Bradley F. Bowden

Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation.

We have applied techniques developed for IR waveguides to fabricate Ag/polystyrene (PS) -coated hollow glass waveguides (HGWs) for transmission of terahertz radiation. A loss of 0.95 dB/m at 119 microm (2.5 THz) was obtained for a 2 mm bore, 90 cm long Ag/PS HGW. We found that TE modes are supported in HGWs with thin PS films, while hybrid (HE) modes dominate when PS film thickness increases. The lowest losses are obtained for the thicker PS films and the propagation of the HE modes.

Low-loss modes in hollow metallic terahertz waveguides with dielectric coatings

Cylindrical hollow-core metallic waveguides with an inner coating of polystyrene (PS) deposited over silver have losses less than 1dB/m for terahertz waves propagating in the HE11 mode. The thickness of the PS film determines whether the hybrid HE11 mode or the transverse TE01 mode exhibits the lowest loss. The mode selection is confirmed by studying mode profiles and transmission losses at 2.5 THz in waveguides with the dielectric coating thickness ranging from 0 to the optimum value of approximately 10 μm.

Waveguide mode imaging and dispersion analysis with terahertz near-field microscopy

Propagation of terahertz waves in hollow metallic waveguides depends on the waveguide mode. Near-field scanning probe terahertz microscopy is applied to identify the mode structure and composition in dielectric-lined hollow metallic waveguides. Spatial profiles, relative amplitudes, and group velocities of three main waveguide modes are experimentally measured and matched to the HE11, HE12, and TE11 modes. The combination of near-field microscopy with terahertz time-resolved spectroscopy opens the possibility of waveguide mode characterization in the terahertz band.

Fabrication and characterization of chalcogenide glass for hollow Bragg fibers

Low- and high-refractive-index chalcogenide glasses are studied for their potential use in the fabrication of one-dimensional hollow Bragg fibers. The low-index glasses are based on the GeSe-glass systems with indices varying from 2.0 to 2.5, while the high-index glasses are formed from the AgAsSe glasses with indices ranging from 2.8 to 3.8. High-purity elemental starting materials are distilled and the surface oxides removed prior to mixing in a rocking furnace. The refractive indices of the AgAsSe glasses, measured using a CO2 laser reflectometer, were near 3.10 for the compositions most compatible with the low-index Ge20Se80 glass (n = 2.46). Spectral measurements show impurity absorption bands between 12 and 16 microm. The loss at 10.6 microm for the Ag25As40Se35 glass measured using CO2 laser calorimetry was 1.16 x 10(-3) cm(-1).

Fabrication of terahertz hollow-glass metallic waveguides with inner dielectric coatings

Hollow-core glass waveguides (HCWs) with inner coatings of silver and polystyrene (PS) have been fabricated for transmission of terahertz radiation. A liquid-phase chemical deposition process was used to deposit silver and PS thin film coatings inside glass tubing. The PS dielectric layer can substantially lower the loss of the HCW compared to the metal-only waveguide. A polymer coating was chosen because it is possible to deposit the dielectric film thickness required for operation in the terahertz regime. Specifically, the dielectric film thickness is proportional to the wavelength so the PS coatings need to be on the order of 10 to 15 μm to minimize transmission losses at terahertz frequencies. This is much thicker than the submicron thick dielectric coatings needed in the IR region. The thickness of the PS film depends on the concentration of PS in the coating solution and the coating rate. Both of these parameters are studied and related to the losses in the waveguides. The lowest loss of 0.95 dB/m a...

Polymer/metal sulfide coated hollow waveguides for delivery of Er:YAG laser radiation

Polystyrene (PS) as a low index dielectric material in silver-coated hollow glass waveguides (HGW) has been studied as an alternative to the more conventional Ag/AgI HGWs. Polystyrene was chosen because it has been well characterized and it is easily dissolved in toluene. The waveguides were made using an electroless, wet chemistry technique. The absorbance spectrum for polystyrene indicates it will transmit from 1 to 3 &mgr;m and thus, it is useful at the Er:YAG laser wavelength of 2.94 &mgr;m. For multilayer HGWs, cadmium sulfide is the high index dielectric material.

Silver/Polystyrene Coated Hollow Glass Waveguides for the Transmission of THz Radiation

We report on the design theory and fabrication of silver/polystyrene(PS) coated hollow glass waveguides (HGWs) for THz radiation. We find that Ag/PS coated HGWs have significantly lower attenuation for 119 mum radiation than Ag-only HGWs.

Hollow CU-coated plastic waveguides for the delivery of THz radiation

Terahertz (THz) radiation has important applications in spectroscopy, imaging, and space science. For some of these applications, in particular spectroscopy, a flexible fiber optic could potentially simplify the THz system and enable the user to transmit radiation to remote locations without excessive absorption by atmospheric moisture. To date THz fiber optics and waveguides have been limited to rigid hollow metallic waveguides, solid wires, or short lengths of solid-core transparent dielectrics such as sapphire and plastic. In this paper we report on flexible, hollow polycarbonate waveguides with interior Cu coatings for broadband THz transmission fabricated using simple liquid-phase chemistry techniques. The losses for these hollow-core guides were measured using a tunable, cw single-mode far IR laser. The losses for the best guides were found to be less than four dB/m and the single mode of the laser was preserved for the smaller bore waveguides. Loss calculations of the loss for the HE11 mode reveal that the metal-coated hollow waveguides have much higher loss than for waveguides coated with both metallic and dielectric thin films.

Chalcogenide glass 1D photonic bandgap hollow fiber

We propose a chalcogenide glass 1-D photonic bandgap (PBG) hollow fiber for the transmission of mid-IR radiation. Its structure consists of an air-core surrounded by alternating rings of high and low refractive index chalcogenide glass supported by a thick polymer coating. Theoretical calculations demonstrate the potential to achieve low transmission losses at 10.6 μm for the HE11 mode. These calculations indicate that the chalcogenide glasses used in the PBG structure should have high index contrast and low extinction coefficients to minimize transmission losses. We present preliminary results of our ongoing efforts to identify a pair of chalcogenide glasses that meet this criterion. A strategy is developed for the fabrication of a drawable chalcogenide glass 1-D PBG hollow fiber preform.

Mode imaging and dispersion analysis in Terahertz waveguides using Terahertz near-field microscopy

Mode structure, transmission loss and dispersion are characterized in low-loss (∼1dB/m) Terahertz (THz) dielectric-lined hollow metallic waveguides. THz near-field probe imaging and spectroscopy is applied for precise mode imaging and selective mode probing.