I. S. Gregory

Optimization of photomixers and antennas for continuous-wave terahertz emission

We have studied terahertz emission from interdigitated finger photomixers coupled to planar antenna structures. Using both pulsed and continuous-wave excitation, polarization measurements reveal that the antenna design dominates the properties of the radiated output at frequencies below 0.6 THz, while the efficiency at higher frequencies is additionally dependent on the design of the photomixer fingers. We have produced terahertz maps of the device, characterizing the photomixer by measuring the generated power as a function of the excitation position. Together, these measurements have allowed us to understand better the distinct roles of the photomixer and antenna in emission at different frequencies and, hence, independently optimize these components.

High resistivity annealed low-temperature GaAs with 100 fs lifetimes

We demonstrate the critical effect of postgrowth annealing temperature on the properties of low-temperature-grown GaAs. By using annealing temperatures substantially below the 500–600 °C commonly reported, GaAs with high resistivity and with carrier lifetimes as short as 100 fs can be routinely obtained. We discuss the optimum, but different, anneal conditions required for terahertz photoconductive emitters and detectors, and illustrate their use in a continuous-wave system.

Transmittance of a tunable filter at terahertz frequencies

A metallic photonic crystal filter has been demonstrated at terahertz frequencies, with the passband tunable over the range of 365–386 GHz. Tuning is achieved by a relative lateral shift of two metallic photonic crystal plates. Each plate is comprised of two orthogonal layers of gratings and integral mounting lugs. The plates are micromachined from silicon wafers then coated in gold to provide metallic electromagnetic behavior. An insertion loss of 3–7 dB and Q in the range of 20–30 was achieved. A shift of 140μm gave a tuning range of 21 GHz, tuning sensitivity of 150 GHz/mm, and a fractional tuning range of 6%.

Terahertz pulsed imaging with 1.06 μm laser excitation

We report the operation of a pulsed terahertz (THz) imaging system that uses a 120 fs pulse-width laser operating at 1.06 μm, and photoconductive InGaAs emitters and detectors. THz images are presented of biological and nonbiological specimens. The signal-to-noise ratio and frequency range of this system are compared to those obtained from existing pulsed imaging systems based on 0.8 μm excitation.

Resonant dipole antennas for continuous-wave terahertz photomixers

We present measurements and numerical simulations for the performance of resonant dipole antennas used in continuous-wave terahertz photomixers. We demonstrate that center-feed dipoles, with inductively choked electrical feeds, are a good choice for a resonant emitter design, and outperform broadband spiral antennas by a factor of five at a resonant frequency of 0.4THz. Excellent agreement is found between experiment and the predictions of finite element simulations. Furthermore, we show that the dipoles can outperform spirals across frequency bands of over 0.5THz, opening the way for their use in terahertz spectroscopy.

Highly resistive annealed low-temperature-grown InGaAs with sub-500 fs carrier lifetimes

We have optimized low-temperature-grown In0.3Ga0.7As for use in ultrafast photoconductive devices. Using low temperature ex situ annealing techniques, we have produced a photoconductive material that is highly resistive (∼104Ωcm), has sub-500fs carrier trapping lifetimes, and is matched to 1.06μm laser excitation.We have optimized low-temperature-grown In0.3Ga0.7As for use in ultrafast photoconductive devices. Using low temperature ex situ annealing techniques, we have produced a photoconductive material that is highly resistive (∼104Ωcm), has sub-500fs carrier trapping lifetimes, and is matched to 1.06μm laser excitation.

Effect of defect saturation on terahertz emission and detection properties of low temperature GaAs photoconductive switches

We present a study into the properties of terahertz (THz) emission and detection using low temperature grown GaAs photoconductive switches over a range of ex situ anneal temperatures. Our analysis focuses on the effect of defect saturation, which has been confirmed in many experiments. However its effect on the THz emission and detection has so far not been fully investigated. In this letter, we examine the dependence of the radiated THz pulse width (full width at half maximum) upon optical power, and show that the differences in the characteristics with annealing can be theoretically accounted for when defect saturation is taken into account. Defect saturation was found to substantially increase the trapping time of photoexcited electrons, which in turn can cause THz pulse broadening at high optical powers. This effect was found to increase with anneal temperature due to the decrease in defect density. The radiated peak THz amplitude from emitters increases monotonically with increasing optical power acr...

Multi-channel homodyne detection of continuous-wave terahertz radiation

We demonstrate the principle of multichannel spectral data capture using an all-optoelectronic terahertz photomixing system. Two independent laser diodes, each emitting in multiple longitudinal modes, are used to generate and detect over ten terahertz frequencies simultaneously. The spectral composition can be altered by tuning the lasers, and filtering is achieved through the use of resonant antennas and tunable photonic crystal filters. Potential applications include gas phase spectroscopy and explosives detection.

Stand-off Explosives Detection Using Terahertz Technology

Terahertz imaging and spectroscopy has been shown to have the potential to use very low levels of this non-ionising radiation to detect and identify objects hidden under clothing at stand-off distances. In this paper we discuss some of the important factors involved in developing systems for the security industry, and describe our recent work on the development of a proto- type terahertz stand-off detection system. Using this system we demonstrate the spectroscopic detection of concealed explosives at a stand-off distance of 1 m, both real time, in reflection, and under normal atmospheric conditions. We believe the technique has the potential for further development towards a practical system for the detection of suicide bombers and mobile subjects.

Enhanced Terahertz Receiver Using a Distributed Bragg Reflector Coupled to a Photoconductive Antenna

We report the use of a terahertz (THz) receiver fabricated using a photoconductive antenna mounted upon a distributed Bragg reflector (DBR). It is shown that the relative performance of the device is in accordance with the reflectivity curves of the DBR. At maximum reflectivity the device improved upon the reference THz signal by up to 30%. In addition, we characterize the saturation of the device with laser pump power and it is shown that the device exhibits an improved optical efficiency when compared to the reference receiver.