An external cavity with a binary phase grating has been developed to achieve the coherent beam addition of five
quantum-cascade lasers emitting at 4.65 μm. The combining of these five emitters is achieved by a binary phase grating
or Dammann grating able to separate an incident beam into five beams of equal intensities with a 75% efficiency. A CW
output power of ~ 0.65 W corresponding to a combining efficiency of 70% with a good beam quality is obtained. More
results concerning output power, combining, efficiency stability and beam quality and spectrum are exposed.
We demonstrate a monolithic Quantum Cascade Laser array. We show phase-locking and single-mode emission
at λ=8.4μm. It consists of narrow ridges buried into InP:Fe. Phase-locking is provided by evanescent coupling between
adjacent ridges. This μ-structuration is simultaneously an answer to the excessive heating and poor beam quality of broad
area lasers. First, it increases the surface of exchange between the multi-layer active region and the InP:Fe, which
presents a higher thermal conductivity. Secondly, by choosing carefully the width of emitters and the distance between
them, we insure phase locking and control of the supermode emission. We have investigated 2μm wide emitters. In order
to study the behavior of evanescent coupling, we have chosen spacing from 1 to 8 microns. The number of emitters
ranges from 1 to 64. Technological feasibility was demonstrated up to 64 emitters, and lasing operation up to 32 emitters.
We have obtained a pure dual-lobe far-field pattern as expected from an anti-symmetrical supermode. The width of each
lobes narrows with an increasing array size as expected from the diffraction theory. The beam quality is insensitive to the
injective current. The optical power scales linearly with the number of emitters.
Since 2005, Thales is successfully manufacturing QWIPs in high rate production through III-V Lab. All the early
claimed advantages of QWIPs are now demonstrated. The versatility of the band-gap engineering allows the custom
design of detectors to fulfill specific application requirements in MWIR, LWIR or VLWIR ranges. The maturity of the
III-V microelectronics based on GaAs substrates gives uniformity, stability and high production rate. In this presentation
we will discuss the specific advantages of this type of detector. An overview of the available performances and
production status will be presented including under-development products such as dual band and polarimetric sensors.
An external cavity with a binary phase grating has been developed to achieve the coherent beam addition of five
quantum-cascade lasers emitting at 4.65 μm. The combining of these five emitters is achieved by a binary phase grating
or Dammann grating able to separate an incident beam into five beams of equal intensities with a 75% efficiency. A CW
output power of ~ 0.5 W corresponding to a combining efficiency of 66% with a good beam quality is obtained. More
results concerning output power, combining, efficiency stability and beam quality and spectrum are exposed.
A review of the III-V Lab activities in the field of quantum well infrared photodetectors (QWIPs) is presented. We discuss the specific advantages of this type of detector and present the production facilities and status. A large section is dedicated to broadband QWIPs for space applications and to QWIPs on InP for mid-wavelength infrared detection. We review the progress of QWIP technology for the next generation (dual band, polarimetric, and multispectral) of thermal imagers. Finally, the state-of-the-art of very long wavelength QWIPs is discussed.
One of the key features of quantum well infrared photodetectors is the narrow absorption band. However, some
applications, as the infrared spectroscopy, require broadband detection. Several approaches have been used to get a
broadband response with QWIPs (superlattices, digital graded barriers, stacks, etc.). In this paper, we focus on the
interlaced configuration and on the coupled wells structure. Both designs exhibit broadband response covering the [11-15
μm] spectral range. The experimental dependencies of the spectral shape versus the temperature and bias voltage are
discussed. Based on numerical model, we propose a specific design strategy which leads to a spectral shape quasiindependent
on the operating conditions.
Integrated terahertz (THz) pulse generation and amplification in a THz quantum cascade laser (QCL) is demonstrated.
Intra-cavity THz pulses are generated by exciting the facet of the quantum cascade laser with an ultrafast Ti:Sapphire
laser (~100fs) and detected using electro-optic sampling. Maximum THz field emission is found with an interband
transition of 1.535eV (809nm) and by narrowing the excitation laser bandwidth to ~3THz. These resonance conditions
correspond to the narrowband excitation of the quantum cascade miniband, indicating that the THz pulse is generated by
the photo-excited carriers that are accelerated by the applied field. The generated pulse is subsequently amplified by the
narrowband gain of the laser as it propagates through the QCL cavity. As an integrated THz generator-amplifier, the
technique avoids the issues associated with the coupling of external THz pulses into sub-wavelength dimensioned
cavities.
Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on GaAs
and related III-V compounds, at the Alcatel-Thales-III-V Lab (formerly part of THALES Research and Technology
Laboratory).
In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and has been the key
parameter for the production to start. Another widely claimed advantage for QWIPs was the so-called band-gap
engineering and versatility of the III-V processing allowing the custom design of quantum structures at various
wavelengths in MWIR, LWIR and VLWIR. An overview of the available performances of QWIPs in the whole infrared
spectrum is presented here. We also discuss about the under-development products such as dual band and
polarimetric structures.
Quantum cascade detectors (QCDs) have been introduced recently as a photovoltaic candidate to infrared
detection. Since QCDs work with no applied bias, longer integration time and different read-out circuits can
be used. Depending on the application, QCDs could be preferred to QWIPs. The systematic comparison
between QCDs and QWIPs is difficult due to the large number of parameters in a thermal imager for a given
application. Here we propose a first comparison between these two devices, starting with several examples,
based on specific cases. In particular, it is shown that QCDs in the 8-12 µm band are an interesting alternative
to QWIPs if higher operating temperature is required.
Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on
GaAs and related III-V compounds, at the Alcatel-Thales-III-V Lab (formerly part of THALES Research and
Technology Laboratory).
In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and has been the key
parameter for the production to start. Another widely claimed advantage for QWIPs was the so-called band-gap
engineering and versatility of the III-V processing allowing the custom design of quantum structures to fulfil the
requirements of specific applications such as very long wavelength (VLWIR) or multispectral detection. In this
presentation, we give the status of our LWIR QWIP production line, and also the current status of QWIPs for MWIR
(<5μm) and VLWIR (>15μm) arrays.
As the QWIP technology cannot cover the full electromagnetic spectrum, we develop other semiconductor
compounds for SWIR and UV applications. We present here the status of our first FPA realization in UV with GaN
alloy, and at 1.5μm with InGaAs photodiodes.
Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on GaAs and
related III-V compounds, at THALES Research and Technology Laboratory. The QWIP technology allows the
realization of large staring arrays for Thermal Imagers (TI) working in the long-wave infrared (LWIR) band (8-12
μm).
In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and has been the key
parameter for the production to start. The 640x512 LWIR focal plane arrays (FPAs) with 20μm pitch was the
demonstration that state of the art performances can be achieved even with small pixels. This opened the field for the
realization of usable and affordable megapixel FPAs. Thales Research & Technology (TRT) has been developing third
generation GaAs LWIR QWIP arrays for volume manufacture of high performance low cost thermal imaging cameras.
In the past, another widely claimed advantage for QWIPs was the so-called band-gap engineering and versatility of the
III-V processing allowing the custom design of quantum structures to fulfil the requirements of specific applications
such as very long wavelength (VLWIR) or multispectral detection. In this presentation, we present the performances of
both our first 384x288, 25 μm pitch, MWIR (3-5μm) / LWIR (8-9 μm) dual-band FPAs, and the current status of
QWIPs for MWIR (< 5μm) and VLWIR (>15μm) arrays.
Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIP) are considered as a technological choice for
3rdgeneration thermal imagers [1], [2].
Since 2001, the THALES Group has been manufacturing sensitive arrays using AsGa based QWIP technology at
THALES Research and Technology Laboratory. This QWIP technology allows the realization of large staring arrays
for Thermal Imagers (TI) working in the Infrared region of the spectrum. The main advantage of this GaAs detector
technology is that it is also used for other commercial devices. The GaAs industry has lead to important improvements
over the last ten years and it reaches now an undeniable level of maturity. As a result the key parameters to reach high
production yield: large substrate and good uniformity characteristics, have already been achieved. Considering
defective pixels, the main usual features are a high operability (> 99.9%) and a low number of clusters having a
maximum of 4 dead pixels.
Another advantage of this III-V technology is the versatility of the design and processing phases. It allows
customizing both the quantum structure and the pixel architecture in order to fulfill the requirements of any specific
applications. The spectral response of QWIPs is intrinsically resonant but the quantum structure can be designed for a
given detection wavelength window ranging from MWIR, LWIR to VLWIR.
Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIP) are now seriously considered as a technological choice for the 3rd generation of thermal imagers.
Since 2001, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on AsGa techniques through THALES Research and Technology Laboratory. This QWIP technology allows the realisation of large staring arrays for Thermal Imagers (TI) working in the Infrared region of the spectrum. A review of the current QWIP products is presented (LWIR, MWIR and dual color FPAs).
The main advantage of this GaAs detector technology is that it is also used for other commercial devices. The duality of this QWIP technology has lead to important improvements over the last ten years and it reaches now an undeniable level of maturity. As a result, the processing of large substrate and a good characteristic uniformity, which are the key parameters for reaching high production yield, are already achieved. Concerning the defective pixels, the main common features are a high operability (above 99.9%) and a low number of clusters including a maximum of 5 dead pixels.
Another advantage of this III-V technology is the versatility of the design and processing phases. It allows customizing both the quantum structure and the pixel architecture in order to fulfill the requirements of any specific applications. The spectral response of QWIPs is intrinsically resonant but the quantum structure can be designed for a given detection wavelength window ranging from MWIR, LWIR to VLWIR.
Quantum Cascade Lasers (QCL), emitting between 5 and 9 μm, have been realised with a view to achieving QCLs fabrication on a production scale. The growth of the structures was carried out in a multi-wafer RIBER 49 system (13 x 2" platen), and the processing sequence involved an Inductively Coupled Plasma (ICP) step for homogeneity and reproducibility purposes. To validate the approach used, a first batch of lasers, emitting around 9μm, based on a design already published [1], has been realised. State of the art performance on these devices (Jth = 4.2 kA cm-2, η = 304 mW A-1, Pmax = 690 mW) has been achieved. A second set of strained balanced structures, emitting around 5.4μm, has been demonstrated, working in pulsed operation at room temperature(Jth = 3.9 kA cm-2, η = 362 mW A-1, Pmax = 420 mW).
Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIP) are from now seriously considered for the 3rd generation of IR imagers for military markets. Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on AsGa
techniques through THALES Research and Technology Laboratory. This QWIP technology allows the realization of large staring arrays for Thermal Imagers (TI) working in the IR band III (8-12 μm). A review of the current QWIP products is presented. In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and is the key parameter for the production start. By presenting our first results of a 640x512 LWIR FPA at a pitch of 20μm we also demonstrate that very high performances can be achieved even with small pixels which opens the field for the realization of usable and affordable
megapixel FPAs. Another advantage widely claimed in the past for QWIPs was the so-called band-gap engineering and versatility of the III-V processing allowing the custom design of quantum structure to fulfill the requirements of specific applications like very long wavelength (VLWIR) or multispectral detection. In this presentation, we present the performances of our first 256x256 MWIR / LWIR two color FPA at a pitch of 25 μm, and also the current status of QWIPs
for VLWIR arrays (>15μm).
Some parameters of integration of a Quantum Cascade Detector (QCD) in an infrared imaging system are studied. Performances of QCD are first presented : absorption and responsivity spectra, peak responsivity (around 44 mA/W), resistivity at zero bias and detectivity. Quantum efficiency and photoconduction gain are deduced from these results. Finally the consequences of an integration of such a detector in a readout circuit are studied in terms of saturation of an external capacitor.
A photovoltaic intersubband detector based on electron transfer on a cascade of quantum levels is presented: a Quantum Cascade Detector (QCD). Optical and electrical performances of a QCD are presented: high responsivity at null bias voltage about 44mA/W, high resistivity. Because they work with no dark current, QCDs are very promising for small pixel and large focal plane array applications. A dark current modelling is explained.
CEA/LETI has been working for several years on the development of HgCdTe-based infrared dual band detectors [3]. Since 2001 CEA/LETI is also involved in a large program for the demonstration of dual band QWIP FPAs presenting large format and small pitch. This study is carried out with the QWIP team of THALES Research and Technology (TRT) in charge of QWIP design, MBE growth and GaAs processing for the detector side. As part of this program TRT investigated different quantum structures and pixel architectures for the realization of two-band FPAs for MWIR/LWIR and LWIR/LWIR applications. At the end of this study a choice of the most appropriate architecture was done. On its side, CEA/LETI designed readout circuits optimized for the selected dual-band QWIP. TRT delivered QWIP arrays and CEA/LETI proceeded to the assembly, integration and electro-optical characterization. The aim of this paper is to describe the architecture of these dual-band demonstrators and to present the first results concerning their electro-optical performances measured at 70K and 65K.
Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIPs) are from now seriously considered for the 3rd generation of IR imagers. Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on AsGa techniques through THALES Research and Technology Laboratory. This QWIP technology allows the realization of large staring arrays for Thermal Imagers (TI) working in the IR band III (8-12 μm). A review of the current QWIP products is presented. In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and was the key parameter for the production start. By presenting our first results of a 640x512 LWIR FPA at a pitch of 20μm, we also demonstrate that very high performances can be achieved even with small pixels which opens the field for the realization of usable and affordable megapixel FPAs. Another advantage widely claimed in the past for QWIPs was the so-called band-gap engineering and versatility of the III-V processing. This allows the custom design of quantum structure to fulfill the requirements of specific applications like very long wavelength or multispectral detection. In this presentation, we present the performances of our first 256x256 MWIR / LWIR and LWIR/LWIR two color FPAs at a pitch of 25 μm.
The quantum cascade laser is a semiconductor light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these devices the principles of operation are not based on the physical properties of the constituent materials, but arise from the layer sequence forming the heterostructure. The quantum design and the control of the layer thickness, down to an atomic mono-layer, allows one to ascribe into a semiconductor crystal, artificial potentials with the desired electronic energy levels and wavefunctions. In recent years the performance of quantum cascade lasers has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid-far infrared lasers in a very wide spectral range (3.5-160 μm). At present, the best performances are reached at wavelength between 5-10 μm, but recent results on new material systems with deeper quantum wells are indicating that this technology will be soon available also in the 3-5 μm spectral region.
A photovoltaic intersubband detector based on electron transfer on a cascade of quantum levels is presented: a Quantum Cascade Detector (QCD). The highest photoresponse of intersubband transition based photovoltaic detectors is demonstrated: 44 mA/W at null bias. Further improvements permit to suppress the leakage current and to increase the resistivity R0A. Useless cross-transitions have been eliminated finally leading to a high resistance narrow band photodetector with a Johnson noise detectivity at 50 K comparable to QWIPs. Because they work with no dark current, QCDs are very promising for small pixel and large focal plane array applications.
Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIP) are coming out from the laboratory. In this presentation we demonstrate that production and research cannot be dissociated in order to make the new generation of thermal imagers benefit as fast as possible from the building blocks developed by researchers. Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on AsGa techniques through THALES Research and Technology Laboratory. This QWIP technology, integrated in IDDCA built by Sofradir, allows the realization of large staring arrays for Thermal Imagers (TI) working in the IR band III (8-12 μm). A review of the current QWIP products, offered by Sofradir, is presented. In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and was the key parameter for the production start. Another advantage widely claimed also for QWIPs was the so-called band-gap engineering, allowing the custom design of quantum structure to fulfill the requirements of specific applications like very long wavelength or multispectral detection. In this presentation, we present the performances for Middle Wavelength InfraRed (MWIR) detection and demonstrate the ability of QWIP to cover the two spectral ranges (3-5 μm and 8-20 μm). At last but not least, the versatility of the GaAs processing appeared for QWIPs as an important gift. This assumption was well founded. We give here some results achieved on building blocks for two color QWIP pixels. We also report the expected performances of focal plane arrays we are currently developing with the CEA-LETI-SLIR.
Standard GaAs/AlGaAs QWIPs are now well established for LWIR detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The realization of large FPAs (up to 640 X 480) drawing on the standard III-V technological process has already been demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIP technology has been growing up over the last ten years and now reaches an undeniable level of maturity. As with all quantum detectors, the operating temperature of QWIPs is limited by the thermal current, particularly in the LWIR range. It is very crucial to achieve an operating temperature as high as possible and at least above 77 K in order to reduce volume and power consumption and to improve the reliability of the detection module. This thermal current offset has three detrimental effects: noise increase, storage capacitor saturation and high sensitivity of FPAs to fluctuations in operating temperature. For LWIR FPAs, large cryocoolers are required, which means volume and power consumption unsuitable for handheld systems. The understanding of detection mechanisms has led us to design and realize high performance 'standard' QWIPS working near 77 K. Furthermore, a new in situ skimmed architecture accommodating this offset has already been demonstrated. In this paper we summarize the contribution of THALES Research & Technology to this progress. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the development of our QWIPs by results on FPAs.
We describe some key growth issues for Mid-Infrared electroluminescent devices based on a quantum-cascade design using InAs/AlSb heterostructures grown on GaSb substrates. Structural and optical properties of antimonide/arsenide interfaces are first investigated on InAs/AlSb multiple quantum well samples with different types of Sb-like interfaces and various InAs thicknesses. We show that X-ray reflectometry is a powerful complementary tool to High Resolution X-ray Diffraction (HRXRD) to extract both individual layer thicknesses and interface roughnesses using only electronic densities as input parameters. The good structural quality of samples is evidenced by the persistence of sharp high order satellite peaks on HRXRD spectra. The associated optical properties are studied by photo-induced intersubband absorption. Strong E12 p- polarized intersubband absorptions are observed with a full- width-at-half-maximum (FWHM) around 12 meV at 77 K showing good material quality. Absorption peak positions are compared to theoretical simulations based on a 2 X 9-band k.p calculation. These results allow us to properly design and fabricate InAs/AlSb quantum cascade light emitting devices in the 3 - 5 micrometers wavelength window taking into account the growth constraints. Well-resolved Mid-Infrared (3.7 - 5.3 micrometers ) electroluminescence peaks are observed up to 300 K with FWHM to emission energy ratio ((Delta) E/E) around 8%.
Nowadays refractive-index engineering has become a challenging area for experimentalists in semiconductor integrated optics, whereas design constraints are often more strict than both standard technology tolerances and model accuracies. In fact, it is crucial to non-destructively evaluate thicknesses and refractive indices of a multilayer waveguide independently, and to this aim we resorted to X-ray reflectometry and effective index measurements on MBE-grown AlGaAs waveguides, respectively. With the first technique interference effects (Kiessig fringes) arise, which are related to layer thicknesses. By standard data processing, thickness accuracies of +/- 0.05 nm are readily achieved. Effective index measurements were performed at several wavelengths on both slab and rib waveguides, through grating-assisted distributed coupling with both photoresist and etched gratings. Effective indices were determined with an absolute precision as good as 1/2000, adequate for phase matching in parametric devices. Merging thickness and effective index evaluations, the refractive indices of the constituent layers were determined with unprecedented accuracies, in substantial agreement with existing models.
The dynamic properties of low-temperature-grown GaAs (LT- GaAs) depend critically on growth and annealing conditions, such as substrate temperature during the MBE process (Ts), annealing temperature (Ta) and duration ((tau) a). Previous empirical models based on carrier rate- equations introduce parameters, such as carrier lifetime, that cannot be directly correlated with growth and annealing conditions. The Schockley-Read-Hall model we use here introduces deep donor (NDD) and acceptor (NA) concentrations, instead of lifetimes. In LT-GaAs, deep donors are in majority constituted by an As-antisite related defect. The acceptors, in absence of doping, are most likely constituted either by Ga vacancy defects or by the residual doping during MBE growth. The samples studied are grown on GaAs semi-insulating substrates at different Ts and annealed under different conditions. For each sample, we first measure NDD using X-ray diffraction analysis. Then we fit both continuous photoconductivity and pump-probe reflectometry measurements using NA as the only adjustable parameter. From the set of data obtained, we can relate Ts, Ta and (tau) a with NDD and NA. This gives us a way to predict LT-GaAs dynamics from growth and annealing continues. This approach has been used to fabricate ultrafast photoconductive switches showing high sensitivity and good insulation.
We describe a mid-IR photovoltaic detector using InAsSb as active material, grown by MBE on a GaSb substrate. The purpose of this study is to show that quantum detectors can offer an alternative to thermal detectors (pyroelectric or resistive bolometers) for high temperature (near room temperature) operation. With a 9% Sb content, InAsSb is lattice matched to GaSb and thus provides an excellent material quality, with Shockley-Read lifetimes of the order of 200 ns as measured by photoconductive gain measurements as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelength of 5 microns at room temperature. This makes InAsSb an ideal candidate for room temperature detection in the 3-5 microns atmospheric window. Photovoltaic structures are characterized by current voltage characteristics as a function of temperature. Using the absorption value obtained on the test samples, a detectivity of 7X109 Jones at 3.5 micrometers is estimated at a temperature of 250 K, which can easily be reached with Peltier cooling. Considering the photovoltaic spectrum, this leads to a NETD lower than 80 mK.
Standard GaAs/AlGaAs QWIPs are now well established for LWIR detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIPS are close to being optimized. The understanding of detection mechanisms has led to high performance QWIPs working at high temperature (above 77 K). However, as with all quantum detectors, the operating temperature of QWIPs is limited by the thermal current. A new skimmed architecture accommodating this offset has already been demonstrated. The optimization of a skimmed structure requires the modeling procedures and the process, to be adapted. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the development of our QWIPs by recent results on FPAs.
We describe a mid-IR photovoltaic detector using InAsSb as active material, grown by MBE on a GaSb substrate. The purpose of this study is to show that quantum detectors can offer an alternative to thermal detectors for high temperature operation. With a 9 percent Sb content, InAsSb is lattice matched to GaSb and thus provides an excellent material quality, with Shokley-Read lifetimes of the order of 200 ns as measured by photoconductive gain measurements as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelengths as well as time resolved photoconductivity experiments. The band gap of InAsSb corresponds to a wavelength of 5 microns at room temperature. This makes InAsSb an ideal candidate for rom temperature detection in the 3-5 microns atmospheric window. Photovoltaic structures are characterized by current voltage characteristics as a function of temperature. Using the absorption value obtained on the test samples, a detectivity of 7 by 109 Jones can be obtained at a temperature of 250 K, which can easily be reached with Peltier cooling. This leads to a NETD lower than 80 mK.
We discuss here the feasibility of an optical parametric oscillator integrated on a GaAs chip, after reviewing the recent frequency conversion experiments using from birefringence in GaAs/oxidized-AlAs (Alox) waveguides. Recently, phase-matching has been demonstrated for the first time in a GaAs-based waveguide, using form birefringence in multilayer heterostructures GaAs/Alox. Birefringence n(TE)- n(TM) from 0.15 to 0.2 have been measured for different GaAs/Alox waveguides, which is sufficient to phase match mid-IR generation between 3 micrometers and 10 micrometers by difference frequency generation form two near-IR beams. A second step was the observation of parametric fluorescence. Results on parametric fluorescence at 2.1 micrometers will be described, in an oxidized AlGaAs form-birefringent waveguide, consisting of a high-index, strongly birefringent GaAs-Alox core embedded in an AlGaAs cladding. One of the most existing perspectives opened with this new type of nonlinear material is the realization of an optical parametric oscillator on a GaAs chip. To this aim, minimization of losses is the most crucial point. A typical calculated value of this threshold is less than 70 mW for 1 cm-1 losses, and with 90 percent reflection coefficients. The level of losses has been reduced from 2 cm-1 in ridges obtained by a standard reactive ion etching technique, to less than 0.5 cm-1 in ridges realized with a more refined reactive ion etching process, using a 'three layer' mask. There is still a need for an improvement of the waveguide fabrication process, before reaching the oscillation threshold.
Standard GaAs/AlGaAs Quantum Well IR Photodetectors (QWIP) are now well established for long wavelength IR (LWIR) detection. The first advantage of this technology is the duality with the technology of commercia GaAs devices. The realization of large focal plane arrays employing the standard III-V technological processes is already demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications like multispectral detection. In this paper, we present electro-optical results on Middle Wavelength IR (MWIR) detectors. We demonstrate the ability of QWIPs to cover the two spectral ranges. As the operating temperature is crucial for commercial thermal images, we report the temperature dependence of the performances of our MWIR QWIP detector up to 150 K. Performances of QWIPs in the MWIR with the implementation of the new skimming architecture are discussed.
Photoreflectance (PR) measurements are performed on specific structures grown by MBE on different substrate orientations: <111>B, <111>B 2 degree(s)off, <111>A and <100>. A strained In0.2Ga0.8As quantum well is grown in the space charge layer of an undoped GaAs layer. On a polar substrate orientation <111>, the strain induced piezoelectric field in the quantum well modifies the field in the space charge layer. PR spectra recorded in such structures exhibit Franz Keldysh oscillations from which we can measure the internal electric field. The piezoelectric field is then deduced from the comparison between two structures differing only by the presence of the strained quantum well. Experimental values range between 110 and 150 kV/cm, and are used to experimentally determine the piezoelectric constant e14 in In0.2Ga0.8As.
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