Zhenxin Yu

GRAPHITIZATION OF NANODIAMOND POWDER ANNEALED IN ARGON AMBIENT

Nanodiamond powder was annealed at each of the following temperatures: 300, 600, 800, 1000, and 1150 °C, for an hour in flowing argon ambient. The variations of x-ray diffraction patterns and Raman spectra of the powder with different annealing temperatures were studied. While being annealed at temperatures higher than 800 °C, the powder can undergo a phase-transition process from cubic diamond to graphite. In addition, the size of nanodiamond crystallites decreased from ∼50 to ∼25 A. The physical mechanism responsible for the variation in Raman spectra is discussed using a phonon-confinement model.

Efficient up‐conversion fluorescence in charge transfer compound crystal

A series of new intramolecular charge transfer compounds with absorption bands in shorter wavelengths have been synthesized. Bright visible light emits from the crystals of these new compounds under infrared pulsed laser pumping. The spectral and pump density dependent properties demonstrate that some of the emissions are up‐conversion fluorescence due to two photon absorption. The fluorescent yields of one‐ and two‐photon absorption are in the same order at high pump density. This work demonstrates that these new intramolecular charge transfer crystals are good candidates for nonlinear optical materials.

Comparison between the temporal characteristics of picosecond SRS from the cell and SRO from the droplet

Abstract We have studied the temporal behavior of picosecond stimulated Raman scattering (SRS) from a liquid cell and stimulated Raman oscillation (SRO) from liquid droplets. Big differences have been found between the multiorder stimulated Raman emissions from the cell and the droplets. There is nearly no delay of SRS emission from the cell, but a long delay and slow decay of the Stokes SRO from the droplets. Basing on the basic properties of the droplet cavity, the experimental results were well explained.

Picosecond transient photoluminescence spectra of ZnSe‐ZnS strained‐layer superlattices grown on GaAs(001) by molecular beam epitaxy

Picosecond transient photoluminescence spectra were carried out on ZnSe‐ZnS strained‐layer superlattices at 77 K. The typical exciton formation time and exciton lifetime are 40 and 100 ps, respectively. The relationship between exciton lifetime and the well width is also given.

Efficient generation of ultrafast broadband radiation in a submillimeter liquid‐core waveguide

An experimental investigation of picosecond laser propagation in a submillimeter liquid‐core waveguide is presented. It is observed that the pump is spectrally broadened towards its Stokes frequency upon passing through the waveguide and the spectral bandwidth is further extended by stimulated raman scattering. This stable ultrafast broadband radiation is shown to be enhanced due to the use of the waveguide.

Thermal instability of molybdenum and silicon tips

Abstract Thermal effects in molybdenum and silicon tips are analyzed with the aim of understanding the initiating mechanism of thermal instability leading to vacuum breakdown. A new theoretical treatment is employed to study the thermal response of tip emitters, which takes into account the correlative effect between field emission and its resultant temperature rise at the apex of tips. The results show that for a typical Si tip, thermal instability would occur before the tip temperature reaches its melting point, while thermal effect is not significant in the case of the Mo tip with the same geometry and dimension of a silicon tip. This remarkable difference between two types of tip is mainly due to their large discrepancies in electrical resistivity, but the Nottingham effect contributes significantly as well.

Stability consideration of metal–diamond–vacuum micro-emission regimes

Abstract Details are given of both experimental and theoretical studies of the vacuum breakdown phenomena associated with field emission from a metal–diamond–vacuum micro-emission regime. Real-time recording in conjunction with the transparent anode imaging technique was employed to study the process of the breakdown event. It was found that such an event can be cathode initiated due to the thermal heating in diamond film emitter. The theoretical study shows that a new mechanism is responsible for thermal instability of a channel: the effect of temperature rise of the channel on field emission from substrates. The results calculated based on the new theory are qualitatively consistent with the experimental findings.

Stimulated Rayleigh Wing Scattering and Stimulated Four-photon Interaction in Liquid-core Waveguides

Abstract In this Letter we show that the power of stimulated Rayleigh wing scattering (SRWS) generated in bulk liquid of anisotropic molecules is reduced by a simultaneous presence of stimulated four-photon interaction (SFPI) because the combined effect of SRWS and SFPI results in a walk-off between the pump beam and the generated signal in the nonguided system and this shortens the nonlinear interaction length. The SRWS may be greatly enhanced in a hollow waveguide filled with a liquid of higher index of refraction than that of the waveguide cladding as the long interaction length is maintained for the whole length of the waveguide. The enhanced SRWS results in a considerable red-shifted frequency super-broadening of an intense optical pulse that passes through the waveguide, and which can be experimentally observed.

Control of self-pumped phase conjugate reflectivity in a photorefractive crystal by another laser beam

Abstract When a photorefractive Ce:BaTiO 3 crystal is suddenly irradiated by an Ar + laser beam, the self-pumped phase conjugate reflectivity of a He-Ne laser beam in the crystal drops to a minimum and then recovers to a steady state value. When the Ar + laser beam is then blocked, a similar time dependence of the phase conjugate reflectivity is observed. The laser beams do not overlap in the crystal.

Dye laser transverse mode control using capillaries

Abstract We propose and demonstrate the use of a hollow circular dielectric waveguide for laser transverse mode control. A capillary inserted in a conventional transversely pumped pulse dye laser is shown to effectively promote the fundamental gaussian beam oscillation. It is shown by direct comparison that capillary dye lasers are superior to pinhole dye lasers in that the beam far-field divergence is small, the power conversion of the pump to TEM 00 mode output is high. Capillary dye lasers are also superior to waveguide dye lasers in that the design is simple and flexible. It is suggested that the use of capillaries for transverse mode control be applicable to other types of lasers.