We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequencyresolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.

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Bendable, low-loss Topas fibers for the terahertz frequency range

We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of ~1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of ~0.1 THz are predicted. Coupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.

Porous polymer fibers for low-loss Terahertz guiding.

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.

Terahertz dielectric waveguides

In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 µm suspended solid core and a 900 µm suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the subwavelength imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm(-1) are demonstrated specifically for the small core fiber. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.

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Suspended core subwavelength fibers: towards practical designs for low-loss terahertz guidance

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.

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THz porous fibers: design, fabrication and experimental characterization

We report a simple subwavelength-diameter plastic wire, similar to an optical fiber, for guiding a terahertz wave with a low attenuation constant. With a large wavelength-to-fiber-core ratio, the fractional power delivered inside the lossy core is reduced, thus lowering the effective fiber attenuation constant. In our experiment we adopt a polyethylene fiber with a 200 µm diameter for guiding terahertz waves in the frequency range near 0.3 THz in which the attenuation constant is reduced to of the order of or less than 0.01 cm−1. Direct free-space coupling efficiency as high as 20% can be achieved by use of an off-axis parabolic mirror. Furthermore, all the plastic wires are readily available, with no need for complex or expensive fabrication.

Low-loss subwavelength plastic fiber for terahertz waveguiding.

To accurately detect minute amounts of substances without disturbing the surroundings of target molecules has been a major goal in bioanalytical technology. Here by integrating an optoelectronic terahertz (THz) microsource into a glass-substrated microchip within the near-field distance, we demonstrate a compact, label-free, noninvasive, and sensitive microbiosensing system with low-power consumption. The demonstrated THz microchip allows us to locally specify various illicit drug powders with weights of nanograms, with a promising future for rapid identification of the static status or even the dynamics of various biomolecules

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Terahertz Microchip for Illicit Drug Detection

We report the theory and experiment on the low-loss subwavelength-diameter plastic THz fibers with attenuation constants ≪0.01cm−1. With a single-mode guiding capability, a direct free-space coupling efficiency ≫20% is achieved.

Low-loss subwavelength THz plastic fibers

Picosecond ultrasonic experiments were performed to generate and detect acoustic pulses in piezoelectric (111) epilayers of InxGa(1−x)As, and the longitudinal sound velocities in the [111] direction of InxGa(1−x)As were thus directly determined by controlling the indium content x. We found that the sound velocity exhibits an obvious bowing with different alloy compositions and can be described as a quadratic function of VL(x)=458x2−1451x+5414. This result supports a nonlinear compositional dependence of the elastic constants of InxGa(1−x)As and indicates that the linear-interpolation approximation is inapplicable for estimating sound velocity in this specific case.

Compositional Dependence of Longitudinal Sound Velocities of Piezoelectric (111) InxGa(1-x)As Measured by Picosecond Ultrasonics

In this study, we investigate the directivity of the terahertz (THz) radiation pattern from a newly designed two-dimensional rampart slot array antenna integrated in an edge-coupled membrane photonic transmitter. The antenna design is based on the array theory which is well-developed in the microwave regime. By means of the array arrangement of rampart slot antennas, we demonstrate that the 3-dB beam width of the THz radiation pattern at 907 GHz is successfully confined within 30 in both and planes.

Tzeng-Fu Kao

Papers

Low-loss subwavelength plastic fiber for terahertz waveguiding.

Terahertz Microchip for Illicit Drug Detection

Low-loss subwavelength THz plastic fibers

Compositional Dependence of Longitudinal Sound Velocities of Piezoelectric (111) InxGa(1-x)As Measured by Picosecond Ultrasonics

Highly Directed Radiation Pattern From a THz Photonic Transmitter With a Two-Dimensional Rampart Slot Array Antenna