Shengjiang Chang

Liquid-crystal-filled photonic crystal for terahertz switch and filter

The effects of magnetic birefringence of liquid crystal E7 on the photonic bandgaps and the transmitting properties in two-dimensional photonic crystal waveguides are investigated by using the plane wave expansion and finite-difference time domain methods. Detailed calculations on the shifts and variations in the frequency width of bandgaps reveal that under the control of an external magnetic field the two-dimensional liquid-crystal-filled photonic crystal waveguide can serve as a switch and continuously tunable bandpass filter at the terahertz wave band.

Magnetically Tunable Terahertz Isolator Based on Structured Semiconductor Magneto Plasmonics

A tunable terahertz (THz) isolator based on a periodically structured semiconductor magneto plasmonics is proposed. The unique photonic band-gap and one-way transmission property of this structure with different magnetic fields and temperature are investigated in the THz regime. The numerical results show the proposed isolator has a bandwidth of 80 GHz with the maximum isolation of higher than 90 dB and a low insertion loss of 5%. The central operating frequency of this isolator can be broadly tuned from 1.4 to 0.9 THz by changing the external magnetic field from 0.6 to 1.6 Tesla at 195 K. This low-loss high isolation broadband nonreciprocal THz transmission mechanism has great potential applications in promoting the performances of THz application systems.

Design and fabrication of micro-cube-corner array retro-reflectors

A novel approach of modeling and analyzing the two types of micro-cube-corner array retro-reflectors is proposed. Light tracing algorithm for determining the maximum incident angle range limited by the critical angle of total reflection, and reverse-projection (RP) algorithm for calculating the available area of an isolated unit are described. Simulation results and fabrication method of micro-cube-corner array by moving-mask technique are also given.

Terahertz modulator based on insulator–metal transition in photonic crystal waveguide

A terahertz modulator based on the insulator-metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8-1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

Eye gaze estimation from the elliptical features of one iris

The accuracy of eye gaze estimation using image information is affected by several factors which include image resolution, anatomical structure of the eye, and posture changes. The irregular movements of the head and eye create issues that are currently being researched to enable better use of this key technology. In this paper, we describe an effective way of estimating eye gaze from the elliptical features of one iris under the conditions of not using an auxiliary light source, a head fixing equipment, or multiple cameras. First, we provide preliminary estimation of the gaze direction, and then we obtain the vectors which describe the translation and rotation of the eyeball, by applying a central projection method on the plane which passes through the line-of-sight. This helps us avoid the complex computations involved in previous methods. We also disambiguate the solution based on experimental findings. Second, error correction is conducted on a back propagation neural network trained by a sample collection of translation and rotation vectors. Extensive experimental studies are conducted to assess the efficiency, and robustness of our method. Results reveal that our method has a better performance compared to a typical previous method.

Terahertz polarization splitter based on orthogonal microstructure dual-core photonic crystal fiber.

A broadband polarization splitter operating in the terahertz (THz) band is proposed based on dual-core photonic crystal fiber with orthogonal microstructure in the core regions. The Index Converse Matching Coupling method is presented to design the THz polarization splitter for the first time, which exhibits several advantages, such as short splitting length, high extinction ratio, low loss, and broad operation bandwidth. By numerical simulation, it has been found that the strong coupling occurs within a frequency range of 0.4-0.7 THz. The operation bandwidth is more than 0.15 THz (equal to 138 μm). The shortest splitting length is only 1.83 cm at 0.4 THz. The extinction ratios for both of x and ypolarization are better than -15 dB when the frequency is larger than 0.51 THz. The lowest material absorption loss is only 0.34 dB at 0.4 THz. Moreover, this structure is simple to design and easy to fabricate over its counterparts in the communication band. Our research offers an effective method to design a broadband THz device and would be of significance for future relevant applications.

Efficient two-photon absorption of CdSe-CdS/ZnS core-multishell quantum dots under the excitation of near-infrared femtosecond pulsed laser

We report that two-photon absorption (TPA) properties of semiconductor CdSe-core CdS/ZnS-multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and 16060 GM (1 GM = 10−50 cm4 s photon−1), respectively. TPA cross section dramatically increases with increased core size, showing a strong size-dependence effect. Furthermore, two-photon excitation (TPE) fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the narrow fluorescence bandwidth make these QDs as promising probes for multicolor two-photon microscopy.

Subwavelength B-shaped metallic hole array terahertz filter with InSb bar as thermally tunable structure

We propose a subwavelength B-shaped metallic hole array filter with an embedded thermally tunable InSb semiconductor bar in the terahertz regime. The resonance frequency of this filter can be actively tuned by controlling the temperature of InSb. The transmissions of the filter are calculated with the finite-difference time domain method at various temperatures. Narrowband THz wave with the full width at half maximum of 235 GHz can be selected in the frequency range from 0.74 to 2.02 THz at temperatures from 160 to 350 K.

Multifunctional photonic crystal cross waveguide for terahertz waves

A multifunctional cross waveguide is designed based on the photonic crystal structure and the liquid crystal material. The different states of the cross waveguide controlled by the electric field make its various functions possible, including a switch with a high extinction ratio, a splitter that divides the terahertz wave into the desired proportions, and a through or 90° turn waveguide. The plane wave expansion method is used to calculate the bandgap in the photonic crystals, and coupling mode theory is adopted to analyze and eliminate the reflection loss. The finite element method is used to get the proper distribution of the external electric field. The properties of the cross waveguide are simulated by the finite difference time domain method, and the results show that the cross waveguide is a multifunctional device with high performance characteristics.

Terahertz Single-Polarization Single-Mode Hollow-Core Fiber Based on Index-Matching Coupling

We have proposed and analyzed a novel terahertz (THz) single-polarization single-mode (SPSM) hollow-core fiber based on the index-matching coupling method. However, its coupling principle is different from the solid-core SPSM fiber. The confinement loss of the index-matched polarization state is increased slightly, while that of the other polarization state is enhanced by nearly two orders of magnitude. In particular, the confinement loss of the index-matched polarization state is only 0.004 dB/m at f=1.675 THz.