Yuye Wang

InP- and GaAs-Based Plasmonic High-Electron-Mobility Transistors for Room-Temperature Ultrahigh-Sensitive Terahertz Sensing and Imaging

This paper reviews recent advances in the design and performance of our original InP- and GaAs-based plasmonic high-electron-mobility transistors (HEMTs) for ultrahighly-sensitive terahertz (THz) sensing and imaging. First, the fundamental theory of plasmonic THz detection is briefly described. Second, single-gate HEMTs with parasitic antennae are introduced as a basic core device structure, and their detection characteristics and sub-THz imaging potentialities are investigated. Third, dual-grating-gate (DGG)-HEMT structures are investigated for broadband highly sensitive detection of THz radiations, and the record sensitivity and the highly-sensitive THz imaging are demonstrated using the InP-based asymmetric DGG-HEMTs. Finally, the obtained results are summarized and future trends are addressed.

Tunable terahertz-wave generation from DAST crystal pumped by a monolithic dual-wavelength fiber laser

For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.

Composite THz materials using aligned metallic and semiconductor microwires, experiments and interpretation

We report fabrication method and THz characterization of composite films containing either aligned metallic (tin alloy) microwires or chalcogenide As2Se3 microwires. The microwire arrays are made by stack-and-draw fiber fabrication technique using multi-step co-drawing of low-melting-temperature metals or semiconductor glasses together with polymers. Fibers are then stacked together and pressed into composite films. Transmission through metamaterial films is studied in the whole THz range (0.1-20 THz) using a combination of FTIR and TDS. Metal containing metamaterials are found to have strong polarizing properties, while semiconductor containing materials are polarization independent and could have a designable high refractive index. Using the transfer matrix theory, we show how to retrieve the complex polarization dependent refractive index of the composite films. Finally, we study challenges in the fabrication of metamaterials with sub-micrometer metallic wires by repeated stack-and-draw process by comparing samples made using 2, 3 and 4 consecutive drawings. When using metallic alloys we observe phase separation effects and nano-grids formation on small metallic wires.

Dual-wavelength single-crystal double-pass KTP optical parametric oscillator and its application in terahertz wave generation

A tunable dual-wavelength double-pass singly resonant optical parametric oscillator based on a single KTP crystal has been proposed and demonstrated. By setting the rear mirror tuning angle precisely, collinear and noncollinear dual phase matching can be established simultaneously and leads to a two-wavelength oscillation at steady state. With high-speed KTP angle tuning, two wavelengths and their frequency difference can be adjusted continuously and accurately. By using this newly developed dual-wavelength optical parametric oscillator to pump a DAST crystal, a monochromatic terahertz wave ranging from 0.5 to 3 THz has been successfully generated in a difference-frequency generation system.

Study of water concentration measurement in thin tissues with terahertz-wave parametric source

Water concentration and distribution in biotissues are important factors in many applications. THz-wave is a viable tool for water content measurement due to its highly sensitivity to water. In this study, the measuring errors of water concentration using THz-wave induced by transmittance and sample thickness were analyzed theoretically. The chosen basis for sample thickness and measuring THz frequency were presented theoretically. Measurements of the water two-dimensional mapping in different animal tissue samples were demonstrated experimentally, which clearly shows the spatial distribution of the tissues.

Ultrahigh sensitive plasmonic terahertz detectors based on an asymmetric dual-grating gate HEMT structure

We report on ultrahigh sensitive, broadband terahertz (THz) detectors based on asymmetric double-grating-gate (A-DGG) high electron mobility transistors, demonstrating a record responsivity of 2.2 kV/W at 1 THz with a superior low noise equivalent power of 15 pW/√Hz using InGaAs/InAlAs/InP material systems. When THz radiation is absorbed strong THz photocurrent is first generated by the nonlinearity of the plasmon modes resonantly excited in undepleted portions of the 2D electron channel under the high-biased sub-grating of the A-DGG, then the THz photovoltaic response is read out at high-impedance parts of 2D channel under the other sub-grating biased at the level close to the threshold. Extraordinary enhancement by more than two orders of magnitude of the responsivity is verified with respect to that for a symmetric DGG structure.

Sensitive water concentration mapping in thin fresh tissues using tunable THz-wave parametric oscillator

Sensitive water concentration mapping in thin animal tissue samples has been demonstrated using tunable monochromatic THz-wave parametric source. A novel sample preparation approach is performed to effectively preserve tissue freshness at room temperature. The time course results show the sample characteristic of water content and distribution can be well measured and excellently repeated in 70minutes with a standard deviation of less than 1%. These results suggest the method of water volume concentration and distribution measurement using THz-wave has good stability with proper sample preparation, which has great potential in the fields of medical and biological diagnosis.

Solution growth of an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal for DFG-THz source

A monochromatic THz radiation source with broadband tunability utilizing N-benzyl-2-methyl-4-nitroaniline (BNA) crystal has been developed. BNA quality was improved by applying solution growth method instead of melt method. BNA-THz source using difference frequency generation was further optimized.

Optimization of Broadly Tunable BNA-DFG Terahertz-Wave Source

Broadly tunable terahertz wave source utilizing difference frequency generation in a organic BNA crystal is developed. By using dual KTP-OPO optimized for phase matching condition, wideband THz-wave generation from sub- to 20 THz is achieved.

Tunable narrow linewidth THz-wave generation using dual-wavelength fiber ring laser and organic DAST crystal

We demonstrated a novel single longitudinal mode dual-wavelength fiber ring laser with continuously tunable wavelength spacing. Continuous-wave Terahertz radiation from sub-THz to 2 THz was generated through difference-frequency generation of two laser wavelengths in a nonlinear DAST crystal.