Jeffrey E. Melzer

Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band

A low-loss and low-dispersive optical-fiber-like hybrid HE₁₁ mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE₁₁ mode low. The experimentally measured loss is below 10 dB/m for 2 < ν < 2.85 THz (~4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ν < 5 THz according to the numerical calculations. At ~1.25 THz, the waveguide shows an absorption peak of ~75 dB/m related to the transition between the TM₁₁-like mode and the HE₁₁ mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further.

Theory and practical considerations of multilayer dielectric thin-film stacks in Ag-coated hollow waveguides.

This analysis explores the theory and design of dielectric multilayer reflection-enhancing thin film stacks based on high and low refractive index alternating layers of cadmium sulfide (CdS) and lead sulfide (PbS) on silver (Ag)-coated hollow glass waveguides (HGWs) for low loss transmission at midinfrared wavelengths. The fundamentals for determining propagation losses in such multilayer thin-film-coated Ag hollow waveguides is thoroughly discussed, and forms the basis for further theoretical analysis presented in this study. The effects on propagation loss resulting from several key parameters of these multilayer thin film stacks is further explored in order to bridge the gap between results predicted through calculation under ideal conditions and deviations from such ideal models that often arise in practice. In particular, the effects on loss due to the number of dielectric thin film layers deposited, deviation from ideal individual layer thicknesses, and surface roughness related scattering losses are presented and thoroughly investigated. Through such extensive theoretical analysis the level of understanding of the underlying loss mechanisms of multilayer thin-film Ag-coated HGWs is greatly advanced, considerably increasing the potential practical development of next-generation ultralow-loss mid-IR Ag/multilayer dielectric-coated HGWs.

Efficient coupling of double-metal terahertz quantum cascade lasers to flexible dielectric-lined hollow metallic waveguides

The growth in terahertz frequency applications utilising the quantum cascade laser is hampered by a lack of targeted power delivery solutions over large distances (>100 mm). Here we demonstrate the efficient coupling of double-metal quantum cascade lasers into flexible polystyrene lined hollow metallic waveguides via the use of a hollow copper waveguide integrated into the laser mounting block. Our approach exhibits low divergence, Gaussian-like emission, which is robust to misalignment error, at distances > 550 mm, with a coupling efficiency from the hollow copper waveguide into the flexible waveguide > 90%. We also demonstrate the ability to nitrogen purge the flexible waveguide, increasing the power transmission by up to 20% at 2.85 THz, which paves the way for future fibre based terahertz sensing and spectroscopy applications.

Investigation of metal sulfide optical thin film growth in low-loss IR hollow glass waveguides

In this study, the film growth kinetics for near and mid-IR reflection enhancing CdS and PbS dielectric thin films in HGWs is experimentally established. Crucial fabrication parameters including solution concentrations, pH, and fluid velocity are optimized. The film thickness of these films in HGWs is studied as a function of deposition time and temperature. Through IR spectral response analysis, the dielectric thin film thicknesses were determined and found to have a strong linear time dependence. Accurate metal sulfide film growth models in HGWs were developed, allowing for direct determination of necessary deposition times to yield metal sulfide HGW thin film coatings having a desired response.

Theoretical and experimental investigation of infrared properties of tapered silver/silver halide-coated hollow waveguides

Silver/silver halide-coated hollow-glass waveguides (HGWs) are capable of low-loss, broadband transmission at infrared wavelengths with the advantage of optical response tunability through alteration of a number of key design parameters. Generally, the design of circular HGWs has primarily involved optimization of the waveguide bore size and deposited film structure in order to obtain the desired optical response, with the waveguide bore size being held constant as a function of length. In this study, the effects of HGW structures consisting of linearly tapered inner diameters on the optical response at infrared wavelengths are theoretically and experimentally investigated. Theoretical analysis involving numerical ray optics methods accounting for the dynamic nature of bore size, and consequently light propagation, along the waveguide length is presented and compared to experimental results in order to gain a deeper understanding of these atypical HGW structures.

Fabrication and characterization of improved Ag/PS hollow-glass waveguides for THz transmission

This study involves the fabrication and characterization of improved quality silver (Ag)/polystyrene (PS) thin-film-coated hollow-glass waveguides for the low-loss transmission of terahertz radiation via modified dynamic liquid phase deposition techniques. High-quality PS thin films were deposited from aqueous PS solutions, and the spectral response of fabricated samples was measured from λ=1-100 μm. Fabricated samples exhibited highly defined spectral responses throughout this entire range indicative of PS films of excellent quality. The spectra of experimental samples were compared to the theoretical and bulk PS spectra in the near-IR and far-IR regions. The thickness of deposited PS thin films was found to depend on total sample length and to vary from approximately 10-16 μm for sample lengths ranging from 115 to 140 cm. Such PS film thicknesses are adequate for low-loss delivery from approximately 2-4 THz. Furthermore, film thickness was found to vary minimally along the waveguide length regardless of total sample length.

Investigation of silver-only and silver/TOPAS coated hollow glass waveguides for visible and NIR laser delivery

Hollow Glass Waveguides (HGWs) present a viable option for the low-loss transmission of radiation over a broad range spanning from visible to far-infrared wavelengths. Cyclic Olefin Copolymer (COC), a commercially available polymer known as TOPAS®, is chosen for this study due to its exceptionally low absorption losses throughout the spectrum, particularly in the visible and near-infrared (NIR) regions. While silver-coated HGWs are capable of transmitting visible and NIR radiation with low losses, theory predicts that the addition of a uniform dielectric thin film of quarter wavelength thickness will reduce these losses for both straight and bent configurations, while additionally providing a potentially more desirable modal output for laser applications. In this paper, the procedures for the deposition of the silver and subsequent COC films are outlined. Spectroscopy is used to obtain the thickness of the polymer film. The theoretical attenuation losses of the silver and Ag/COC HGWs are explored and experimental values are obtained using various visible and IR lasers. Moreover, the modal output of the silver and Ag/COC HGWs is qualitatively compared. The possibility of use of these Ag/COC HGWs at mid- and far-IR wavelengths is discussed.

Silver-coated Teflon hollow waveguides for the delivery of terahertz radiation

Significant research exists regarding the successful implementation of hollow waveguides for the low-loss transmission of infrared radiation in applications ranging from laser power delivery to spectroscopy. With the continued development of terahertz (THz) technologies and applications, it is often advantageous to have a waveguide for the transmission of THz radiation. This study focuses on the fabrication of novel silver-coated polytetrafluoroethylene (PTFE) waveguides for the transmission of terahertz radiation. The hollow structure described in this paper is made by depositing a thin film of Ag on the outer surface of a dielectric tube. This is in contrast to depositing metallic and dielectric thin film coatings on the inner surface of capillary tubing as is commonly done for IR and some THz transmissive waveguides. In this work, the Teflon tubing itself is the dielectric layer that is used to enhance the reflectivity of the Ag. Theoretical loss calculations will be presented and compared to the loss obtained for the guides measured at THz frequencies. In addition the spectra of the guides in the infrared region are also measured as a means to study the uniformity of the Teflon “layer” and to confirm the wall thickness of the Teflon tubing. The surface topography of the silver / PTFE waveguides is obtained and the resulting surface roughness related scattering losses are calculated. The implications of the terahertz fiber for applications ranging from nondestructive evaluation (NDE), security, and medical imaging are briefly discussed.

Fiber optic probes based on silver-only coated hollow glass waveguides for ionizing beam radiation dosimetry

Čerenkov contamination is a significant issue in radiation detection by fiber-coupled scintillators. To enhance the scintillation signal transmission while minimizing Čerenkov contamination, we designed a fiber probe using a silver-only coated hollow waveguide (HWG). The HWG tip with inserted scintillator, embedded in tissue mimicking phantoms, was irradiated with clinical electron and photon beams. Optical spectra of irradiated tips were taken using a fiber spectrometer, and the signal was deconvolved with a linear fitting algorithm. The resultant decomposed spectra of the scintillator with and without Čerenkov correction were in good agreement with measurements performed by an electron diode and ion chamber for electron and photon beam dosimetry, respectively, indicating the minimal effect of Čerenkov contamination. Compared with a silver/dielectric coated HWG fiber dosimeter design we observed higher signal transmission in our design based on the use of silver-only HWG.

Investigation of hollow cylindrical metal terahertz waveguides suitable for cryogenic environments.

The field of terahertz (THz) waveguides continues to grow rapidly, with many being tailored to suit the specific demands of a particular final application. Here, we explore waveguides capable of enabling efficient and accurate power delivery within cryogenic environments (< 4 K). The performance of extruded hollow cylindrical metal waveguides made of un-annealed and annealed copper, as well as stainless steel, have been investigated for bore diameters between 1.75 - 4.6 mm, and at frequencies of 2.0, 2.85 and 3.4 THz, provided by a suitable selection of THz quantum cascade lasers. The annealed copper resulted in the lowest transmission losses, < 3 dB/m for a 4.6 mm diameter waveguide, along with 90° bending losses as low as ~2 dB for a bend radius of 15.9 mm. The observed trends in losses were subsequently analyzed and related to measured inner surface roughness parameters. These results provide a foundation for the development of a wide array of demanding low-temperature THz applications, and enabling the study of fundamental physics.