High power quantum cascade lasers operating at λ 87 and 130 μ mLassaad Ajili, Giacomo Scalari, Jérôme Faist, Harvey Beere, Edmund Linfield, David Ritchie, and Giles Davies Citation: Applied Physics Letters 85, 3986 (2004); doi: 10.1063/1.1810217 View online: http://dx.doi.org/10.1063/1.1810217 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/85/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Double metal waveguide InGaAs/AlInAs quantum cascade lasers emitting at 24μm Appl. Phys. Lett. 105, 121115 (2014); 10.1063/1.4896542 High power operation of λ5.2–11μm strain balanced quantum cascade lasers based on the same materialcomposition Appl. Phys. Lett. 105, 071106 (2014); 10.1063/1.4893746 Low frequency terahertz quantum cascade laser operating from 1.6 to 1.8 THz Appl. Phys. Lett. 89, 231121 (2006); 10.1063/1.2404598 Room-temperature, high-power, and continuous-wave operation of distributed-feedback quantum-cascade lasersat λ 9.6 μ m Appl. Phys. Lett. 88, 201114 (2006); 10.1063/1.2205730 2.9 THz quantum cascade lasers operating up to 70 K in continuous wave Appl. Phys. Lett. 85, 1674 (2004); 10.1063/1.1784874

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High power quantum cascade lasers operating at l¶87 and 130 mmLassaad Ajili,a) Giacomo Scalari, and Jérôme Faistb)

Institute of Physics, University of Neuchâtel, ACH-2000 Neuchâtel, Switzerland

Harvey Beere, Edmund Linfield, and David RitchieCavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom

Giles DaviesSchool of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

(Received 25 February 2004; accepted 26 August 2004)

We report high power quantum cascade lasers operating above liquid nitrogen temperature atl.87 and 130mm based on a bound-to-continuum transition. Forl.87 mm, 56 mW peak powerin pulsed operation and 50 mW continuous wave operation at 10 K are demonstrated. Atl.130 mm, a peak power of 50 mW was achieved and devices operated in continuous wave reacheda maximum temperature of 53 K with an optical power of 11.5 mW atT=10 K. Lifetimes areextracted from the scaling of the transport and laser parameters as a function of size using a simplerate equation model. ©2004 American Institute of Physics. [DOI: 10.1063/1.1810217]

Terahertz quantum cascade lasers(QCLs) were reportedusing intersubband transitions in quantum wells1–4 reachingwavelengths longer than 100mm (Ref. [5]) and operatingtemperatures above 100 K.6 Terahertz QCLs based on abound-to-continuum transition have recently demonstratedhigh performance operation7 with maximum temperatures of55 K in continuous-wave and 90 K in pulsed operation.These high performance levels were attributed to the goodinjection efficiency in the upper state and the efficient extrac-tion achieved from the lower state of the bound-to-continuum design, features that were also demonstrated inmid-infrared devices.8,9

In this letter we demonstrate that the bound-to-continuum approach enables the extension of the wavelengthrange as well as the achievement of high power operation,and high power QCLs emitting atl.87 and 130mm aredemonstrated. Such devices would be potentially importantfor applications such as terahertz imaging and telecommuni-cation.

Our GaAs/Al0.15Ga0.85As lasers, grown by molecularbeam epitaxy on a semi-insulating GaAs substrate consists of120 periods embedded in a waveguide based on a singleinterface plasmon. The design of the sample A2986sl.87 mmd is the same as in Ref. 7, but with an injectionbarrier thickness increased from 3.5 to 4.2 nm, in order tolimit the leakage current, and the doping level in the activeregion increased, with a Si concentration of 331016 cm−3

instead of 2.531016 cm−3. Finally, the samples were pro-cessed as described in Ref. 3, soldered with indium on acopper heatsink, the backfacet coated by ZnSe/Aus100/50 nmd evaporation.

For the experimental measurement, the devices aredriven with 160-ns-long current pulses and a duty cycle of2%. The light–current characteristics of a 210-mm-wide and1.6-mm-long device are shown in Fig. 1(a) for temperaturesranging from 10 up to 98 K. The terahertz emission is col-lected (with a calculated collection efficiency of<29%)

through a light pipe and sent to a broadband thermopilepower meter.

The collected peak power at 10 K reaches a value of56 mW, with a threshold current density ofJth=205 A/cm2,compared to the 22 mW andJth=248 A/cm2 obtained in theprevious devices7 of the same size. Still more than 2 mW isdetected at 98 K. A 2.7-mm-long and 210-mm-wide deviceoperated in pulsed mode up to 103 K temperature, which isan improvement of about 10 K.

a)Electronic mail: lassaad.ajili@unine.chb)Electronic mail: jerome.faist@unine.ch

FIG. 1. (a) Peak optical output power vs injected current in pulsed mode ata duty cycle of 2% at various temperatures, as indicated. The sample isprocessed into a 1.6-mm-long and 210-mm-wide waveguide with backfacetcoating. The inset shows pulsed threshold current as a function of inverselaser length with backfacet coating measured at 10 K.(b) cw optical powerfrom a single facet and bias of the same device as function of drive currentfor various heat sink temperatures. Inset: Bias voltage as a function of in-jection current and differential resistance deduced from theV–I curve of theA2986 ( continuous line) and A2771( dashed line) samples, the horizontalstraight curve at 0.34310−3 V cm2 is the residual series resistance of theburied contact layer.

APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 18 1 NOVEMBER 2004

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Assuming a linear dependence of the peak gain in cur-rent density, systematic threshold current density measure-ments of devices of various cavity lengthsL enable the ex-traction of the material differential gaingd and thewaveguide lossaw.10 Using the data shown in the inset ofFig. 1(a), assuming a reflectivity coefficientR1=0.31 andR2=0.98, and the computed value of the overlap factorG=38%, we can extractgd=105 cm/kA andaw=5.1 cm−1,close to the calculated values6.5 cm−1d.

As shown in Fig. 1(b), the same device could be oper-ated in continuous wave with a threshold current density ofJth=235 A/cm2 and a maximum collected power of 50 mW.

The specific differential resistance, obtained by numeri-cally differentiating the current–voltage characteristics, isplotted as a function of injected current density in the inset ofFig. 1(b) as a function of current density. For comparison,the same data taken in the previous device A2771,7 are alsodisplayed in the same graph. The data demonstrate that thethicker injection barrier reduces the current injected in thelower miniband before the devices achieve the proper bandalignment, with the conductance peak reduced from140 to 99 A/cm2.

We interpret these measurements assuming a simplifedthree level model of the active region wheren=3 andn=2are the upper and lower states of the laser transition, respec-tively.

Inspection of the rate equations that includes the photonfield demonstrates the strong link between electron transportand photon emission in quantum cascade lasers. The deriva-tive of the upper state population, as expressed in Ref. 11, asa function of injected current density can be expressed as

dn3

dJ=

t3

qS1 − c̄atot

dS

dJD , s1d

whereS is the photon flux density,c̄ is the effective velocityof light in the waveguide, andatot the total optical loss. As-suming that differential resistanceRd is proportional todn3/dJ, and recalling the expression fordS/dJ:

dS

dJ=

1

qc̄atot

teff

teff + t2. s2d

In Eq. (2), the effective lifetime may be extracted from themeasurement of the differential gaingd,

12 and yieldsteff=t3s1−t2/t32d=1.6 ps. The ratio of the differential conduc-tance above and below threshold is just:

Rd,S.0/Rd,S=0 =t2

teff + t2. s3d

It is a measurement of the population inversion obtaineddirectly from the I–V characteristics. After a substraction ofthe series resistance of the device, measured from the scalingof the I –V characteristics with stripe width, a value ofRd,S.0/Rd,S=0=0.41 is obtained. From this value, a direct es-timate of the lower state lifetimet2=1.1 ps is obtained. Thevalue of the population inversion at threshold yields a pre-dicted value of the slope efficiency:

dP

dI= Np

hn

q

am

atot

teff

teff + t2= 0.4W/A s4d

significantly larger than the measured one(dP/dI=0.17 W/A, after correction of the collection efficiencyhcoll<0.29). We attribute the discrepancy in the measured

slope efficiency to a poor estimate of facet transmission andof the collection efficiency. Indeed, comparison with the pre-vious devices A2771 whereRd,S.0/Rd,S=0=0.58 show that animprovement of the quantitys1−Rd,S.0/Rd,S=0d, proportionalto the slope efficiency, by a factor 1.44 is indeed accompa-nied by improvement of the slope efficiency by a factor of1.55.

Spectral measurements in cw above and belowJ=450 A/cm2 show that theJ–V behavior at this current den-sity is due to the appearence of a new series of modes.

As shown in Fig 2, the structure of sample A2985, de-signed for emission at about 9.6 meVs.130 mmd is alsobased on a bound-to-continuum transition. Typical light ver-sus currents sL–Id curves from a 430-mm-wide and3-mm-long device with backfacet coating are shown in Fig.3(a) for temperatures ranging from 10 up to 78 K. A verylow threshold current density ofJth=99 A/cm2 is achievedat 10 K. The maximum peak power at this temperature is50 mW with a slope efficiencydP/dI=20 mW/A. The de-vice operates up to 80 K with a peak power of 30mW.Analysis of the threshold current density versus cavitylengths [see inset of Fig. 3(a)], assumingG=16%, yieldsaW=4.5 cm−1, close to the experimental value ofaW=3.3 cm−1.

The continuous wave operation was demonstrated in a210-mm-wide and 2.1-mm-long device with backfacet coat-ing sZnSe/Aud [Fig. 3(b)]. At 10 K the laser exhibited athreshold current of 656 mA(Jth=148 A/cm2 at U=8.3 V)and a slope efficiencydP/dI=30 mW/A. A maximum opti-cal power of 11.5 mW at 10 K was detected at a drivingcurrent of 1.2 A. Continuous wave operation was observedup to 53 K. A thermal resistance of 6 K/W is deduced froma comparison of the pulsed an continuous wave thresholdcurrent density.

As displayed in Fig. 3(b) the voltage at which the devicestarts to lase is much largers8.3 Vd than the calculated values2.9 Vd. We believe that this disagreement is caused by aresidual series resistance of the order of 8.2V s3.6310−2 V cm2d originated from the bottom contact layer. The

FIG. 2. Computed conduction band profile atT=40 K of one stage of thestructures130 mmd under an average applied electric field of 2.55 kV/cm.The optical transition occurs between leveln=8, isolated in the minigap,and the staten=6 of the lower miniband. The GaAs/Al0.15Ga0.85As layersequence of one period of the active layers starting from the injection barrieris as follows: 4.0/12/0.5/22.3/0.8/21.5/0.9/17.8/1/14.5/1.3/13.2I /2.2/13.4/2.8/17 where Al0.15Ga0.85As layers are in bold, GaAs in Roman, andthe underlined number corresponds to the Si doped layer withn=2.531016 cm−3.

Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004 Ajili et al. 3987

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high resistance is explained by the lower dopingNd=931017 cm−3 of the 0.6-mm-thick bottom contact layerneeded to minimize the waveguide losses. For this reason wecannot deducet2 from the ratio ofdN3/dJ (thereforedV/dJ)below and above threshold in this device.

Spectral measurements are performed in cw operation athigh-resolutions0.09 cm−1d with a Nicolet Fourier transforminfrared spectrometer in rapid-scan mode. The inset of Fig.3(b) shows the lasing spectra measured at different heat sinktemperature. Single mode operation is observed atI

=385 mA for all temperatures. The emission wavelengthshifts toward smaller energies as the temperature increasefrom 15 to 47 K. The temperature tuning coefficient of thelasing peak is constant over the entire temperature range, andits magnitude is 1/l3Dl /DT=5.2310−5 K−1. Multimodeemission for the same device was observed atT=14 K atvarious currents between 283 and 364 mA. The longitudinalmode spacing isDl=2.23mm, knowing the cavity length,we can also compute an approximate value for the groupeffective refractive index in the laser cavity. This procedureyields ng=l2/2DlL=3.61, which is in excellent agreementwith the value reported in literature13 sn=3.61d.

This work was supported in part by the European ISTproject WANTED and by the Swiss National Science foun-dation center NCCR Quantum Photonics.

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12The effective state lifetimeteff can be calculated fromgG by the followingequation(Ref. 14): teff=se0n/4pqdf2n23l / sZ23d2GgLpg, where Lp is thelength of each stage,l is the emission wavelength,Z23 is the transitionmatrix element,n is the mode refractive index,e0 is the vacuum permit-tivity, q is the electronic charge, and 2ni j =2.28 meV is the full width athalf maximum (FWHM) of the nonamplifed luminescence. For sampleA2986 we usegG=40 cm/kA, G=38%, Lp=116.2 nm,l=87 mm, Z23

=8.8 nm,n 3.6, and 2n23=4.57 meV.13E. Palik, inHandbook of Optical Constants of Solids, edited by E. D. Palik

(Academic, Orlando, FL, 1985), pp. 429–443.14J. Faist, F. Capasso, C. Sirtori, D. Sivco, A. Hutchinson, S. Chu, and A.

Cho, Superlattices Microstruct.19, 337 (1996).

FIG. 3. (a) Light–current characteristics at different temperatures as mea-sured from a 3-mm-long and 430-mm-wide laser stripe with backfacet coat-ing. In the inset, threshold current density vs inverse laser length with back-facet coating measured at 10 K.(b) cw optical power from a single facet andbias of a 2.1-mm-long and 200-mm-wide laser stripe as function of drivecurrent for various heat sink temperatures. The inset shows the lasing spec-tra of a 150-mm-wide and 1-mm-long QCL with high reflectivity back- andfrontfacet coating at different temperatures between 15 and 47 K. Thecurves are vertically displaced for clarity.

3988 Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004 Ajili et al.

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