A compact, single-pass and low-power-consumption methane sensing system is presented and its sensitivity is investigated for a set of reference cells with pressures of 7.4, 74 and 740 Torr and the same methane concentration of 1351 ppm. A single-mode GaSb-based continuous-wave (CW) distributed feedback (DFB) tunable diode laser at 3270 nm and wavelength modulation spectroscopy were employed to collect 2f spectra in the ν3,R3 band of 12CH4. A modulated Voigt line profile model was used to fit the collected 2f spectra. The best detectivity of 4 ppbm is obtained for the highest pressure cell using an Allan-Werle variance analysis.
Interband cascade lasers (ICLs) are becoming a leading semiconductor laser technology for the mid-infrared because of their high efficiency and low power consumption, especially as compared with conventional diode lasers and intersubband quantum cascade lasers (QCLs) in the wavelength range from 3-5 μm. Although a greater effort has been directed towards GaSb-based ICLs in the ~3-5μm range, recent work has highlighted the exciting potential for InAs-based ICLs for reaching longer emission wavelengths.
In this work we report the development of low-threshold InAs-based ICLs with a room-temperature emission wavelength of 6.3μm. The devices were grown on n+-InAs (100) substrates by solid-source molecular beam epitaxy in a custom V90 system using valved crackers for Sb2 and As2. The ICL structures employ an improved waveguide design using intermediate AlAs/AlSb/InAs strain-balanced superlattice cladding layers surrounded by heavily-doped n+-InAs plasmonic claddings. The active region includes 15-stages with AlSb/InAs/In(0.35)Ga(0.65)Sb/InAs/AlSb type-II “W” quantum wells and optimized electron injector doping.
In pulsed mode, broad-area devices lased at 300 K at a lasing wavelength of 6.26 μm and a threshold current density of 395 A/cm2 which is the lowest ever reported among semiconductor lasers at similar wavelengths. The broad-area devices lased up to 335K in pulsed mode at a wavelength of 6.45 μm. These results provide strong evidence of the potential for InAs-based ICLs as efficient sources in the mid-IR.
InAs-based interband cascade (IC) lasers with improved optical confinement have achieved high-temperature operation with a threshold current density as low as 333 A/cm2 at 300 K for emission at 6003 nm. The threshold current density is the lowest ever reported among semiconductor midinfrared lasers at similar wavelengths. These InAs-based IC devices lased in pulsed mode at temperatures up to 357 K near 6.28 μm. A narrow-ridge device was able to operate in continuous-wave mode at temperatures up to 293 K at 6.01 μm.
The family of interband cascade (IC) IR devices includes: interband cascade lasers (ICLs), interband cascade IR photodetectors (ICIPs), and thermophotovoltaics (ICTPVs). To date, developments at the component level have resulted in power-efficient mid-IR ICLs with CW operation at room temperature and above as well as uncooled mid-IR low-noise and high-speed ICIPs. However, there has been little effort to integrate these devices on a single chip for an IR photonic system. Since an appropriately designed ICL can operate as an IR photodetector at zero bias, ICLs and ICIPs can be grown and fabricated on a single chip, enabling the on-chip integration of IR lasers and photodetectors for mid- and long-IR wavelengths.
We report the first demonstration of monolithically integrated mid-IR IC devices operating at room temperature. The unit consists of a monolithically integrated ICL and ICIP fabricated using focused ion beam (FIB) milling. The base structure is a type-I ICL with quaternary GaInAsSb active regions. The laser peak emission wavelength is 3.1 μm at 20 ◦C and the 10% cut-off wavelength of the corresponding ICIP is 3.3 μm, which ensures sufficient photon absorption at the lasing wavelength. For a laser/detector unit (at 20 ◦C) with a 12 μm gap between laser mirror and detector, the open-circuit voltage of the ICIP is 1.06 V and its short-circuit current is 106 μA, resulting from the laser emission (2.6 mW/facet). These preliminary results demonstrate the practical application of integrated IC devices for high-temperature, high-bandwidth and power-efficient on-chip sensors and optical communication mid-IR photonic systems.
GaSb-based tunable single-mode diode lasers can enable rapid, highly-selective and highly-sensitive absorption spectroscopy systems for gas sensing. In this work, single-mode distributed feedback (DFB) laser diodes were developed for the detection of various trace gases in the 2-3.3um range, including CO2, CO, HF, H2S, H2O and CH4. The lasers were fabricated using an index-coupled grating process without epitaxial regrowth, making the process significantly less expensive than conventional DFB fabrication.
The devices are based on InGaAsSb/AlGaAsSb separate confinement heterostructures grown on GaSb by molecular beam epitaxy. DFB lasers were produced using a two step etch process. Narrow ridge waveguides were first defined by optical lithography and etched into the semiconductor. Lateral gratings were then defined on both sides of the ridge using electron-beam lithography and etched to produce the index-grating.
Effective index modeling was used to optimize the ridge width, etch depths and the grating pitch to ensure single-lateral-mode operation and adequate coupling strength. The effective index method was further used to simulate the DFB laser emission spectrum, based on a transfer matrix model for light transmission through the periodic structure.
The fabricated lasers exhibit single-mode operation which is tunable through the absorption features of the various target gases by adjustment of the drive current. In addition to the established open-path sensing applications, these devices have great potential for optoelectronic integrated gas sensors, making use of integrated photodetectors and possibly on-chip Si photonics waveguide structures.
Four-junction solar cells for space and terrestrial applications require a junction with a band gap of ∼1 eV for optimal performance. InGaAsN or InGaAsN(Sb) dilute nitride junctions have been demonstrated for this purpose, but in achieving the 14 mA/cm2 short-circuit current needed to match typical GaInP and GaAs junctions, the open-circuit voltage (VOC) and fill factor of these junctions are compromised. In multijunction devices incorporating materials with short diffusion lengths, we study the use of thin junctions to minimize sensitivity to varying material quality and ensure adequate transmission into lower junctions. An n-i-p device with 0.65-μm absorber thickness has sufficient short-circuit current, however, it relies less heavily on field-aided collection than a device with a 1-μm absorber. Our standard cell fabrication process, which includes a rapid thermal anneal of the contacts, yields a significant improvement in diffusion length and device performance. By optimizing a four-junction cell around a smaller 1-sun short-circuit current of 12.5 mA/cm2, we produced an InGaAsN(Sb) junction with open-circuit voltage of 0.44 V at 1000 suns (1 sun=100 mW/cm2), diode ideality factor of 1.4, and sufficient light transmission to allow >12.5 mA/cm2 in all four subcells.
We investigate high-temperature and high-frequency operation of interband cascade infrared photodetectors (ICIPs)-two
critical properties. Short-wavelength ICIPs with a cutoff wavelength of 2.9 μm had Johnson-noise limited detectivity of
5.8×109 cmHz1/2/W at 300 K, comparable to the commercial Hg1-xCdxTe photodetectors of similar wavelengths. A
simple but effective method to estimate the minority carrier diffusion length in short-wavelength ICIPs is introduced.
Using this approach, the diffusion length was estimated to be significantly shorter than 1 μm at high temperatures,
indicating the importance of a multiple-stage photodetector (e.g., ICIPs) at high temperatures. Recent investigations on
the high-frequency operation of mid-wavelength ICIPs (λc=4.3 μm) are discussed. These photodetectors had 3-dB
bandwidths up to 1.3 GHz with detectivities exceeding 1x109 cmHz1/2/W at room temperature. These results validate the
ability of ICIPs to achieve high bandwidths with large sensitivity and demonstrate the great potential for applications
such as: heterodyne detection, and free-space optical communication.
Semiconductor nanostructures, such as quantum wells and quantum dots, are well known, and some have been
incorporated in applications. Here we propose a new general approach to make use of polar optical phonons in quantum
wells for terahertz (THz) devices. As the first example, we show the coupling of phonon and intersubband transition
leading to Fano resonance in photocurrent spectra. We investigate the phenomenon experimentally in specially designed
GaAs/AlGaAs quantum well infrared photodetectors. Finally, we discuss the future research and potentials.
Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well
active regions grown by molecular beam epitaxy. The devices were fabricated
using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide.
High sidemode suppression was achieved, and in 20°C continuous-wave operation, devices with a 400μm-long cavity provided 4.5mW total output power
at the 2396nm target wavelength.
Anti-reflection and high-reflection facet coatings exhibited no deleterious effects on the laser tunability or mode quality, thus allowing
the preferential extraction of output power from a single laser facet.
A detailed study of the high-power pulsed operation of C-band optically-pumped GaInNAsSb vertical cavity surface emitting lasers is reported.
The devices employ a resonant periodic gain structure grown by molecular beam epitaxy on a GaAs substrate with a 31-pair GaAs/AlAs bottom
distributed Bragg reflector and a 4-λ,
GaAs-based resonant cavity containing 10 GaInNAsSb quantum wells distributed among the 7 antinodes of the electric field.
A dual-pump-band SiO2/TiO2 dielectric top mirror allows efficient optical pumping via low reflectivities at 808nm and 1064nm
while providing very high reflectivity at the 1.55μm target emission wavelength. The laser characteristics were evaluated using both a Q-switched Nd:YAG
1064nm pump and a 20W-peak 180ns-pulsed 850nm diode laser. The importance of the gain-cavity detuning was evident from time-dependent spectral
measurements of laser material
subjected to post-growth annealing at different temperatures between 725 and 775°C. The highest annealing temperature produces the largest blue shift of the
gain peak relative to the cavity resonance, resulting in the best power transfer characteristics as well as reduced temperature sensitivity.
Semiconductor nanostructures, such as quantum wells and quantum dots (QD), are well known, and some have been
incorporated in applications. Here will focus on novel structures made of QDs and related devices for terahertz (THz)
generation. Their potential advantages, such as low threshold current density, high characteristic temperature, increased
differential gain, etc., make QDs promising candidates for light emitting applications in the THz region. Our idea of
using resonant tunneling through QDs is presented, and initial results on devices consisting of self assembled InAs QDs
in an undoped GaAs matrix, with a design incorporating GaInNAs/GaAs short period superlattice, are discussed.
Moreover, shallow impurities are also being explored for possible THz emission: the idea is based on the tunneling
through bound states of individual donor or acceptor impurities in the quantum well. Initial results on devices having an
AlGaAs/GaAs double barrier resonant tunneling structure are discussed.
The room-temperature 1.55 &mgr;m continuous-wave (CW) operation of single-lateral mode GaInNAsSb ridge
waveguide lasers grown on GaAs is reported.
Detailed measurements of the light output power and spectral properties were used to assess the device characteristics
as a function of applied current and temperature in both CW and pulsed operation. An exemplary, 3&mgr;×750&mgr;m,
device with a 92% high-reflectivity back facet coating exhibited a record low CW threshold current of 63~mA, with a peak output power of 15~mW.
High-resolution modal gain spectra were extracted from amplified spontaneous emission measurements yielding the
internal loss (8.0~cm-1, transparency current (50~mA) and the wavelength dependence of the differential gain.
The latter was used with careful measurements of the Fabry-Perot mode shift with injection current to determine
the linewidth enhancement factor of 2.8 at the transparency current. The first measurement of intrinsic modulation frequency in 1.55 &mgr;m GaInNAsSb lasers is
reported, based on the observed relative intensity noise (RIN). The RIN measurements indicate a maximum modulation frequency of 7.2~GHz,
which is a promising result for future telecommunications applications.
The properties of a 1.3μm GaInNAs Double Quantum Well (QW) ridge waveguide (RWG) laser have been systematically studied for GaAs based uncooled long wavelength lasers. The threshold current, transparency current, optical gain, internal loss and quantum efficiency characteristics were assessed by light-current (L-I) measurement using devices with different geometries. Measurements of gain spectra versus injection current and temperature were taken and used to analyze GaInNAs as an active material in terms of gain, loss and transparency. The experimental observations are discussed. The results are compared with those obtained from lasers made by other conventional materials. The high characteristic temperature (T0=155K from 20°C to 75°C) and comparable stimulated emission to InP based lasers offer the promise of application as a light source for low cost data communication systems.
High hydrostatic pressure can be used for wavelength tuning of semiconductor laser diodes in a wide spectral range. Coupling the laser with external grating leads to wavelength tuning within the gain spectrum (i.e. in a narrower range than with pressure) but allows for a narrow emission line and nearly continuous tuning (mode-hop free if anti-reflecting coating is applied). Here we demonstrate a combination of pressure and external-resonator tuning for the GaInNAs laser emitting at 1343 nm at ambient conditions. Using the specially designed liquid pressure cell working up to 20 kbar we shift the emission down to 1170 nm while the external grating (used in Littrow configuration) allows for fine tuning in the ~10 nm range (at each pressure).
Transparency current density (Jtr) was studied in GaInNAs ridge waveguide lasers. The devices employ Ga1-xInxNyAs1-y multiple quantum wells and were grown on GaAs substrates using solid-source molecular beam epitaxy (MBE) with an RF plasma cell. The transparency current density is sensitive to material quality: defects, traps and other sources of non-radiative recombination. It is also dependent on the rate of thermionic emission from quantum wells. Wavelength, polarization and temperature dependence of transparency carrier density of annealed material was studied. Record low transparency carrier densities of 20 and 90 A/cm2/well were observed (for TM and TE polarizations) in devices based on GaInNAs material designed for emission at 1340 nm after optimized rapid thermal annealing. This low value of Jtr confirms the excellent quality of the GaInNAs material and demonstrates that GaInNAs lasers with excellent material properties can be grown for long wavelength applications provided appropriate annealing is applied. It is believed that the low transparency current density is a unique feature of GaInNAs and is due to the band structure and band alignment of the material system.
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