Design and 3D printing of the Powell lens for sub-terahertz imaging.

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100 GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

Two-photon-absorption enhanced terahertz generation from KTP optically pumped in the visible-to-UV range.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν1 (A1g) mode.

Video-Rate Identification of High-Capacity Low-Cost Tags in the Terahertz Domain.

In this article, we report on video-rate identification of very low-cost tags in the terahertz (THz) domain. Contrary to barcodes, Radio Frequency Identification (RFID) tags, or even chipless RFID tags, operate in the Ultra-Wide Band (UWB). These THz labels are not based on a planar surface pattern but are instead embedded, thus hidden, in the volume of the product to identify. The tag is entirely made of dielectric materials and is based on a 1D photonic bandgap structure, made of a quasi-periodic stack of two different polyethylene-based materials presenting different refractive indices. The thickness of each layer is of the order of the THz wavelength, leading to an overall tag thickness in the millimetre range. More particularly, we show in this article that the binary information coded within these tags can be rapidly and reliably identified using a commercial terahertz Time Domain Spectroscopy (THz-TDS) system as a reader. More precisely, a bit error rate smaller than 1% is experimentally reached for a reading duration as short as a few tens of milliseconds on an 8 bits (~40 bits/cm2) THID tag. The performance limits of such a tag structure are explored in terms of both dielectric material properties (losses) and angular acceptance. Finally, realistic coding capacities of about 60 bits (~300 bits/cm2) can be envisaged with such tags.

Terahertz generation from ZnTe optically pumped above and below the bandgap.

We report on the generation of THz waves through optical rectification in ZnTe of femtosecond laser pulses whose photon energy is tuned from below to above the ZnTe bandgap energy. The THz signal exhibits a pronounced peak at the bandgap energy, at THz frequencies for which losses in ZnTe remain small. This peak is likely due to the resonance of the ZnTe nonlinear susceptibility in the vicinity of the bandgap.

Electrodynamic resonance of surface conduction and THz transmission through arrays of rectangular apertures in opaque metallic thin films.

A modal method is developed analytically to investigate the THz optical transmission and reflection of a metallic thin film perforated by a 2D array of rectangular apertures. For subwavelength apertures, this optical model is interpreted in terms of passive electrical circuits, with interface admittances accounting for the THz surface conduction properties of the metallic film. The reactive component of the admittance of the evanescent diffraction cloud is shown to exhibit resonant behavior governed by the shape factors of the array. Interaction of such an electrodynamic resonance with the Rayleigh diffraction orders may alter their standard Fano profiles. Experimental evidence of the resonance is obtained owing to lineshape analysis of transmittance measurements in the THz range on metallic thin films deposited on a dielectric substrate, both above and below the first Wood-Rayleigh anomaly.

Observation of terahertz beam diffraction by fabrics.

The structure of most of fabrics is an almost periodic network of interlaced yarns or threads, whose periodicity is of the order of the terahertz wavelength. We report here on the diffraction of terahertz electromagnetic waves by the yarn network in common cloths. In the case of linen, this effect could lead to overestimating (by a factor as large as 3) the absorption when determined from the classical terahertz time-domain experiment. Our results are confirmed by a HFSS numerical simulation.

Evaluation of the shadow effect in terahertz kinoform gratings.

The experimental and numerical evaluation of the shadow effect in kinoform diffractive gratings for the terahertz (THz) range is given. This effect limits the diffractive efficiency of dense gratings, which are the base of the elements suited for convenient beam focusing and imaging in THz. The observed effect of redirecting most of the incident energy into stray -1st diffractive order is observed and discussed. The presented results show the great significance of the shadow effect in selected kinoform gratings and prove the utility of the used methodology of numerical simulations.

Imaging of broadband terahertz beams using an array of antenna-coupled microbolometers operating at room temperature.

We present results of 2D real-time imaging of terahertz (THz) beam generated by a photoconductive antenna driven by a femtosecond oscillator. The detector, operating at room temperature, is a 320 x 240 array of antenna-coupled microbolometers with integrated CMOS read-out electronics delivering 25 images per second. High quality images of broadband THz beams covering the 0.1-2 THz range are recorded while maintaining a signal-to-noise ratio of 10 for detected THz power as low as 25 nW. The compactness of the easy-to-use uncooled camera makes it very useful for the alignment of systems such as THz time-domain spectrometers and for the characterization of emitters, optics and other components.

Narrow band, large angular width resonant reflection from a periodic high index grid at terahertz frequency.

The property of a thin silicon membrane with periodic air slits of definite depth and width to exhibit under normal incidence a close to 100% ultra-narrow band reflection peak is demonstrated experimentally in the terahertz frequency range on a single-crystal silicon grid fabricated by submillimeter microsystem technology. An analysis based on the true modes supported by the grid reveals the nature of such resonances and permits to sort out those exhibiting ultra-narrow band.

Diffractive paper lens for terahertz optics.

Passive terahertz (THz) setups require optical elements with large diameters for optimal harvesting of weak signals. High f-number implies sophisticated aspheric designs to ensure optimal resolution and good energetic efficiency. Trial and error testing of such optics is expensive and numerical modeling is time consuming; hence, we propose extremely cheap diffractive lenses for THz made of regular paper. They are easy to manufacture even with large diameters, and the optical function can be easily customized, which can be used for initial experimental testing of THz setups. Characterization of the proposed diffractive lenses with time-domain spectroscopy is presented and discussed.

Highly efficient broadband double-sided Fresnel lens for THz range.

Modern passive THz setups require effective optical elements with a large numerical aperture. Here we propose a new type of the optical element for THz applications, which is a broadband double-sided Fresnel-like lens with an optimized thickness. The optimization is performed to obtain a very low attenuation, low material cost, and small weight in the element media. It also provides achromatic properties for the assumed wavelength range. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Terahertz encoding approach for secured chipless radio frequency identification.

In this article, we present a new family of chipless tags, which permit encoding of digital data in the terahertz domain. These devices consist of stacked dielectric media whose thicknesses are of the same order as terahertz wavelengths. Since the information is encoded in the volume of these multilayer terahertz tags, they can easily be associated with classical identification techniques (e.g., barcode, radio frequency identification), where information is encoded at the surface of the tag, to provide higher data security. The principle of this encoding approach is studied and experimentally demonstrated in this paper. A 2 bit tag prototype has been realized and measured for validation purposes.

Off-axis metallic diffractive lens for terahertz beams.

A diffractive optical element for off-axis focusing of terahertz radiation is presented. It was designed in a nonparaxial regime and manufactured in a metal slab by laser cutting of curved stripes. The optical function of the structure includes focusing and deflecting the illuminating beam of a chosen frequency in a particular place. Therefore, the element acts as both a spatial and a spectral filter; hence it is especially suitable for separating the terahertz signal from a broadband thermal load in passive detection devices. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Time-domain terahertz study of defect formation in one-dimensional photonic crystals.

One-dimensional photonic crystals composed of silicon and air layers with and without twinning defect (i.e., a periodicity break where one half of the photonic structure is a mirror image of the other one) are studied by means of terahertz time-domain transmission and reflection spectroscopy. The structure with defect is decomposed into building blocks: two twins and a defect. A phase-sensitive characterization in transmission and reflection allows us to fully determine the transfer matrices of any block and consequently to predict the properties of composed structures regardless of the microstructure of the constituting blocks. It is shown and experimentally demonstrated that the defect level position is controlled by the reflectance phase of the twins. Possible approach of the reflectance phase determination by use of Kramers-Kronig analysis is also discussed.

Grating-assisted coupling of terahertz waves into a dielectric waveguide studied by terahertz time-domain spectroscopy.

We report on the efficient coupling of terahertz (THz) waves into a dielectric waveguide by means of a diffraction grating engraved at the top of the waveguide. The waveguide is made of a 201-microm-thick high-resistivity silicon wafer. The transmission of the device, measured versus frequency by terahertz time-domain spectroscopy, shows usual m lines when a frequency component of the THz pulse spectrum satisfies the phase-matching condition and is coupled into the waveguide. The experimental data are well modeled with the differential electromagnetic method to compute the diffraction pattern of the grating device. The dispersion curve of the first four modes of propagation is determined from the frequency position of the m lines recorded for different angles of incidence of the THz beam. The waveguide exhibits a weak group velocity dispersion at high frequencies.