be absorbed may be enhanced by constructing an optical cavity for the pump light so that the pump light makes multiple vertical passes through the cascade/barrier substructures of the OPL. This type of optical cavity is described in Applied Physics Letters, v.75, n.19, pp. 2876-2878 (1999) and incorporated herein in its entirety. The optical reflectors for the pump light may be located above and below the substructures40. These reflectors may be incorporated into the cladding layers30and50or may be separate from those cladding layers.

In another exemplary embodiment, an optical pump laser may be integrated into the IC-OPL10and the pump light co-propagates, in the same longitudinal directions, with the light emitted by the IC-OPL10. In this exemplary embodiment, the pump light makes multiple longitudinal passes through the OPICL substructures40. The optical reflectors for the pump light may be located at the ends of the OPL substrate, perpendicular to the longitudinal axis. The integration of an optical pump into the OPL structure is described in the U.S. application Ser. No. 11/090,453 which is incorporated herein in its entirety.