Effect of ion implantation on quantum well infrared photodetectors

Ion implantation is a postgrowth processing technique which, when combined with annealing, can be used to tune the absorption wavelength of quantum well devices. We have implanted and annealed, three different quantum well infrared photodetector structures, and measured the absorption spectra of the samples by Fourier transform spectroscopy. The peak absorption wavelength shift of each structure has been calculated as a function of diffusion length by simulating the diffusion processes. We found different diffusion rates for the structures and attribute this to different numbers of as-grown defects. Our results indicate that agglomeration of single defects into defect clusters limits the ability of ion implantation to tune the wavelength of a structure with a higher number of as-grown defects. Thus, a structure with the lowest number of as-grown defects is most useful for fabricating a multi-color quantum well photodetector by ion implantation, because in this case ion implantation can enhance the diffusion rate considerably leading to large red- shift in peak absorption wavelength.     

Terahertz emission from quantum cascade lasers in the quantum Hall regime: evidence for many body resonances and localization effects

A terahertz quantum cascade laser, operating at lambda=159 microm and exploiting the in-plane confinement arising from perpendicular magnetic field, is used to investigate the physics of electrons confined on excited subbands in the regime of a large ratio of the magnetic field confinement energy to the photon energy. As the magnetic field is increased above about 6 T, and the temperature lowered below 20 K, the devices are characterized by a very low threshold current density, with values as low as J(th)=1A/cm(2), and an increase of gain by five times the low field value. We show that, as with the quantum Hall effect, the key physical process is the localization of the carriers. Evidences for resonant electron-electron scattering processes are directly obtained from light intensity and transport measurements.     

Integrated injection seeded terahertz source and amplifier for time-domain spectroscopy

We used a terahertz (THz) quantum cascade laser (QCL) as an integrated injection seeded source and amplifier for THz time-domain spectroscopy. A THz input pulse is generated inside a QCL by illuminating the laser facet with a near-IR pulse from a femtosecond laser and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime.     

Application of terahertz quantum-cascade lasers to semiconductor cyclotron resonance

Quantum-cascade lasers operating at 4.7, 3.5, and 2.3 THz have been used to achieve cyclotron resonance in InAs and InSb quantum wells from liquid-helium temperatures to room temperature. This represents one of the first spectroscopic applications of terahertz quantum-cascade lasers. Results show that these compact lasers are convenient and reliable sources with adequate power and stability for this type of far-infrared magneto-optical study of solids. Their compactness promises interesting future applications in solid-state spectroscopy.     

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 sample's 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.     

Phase-locking of a 2.5 THz quantum cascade laser to a frequency comb using a GaAs photomixer

We report the heterodyne detection and phase locking of a 2.5?THz quantum cascade laser (QCL) using a terahertz frequency comb generated in a GaAs photomixer using a femtosecond fiber laser. With 10?mW emitted by the QCL, the phase-locked signal at the intermediate frequency yields 80?dB of signal-to-noise ratio in a bandwidth of 1?Hz.     

On-chip pulsed terahertz systems and their applications

The generation and detection of guided wave terahertz (THz) transients in microstrip transmission line systems is demonstrated at both room and cryogenic (∼ 4 K) temperatures using thin film low-temperature-grown GaAs (LT-GaAs) switches, excited by a 100 fs, 80 MHz repetition rate pulsed Ti:Sapphire laser. The characterisation of passive filter elements formed in the microstrip line is reported, together with their response to the application of dielectric loads of varying thickness at room temperature.     

Metallic behavior in dilute two-dimensional hole systems

We have studied the metallic behavior in low-density two-dimensional p-GaAs systems, close to the apparent metal-insulator transition. Two observations are made concerning the origins of the metallic-like behavior. Within a given sample the strength of the metallic behavior is almost independent of the asymmetry of the confining potential, and is predominantly determined by the low-temperature resistivity (i.e., by k(F)l). In all our samples we find that at low densities, close to the transition from insulating to metallic behavior, the fractional decrease in conductivity with increasing temperature scales as T/T(F).