Victoria Astley

Characterization of terahertz field confinement at the end of a tapered metal wire waveguide

We present experimental verification of the possibility of strong subwavelength confinement of the terahertz electric field at the end of a tapered metal wire waveguide. The axial field component at the end of the tapered waveguide shows a lateral confinement that is an order of magnitude greater than an untapered waveguide, and over 100 times greater than the free-space wavelength. The axial component is also strongly confined in the propagation direction, in contrast to the radial field component. Comparison to numerical simulation yields excellent agreement when the effect of the detecting probe is included in the analysis.

Mechanically flexible polymeric compound one-dimensional photonic crystals for terahertz frequencies

We describe the fabrication and characterization of fully flexible one-dimensional photonic crystals for terahertz frequencies based on alternating layers of a highly refracting polymericcompound and a pure polymer. Due to the high permittivity contrast and low intrinsic material absorption, even a low number of layers yields structures with a pronounced artificial band gap, with a center frequency reflectivity close to unity. Besides the high filter performance, the polymericcompound one-dimensional photonic crystals can be employed for beam shaping by applying a curvature to the mechanically flexible structures, since the spectral characteristics remain nearly unchanged when the structure is flexed.

High-contrast terahertz modulator based on extraordinary transmission through a ring aperture.

We demonstrated extraordinary THz transmission through ring apertures on a metal film. Transmission of 60% was obtained with an aperture-to-area ratio of only 1.4%. We show that the high transmission can be suppressed by over 18 dB with a thin layer of free carriers in the silicon substrate underneath the metal film. This result suggests that CMOS-compatible terahertz modulators can be built by controlling the carrier density near the aperture.

The metal-insulator transition in VO2 studied using terahertz apertureless near-field microscopy

We have studied the metal-insulator transition in a vanadium dioxide (VO2) thin film using terahertz apertureless near-field optical microscopy. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of micron-scale metallic domains in inhomogeneous VO2 thin films.

Bending and coupling losses in terahertz wire waveguides.

We present an experimental study of several common perturbations of wire waveguides for terahertz pulses. Sommerfeld waves retain significant signal strength and bandwidth even with large gaps in the wire, exhibiting more efficient recoupling at higher frequencies. We also describe a detailed study of bending losses. For a given turn angle, we observe an optimum radius of curvature that minimizes the overall propagation loss. These results emphasize the impact of the distortion of the spatial mode on the radiative bend loss.

A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging

Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting extreme subwavelength spatial resolution. This technique has recently been adapted for use in the terahertz region of the spectrum. Here, the interpretation of the measured signals and the suppression of background scattering can be complicated by the extremely broad bandwidth of the terahertz source and by the coherent (i.e., phase-sensitive) detection of the scattered radiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus are relevant when phase-sensitive detection is used. We use a dipole antenna model to explain the spatial variation in the scattered signal. Since t...

Analysis of rectangular resonant cavities in terahertz parallel-plate waveguides

We describe an experimental and theoretical characterization of rectangular resonant cavities integrated into parallel-plate waveguides, using terahertz pulses. When the waveguide is excited with the lowest-order transverse-electric mode, these cavities exhibit resonances with narrow linewidths. Broadband transmission spectra are compared with the results of mode-matching calculations, for various cavity dimensions.

A terahertz dual wire waveguide

We numerically model the propagation of terahertz radiation along a double wire waveguide using finite element analysis. This is a promising alternative configuration for terahertz waveguiding.

Fully flexible terahertz Bragg reflectors based on titania loaded polymers

We demonstrate fully flexible, high performance terahertz Bragg reflectors and filters based on TiO2-loaded polymers. Broadband terahertz reflection measurements with corresponding transfer matrix simulations are shown and the effect of curvature is analyzed.

A mode-matching analysis of dielectric-filled resonant cavities coupled to terahertz parallel-plate waveguides

We use the mode-matching technique to study parallel-plate waveguide resonant cavities that are filled with a dielectric. We apply the generalized scattering matrix theory to calculate the power transmission through the waveguide-cavities. We compare the analytical results to experimental data to confirm the validity of this approach.