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Most people with diabetes suffer some deterioration of the feet. Diabetic foot syndrome causes ulceration in about 15% of cases and such deterioration leads to amputation in about 2.5% of diabetic patients, diminishing their quality of life and generating extraordinary costs for patients and public health systems. Currently, there is no objective method for the detection of diabetic foot syndrome in its early stages. We propose terahertz imaging as a method for the evaluation of such deterioration. This screening method could aid the prevention and medical treatment of this condition in the future.

The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy measurements, coupled with Monte Carlo simulations of THz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump THz-probe spectroscopy, and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, we have demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch THz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast THz photonic devices.

We have developed a terahertz radiation detector that measures both the amplitude and polarisation of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric field components of an arbitrary polarised electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe + ion-implanted InP. Polarisation-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum.PACS numbers: 07.57.Kp , 07.57.Pt, 07.60.Fs, 42.25.Ja, 71.55.Eq , 78.20.Ek, 78.20.Fm The far-infrared, or terahertz (THz), region of the electromagnetic spectrum encompasses the energy range of many collective processes in condensed matter physics and macromolecular chemistry. However, in the past this spectral region has been relatively unexplored owing to a lack of bright radiation sources and appropriate detectors. The technique of THz time domain spectroscopy (TDS), 1,2 which has developed rapidly as a result of advances in ultra-short pulsed laser technology, now provides a very sensitive probe across the THz band. TDS is currently an invaluable tool in condensed matter physics 3,4,5 and macromolecular chemistry. 6,7To date THz-TDS techniques have relied on linearly polarised emitters and detectors. However, for spectroscopy of birefringent and optically active materials it is also important to measure the polarisation state of radiation before and after it has interacted with the material. Here we report on a detector that enables such a THz-TDS system to be realised.Vibrational circular dichroism (VCD) spectroscopy is a new technique which has substantial potential in the fields of macromolecular chemistry and structural biology. 8 Akin to the established technique of (ultraviolet) circular dichroism, VCD is used to analyse the structure of chiral molecules. It is predicted that VCD will be more powerful than conventional circular dichroism for stereo-chemical structure determination. 8 However the technique is currently limited by insensitive and narrow band spectrometers.Of particular interest to biochemists is the structure and function of proteins and nucleic acids. These chiral biomolecules have vibrational and librational modes in the THz region and the THz optical activity of these modes are starting to be studied experimentally. 9,10 THz frequency VCD is already finding application in fields as distinct as biochemical research 11 and astrobiology. 10 In the future the ability to perform VCD using a polarisation sensitive THz-TDS technique should enhance the bandwidth and sensitivity of measurements, and allow dynamic time-resolved studies to be performed.In order to perform polarisation sensitive THz-TDS, it is necessary to be able to measure two (preferably orthogonal) electric field components of a terahertz transient. Theoretically it is possible to do this using a ...

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photo-excitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analysed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (< 40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (> 40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes.