Dahe Liu

Measurement of the bulk viscosity of liquid by Brillouin scattering

The aim is to develop a rapid and direct method for measuring the bulk viscosity of a liquid as a function of temperature. Brillouin scattering of a laser beam in fresh water and salt water at different temperatures has been studied. The results show that there exists a close temperature-dependent relationship among the Brillouin frequency shift, the Brillouin linewidth, and the bulk viscosity of water. Thus the bulk viscosity of water can be determined directly from Brillouin-scattering measurements. The method has a high signal-to-noise ratio and high accuracy.

Measurements of sound speed in the water by Brillouin scattering using pulsed Nd:YAG laser

Abstract The principles of measuring sound speed in the water by Brillouin scattering are analyzed. Experimental measurements of sound speed in the water were implemented in the laboratory. The experimental conditions, which covered a temperature range of 0–30 °C and the salinity of 0‰ and 35‰ , were equivalent to actual conditions of temperature and salinity. The measured results using Brillouin scattering are in good agreement with classical methods.

Method for measuring the threshold value of stimulated Brillouin scattering in water

A new method for accurate measurement of the threshold value of stimulated Brillouin scattering (SBS) in water in terms of pump laser intensity is investigated. The threshold value of SBS is determined by the point of the deviation of the value of the attenuation coefficients of wide- and narrow-linewidth lasers rather than the intensity of the backscattered SBS signal in the material.

Controlling magnetic dipole transition with magnetic plasmonic structures

A plasmonic structure with double gold patches is proposed for enhancing the spontaneous emission of a magnetic dipole transition through a magnetic hot area. A Purcell factor of nearly 2000 can be obtained at optical frequencies together with a low sensitivity in spatial and spectral mismatches between the light emitter and the resonance mode. The associated resonance can be tuned from the visible to the IR frequencies, enabling efficient control of forbidden transitions using plasmonic structures.

Compact Color Filter and Polarizer of Bilayer Metallic Nanowire Grating Based on Surface Plasmon Resonances

Surface plasmon resonances on bilayer aluminum nanowire gratings are studied in both theory and experiment. It is found that there are two kinds of surface plasmon on the bilayer metallic gating: longitudinal aluminum/dielectric/aluminum slit and lateral aluminum/dielectric interface waveguide mode. The surface plasmon waveguide mode resonance in the slits makes the grating act as a transverse magnetic (TM)-passing polarizer. With the lateral waveguide mode resonance, certain wavelengths of the incident TM light are translated to aluminum/air or aluminum/substrate waveguide light, and the grating acts as a color filter. With both resonances, the bilayer nanowire grating can be a compact-integrated polarizer and color filter.

High performance plasmonic random laser based on nanogaps in bimetallic porous nanowires

A plasmonic random laser is fabricated using gold-silver bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules. The strong confinement of the nanogaps allows the bimetallic porous nanowire-based random laser, which is pumped by ns pulses, to operate with a very low threshold and extremely low concentrations of Rhodamine 6 G (as low as 0.067 mM). This random laser can be used as a pump source for another coherent random laser based on oxazine. These results provide a basis for studies of coherent random lasing pumped by another random laser.

White light emission with red-green-blue lasing action in a disordered system of nanoparticles

White light emission from a disordered system with intensity feedback is investigated. The gain material is an ethanol solution with three laser dyes (Coumarin 440, Coumarin 6, Oxazine), and titanium dioxide (TiO2) nanoparticles to provide feedback. A single pulsed laser beam at 355 nm with 8 ns pulse duration is used to pump the dyes. Coumarin 440 and Coumarin 6 are excited first by the pump beam, and part of the Coumarin 6 peak pumps Oxazine. Bright white light emission is obtained by combining blue, green, and red beams with a threshold effect. The working properties and system emission characteristics are discussed.

Nonlinear analysis for a reflection hologram

Abstract The diffraction of planar reflective hologram gratings was analyzed with a nonlinear differential equation based on the coupled wave theory. Under special conditions of low and high reflectivity, an analytic solution and its relationship to the fourier transform was obtained, which showed that the rexonstruction of a volume hologram is just a Fresnel diffraction process. Through a numerical solution of the equation, using the Runge-Kutta method, some new phenomena were discovered, and the physical pictures that cause these phenomena were discussed. Also, the properties of the spectrum based on nonlinear theory were analyzed. Finally, experimental verifications were given.

Surface plasmon-mediated transmission in double-layer metallic grating polarizers

We study the mechanism of polarization-selective transmission in double-layer gratings. For TM polarized light, the coupling between the surface plasmon waveguide modes of slits in the two metal layers causes most of the energy to pass the gratings. For TE polarized light, there is no surface plasmon coupling effect, and most of the energy in the photoresist waveguide is reflected back because of the cutoff of waveguide modes. In view of the characteristics of surface plasmon waveguide modes, a new polarizer with an undercut profile of the photoresist is proposed to achieve better performance by improving the coupling efficiency. At the pitch of 200nm, the TM transmissions can be over 70% in the visible spectrum and over 80% in the wavelengths from 470nm to 800nm for a bottom width w2<60nm.

Structures of photonic crystals and band gaps in volume hologram

Abstract The concept and method of analyzation of photonic crystals and band gaps are introduced into optical holography. The volume holograms are treated theoretically as a photonic crystal. All properties of volume holograms can be explained accurately with the band structure of photonic crystals. Different from superlattice photonic crystal, the dielectric constant of holographic photonic crystals is a continuous rather than a step function. The band gap structures with changing incident angles (not for a fixed angle only) are discussed. Experimental verifications are implemented by using elementary reflection holograms, which show that the analysis of band gaps for volume holograms are consistent with the experimental results.