Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm.     

Influence of circular aperture on high-order harmonic generation

The influence of circular aperture on the intensity of high-order harmonic generation (HHG) with intense femtosecond laser pulse was studied both experimentally and theoretically. The intensity variety of HHG with the diameter of circular aperture was observed in pulsed Ar gas. The result was discussed and interpreted in terms of the theory of Hankel transform. It is found that using the Gaussian beam truncated by an aperture could enhance the conversion efficiency of HHG at certain conditions.     

Fractional Fourier transform for partially coherent beam in spatial-frequency domain

By using Fourier transform and the tensor analysis method, the fractional Fourier transform (FRT) in the spatial-frequency domain for partially coherent beams is derived. Based on the FRT in the spatial-frequency domain, an analytical transform formula is derived for a partially coherent twisted anisotropic Gaussian－Schell model (GSM) beam passing through the FRT system. The connections between the FRT formula and the generalized diffraction integral formulae for partially coherent beams through an aligned optical system and a misaligned optical system in the spatial-frequency domain are discussed, separately. By using the derived formula, the intensity distribution of partially coherent twisted anisotropic GSM beams in the FRT plane are studied in detail. The formula derived provide a convenient tool for analysing and calculating the FRTs of the partially coherent beams in spatial-frequency domain.     

Generation and application of the twisted beam with orbital angular momentum

The twisted Laguerre-Gaussian beam was generated by transforming of Hermite-Gaussian beams through an optical system consisting of three rotated cylindrical lenses. The intensity distribution and phase structure of the twisted hollow beam were theoretically analyzed by using Collins diffraction integral. By utilizing the method of mode decomposition, the theory of transformation was analyzed. In the experiment, micro particles were trapped and rotated by this twisted beam.     

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a(0001) GaN film on a c-plane sapphire substrate.The growth was carried out in a radio-frequency molecular beam epitaxy system.The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images,as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer.The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra.The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves.Meanwhile,the threading dislocation density in the quantum dots-templated film was estimated to be 7.1×107cm-2,which was significantly suppressed compared with that of the AlN-buffered GaN film.The roomtemperature Hall measurement showed an electron mobility of up to 1860cm2 /V·s in the two-dimensional electron gas at the interface of the Al 0.25Ga0.75 N/GaN heterojunction.     

Relations between chirp transform and Fresnel diffraction, Wigner distribution function and a fast algorithm for chirp transform

Two physical interpretations of chirp transform related to Fresnel diffraction and Wigner distribution function are given.The chirp transform can be regarded as a Fresnel diffraction observed on a spherical tangent to the diffraction plane,or a rotation and stretching transformation of the Wigner distribution function space.A general fast algorithm for the numerical calculation of chirp transform is developed by employing two fast Fourier transform algorithms.The algorithm,by which a good evaluation can be achieved,unifies the calculations of Fresnel diffraction,arbitrary fractionalorder Fourier transforms and other scalar diffraction systems.The algorithm is used to calculate the Fourier transform of a Gaussian function and the Fourier transform,the Fresnel transform,the Fractional-order Fourier transforms of a rectangle function to evaluate the performance of this algorithm.The calculated results are in good agreement with the analytical results,both in the amplitude and phase.     

Quantum spectra and classical periodic orbit in the cubic billiard

Quantum billiards have attracted much interest in many fields. People have made a lot of researches on the two-dimensional (2D) billiard systems. Contrary to the 2D billiard, due to the complication of its classical periodic orbits, no one has studied the correspondence between the quantum spectra and the classical orbits of the three-dimensional (3D) billiards. Taking the cubic billiard as an example, using the periodic orbit theory, we find the periodic orbit of the cubic billiard and study the correspondence between the quantum spectra and the length of the classical orbits in 3D system. The Fourier transformed spectrum of this system has allowed direct comparison between peaks in such plot and the length of the periodic orbits, which verifies the correctness of the periodic orbit theory. This is another example showing that semiclassical method provides a bridge between quantum and classical mechanics.     

Beam quality improvement and focused peak intensity measurement of an intense femtosecond Ti:sapphire laser system

Micro-lens arrays were adopted to homogenize the beam profile of 532-nm pumping laser for the main amplifier of an intense femtosecond, chirped pulse amplification (CPA) Ti:sapphire laser. Experimental measurements showed a great improvement of the near-field pattern of the CPA beam after the main amplifier and the size of the focal spot was improved from 2.7 times diffraction limitation (DL) to 1.6 DL. The spot size focused by an f/4 off-axis parabola (OAP) in the target chamber was measured to be 5.8μm (full-width at half-maximum (FWHM)), and a peak intensity of 2.6×1020 W/cm2 was obtained at the output power of 120 TW. Peak intensity exceeding 1021 W/cm2 or even 1022 W/cm2 can be expected with smaller f-number focusing configuration and wavefront correction.     

The compensability between the longitudinal andtransverse mismachining tolerance of gratingin the optical pick-up head

In this paper numeric analysis is made to the influence of the longitudinal mismachining tolerance (LMT) and the transverse mismachining tolerance (TMT) of the grating in the optical pick-up head (GOPH) of VCD, DVD, and CD-ROM on the transmissivity and the intensity ratio of the auxiliary light beam to the main read-write light beam (IRAM) by using the general expression of diffraction efficiency obtained from the scalar diffraction theory. On solving GOPH problem, the scalar diffraction theory and the vector diffraction theory are coincident, and the scalar diffraction theory is reliable. The result shows that LMT and TMT can compensate for the inverse effect of IRAM, however, at the expense of reducing transmissivity. As far as GOPH is concerned, the goodness that LMT and TMT can be effectively compensated for is very advantageous in manufacturing of the grating.     

Structure and photoluminescence properties of Er3+-doped TiO2-SiO2 powders prepared by sol-gel method

Er 3+-doped TiO 2-SiO 2 powders are prepared by the sol-gel method,and they are characterized by high resolution transmission electron microscopy (HR-TEM),X-ray diffraction (XRD) spectra,and Raman spectra of the samples.It is shown that the TiO 2 nanocrystals are surrounded by an SiO 2 glass matrix.The photoluminescence (PL) spectra are recorded at room temperature.A strong green luminescence and less intense red emission are observed in the samples when they are excited at 325 nm.The intensity of the emission,which is related to the defect states,is strongest at the annealing temperature of 800 C.The PL intensity of Er 3+ ions increases with increasing Ti/Si ratio due to energy transfer between nano-TiO 2 particles and Er 3+ ions.     

Absorption and Recurrence Spectra of Sodium Rydberg Atom in a Strong External Magnetic Field

Using core-scattered closed-orbit theory, we calculate the photoabsorption and the scaled recurrence spectra of sodium Rydberg atom in strong magnetic fied below ionization threshoM. The non-Coulombic nature of the ionic core have been modified by a model potential, which includes an attractive Coulomb potential and a short-ranged core potential. A family of core-scattered nonhydrogenic closed orbits have also been discovered. The Fourier transformed spectra of sodium atom have allowed direct comparison between peaks in such plot and the scaled action values of closed orbits. The new peaks in the recurrence spectra of sodium atom have been considered as effects caused by the core scattering of returning waves at the ionic core. The results are compared with those of hydrogen case, which show that the core-scattered effects play an important role in alkali-metal atoms.     

System validation of CO_2-laser-based phase contrast imaging on HL-2A tokamak

System validation and density fluctuation calibration of phase contrast imaging(PCI) on an HL-2 A tokamak are presented. Signals from different channels show not only a pronounced modulation of an incident laser beam induced by a sound wave, but also an excellent magnification and low image distortion of optics. The frequencywavenumber spectrum is achieved by using two-dimensional Fourier transform of time series signals. The conversion coefficient between detected signal amplitude and chord integral plasma density fluctuation is 2.35 × 10~(13) m~(-2)/mV, which promises a suitable signal level at the order of volts of the PCI in typical HL-2 A discharges.     

Output beam analysis of high power COIL

As the output power of a chemical oxygen iodine laser (COIL) increases, the output laser beam instability appears as the far-field beam spot drift and deformation for the large Presnel number unstable resonator. In order to interpret this phenomenon, an output beam mode simulation code was developed with the fast Fourier transform method. The calculation results show that the presence of the nonuniform gain in COIL produces a skewed output intensity distribution, which causes the mirror tilt and bulge due to the thermal expansion. With the output power of COIL increases, the mirror surfaces, especially the back surface of the scraper mirror, absorb more and more heat, which causes the drift and deformation of far field beam spot seriously. The initial misalignment direction is an important factor for the far field beam spot drifting and deformation.     

Detecting emboli from Doppler ultrasound signals with the wavelet packet analysis method

A Doppler ultrasound analysis method based on wavelet package transform was proposed for embolic detection. The embolic Doppler signal was firstly decomposed using the wavelet packet. Then the sensitive characteristics were calculated from each sub-band signal and used in the emboli classification. This method was applied to analyze 300 cases simulated and 163 cases clinical Doppler signals. The minimum error ratio of embolic detection using this method was 13 percents lower than that using the traditional spectrogram analysis method.It was shown that this method overcame the limit between the time and frequency resolution in the short time Fourier transform, improved the accuracy of embolic detection greatly and extracted more reliable parameters for the clinical diagnosis.     

Characterization of ZnS nanoparticles synthesized by co-precipitation method

ZnS nanoparticles are prepared by homogeneous chemical co-precipitation method using EDTA as a stabilizer and capping agent. The structural, morphological, and optical properties of as-synthesized nanoparticles are investigated using x-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, ultraviolet-visible(UV-Vis)absorption, and photoluminescence spectroscopy. The x-ray diffraction pattern exhibits a zinc-blended crystal structure at room temperature. The average particle size of the nanoparticles from the scanning electron microscopy image is about50 nm. The ultraviolet absorption spectrum shows the blue shift in the band gap due to the quantum confinement effect.The photoluminescence spectrum of Zn S nanoparticles shows a blue visible spectrum.     

Fractional Fourier transform of apertured paraboloid refracting system

The limitation of paraxial condition of paraboloid refracting system in performing fractional Fourier trans-form acts like an aperture, which makes the system different from ideal systems. With aperture expanded as the sum of finite complex Gaussian terms, a more practical approximate analytical solution of frac-tional Fourier transform of Gaussian beam in an apertured paraboloid refracting system is obtained and also numerical investigation is presented. Complicated and practical fractional Fourier transform systems can be constructed by cascading several apertured paraboloid refracting systems which are the simplest and the most basic units for performing more precise transform.     

1.57 times diffraction-limit high-energy laser based on a Nd:YAG slab amplifier and an adaptive optics system

In this Letter, a 116-actuator deformable mirror(DM) was used to correct the wavefront distortion in a 10.7 J,10 Hz neodymium-doped yttrium aluminum garnet(Nd:YAG) slab amplifier. By applying a pump-light homogenizer to transform the 3 × 1 near-field beam array into an integrated beam, the beam quality was greatly improved from 5.54 times diffraction limit(TDL) to 1.57 TDL after being corrected by the DM. To the best of our knowledge, this is the first investigation on beam quality control of an arrayed near-field beam in high-energy diode-pumped solid-state lasers.     

Raman spectrum study of δ-doped GaAs/AlAs multiple-quantum wells

Three samples of GaAs/AlAs multiple-quantum wells with different quantum well widths and δ-doped with Be acceptors at the well center were grown on(100) Ga As substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4-50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical(LO) phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.