Data are presented on a monolithic chip that integrates a quantum well semiconductor laser with a high efficiency light emitting diode (LED), with the LED used to optically pump the laser. The LED operates in the thermoelectrophotonic regime that can produce heat absorption, offering the possibility of heat pump action. The internal optical pumping also offers the possibility of very low internal loss in the laser and the prospect of reaching greater than unity power conversion efficiency in the laser chip. The integrated laser chip is operated and tested under continuous-wave room temperature operation

Deppe, D. G.

Liu, X.

Zhang, Y.

Zhao, G.

English

X. Liu

G. Zhao

Y. Zhang

D. G. Deppe

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Applied Physics Letters

Semiconductor laser monolithically pumped with a light emitting diode operating in the thermoelectrophotonic regime

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University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2013 Semiconductor laser monolithically pumped with a light emitting diode operating in the thermoelectrophotonic regime X. Liu University of Central Florida G. Zhao University of Central Florida Y. Zhang University of Central Florida D. G. Deppe University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Liu, X.; Zhao, G.; Zhang, Y.; and Deppe, D. G., "Semiconductor laser monolithically pumped with a light emitting diode operating in the thermoelectrophotonic regime" (2013). Faculty Bibliography 2010s. 4324. https://stars.library.ucf.edu/facultybib2010/4324 Appl. Phys. Lett. 102, 081116 (2013); https://doi.org/10.1063/1.4793656 102, 081116© 2013 American Institute of Physics.Semiconductor laser monolithically pumpedwith a light emitting diode operating in thethermoelectrophotonic regimeCite as: Appl. Phys. Lett. 102, 081116 (2013); https://doi.org/10.1063/1.4793656Submitted: 19 November 2012 . Accepted: 14 February 2013 . Published Online: 28 February 2013X. Liu, G. Zhao, Y. Zhang, and D. G. DeppeARTICLES YOU MAY BE INTERESTED INThermally enhanced blue light-emitting diodeApplied Physics Letters 107, 121109 (2015); https://doi.org/10.1063/1.4931365Room temperature thermo-electric pumping in mid-infrared light-emitting diodesApplied Physics Letters 103, 183513 (2013); https://doi.org/10.1063/1.4828566Photon recycling effect on electroluminescent refrigerationJournal of Applied Physics 111, 014511 (2012); https://doi.org/10.1063/1.3676249Semiconductor laser monolithically pumped with a light emitting diodeoperating in the thermoelectrophotonic regimeX. Liu, G. Zhao, Y. Zhang, and D. G. DeppeCollege of Optics and Photonics, CREOL, College of Optics and Photonics, University of Central Florida,Orlando, Florida 32816, USA(Received 19 November 2012; accepted 14 February 2013; published online 28 February 2013)Data are presented on a monolithic chip that integrates a quantum well semiconductor laser with ahigh efficiency light emitting diode (LED), with the LED used to optically pump the laser. The LEDoperates in the thermoelectrophotonic regime that can produce heat absorption, offering thepossibility of heat pump action. The internal optical pumping also offers the possibility of very lowinternal loss in the laser and the prospect of reaching greater than unity power conversion efficiencyin the laser chip. The integrated laser chip is operated and tested under continuous-wave roomtemperature operation.VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793656]Semiconductor light emitting diodes (LEDs) can operatein a thermoelectrophotonic (TEP) regime for which theirpower conversion efficiency may exceed unity. In such oper-ation the LED radiates more optical power than the power itdraws from an external electrical power source. While theTEP regime physics that can produce greater than unitypower conversion efficiency has been understood for sometime,1–5 only recently has it been demonstrated in an LED.6The experimental demonstration of the greater thanunity efficiency so far suffers major drawbacks for applica-tions mainly because the LED scheme that has been demon-strated requires extracting the spontaneous emission fromthe semiconductor chip.6 This light extraction suffers fromSnell’s law and a small escape cone from the LED chip. Onthe other hand, a laser that operates with greater than unityefficiency would be much more useful and quite likely atechnological revolution because of its directional emissionand high extraction efficiency. Such a device could thereforeproduce usable power levels while operating in the self-cooling regime. However, as opposed to an LED, a diodelaser requires a voltage drive that exceeds the laser photonenergy, while the TEP regime needed for greater than unityefficiency in an LED requires a voltage drive that is less thanthe photon energy. Therefore it does not appear possible thata diode laser operating alone can break the unity efficiencybarrier.Modern semiconductor technology, however, enablesthe monolithic integration of an LED that can operate in theTEP regime with greater than unity efficiency with a semi-conductor laser, for which the LED acts as an optical pumpas well as a TEP heat pump to the laser. In fact, as we showbelow, this type of integration eliminates the need to extractthe LED emission from the chip and instead benefits fromSnell’s law. Our calculations based on material parametersindicate that the combination could result in an integratedlaser chip for which the laser output power exceeds unityefficiency with respect to the chip’s electrical input power.In this letter we present the demonstration of the mono-lithic integration of such an LED pumped laser chip andpresent data on its operation. We describe advantages ofusing the TEP effect that comes from absorbing the LEDlight internal to the chip, mainly avoiding the usual lowextraction efficiency caused by Snell’s law of the LED light.We also describe further advantages of the integrated chipfor producing very low internal optical loss in the semicon-ductor laser waveguide, due to integration of the monolithicLED optical pump.Figure 1 shows a schematic of the integrated laser chip.The LED is either bulk GaAs or AlGaAs, and semiconductorlaser uses a single InGaAs quantum well. Both the LED andlaser are clad between two mirrors. The p-type mirror iscomprised of two p-type pairs of quarter-wave layers ofAl0.1Ga0.9As/Al0.67Ga0.33As, followed by phase matching p-type Al0.1Ga0.9As and pþ GaAs contact layers and a post-growth deposited Ag mirror. The Ag mirror also serves as anelectrical p-contact to the LED. The n-type mirror is basedon 16 quarter wave pairs of Al0.1Ga0.9As/AlAs. The semi-conductor laser uses Al0.3Ga0.7As cladding layers, a 0.5 lmthick GaAs waveguide layer, and a 60 A˚ In0.2Ga0.8As quan-tum well active region. All layers of the laser except thequantum well active layer are doped n-type at 1017 cm3.Above the laser is grown a p-n bulk LED with an undopedactive thickness of 1.5 lm and cladding p-n Al0.1Ga0.9Aselectrical injection layers. The two mirror stacks trap thespontaneous emission of the LED, which undergoes photonrecycling and ultimately absorption in the laser waveguide.Figure 2 shows 300K and 77K photoluminescence fromthe as-grown structure. The epitaxial structure is excitedusing a 15mW HeNe laser operated at 0.632 lm with a1mm diameter excitation spot. The HeNe pump is mainlyabsorbed in the LED layers. The GaAs LED active regionthen also pumps the laser waveguide and quantum well inphotoluminescence. The comparison is made between the300K and 77K photoluminescence to estimate the GaAsemission efficiency. The emission spectra, taken from theepitaxial surface, show the GaAs LED emission peak at0.87 lm at 300K and 0.83 lm at 77K, and the In0.2Ga0.8Asquantum well emission of the laser at 0.98lm at 300K and0.93 lm at 77K. This emission is spectrally modified by thevertical cavity structure of the upper and lower mirrors butclearly shows both the GaAs LED emission and theIn0.2Ga0.8As quantum well emission. Because of the thick-nesses of the LED layers including the p and n Al0.1Ga0.9Asinjection layers, the In0.2Ga0.8As quantum well emission can0003-6951/2013/102(8)/081116/3/$30.00 VC 2013 American Institute of Physics102, 081116-1APPLIED PHYSICS LETTERS 102, 081116 (2013)be assumed to result from optical pumping by the GaAsLED emission towards the substrate. Electrical contacting tothe GaAs LED is through the Ag mirror/contact placed onthe upper p-side of the integrated chip and through a GeAualloyed contact formed on the GaAs nþ substrate. The elec-tron current of the n-side is therefore passed through the laserwaveguide and quantum well.The laser and LED spectral emission are measured driv-ing the chip quasi-continuous-wave without heat sinking,simply probing the chip placed on a metal contact. Nosoldering or other heat sinking is used so that the 40mselectrical pulses of the quasi-continuous-wave operationcause some heating of the chip. Figure 3 shows the electrolu-minescence spectra measured from one facet of the chip bothbelow and above threshold. While GaAs LED emission canbe measured below threshold, only In0.2Ga0.8As quantumwell lasing is measured above threshold. In fact the GaAsLED injection current density at threshold of 333A/cm2 isfar below the estimated value of 9000A/cm2 needed to invertthe electron-hole population of the GaAs LED. If the GaAsLED region did become inverted, its thickness would easilysupport lasing end-to-end in the GaAs LED.Numerous laser and LED designs have been investi-gated so far, with most showing room temperature laseroperation. The light vs. current curve of one of the higherpower integrated chips is shown in Figure 4. This sample hasAl0.02Ga0.98 in LED active region, three quantum wells inlaser structure, and 10 pairs of p mirror layers. The laseremission is measured driving the chip continuous-wave withBeO heat sink, and the chip is p-side down mounted on theBeO heat sink with AuSn solder. The lasing threshold of theIn0.2Ga0.8As quantum well for a 120 lm wide electrical con-tact on the LED and 0.5 cm long cavity is measured to be2A under room temperature operation. The 2A lasingthreshold corresponds to a current density of 333A/cm2,which is only a factor of two higher than for an electricallyinjected p-n diode laser of similar waveguide and quantumwell design and contacting area. The slope efficiency of thisdevice is 12%.Finally we note that optical absorption of the LED lightby the laser waveguide is largely due to off-normal radiationmodes due to their much larger solid angle of emission.These off-normal radiation modes are more readily trappedbetween the LED and laser waveguide to increase absorp-tion. We have estimated this absorption in the currentFIG. 2. Photolumenescence measured at 300K and 77K from the as-grownepitaxial sample. The HeNe pump light is absorbed in the GaAs LED region,which is shown at 870 nm at 300K (dashed curve) and 830 nm at 77K (solidcurve). The GaAs spontaneous emission pumps the quantum well at 980 nmat 300K (dashed curve) and at 930 nm at 77K (solid curve).FIG. 3. Electroluminescence measured from the chip at two different biaslevels. The below threshold level at 0.5A shows both the GaAs emissionand the quantum well emission. At 2A only laser emission from the quan-tum well occurs.FIG. 4. Light vs. current curve from the chip measured CW from both fac-ets. Driving the LED electrically optically pumps the quantum well laser,which produces the laser emission shown in the inset. The stripe dimensionsare 120lm 0.5 cm.FIG. 1. Schematic illustration of the GaAs LED integrated with the semi-conductor laser. The LED is electrically biased and optically pumps the laserwaveguide and quantum well. The LED is designed to operate in the regimeof thermoelectrophotonic heat pump, absorbing heat as it optically pumpsthe laser.081116-2 Liu et al. Appl. Phys. Lett. 102, 081116 (2013)structures to be greater than 90%. Further optimization andsimulation indicate that the absorption between the LED andlaser quantum well active region could be increased poten-tially to as high as >98%.In summary, we have shown that an internal LED opti-cal pump can be monolithically integrated with a semicon-ductor laser and operated in a regime for which the LED canbe optically absorbing while pumping the laser into continu-ous wave operation at room temperature. Heat absorption isexpected to result for sufficiently high internal quantum effi-ciency in the bulk LED, and this is currently being explored.Simulations also show that with sufficiently low threshold inthe laser and sufficiently small quantum defect between theLED pump and quantum well lasing transition, it may bepossible for the integrated laser chip to enter the self-coolingregime with greater than unity power conversion efficiency.When optimized, low internal optical loss is expected for thesemiconductor laser due to elimination of p-doping from thecladding layers and removal of the need for hole injection,giving the prospect for increased brightness from the semi-conductor laser.Parts of this research have been supported by DARPAthrough the Army Research Laboratory under Grant No.W31P4Q1110013 and by the National Science Foundationthrough Award No. 1034187.1R. J. Keyes and T. M. Quist, Proc. IRE 50, 1822 (1962).2G. C. Dousmanis, C. W. Mueller, H. Nelson, and K. G. Petzinger, Phys.Rev. 133, A316 (1964).3P. Berdahl, J. Appl. Phys. 58, 1369 (1985).4K. C. Lee and S. T. Yen, J. Appl. Phys. 111, 014511 (2012).5H. Oskari, J. Oksanen, and J. Tulkki, J. Appl. Phys. 107, 033105 (2010).6P. Santhanam, D. J. Gray, and R. J. Ram, Phys. Rev. Lett. 108, 097403(2012).081116-3 Liu et al. Appl. Phys. Lett. 102, 081116 (2013)

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Semiconductor laser monolithically pumped with a light emitting diode operating in the thermoelectrophotonic regime

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