E. Castro-Camus

Extraction of the anisotropic dielectric properties of materials from polarization-resolved terahertz time-domain spectra

The anisotropic complex dielectric properties of materials in the terahertz band is a topic that has attracted considerable attention recently in the fields of physics, chemistry and biochemistry. The mathematical formalism for analysing polarization-resolved terahertz time-domain data is presented, and particular cases including birefringence, optical activity and circular dichroism are discussed.

Beating the wavelength limit: three-dimensional imaging of buried subwavelength fractures in sculpture and construction materials by terahertz time-domain reflection spectroscopy

We use reflection terahertz spectroscopy to locate and produce three-dimensional images of air gaps between stones that resemble fractures, even of subwavelength thicknesses. This technique is found to be promising tool for sculpture and building damage evaluation as well as structural quality control in other dielectric materials.

Quality control of leather by terahertz time-domain spectroscopy

We use terahertz time-domain spectroscopy, combined with effective-medium theory, to measure the moisture content and thickness of leather simultaneously. These results demonstrate that this method could become a standard quality control test for the industrial tanning process.

Fabrication of gradient-refractive-index lenses for terahertz applications by three-dimensional printing

In this article we present the design, fabrication, and characterization of gradient-refractive-index lenses for terahertz (THz) applications. The fabrication was performed by three-dimensional printing. These new low-cost lenses were tested using THz time-domain spectroscopy and imaging in order to measure their optical properties. The results show a focusing capacity within the diffraction limit for frequencies below 700 GHz.

Attenuated Total Reflection Terahertz Time-Domain Spectroscopy: Uncertainty Analysis and Reduction Scheme

Terahertz time-domain spectroscopy (THz-TDS) in the attenuated total reflection (ATR) configuration is ideally suited to characterize highly absorptive media. In this paper, we analyze the impact of random phase and amplitude errors and prism misalignment on the optical constants extracted from the ATR THz-TDS measurements. Analytical models together with empirical data suggest that, among those factors, small prism misalignment has a relatively strong impact on the optical constants. We propose an alternative configuration for ATR THz-TDS that significantly reduces the impact of prism misalignment between the reference and sample measurements. As a result, this configuration facilitates repetitive reference and sample scans to mitigate the effect from long-term signal drifts. Based on this proposed method, the measured optical constants of distilled water and pure ethanol have a significantly reduced uncertainty. The presented analysis and method can be used to improve the measurement accuracy in standard ATR THz-TDS setups.

Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers

A semi-classical Monte Carlo simulation of carrier dynamics in photoconductive detectors of terahertz (THz) radiation is presented. We have used this simulation to elucidate the importance of carrier trapping in the operation of photoconductive detectors. Simulations of the detection of single-cycle THz pulses by photoconductive antennas based on GaAs with trap densities between 2 × 10 17 and 2 × 10 18 cm −3 are presented. We show that the high frequency (>1 THz) spectral response of photoconductive devices decreases with increasing excitation fluence. Our simulations reveal that this effect is a direct consequence of the saturation of trapping centres (Some figures may appear in colour only in the online journal)

Highly sensitive terahertz dielectric sensor for small-volume liquid samples

We present a resonator-based sensor for the measurement of the refractive index of dielectric liquid samples. The proposed sensor operates on the basis of an electromagnetic resonance between a thin metallic grating and a reflecting ground plane. The fluid whose refractive index is to be measured fills the region between the metallic grating and the ground plane and causes a considerable shift in the resonance frequency (>500 GHz/RIU). The sensor has a relatively simple structure; therefore, it can be manufactured economically on industrial scales.

Roadmap on optical sensors

Sensors are devices or systems able to detect, measure and convert magnitudes from any domain to an electrical one. Using light as a probe for optical sensing is one of the most efficient approaches for this purpose. The history of optical sensing using some methods based on absorbance, emissive and florescence properties date back to the 16th century. The field of optical sensors evolved during the following centuries, but it did not achieve maturity until the demonstration of the first laser in 1960. The unique properties of laser light become particularly important in the case of laser-based sensors, whose operation is entirely based upon the direct detection of laser light itself, without relying on any additional mediating device. However, compared with freely propagating light beams, artificially engineered optical fields are in increasing demand for probing samples with very small sizes and/or weak light-matter interaction. Optical fiber sensors constitute a subarea of optical sensors in which fiber technologies are employed. Different types of specialty and photonic crystal fibers provide improved performance and novel sensing concepts. Actually, structurization with wavelength or subwavelength feature size appears as the most efficient way to enhance sensor sensitivity and its detection limit. This leads to the area of micro- and nano-engineered optical sensors. It is expected that the combination of better fabrication techniques and new physical effects may open new and fascinating opportunities in this area. This roadmap on optical sensors addresses different technologies and application areas of the field. Fourteen contributions authored by experts from both industry and academia provide insights into the current state-of-the-art and the challenges faced by researchers currently. Two sections of this paper provide an overview of laser-based and frequency comb-based sensors. Three sections address the area of optical fiber sensors, encompassing both conventional, specialty and photonic crystal fibers. Several other sections are dedicated to micro- and nano-engineered sensors, including whispering-gallery mode and plasmonic sensors. The uses of optical sensors in chemical, biological and biomedical areas are described in other sections. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed. Advances in science and technology required to meet challenges faced in each of these areas are addressed, together with suggestions on how the field could evolve in the near future.

Modelling the carrier dynamics of semiconductors to understand their terahertz emission (Conference Presentation)

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics on bulk InAs, InGaAs and GaAs that leads to terahertz emission after ultrafast photoexcitation. This detailed model has allowed us to understand various aspects of the THz emission process, including the near-field distribution which has been experimentally observed, the role of the excess excitation photon energy, and the relative importance of the surface field driven, diffusive (photo-Dember) and ballistic currents. In order to understand the near-field emission we coupled a finite-difference time-domain routine to the carrier dynamics simulation, by doing this, we were able to analyse the near terahertz field emission caused by the motion of such carriers even when the excitation is performed at normal incidence. We found that both the current parallel, which has traditionally been assumed not to take part in the emission, and normal to the interface take a relevant role in the terahertz generation. We performed another set of simulations for different bandgaps and excitation-photon energies in order to compare the emission power of all three semiconductors as function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their motilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

Diabetic foot early diagnosis and statistical analysis by spectral terahertz reflection images

In this work, the development of a technique for the early diagnosis of diabetic foot using terahertz spectroscopic images is presented. The degree of hydration of the skin on the sole of the foot of diabetic and non-diabetic subjects was obtained and related to the degree of deterioration. The hydration information was coded in three- color (red, yellow and greed) images which allow to easily identify areas in risk of ulceration. The hydration images together with the three-color images represent a quantitative indicator of the deterioration caused by the diabetic foot syndrome.