Enrique Castro-Camus

• PhD
• Group Leader at Centro de Investigaciones en Optica

As the need for higher data rates for communication increases, the terahertz (THz) band has drawn considerable attention. This spectral region promises a much wider bandwidth and the transmission of large amounts of data at high speeds. However, there are still challenges that need to be addressed before the THz telecommunications technology hits t...

The frequency-dependent signal-to-noise ratio of terahertz time-domain signals is a relevant source of uncertainty for parameters measured with it. It also limits the total usable bandwidth of such signals. In the great majority of cases, the processes to establish the limits of this usable bandwidth are determined based on the experience of the us...

Here we propose a 3D printed form-birefringent achromatic quarter-waveplate targeting the lower terahertz frequency range. The monolithically fabricated waveplate consists of three individual layers of different thickness and orientation of the stratified structure resulting in the achromatic response. The experimental results show that the perform...

One of the key distinctions between legacy low-frequency wireless systems and future THz wireless transmissions is that THz links will require high directionality, to overcome the large free-space path loss. Because of this directionality, optical phenomena become increasingly important as design considerations. A key example lies in the strong dep...

Terahertz (THz) radiation has shown enormous potential for non-destructive inspection in many contexts. Here, we present a method for imaging defects in chocolate bars that can be extended to many other materials. Our method requires only a continuous wave (CW) monochromatic source and detector at relatively low frequencies (280 GHz) corresponding...

The molecular beam epitaxial growth and physical properties of In graded-composition layers of InGaAs on GaAs (100) substrates varying nominal In concentration from 12 to 97% and vice versa is analyzed along the growth front. The substrate temperature was varied as the In concentration was changed in order to propitiate the near-optimal growth cond...

We use terahertz (THz) time-domain spectroscopy (TDS) to assess the crystalline state of indomethacin (IM) when loaded in smartFilms®. We found that smartFilms favour the amorphous IM (A-IM) for low loading concentrations. For higher concentrations, IM recrystallizes in its α- crystalline form and the amount of A-IM in the smartFilms reduces. Both,...

The growth and characterization of InxGa1-xAs layers with hyperbolic tangent In concentration profiles is presented. Along the growth the molecular beam fluxes of Ga and In, and the substrate temperature were varied. Since the concentration of In is varied during growth process the strain is expected to increase monotonically, and eventually be rel...

Terahertz (THz) technology has experienced rapid development in the past two decades. Growing numbers of interdisciplinary applications are emerging, including materials science, physics, communications, and security as well as biomedicine. THz biophotonics involves studies applying THz photonic technology in biomedicine, which has attracted attent...

Diabetic foot syndrome, a long term consequence of Diabetes Mellitus, is the most common cause of non-traumatic amputations. Around 8% of the world population suffers from diabetes, 15% of diabetic patients present a diabetic foot ulcer which leads to amputation in 2.5% of the cases. There is no objective method for the early diagnosis and preventi...

In this contribution, we demonstrate the first referenceless measurement of a THz wavefront by means of shear-interferometry. The technique makes use of a transmissive Ronchi phase grating to generate the shear. We fabricated the grating by mechanical machining of high-density polyethylene. At the camera plane, the +1 and −1 diffraction orders are...

In order for terahertz devices to reach technological maturity, robust characterization methods and reliable metrics for comparison between studies must be defined. In this comment, the authors describe the challenges faced in obtaining robust power measurements in the terahertz regime, and summarize recent efforts to establish standards for this f...

We discuss the progress in the field of THz imaging based on time-domain spectroscopy during the last 20 years emphasizing several highlights. These include 3D mapping of the water distribution of plants, THz reflection imaging of samples with arbitrary shape, burn wound imaging and the early diagnosis of diabetic foot disease. These applications g...

We demonstrate the first “nonlocal” pump-probe measurement using a scattering scanning near-field microscopy technique by laterally displacing the pump and probe spots. We observe nonlocal effects associated with carrier drift into the probe region.

We present an experimental study of the terahertz emission from InxGa1−xAs epitaxial layers that were grown while varying the alloy fraction x. We observe the terahertz emission that is significantly different depending on the variation direction of the alloy fraction. We attribute the difference to the significant change of the band bending induce...

The worldwide shortage of medical-grade ventilators is a well-known issue, that has become one of the central topics during the COVID-19 pandemic. Given that these machines are expensive and have long lead times, one approach is to vacate them for patients in critical conditions while patients with mild to moderate symptoms are treated with strippe...

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 XO...

As part of a plethora of global efforts to minimize the negative effects of the SARS-CoV2 (COVID-19) pandemic, we developed two different mechanisms that, after further development, could potentially be of use in the future in order to increase the capacity of ventilators with low-cost devices based on single-use-bag-valve mask systems. We describe...

Currently a significant fraction of the world energy is still produced from the combustion of mineral coal. The extraction of coal from mines is a relatively complex and dangerous activity that still requires the intervention of human miners, and therefore in order to minimize risks, automation of the coal mining process is desirable. An aspect tha...

The worldwide shortage of medical-grade ventilators is a well-known issue, that has become one of the central topics during the COVID-19 pandemic. Given that these machines are expensive and have long lead times, one approach is to vacate them for patients in critical conditions while patients with mild to moderate symptoms are treated with strippe...

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 whil...

In this article, we propose variable-period diffraction gratings for terahertz frequencies. The design, fabrication and characterization of such devices are presented. Our measurements show the possibility to actively shift of the deflection angle for each frequency using this device. We also demonstrated that, when driven by a speaker, these varia...

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...

As part of a plethora of global efforts to minimize the negative effects of the SARS-CoV2 (COVID-19) pandemic, we developed two different mechanisms that, after further development, could potentially be of use in the future in order to increase the capacity of ventilators with low-cost devices based on single-use-bag-valve mask systems. We describe...

While terahertz imaging has been used before for the determination of water content in vegetative tissue, most studies have either presented measurements of the temporal evolution of water content at a single point of the plant or have presented two-dimensional images of leaves, demonstrating the potential of the technique, but relatively little of...

We study the use of three effective medium theory models, namely Maxwell–Garnett, Bruggeman, and Landau–Lifshitz–Looyenga, for the dielectric response of biological tissue in the terahertz band of the electromagnetic spectrum. In order to accomplish our objectives, we performed measurements on water-dehydrated basil binary mixtures encompassing the...

On site measurements of historically accumulated paint layers on the kiosk of Guadalajara were taken using a terahertz time-domain spectrometer. Sub-surface air gaps produced by delamination were found on various regions of the structure. The occurrence of the air gaps correlates with the position in the structure, being more frequent on the southe...

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, T...

In this article, we present the design and characterization of the first 3D-printed guided-mode filter for terahertz frequencies. The device consist on a diffractive grating fabricated on top of a rectangular dielectric waveguide by 3D printing. Finite difference numerical modeling shows this device acts as a filter at a frequency that can be geome...

In this article, we present a series of hydration mapping images of the foot soles of diabetic and non-diabetic subjects measured by terahertz reflectance. In addition to the hydration images, we present a series of RYG-color-coded (red yellow green) images where pixels are assigned one of the three colors in order to easily identify areas in risk...

Very few real easel paintings have been scanned by means of terahertz time-domain imaging (THz-TDI) up to now. In this study, four different areas of an eighteenth century easel painting have been scanned by THz-TDI with the aim of proving useful information to the conservators for its imminent restoration treatment, owing to the complexity of its...

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 out...

Here, we present a 3D printed prism for THz waves made out of an artificial dielectric material in which the dispersion can be tuned by external compression. The artificial material consists of thin dielectric layers with variable air spacings which has been produced using a fused deposition molding process. The material properties are carefully ch...

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). Th...

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length...

We use terahertz transmission through limestone sedimentary rock samples to assess the macro and micro porosity. We exploit the notable water absorption in the terahertz spectrum to interact with the pores that are two orders of magnitude smaller (<1μm) than the terahertz wavelength. Terahertz water sensitivity provides us with the dehydration prof...

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,...

We present the design, fabrication, and characterization of a q-plate with continuous birefringence variation at terahertz frequencies. This q-plate was fabri- cated by three-dimensional printing and is a simple solution for the generation of cylindrical vector beams. This device can find a number of applications in future terahertz technologies su...

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 cent...

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 simulate...

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...

Four easel paintings attributed to Hermenegildo Bustos (Purísima del Rincón, 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 radio...

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 m...

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 t...

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) 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...

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 o...

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...

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)...

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 fo...

Photoconductive switches were the key components that allowed the generation and detection of coherent broadband electromagnetic pulses at terahertz frequencies, opening the possibility for performing spectroscopy and, therefore, measuring complex dielectric properties of materials in this band, which was mostly unexplored. In this paper, we presen...

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 th...