Light-emitting device having self-cooled semiconductor laser

Abstract

A light-emitting device having a self-cooled semiconductor laser having a laser cavity.

Claims

1. A light-emitting device comprising: a self-cooled semiconductor laser, said self-cooled semiconductor laser comprising: a laser resonator with a waveguide having a core and bound by a slab photonic-crystal structure that is defined in a cubic, hexagonal, or complex photonic-crystal lattice fabricated in the same structure as said semiconductor laser; nanoemitters having anti-Stokes photoluminescence when excited by an internal laser emission, said nanoemitters are located in said core of said waveguide; said waveguide made of a wider-bandgap semiconductor material than said nanoemitters; and wherein said nanoemitters are quantum dots.

2. The light-emitting device of claim 1, wherein said laser resonator is a Fabry-Perot cavity.

3. The light-emitting device of claim 1, wherein said laser resonator is a distributed-feedback cavity.

4. The light-emitting device of claim 1, wherein said laser resonator is a distributed Bragg reflector cavity.

5. The device of claim 1, where said laser resonator is a ring cavity.

6. The light-emitting device of claim 1, wherein said self-cooled semiconductor laser is an injection-lockable ring laser, comprising a ring laser, an injecting waveguide used to collect light from an external laser source and to deliver it to the ring laser, a waveguide directional output coupler proximate, in the lateral direction, to the cavity of the ring laser and used to collect its output; all monolithically integrated on the same substrate, and said ring laser has whistle geometry.

7. The light-emitting device of claim 1, wherein said laser resonator and said slab photonic-crystal structure are monolithically integrated on a substrate made of III-V or II-VI semiconductor materials.

8. The light-emitting device of claim 1, wherein said laser resonator and said slab photonic-crystal structure are monolithically integrated on a silicon-on-insulator substrate.

9. The device of claim 1, wherein said spontaneous emission is further confined in the vertical direction by one-dimensional stacks of distributed-Bragg-reflectors fabricated below and above said slab photonic-crystal laser cavity.

10. The light-emitting device of claim 9, wherein the peak reflectivity and bandwidth of the said distributed-Bragg-reflectors spectrally match the spontaneous emission circulating inside the photonic-crystal laser cavity.

11. A light-emitting device comprising: a self-cooled semiconductor laser, said self-cooled semiconductor laser comprising; a laser resonator with a waveguide having a core and bound by a slab photonic-crystal structure that is defined in a cubic, hexagonal, or complex photonic-crystal lattice fabricated in the same structure as said semiconductor laser; nanoemitters having anti-Stokes photoluminescence when excited by an internal laser emission, said nanoemitters are located in said core of said waveguide; said waveguide made of a wider-bandgap semiconductor material than said nanoemitters; and wherein said nanoemitters are multiple quantum wells.

12. The light-emitting device of claim 11, wherein said laser resonator is a Fabry-Perot cavity.

13. The light-emitting device of claim 11, wherein said laser resonator is a distributed-feedback cavity.

14. The light-emitting device of claim 11, wherein said laser resonator is a distributed Bragg reflector cavity.

15. The device of claim 11, where said laser resonator is a ring cavity.

16. The light-emitting device of claim 11, wherein said self-cooled semiconductor laser is an injection-lockable ring laser, comprising a ring laser, an injecting waveguide used to collect light from an external laser source and to deliver it to the ring laser, a waveguide directional output coupler proximate, in the lateral direction, to the cavity of the ring laser and used to collect its output; all monolithically integrated on the same substrate, and said ring laser has whistle geometry.

17. The light-emitting device of claim 11, wherein said laser resonator and said slab photonic-crystal structure are monolithically integrated on a substrate made of III-V or II-VI semiconductor materials.

18. The light-emitting device of claim 11, wherein said laser resonator and the slab photonic-crystal structure are monolithically integrated on a silicon-on-insulator substrate.

19. The device of claim 11, wherein said spontaneous emission is further confined in the vertical direction by one-dimensional stacks of distributed-Bragg-reflectors fabricated below and above said slab photonic-crystal laser cavity.

20. The light-emitting device of claim 19, wherein the peak reflectivity and bandwidth of the said distributed-Bragg-reflectors spectrally match the spontaneous emission circulating inside the photonic-crystal laser cavity.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.

(2) FIG. 1 shows an energy-level scheme for a radiation-balanced laser.

(3) FIG. 2 is a schematic diagram of a whistle-geometry ring laser.

(4) FIG. 3 is a schematic diagram of an optically injection-locked (strong injection) whistle-geometry ring laser monolithically integrated with a single-frequency master laser.

(5) FIG. 4 illustrates an embodiment of the present invention providing a self-cooled semiconductor laser implemented in a dielectric-waveguide platform with multiple quantum wells (MQW) and nanoemitters such as quantum dots (QDs).

(6) FIG. 5 shows the cleaved end of a single-line-defect slab photonic-crystal waveguide.

(7) FIG. 6A shows a self-cooled PhC semiconductor laser implemented in a 2D PhC-waveguide platform prior to metallization for an embodiment of the present invention.

(8) FIG. 6B shows a self-cooled PhC semiconductor laser implemented in a 3D PhC-waveguide platform prior to metallization for an embodiment of the present invention.

(9) FIG. 7 shows a photonic-crystal ring cavity of a 1.55-m InGaAsP/InP-based PhC ring laser.

(10) FIG. 8 shows a fabricated 2D-PhC slab directional coupler.

(11) FIG. 9 shows a strongly injection-locked RL implemented in a 2D PhC-waveguide platform for an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(12) Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.

(13) The present invention concerns various preferred embodiments of optical designs for a self-cooled edge-emitting Fabry-Perot (FP) semiconductor laser and for a strongly injection-locked ring laser (RL) with minimized internal heating that would efficiently address the problem of directional extraction of spontaneous emission. The self-cooled semiconductor laser embodiments may be implemented in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence. In a preferred embodiment, a semiconductor material with high efficiency of anti-Stokes photoluminescence is directly used as the active region material providing optical gain for lasing and, at the same time, providing cooling effect through the anti-Stokes photoluminescence. In addition, the properties/quality of the active region material (such as high optical gain and high efficiency of anti-Stokes luminescence) are not easy to meet in one specific material. To overcome this problem, the present invention provides an alternative embodiment of a self-cooled semiconductor laser, where colloidal quantum dot (QD) emitters grown in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence are inserted into nanocavities prefabricated in the optical waveguide layer of the epitaxial laser structure waveguide made of a wider-bandgap semiconductor material. Thus, the QD nanoemitters will be optically pumped by the internal laser emission and provide cooling, similar to the laser cooling scheme of rare-earth doped glasses optically pumped with external laser light.

(14) To date, anti-Stokes spontaneous luminescence caused by single-photon phonon-assisted carrier excitation has been reported for InP, CdSe, CdTe, PbS, and PbSe QDs. The embodiments of the present invention also overcome the known problem of very low absorption of the pump light in the latter scheme by putting nanoemitters, such as QD emitters, inside the laser cavity and, thus, employing the effect of cavity-enhanced absorption of the pump light.

(15) Implementation in Dielectric-Waveguide-Based Platforms

(16) Self-cooled injection-locked whistle-geometry ring laser

(17) In another embodiment, the strongly injection-locked WRL shown in FIG. 3 may be implemented in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence. The semiconductor material with high efficiency of anti-Stokes photoluminescence may be directly used as the active region material providing optical gain for lasing and, at the same time, providing cooling effect through the anti-Stokes photoluminescence. The size of the microring cavity should be small enough for a substantial part of anti-Stokes spontaneous emission to couple into guided modes/resonances of the cavity for efficient removal from the cavity through a properly designed directional outcoupler and, eventually, for directing it out from the cryogenic environment. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(18) In another embodiment, as shown in FIG. 4, strongly injection-locked WRL 400 may have colloidal QD emitters 410 grown in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence inserted into nanocavities prefabricated in the optical waveguide layer 420 of the epitaxial laser structure made of a wider-bandgap semiconductor material. The nanoemitters or QDs are optically pumped by the internal laser emission and provide cooling. The size of the microring cavity should be small enough for a substantial part of anti-Stokes spontaneous emission to couple into guided modes of the cavity for efficient removal from the cavity through a properly designed directional outcoupler and, eventually, for directing it out from the cryogenic environment. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(19) Self-Cooled PhC Edge-Emitting Semiconductor Laser

(20) In other embodiments, the present invention provides a self-cooled 2D and 3D PhC edge-emitting FP lasers to ensure significantly improved directional output of spontaneous emission. When implemented in a 2D PhC-waveguide platform, as shown in FIG. 5, the edge-emitting FP lasers are realized as slab PhC waveguides 500 fabricated e.g. in cubic photonic lattices and terminated with mirror facets. Based on PBG confinement, ultra-compact lasers can be fabricated. As shown in FIG. 5, PhC waveguides can be formed by removing one or more rows of PhC air holes 520. The anti-Stokes spontaneous emission inside the cavity can be further controlled by a 3D PhC structure. In addition to the in-plane 2D confinement provided by the slab design described above, the spontaneous emission is also confined in the vertical direction by 1D stacks of distributed Bragg reflectors (DBRs) located below and above the 2D PhC laser. The peak reflectivity and bandwidth of the DBRs should match spontaneous emission spectrum of the active region of the laser and that of the embedded nanoemitters or QDs, thus confining both the Stokes and anti-Stokes components of the spontaneous emission to the laser cavity.

(21) In other embodiments, the present invention provides 2D or 3D PhC edge-emitting FP semiconductor lasers implemented in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence. The semiconductor material with high efficiency of anti-Stokes photoluminescence is directly used as the active region material providing optical gain for lasing and, at the same time, providing cooling effect through the anti-Stokes photoluminescence. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(22) In other embodiments, as shown in FIGS. 6A-6B, the present invention provides 2D or 3D PhC edge-emitting FP semiconductor lasers where colloidal QD emitters grown in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence are inserted into nanocavities prefabricated in the optical waveguide layer of the epitaxial laser structure made of a wider-bandgap semiconductor material. The QDs are optically pumped by the internal laser emission and provide cooling. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(23) As shown in FIG. 6A, 2D PhC edge-emitting FP laser 600 is formed using e-beam lithography and fabricated using the ICP etching. The 2D photonic crystal lattice is defined in the epitaxial wafer by etching holes 605-606, with the row defects representing the slab photonic waveguides. The holes will be etched through the top contact layer 610, upper cladding layers 611-12, active region 613, and down to the lower cladding layer 614. The 2D PhC lasers with different cavity length are obtained first by cleaving the processed laser wafer into bars and then by cleaving the bars into individual laser devices.

(24) As shown in FIG. 6B, the fabrication process for 3D PhC Fabry-Perot laser 650 is similar to that of 2D PhC Fabry-Perot lasers. In this case, holes 625-626 defining the 2D photonic crystal lattice and the 2D slab photonic waveguide are etched through the top DBR layer 640, upper spacer layers 642, active region 643, and lower spacer layer 645. The term spacer is used as it is commonly used in vertical-cavity surface-emitting (VCSEL) literature, emphasizing the critical role of the spacer layer thickness in the vertical-cavity design. For preferred embodiment, the thickness is selected such as to reflect most of the anti-Stokes emission back into the waveguide core. The 3D PhC lasers with different cavity length are obtained first by cleaving the processed laser wafer into bars and then by cleaving the bars into individual laser devices.

(25) In alternate embodiments, 2D PhC edge-emitting FP lasers may be formed using e-beam lithography and fabricated using the ICP etching. The 2D photonic crystal lattice is defined in the epitaxial wafer by etching holes, with the row defects representing the slab photonic waveguides. The holes will be etched through the top contact layer, upper cladding layer, active region, and down to the lower cladding layer. The 2D PhC lasers with different cavity length are obtained first by cleaving the processed laser wafer into bars and then by cleaving the bars into individual laser devices. The fabrication process for 3D PhC Fabry-Perot lasers is similar to that of 2D PhC Fabry-Perot lasers. In this case, the holes defining the 2D photonic crystal lattice and the 2D slab photonic waveguide are etched through the top DBR layer, upper spacer layer, active region, and lower spacer layer. The 3D PhC lasers with different cavity length are obtained first by cleaving the processed laser wafer into bars and then by cleaving the bars into individual laser devices.

(26) In all PhC FP laser embodiments, a laser performance with minimized internal heating will be achieved by extracting a substantial part of the anti-Stokes spontaneous emission from the laser cavity and, eventually, from the cryogenic environment. NIR-emitting PbS and PbSe colloidal QDs are good candidates for many application, as their size-tunable absorption/emission spectra can be closely matched to 1.55-m emission wavelength of the laser by a proper control of the colloidal synthesis. Many military and commercial applications exist for high-power lasers operating in this eye-safe spectral region, such as target identification in 3D lidars, ultra-high power lasers for missile defense systems, free-space optical communication, laser illumination, and laser machining in manufacturing.

(27) The feasibility of achieving a substantial cooling power may be estimated by extracting the anti-Stokes spontaneous emission of the embedded nanoemitters or QDs from the laser cavity as follows. First, the concentration of embedded nanoemitters or QDs necessary to achieve a significant level of absorption of 1.55-m internal laser emission by the nanoemitters or QDs is estimated. The absorption cross section for PbSe QDs of 5-nm diameter doped in glass at 1.55-m excitation wavelength was extracted from FIG. 3 in to be 6.sub.a=110.sup.16 cm.sup.2 per one QD. The absorption coefficient of the material a [cm.sup.1] can then be calculated as =6.sub.a N, where N[cm.sup.3] is concentration of QDs. With the typical value of threshold modal gain in semiconductor lasers g.sub.mod10-20 cm.sup.1, the modal absorption caused by QDs should be .sub.mod1-2 cm.sup.1 for the absorption in QDs to be substantial and for the threshold current not to be strongly affected. Assuming one percent optical confinement for the layer of embedded QDs, the material absorption coefficient 100-200 cm.sup.1 and the corresponding QD concentration N110.sup.18-210.sup.18 cm.sup.3, which corresponds, for example to 110.sup.12-210.sup.12 cm.sup.2 sheet density of QDs for a 10-nm QD layer may be determined.

(28) The condition .sub.mod0.1 g.sub.mod means the optical power absorbed by QDs, P.sub.abs0.1 P.sub.out, since the modal gain is mostly determined by the outcoupling mirror losses. The cooling power is defined as P.sub.cool=P.sub.abs .sub.cool. Here .sub.cool=.sub.ext .sub.abs .sub.1/.sub.f1 is the cooling efficiency. The external quantum efficiency .sub.ext represents the probability for an excited carrier to recombine by the desired radiative process and for the emitted fluorescence photon not to be reabsorbed. .sub.abs=.sub.r/(.sub.r+.sub.b) is the absorption efficiency representing the ratio of resonant absorption efficiency .sub.r of QDs to total absorption efficiency .sub.r+.sub.b. .sub.b is the background absorption efficiency. .sub.l is the wavelength of the laser and .sub.f is the mean wavelength of spontaneous emission. Assuming .sub.ext .sub.abs (100% quantum efficiency of QDs, no photon trapping in the proposed PhC laser design, and negligible background absorption), and the demonstrated anti-Stokes shift in PbS QDs of 132 nm, .sub.cool0.093 and P.sub.cool0.093 P.sub.abs0.01P.sub.out are determined.

(29) To mitigate the thermal rollover, one needs to compensate for the excess over-threshold Joule heating from the laser series resistance R.sub.s. The ratio .sub.cool/P.sub.diss for a single-emitter device characterized may be estimated. The incremental values P.sub.cool and P.sub.diss were calculated as:
P.sub.cool=0.01 P.sub.out=0.01(dP.sub.out/dI)I
P.sub.diss=2IIR.sub.s+(I).sup.2R.sub.s

(30) Series resistance was extracted from FIG. 2 to be R.sub.s0.015 dP.sub.out/dI=1.1 W/A, as shown in FIG. 3, at I=2 mA, where the thermal rollover starts. The calculated value of P.sub.cool/P.sub.diss is 0.18. Better efficiency of mitigating the thermal rollover through optical cooling can be expected in high-power lasers as the laser technology improves further to allow for even smaller values of R.sub.s. For example, it is estimated that P.sub.cool/P.sub.diss0.55 for R.sub.s=0.005, and P.sub.cool/P.sub.diss0.92 for R.sub.s=0.003, which implies 3 to 5 times reduction in R.sub.s compared to the current state-of-the-art.

(31) Self-Cooled Injection-Locked PhC Ring Laser

(32) The proposed concept is based on frequency selectivity of microring resonators and their ability to efficiently control spontaneous emission. The extremely small diameters of microring resonators, necessary for efficient control of spontaneous emission, translate into a very hard requirement for the optical waveguide: to make a compact ring, a small bend radius is required, and this in turn is only possible with high-refractive-index-contrast waveguides with strong optical confinement. Photonic crystal structures can overcome this challenge as they have the potential to achieve high-Q, low-loss resonators in ultra-compact cavities several times smaller than the minimum-possible-size dielectric-waveguide-based rings. In dielectric-waveguide-based resonators, the guided modes are supported by the total internal reflection, which sets the ultimate limit for size reductionthe radiation losses increase very rapidly with reduction in the ring radius. By contrast, the resonant modes in photonic crystal ring resonators (PCRRs) are supported by the photonic bandgap, which is much more efficient for optical confinement. The smallest PCRR can be a single point-defect cavity, which offers a very low loss with extremely high Q and ultra-small cavity volume.

(33) The strongly injection-locked RL can be implemented in a 2D PhC-waveguide platform. All the functional elements of the injection-locked RL have been successfully realized using slab PhC waveguides. Based on PBG confinement, ultra-compact PCRRs can be fabricated in cubic, hexagonal, and other complex photonic lattices. The choice of the ring size is determined by the desired resonant wavelength, and the tradeoff between the cavity quality factor Q and the modal volume V. PhC waveguides (FIG. 5) and PCRRs (FIG. 7) can be formed by removing one or more rows of PhC dielectric columns/air holes or a ring (or racetrack) shape of dielectric rods/air holes, respectively. In general, dielectric-rod-type PhC waveguides can be easily operating in single mode while air-hole-type PhC waveguides tend to be multimode.

(34) Photonic crystal directional couplers (FIG. 8) capable of short coupling length and wide bandwidth have been demonstrated. An ultra-short PhC waveguide coupler has been designed with coupling length less than 3a, where a is the PhC lattice constant. In other aspects, the present invention uses strong coupling efficiency of PhC directional couplers to implement the injecting waveguide.

(35) The coupling properties of PhC directional couplers have been shown to be strongly wavelength dependent due to the large group velocity dispersion in the presence of the PBG. The coupling lengths are on a wavelength scale and show strong wavelength dependence, allowing for the design of compact wavelength-selective optical filters. In the present invention, this wavelength-selectivity of PhC directional couplers is used to design the outcoupling waveguide of FIG. 2 in such a way as to outcouple as much of the anti-Stokes spontaneous emission as possible, and to extract only a small portion of the injected/lasing Stokes emission (e.g., with 5% coupling efficiency), thus maintaining high Q of the ring cavity at the injected/lasing wavelength.

(36) The PhC directional couplers have been demonstrated in various semiconductor material systems, such as InAlGaAs, InP, and GaN.

(37) The general concept of a strongly injection-locked RL implemented in a 2D photonic-crystal-waveguide platform is illustrated in FIG. 9. Ultra-compact PCRRs of circular, oval, racetrack, hexagonal, and other shapes can be fabricated in cubic, hexagonal, and other complex photonic lattices.

(38) The period of the PhC lattice should is designed to achieve a wide PBG, covering the entire spontaneous emission spectrum of the active laser material. In this way, the spontaneous emission generated inside the PCRR will be directed only into the PCRR guided modes.

(39) The wavelength-selectivity of PhC directional couplers will be also used, to the maximum possible extent, to prevent the anti-Stokes spontaneous emission from leaving the PCRR cavity through the injecting waveguide. Additionally, the core of the injecting waveguide can be modified with periodic air holes, making a PhC lattice with a period different from that of the main PhC lattice defining the PCRR. The purpose of that heterostructure PhC design is to reject the anti-Stokes spontaneous emission by making PBG in the core of the injecting waveguide narrower, thus confining the anti-Stokes spontaneous emission to the PCRR cavity and the outcoupling waveguide.

(40) The anti-Stokes spontaneous emission inside the PCRR cavity can be further controlled by a 3D PhC structure. In addition to the in-plane 2D confinement provided by the slab PCRR design, the spontaneous emission is also confined in the vertical direction by 1D stacks of distributed Bragg reflectors (DBRs) located below and above the 2D PCRR. The peak reflectivity and bandwidth of the DBRs should match spontaneous emission spectrum of the active region of the PCRR, thus confining both the Stokes and anti-Stokes components of the spontaneous emission to the PCRR.

(41) In other embodiments, the present invention provides 2D or 3D PhC injection-lockable self-cooled ring lasers implemented in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence. The semiconductor material with high efficiency of anti-Stokes photoluminescence is directly used as the active region material providing optical gain for lasing and, at the same time, providing cooling effect through the anti-Stokes photoluminescence. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(42) In other embodiments, the present invention provides 2D or 3D PhC injection-lockable self-cooled ring lasers where colloidal QD emitters grown in any semiconductor material system (including III-V, II-VI, or IV-VI semiconductors) with high efficiency of anti-Stokes photoluminescence are inserted into nanocavities prefabricated in the optical waveguide layer of the epitaxial laser structure made of a wider-bandgap semiconductor material (FIG. 6). The nanoemitters or QDs are optically pumped by the internal laser emission and provide cooling. The device can be integrated on a silicon-on-insulator (SOI) substrate.

(43) In all embodiments, the PhC RL performance with minimized internal heating will be achieved by extracting a substantial part of the anti-Stokes spontaneous emission from the RL cavity through a properly designed directional coupler and, eventually, from the cryogenic environment. In particular, this energy-efficient directly modulated laser source with very high modulation bandwidth of up to 100 GHz is particularly attractive for the cryogenic optical data link application.

(44) While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.