Yang Guozhen

Laser molecular beam epitaxy system and its key technologies

The laser molecular beam epitaxy system and its key technologies have been successfully developed in China. Atomically regulated unit-cell by unit-cell homoepitaxial SrTiO3 (STO) and heteroepitaxial BaTiO3 (BTO) films and STO/BTO superlattices were fabricated on STO (100) substrates by the laser molecular beam epitaxy. The fine streak patterns and more than 1000 cycles of undamping intensity oscillation were obtained by using in-situ reflection high-energy electron diffraction. The surfaces of films are atomically smooth.

Design of diffractive-phase axicon illuminated by a Gaussian-profile beam

The diffractive-phase axicon can convert the Gaussian-profile beam into axial uniform intensity distribution with long focal depth and high lateral resolution. Two types of phase-retardation functions for the nonuniform-illuminating axicon are derived in terms of the ray tracing and the geometrical law of energy conservation. Based on the general theory of the amplitude-phase retrieval in optical system and the iteration algorithm, the optimization design of the phase distribution of the diffractive-phase axicon can be achieved. The simulation calculations show that the new approach may successfully offer the design of the desired diffractive-phase axicon with long focal depth and high lateral resolution. A comparison of the performances of the holographic axicon with the phase-retardation functions from the geometrical optics prediction and the diffractive-phase axicon designed by the new approach is also presented.

Polycrystalline β-C3N4 Thin Films Deposited on Single-Crystal KCl(100) Using rf Sputtering

The carbon-nitride thin films were deposited on the (100) oriented single-crystal KCl wafers at ambient temperatures by using rf-plasma sputtering. The IR spectrum showed that the films contained carbon-nitride bonds. The transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements indicated the existence of predicted polycrystalline β-C3N4 films on the KCl(100) wafers. And the TEM and XRD measured lattice spacings well match the calculated data.

Femtosecond Optical Pump Induced Enhancement of Terahertz Radiation from GaAs Surface

Terahertz (THz) far infrared radiation with a subpicosecond pulse duration is generated from GaAs(100) surface illuminated by femtosecond laser pulses. By illuminating the sample with another pump pulse, we observe an enhancement of THz emission. A mechanism of photocarriers trapping by impurities and defects in the depletion layer is proposed.

Quantum well infrared detector with grating enhancement

Abstract We have investigated theoretically enhanced quantum well IR detector. The field enhancement expressions are given by means of the perturbation method. The quantum efficiency and enhancement ratio have been obtained for a doped GaAs/AlGaAs quantum well structure at λ = 10 μm for different grating periods. We explained why Goossen and Lyon only observed a small enhancement.

Performing a 2-dimensional linear transform with a holographic mask

Based on the configuration of coherent optical system composed of a holographic mask and two Fourier transform lenses, an approach of performing a 2-dimensional (2-D) linear transform is suggested. The 2-D Walsh-Hadamard transform of order 32 is tested, and the experimental results are given.

Growth mode of Cu-rich particles on laser-ablated YBa2Cu3O7 thin films

The microstructure of the laser-ablated YBa2Cu3O7 (YBCO) thin film deposited on heated (100) SrTiO3 substrate was examined by transmission electron microscope. The particles on the film mainly consist of CuO and few CuYO2. Most of these non-superconducting particles nucleate on or near the surface of the film and protrude about 100-400 nm in height. A large amount of a-axis YBCO grains also exist in the film, which nucleate at the substrate surface and grow perpendicularly above the c-axis YBCO film. The YBCO thin film deposited under low oxygen pressure has very different microstructure compared with YBCO thin film deposited under high oxygen pressure.

Enhanced Stark Shift in GaAs/AlGaAs Step Quantum Well Under an Applied Electric Field

Theoretical calculations of the quantum confined Stark shift for the first heavy hole exciton (hh1-e1) in step quantum well are given. A significant change in Stark shift is achieved in specially stepped potential shape, which is more than two times larger than the conventional rectangular quantum well. The structure has the advantage of a large change in absorption, which is one of the best candidates for the optical modulators utilizing the quantum confined Stark effect (QCSE) in GaAs/AlGaAs. Relations between QCSE and stepped structure are discussed in the optimization for large Stark shift.

Intersubband Third-Order Nonlinearities in a GaAs/AlGaAs Step Quantum Well Structure by Phase Conjugation Method

In this letter, we present the observation of phase conjugation at 9.27 μm in a GaAs/AlGaAs multiple step quantum well structure. The response is caused by the nearly resonant intersubband transition. The magnitude of χ(3) determined by this phase conjugation method is about 8 × 10-5 esu and the phase conjugate reflectivity η is about 6 × 10-3 uncorrected for Fresnel reflections.

CRYSTALLIZATION OF DIELECTRIC SPHERES IN LASER STANDING WAVE FIELD

Experimental study on crystallization of polystyrene spheres and SrTiO3 particles in laser standing wave field is presented. The diameter of polystyrene, which are able to be trapped, is 7 times smaller than that reported by Burns, the corresponding lattice constant is reduced to 1.5 μm. The crystallization effect of SrTiO3 particles with a refractive constant of 2.5 in the visible range is observed for the first time. Based on the results above we believe that it is possible to construct photonic crystal with lattice constant extended to visible wave range by this method.