As continuous wave (cw) THz spectroscopy advances rapidly, its high potential for sensing and non-destructive testing is becoming increasingly apparent. This is demonstrated in two recent developments: First, I will present our novel terahertz receivers for frequency-domain spectroscopy based on rhodium-doped InGaAs grown by molecular beam epitaxy. These new devices provide a peak dynamic range of 130 dB, an 18 dB improvement over the state of the art. Secondly, I will present a compact terahertz spectroscopy system with a measurement rate of 1 kHz, made possible by a single photonic integrated circuit that acts as the optical driver engine.
These new results demonstrate the excellent performance and flexibility of frequency-domain spectroscopy, paving the way for compact and task-specific terahertz systems for science and industry.
We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene with a computer numerical control machine. It acts as a shearing element that generates two diffraction orders, thereby, creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.
We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene (HDPE) with a computer numerical control (CNC) machine. It acts as a shearing element that generates two diffraction orders, thereby creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.
Diabetic foot syndrome, a consequence of Diabetes is, after traumatic accidents, the main cause of amputations. Currently there is no established method for its early diagnosis and prevention of its consequences. Terahertz imaging is capable of mapping the cutaneous hydration, an indicator of the diabetic foot deterioration. We performed imaging on 198 patients including non-diabetic, controlled and deteriorated diabetics, which is, as far as we know, the largest human population ever imaged with terahertz. Our terahertz measurements combined with neurological and vascular assessment of the patients indicates that the dehydration is related to neuropathy without a significant vascular cause.
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.
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.
We use terahertz time domain imaging for the evaluation of the effects of skin-moisturizers in vivo. We evaluate three principal substances used in commercial moisturizers: glycerin, hyaluronic acid and lanolin. We image the interaction of the forearm with each of the substances taking terahertz spectra at sequential times. With this, we are able to measure the effect of the substances on the hydration level of the skin in time, determining the feasibility of using THz imaging for the evaluation of the products and their effects on the hydration levels of the skin.
The design, fabrication and characterization of space-variant Pancharatnam-Berry phase optical elements is presented for the terahertz regime (THz). These PBOEs are made out of polystyrene and were fabricated by commercially available three-dimensional printers, providing a simple and inexpensive solution for the generation of THz vector beams. The polarization structure was characterized by using a THz time-domain imaging system. These devices can find applications in future THz technologies and provide new tools for the study of polarization morphologies
We present a straight forward and practical method for joining pneumatically floated optical tables with no
previous preparation. In order to demonstrate this method we joined two optical tables in an uncentered "T-shape"
using twenty four stainless steel plates (SSP), and used a Michelson interferometer to compare the stability
of the entire "T"-structure versus one of its parts alone, finding that they both show similar rigidity. We also
evaluated the performance of two different master-salve leg configurations by calculating the stress on the joint
and confirmed the calculations by Michelson interferometry. In terms of floor vibration damping, it was observed
that the performance of the system for the joined "T"-table seemed to be comparable to that of a single segment.
This method can significantly reduce costs of large optical tables and will be useful to extend existing optical
tables without manufacturer modification.
We present a simple and practical method for joining pneumatically floated optical tables. In order to demonstrate
this method we joined two optical tables in an uncentered "T-shape", and used a Michelson interferometer to
compare the stability of the entire "T-structure" versus one of its parts alone finding that they both show similar
rigidity. We also found the optimal master-slave leg configuration by calculating the stress on the joint and
confirmed the calculations by Michelson interferometry. The vibration damping for the "T-structure" against
the unjoined tables was measured finding comparable results. This method can significantly reduce costs of large
optical tables and will be useful to extend existing optical tables without manufacturer modification.
We have developed a detector which records the full polarization state of a terahertz (THz) pulse propagating
in free space. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic
pulse in two orthogonal directions as a function of time. A prototype device fabricated on Fe+ ion
implanted InP exhibited a cross polarized extinction ratio better than 390:1. The design and optimization of
this device are discussed along with its significance for the development of new forms of polarization sensitive
time domain spectroscopy, including THz circular dichroism spectroscopy.
We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with arsenic ions, and InGaAs and InP with Fe+ iron ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and InAs based photoconductive emitters.
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