High power continuous wave operation of single mode quantum cascade lasers up to 5 W spanning λ∼3.8-8.3 µm.

In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.

High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm.

We report a room-temperature eight-element phase-locked quantum cascade laser array emitting at 8 µm with a high continuous-wave power of 8.2 W and wall plug efficiency of 9.5%. The laser array operates primarily via the in-phase supermode and has single-mode emission with a side-mode suppression ratio of ~20 dB. The quantum cascade laser active region is based on a high differential gain (8.7 cm/kA) and low voltage defect (90 meV) design. A record high wall plug efficiency of 20.4% is achieved from a low loss buried ridge type single-element Fabry-Perot laser operating in pulsed mode at 20 °C.

nBn extended short-wavelength infrared focal plane array.

An extended short-wavelength nBn InAs/GaSb/AlSb type-II superlattice-based infrared focal plane array imager was demonstrated. A newly developed InAs0.10Sb0.90/GaSb superlattice design was used as the large-bandgap electron barrier in this photodetector. The large band gap electron-barrier design in this nBn photodetector architecture leads to the device having lower dark current densities. A new bi-layer etch-stop scheme using a combination of InAs0.91Sb0.09 bulk and AlAs0.1Sb0.9/GaSb superlattice layers was introduced to allow complete substrate removal and a shorter wavelength cut-on. Test pixels exhibit 100% cutoff wavelengths of ∼2.30 and ∼2.48 µm at 150 and 300 K, respectively. The devices achieve saturated quantum efficiency values of 59.7% and 63.8% at 150 and 300 K, respectively, under backside illumination and without any antireflection coating. At 150 K, photodetectors exhibit dark current density of 8.75×10-8 A/cm2 under -400 mV applied bias, providing specific detectivity of 2.82×1012 cm·Hz1/2/W at 1.78 µm. At 300 K, the dark current density reaches 4.75×10-2 A/cm2 under -200 mV bias, providing a specific detectivity of 8.55×109 cm·Hz1/2/W 1.78 µm.

Bias-selectable three-color short-, extended-short-, and mid-wavelength infrared photodetectors based on type-II InAs/GaSb/AlSb superlattices.

A bias-selectable, high operating temperature, three-color short-, extended-short-, and mid-wavelength infrared photodetector based on InAs/GaSb/AlSb type-II superlattices on GaSb substrate has been demonstrated. The short-, extended-short-, and mid-wavelength channels' 50% cutoff wavelengths were 2.3, 2.9, and 4.4 µm, respectively, at 150 K. The mid-wavelength channel exhibited a saturated quantum efficiency of 34% at 4 µm under +200 mV bias voltage in a front-side illumination configuration and without any antireflection coating. At 200 mV, the device exhibited a dark current density of 8.7×10-5 A/cm2 providing a specific detectivity of ∼2×1011 cm·Hz1/2/W at 150 K. The short-wavelength channel achieved a saturated quantum efficiency of 20% at 1.8 µm. At -10 mV, the device's dark current density was 5.5×10-8 A/cm2. At zero bias, its specific detectivity was 1×1011 cm·Hz1/2/W at 150 K. The extended short-wavelength channel achieved a saturated quantum efficiency of 22% at 2.75 µm. Under -2 V bias voltage, the device exhibited a dark current density of 1.8×10-6 A/cm2 providing a specific detectivity of 6.3×1011 cm·Hz1/2/W at 150 K.

Type-II superlattice-based extended short-wavelength infrared focal plane array with an AlAsSb/GaSb superlattice etch-stop layer to allow near-visible light detection.

A versatile infrared imager capable of imaging the near-visible to the extended short-wavelength infrared (e-SWIR) is demonstrated using e-SWIR InAs/GaSb/AlSb type-II superlattice-based photodiodes. A bi-layer etch-stop scheme consisting of bulk InAs0.91Sb0.09 and AlAs0.1Sb0.9/GaSb superlattice layers is introduced for substrate removal from the hybridized back-side illuminated photodetectors. The implementation of this new technique on an e-SWIR focal plane array results in a significant enhancement in the external quantum efficiency (QE) in the 1.8-0.8 µm spectral region, while maintaining a high QE at wavelengths longer than 1.8 µm. Test pixels exhibit 100% cutoff wavelengths of ∼2.1 and ∼2.25 µm at 150 and 300 K, respectively. They achieve saturated QE values of 56% and 68% at 150 and 300 K, respectively, under back-side illumination and without any anti-reflection coating. At 150 K, the photodetectors (27 µm×27 µm area) exhibit a dark current density of 4.7×10-7 A/cm2 under a -50 mV applied bias providing a specific detectivity of 1.77×1012 cm·Hz1/2/W. At 300 K, the dark current density reaches 6.6×10-2 A/cm2 under -50 mV bias, providing a specific detectivity of 5.17×109 cm·Hz1/2/W.

Impact of scaling base thickness on the performance of heterojunction phototransistors.

In this letter we report the effect of vertical scaling on the optical and electrical performance of mid-wavelength infrared heterojunction phototransistors based on type-II InAs/GaSb/AlSb superlattices. The performance of devices with different base thickness was compared as the base was scaled from 60 down to 40 nm. The overall optical performance shows enhancement in responsively, optical gain, and specific detectivity upon scaling the base width. The saturated responsivity for devices with 40 nm bases reaches 8845 and 9528 A W-1 at 77 and 150 K, respectively, which is almost five times greater than devices with 60 nm bases. The saturated optical gain for devices with 40 nm bases is measured as 2760 at 77 K and 3081 at 150 K. The devices with 40 nm bases also exhibit remarkable enhancement in saturated current gain, with 17690 at 77 K, and 19050 at 150 K.

Nanoselective area growth of defect-free thick indium-rich InGaN nanostructures on sacrificial ZnO templates.

Nanoselective area growth (NSAG) by metal organic vapor phase epitaxy of high-quality InGaN nanopyramids on GaN-coated ZnO/c-sapphire is reported. Nanopyramids grown on epitaxial low-temperature GaN-on-ZnO are uniform and appear to be single crystalline, as well as free of dislocations and V-pits. They are also indium-rich (with homogeneous 22% indium incorporation) and relatively thick (100 nm). These properties make them comparable to nanostructures grown on GaN and AlN/Si templates, in terms of crystallinity, quality, morphology, chemical composition and thickness. Moreover, the ability to selectively etch away the ZnO allows for the potential lift-off and transfer of the InGaN/GaN nanopyramids onto alternative substrates, e.g. cheaper and/or flexible. This technology offers an attractive alternative to NSAG on AlN/Si as a platform for the fabrication of high quality, thick and indium-rich InGaN monocrystals suitable for cheap, flexible and tunable light-emitting diodes.

Recent progress of quantum cascade laser research from 3 to 12 µm at the Center for Quantum Devices [Invited].

The quantum cascade laser (QCL) is becoming the leading laser source in the mid-infrared (mid-IR) range, which contains two atmospheric transmission windows and many molecular fingerprint absorption features. Since its first demonstration in 1994, the QCL has undergone tremendous development in terms of the output power, wall plug efficiency, wavelength coverage, tunability and beam quality. At the Center for Quantum Devices, we have demonstrated high-power continuous wave operation of QCLs covering a wide wavelength range from 3 to 12 µm, with power output up to 5.1 W at room temperature. Recent research has resulted in power scaling in pulsed mode with up to 203 W output, electrically tunable QCLs based on monolithic sampled grating design, heterogeneous QCLs with a broad spectral gain, broadly tunable on-chip beam-combined QCLs, QCL-based mid-IR frequency combs, and fundamental mode surface emitting quantum cascade ring lasers. The developed QCLs will be the basis for a number of next-generation spectroscopy and sensing systems.

Demonstration of type-II superlattice MWIR minority carrier unipolar imager for high operation temperature application.

An InAs/GaSb type-II superlattice-based mid-wavelength infrared (MWIR) 320×256 unipolar focal plane array (FPA) using pMp architecture exhibited excellent infrared image from 81 to 150 K and ∼98% operability, which illustrated the possibility for high operation temperature application. At 150 K and -50 mV operation bias, the 27 µm pixels exhibited dark current density to be 1.2×10(-5) A/cm(2), with 50% cutoff wavelength of 4.9 µm, quantum efficiency of 67% at peak responsivity (4.6 µm), and specific detectivity of 1.2×10(12) Jones. At 90 K and below, the 27 µm pixels exhibited system limited dark current density, which is below 1×10(-9) A/cm(2), and specific detectivity of 1.5×10(14) Jones. From 81 to 100 K, the FPA showed ∼11 mK NEDT by using F/2.3 optics and a 9.69 ms integration time.

Advances in mid-infrared detection and imaging: a key issues review.

It has been over 200 years since people recognized the presence of infrared radiation, and developed methods to capture this signal. However, current material systems and technologies for infrared detections have not met the increasing demand for high performance infrared detectors/cameras, with each system having intrinsic drawbacks. Type-II InAs/GaSb superlattice has been recently considered as a promising candidate for the next generation of infrared detection and imaging. Type-II superlattice is a man-made crystal structure, consisting of multiple quantum wells placed next to each other in a controlled way such that adjacent quantum wells can interact. The interaction between multiple quantum wells offers an additional degree of freedom in tailoring the material's properties. Another advantage of type-II superlattice is the experimental benefit of inheriting previous research on material synthesis and device fabrication of bulk semiconductors. It is the combination of these two unique strengths of type-II superlattice--novel physics and easy manipulation--that has enabled unprecedented progress in recent years. In this review, we will describe historical development, and current status of type-II InAs/GaSb superlattice for advanced detection and imaging in the mid-infrared regime (λ = 3-5 µm).

QEPAS based ppb-level detection of CO and N2O using a high power CW DFB-QCL.

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 µm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm(-1), a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.

Active and passive infrared imager based on short-wave and mid-wave type-II superlattice dual-band detectors.

A versatile dual-band detector capable of active and passive use is demonstrated using short-wave (SW) and mid-wave (MW) IR type-II superlattice photodiodes. A bilayer etch-stop scheme is introduced for back-side-illuminated detectors, which enhanced the external quantum efficiency both in the SWIR and MWIR spectral regions. Temperature-dependent dark current measurements of pixel-sized 27 µm detectors found the dark current density to be ~1 × 10(-5) A/cm(2) for the ~4.2 µm cutoff MWIR channel at 140 K. This corresponded to a reasonable imager noise equivalent difference in temperature of ~49 mK using F/2.3 optics and a 10 ms integration time (t(int)), which lowered to ~13 mK at 110 K using t(int)=30 ms, illustrating the potential for high-temperature operation. The SWIR channel was found to be limited by readout noise below 150 K. Excellent imagery from the dual-band imager exemplifying pixel coincidence is shown.

Al(x)Ga(1-x)N-based deep-ultraviolet 320×256 focal plane array.

We report the synthesis, fabrication, and testing of a 320×256 focal plane array (FPA) of back-illuminated, solar-blind, p-i-n, Al(x)Ga(1-x)N-based detectors, fully realized within our research laboratory. We implemented a pulse atomic layer deposition technique for the metalorganic chemical vapor deposition growth of thick, high-quality, crack-free, high Al composition Al(x)Ga(1-x)N layers. The FPA is hybridized to a matching ISC 9809 readout integrated circuit and operated in a SE-IR camera system. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower, and falling off three orders of magnitude by ~285 nm. By developing an opaque masking technology, the visible response of the ROIC is significantly reduced; thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allows the FPA to achieve high external quantum efficiency (EQE); at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.

Highly selective two-color mid-wave and long-wave infrared detector hybrid based on Type-II superlattices.

We report a two-color mid-wave infrared (MWIR) and long-wave infrared (LWIR) co-located detector with 3 µm active region thickness per channel that is highly selective and can perform under high operating temperatures for the MWIR band. Under back-side illumination, a temperature evolution study of the MWIR detector's electro-optical performance found the 300 K background-limit with 2π field-of-view to be achieved below operating temperatures of 160 K, at which the temperature's 50% cutoff wavelength was 5.2 µm. The measured current reached the system limit of 0.1 pA at 110 K for 30 µm pixel-sized diodes. At 77 K, where the LWIR channel operated with a 50% cutoff wavelength at 11.2 µm, an LWIR selectivity of ~17% was achieved in the MWIR wave band between 3 and 4.7 µm, making the detector highly selective.

Low irradiance background limited type-II superlattice MWIR M-barrier imager.

We report a type-II superlattice mid-wave infrared 320×256 imager at 81 K with the M-barrier design that achieved background limited performance (BLIP) and ∼99% operability. The 280 K blackbody's photon irradiance was limited by an aperture and a band-pass filter from 3.6 µm to 3.8 µm resulting in a total flux of ∼5×10(12) ph.cm(-2).s(-1). Under these low-light conditions, and consequently the use of a 13.5 ms integration time, the imager was observed to be BLIP thanks to a ∼5 pA dark current from the 27 µm wide pixels. The total noise was dominated by the photon flux and read-out circuit which gave the imager a noise equivalent input of ∼5×10(10) ph.cm(-2).s(-1) and temperature sensitivity of 9 mK with F/2.3 optics. Excellent imagery obtained using a 1-point correction alludes to the array's uniform responsivity.

Ultrafast Pulse Generation from Quantum Cascade Lasers.

Quantum cascade lasers (QCLs) have broken the spectral barriers of semiconductor lasers and enabled a range of applications in the mid-infrared (MIR) and terahertz (THz) regimes. However, until recently, generating ultrashort and intense pulses from QCLs has been difficult. This would be useful to study ultrafast processes in MIR and THz using the targeted wavelength-by-design properties of QCLs. Since the first demonstration in 2009, mode-locking of QCLs has undergone considerable development in the past decade, which includes revealing the underlying mechanism of pulse formation, the development of an ultrafast THz detection technique, and the invention of novel pulse compression technology, etc. Here, we review the history and recent progress of ultrafast pulse generation from QCLs in both the THz and MIR regimes.

Type-II superlattice dual-band LWIR imager with M-barrier and Fabry-Perot resonance.

We report a high performance long-wavelength IR dual-band imager based on type-II superlattices with 100% cutoff wavelengths at 9.5 µm (blue channel) and 13 µm (red channel). Test pixels reveal background-limited behavior with specific detectivities as high as ~5×10¹¹ Jones at 7.9 µm in the blue channel and ~1×10¹¹ Jones at 10.2 µm in the red channel at 77 K. These performances were attributed to low dark currents thanks to the M-barrier and Fabry-Perot enhanced quantum efficiencies despite using thin 2 µm absorbing regions. In the imager, the high signal-to-noise ratio contributed to median noise equivalent temperature differences of ~20 milli-Kelvin for both channels with integration times on the order of 0.5 ms, making it suitable for high speed applications.

Band-structure-engineered high-gain LWIR photodetector based on a type-II superlattice.

The LWIR and longer wavelength regions are of particular interest for new developments and new approaches to realizing long-wavelength infrared (LWIR) photodetectors with high detectivity and high responsivity. These photodetectors are highly desirable for applications such as infrared earth science and astronomy, remote sensing, optical communication, and thermal and medical imaging. Here, we report the design, growth, and characterization of a high-gain band-structure-engineered LWIR heterojunction phototransistor based on type-II superlattices. The 1/e cut-off wavelength of the device is 8.0 µm. At 77 K, unity optical gain occurs at a 90 mV applied bias with a dark current density of 3.2 × 10-7 A/cm2. The optical gain of the device at 77 K saturates at a value of 276 at an applied bias of 220 mV. This saturation corresponds to a responsivity of 1284 A/W and a specific detectivity of 2.34 × 1013 cm Hz1/2/W at a peak detection wavelength of ~6.8 µm. The type-II superlattice-based high-gain LWIR device shows the possibility of designing the high-performance gain-based LWIR photodetectors by implementing the band structure engineering approach.

Mid-wavelength infrared avalanche photodetector with AlAsSb/GaSb superlattice.

In this work, a mid-wavelength infrared separate absorption and multiplication avalanche photodiode (SAM-APD) with 100% cut-off wavelength of ~ 5.0 µm at 200 K grown by molecular beam epitaxy was demonstrated. The InAsSb-based SAM-APD device was designed to have electron dominated avalanche mechanism via the band structure engineered multi-quantum well structure based on AlAsSb/GaSb H-structure superlattice and InAsSb material in the multiplication region. The device exhibits a maximum multiplication gain of 29 at 200 K under -14.7 bias voltage. The maximum multiplication gain value for the MWIR SAM-APD increases from 29 at 200 K to 121 at 150 K. The electron and hole impact ionization coefficients were derived and the large difference between their value was observed. The carrier ionization ratio for the MWIR SAM-APD device was calculated to be ~ 0.097 at 200 K.

Surface Emitting, Tunable, Mid-Infrared Laser with High Output Power and Stable Output Beam.

A reflective outcoupler is demonstrated which can allow for stable surface emission from a quantum cascade laser and has potential for cost-effective wafer-scale manufacturing. This outcoupler is integrated with an amplified, electrically tunable laser architecture to demonstrate high power surface emission at a wavelength near 4.9 µm. Single mode peak power up to 6.7 W is demonstrated with >6 W available over a 90 cm-1 (215 nm) spectral range. A high quality output beam is realized with a simple, single-layer, anti-reflective coating. The beam shape and profile are shown to be independent of wavelength.