Hichem Guerboukha

Hybrid metal wire–dielectric terahertz waveguides: challenges and opportunities [Invited]

In this review we evaluate recent experimental and theoretical progress in the development of wire-based waveguides used for practical low-loss and low-dispersion delivery of terahertz radiation. Waveguides considered in this review utilize plasmonic modes guided in the air gap between two parallel wires. The two parallel wires are, in turn, encapsulated inside of a low-loss, low-refractive-index micro- or nano-structured cladding that provides mechanical stability and isolation from the environment. We describe two alternative techniques that may be used to encapsulate the two-wire waveguides while minimizing the negative impact of dielectric cladding on the optical properties of the waveguide. The first technique uses low-density foam as a cladding material, while the other uses air-filled microstructured plastic claddings to support metallic wires. Additionally, we offer a detailed analysis of the modal properties of wire-based waveguides, compare them with the properties of a classic two-wire waveguide, and present several strategies for the improvement of hybrid waveguide performance. Using the resonant dependence of the confinement properties of some hybrid plasmonic modes also allows us to propose their use in terahertz refractometry. Finally, we demonstrate that wire-based porous waveguides can have a very large operational bandwidth while supporting tightly confined, air-bound modes at both high and low frequencies. This is possible as, at higher frequencies, hybrid fibers can support ARROW-like low-loss air-bound modes while changing their guidance mechanism to plasmonic confinement in the inter-wire air gap at lower frequencies.

Time Resolved Dynamic Measurements at THz Frequencies Using a Rotary Optical Delay Line

Fabrication, characterization, and applications of a fast rotary linear optical delay line (FRLODL) for THz time-domain spectroscopy are presented. The FRLODL features two reflective surfaces with spatially separated incoming and outgoing beams. It has been manufactured using CNC machining. A linear dependence of the optical delay on the rotation angle allows a straightforward extraction of the conversion factor between the acquisition time (in ms) and the terahertz pulse time (in ps). We also discuss the accuracy of the rotary delay line detailing the possible sources of imprecision. The FRLODL has been tested using rotation speeds of up to 48 Hz, corresponding to an acquisition rate of up to 192 Hz with four blades incorporated on the same disk. At high speeds we observe a decrease of the bandwidth due to the limitations of the electronics, in particular, the transimpedance amplifier. An error analysis is performed by experimentally evaluating the signal-to-noise ratio and the dynamic range. With regard to the applications of the FRLODL, we first present observation of the evaporation of liquids, namely water, acetone and methanol. We then demonstrate monitoring of the spray painting process. Finally, detection of fast moving objects at 1 m/s and their thickness characterization are presented.

Exploiting k-space/frequency duality toward real-time terahertz imaging

Imaging at terahertz frequencies has recently received considerable attention because many materials are semitransparent to THz waves. The principal challenge that impedes a widespread use of THz imaging is the slow acquisition time of a conventional point-by-point raster scan. In this work, we present a theoretical formulation and an experimental demonstration of a novel technique for fast compressionless terahertz imaging based on broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics that allows the use of a single-pixel detector to perform angular scans along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compressionless image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with an image’s linear size, thus promising real-time acquisition of high-resolution THz images. Additionally, our imaging technique handles equally well and on an equal theoretical footing amplitude contrast and phase contrast images, which makes this technique useful for many practical applications. A detailed analysis of the technique’s advantages and limitations is presented, and its place among other existing THz imaging techniques is clearly identified.

Analog signal processing in the terahertz communication links using waveguide Bragg gratings: example of dispersion compensation

We study the possibility of analog signal processing for the upcoming terahertz (THz) high-bitrate communications using as an example the problem of dispersion compensation in the THz communication links. In particular, two Waveguide Bragg Grating devices (WBGs) operating in the transmission mode are detailed. WBGs are designed by introducing periodic corrugation onto the inner surface of the metalized tubes. The resultant devices operate in a single mode regime either in the vicinity of the modal cutoff or in the vicinity of a bandgap edge, featuring large negative group velocity dispersions (GVD). We fabricate the proposed WBGs using 3D stereolithography, and metallize them using wet chemistry. Optical properties of the fabricated WBGs are investigated both theoretically and experimentally. The results confirm single mode guidance, relatively high coupling efficiency, as well as large negative group velocity dispersions in the range of several -100s ps/(THz · cm) in the vicinity of 0.14THz. This makes the short sections of proposed WBGs suitable for compensating positive dispersion incurred in the THz wireless links or fiber-assisted THz interconnects for signals of several-GHz bandwidth. Finally, we comment on the challenges associated with the analog signal processing in the THz spectral range.

Silk foam terahertz waveguides

Silk foam-based terahertz waveguides are fabricated using lyophilisation and casting techniques. This work is motivated by the lack of biocompatible waveguides for low-loss guidance of THz for applications in remote sensing in biomedical and agro-alimentary industries.

A complementary study to “Hybrid hollow core fibers with embedded wires as THz waveguides” and “Two-wire terahertz fibers with porous dielectric support:” comment

In a recent paper, Anthony et al. [Opt. Express 21, 2903 (2013)] demonstrated broadband terahertz pulse propagation through the hollow core fibers with two embedded Indium wires. In another paper by A. Markov et al. [Opt. Express 21, 12728 (2013)], we proposed a plasmonic THz fiber featuring two metallic wires held in place by the porous dielectric cladding functioning as a mechanical support. Although the cross sections of the two waveguides look very similar, we were surprised to find that the guidance mechanisms for these two waveguides are quite different. In fact, waveguide considered by A. Markov et al. was guiding a plasmonic mode, while the waveguide presented by Anthony et al. was guiding a dielectric waveguide-like mode. Finally, we have realized that by reducing the waveguide dimensions by a factor of ~10-20 one can transition from the dielectric waveguide guidance as it is demonstrated by Anthony et al. to plasmonic guidance as reported in A. Markov et al. Therefore, we conclude that both waveguide are essentially identical, while their guidance mechanism changes as a function of the waveguide overall size.

Exploiting k-space/frequency duality in Fourier optics towards real-time compression-less terahertz imaging

We present theoretical formulation and experimental demonstration of a novel technique for the fast compression-less terahertz imaging based on the broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics which allows using a single-pixel detector to perform angular scan along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compression-less image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with the image linear size, thus promising real-time acquisition of high-resolution THz images. Additionally, our imaging technique handles equally well and on the equal theoretical footing the amplitude contrast and the phase contrast images, which makes this technique useful for many practical applications. A detailed analysis of the novel technique advantages and limitations is presented, as well as its place among other existing THz imaging techniques is clearly identified

Linear rotary optical delay lines

We present a semi-analytical solution for the design of a high-speed rotary optical delay line that use a combination of two rotating curvilinear reflectors. We demonstrate that it is possible to design an infinite variety of the optical delay lines featuring linear dependence of the optical delay on the rotation angle. This is achieved via shape optimization of the rotating reflector surfaces. Moreover, a convenient spatial separation of the incoming and outgoing beams is possible. For the sake of example, we present blades that fit into a circle of 10cm diameter. Finally, a prototype of a rotary delay line is fabricated using CNC machining, and its optical properties are characterized.

Dynamic measurements at terahertz frequencies with a fast rotary delay line

A fast rotary delay line is fabricated and characterized for terahertz (THz) frequencies. With this new device, we present dynamic measurements of spray painting process and fast moving objects detection along with thickness determination.

Hybrid metal wire-dielectric THz fibers: Design and perspectives

We investigate the optical characteristics of terahertz two-wire plasmonic waveguides, porous, and foam-based dielectric waveguides. Our group demonstrates a low-loss and low-dispersion hybrid metal wire-dielectric fiber resulting from fusion of the aforementioned types of waveguides.