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1.  Dual section quantum cascade lasers with wide electrical tuning
S. Slivken, N. Bandyopadhyay, S. Tsao, S. Nida, Y. Bai, Q.Y. Lu and M. Razeghi
SPIE Proceedings, Vol. 8631, p. 86310P-1, Photonics West, San Francisco, CA-- February 3, 2013
This paper describes our development efforts at Northwestern University regarding dual-section sampled grating distributed feedback (SGDFB) QCLs. These devices are the same size, but have much wider electrical tuning, than a traditional DFB laser. In this paper, I will show how we have dramatically extended the monolithic tuning range of high power quantum cascade lasers with high side mode suppression. This includes individual laser element tuning of up to 50 cm-1 and 24 dB average side mode suppression. These lasers are capable of room temperature continuous operation with high power (>100 mW) output. Additionally, we have demonstrated a broad spectral coverage of over 350 cm-1 on a single chip, which is equivalent to 87.5% of the gain bandwidth. The eventual goal is to realize an extended array of such laser modules in order to continuously cover a similar or broader spectral range, similar to an external cavity device without any external components. reprint
 
2.  Continuous wave, room temperature operation of λ ~ 3μm quantum cascade laser
N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken and M. Razeghi
SPIE Proceedings, Vol. 8631, p. 86310M-1, Photonics West, San Francisco, CA-- February 3, 2013
Quantum Cascade Lasers (QCLs), operating in continuous wave (CW) at room temperature(RT) in 3-3.5 μm spectral range, which overlaps the spectral fingerprint region of many hydrocarbons, is essential in spectroscopic trace gas detection, environment monitoring, and pollution control. A 3 μm QCL, operating in CW at RT is demonstrated. This initial result makes it possible, for the most popular material system (AlInAs/GaInAs on InP) used in QCLs in mid-infrared and long-infrared, to cover the entire spectral range of mid-infrared atmospheric window (3-5 μm). In0.79Ga0.21As/In0.11Al0.89As strain balanced superlattice, which has a large conduction band offset, was grown. The strain was balanced with composite barriers (In0.11Al0.89As /In0.4Al0.6As) in the injector region, to eliminate the need of extremely high compressively strained GaInAs, whose pseudomorphic growth is very difficult. reprint
 
3.  Room temperature continuous wave operation of λ ~ 3-3.2 μm quantum cascade lasers
N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken and M. Razeghi
Applied Physics Letters, Vol. 101, No. 24, p. 241110-1-- December 10, 2012
We demonstrate quantum cascade lasers emitting at wavelengths of 3–3.2 μm in the InP-based material system. The laser core consists of GaInAs/AlInAs using strain balancing technique. In room temperature pulsed mode operation, threshold current densities of 1.66 kA∕cm² and 1.97 kA∕cm², and characteristic temperatures (T0) of 108 K and 102 K, are obtained for the devices emitting at 3.2 μm and 3 μm, respectively. Room temperature continuous wave operation is achieved at both wavelengths. reprint
 
4.  Sampled grating, distributed feedback quantum cascade lasers with broad tunability and continuous operation at room temperature
S. Slivken, N. Bandyopadhyay, S. Tsao, S. Nida, Y. Bai, Q.Y. Lu and M. Razeghi
Applied Physics Letters, Vol. 100, No. 26, p. 261112-1-- June 25, 2012
A dual-section, single-mode quantum cascade laser is demonstrated in continuous wave at room temperature with up to 114 nm (50 cm−1) of tuning near a wavelength of 4.8 μm. Power above 100 mW is demonstrated, with a mean side mode suppression ratio of 24 dB. By changing the grating period, 270 nm (120 cm−1) of gap-free electrical tuning for a single gain medium has been realized. reprint
 
5.  Substrate emission quantum cascade ring lasers with room temperature continuous wave operation
Y. Bai, S. Tsao, N. Bandyopadhyay, S. Slivken, Q.Y. Lu, and M. Razeghi
SPIE Proceedings, Vol. 8268, p. 82680N-- January 22, 2012
We demonstrate room temperature, continuous wave operation of quantum cascade ring lasers around 5 μm with single mode operation up to 0.51 W output power. Single mode operation persists up to 0.4 W. Light is coupled out of the ring cavity through the substrate with a second order distributed feedback grating. The substrate emission scheme allows for epilayer-down bonding, which leads to room temperature continuous wave operation. The far field analysis indicates that the device operates in a high order mode. reprint
 
6.  High power, continuous wave, quantum cascade ring laser
Y. Bai, S. Tsao, N. Bandyopadhyay, S. Slivken, Q.Y. Lu, D. Caffey, M. Pushkarsky, T. Day and M. Razeghi
Applied Physics Letters, Vol. 99, No. 26, p. 261104-1-- December 26, 2011
We demonstrate a quantum cascade ring laser with high power room temperature continuous wave operation. A second order distributed feedback grating buried inside the waveguide provides both in-plane feedback and vertical power outcoupling. Total output power reaches 0.51 W at an emission wavelength around 4.85 μm. Single mode operation persists up to 0.4 W. The far field analysis indicates that the device operates in a high order mode. The magnetic and electric components of the ring-shaped lasing beam are in radial and azimuthal directions, respectively. reprint
 
7.  Widely Tunable, Single-Mode, High-Power Quantum Cascade Lasers
M. Razeghi, B. Gokden, S. Tsao, A. Haddadi, N. Bandyopadhyay, and S. Slivken
SPIE Proceedings, Intergreated Photonics: Materials, Devices and Applications, SPIE Microtechnologies Symposium, Prague, Czech Republic, April 18-20, 2011, Vol. 8069, p. 806905-1-- May 31, 2011
We demonstrate widely tunable high power distributed feedback quantum cascade laser array chips that span 190 nm and 200 nm from 4.4 um to 4.59 um and 4.5 um to 4.7 um respectively. The lasers emit single mode with a very narrow linewidth and side mode suppression ratio of 25 dB. Under pulsed operation power outputs up to 1.85 W was obtained from arrays with 3 mm cavity length and up to 0.95 W from arrays with 2 mm cavity length at room temperature. Continuous wave operation was also observed from both chips with 2 mm and 3 mm long cavity arrays up to 150 mW. The cleaved size of the array chip with 3 mm long cavities was around 4 mm x 5 mm and does not require sensitive external optical components to achieve wide tunability. With their small size and high portability, monolithically integrated DFB QCL Arrays are prominent candidates of widely tunable, compact, efficient and high power sources of mid-infrared radiation for gas sensing. reprint
 
8.  Room temperature quantum cascade lasers with 27% wall plug efficiency
Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken and M. Razeghi
Applied Physics Letters, Vol. 98, No. 18, p. 181102-1-- May 3, 2011
Using the recently proposed shallow-well design, we demonstrate InP based quantum cascade lasers (QCLs) emitting around 4.9 μm with 27% and 21% wall plug efficiencies in room temperature (298 K) pulsed and continuous wave (CW) operations, respectively. The laser core consists of 40 QCL-stages. The highest cw efficiency is obtained from a buried-ridge device with a ridge width of 8 μm and a cavity length of 5 mm. The front and back facets are antireflection and high-reflection coated, respectively. The maximum single facet cw power at room temperature amounts to 5.1 W. reprint
 
9.  High power 1D and 2D photonic crystal distributed feedback quantum cascade lasers
B. Gokden, Y. Bai, S. Tsao, N. Bandyopadhyay, S. Slivken and M. Razeghi
SPIE Proceedings, San Francisco, CA (January 22-27, 2011), Vol. 7945, p. 79450C-- January 23, 2011
For many practical applications that need bright sources of mid-infrared radiation, single mode operation and good beam quality are also required. Quantum cascade lasers are prominent candidates as compact sources of mid-infrared radiation capable of delivering very high power both CW and under pulsed operation. While 1D photonic crystal distributed feedback structures can be used to get single mode operation from quantum cascade lasers with narrow ridge widths, novel 2D photonic crystal cavity designs can be used to improve spectral and spatial purity of broad area quantum cascade lasers. In this paper, we demonstrate high power, spatially and spectrally pure operation at room temperature from narrow ridge and broad area quantum cascade lasers with buried 1D and 2D photonic crystal structures. Single mode continuous wave emission at λ = 4.8 μm up to 700 mW in epi-up configuration at room temperature was observed from a 11 μm wide 5 mm long distributed feedback quantum cascade laser with buried 1D gratings. High peak powers up to 34 W was obtained from a 3mm long 400 μm wide 2D photonic crystal distributed feedback laser at room temperature under pulsed operation. The far field profile had a single peak normal to the laser facet and the M2 figure of merit was as low as 2.5. Emission spectrum had a dominating single mode at λ = 4.36 μm. reprint
 
10.  Highly temperature insensitive quantum cascade lasers
Y. Bai, N. Bandyopadhyay, S. Tsao, E. Selcuk, S. Slivken and M. Razeghi
Applied Physics Letters, Vol. 97, No. 25-- December 20, 2010
An InP based quantum cascade laser (QCL) heterostructure emitting around 5 μm is grown with gas-source molecular beam epitaxy. The QCL core design takes a shallow-well approach to maximize the characteristic temperatures, T(0) and T(1), for operations above room temperature. A T(0) value of 383 K and a T(1) value of 645 K are obtained within a temperature range of 298–373 K. In room temperature continuous wave operation, this design gives a single facet output power of 3 W and a wall plug efficiency of 16% from a device with a cavity length of 5 mm and a ridge width of 8 μm. reprint
 
11.  Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ ∼ 3.76 μm
N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken and M. Razeghi
Applied Physics Letters, Vol. 97, No. 13-- September 27, 2010
An InP-based quantum cascade laser heterostructure emitting at 3.76 μm is grown with gas-source molecular beam epitaxy. The laser core is composed of strain balanced In0.76Ga0.24As/In0.26Al0.74As. Pulsed testing at room temperature exhibits a low threshold current density (1.5 kA/cm²) and high wall plug efficiency (10%). Room temperature continuous wave operation gives 6% wall plug efficiency with a maximum output power of 1.1 W. Continuous wave operation persists up to 95 °C. reprint
 
12.  High performance quantum dot-quantum well infrared focal plane arrays
S. Tsao, A. Myzaferi, and M. Razeghi
SPIE Proceedings, San Francisco, CA (January 22-28, 2010), Vol. 7605, p. 76050J-1-- January 27, 2010
Quantum dot (QD) devices are a promising technology for high operating temperature detectors. We have studied InAs QDs embedded in an InGaAs/InAlAs quantum well structure on InP substrate for middle wavelength infrared detectors and focal plane arrays (FPAs). This combined dot-well structure has weak dot confinement of carriers, and as a result, the device behavior differs significantly from that in more common dot systems with stronger confinement. We report on our studies of the energy levels in the QDWIP devices and on QD-based detectors operating at high temperature with D* over 1010 cm·Hz½/W at 150 K operating temperature and high quantum efficiency over 50%. FPAs have been demonstrated operating at up to 200 K. We also studied two methods of adapting the QDWIP device to better accommodate FPA readout circuit limitations. reprint
 
13.  Gain-length scaling in quantum dot/quantum well infrared photodetectors
T. Yamanaka, B. Movaghar, S. Tsao, S. Kuboya, A. Myzaferi and M. Razeghi
Virtual Journal of Nanoscale Science & Technology-- September 14, 2009reprint
 
14.  Gain-length scaling in quantum dot/quantum well infrared photodetectors
T. Yamanaka, B. Movaghar, S. Tsao, S. Kuboya, A. Myzaferi and M. Razeghi
Applied Physics Letters, Vol. 95, No. 9-- August 31, 2009
The gain in quantum dot/quantum well infrared photodetectors is investigated. The scaling of the gain with device length has been analyzed, and the behavior agrees with the previously proposed model. We conclude that we understand the gain in the low bias region, but in the high field region, discrepancies remain. An extension of the gain model is presented to cover the very high electric field region. The high field data are compared to the extended model and discussed. reprint
 
15.  Quantum dot in a well infrared photodetectors for high operating temperature focal plane arrays
S. Tsao, T. Yamanaka, S. Abdollahi Pour, I-K Park, B. Movaghar and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7234-0V-- January 25, 2009
InAs quantum dots embedded in InGaAs quantum wells with InAlAs barriers on InP substrate grown by metalorganic chemical vapor deposition are utilized for high operating temperature detectors and focal plane arrays in the middle wavelength infrared. This dot-well combination is unique because the small band offset between the InAs dots and the InGaAs well leads to weak dot confinement of carriers. As a result, the device behavior differs significantly from that in the more common dot systems that have stronger confinement. Here, we present energy level modeling of our QD-QW system and apply these results to interpret the detector behavior. Detectors showed high performance with D* over 1010 cm·Hz1/2W-1 at 150 K operating temperature and with high quantum efficiency over 50%. Focal plane arrays have been demonstrated operating at high temperature due to the low dark current observed in these devices. reprint
 
16.  Gain and recombination dynamics in photodetectors made with quantum nanostructures: the quantum dot in a well and the quantum well
B. Movaghar, S. Tsao, S. Abdollahi Pour, T. Yamanaka, and M. Razeghi
Virtual Journal of Nanoscale Science & Technology, Vol. 18, No. 14-- October 6, 2008reprint
 
17.  Gain and recombination dynamics in photodetectors made with quantum nanostructures: The quantum dot in a well and the quantum well
B. Movaghar, S. Tsao, S. Abdollahi Pour, T. Yamanaka, and M. Razeghi
Physical Review B, Vol. 78, No. 11-- September 15, 2008
We consider the problem of charge transport and recombination in semiconductor quantum well infrared photodetectors and quantum-dot-in-a-well infrared detectors. The photoexcited carrier relaxation is calculated using rigorous random-walk and diffusion methods, which take into account the finiteness of recombination cross sections, and if necessary the memory of the carrier generation point. In the present application, bias fields are high and it is sufficient to consider the drift limited regime. The photoconductive gain is discussed in a quantum-mechanical language, making it more transparent, especially with regard to understanding the bias and temperature dependence. Comparing experiment and theory, we can estimate the respective recombination times. The method developed here applies equally well to nanopillar structures, provided account is taken of changes in mobility and trapping. Finally, we also derive formulas for the photocurrent time decays, which in a clean system at high bias are sums of two exponentials. reprint
 
18.  InP-based quantum-dot infrared photodetectors with high quantum efficiency and high temperature imaging
S. Tsao, H. Lim, H. Seo, W. Zhang and M. Razeghi
IEEE Sensors Journal, Vol. 8, No. 6, p. 936-941-- June 1, 2008
We report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled InAs quantum dots and the device structure were grown by low-pressure metalorganic chemical vapor depositon. The detectivity was 6 x 1010cm·Hz1/2·W-1 at 150 K and a bias of 5 V with a peak detection wavelength around 4.0 micron and a quantum efficiency of 48%. Due to the low dark current and high responsivity, a clear photoresponse has been observed at room temperature. A 320 x 256 middle wavelength infrared focal plane array operating at temperatures up to 200 K was also demonstrated. The focal plane array had 34 mA/W responsivity, 1.1% conversion efficiency, and noise equivalent temperature difference of 344 mK at 120 K operating temperature. reprint
 
19.  Thermal imaging based on high-performance InAs/InP quantum-dot infrared photodetector operating at high temperature
M. Razeghi; H. Lim; S. Tsao; H. Seo; W. Zhang
Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS.15-16:[4382251] (2007).-- October 21, 2007
We report a room temperature operating and high-performance InAs quantum-dot infrared photodetector on InP substrate and thermal imaging of 320times256 focal plane array based on this device up to 200 K. reprint
 
20.  High operating temperature 320 x 256 middle-wavelength infrared focal plane array imaging based on an InAs/InGaAs/InAlAs/InP quantum dot infrared photodetector
S. Tsao, H. Lim, W. Zhang, and M. Razeghi
Virtual Journal of Nanoscale Science and Technology-- May 28, 2007reprint
 
21.  High operating temperature 320 x 256 middle-wavelength infrared focal plane array imaging based on an InAs/InGaAs/InAlAs/InP quantum dot infrared photodetector
S. Tsao, H. Lim, W. Zhang, and M. Razeghi
Applied Physics Letters, Vol. 90, No. 20, p. 201109-- May 14, 2007
This letter reports a 320×256 middle-wavelength infrared focal plane array operating at temperatures up to 200 K based on an InAs quantum dot/InGaAs quantum well/InAlAs barrier detector grown on InP substrate by low pressure metal organic chemical vapor deposition. The device's low dark current density and the persistence of the photocurrent up to room temperature enabled the high temperature imaging. The focal plane array had a peak detection wavelength of 4 µm, a responsivity of 34 mA/W, a conversion efficiency of 1.1%, and a noise equivalent temperature difference of 344 mK at an operating temperature of 120 K. reprint
 
22.  Self-assembled semiconductor quantum dot infrared photodetector operating at room temperature and focal plane array
Ho-Chul Lim; Stanley Tsao; Wei Zhang; Manijen Razeghi
Proc. SPIE 6542, Infrared Technology and Applications XXXIII, 65420R (May 14, 2007)-- May 14, 2007
Self-assembled semiconductor quantum dots have attracted much attention because of their novel properties and thus possible practical applications including the lasers, detectors and modulators. Especially the photodetectors which have quantum dots in their active region have been developed and show promising performances such as high operation temperature due to three dimensional confinement of the carriers and normal incidence in contrast to the case of quantum well detectors which require special optical coupling schemes. Here we report our recent results for mid-wavelength infrared quantum dot infrared photodetector grown by low-pressure metalorganic chemical vapor deposition. The material system we have investigated consists of 25 period self-assembled InAs quantum dot layers on InAlAs barriers, which are lattice-matched to InP substrates, covered with InGaAs quantum well layers and InAlAs barriers. This active region was sandwiched by highly doped InP contact layers. The device operates at 4.1 μm with a peak detectivity of 2.8×1011 cm·Hz1/2/W at 120 K and a quantum efficiency of 35 %. The photoresponse can be observed even at room temperature resulting in a peak detectivity of 6×107 cm·Hz1/2/W. A 320×256 focal plane array has been fabricated in this kind of device. Its performance will also be discussed here. reprint
 
23.  High-performance InAs quantum-dot infrared photodetectors grown on InP substrate operating at room temperature
H. Lim, S. Tsao, W. Zhang, and M. Razeghi
Applied Physics Letters, Vol. 90, No. 13, p. 131112-1-- March 26, 2007
The authors report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled InAs quantum dots and the device structure were grown by low-pressure metal-organic chemical vapor deposition. The detectivity was 2.8×1011 cm·Hz1/2/W at 120 K and a bias of −5 V with a peak detection wavelength around 4.1 μm and a quantum efficiency of 35%. Due to the low dark current and high responsivity, a clear photoresponse has been observed at room temperature, which gives a detectivity of 6.7×107 cm·Hz1/2/W. reprint
 
24.  Gain and recombination dynamics of quantum-dot infrared photodetecto
H. Lim, B. Movaghar, S. Tsao, M. Taguchi, W. Zhang, A.A. Quivy, and M. Razeghi
Virtual Journal of Nanoscale Science & Technology-- December 4, 2006reprint
 
25.  Gain and recombination dynamics of quantum-dot infrared photodetectors
H. Lim, B. Movaghar, S. Tsao, M. Taguchi, W. Zhang, A.A. Quivy, and M. Razeghi
Physical Review B, 74 (20)-- November 15, 2006
In this paper we present a theory of diffusion and recombination in QDIPs which is an attempt to explain the recently reported values of gain in these devices. We allow the kinetics to encompass both the diffusion and capture rate limited regimes of carrier relaxation using rigorous random walk and diffusion methods. The photoconductive gains are calculated and compared with the experimental values obtained from InGaAs/InGaP/GaAs and InAs/InP QDIPs using the generation-recombination noise analysis. reprint
 

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