Vincent Berger

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.

Advantages of quantum cascade detectors

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.

Quantum cascade detection

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.

Third-order mode laser diode for twin photon generation

A third-order-mode-emitting laser diode is demonstrated. The AlGaAs/GaAs hetero-structure is engineered to emit a photon pair through intra-cavity modal phase-matched parametric down-conversion. Device operations and twin photon generation experimental issues are discussed.

Parametric processes in GaAs/Alox structures

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.

Mid-IR InAsSb photovoltaic detectors

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.

Magnetotransport in very long wave infrared quantum cascade detectors: Analyzing the current with and without illumination

Current measurements of current have been performed on a very long wave infrared quantum cascade detector under magnetic field under both dark and light conditions. The analysis of dark current as a function of temperature highlights three regimes of transport. Under illumination, the model developed is in agreement with the oscillatory component of the experimental magnetophotocurrent. It allows to identify the key points controlling the electronic transport: crucial role of extraction, location of ionized impurities and scattering mechanisms involved in the structure. This work is valuable for the future conception of high-performance quantum cascade detectors in the infrared range.

Modal birefringences measurement of multilayer multimodal AlGaAS/AlAs waveguides

We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective AlAs oxidation. By end-fire coupling linearly-polarized laser pulses into ridge waveguides, both forward- and back-propagating eigenpolarizations were excited due to Fresnel reflection at the output facet. Several TE-TM pairs of counterpropagating modes then interact through the quadratic nonlinearity, giving rise to interference of SESHG fields. With a single image acquisition of the SESHG far field by a CCD camera, we could evaluate the modal birefringences between all the excited TE- TM mode pairs at the fundamental frequency. This simple approach led us to estimate form-birefringence of our multilayer quadratic waveguides with the high accuracy required by optimized phase-matched interactions in parametric generators and oscillators. This technique is a valuable complement to standard m-line effective index evaluation, and a versatile one-shot tool for waveguide diagnostics.

X-ray and optical characterization of multilayer semiconductor waveguides

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.

Noncryogenic quantum detection in the mid-IR using InAsSb photovoltaic structures

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.