Qirong Xing

Terahertz superconductor metamaterial

We characterize the behavior of split ring resonators made up of high transition temperature yttrium barium copper oxide superconductor using terahertz time-domain spectroscopy measurements and numerical simulations. The superconductor metamaterial is found to show a remarkable change in the transmission spectra at the fundamental inductive-capacitive resonance as the temperature dips below the critical transition temperature. This resonance switching effect is normally absent in traditional metamaterials made up of regular metals. The temperature-dependent resonance behavior of the superconducting metamaterial would lead to development of low loss terahertz switches at cryogenic temperatures.

Broadband resonant terahertz transmission in a composite metal-dielectric structure

We present a systematic numerical study of a metal-dielectric-metal sandwich plasmonic structure for broadband resonant transmission at terahertz frequencies. The proposed structure consists of periodic slotted metallic arrays on both sides of a thin dielectric substrate and is demonstrated to exhibit a broad passband transmission response. Various design considerations have been investigated to exploit their influence on the transmission passband width and the center resonance frequency. The structure ensures a broadband transmission over a wide range of incident angles.

A close-ring pair terahertz metamaterial resonating at normal incidence

We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by double-layer, self-aligned photolithography. Terahertz time-domain spectroscopy transmission measurements and numerical simulations have revealed negative index of refraction in the frequency range of 0.66-0.90 THz under normal wave incidence. The observed resonance behaviors can be well explained by a theoretical circuit model. The electromagnetic properties and the figure of merit of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell with a goal of providing optimized design for three-dimensional metamaterials and potential device applications.

Environmentally Stable, High Pulse Energy Yb-Doped Large-Mode-Area Photonic Crystal Fiber Laser Operating in the Soliton-Like Regime

A high pulse energy passively mode-locking fiber laser operating in the soliton-like regime is demonstrated. The laser is based on a linear cavity design. A segment of Yb-doped single-polarization large-mode-area photonic crystal fiber serves as the gain medium, and the self-starting mode-locking is achieved by a high contrast semiconductor saturable absorber mirror. The laser directly generates 600-fs pulses with 900 mW of average power at a repetition rate of 47.3 MHz, corresponding to a single pulse energy of 19 nJ. Furthermore, this fiber laser is directly used for pumping ZnTe to generate broadband terahertz radiation, resulting in a compact terahertz source.

Terahertz superconducting plasmonic hole array

We demonstrate a superconductor array of subwavelength holes with active thermal control over the resonant transmission induced by surface plasmon polaritons. The array was lithographically fabricated on a high-temperature yttrium barium copper oxide superconductor and characterized by terahertz time-domain spectroscopy. We observe a clear transition from a virtual excitation of the surface plasmon mode to a real surface plasmon mode. The highly controllable superconducting plasmonic crystals may find promising applications in the design of low-loss, large- dynamic-range amplitude modulation and surface-plasmon-based terahertz devices.

Femtosecond laser-induced cell fusion

Femtosecond laser was employed to induce cell fusion. The interface between two protoplasts of Phaffia rhodozyma in contact was irradiated by femtosecond laser pulses with 1.38×104W for 0.25s. After about 20min, the fusion of cells became visible and finally two cells merged into one larger cell within 160min. Fusion rate at this power level was 80%. The experiments suggested the existence of threshold power for cell fusion induced by femtosecond laser. Regarding femtosecond laser pulses as a trigger, we present a hypothetical model for femtosecond laser-induced cell fusion.

Regular, period-doubling, quasi-periodic, and chaotic behavior in a self-mode-locked Ti:sapphire laser

Regular, period-doubling, quasi-periodic and chaotic behavior in a self-mode-locked Ti:sapphire laser was observed under fixed pump power during alignment of the laser cavity.

Broadband Terahertz Pulses Generated by a Compact Femtosecond Photonic Crystal Fiber Amplifier

We demonstrate a scalable, compact, high-power, and broadband terahertz (THz) source based on a cutting-edge large-mode-area photonic-crystal-fiber amplifier system. A 3-mm (110)-cut bulk GaP crystal was used as the THz emitter based on an optical rectification technique. Systematic optimization of the operation parameters allowed a THz output up to 150 μW to be achieved with an input laser power of 12 W in the GaP crystal.

Enhanced zero-order transmission of terahertz radiation pulses through very deep metallic gratings with subwavelength slits

Very deep metallic grating structures with subwavelength slits are processed to study the transmission properties of terahertz radiation pulses. The experiments have been performed with two samples. The delay of the terahertz pulses and the corresponding enhanced unresonant transmission spectra through the samples are observed. A simple interpretation is presented based on the combination of zero-order diffraction of the gratings with the coupling of the surface plasmon polaritons formed on the top and bottom of the gratings. The experimental results are in good agreement with theoretical calculations.

Large dynamic resonance transition between surface plasmon and localized surface plasmon modes

We present resonant terahertz transmission in a composite plasmonic film comprised of an array of subwavelength metallic patches and semiconductor holes. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The reversal in transmission amplitude from a stop-band to a pass-band and up to pi/2 phase shift achieved in the composite plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz plasmonics.