Shaghik Atakaramians

Porous fibers: a novel approach to low loss THz waveguides.

We propose a novel class of optical fiber with a porous transverse cross-section that is created by arranging sub-wavelength air-holes within the core of the fiber. These fibers can offer a combination of low transmission loss and high mode confinement in the THz regime by exploiting the enhancement of the guided mode field that occurs within these sub-wavelength holes. We evaluate the properties of these porous fibers and quantitatively compare their performance relative to that of a solid core air cladded fiber (microwire). For similar loss values, porous fibers enable improved light confinement and reduced distortion of a broadband pulse compared to microwires.

T-Ray Sensing and Imaging

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.

THz porous fibers: design, fabrication and experimental characterization

Porous fibers have been identified as a means of achieving low losses, low dispersion and high birefringence among THz polymer fibers. By exploiting optical fiber fabrication techniques, two types of THz polymer porous fibers--spider-web and rectangular porous fibers--with 57% and 65% porosity have been fabricated. The effective refractive index measured by terahertz time domain spectroscopy shows a good agreement between the theoretical and experimental results indicating a lower dispersion for THz porous fiber compared to THz microwires. A birefringence of 0.012 at 0.65 THz is also reported for rectangular porous fiber.

Terahertz dielectric waveguides

Several classes of non-planar metallic and dielectric waveguides have been proposed in the literature for guidance of terahertz (THz) or T-ray radiation. In this review, we focus on the development of dielectric waveguides, in the THz regime, with reduced loss and dispersion. First, we examine different THz spectroscopy configurations and fundamental equations employed for characterization of THz waveguides. Then we divide THz dielectric waveguides into three classes: solid-core, hollow-core, and porous-core waveguides. The guiding mechanism, fabrication steps, measured loss, and dispersion are presented for the waveguides in each class in chronological order. The goal of this review is to compare and contrast the current solutions for guiding THz radiation.

Low loss, low dispersion and highly birefringent terahertz porous fibers

We demonstrate that porous fibers in addition to low loss and high confinement, have near zero dispersion for 0.5–1 THz resulting in reduced terahertz signal degradation compared to microwires. We also show for the first time that these new fibers can be designed, introducing asymmetrical sub-wavelength air-holes within the core, to achieve high birefringence �0.026. This opens up the potential for realization of novel polarization preserving fibers in the terahertz regime.

Direct probing of evanescent field for characterization of porous terahertz fibers

We develop a technique based on a micromachined photoconductive probe-tip to characterize a terahertz (THz) porous fiber. Losses less than 0.08 cm−1 are measured in the frequency range from 0.2 to 0.35 THz, with the minimum of 0.003 cm−1 at 0.24 THz. Normalized group velocity greater than 0.8, which corresponds to dispersion values in between −1.3 and −0.5 ps/m/μm for 0.2<f<0.35 THz are obtained. Moreover, we directly measure the evanescent electric field as a function of frequency. Good agreement between the measured curves and expected theoretical values indicates the low invasiveness of the applied probe-tip.

Hollow-core waveguides with uniaxial metamaterial cladding: modal equations and guidance conditions

We discuss the conditions in which guided modes exist in a circular waveguide with an anisotropic, uniaxial metamaterial cladding. These hollow-core waveguides can guide modes at deep subwavelength dimensions, with core diameters more than 20 times smaller than the operating wavelength.

Fiber-drawn double split ring resonators in the terahertz range

We present a novel method for producing metamaterials based on double split ring resonators with a magnetic resonance at terahertz (THz) frequencies. The resonators were made by fiber drawing, a scalable method capable of producing large volumes of metamaterials, demonstrating that this technique can be extended to complex meta-atoms. The observed resonances occur at larger wavelengths relative to the resonator size, compared to single split ring resonators, and are in good agreement with simulations.

Cleaving of Extremely Porous Polymer Fibers

Different cleaving techniques, based on the use of a semiconductor dicing saw, focused-ion-beam milling, and a 193-nm ultraviolet laser, have been exploited to cleave highly porous polymer fibers developed for guiding terahertz radiation. Porous fibers made up of two different polymer materials have been cleaved with the proposed methods and compared with those achieved from the conventional cleaving method. Regardless of the polymer material used for fabricating terahertz porous fibers, using an ultraviolet laser for cleaving and rotating the fiber during the process rapidly provides smooth and reproducible cleaves across the entire fiber cross section.

Hollow-core uniaxial metamaterial clad fibers with dispersive metamaterials

We present a comprehensive study of the effect of material dispersion on the guided modes of a circular waveguide with an anisotropic, uniaxial metamaterial cladding, including in particular claddings with dispersive hyperbolic or indefinite material properties, both magnetic and electric. We show that the transverse components of material parameters have dominating effects compared to that of the longitudinal material components. We derive the condition for the existence of frequencies at which propagation constants diverge and study the modes’ behavior around such points. In particular we show these modes can be strongly confined in an air core with significantly subwavelength dimensions.