Continuous wave operation of quantum cascade lasers above room temperature

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1 Invited Paper Continuous wave operation of quantum cascade lasers above room temperature Mattias Beck *a, Daniel Hofstetter a,thierryaellen a,richardmaulini a,jérômefaist a,emiliogini b a Institute of Physics, University of Neuchatel, Switzerland b Center for Micro- and Nanosciences, ETH Zurich, Switzerland ABSTRACT Continuous wave operation of quantum cascade lasers is reported up to a temperature of 311 K. Fabry-Perot and DFB devices were fabricated as buried heterostructure lasers with high-reflection facet coatings. Junction-down mounted FPlasers emitted up to 17 mw and 3 mw of optical power per facet under continuous wave operation at 292 K and 311 K, respectively. The DFB-devices could be operated up to T=253 K on a thermoelectric cooler at an emission frequency of 1114 cm -1 with a side mode suppression rate better than 30 db. Grating-coupled external cavity quantum cascade lasers based on a bound-to-continuum active region featuring a broad gain spectrum demonstrated frequency tuning of ~10% from 1036 cm -1 to 1142 cm -1 with average output power between 0.15 and 0.85 mw under pulsed operation at room temperature. Keywords: Quantum cascade laser, buried heterostructure, external cavity 1. INTRODUCTION The main applications of semiconductor lasers are currently focused in the visible and near-infrared spectral range, where semiconductor-based interband lasers are produced very economically with continuous wave (CW) output power of tens of milliwatts. Extension of semiconductor optoelectronics into the mid-infrared has until recently been limited by the lack of suitable sources, due to materials problems with narrow bandgap semiconductors and their fundamental limitations on high temperature performance. In the mid-infrared, a new class of semiconductor lasers quantum cascade (QC) lasers 1 hasbecomeapromisingalternativetoconventionaldiodelasers.inqclasers,photonemission is obtained only by electrons propagating through a potential staircase of coupled quantum wells (QW) and making optical transitions between energy levels in the conduction band created by quantum confinement. The unipolarity and the cascading scheme, where electrons are recycled from period to period, are fundamental and unique aspects of intersubband QC lasers. One of the key advantages of QC laser technology is that the emission wavelength can be tailored over a wide range by changing only the layer thicknesses of the active region. Up to now, InP-based QC lasers cover the wavelength range from 3.4 to 24 µm 2,3 relying on the same InGaAs/InAlAs material system. Pulsed operation was reported above room temperature between 3.4 to 16 µm 2,4.However,CWoperationofQClasershasremained limited to cryogenic temperatures below 210 K imposing a strong technological limitation for all applications that require narrow linewidths and/or high-frequency modulation. The maximum CW operating temperature increased only for 60 K (from 145 to 210 K between 1995 and 2001). Different methods like junction down mounting or dielectricmetal-based back facet coatings 5 had been tried to push the CW and pulsed operating temperatures towards higher values. However, all attempts failed quite systematically due to very high threshold current densities which led to overheating of the device. For this reason, it was essential to (i) minimize the threshold current density and (ii) use a laser geometry that maximizes heat dissipation from the active region and minimizes thermal stress within the active region. 2. FABRICATION The laser structure was grown by molecular beam epitaxy (MBE) using ternary InGaAs and InAlAs alloys lattice matched to an n-doped InP substrate. The laser core consists of 35 periods, each comprising a partially n-doped injector region and an undoped active region, embedded in an optical waveguide formed on one side by the InP substrate and a 0.2 µm thickingaaslayerandontheothersidebyanupperingaaslayerandaninptopcladdinglayer,thelatter * Mattias.Beck@unine.ch; phone: ; fax: ; Institute of Physics, University of Neuchatel, Rue Breguet 1, CH-2000 Neuchatel, Switzerland; ** gini@iqe.phys.ethz.ch; phone: ; fax: ; Center for Micro- and Nanosciences, ETH Zurich, CH-8093 Zurich, Switzerland Diode Lasers and Applications in Atmospheric Sensing, Alan Fried, Editor, Proceedings of SPIE Vol (2002) 2002 SPIE X/02/$

2 grown by metal-organic chemical vapour phase epitaxy (MOVPE). For DFB devices, the grating was etched into the upper InGaAs layer prior to MOVPE regrowth of the InP top cladding layer. To reduce the room temperature threshold current density, active region designs based on a double-phonon resonance 6 were developed. Additionally, growth conditions and the doping concentration of the active region were optimized. As aconsequence,thethresholdcurrentdensityat300kdroppedtoavalueaslowas3ka/cm 2.Aschematicconduction band diagram of one period of the laser core is shown in Fig. 1. The active region is composed of four quantum wells which results in three coupled lower energy states (levels 1, 2, and 3 in Fig. 1) separated from each other by one phonon energy. The active region was designed for a lasing transition at an energy of 135 mev (corresponding to an emission frequency of 1089 cm -1 )betweentheupperandlowerlasingstates(levels4and3).theactiveregionusedanarrow QW-barrier pair just after the injection barrier that enhances the injection efficiency into the upper lasing level by increasing locally the magnitude of the upper state wave function and by reducing the overlap of the injector ground state with the lower levels 1, 2, and 3. Fig.1: Schematic conduction-band diagram of one period of the laser core with moduli squared relevant wave functions in the four-qw active region based on a double-phonon resonance. The wavy arrow indicates the transition responsible for laser action, the others represent optical phonon transitions. The layer sequence of one period, in nanometers, from left to right starting with the injection barrier, is as follows: 4.0/1.9/0.7/5.8/0.9/5.7/0.9/5.0/2.2/3.4/ 1.4/3.3/1.3/3.2/1.5/3.1/1.9/3.0/2.3/2.9/2.5/2.9, where InAlAs barrier layers are in bold, InGaAs well layers are in roman, and n-doped layers ( cm -3 )areunderlined. After growth of the QC structure, Fabry-Perot (FP) and DFB lasers were processed from the same active material in a narrow stripe, planarized buried heterostructure geometry 7 in which the gain region was vertically and laterally buried within InP cladding layers. Lasers were fabricated by chemical wet etching of 5 µm deep and 9-15 µm narrow ridge waveguides using a SiO 2 mask followed by regrowth of 5 µm of non-intentionally doped InP by MOVPE to form the planarized buried heterostructure. The SiO 2 layer on top of the laser ridge was removed after this last regrowth and ohmic contacts were evaporated on top of the highly doped cladding layer. Substrate thinning and standard back metalization completed the laser fabrication. Devices were then cleaved into 0.75 mm long lasers, soldered junctiondown onto a diamond platelet and finally facet-coated by a ZnSe/PbTe high-reflectivity (R = 0.7) layer pair. Since the refractive index difference in this material combination is very large, one layer of each material is sufficient to achieve 70% reflectivity. With two mirrors of this kind, the threshold current density of a 750 µm long device is equal to the one of a 3 mm long uncoated laser. 2 Proc. SPIE Vol. 4817

3 The choice of a buried stripe greatly improves the heat transport by allowing heat flow from all sides of the active region due to the much higher thermal conductivity of InP compared to ternary III-V compounds and insulating SiN layers used in the conventional ridge geometry of QC lasers. Moreover, the small refractive index difference between the waveguide core and lateral InP allowed to make very narrow waveguides without having excessive losses. Additionally, the narrow stripe geometry also decreases the total amount of strain that builds up in a material subjected to a very strong temperature gradient. A finite-element simulation of the temperature and the thermally induced stress showed that the stress in a 12 µm wide buried laser stripe was only 20% (3.6 MPa) of the one in a normal, 28 µm wide standard ridge waveguide without lateral InP layers (22 MPa). We calculated a thermal conductance of 820 W/Kcm 2 for the buried, 12 µm wide, junction-down mounted device, as compared to the calculated value of 510 W/Kcm 2 for the 28 µm wide, junction-down mounted standard ridge device RESULTS The optical output power emitted from one facet was measured using a calibrated thermopile detector which was mounted directly in front of the laser facet. At room temperature (292 K), 15 µm wide FP-lasers emitted up to 17 mw of continuous optical power per facet at a drive current of 600 ma (Fig. 2) with a slope efficiency η =dp/di of 99 mw/a. This device could be operated up to 311 K with a maximum optical power of 3 mw and a slope efficiency η of 52 mw/a. The threshold current I th increased from 415 ma at 292 K (corresponding to a threshold current density J th of 3.7 ka/cm 2 )to540maat311k(j th =4.8kA/cm 2 ). The threshold voltage V th increased from 7.5 V to 8.1 V and the wall plug efficiency decreased from 0.33% to 0.06% per facet over the measured temperature range. Fig.2: Voltage bias and CW optical power from a single laser facet as a function of drive current for various heat sink temperatures. The laser is 0.75 mm long and 15 µm wide. The power was measured with near unity collection efficiency and a calibrated thermopile detector. In pulsed mode, we measured a threshold current I th as low as 280 ma for 12 µm wide, HR-coated FP-devices (J th =3.1kA/cm 2 )atroomtemperature.thepulsedj th,measuredbetween240kand320k,canbefittedbythe expression J th =J 0 exp(t act /T 0 )withat 0 =171K and J 0 =560A/cm 2.InCWmode,thislaserexhibitedathreshold current of 390 ma (J th =4.3kA/cm 2 )atavoltagebiasofv th =7.6V.Thisdeviceemitted13mWofopticalpowerfrom asinglefacetatadrivingcurrentof550ma(η =101mW/A)at292Kresultinginawallplugefficiencyof0.275%. Continuous wave operation was observed up to 312 K with still more than 1 mw of output power. The CW spectral properties of this device were analyzed with a Fourier transform infrared (FTIR) spectrometer (Fig. 3). The emission spectra collected at a constant heat sink temperature of 292 K and various currents between 395 ma and 530 ma reveal frequency tuning from cm -1 to cm -1 linear with the electrical input power (inset of Fig. 3A) with a tuning coefficient of Δυ/ΔP =-1.51cm -1 /W. At a fixed current of 530 ma, the emission frequency of the laser shifts from cm -1 at 292 K to cm -1 at 313 K (Fig. 3B). The measured center frequencies are Proc. SPIE Vol

4 well fitted by a linear function (inset of Fig. 3B) with a tuning coefficient of Δυ/ΔT =-0.078cm -1 /K. Single mode emission was observed for this particular device over the whole investigated current and temperature range with a side mode suppression ratio better than 30 db. This rather surprising fact can be explained by a small defect within the laser cavity, as indicated by an intensity modulation of the sub-threshold Fabry-Perot fringes at twice the cavity mode spacing. Assuming that the emission frequency υ is a function only of the temperature of the active region, we can deduce a thermal resistance R th of the device from the variation of the emission frequency and get a thermal resistance of 19.4 K/W in the range between 292 K and 313 K (corresponding to a thermal conductance G th of 574 W/Kcm 2 ). Fig.3: (a) CW spectra as a function of injection current measured at a constant temperature of 292 K for various drive currents ranging from 395 ma up to 525 ma in steps of 10 ma. (Inset) Emitted peak frequency in dependence of the electrical input power. (b) CW emission spectra at a constant drive current of 530 ma for various temperatures between 292 K and 313 K. (Inset) Measured temperature dependence of the center emission frequency at I=530 ma. The grating of the DFB lasers with a µm periodforanemissionfrequencyat1106cm -1 was holographically defined using a 488 nm Ar-ion laser and a 90 corner reflector mounted on a rotational stage for the grating exposure. The grating was structured by wet chemical etching to a depth of 180 nm into the upper 200 nm thick InGaAs waveguide layer of the laser structure and then overgrown with the InP top cladding layer by MOVPE. The DFB devices were fabricated as buried heterostructure lasers as described in the previous section but with a narrower active region width of 9 µm(comparedto12 µm or15 µmofthefplasers)toreducethecurrentconsumption. In pulsed mode, the threshold current at room temperature was as low as 300 ma measured for a 0.75 mm long uncoated laser and even 250 ma for the same laser with facet coatings. DFB lasers could be operated in continuous mode in a temperature range between 243 K up to 253 K on a thermoelectric cooler (Fig. 4). Lasing action was 4 Proc. SPIE Vol. 4817

5 observed above a threshold current of 310 ma at 243 K with a maximum measured optical power of 4.6 mw per facet. At 253 K, the corresponding values are 345 ma and 1 mw of optical power per facet. The bias voltage of these devices were slightly higher than the bias of the FP-devices due to increased resistivity of the ohmic contacts and probably also due to higher waveguide losses of the narrow laser geometry. Fig.4: Bias voltage and optical power from a single laser facet of a HR-coated DFB-laser as a function of drive current for various heat sink temperatures. The 0.75 mm long and 9 µm wide QC device was junction-down mounted. Fig. 5 shows the lasing spectra of the uncoated DFB device in pulsed mode and of the same device with HR-coatings driven in continuous wave. The spectra for the uncoated laser were measured at a duty cycle of 1.5% for various injection currents at T = 300K and showed single mode emission over the whole current range. The emission frequency at I = 380 ma was cm -1 with a linewidth of 0.25 cm -1 and a side mode suppression mode (SMSR) better than 30 db. In CW mode, we observed single mode emission over the whole temperature range ( K) and current range ( ma) with a much narrower linewidth (below the resolution limit of our FTIR) and a high SMSR (>35 db). The shift in the emission frequency towards lower wavelengths can be explained by competing modes between the DFB mode and the longitudinal modes from the HR-coated laser. Indeed, we measured a shift of the center frequency of Δυ =3.5cm -1 between the uncoated and the HR-coated DFB-device, both being operated under the same conditions (pulsed mode, 1.5% duty cycle, T = 300K). Fig.5: Emission spectra of a DFB-device before and after facet coating. The left peak at cm -1 belongs to the spectrum of the uncoated laser measured in pulsed mode at room temperature. The same laser with HR-coatings operated in CW shows single mode emission with a narrow peak around cm -1 at the maximum operating current and T = 248 K. Proc. SPIE Vol

6 At high duty cycles, the temperature of the active region T act is much higher than the heat sink temperature T sink.ina simple model, the heat transport between active region and heat sink is characterized by the thermal conductance G th per unit of area of the active region, i.e. T act =T sink + U I/G th. With the above value of the thermal conductance (G th =574W/Kcm 2 )wecalculateforthecw-operateddeviceanactiveregiontemperaturet act =321KatT sink =248K and I = 380 ma (U = 10.5 V). Using the above tuning coefficients (Δυ/ΔP =-1.51cm -1 /W, Δυ/ΔT=-0.078cm -1 /K), the shift in the emission frequency between pulsed and CW operation results in a theoretical value Δυ =-1.37cm -1, compared to the experimental value Δυ =1.5cm APPLICATION Mid-infrared lasers have traditionally an important field of applications in the sensor area like industrial process monitoring, medical diagnostics, and environmental sensing. This is especially due to most gas molecules which have their vibrational modes in this wavelength range the socalled fingerprint region of the molecules. For this reason, spectroscopists are among the main users of this relatively new technology. Obviously, they will benefit from the dramatic reduction in laser linewidth of the CW operated laser which is no longer coupled with cryogenic cooling. Widely tunable mid-infrared lasers are useful for broadband spectroscopic applications. Because the emission frequency of QC lasers can only be tuned over a small range of a few cm -1 by changing the injection current or/and the temperature different approaches have been demonstrated for achieving broadband laser action such as supercontinuum QC lasers 9 or external QC laser cavity configuration 10.Westudiedthefirst-orderdirectfeedbackconfiguration(Littrow configuration), as shown in Fig. 6, using a QC laser based on a bound-to-continuum active region with a broad gain spectrum to achieve broad tunability. Fig.6: Littrow configuration of the grating coupled external cavity QC laser. For this experiment, QC devices were processed into standard-ridge lasers 28 µm wide and 1.5 mm long with a ZnSe/Au high-reflection coating on the back facet (reflectivity of ~97%) and a PbTe/ZnSe anti-refelction (AR) coating on the front facet (reflectivity of ~6%). The laser was mounted on a thermo-electric cooler and operated in pulsed mode at a duty cycle of 4% (50 ns long pulses at a repetition rate of 800 khz) at room temperature. Under these operating conditions, the free-running QC laser emitted up to 300 mw of peak power with multimode emission between cm -1 and cm -1 and a measured longitudinal mode separation of 0.91 cm -1.Thelightwascollimatedbyan aspheric Ge lens designed for f/0.6 at 9 µm andcoatedwith3-12µm BBARcoatingsonbothsurfaces.Thegratingwas aruledplane,150l/mm,8µm blazedechellemountedonamanuallycontrolledrotationalstageforfrequencytuning. The grating first-order was used for cavity feedback, and the output was taken from the zeroth-order. The spectrum was measured with a Fourier transform IR (FTIR) spectrometer having a resolution of cm -1 and the average output power was measured with a calibrated thermopile detector using a ZnSe (f = 10) lens. With this configuration, the emission frequency could be tuned over a range from 1036 cm -1 to 1147 cm -1 (Fig. 7) with an average optical power between 0.15 mw and 0.85 mw, corresponding to peak power of 4-20 mw. The measured 6 Proc. SPIE Vol. 4817

7 SMSR was better than 20 db within the frequency range from 1068 cm -1 to 1109 cm -1 and still better than 10 db between cm -1. Fig.7: Average power and side mode suppression ration as a function of the emission frequency of the external cavity QC laser. Improvements in the AR-coating of the laser front facet, the use of a 9 µm gratingandasteppingmotorforthe rotational stage of the grating, and the use of a CW-operated device with its narrow linewidth will certainly improve tunability, output power, and side mode suppression ratio of the external cavity QC laser. ACKNOWLEDGMENTS This work was financially supported by the Swiss National Science Foundation and the Science Foundation of the European Community under the IST project SUPERSMILE. REFERENCES 1. J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, and A.Y. Cho, Quantum cascade laser, Science, 264,pp , J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, S.-N.G. Chu, and A.Y. Cho, Short wavelength (λ~3.4µm) quantum cascade laser based on strained compensated InGaAs/AlInAs, Appl. Phys. Lett., 72, pp , R. Colombelli, F. Capasso, C. Gmachl, A.L. Hutchinson, D.L. Sivco, A. Tredicucci, M.C. Wanke, A.M Sergent, and A.Y. Cho, Far-infrared surface-plasmon quantum cascade lasers at 21.5µmand24µmwavelengths,Appl. Phys. Lett., 78, pp , M. Rochat, D. Hofstetter, M. Beck, and J. Faist, Long-wavelength (λ~16µm), room-temperature, single-frequency quantum cascade lasers based on a bound-to-continuum transition, Appl. Phys. Lett., 79, pp , C. Gmachl, A.M. Sergent, A. Tredicucci, F. Capasso, A.L. Hutchinson, D.L. Sivco, J.N. Baillargeon, S.-N.G. Chu, and A.Y. Cho, Improved cw operation of quantum cascade lasers with epitaxial-side heat-sinking, IEEE Photon. Technol. Lett., 11, pp , D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, Continuous wave operation of a 9.3µmquantumcascadelaseronapeltiercooler,Appl. Phys. Lett., 78, pp , M. Beck, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, Buried heterostructure quantum cascade lasers with a large optical cavity waveguide, IEEE Photon. Technol. Lett., 12,pp , M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, Continuous wave operation of a mid-infrared semiconductor laser at room temperature, Science, 295,pp , C. Gmachl, D.L. Sivco, R. Colombelli, F. Capasso, and A.Y. Cho, Ultra-broadband semiconductor laser, Nature, 415,pp , G. Luo, C. Peng, H.Q. Le, S.-S. Pei, H. Lee, W.-Y. Hwang, B. Ishaug, and J. Zheng, Broadly wavelength-tunable external cavity mid-infrared quantum cascade lasers, IEEE J. Quantum Electron., 38,pp ,2002. Proc. SPIE Vol