Continuous-wave mid-infrared intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate

This paper reports a continuous-wave （CW） mid-infrared intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate （PPLN） pumped by a diode-end-pumped CW Nd：YVO4 laser. Considering the thermal lens effects, it adopted an optical ballast lens and the near-concentric cavity for better operation. At the PPLN＇s grating period of 28.5 μm and the temperature of 140℃, the maximum idler output power of 155 mW at 3.86 μm has been achieved when the 808 nm pump power is 8.5 W, leading to an optical-to-optical conversion efficiency of 1.82%.

Two-Dimensional Photoacoustic Imaging of Blood Vessel Networks within Biological Tissues

Photoacoustic tomography （PAT） is a powerful imaging merits and most compelling features of light and sound, technique for medical diagnosis because it combines the We describe a PAT experimental system constructed in our laboratory which consists of a Q-switched Nd：YAG pulse laser operating at 532nm with a 8-ns pulse width to generate the photoacoustic signals from a biological sample. Two-dimensional photoacoustic imaging of blood vessel networks lcm below the tissue surface is achieved, We also successfully demonstrate that the system is capable of imaging the blood vessels over the ex vivo rat brain with skull and skin intact.

Selective reflection combined with Fabry-Perot effects from two-level atoms confined between two dielectric walls

The coefficient of selective reflection at oblique incidence from two-level atoms confined between two dielectric walls is calculated in this paper. It is found to be related to the transient behaviour of atoms after colliding with the wall and the distribution of the field inside the vapour corresponds to L/λ, with L the thickness of the film and λ the incident wavelength. We find that the sub-Doppler structure is manifest both for normal incidence and small angle oblique incidence, It is feasible to detect the real part of selective reflection in several cases that have not been achieved before.

High efficiency continuous-wave tunable signal output of an intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate

In this paper we report on a continuous-wave （CW） intracavity singly resonant optical parametric oscillator （ICSRO） based on periodically poled LiNbO3 （PPLN） pumped by a diode-end-pumped CW Nd：YVO4 laser. Considering the thermal lens effects and diffraction loss, an optical ballast lens and a near-concentric cavity are adopted for better operation. Through varying the grating period and the temperature, the tunable signal output from 1406 nm to 1513 nm is obtained. At a PPLN grating period of 29 pm and a temperature of 413 K, a maximum signal output power of 820 mW at 1500 nm is achieved when the 808 nm pump power is 10.9 W, leading to an optical-to-optical conversion efficiency of 7.51%.

High-power terahertz radiation from surface-emitted THz-wave parametric oscillator

We report a pulsed surface-emitted THz-wave parametric oscillator based on two MgO：LiNbO3 crystals pumped by a multi-longitudinal mode Q-switched Nd：YAG laser. Through varying the phase matching angle, the tunable THzwave output from 0.79 THz to 2.84 THz is realized. The maximum THz-wave output was 193.2 n J/pulse at 1.84 THz as the pump power density was 212.5 MW/cm2, corresponding to the energy conversion efficiency of 2.42Х10-6 and the photon conversion efficiency of about 0.037%. When the pump power density changed from 123 MW/cm^2 to 148 MW/cm^2 and 164 MW/cm^2, the maximum output of the THz-wave moved to the high frequency band. We give a reasonable explanation for this phenomenon.

Location algorithm for seal imprints on Chinese bank-checks based on region growing

The paper focuses on location of seal imprints on Chinese bank-checks based on region growing.Region growing method can be applied to searching and locating connection region in an image.A seal imprint,however,is generally composed of various connection regions which are unconnected to each other.In order to locate the seal imprint,these connection regions must be fused together.In the paper,an algorithm for locating seal imprints on Chinese bankchecks based on region growing is proposed, of which a fusion criterion for connection regions in a seal imprint is put forth based on the image feature of Chinese bankcheck,and a center-rays model is proposed to find the topological relationship between connection regions,for which externally-tangent rectangle of region is used as the mark of location of region.The location experiment is achieved with a false-acceptance rate of 7.1% and a false-rejection rate of 0% on Chinese bankcheck.

All-fiber optical modulator based on no-core fiber and magnetic fluid as cladding

An all-fiber optical modulator, which is composed of a piece of no-core fiber spliced between two sections of singlemode fibers and uses magnetic fluid（MF） as the cladding of the no-core fiber section, is proposed and investigated experimentally. Due to the tunable refractive index and absorption coefficient of MF, the output intensity can be modulated by controlling an applied magnetic field. The dependences of the modulator＇s temporal response on the working wavelength,the magnetic field strength（H）, and the MF＇s concentration are investigated experimentally. The results are explained qualitatively by the dynamic response process of MF under the action of a magnetic field. The findings are helpful for optimizing this kind of modulator.

Interference of selective higher-order modes in optical fibers

The interference of selective higher-order modes in optical fibers is investigated both theoretically and experimentally. It has been demonstrated that by coupling the LP01 mode in a step-index single-mode fiber (SMF) to the LP0m modes in step-index multimode fibers (MMFs) with different parameters, one can selectively generate higher-order modes and construct all-fiber interferometers. The research presented in this paper forms a basis of a new type of fiber devices with potential applications in fiber sensing, optical fiber communications, and optical signal processing.

Color Tunable Light-Emitting Diodes Based on Polymer/Inorganic Heterojunction

By using p-type polymer poly(2,5-Didodecyloxy-1,4-Phenylenylenevinylene)(PDDOPV)and n-type inorganic material ZnO:Zn,the electronluminescent(EL)emission from bilayer device based on polymer/inorganic hetero-junction was successfully demonstrated.In comparison with single layer polymer device,with dc voltage of 10V,the brightness and luminous efficiency of bilayer device were enhanced 14 and 26 times,respectively.The EL spectrum of bilayer device is not identical to the characteristic emission spectrum of either PDDOPV or ZnO:Zn,and shifts continuously towards to short wavelength direction as the applied voltage increases.

Investigation of pump-wavelength dependence of terahertz-wave parametric oscillator based on LiNbO_3

This paper investigates the performances of terahertz-wave parametric oscillators （TPOs） based on the LiNbO3 crystal at different pump wavelengths.The calculated results show that TPO characteristics,including the frequency tuning range,the THz-wave gain and the stability of THz-wave output direction based on the Si-prism coupler,can be significantly improved by using a short-wavelength pump.It also demonstrates that a long-wavelength-pump allows the employment of a short TPO cavity due to an enlarged phase-matching angle,that is,an increased angular separation between the pump and oscillated Stokes beams under the THz-wave generation at a specific frequency. The study provides an useful guide and a theoretical basis for the further improvement of TPO systems.

Theoretical study of phase-matching properties for tunable terahertz-wave generation in isotropic nonlinear crystals

Theoretical investigation of the phase-matching properties for the tunable coherent terahertz wave generation in the isotro- pic semiconductor nonlinear materials of CdTe and GaP is presented according to their optical dispersion properties, based on a pulsed dual-wavelength KTP-OPO operating near degenerate point in the process of the difference frequency mixing. The cross-Reststrahlen band dispersion compensation phase-matching technique involved in this interaction is introduced concisely. The optimum lengths of DFG crystals are decided according to the minimum coherence lengths of the crystals in the given pump wavelength ranges. The results of analysis and calculation provide the useful and necessary theoretical basis and guidance for the future experiments.

Reflection-type electromagnetically induced transparency analogue in terahertz metamaterials

A reflection-type electromagnetically induced transparency（EIT） metamaterial is proposed, which is composed of a dielectric spacer sandwiched with metallic patterns and metallic plane. Experimental results of THz time domain spectrum（THz-TDS） exhibit a typical reflection of EIT at 0.865 THz, which are in excellent agreement with the full-wave simulations. A multi-reflection theory is adopted to analyze the physical mechanism of the reflection-type EIT, showing that the reflection-type EIT is a superposition of multiple reflection of the transmission EIT. Such a reflection-type EIT provides many applications based on the EIT effect, such as slow light devices and nonlinear elements.

Ultrabroad Terahertz Bandpass Filter Based on a Multiple-Layered Metamaterial with Flexible Substrates

An ultrabroad and sharp transition bandpass flexible terahertz(THz)filter is designed using a multiple-layered metamaterial.This bandpass filter has excellent filtering capability,with a 3 dB bandwidth of about 0.47 THz and sharp band-edge transitions of 80 dB/THz and 96 dB/THz,respectively,and it can be realized by a coupling individual resonance mode.We find that the geometry parameters have an influence on the transmission profile,which are capable of giving us meaningful guidance in design of high profile bandpass THz filters.The numerical results show that the multiple-layered flexible metamaterial provides an effective way to achieve ultrabroad THz devices.

Ultrafast Carrier Dynamics and Terahertz Photoconductivity of Mixed-Cation and Lead Mixed-Halide Hybrid Perovskites

Using time-dependent terahertz spectroscopy, we investigate the role of mixed-cation and mixed-halide on the ultrafast photoconductivity dynamics of two different methylammonium(MA) lead-iodide perovskite thin films. It is found that the dynamics of conductivity after photoexcitation reveals significant correlation on the microscopy crystalline features of the samples. Our results show that mixed-cation and lead mixed-halide affect the charge carrier dynamics of the lead-iodide perovskites. In the(5-AVA)_(0.05)(MA)_(0.95) PbI_(2.95) Cl_(0.05)/spiro thin film, we observe a much weaker saturation trend of the initial photoconductivity with high excitation fluence, which is attributed to the combined effect of sequential charge carrier generation, transfer, cooling and polaron formation.

Efficient Nanosecond Dual-Signal Optical Parametric Generator with a Periodically Phase Reversed PPMgLN

We report an efficient nanosecond optical parametric generator （OPG） with a periodically-phase-reversed periodically poled MgO：LINbO3 （ppr-PPMgLN）, which produces two pairs of signal and idler waves. The OPG is pumped by a 1.064μm Q-switched Nd：YVO4 laser. When the repetition rate is set at 10kHz, the maximum average total output power of 570mW is achieved, including 410mW of dual-signal radiations and 160mW of dual-idler radiations. The total conversion efficiency is 32.5%. The tunable dual-signal wavelengths in the range of 1.474-1.518μm and 1.490-1.539μm and the dual-idler of 3.826-3.558 μm and 3. 726-3.451 μm are obtained by changing the crystal temperature from 30℃ to 200℃.

Low-loss terahertz waveguide with InAs-graphene-SiC structure

We demonstrate a low-loss terahertz waveguide based on the InAs-graphene-SiC structure. By analyzing the terahertz waveguide proposed in this paper, we can obtain that it is the characteristic of a low transmission loss coefficient （αloss 0.55 dB/m） for fundamental mode （LP01） when the incident frequency is larger than 3.0 THz. The critical radii of the inside and outside cylinders have been found for the high-quality transmission. The large inside radius and the high transmission frequency result in a fiat transmission loss coefficient curve. As a strictly two-dimensional material, the double graphene surface rings perform better to improve the quality of transmission mode. These results provide a new idea for the research of the long-distance THz waveguide.

Tunable ultra-wideband terahertz filter based on three-dimensional arrays of H-shaped plasmonic crystals

A face-to-face system of double-layer three-dimensional arrays of H-shaped plasmonic crystals is proposed, and its transmission and filtering properties are investigated in the terahertz regime. Simulation results show that our design has excellent filtering properties. It has an ultra-wide bandgap and passband with steep band-edges, and the transmittance of the passband and the forbidden band are very close to 1 and 0, respectively. As the distance between the two face-to-face plates increases, the resonance frequency exhibits a gradual blueshift from 0.88 THz to 1.30 THz. Therefore, we can dynamically control the bandwidths of bandgap and passband by adding a piezoelectric ceramic plate between the two crystal plates. Furthermore, the dispersion relations of modes and electric field distributions are presented to analyze the generation mechanisms of bandgaps and to explain the location of bandgaps and the frequency shift phenomenon. Due to the fact that our design can provide many resonant modes, the bandwidth of the bandgaps can be greatly broadened. This paper can serve as a valuable reference for the design of terahertz functional devices and three-dimensional terahertz metamaterials.

An all-solid-state high power quasi-continuous-wave tunable dual-wavelength Ti：sapphire laser system using birefringence filter

This paper describes a tunable dual-wavelength Ti：sapphire laser system with quasi-continuous-wave and high-power outputs. In the design of the laser, it adopts a frequency-doubled Nd：YAG laser as the pumping source, and the birefringence filter as the tuning element. Tunable dual-wavelength outputs with one wavelength range from 700 nm to 756.5 nm, another from 830 nm to 900mn have been demonstrated. With a pump power of 23 W at 532 nm, a repetition rate of 7 kHz and a pulse width of 47.6 ns, an output power of 5.1 W at 744.8 nm and 860.9 nm with a pulse width of 13.2 ns and a line width of 3 nm has been obtained, it indicates an optical-to-optical conversion efficiency of 22.2%.

Effect of optical pumping on the momentum relaxation time of graphene in the terahertz range

The momentum relaxation time of a photoexcited graphene in the THz frequency range has been studied by using terabertz time domain spectroscopy under optical pumping at room temperature. It is found that the momentum relaxation time of the graphene as a function of the optical pumping intensity exhibits a threshold behavior. The features of the momentum relaxation time as a function of the optical pumping intensity are also investigated. The results are useful for understanding the basic underlying physics of graphene scattering as well as finding the possible applications in carbon- based electronics.

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors （HEMTs） have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current-voltage （I-V） measurements. A non-linear fitting method was used to extract the contact parameters from the I-V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450 ℃ in N2 ambience.