Hong Su

Generation of coherent terahertz radiation with multifrequency modes in a Fibonacci optical superlattice

We investigate theoretically the generation of coherent narrow-band terahertz (THz) radiation with multifrequency modes in an optical superlattice produced by quasiperiodically poled ferroelectric crystal. The superlattice consists of two building blocks A and B, in which each containing a pair of antiparallel 180° ferroelectric domains, arranged as a Fibonacci sequence. Second-order nonlinear optical rectification of femtosecond pulses gives rise to a THz wave form whose multifrequency modes are determined by the quasiperiodic structure properties. The relationship between multifrequency modes terahertz generation and Fibonacci structure parameters is also analyzed and studied.

Enhanced transmission of THz radiation through sub-wavelength, asymmetry metallic hole arrays

Strongly enhanced transmission of terahertz radiation pulses through asymmetry rectangular hole arrays with various hole lengths are obtained experimentally. The transmission magnitudes increase and their positions shift to lower frequency with increasing hole length. The maximum transmittance peak is observed for the sample with a hole length of 400 μm. Apparent angle-dependent behavior is observed in the transmission spectra of the 200 × 400 μm2 hole array, which due to the size effect of the rectangular hole along the two orthogonal axes. The resonance frequencies are deduced according to the surface plasmon polaritons modes. Our experiment results closely match the theoretical calculations.

Surface plasmon-polaritons on very thin metal tubes

A thin metal tube is proposed to guide surface plasmon-polaritons (SPPs). We derive the dispersion equation of the SPPs, and then solve the equation as a function of the tube thickness. It is indicated that both long-range (LR) SPPs and short-range (SR) SPPs can be guided by the metal tube. For the LR-SPPs the loss coefficient goes to zero as the thickness of the tube decreases, thus the propagation distance can be much longer than that of the SPPs propagating on a metal wire. For the SR-SPPs, compared with the SPPs on the wire, the mode distribution is much sharper.

Multigrating quasi-phase-matched optical parametric oscillation in periodically poled MgO:LiNbO3 device

A widely tunable middle-infrared quasi-phase-matched optical parametric oscillation that uses a periodically poled magnesium-oxide-doped lithium niobate with a multigrating structure is reported. The optical parametric oscillation device is tuned by translating the crystal through the resonator and the pump beam, with no realignment needed. The results show that the idler range lies in 3.6–4.1μm in 0.1μm increment as the pump wavelength is fixed at 1.064μm. The grating periods on the crystal and the threshold pump energy in this experiment are verified, respectively.

Periodically poled LiNbO3: Optical parametric oscillation at wavelengths larger than 4.0μm with strong idler absorption by focused Gaussian beam

Coupled-wave equations for focused Gaussian beams in an optical superlattice are derived. By solving the coupled equations, we investigate theoretically the optimum design parameters for the middle-infrared wavelength generation under the configuration of a singly resonating optical parametric oscillation with strong idler absorption. A positive dephasing is required due to the Gouy effect for optimal idler generation. For idler wavelengths of up to 5.0μm, the threshold power can be kept at a lower level over a wider range of dephasing. Beyond this, the threshold power increases with the absorption coefficient of the idler wave. This model accurately predicts the current experimental results with weaker idler absorption. Validation with strong idler absorption is also verified.

Self- Q -switching phase-conjugation Nd:YAG laser with a stimulated-Brillouin-scattering mirror

We present a numerical and experimental study of a self-Q-switching phase-conjugation Nd:YAG laser oscillator with a stimulated-Brillouin-scattering mirror. Q-switching is provided only by a nonlinear liquid in the cavity without further need for any shutter. During the stimulated-Brillouin-scattering process, through switching by intracavity radiation, laser beam spot-size widening occurs because of imperfect phase conjugation. It is demonstrated that the results of numerical simulation of the intracavity photon number, pulse duration, and laser beam spot size at the output coupling mirror for the laser are comparable with those observed experimentally.

Terahertz generation in an aperiodically poled lithium niobate parametric oscillator

In this report, a required aperiodically poled structure is designed, where two quasi-phase-matched conditions of optical parametric oscillator and frequency difference are required to be satisfied simultaneously. By numerical simulation, we analyze the effects of crystal structure, the cavity parameter of optical parametric oscillator, and terahertz absorption coefficient on terahertz generation. And, the terahertz wave is generated via cascaded processes, which is of great interest in fundamental and applied sciences, and is a great demand for many applications in spectroscopy, sensing, communication, medial diagnoses and biomedical imaging. In addition, the design method for poled ferroelectric crystal is universal and applicable to many other fields, particularly in those processes where multiple quasi-phase-matched conditions are required to be satisfied simultaneously.

Bandwidth enhancement in difference frequency generation by domain-shifted quasi-phase-matching structure

Bandwidth enhancement in difference frequency generation (DFG) has been investigated using domain-shifted quasi-phase-matching (QPM) structure of electrically poled ferroelectric crystals. The exact periodic QPM condition is relaxed by extending the periodic structure to a more flexible domain-shifted structure. The dependence of bandwidth enhancement and flatness on the positions and number of shifted domains are systematically studied for optimum output efficiency. The results presented here are useful for the fabrication of wide-bandwidth wavelength conversion devices based on the DFG process.