Philippe Bois

Intersubband Transitions in Quantum Wells

The potential energy profile in semiconductor heterostructures can now be controlled in a fascinating way that could barely be dreamed of twenty years ago 1. When dealing with interband optical transitions, additional features related to electron-hole interactions (see for instance exciton descriptions in this book) are coming into play and the one-electron wavefunctions and energy levels may fail to describe or predict experimental results. Moreover, the quantization energy is usually small compared to the forbidden band gap, so that typical interband transitions always occur in the same energy range for a given materials pair. On the contrary, intersubband transitions (ISBT) are very sensitive to the exact potential profile and transitions have been observed at wavelengths between lμm and 100μm. In addition, they can be quantitatively described by a simple formalism based on one-electron approaches and many-body effects usually appear as small corrections only. Since 19852, many devices have been designed according to this quantum engineering and have shown unsurpassed properties3. Various materials have been successfully used for these quantum well (QW) heterostructures: GaAs/AlGaAs, InP/InGaAs/InAlAs, Si/SiGe, D/VI compounds…We will focus here on the GaAs/AlGaAs system which has been the most widely studied. First, the calculation of the ISBT matrix element will evidence two major characteristic properties: the optical transitions take advantage of giant dipoles but must verify in the same time a rather drastic selection rule. Then, examples will be given in different fields of application: detection, modulation and emission. Some interesting aspects of coupling and propagation in these structures involve a photon mode density alteration. Finally, a detailed study of second order non linearities will exemplify the beauty of quantum engineering for improving optical properties.

Observation of nonlinear optical rectification at 10.6 μm in compositionally asymmetrical AlGaAs multiquantum wells

We report the first experimental evidence of a nonlinear optical effect due to intersubband transitions in compositionally asymmetrical multiquantum wells. The effect is detected as an optical rectification signal appearing at the structure terminals when irradiated by a continuous 10.6 μm CO2 laser. The net electro‐optical coefficient of the structure is found to be 7.2 nm/V which is more than three orders of magnitude higher than for bulk GaAs. The results are in good agreement with theoretical predictions.

Detailed analysis of second‐harmonic generation near 10.6 μm in GaAs/AlGaAs asymmetric quantum wells

We report on the observation of resonant intersubband second‐harmonic generation in asymmetric GaAs/AlGaAs quantum wells using a cw or Q‐switched tunable CO2 laser as the pumping source. The dependence of the second‐harmonic intensity with the pump photon wavelength is presented for the first time. A Lorentzian‐like second‐harmonic line shape is found with a maximum at 10.9 μm and a linewidth of 0.4 μm (4.1 meV). These results are in good agreement with theoretical predictions. The expected quadratic dependence of the second‐harmonic conversion efficiency with pump intensity is well verified for intensities up to 150 kW/cm2. The calibrated second‐harmonic power reaches 0.13 μW for a cw pump power of 0.8 W. The value of 7.2×10−7 m/V deduced for the second‐order nonlinear susceptibility is about 1900 times greater than that found in bulk GaAs.

Capture time versus barrier thickness in quantum‐well structures measured by infrared photoconductive gain

Photoconductive gain measurements in quantum‐well (QW) infrared detectors are used to determine the variation of the capture time of electrons in QWs as a function of barrier thickness. The capture time is shown to be proportional to the multi‐quantum‐well period, which is consistent with a quantum mechanical description of the capture process. The measured values are far higher than the ones measured by time‐resolved photoluminescence, ranging from 8 to 150 ps, depending on the applied electric field and barrier thickness. The reasons for this discrepancy are discussed.

Switchable bicolor infrared detector using an electron transfer infrared modulator

In this study, we have realized a bicolor switchable detector, in which tunneling between two quantum well detectors of different widths is used to populate or deplete their ground level. The behavior of the bicolor detector is simulated using a self‐consistent model, which shows that under electric field, only the downstream well detects. The performance of the sample as a modulator and as a bicolor detector are analyzed in a Fourier transform infrared spectrometer.

Electron relaxation time measurements in GaAs/AlGaAs quantum wells: Intersubband absorption saturation by a free‐electron laser

The intersubband absorption saturation in GaAs/AlGaAs quantum wells as a function of the incident power has been measured, using picosecond micropulses with a power density up to 1 GW/cm2 delivered by a free‐electron laser. The absorption in a sample with a bound‐to‐bound transition was compared to the absorption in a sample with a bound‐to‐resonant transition, and it was found that the electron relaxation time in the bound‐to‐bound transition is about four times shorter than for the bound‐to‐resonant transition.

THALES long-wave advanced IR QWIP cameras

THALES have developed for volume manufacture two high performance low cost thermal imaging cameras based on the THALES Research & Technology (TRT) 3rd generation gallium arsenide long wave Quantum Well Infrared Photodetector (QWIP) array. Catherine XP provides 768 x 575 CCIR video resolution and Catherine MP provides 1280 x 1024 SXGA video resolution. These compact and rugged cameras provide 24-hour passive observation, detection, recognition, and identification in the 8 to 12μm range, providing resistance to battlefield obscurants and solar dazzle, and are fully self-contained with standard power and communication interfaces. The cameras have expansion capabilities to extend functionality (for example automatic target detection) and have network battlefield capability. Both cameras benefit from the high quantum efficiency and freedom from low frequency noise of the TRT QWIP, allowing operation at 75 K, low integration times and non-interruptive non-uniformity correction. The cameras have successfully reached technology readiness level 6/7 and have commenced environmental qualification testing in order to complete the development programmes. These latest additions to the THALES Catherine family provide high performance thermal imaging at an affordable cost.

QWIP products and building blocks for high-performance systems

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.

State of the art of quantum cascade photodetectors

The Quantum Cascade Detector (QCD) is a multiple quantum well photodetector working at low bias or zero bias. It has a zero dark current occurring at 0V, together with a high photovoltaic photoresponse, since the QCD does not need any applied field to improve the collection of electrons. QCDs have been tested at various wavelengths, from short wavelengths (1.5 microns) up to THz waves, through the entire infrared spectrum (middle and long wavelengths). Theory of transport in QCD is now well established, and leads to accurate calculations of current and noise in QCDs, with a very good agreement with experimental results. Latest results and state of the art of performances of QCDs are presented.

Spectral cross-talk in dual-band quantum well infrared detectors

We propose a general definition of the spectral cross-talk in dual-band infrared (IR) photodetectors, based on the common information carried by the spectral channels. This definition includes detector characteristics as well as scene characteristics and can be applied to any real configuration. We use it to evaluate narrowband and wideband quantum well infrared photodetector structures and set up their interest for dual-band imaging. The spectral cross-talk is negligible for interband IR (3–5μm∕8–12μm) applications and can be minimized for intraband IR (8–12μm) applications by properly tailoring the responsivity peaks.