Tzeng-Fu Kao

A simple terahertz spectrometer based on a low-reflectivity Fabry-Perot interferometer using Fourier transform spectroscopy

A simple terahertz (THz) Fourier transform spectrometer is theoretically and experimentally demonstrated with a low-reflectivity Fabry-Perot interferometer. Composed with only two parallel low reflectivity surfaces, this simple spectrometer has the ability to measure the spectrum over more than one octave with a controllable resolution. The emission spectra of a wavelength-tunable photonic transmitter excited by an optical coherent control system are determined by the spectrometer. With a simple algorithm for the spectral reconstruction and a compact structure with easy alignment, the demonstrated spectrometer will meet the needs of many important applications where a compact and convenient spectrometer is required in the frequency range from sub-THz wave up to even visible light.

Frequency Tunability of Terahertz Photonic Transmitters

In this study we investigated the frequency tunability of an edge-coupled membrane photonic transmitter. Its frequency tunability was found to be determined, not only by the radiation loss of the device and antenna pattern, but also by the frequency response of the photodetector. With a properly designed device pattern with a slot dipole antenna, which is known to have a strong resonant peak, both high-power conversion efficiency and a broadband response can be achieved. A wide frequency tuning range from <100GHz up to 1.1THz is demonstrated from a single terahertz photonic transmitter. Combined with coherent-controlled quasicontinuous-wave optical excitation, this optoelectronic-based device also exhibits a record-high light-THz power-conversion efficiency of 0.33%.

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

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.

Compositional dependence of longitudinal sound velocities of piezoelectric (111) InxGa(1−x)As measured by picosecond ultrasonics

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.

Low-loss Subwavelength Plastic Fiber for Terahertz Wave Guiding

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 microm 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.

Terahertz Fourier transform spectrometer based on a low-reflectivity Fabry-Perot interferometer

A simple THz Fourier transform spectrometer is demonstrated by using a low-reflectivity Fabry-Perot Interferometer. The emission spectra of a wavelength-tunable photonic transmitter excited by an optical coherent control system are determined by the spectrometer.

Terahertz biochip based on optoelectronic devices

The accurate detection of minute amounts of chemical and biological substances has been a major goal in bioanalytical technology throughout the twentieth century. Fluorescence dye labeling detection remains the effective analysis method, but it modifies the surroundings of molecules and lowering the precision of detection. An alternative label free detecting tool with little disturbance of target molecules is highly desired. Theoretical calculations and experiments have demonstrated that many biomolecules have intrinsic resonance due to vibration or rotation level transitions, allowing terahertz (THz)-probing technique as a potential tool for the label-free and noninvasive detection of biomolecules. In this paper, we first ever combined the THz optoelectronic technique with biochip technology to realize THz biosensing. By transferring the edge-coupled photonic transmitter into a thin glass substrate and by integrating with a polyethylene based biochip channel, near field THz detection of the biomolecules is demonstrated. By directly acquiring the absorption micro-spectrum in the THz range, different boiomecules can then be identified according to their THz fingerprints. For preliminary studies, the capability to identity different illicit drug powders is successfully demonstrated. This novel biochip sensing system has the advantages including label-free detection, high selectivity, high sensitivity, ease for sample preparation, and ease to parallel integrate with other biochip functionality modules. Our demonstrated detection capability allows specifying various illicit drug powders with weight of nano-gram, which also enables rapid identification with minute amounts of other important molecules including DNA, biochemical agents in terrorism warfare, explosives, viruses, and toxics.

Highly-directed terahertz photonic transmitter by using the design of planar antenna arrays

By adopting a novel CPW-fed rampart slot array antenna, we demonstrate a compact highly-directed THz photonic transmitter with a single photodetector source. 3dB radiation beam widths in both E- and H-planes are significantly reduced.

Design and analysis of surface plasmon-enhanced metal-semiconductor-metal traveling wave photodetectors

We propose and analyze the bandwidth and efficiency issues of surface-plasmon-resonance (SPR) enhanced metal-semiconductor-metal traveling-wave photodetectors. With strong field confinement near the nano-sized metal fingers, SPR-effect is found to greatly enhance the devices highspeed performance.

Low-loss subwavelength THz plastic fibers

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