APPLIED PHYSICS LETTERS 98, 141101 共2011兲
Two-dimensional broadband distributed-feedback quantum cascade laser arrays Elvis Mujagić,1,a兲 Clemens Schwarzer,1 Yu Yao,2 Jianxin Chen,2,3 Claire Gmachl,2 and Gottfried Strasser1 1
Institute for Solid State Electronics, TU Vienna, Floragasse 7, A-1040 Vienna, Austria Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA 3 Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China 2
共Received 17 February 2011; accepted 16 March 2011; published online 4 April 2011兲 We present two-dimensional broadband quantum cascade laser arrays based on distributed-feedback 共DFB兲 ring cavity surface emitting lasers. The 16-element arrays exhibit a linear tuning range of 180 cm−1 centered at a wavelength of 8.2 m when operated in pulsed mode at room temperature. The devices show single-mode emission with a side mode suppression ratio of 30 dB. Given by the facetless nature of the single emitters, the spectral dependent threshold current densities and optical power reflect the gain profile of the incorporated material and are not impaired by the diversity of underlying DFB designs. © 2011 American Institute of Physics. 关doi:10.1063/1.3574555兴 Over the last two decades quantum cascade lasers 共QCLs兲 have emerged as reliable sources of coherent light in the mid infrared spectral region.1,2 The demonstration of high power, continuous wave 共cw兲 and high temperature operation of QCLs,3–6 has attracted the attention of real world’s applications, such as gas analysis and chemical sensing.7 For these uses a broad spectral tuning range and single-mode emission are of great importance. These properties allow multianalyte spectroscopy along with a high selectivity and sensitivity of such systems. Broadly tunable monomode emission can be provided by processing linear arrays of distributed feedback 共DFB兲 lasers 共Ref. 8兲 or with QCLs operated in an external cavity 共EC兲 configuration.9,10 EC QCLs have shown tremendous tuning ranges by exploiting the entire gain region of the incorporated material. This setup requires high quality antireflection coatings, piezoelectric controllers, and well-aligned external optical components such as gratings for tuning. Hence, the EC represents a rather bulky, vibration-sensitive, and a complex-to-build system. A monolithically integrated DFB laser array serves as a compact version of a widely tunable light source. Such arrays, however, show significant grating period induced variations in threshold currents and radiation efficiencies.8 Due to the edge emitting nature this configuration allows one-dimensional arrangement of individual lasers only. A two-dimensional 共2D兲 integration of such coherent emitters is appealing since it allows for on-wafer testing and scalability at the wafer level, which consequently reduces the fabrication costs and effort. Recently, our group demonstrated single-mode operation of ring cavity surface emitting QCLs 共ringCSELs兲.11 Its facetless ring shaped resonator combined with the large overall emission surface leads to reduced threshold currents, enhanced optical output power and a highly reduced beam divergence as compared to conventional Fabry-Pérot emitters.12 Due to the surface emitting design, this device qualifies as an attractive fundamental building block for 2D a兲
Electronic mail:
[email protected].
0003-6951/2011/98共14兲/141101/3/$30.00
QCL arrays. In this letter, we present vertically emitting 2D broadband laser arrays based on ringCSELs. The QCL structure used in this letter is based on an In0.53Ga0.47As/ In0.52Al0.48As so-called continuum-to-bound design for a broadband emission in the wavelength range of ⬇ 7 – 9 m, as described in Ref. 13. The 40 period active core is sandwiched between two InGaAs 共Si, n = 5 ⫻ 1016 cm−3, 500 nm兲 spacers and grown on a low doped InP-substrate. The upper waveguide cladding consists of an InAlAs-layer 共Si, n = 1 ⫻ 1017 cm−3, 2800 nm兲 followed by an InGaAs plasmon layer 共Si, n = 1 ⫻ 1019 cm−3, 600 nm兲 grown on top. An InGaAs 共Si, n = 2 ⫻ 1019 cm−3, 30 nm兲 contact layer completed the QCL growth. In order to determine the width of the gain spectrum circular mesa structures with a 400 m diameter were processed. Electroluminescence 共EL兲 was then measured from cleaved semicircular mesas under pulsed condition 共80 kHz/ 500 ns兲 at 293 K. From the recorded data a full-width at half-maximum of ⬃200 cm−1 was extracted with a central frequency of 1230 cm−1 共8.13 m兲. Device fabrication started with the definition of Ti/Au 共10/200 nm兲 radial second-order Bragg gratings via electronbeam lithography and lift-off technique. The gratings were arranged in a 16-element 2D configuration and served as hard mask for the subsequent etching 共Fig. 1兲. Nominal grating periods in a range between 2.356 and 2.753 m periods were realized in order to cover emission frequencies between 1135 and 1326 cm−1, when taking into account a calculated effective refractive index of 3.2. This configuration gives a separation between individual single modes of ⬃12.7 cm−1. A duty cycle of 60% was chosen for the gratings 共inset of Fig. 1兲. Etching of the 1.9 m deep gratings was done by means of reactive ion etching 共RIE兲. For this particular duty cycle and grating depth a coupling coefficient of 12.2 cm−1 was calculated. Ring waveguides, 10 m wide and 6 m deep, with an outer radius of 200 m were defined by RIE also. Individual emitters are spaced 600 m apart. Deposition of a 300 nm thick silicon nitride layer 共SiNx兲 was used for electrical insulation. Extended contact pads, as shown in Fig. 1, were formed by sputtering of Ti/Au 共10/250 nm兲. To
98, 141101-1
© 2011 American Institute of Physics
141101-2
Mujagić et al.
Appl. Phys. Lett. 98, 141101 共2011兲
FIG. 1. Scanning electron microscopy picture of a two-dimensional 16element QCL array based on ring cavity surface emitting lasers. The entire array is ⬃4 ⫻ 3 mm2 in size. The inset shows a waveguide-section illustrating the etched second-order gratings to allow for vertical light emission.
allow for surface emission the insulation as well as the extended contacts were opened on top of the ridge. Substrate thinning to approximately 200 m, polishing and evaporation of a Ge/Au/Ni/Au 共15/30/14/150 nm兲 back contact completed the fabrication. The wire bonded ringCSEL array, with a size of ⬃4 ⫻ 3 mm2, was mounted on a Peltier cooler. The peak optical power was measured in pulse-mode operation with a pulse length of 100 ns and a repetition rate of 5 kHz at 293 K. Mid infrared spectra were recorded by means of a Fourier-transform spectrometer with a resolution of 0.2 cm−1. In the case of spectral characterization the pulsing conditions of 20 ns/100 kHz were used. Figure 2 shows the recorded spectra of the entire laser array at 293 K and indicates lasing of all 16 emitters. The lasers operate in single-mode at a frequency corresponding to its grating period and show a regular distribution without any mode hopping. The resonances are spaced ⬃12.3 cm−1 apart and cover a range from 1140 to 1322 cm−1 共corresponds to a wavelength range of 7.56– 8.77 m兲. A side mode suppression ratio of 30 dB was achieved for all frequencies and operation currents 共inset of Fig. 2兲. In index coupled edge emitting DFBs lasing can take place on two modes 共with
FIG. 2. 共Color online兲 Spectra of 16-surface emitting DFB lasers of the 2D array recorded at 293 K. Individual wavelengths are separated by ⬃12.3 cm−1 and cover an entire spectral range of ⬃180 cm−1. The inset shows a single-mode spectrum at 1261 cm−1 with a side mode suppression ratio of 30 dB.
FIG. 3. 共Color online兲 共a兲 Peak optical power and voltage versus current density characteristics for eight individual lasers, located at the low frequency side 共frequencies shown as inset兲. 共b兲 Threshold current density 共squares兲 and peak optical power 共circles兲 as a function of the wavelength. For comparison the measured electroluminescence from a mesa 共dashed line兲 indicates the gain spectrum of the incorporated QC material. All data were recorded at 293 K.
equal losses兲 at both edges of the photonic gap centered at the Bragg wavelength. Depending on the position of the end mirror facets relative to the laser ridge gratings 共i.e., phase difference兲 loss difference is induced and lasing on one of these modes is preferred. Given by the random variation in the phase difference in a DFB array this fact might result in an irregular frequency distribution.14,15 In surface emitting second-order DFB lasers, such as ringCSELs, large loss discrimination between the two resonator modes is inherently present which arises from different radiation losses for these two modes. For this reason, ringCSELs will necessarily operate at the same side of the photonic gap 共i.e., antisymmetric mode兲 irrespective of the grating period.14,16 Consequently, surface emitting devices provide a robust and predictable single-mode operation resulting in a DFB array with a uniformly spaced frequency comb as shown in Fig. 2. Moreover, any undesirable mirror induced influence on the spectral behavior, as observed in edge emitting arrays, is eliminated by the absence of facets in ring resonators. Figure 3共a兲 shows the peak optical power and voltage versus current density curves of eight lasers situated at the low frequency side of the array. A certain grating period induced dependency of threshold currents and optical power is observed. Figure 3共b兲 shows the measured threshold current density and peak optical power versus the entire emission range. For comparison also the recorded EL spectrum from a semicircular mesa is plotted. A maximum power of
141101-3
Appl. Phys. Lett. 98, 141101 共2011兲
Mujagić et al.
60 mW was observed at 1237.5 cm−1 which is positioned in the gain maximum. Optical power gradually decreases toward the edges of the EL curve and reaches a minimal emission of ⬃20 mW. Similarly a minimal threshold current density Jth of 4.6 kA/ cm2 was measured in the center of the recorded EL profile and increases regularly toward the EL edge with highest thresholds around 5.5 kA/ cm2. These values are then compared to a Fabry–Pérot laser, fabricated from the same material and comparable dimensions 共1.2 mm⫻ 10 m兲 with a peak emission of 140 mW and Jth of 3.5 kA/ cm2 at 293 K. This reduced performance for ringCSELs is ascribed to the used waveguide structure which was not optimized for surface emission. Here, in order to obtain sufficient coupling relatively deep gratings were etched at the expense of increased losses. With an optimized waveguide ringCSELs are capable of significantly improved performance as compared to conventional edge-emitting QCLs, as demonstrated in Ref. 12. As for the laser array we observed a change in slope efficiency with values between 59 and 124 mW/A 关Fig. 3共a兲兴. This is attributed to a grating induced nonuniform radiation loss 共surface loss兲 and substrate loss distribution across the entire spectral region. By accurately designing the cladding layers also this effect can be significantly minimized. Currently, we are investigating ringCSEL design strategies by performing waveguide simulations, where the results will be reported in another publication. Both the observed emission characteristics as well as the thresholds of the 2D array reflect the gain profile without any significant scattering. In contrary, the performance of linear DFBs shows a strong dependence on the position of the end mirror facet with respect to the laser ridge gratings.8 In particular, a substantial scattering of the amplitude of emitted light and threshold currents was observed in tunable DFB arrays with different grating periods. Due to its facetless nature, ringCSELs represent an attractive element for laser arrays, with a performance which is determined by the incorporated gain material and is not affected by the underlying frequency selection mechanism. In conclusion, we presented the operation of twodimensional QCL arrays based on ring cavity surface emitting lasers. When operated in pulsed mode the 16-element
array shows a linear tuning range of 180 cm−1 at a center wavelength of 8.2 m. At room temperature a side mode suppression ratio of 30 dB was measured. Spectral evolution of emitted radiation and threshold currents reflect the measured gain profile of the used material. The authors acknowledge the support by the Austrian projects IR-ON and ADLIS, the Austrian Nanoinitiative project PLATON, the “Gesellschaft für Mikro- und Nanoelektronik” GMe and MIRTHE 共NSF-ERC, EEC 0540832兲. 1
J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science 264, 553 共1994兲. 2 C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, Rep. Prog. Phys. 64, 1533 共2001兲. 3 A. Lyakh, C. Pflügl, L. Diehl, Q. J. Wang, F. Capasso, X. J. Wang, J. Y. Fan, T. Tanbun-Ek, R. Maulini, A. Tsekoun, R. Go, C. Kumar, and N. Patel, Appl. Phys. Lett. 92, 111110 共2008兲. 4 R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflügl, L. Diehl, F. Capasso, and C. K. N. Patel, Appl. Phys. Lett. 95, 151112 共2009兲. 5 Y. Bai, S. Slivken, S. R. Darvish, A. Haddadi, B. Gokden, and M. Razeghi, Appl. Phys. Lett. 95, 221104 共2009兲. 6 A. Wittmann, Y. Bonetti, M. Fisher, J. Faist, S. Blaser, and E. Gini, IEEE Photon. Technol. Lett. 21, 814 共2009兲. 7 A. Kosterev, G. Wysocki, Y. Bakhirkin, S. So, R. Lewicki, M. Fraser, F. Tittel, and R. F. Curl, Appl. Phys. B: Lasers Opt. 90, 165 共2008兲. 8 B. G. Lee, M. A. Belkin, R. Audet, J. MacArthur, L. Diehl, C. Pflügl, F. Capasso, D. C. Oakley, D. Chapman, A. Napoleone, D. Bour, S. Corzine, G. Höfler, and J. Faist, Appl. Phys. Lett. 91, 231101 共2007兲. 9 R. Maulini, I. Dunayevskiy, A. Lyakh, A. Tsekoun, C. K. N. Patel, L. Diehl, C. Pflügl, and F. Capasso, Electron. Lett. 45, 107 共2009兲. 10 A. Hugi, R. Terazzi, Y. Bonetti, A. Wittmann, M. Fischer, M. Beck, J. Faist, and E. Gini, Appl. Phys. Lett. 95, 061103 共2009兲. 11 E. Mujagić, L. K. Hoffmann, S. Schartner, M. Nobile, W. Schrenk, M. P. Semtsiv, M. Wienold, W. T. Masselink, and G. Strasser, Appl. Phys. Lett. 93, 161101 共2008兲. 12 E. Mujagić, M. Nobile, H. Detz, W. Schrenk, J. Chen, C. Gmachl, and G. Strasser, Appl. Phys. Lett. 96, 031111 共2010兲. 13 Y. Yao, W. O. Charles, T. Tsai, J. Chen, G. Wysocki, and C. Gmachl, Appl. Phys. Lett. 96, 211106 共2010兲. 14 R. F. Kazarinov and C. H. Henry, IEEE J. Quantum Electron. 21, 144 共1985兲. 15 B. G. Lee, M. A. Belkin, C. Pflügl, L. Diehl, H. A. Zhang, R. Audet, J. MacArthur, D. P. Bour, S. Corzine, G. Höfler, and F. Capasso, IEEE J. Quantum Electron. 45, 554 共2009兲. 16 R. J. Noll and S. H. MaComber, IEEE J. Quantum Electron. 26, 456 共1990兲.
