Paul F. Liao

Interaction of metal particles with adsorbed dye molecules: absorption and luminescence.

Absorption, luminescence, and excitation luminescence studies of dye molecules adsorbed onto ultrathin, variable-thickness silver films show strong coupling of the particle plasma resonances to the molecules. Luminescence from the dye is increased by excitation transfer from the silver particles for certain film thickness. Increased luminescence has also been observed from dyes on Au and Cu films.

Surface-enhanced raman scattering from microlithographic silver particle surfaces

Abstract Surface-enhanced Raman scattering is observed from uniformly sized and shaped silver particles produced by evaporation of silver onto a lithographically produced microstructure. An excitation resonance is observed which can be shifted by changes in particle shape or surrounding dielectric constant in agreement with particle plasmon theories of surface Raman enhancement.

Observation of amplified reflection by degenerate four-wave mixing in atomic sodium vapor

This Letter is the first report of the observation of amplified reflection and self-oscillation produced by degenerate optical four-wave mixing by using a resonantly enhanced Kerr nonlinearity in atomic sodium vapor. Phase-conjugated reflected waves with intensities 100 times greater than input-signal intensities were achieved with only 40-kW/cm2 pump intensity.

cw optical wave‐front conjugation by saturated absorption in atomic sodium vapor

Reconstructed images have been obtained by direct wave‐front conjugation in atomic sodium vapor using a cw tunable dye laser. With a pump power of only 15 mW a conversion efficiency of 0.2% has been observed.

Surface-enhanced Raman scattering on gold and aluminum particle arrays.

The intensity and excitation wavelength dependence of the surface-enhanced Raman effect is measured on microlithographically prepared gold and aluminum particle surfaces. Comparison of enhancement relative to identically fabricated silver surfaces gives good agreement with particle-plasmon theories of surface-enhanced Raman scattering.

Relationship between surface-enhanced Raman scattering and the dielectric properties of aggregated silver films

By using an evaporated silver film that has a continuously varying thickness, we have demonstrated the dependence of surface-enhanced Raman scattering (SERS) on the dielectric properties of the film. These results support local field models that are based on metal-particle resonances for SERS.

Emission cross section and fluorescence efficiency of Nd-pentaphosphate

The room-temperature cross sections for the Nd3+4 F_{3/2} levels to the4 I_{11/2} and4 I_{9/2} manifolds (lower laser state and ground state, respectively) in NdP 5 O 14 are measured by two spectroscopic methods. A value for the largest cross section of \sigma(R_{1} - Y_{2}) = 1.7 \times 10^{-19} cm2is found. The highest effective cross section, resulting from superposition of two lines at 1.051 μm, gives a laser gain per Nd ion which is about 2/3 of the maximum gain in YAG:Nd. The relative branching ratio into the4 I_{11/2} and4 I_{9/2} manifolds is 0.65:0.35. Comparison of the integrated cross sections with the measured lifetime for 1-percent Nd in LaP 5 O 14 indicates a combined efficiency I_{13/2} and4 I_{15/2} manifolds and multiphonon quenching. A measurement of temperature dependence of fluorescence lifetime supports this last result.

Angular dependence of line shape and strength of degenerate four-wave mixing in a Doppler-broadened system with optical pumping

Results of experiments investigating the effects of optical pumping and atomic motion on cw degenerate four-wave mixing in atomic sodium vapor are presented. The signal strength is found to decrease as (sin θ)−2, where θ is the angle between the object and pump waves; however, the linewidth for generation is Doppler free and is independent of θ. Agreement is found with a simple model of the mixing process. Distortions of the atomic velocity distribution (produced in our experiments by optical pumping) are found to be essential for efficient cw generation. A new modulation scheme for the discrimination against background is used.

The POPOP dye vapor laser

We report a comprehensive study of the operation of a laser using the organic scintillator dye POPOP in the vapor phase. Laser operating parameters have been determined, and we describe tuning experiments which demonstrate an oscillation range of 25 nm. Gain measurements as a function of wavelength and time are reported for transverse pumping with the 337.1-nm output of a pulsed N 2 laser. The quantum efficiency has been measured as a function of temperature and pump wavelength, and a preliminary study has been made of the effect of buffer gases on the quantum efficiency and the gain. Spectral condensation measurements show 90 percent of the broad-band laser output can be obtained in a 1-nm band-indicating that even in a dilute vapor, the thermalization time of the laser levels is \gg1 ns. We conclude by discussing the implications of these results for the achievement of an electrically pumped dye vapor laser.

Optical metal-oxide tunnel detectors with microstructured electrodes.

Order-of-magnitude improvement of the quantum efficiency of metal-oxide-metal detectors is achieved in the visible spectrum by fabricating the devices on silica substrates structured with a periodic array of bumps separated by about 0.25 microm. Structuring metallic films affords improved coupling of the detector to visible light by excitation of localized plasma resonances and surface-plasmon polariton modes. Voltage responsivities of 5 mV/W into a 50-Omega impedance has been demonstrated in preliminary devices. In suitable geometries, such detectors have the potential of subpicosecond response times.