M. Lachab

Terahertz vibrational absorption spectroscopy using microstrip-line waveguides

We demonstrate that terahertz microstrip-line waveguides can be used to measure absorption spectra of polycrystalline materials with a high frequency resolution (∼2 GHz) and with a spatial resolution that is determined by the microstrip-line dimensions, rather than the free-space wavelength. The evanescent terahertz-bandwidth electric field extending above the microstrip line interacts with, and is modified by, overlaid dielectric samples, thus enabling the characteristic vibrational absorption resonances in the sample to be probed. As an example, the terahertz absorption spectrum of polycrystalline lactose monohydrate was investigated; the lowest lying mode was observed at 534(±2) GHz, in excellent agreement with free-space measurements. This microstrip technique offers both a higher spatial and frequency resolution than free-space terahertz time-domain spectroscopy and requires no contact between the waveguide and sample.

Efficient prediction of terahertz quantum cascade laser dynamics from steady-state simulations

Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light–current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs.

Terahertz inverse synthetic aperture radar imaging using self-mixing interferometry with a quantum cascade laser

We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit.

Generation of Bessel beams using a terahertz quantum cascade laser

We report the generation of Bessel beams using polytetrafluoroethene conical lenses and a quantum cascade laser emitting at 2.8 THz. The formation of a central beam spot that retains its size over distances exceeding the characteristic Rayleigh range is demonstrated, and the power transport properties of these beams are compared with those obtained using parabolic reflectors. These lenses could provide an attractive alternative to parabolic reflectors for terahertz imaging applications where a large depth of focus and/or efficient and controllable coupling of radiation onto a small target are desirable.

Radiative recombination spectra of p-type δ-doped GaAs∕AlAs multiple quantum wells near the Mott transition

Photoluminescence (PL) spectra of beryllium δ-doped GaAs∕AlAs multiple quantum wells are studied over a range of doping concentrations. Possible mechanisms for carrier recombination, both above and below the Mott metal-insulator transition, are discussed. In 15nm width Be δ-doped GaAs∕AlAs quantum wells, it is found that the Mott transition can be observed if the acceptor concentration (NBe)⩾3×1012cm−2. At doping concentrations near the Mott transition band the PL spectra are dominated by excitons-bound-to-acceptor impurity recombinations. Above the Mott transition, the radiative recombination of free electrons with a two-dimensional hole gas is found to be the dominant recombination mechanism.

Impurity bound-to-unbound terahertz sensors based on beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells

Beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells (MQWs) were designed and fabricated for selective sensing of terahertz radiation. Optical characterization of the structures by photoreflectance spectroscopy has indicated a presence of built-in electric fields–from 18 up to 49kV∕cm depending on the structure design–located mainly in the cap and buffer layers, but do not penetrating into the MQWs region. Terahertz sensing under normal incidence via impurity bound-to-unbound transitions is demonstrated in photocurrent experiments within 0.6–4.2THz in silicon-doped MQWs and 3.5–7.3THz range in beryllium-doped MQWs at low temperatures.

Measurement and analysis of the diffuse reflectance of powdered samples at terahertz frequencies using a quantum cascade laser

We report terahertz (THz) diffuse reflectance measurements of bulk powdered samples at a frequency of 2.83 THz using a narrowband quantum cascade laser. Samples studied comprise polydisperse powders with absorption coefficients extending over two orders of magnitude from ∼3 cm(-1) to >200 cm(-1). Diffuse reflectance measurements are used to obtain the effective absorption coefficient of these samples from the backscattering cross-section, predicted under the quasi-crystalline approximation (QCA) in the T-matrix formulation and in conjunction with the Percus-Yevick pair distribution function. Results are compared with effective absorption coefficients obtained from THz time-domain spectroscopy measurements on pressed pellet samples, and show good agreement over the range of effective absorption coefficients studied. We observe that the backscattering cross-section predicted under the QCA is strongly dependent on both the real and imaginary components of the complex permittivity of the sample, and we show that reliable determination of the absorption coefficient from diffuse reflectance measurements therefore requires knowledge of the samples refractive index. This work demonstrates the applicability of diffuse reflectance measurements, using a THz frequency quantum cascade laser, to the high-resolution spectroscopic analysis of bulk powdered samples at THz frequencies.

Dopant Migration-Induced Interface Dipole Effect in n-Doped GaAs/AlGaAs Terahertz Detectors

A heterojunction interfacial workfunction internal photoemission (HEIWIP) terahertz detector with ~1times1018cm-3 n-type doped GaAs emitters in a multilayer GaAs/Al0.13Ga0.87 As heterostructure is presented. The detection mechanism is based on free carrier absorption with a broad response extending to ~ 5.26 THz (57 mum), corresponding to an effective workfunction of ~ 21.8 meV, which is much smaller than the offset expected for an Al fraction of x = 0.13 at a 1times1018 cm-3 doping. This is attributed to a reduction of the conduction band offset by interface dipole formation between the accumulated negative charges at the interface states and migrated positively charged donors in the barrier. The device has a peak responsivity of 0.32 A/W at ~ 26 mum at 5 K. It is demonstrated that the dopant migration-induced interface dipole effect can be used to extend the zero response threshold frequency (f 0) of n-type HEIWIP detectors.

Monolithic integration of low-temperature-grown GaAs with a two-dimensional electron gas

A scheme is presented for the monolithic integration of low-temperature- grown gallium arsenide (LT-GaAs) with a two-dimensional electron gas. Pulsed terahertz (THz) emission (with a 3 THz bandwidth) is demonstrated from a LT-GaAs layer grown epitaxially above a GaAs–AlGaAs heterojunction containing a two-dimensional electron gas, using photoconductive antennae patterned on the LT-GaAs, and excited with 800 nm, 12 fs laser pulses. Electrical transport measurements of the two-dimensional electron gas at cryogenic temperatures, and in high magnetic fields, reveal quantum Hall behaviour. The THz emission characteristics of the LT-GaAs, and the transport properties of the two-dimensional electron gas demonstrate that this integrated approach compromises neither the quality of the LT-GaAs nor the two-dimensional electron gas, and indicate the potential of this monolithic integration for sub-picosecond electronic measurements of mesoscopic systems.

Terahertz quantum cascade lasers operating up to 178 K with copper metal-metal waveguides

We report terahertz quantum cascade lasers, with a three-quantum-well active region design and copper metal-metal waveguides, operating up to a record temperature of 178 K. Their performance is compared to devices with gold metal-metal waveguides.