The optical properties of GaBi x As 1-x (0.04 < x < 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T 2 states of the Bi atoms.

GaBiAs layers have been grown by molecular beam epitaxy at low (270–330°C) temperatures and were characterized by several experimental techniques. It was shown that the spectral photosensitivity cutoff wavelength reaches ∼1.4μm when the growth temperature is as low as 280°C. Optical pump–terahertz probe measurements made on these layers have evidenced that the electron trapping time decreases with decreasing growth temperature from 20 to about 1ps. GaBiAs layers were used for manufacturing photoconductive terahertz emitters and detectors, which, when excited with Ti:sapphire laser pulses, have demonstrated a signal bandwidth of 3THz.

Terahertz radiation from differently doped n- and p-type InAs crystal surfaces was investigated by time-resolved measurement. Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of approximately 1016–1017cm−3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in this layer.

Compact and tunable semiconductor terahertz sources providing direct electrical control, efficient operation at room temperatures and device integration opportunities are of great interest at the present time. One of the most well-established techniques for terahertz generation utilises photoconductive antennas driven by ultrafast pulsed or dualwavelength continuous wave laser systems, though some limitations, such as confined optical wavelength pumping range and thermal breakdown, still exist. The use of quantum dotbased semiconductor materials, having unique carrier dynamics and material properties, can help to overcome limitations and enable efficient optical-to-terahertz signal conversion at room temperatures. Here we discuss the construction of novel and versatile terahertz transceiver systems based on quantum dot semiconductor devices. Configurable, energy-dependent optical and electronic characteristics of quantum-dot-based semiconductors are described, and the resonant response to optical pump wavelength is revealed. Terahertz signal generation and detection at energies that resonantly excite only the implanted quantum dots opens the potential for using compact quantum dot-based semiconductor lasers as pump sources. Proof-ofconcept experiments are demonstrated here that show quantum dot-based samples to have higher optical pump damage thresholds and reduced carrier lifetime with increasing pump power.

Abstract:Generation and measurement of ultrashort, sub-picosecond pulses of electromagnetic radiation with their characteristic Fourier spectra that reach far into terahertz (THz) frequency range has recently become a versatile tool of the far-infrared spectroscopy and imaging. This technique -THz time-domain spectroscopy, in addition to a femtosecond pulse laser, requires semiconductor components manufactured from materials with a short photoexcited carrier lifetime, high carrier mobility, and large dark resistivity. Here we will review most important developments in the field of investigation of such materials. Main characteristics of low-temperature-grown or ion-implanted GaAs and semiconducting compounds sensitive in the wavelength ranges around 1 µm and 1.5 µm will be surveyed. The second part of the paper is devoted to the effect of surface emission of THz transients from semiconductors illuminated by femtosecond laser pulses. Main physical mechanisms leading to this emission as well as their manifestation in various crystals will be described.

Nonstoichiometric GaAs obtained by implantation with 2 MeV arsenic ions at 1015 cm−2 dose is studied. As-implanted samples show a &lt;200 fs lifetime of photocarriers and low resistivity due to hopping, with mobility less than 1 cm2/V s. Annealing of the samples at 600 °C leads to substantial recovery of postimplant damage, as seen from Rutherford backscattering channeling spectra and mobility increase to about 2000 cm2/V s, but photocarrier lifetime is still about 1 ps. These parameters are similar to those of low-temperature GaAs annealed at 600 °C, and make arsenic implanted GaAs an interesting material for optoelectronic applications.

Spectral dependences of the THz radiation from the laser-illuminated surfaces of InAs and InSb have been investigated experimentally at high optical fluences for the laser wavelengths ranging from 0.6 to 2μm. Efficient THz generation was discovered in the excitation range around 1.6μm. The influence of the intervalley scattering was clearly evidenced. The energy position of the subsidiary conduction band valleys was evaluated from this study to be equal 1.08 and 0.53 eV for InAs and InSb, respectively. It has been concluded that THz emission at high excitation fluencies is dominated by the shift current effect.

Single-and multi-quantum well (QW) structures of Ga(AsBi)/GaAs with up to 10% Bi were grown by molecular beam epitaxy (MBE) at 300-330°C substrate temperature. The photoluminesce measurements of QW structures demonstrated room temperature emission up to wavelengths of ∼1.43 μm. In the structures obtained using a combined growth approach -an active layer with three QWs with ∼6% Bi was grown by MBE, whereas (AlGa)As claddings were grown by the metal organic vapour phase epitaxy technique -room temperature lasing at 1060 nm was documented.Introduction: It is expected that the introduction of novel dilute bismide alloys will lead to the achievement of efficient laser diodes at telecom and longer wavelengths with reduced power consumption, thus providing the benefits of a wide range of applications. For Ga(AsBi) alloys with Bi content larger than 10% grown on GaAs substrates, e.g. very large valence band spin-orbit-splitting exceeding the energy band gap E g can be achieved [1], which should help to suppress inter-valence-band absorption and dominant Auger recombination processes in telecom lasers. Optically pumped Ga(AsBi) lasers have already demonstrated promising device characteristics such as reduced temperature dependence of the lasing wavelength [2]. The first Ga (AsBi)/(AlGa)As single-quantum well (SQW) electrical injection laser was presented in [3]. The laser structure with 2.2% Bi in the well layer was grown by metal organic vapour phase epitaxy (MOVPE), and a room temperature emission wavelength of ∼947 nm was achieved. In this Letter, we present investigations of Ga(AsBi)/GaAs QWs and laser diode structures grown by molecular beam epitaxy (MBE). Strong photoluminescence (PL) signals have been observed in QWs with up to 10% Bi in the well region; room temperature lasing at the wavelength of 1060 nm was documented from the diode with three QWs with ∼6% Bi.