Yew Li Hor

Nondestructive evaluation of cork enclosures using terahertz/millimeter wave spectroscopy and imaging

Natural cork enclosures, due to their cell structure, composition, and low moisture are fairly transparent to terahertz (THz) and millimeter waves enabling nondestructive evaluation of the corks surface and interior. It is shown that the attenuation coefficient of the defect-free cork can be modeled with a Mie scattering model in the weakly scattering limit. Contrast in the THz images is a result of enhanced scattering of THz radiation by defects or voids as well as variations in the cork cell structure. The presence of voids, defects, and changes in grain structure can be determined with roughly 100-300 microm resolution.

Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy.

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.

Terahertz response of microfluidic-jetted three-dimensional flexible metamaterials

We demonstrate the fabrication and characterization of three-dimensional (3D) metamaterials in the terahertz (THz) range using the microfluidic-jetted technique. This technique has proven a convenient technique to fabricate metamaterial structures at the micrometer scale. The metamaterials are fabricated using dodecanethiol functionalized gold nanoparticles on flexible polyimide substrates. The metamaterials consist of alternate layers of single split-ring resonator and microstrip arrays that are stacked to form a 3D metamaterial medium. The fabricated metamaterials, with lattice sizes of 180 microm, are characterized using THz time-domain spectroscopy within 0.1 to 2 THz in the transmission mode. Numerical simulation is performed to calculate the effective metamaterials parameter.

Square-shaped metal screens in the infrared to terahertz spectral region : Resonance frequency, band gap, and bandpass filter characteristics

Resonance frequency of freestanding, square-shaped thick metal screens have been studied here in the wavelength range of infrared (IR) to mm (20 to 0.2 THz). It was found that their peak transmission has a linear relationship to the screen’s pitch. An experimental spectral feature, unaccounted for in typical simulations with plane parallel incident beams, was observed in the transmittance envelope for measurements in focused beams. In the past, this spectral feature was assigned to Wood’s anomaly. Yet, unlike the latter, the observed spectral feature appears here in the long wavelength regime as well. We investigated this phenomenon for a large frequency range and assigned the spectral feature to the formation of a photonic band gap at oblique incidence. Many IR Fourier transform spectrometers use a noncollimated incident beam and such spectral features will appear whenever the local state of polarization includes components which are parallel to the plane of incidence.

Veselago-Pendry superlens imaging modeled with a spectral waveguide approach

A compact spectral waveguide model of a superlens imaging system based on the Fourier decomposition of the source field into waveguide modes is presented. This model maps the electromagnetics of the superlens into that of the well-understood waveguide with concomitant advantages in the analysis of dynamics and reality effects and in accurate numerical simulation. Insights into the dynamic response of the superlens, gained from both theoretical and numerical studies, are presented. The effect of loss on the dynamic properties is investigated. In addition, the proposed model leverages a wealth of expertise available for the design of filters, artificial dielectrics and backward wave structures and could possibly aid in the engineering of practical super-resolution imaging systems that will be an enabling technology for future nanoelectronics systems.

Below-Cutoff Properties of Uniform and Tapered Nonradiating Dielectric Waveguides

Abstract This paper aims to explore the suitability of Non-Radiating Dielectric (NRD) waveguide for probing the near-field of sub-wavelength sized objects. The NRD wave properties in the propagating regime of the dominant LSM mode are well known and documented in the literature. However, its transmission behavior below cutoff has hardly been explored. In this paper we first characterize the wave properties of NRD waveguides below cutoff by solving the transverse resonance condition for imaginary longitudinal propagation constants. We then investigate the transmission properties of linear NRD tapers using full-wave time domain simulations. These simulations indicate that tapered NRD sections can indeed transfer evanescent near-field information into the far-field and may be suitable as probes for high-resolution imaging.

Terahertz response of microfluidic-jetted fabricated 3D multilayer metamaterials

The metamaterials (MTMs) are defined as artificial materials which will achieved extraordinary electromagnetic properties that are not available in natural material. Due to their embryonic nature, MTMs evolve in rapid speed in this few years. The science of MTMs, particularly left-handed metamaterials (LH MTMs), has been well established theoretically, numerically, and experimentally in the literature.