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In this letter, we present the design, fabrication, and characterization of a quasi-Wollaston prism for terahertz frequencies based on form birefringence. The prism uses the birefringence induced in a sub-wavelength layered plastic-air structure that produces refraction in different directions for different polarizations. The component was simulated using the finite-difference-time-domain method, fabricated by 3D printing and subsequently tested by terahertz time-domain spectroscopy showing a polarization separation around of 23° for frequencies below 400 GHz, exhibiting cross polarization power extinction ratios better than 1.6 × 10−3 at 200 GHz.

Four easel paintings attributed to Hermenegildo Bustos (Purisima del Rincon, Guanajuato, Mexico), one of the most renowned painters of the late nineteenth and early twentieth century Mexican art, have been investigated by means of terahertz time-domain imaging (THz-TDI) and standard imaging techniques, such as near-IR reflectography and X-ray radiography. The archival sources and the recent studies on the paintings suggest that the artworks were created in the eighteenth century and underwent several modifications since then until the intervention of Bustos who authored the currently visible depictions. By combining the records of the paintings obtained by imaging with the different methodologies, aspects of the previous depictions and further details on the paintings’ history have been revealed, with THz-TDI playing a key role in attributing a chronological evolution of the images. The paintings of Purisima are the first THz-TDI-scanned paintings belonging to the Mexican cultural heritage.

After conservation works were done, we performed on-site THz TDS measurements of Roman paintings on the facade of Roman Imperial Throne Room in Trier and compared them to in-lab studies on mockup samples. Results indicate that an intermediate layer of cyclododecan sublimated.

We present 3D water distribution for Agave victoria reginae leaves employing terahertz time-domain imaging spectroscopy.

In this work, we present the comparison of three effective medium theory models for obtaining the dielectric function of biological tissue in the terahertz range.

We use terahertz spectroscopic imaging to visualize skin-hydration processes of moisturizing agents over stratum corneum in-vitro. The occlusive nature of lanolin as well as the hyper-osmotic hydration mechanism of glycerin were observed.

The damage caused by the environment to exposed glazed terracotta objects is usually not externally visible. For instance, the detachment of the glaze owing to subsurface crack formation or whole cavities in the area of the terracotta cannot be located visually. In this article, we demonstrate that terahertz time-of-flight spectroscopy is suitable to locate and measure the air gaps under the glaze detachments which could only be done by X-ray axial tomography before. This tool will be very useful to guide the restoration process, particularly for pieces that are still attached to buildings or other structures that cannot be transported to a tomography facility.

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.

The so-called terahertz band of the electromagnetic spectrum falls in a spectral region where traditional electronic technologies fail to operate owing to the extremely short switching times required. In this paper, we demonstrate the design of three optical logic gates fabricated by three-dimensional printing, which can perform the OR, AND, and XOR logic operations at 130 GHz. In order to optimize our designs we used a finite element electromagnetic simulation. The amplitude spectra of the experimental measurements of the three devices are presented where good agreement between the numerical results and experimental data can be seen. These results lead to the possibility of building more complex logic circuits based on these three photonic logic gates.

We present a broad literature review on the uses of terahertz radiation for the inspection architectural art, as well as building structures. As an example of the uses of terahertz in this field, we also include original results on a non-destructive assessment of a conservation intervention on murals of the Konstantinbasilika in Trier, Germany while using terahertz time-of-flight spectroscopy.

In this Perspective contribution, we present a brief review of the literature available on optical devices for terahertz frequencies, followed by an analysis of the challenges faced by this technology and its future potential to generate complex photonic systems, and in principle the possibilities of this technique for the production of components for the infrared and visible band.

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 study the hydration dynamics of polysaccharides of commercial and biological significance, namely, agave fructans, inulin, and maltodextrin, employing terahertz time-domain spectroscopy. In addition, we employ differential scanning calorimetry as a supporting technique. With the two techniques, we extract the hydration number for these polysaccharides in order to estimate their hydration ability. We further discuss characteristics of the H-bond network in the hydration shell.

In this paper we present three-color images of the foot soles of diabetic and non-diabetic subjects. Each color represents the hydration degree of the skin within value bands that statistically separate control and diabetic patients.

In this talk I will present a series of applied research projects that my group has worked in the last few years that involve the use of terahertz radiation in many fields including cultural heritage evaluation, industrial quality control, materials science, biology and medicine. I will also try to summarize the work of some other groups in terahertz applications and discuss the challenges and opportunities I see in the implementation of applications of terahertz radiation in many fields.

In this paper we theoretically and experimentally demonstrate a stepped-refractive-index convergent lens made of a parallel stack of metallic plates for terahertz frequencies based on artificial dielectrics. The lens consist of a non-uniformly spaced stack of metallic plates, forming a mirror-symmetric array of parallel-plate waveguides (PPWGs). The theoretical and experimental results show that this structure is capable of focusing a 1 cm diameter beam to a line focus of less than 4 mm for the design frequency of 0.18 THz. This structure shows that this artificial-dielectric concept is an important technology for the fabrication of next generation terahertz devices.

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.

In this paper we theoretically and experimentally demonstrate a stepped-refractive-index convergent lens made of a parallel stack of metallic plates for terahertz frequencies based on artificial dielectrics. The lens consist of a non-uniformly spaced stack of metallic plates, forming a mirror-symmetric array of parallel-plate waveguides (PPWGs). The operation of the device is based on the TE1 mode of the PPWG. The effective refractive index of the TE1 mode is a function of the frequency of operation and the spacing between the plates of the PPWG. By varying the spacing between the plates, we can modify the local refractive index of the structure in every individual PPWG that constitutes the lens producing a stepped refractive index profile across the multi stack structure. The theoretical and experimental results show that this structure is capable of focusing a 1 cm diameter beam to a line focus of less than 4 mm for the design frequency of 0.18 THz. This structure shows that this artificial-dielectric concept is an important technology for the fabrication of next generation terahertz devices.

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.

A mechanically tunable prism is constructed using a conventional 3D printer. First, by printing periodical layers of polystyrene, a dispersive element is obtained. Later, a tunable dispersive element is obtained by linking these layers with 3D printed bow springs. We show how this tunable dispersive element spatially resolves the different frequency components for terahertz radiation.

We report the first outdoor measurements for continuous in vivo leaf-water monitoring using THz spectroscopy. For this, we have developed a compact and portable THz quasi time-domain spectrometer which can be powered by a battery. We monitor the water status of a corn plant (Zea mays) and discuss the influence of the day-night variations of the outdoor temperature.

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

Environmental damages such as the detachment of original surfaces caused by cavity formation under the surface of natural stone and ceramic objects are usually not externally visible. Envirromental influences as temperature and humidity fluctuations, particularly when frost or saline accumulation forms can result in the generation of cracks, gaps, and even full detachment of parts of artwork in the exterior area. By using terahertz time domain spectroscopy (THz-TDS), we illustrate this promising method to locate and measure small subsurface cavities. This method is applied to produce three-dimensional images and pulsed terahertz “time-of-flight” measurements of the cavities.

Plasmonic contact electrodes are very effective for enhancing the radiation power of photoconductive terahertz sources. Au is a preferred metal for plasmonic contact electrodes due to the high plasmonic enhancement factors it offers in the near-infrared regime. However, Au requires an adhesion layer to stick well to photoconductive substrates. We investigate the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on GaAs. Our analysis suggests that Cr is the most suitable adhesion layer, offering up to 80% higher radiated terahertz powers than the previously demonstrated plasmonic sources that use Ti as the adhesion layer.

In this work we present, the design, fabrication and characterization of the first continuously varying birefringent q-plate at terahertz frequencies. This q-plate was fabricated by three-dimensional printing. The device works at 150 GHz and it is capable to produce radial, azimuthal and spiral polarization.

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics of InAs, InGaAs, and GaAs that leads to terahertz emission. We compare the emission power of all three semiconductors as a function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their mobilities. 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 paper we present the development of a noninvasive method for the evaluation of the degree deterioration of the diabetic foot using terahertz time domain spectroscopy. The study is based on the monitoring of hydration of the skin on the sole of the foot.

Four paintings attributed to Hermenegildo Bustos were scanned by terahertz time-domain imaging. The THz records clearly show previous depictions underneath the current ones.

In this work we present the design and characterization of a quiasi-Wollaston prism polarization splitter for frequencies between 100 GHz and 400 GHz based on form birrefringence. This novel device is simulated using the finite-difference-time-domain method. The experimental results and the numerical simulation show the polarization separation of approximately 23° for this range of frequencies and cross polarization power extintion ratio higher than 1.6×10−3 at 200 GHz.

The use of plasmonic contact electrodes in photoconductive emitters is very effective for generating high terahertz powers. This is because plasmonic electrodes concentrate a major portion of photo-generated carriers in their close proximity when excited by an incident optical pump beam. As a result, a large number of photocarriers drift to the terahertz radiating elements of the emitter within a sub-picosecond time-scale to efficiently contribute to terahertz generation. Au is a desired choice of metal for plasmonic contact electrodes due to its strong plasmonic enhancement factors at near-infrared wavelengths. However, it requires an adhesion layer to stick well to device substrates. We show that optical and electrical characteristics of the Au adhesion layer have significant impact on performance of plasmonic photoconductive terahertz emitters. We demonstrate that use of Cr adhesion layer instead of Ti, which is used in most existing plasmonic terahertz emitters, offers 50% enhancement in the generated terahertz powers.

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.

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz-30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies. OA hybrid

Monte-Carlo simulations are used to show that the transit time in ∼100 nm gap photoconductive emitters of terahertz radiation is short enough to produce broad-bandwidth pulses. Furthermore, with these calculations we demonstrate that nanostructured contacts remove the need for low-temperature-grown or ion-implanted materials for broad-band terahertz devices.

In this article we discuss the influence of temporal stability on the value obtained for dielectric properties of materials measured by terahertz time-domain spectroscopy with particular emphasis on attenuated total reflection. The stability of three different terahertz attenuated total reflection spectroscopy systems is carefully characterized. The formalism for the complex refractive index extraction is presented and the effect of delay errors is calculated numerically. We found that good thermal stability of the terahertz system helps to minimize delay fluctuations and therefore the uncertainty of the resulting complex refractive index.

We use terahertz time-domain spectroscopic (THz-TDS) imaging for the evaluation of moisturizing-substances effects over stratum corneum (SC) samples. Excised SC of porcine skin is used as an in vitro skin model. We evaluate the interaction of SC samples with glycerine and lanolin, two substances commonly used in moisturizers. In order to do this, THz images of SC samples after deposition of the substances are scanned. The response of the SC samples to a commercial moisturizer is also analyzed. Our results show that THz imaging is capable of sensing the distinct interaction mechanisms of the substances with the SC samples. The occlusive nature of lanolin, the hyperosmotic behavior of glycerine, and the moisturizing effect of the commercial moisturizer can be observed using THz-TDS imaging.

We present a numerical method to determine the volumetric water content of leaves from transmission terahertz time-domain spectroscopy data. The method is based on iterative optimization of parameters of an effective medium model for vegetative tissue. We found a very good agreement between measurements performed with this method and direct fresh/dry leaf weight-based water measurements.

We study the impact of phase and amplitude uncertainties on the estimated properties of a material measured with ATR THz-TDS. In addition, we investigate the deviation of the refractive index and extinction coefficient due to the small angular misalignment of the prism used in ATR THz-TDS. We also propose a modified configuration for ATR THz-TDS to reduce the impact of amplitude and phase uncertainties as well as of small angular misalignment.

We present a novel 3D printed gradient-refractive-index (GRIN) lens for terahertz frequencies. We use polystyrene for the lenses material because it is highly transparent in the terahertz region. This low cost lenses are demonstrated to operate below 800 GHz within the diffraction limit.

We demonstrate a refractive index sensor for liquid samples for the THz range. The sensor presents a refractive index dependent resonance which shifts in frequency at a rate that exceeds 500 GHz/RIU. The simple structure of the sensor and the low liquid volume requirement make it viable for scientific and industrial applications.

Monte Carlo simulations of photoexcited carrier dynamics combined with time domain electromagnetic modeling of the radiated field reveal different roles of the photocurrent vector components in the process of THz pulse generation upon ultrashort optical excitation of semiconductor surfaces. We find that only low frequency components (below 0.3 THz) are generated mainly by the initial transient current normal to the surface. Higher frequencies (0.6–1.0 THz) have a stronger contribution from the current transients in the plane of the semiconductor surface, whereas photo-excited plasma oscillations in the direction normal to the surface are found significant above 1.5 THz. The simulation results are verified using experimental near-field mapping.

The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.

Photoconductive detectors are convenient devices for detecting pulsed terahertz radiation. We have optimized Fe+ ion-damaged InP materials for photoconductive detector signal to noise performance using dual-energy doses in the range from 2.5× 1012 to 1.0× 1016 cm-2. Ion implantation allowed the production of semiconducting materials with free-carrier lifetimes between 0.5 and 2.1 ps, which were measured by optical pump terahertz probe spectroscopy. The time resolved photoconductivity of the detector substrates was acquired as a function of time after excitation by 2 nJ pulses from a laser oscillator. These data, when combined with a deconvolution algorithm, provide an excellent spectral response correction to the raw photocurrent signal recorded by the photoconductive detectors. © 2008 American Institute of Physics.

We report on the construction, optical alignment and performance of a receiver which is capable of recording the full polarization state of coherent terahertz radiation. The photoconductive detector was fabricated on InP which had been implanted with Fe(+) ions. The device operated successfully when it was gated with near infrared femtosecond pulses from either a Ti:sapphire laser oscillator or a 1 kHz regenerative laser amplifier. When illuminated with terahertz radiation from a typical photoconductive source, the optimized device had a signal to noise figure of 100:1 with a usable spectral bandwidth of up to 4 THz. The device was shown to be very sensitive to terahertz polarization, being able to resolve changes in polarization of 0.34 degrees. Additionally, we have demonstrated the usefulness of this device for (i) polarization sensitive terahertz spectroscopy, by measuring the birefringence of quartz and (ii) terahertz emission experiments, by measuring the polarization dependence of radiation generated by optical rectification in (110)-ZnTe.

Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentionmetry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer., 4 pages, 3 figures

Observing the structural dynamics of proteins under conditions as close as possible to those in a living organism is essential for understanding the biological functions of proteins accurately. Here we demonstrate that terahertz spectroscopy is a convenient probe of conformational changes in proteins suspended in physiological buffer solution. We have observed that the partial unfolding of photoactive yellow protein leads to a clear increase in absorption at terahertz frequencies. Using normal mode and molecular dynamics simulations we show that this increase in absorption is related to an increase in the density of delocalised vibrational modes in the more flexible partially unfolded state. © 2008 Elsevier B.V. All rights reserved.

We have developed a terahertz radiation detector that measures both the amplitude and polarization 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 polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-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., 3 pages, 3 figures

We simulated the carrier dynamics in InGaAs after ultrafast photoexcitation. By using a finite-difference time-domain approach we were able to analyze the near terahertz field emission caused by the motion of such carriers. We found that both the current parallel and normal to the interface take a relevant role in the terahertz emission. We also found that the ballistic motion of the carriers after photoexcitation dominates the emission rather than diffusion.

Measuring the full polarization state of radiation in terahertz time-domain spectroscopy has allowed scientists to study a number of complex dielectric anisotropic properties of materials that could not be easily measured before. Novel polarization sensitive photoconductive detectors have simplified this task and their development has been a significant challenge. In this review I will present some of these devices and will also discuss some of the most recent studies that involve the use of polarization resolved terahertz spectroscopy.

During the last decade there has been a great development in nanoscience and nanotechnology. The technology of nanostructures synthesis and characterization has grown rapidly and optical spectroscopy has become a very useful characterization technique, since it provides information on the structural, electronic, optical and dynamical properties of materials. Nanostructures have unique physical properties that are different from bulk materials. A wealth of interesting and new phenomena are associated with nanometer-sized structures, such as size-dependent emission or excitation, metallic and semiconductor behavior, etc. Here we present an overview of the linear optical response of one-dimensional silicon nanostructures. In particular, we make a theoretical study of the effects of the size and shape of one-dimensional silicon structures on the absorption spectrum, focusing on the calculation of the linear optical response of clean and hydrogen-adsorbed armchair (6,6) silicon nanotubes. We discuss the changes of the absorption spectrum of silicon nanowires with different diameters and analyze the behavior of the band gap as we go from bulk silicon to one-dimensional silicon nanostructures with nanometer-size diameters.