Active control of terahertz waves based on hybrid VO periodic corrugated waveguides.

We describe a method for the active control of terahertz (THz) waves using hybrid vanadium dioxide (VO) periodic corrugated waveguide. Unlike liquid crystals, graphene and semiconductors and other active materials, VO exhibits a unique insulator-metal transition characteristic by the electric fields, optical, and thermal pumps, resulting in five orders of magnitude changes in its conductivity. Our waveguide consists of two gold coated plates with the VO-embedded periodic grooves, which are placed in parallel with the grooves face to face.

Internal Cylinder Identification Based on Different Transmission of Longitudinal and Shear Ultrasonic Waves.

We have built a Fizeau fiber interferometer to investigate the internal cylindrical defects in an aluminum plate based on laser ultrasonic techniques. The ultrasound is excited in the plate by a Q-switched Nd:YAG laser. When the ultrasonic waves interact with the internal defects, the transmitted amplitudes of longitudinal and shear waves are different.

Wave packet interactions in a thin aluminum plate partially immersed in water.

When investigating the wave propagation and mode conversions in a thin aluminum plate partially immersed in water, a kind of wave packet interaction was observed. It was found that the transmitted ultrasonic signal consists of different wave packets, which contain essential information of different wave types. When the incident angle is very small, the signals can be identified as the major wave packet followed by its tail.

Vanadium dioxide-assisted broadband tunable terahertz metamaterial absorber.

Tunable terahertz (THz) functional devices have exhibited superior performances due to the use of active materials, such as liquid crystals, graphene, and semiconductors. However, the tunable range of constitutive parameters of materials is still limited, which leads to the low modulation depth of THz devices. Here, we demonstrate a broadband tunable THz absorber based on hybrid vanadium dioxide (VO) metamaterials.

Acoustic extraordinary transmission manipulation based on proximity effects of heterojunctions.

Heterojunctions between two crystalline semiconductor layers or regions can always lead to engineering the electronic energy bands in various devices, including transistors, solar cells, lasers, and organic electronic devices. The performance of these heterojunction devices depends crucially on the band alignments and their bending at the interfaces, which have been investigated for years according to Anderson's rule, Schottky-Mott rule, Lindhard theory, quantum capacitance, and so on. Here, we demonstrate that by engineering two different acoustic waveguides with forbidden bands, one can achieve an acoustic heterojunction with an extraordinary transmission peak arising in the middle of the former gaps.

Single-mode interface states in heterostructure waveguides with Bragg and non-Bragg gaps.

Interface states can always arise in heterostructures that consist of two or more (artificial) materials with topologically different energy bands. The gapped band structure can be classified by the Chern number (a topological invariant) generally or the Zak phase in one-dimensional periodic systems. Recently, topological properties have been employed to investigate the interface states occurring at the connecting regions of the heterostructures of mechanical isostatic lattices and acoustical waveguides.

Orthogonality breaking induces extraordinary single-mode transparency in an elaborate waveguide with wall corrugations.

Orthogonality plays a fundamental role in various mathematical theorems and in physics. The orthogonal eigenfunctions that represent the intrinsic motions of various physical systems can also be regarded as transverse wave modes in a straight waveguide. Because of their orthogonality, these modes propagate independently, without mutual interference.

Unidirectional optical transmission in dual-metal gratings in the absence of anisotropic and nonlinear materials.

We predict the unidirectional optical transmission in dual-metal grating structures composed of two gratings with different structures in the absence of anisotropy and nonlinearity. The zero-order unidirectional transmission is achieved. Based on the unique property and by modulating the structural parameters, the transmittance approaches to 0% and 60% in the two opposite directions, respectively.

Flat-plateau supercontinuum generation in liquid absorptive medium by femtosecond filamentation.

We have studied filamentation and supercontinuum generation by focusing the intense femtosecond laser pulses into an absorptive medium (CuSO(4) aqueous solution). A broad spectrum from 350 to 950 nm with a flat plateau spanning approximately from 450 to 700 nm with a flatness of 9% is obtained without any additional filters. The results indicate that the absorptive medium not only suppresses the strong surplus femtosecond laser signal but also flattens the supercontinuum spectrum efficiently.

High-efficiency continuous-wave Raman conversion with a BaWO(4) Raman crystal.

We report a high-efficiency cw Raman conversion with a BaWO(4) Raman crystal in a diode-end-pumped Nd:YVO(4) laser. The Raman threshold is as low as 3.6 W of diode power at 808 nm.

FDTD approach to optical forces of tightly focused vector beams on metal particles.

We propose an improved FDTD method to calculate the optical forces of tightly focused beams on microscopic metal particles. Comparison study on different kinds of tightly focused beams indicates that trapping efficiency can be altered by adjusting the polarization of the incident field. The results also show the size-dependence of trapping forces exerted on metal particles.

Three-wave shearing interferometer based on spatial light modulator.

The use of a SLM for the three-wave lateral shearing interference is proposed, and an eight-step phase-shifting scheme is developed for extracting phase information from three-wave interferograms. The two-dimensional phase of object is reconstructed from two phase differences which are calculated from two orthogonal sheared interferograms. The flexibility of SLM can be fully utilized in the sense of dynamical controlling of the direction and amount of shear, as well as phase shift.

A new planar left-handed metamaterial composed of metal-dielectric-metal structure.

An improved planar structure of left-handed (LH) metamaterial is presented, and then designed and analyzed in microwave regime. In the anticipated LH frequency regime, the LH property is validated from the phenomena of backward wave propagation and negative refraction. To characterize the electromagnetic property of the planar metamaterial, we introduce the wedge method by constructing a wedge-shaped bulk LH metamaterial by stacking the planar LH metamaterials.

Saturation effect and forward-dominant second-harmonic generation in single-defect photonic crystals with dual localizations.

We study the forward-dominant output of second-harmonic generation (SHG) in single-defect and dual-localized photonic crystals within the saturation limit. We propose that an asymmetric structure can be used to improve the performance of the enhanced SHG. We get two empirical expressions for the total saturation SHG efficiency and the forward factor and provide a reasonable explanation.

Large lateral shift near pseudo-Brewster angle on reflection from a weakly absorbing double negative medium.

We focus on the lateral shift for an electromagnetic wave reflected from a weakly absorbing double negative medium (DNM). A large lateral shift near the pseudo-Brewster angle is found, which may be negative or positive. We give an analytic expression for such a kind of lateral shift, from which the critical transition point for sign-changing of the lateral shift can be easily obtained.

Z-scan theory for material with two- and three-photon absorption.

We present a theoretical study on the Z-scan characteristics of thin nonlinear optical media with simultaneous two- and three-photon absorption, a situation that exists, for example, in polydiacetylenes. With the introduction of a coupling function between two- and three-photon absorption, we find a quasi-analytic expression for open aperture Z-scan traces. We make a comparison of the analytic solutions with numerical solutions in detail, showing that they are in good agreement.

Passively mode-locking Nd:Gd0.5Y0.5VO4 laser with an In0.25Ga0.75As absorber grown at low temperature.

We have demonstrated stable self-starting passive mode locking in a diode-end-pumped Nd:Gd0.5Y0.5VO4 laser by using an In0.

4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror.

We have demonstrated a passively mode-locked diode end-pumped all-solid-state laser, which is composed of a Nd:Gd0.5Y0.5VO4 crystal and a folded cavity with a semiconductor saturable-absorber mirror grown by metal-organic chemical-vapor deposition.

Tunable high-peak-power, high-energy hybrid Q-switched double-clad fiber laser.

A cw laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime was demonstrated. The output pulses had a duration as short as 4.2 ns, a tunable wavelength range from 1080.

Narrow-linewidth widely tunable hybrid Q-switched double-clad fiber laser.

A laser-diode-pumped Yb-doped double-clad fiber laser operating in a hybrid Q-switched regime has been demonstrated. With pulsed pump light and stimulated Brillouin scattering of the gain fiber as the Q-switching mechanisms, the laser generated nanosecond pulses with a stable repetition rate. A single-pulse energy of as much as 143.