Noise in quantum cascade detectors is studied experimentally and theoretically. Measurements were performed in dark
conditions on a quantum cascade detector operating at 14.5 μm, in the very long wave infrared range. To investigate the
signal-to-noise contributions of each intersubband transition involved in the transport, a model of noise has been
developed. It is based on a noise equivalent electrical circuit of the quantum cascade detector. Non-radiative diagonal
transitions (fundamental state to levels of the cascade structure) are identified as dominant contributions to the dark
current and noise in the measured device. Based on these theoretical considerations, new optimized structures for the
very long wave-infrared range are designed and exhibit a noise reduction down to a factor three at optimum responsivity.
Electronic transport in AlGaAs/GaAs THz Quantum Wells Intersubband Photodetectors (QWIPs) exhibits two
different regimes separated by huge discontinuities (up to five orders of magnitude) in the resistivity. They are
interpreted in terms of band structure reorganizations triggered by intersubband impact ionization. We will
analyze and model their in
uence on the electronic transport. The magnitude of the transport modifications is
explained by the small transition energy and the sharpness of the electrons distribution at stake in THz QWIPs.
Measurements under magnetic field or temperature show that the broadening of the electron distribution damps
the effects of impact ionization. Some experimental features of the electronic transport of shorter wavelength
detectors are then reproduced. The use of intersubband impact ionization in THz QWIPs to design high gain
and fast novel detectors is discussed.
Most of optoelectronic semiconductor devices, especially quantum well based ones, make use of a grating to
couple the active layer to free space. To go beyond the simplistic coupling role of the grating we propose a
specifically designed metal-dielectric corrugated interface that squeezes normal incidence light in subwalength
scale, taking advantage of the very active work achieved over the last few years in near field electromagnetism.
This structure coherently combines three surface plasmon engineering tools: Bragg reflection, microcavity, and
grating coupling. These electromagnetic properties are demonstrated experimentally in the gigahertz regime, as
a function of design parameters. Light squeezing is observed down to a quarter of a wavelength.
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
A semiconductor ridge microcavity is designed to generate counterpropagating twin photons by parametric fluorescence.
This device is suitable as a narrow bandwidth source of twin photons at 1.55 μm working at room temperature. A
sensible efficiency improvement due to the presence of the vertical cavity is demonstrated. The degree of frequency
correlation can be controlled through the pump field spatial and spectral profiles.
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.
We demonstrate an integrated semiconductor source of counterpropagating twin photons in the telecom range. A pump
beam impinging on top of an AlGaAs waveguide generates parametrically two counterpropagating, orthogonally
polarized signal/idler guided modes. A 2 mm long waveguide emits at room temperature one average photon pair per
pump pulse, with a spectral linewidth of 0.15 nm. The twin nature of the emitted photons is tested through a time-correlation
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.
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.
A study of the optimisation of the detectivity of a mid infrared double heterostructures photovoltaic detector is proposed. Simple approximate analytic expressions for the dark current are compared with full numerical calculations, and give physical insight on the mechanisms dominating the dark current. The analysis is performed step by step in different structures, from a simple p-n junction to the full double heterostructures. The influence of temperature, barrier band gap energy and absorbing layer thickness in a double heterostructures, doping density in the active region on diffusion and generation-recombination mechanisms is analysed.
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.
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.
We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) <SUP>(2</SUP>) 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.
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 7X10<SUP>9</SUP> 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.
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 10<SUP>9</SUP> 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<SUP>-1</SUP> losses, and with 90 percent reflection coefficients. The level of losses has been reduced from 2 cm<SUP>-1</SUP> in ridges obtained by a standard reactive ion etching technique, to less than 0.5 cm<SUP>-1</SUP> 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.
With the transition of DUV lithography into mass production, the economics of the excimer laser light sources is getting more important. The efforts in the development are directed towards an increase of the laser's repetition rate and output power for higher wafer throughput and an improvement of the component lifetime in order to reduce the cost of laser operation. Here we describe advanced 248 nm and 193 nm laser systems which operate with repetition rates of 2 kHz to be used in conjunction with refractive, partially achromatic refractive and catadioptric lithographic lenses, respectively.
Quantum cascade lasers are new light sources in the mid- infrared (3.5 - 12 micrometer), based on resonant tunneling and optical transitions between quantised conduction band states. Quantum engineering of electronic energy levels and tailoring of the wavefunctions are used to obtain population inversion and optimize the overall laser performance. Advanced structures from a point of view of the quantum design, such as two wavelength emitters, are presented. New waveguide confinement based on surface plasmon are also discussed. In this configuration it is shown that optical confinement can be achieved without the growth of cladding layers. The paper concludes with a general comparative discussion between GaAs/AlGaAs and GaInAs/AlInAs material systems for the new generations of quantum cascade lasers.