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.

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.

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.

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.

Room temperature terahertz semiconductor frequency comb.

A terahertz (THz) frequency comb capable of high-resolution measurement will significantly advance THz technology application in spectroscopy, metrology and sensing. The recently developed cryogenic-cooled THz quantum cascade laser (QCL) comb has exhibited great potentials with high power and broadband spectrum. Here, we report a room temperature THz harmonic frequency comb in 2.2 to 3.3 THz based on difference-frequency generation from a mid-IR QCL. The THz comb is intracavity generated via down-converting a mid-IR comb with an integrated mid-IR single mode based on distributed-feedback grating without using external optical elements. The grating Bragg wavelength is largely detuned from the gain peak to suppress the grating dispersion and support the comb operation in the high gain spectral range. Multiheterodyne spectroscopy with multiple equally spaced lines by beating it with a reference Fabry-Pérot comb confirms the THz comb operation. This type of THz comb will find applications to room temperature chip-based THz spectroscopy.

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.

Type-II superlattices base visible/extended short-wavelength infrared photodetectors with a bandstructure-engineered photo-generated carrier extractor.

Visible/extended short-wavelength infrared photodetectors with a bandstructure-engineered photo-generated carrier extractor based on type-II InAs/AlSb/GaSb superlattices have been demonstrated. The photodetectors are designed to have a 100% cut-off wavelength of ~2.4 µm at 300K, with sensitivity down to visible wavelengths. The photodetectors exhibit room-temperature (300K) peak responsivity of 0.6 A/W at ~1.7 µm, corresponding to a quantum efficiency of 43% at zero bias under front-side illumination, without any anti-reflection coating where the visible cut-on wavelength of the devices is <0.5 µm. With a dark current density of 5.3 × 10-4 A/cm2 under -20 mV applied bias at 300K, the photodetectors exhibit a specific detectivity of 4.72 × 1010 cm·Hz1/2/W. At 150K, the photodetectors exhibit a dark current density of 1.8 × 10-10 A/cm2 and a quantum efficiency of 40%, resulting in a detectivity of 5.56 × 1013 cm·Hz1/2/W.

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.

Strain-Induced Metastable Phase Stabilization in Ga2O3 Thin Films.

It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin-film Ga2O3, where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga2O3/Al2O3 film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope. Just a few layers of metastable α-Ga2O3 structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial α-Ga2O3 structure of about couple unit cells, several layers (4-5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable κ-Ga2O3 phase is stabilized as the major thin-film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga3+ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga3+ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high-resolution electron microscopy observation, complemented by the density functional theory calculations. This study provides insights from the atomic scale and their implications for the design of functional thin-film materials using epitaxial engineering.

Single-mode, high-power, mid-infrared, quantum cascade laser phased arrays.

We demonstrate single-mode, 16-channel, optical phased arrays based on quantum cascade laser technology, with emission wavelengths around 4.8 µm. The integrated device consists of a distributed feedback seed section, a highly-efficient tree array multi-mode interferometer power splitter, and a 16-channel amplifier array with a 4° angled facet termination. With a single layer Y2O3 coating, the angled facet reflectivity is estimated to be less than 0.1% for suppressing amplifier self-lasing. A peak output power of 30 W is achieved with an emission spectrum narrower than 11 nm and a side mode suppression ratio over 25 dB. Far field distribution measurement result indicates a uniform phase distribution across the array output. Using the same phased array architecture, we also demonstrate single-mode 3.8 µm QCL amplifier arrays with up to 20 W output power.

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.

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.

Author Correction: Radiative recombination of confined electrons at the MgZnO/ZnO heterojunction interface.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

Dark current reduction in microjunction-based double electron barrier type-II InAs/InAsSb superlattice long-wavelength infrared photodetectors.

Microjunction InAs/InAs1-xSbx type-II superlattice-based long-wavelength infrared photodetectors with reduced dark current density were demonstrated. A double electron barrier design was employed to reduce both bulk and surface dark currents. The photodetectors exhibited low surface leakage after passivation with SiO2, allowing the use of very small size features without degradation of the dark current. Fabricating microjunction photodetectors (25 × 25 µm2 diodes with 10 × 10 µm2 microjunctions) in combination with the double electron barrier design results in a dark current density of 6.3 × 10-6 A/cm2 at 77 K. The device has an 8 µm cut-off wavelength at 77 K and exhibits a quantum efficiency of 31% for a 2 µm-thick absorption region, which results in a specific detectivity value of 1.2 × 1012 cm·Hz1/2/W.

Radiative recombination of confined electrons at the MgZnO/ZnO heterojunction interface.

We investigate the optical signature of the interface in a single MgZnO/ZnO heterojunction, which exhibits two orders of magnitude lower resistivity and 10 times higher electron mobility compared with the MgZnO/Al2O3 film grown under the same conditions. These impressive transport properties are attributed to increased mobility of electrons at the MgZnO/ZnO heterojunction interface. Depth-resolved cathodoluminescence and photoluminescence studies reveal a 3.2 eV H-band optical emission from the heterointerface, which exhibits excitonic properties and a localization energy of 19.6 meV. The emission is attributed to band-bending due to the polarization discontinuity at the interface, which leads to formation of a triangular quantum well and localized excitons by electrostatic coupling.

Monolithic beam steering in a mid-infrared, surface-emitting, photonic integrated circuit.

The mid-infrared (2.5 < λ < 25 µm) spectral region is utilized for many purposes, such as chemical/biological sensing, free space communications, and illuminators/countermeasures. Compared to near-infrared optical systems, however, mid-infrared component technology is still rather crude, with isolated components exhibiting limited functionality. In this manuscript, we make a significant leap forward in mid-infrared technology by developing a platform which can combine functions of multiple mid-infrared optical elements, including an integrated light source. In a single device, we demonstrate wide wavelength tuning (240 nm) and beam steering (17.9 degrees) in the mid-infrared with a significantly reduced beam divergence (down to 0.5 degrees). The architecture is also set up to be manufacturable and testable on a wafer scale, requiring no cleaved facets or special mirror coating to function.

Bias-selectable nBn dual-band long-/very long-wavelength infrared photodetectors based on InAs/InAs1-xSbx/AlAs1-xSbx type-II superlattices.

Type-II superlattices (T2SLs) are a class of artificial semiconductors that have demonstrated themselves as a viable candidate to compete with the state-of-the-art mercury-cadmium-telluride material system in the field of infrared detection and imaging. Within type-II superlattices, InAs/InAs1-xSbx T2SLs have been shown to have a significantly longer minority carrier lifetime. However, demonstration of high-performance dual-band photodetectors based on InAs/InAs1-xSbx T2SLs in the long and very long wavelength infrared (LWIR & VLWIR) regimes remains challenging. We report the demonstration of high-performance bias-selectable dual-band long-wavelength infrared photodetectors based on new InAs/InAs1-xSbx/AlAs1-xSbx type-II superlattice design. Our design uses two different bandgap absorption regions separated by an electron barrier that blocks the transport of majority carriers to reduce the dark current density of the device. As the applied bias is varied, the device exhibits well-defined cut-off wavelengths of either ∼8.7 or ∼12.5 µm at 77 K. This bias-selectable dual-band photodetector is compact, with no moving parts, and will open new opportunities for multi-spectral LWIR and VLWIR imaging and detection.

High efficiency quantum cascade laser frequency comb.

An efficient mid-infrared frequency comb source is of great interest to high speed, high resolution spectroscopy and metrology. Here we demonstrate a mid-IR quantum cascade laser frequency comb with a high power output and narrow beatnote linewidth at room temperature. The active region was designed with a strong-coupling between the injector and the upper lasing level for high internal quantum efficiency and a broadband gain. The group velocity dispersion was engineered for efficient, broadband mode-locking via four wave mixing. The comb device exhibits a narrow intermode beatnote linewidth of 50.5 Hz and a maximum wall-plug efficiency of 6.5% covering a spectral coverage of 110 cm-1 at λ ~ 8 µm. The efficiency is improved by a factor of 6 compared with previous demonstrations. The high power efficiency and narrow beatnote linewidth will greatly expand the applications of quantum cascade laser frequency combs including high-precision remote sensing and spectroscopy.