Chen Zhenghao

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.

Enhanced nonlinear optical properties of laser deposited Ag/BaTiO3 nanocomposite films

The composite films containing Ag particles embedded in BaTiO3 matrix were deposited on MgO (100) substrates by using the pulsed laser deposition technique and characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The absorption peak resulting from the surface plasmon resonance of Ag particles is found at the wavelength of 430 nm. The third-order nonlinear optical characteristics of the films are determined by z-scan technique using a nanosecond laser. The results show that the Ag/BaTiO3 nanocomposite films exhibit great optical nonlinearities and the real and imaginary parts of the third-order nonlinear susceptibility χ(3) are calculated to be 4.544×10-6 esu and 2.352×10-7 esu, respectively. The enhancement mechanism of χ(3) is discussed.

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.

Study of resonant excitation of infrared surface second harmonic at the GaAs-Al interface.

By using a tunable TEA CO(2) laser and by grating coupling at the interface of GaAs-Al, we have obtained, for the first time to our knowledge, second-harmonic generation from a resonantly excited infrared surface polariton. The experimental results provided a dispersion relation that compares well with theoretical calculations. The deduced value of chi(2), 5.9 x 10(-7) esu, agrees well with other measurements.

Highly Conductive Nb-Doped BaTiO3 Epitaxial Thin Films Grown by Laser Molecular Beam Epitaxy

The n-type conductive BaNb0.3Ti0.7O3 thin films were grown on SrTiO3(001) substrates by computer-controlled laser molecular beam epitaxy. The BaNb0.3Ti0.7O3 films with (001) orientation are epitaxially grown on SrTiO3 substrates, as confirmed by x-ray diffraction techniques. The root-mean-square surface roughness of the deposited thin films is measured to be 0.24 nm by atomic force microscopy. The resistivity, carrier concentration and mobility of the BaNb0.3Ti0.7O3 thin film are 5.9×10-4 Ω cm, 1.8×1021cm-3 and 10.7 cm2V-1s-1 at room temperature, respectively, which are the best values in Nb-doped BaTiO3 thin films reported so far to our knowledge.

Laser molecular-beam epitaxy and second-order optical-nonlinearity of BaTiO 3 /SrTiO 3 superlattices

A series of c-axis oriented BaTiO3/SrTiO3 superlattices with the atomic-scale precision were epitaxially grown on single-crystal SrTiO3(l00) substrates using laser molecular-beam epitaxy (LMBE). A periodic modulation of the intensity of reflection high-energy electron diffraction (RHEED) in BaTiO3 and SrTiO3 layers was observed and attributed to the lattice-misfit-induced periodic variation of the terrace density in film surface. The relationship between the second-order nonlinear optical susceptibilities and the superlattice structure was systematically studied. The experimental and theoretical fitting results indicate that the second-order nonlinear optical susceptibilities of BaTiO3/SrTiO3 superlattices were greatly enhanced with the maximum value being more than one order of magnitude larger than that of bulk BaTiO3 crystal. The mechanism of the enhancement of the second-order optical nonlinearity was discussed by taking into account the stress-induced lattice distortion and polarization enhancement.A series of c-axis oriented BaTiO 3 /SrTiO 3 superlattices with the atomic-scale precision were epitaxially grown on single-crystal SrTiO 3 (100) substrates using laser molecular-beam epitaxy (LMBE). A periodic modulation of the intensity of reflection high-energy electron diffraction (RHEED) in BaTiO 3 and SrTiO 3 layers was observed and attributed to the lattice-misfit-induced periodic variation of the terrace density in film surface. The relationship between the second-order nonlinear optical susceptibilities and the superlattice structure was systematically studied. The experimental and theoretical fitting results indicate that the second-order nonlinear optical susceptibilities of BaTiO 3 /SrTiO 3 superlattices were greatly enhanced with the maximum value being more than one order of magnitude larger than that of bulk BaTiO 3 crystal. The mechanism of the enhancement of the second-order optical nonlinearity was discussed by taking into account the stress-induced lattice distortion and polarization enhancement.

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.

Study of the properties of infrared intersubband transition in doped GaAs/AlxGa1-xAs multiple quantum wells

The properties of intersubband transition of GaAs/AlxGa1-xAs multiple quantum wells with various well widths and doped-well concentrations have been studied. Both theoretical and experimental results are in good agreement. For the appropriate well width and higher doping concentration, we directly observed two intersubband absorption peaks from E1 → E2 and E2 → E3 transitions in well. The experimental results and theoretical analysis are given.