Surface-enhanced optical third-harmonic generation in Ag island films.

Surface-enhanced optical third-harmonic generation (THG) is observed in silver island films. The THG intensity from Ag nanoparticles is enhanced by more than 2 orders of magnitude with respect to the THG intensity from a smooth and homogeneous silver surface. This enhancement is attributed to a local plasmon excitation and resonance of the local field at the third-harmonic wavelength. The diffuse and depolarized component of the enhanced THG is associated with the third-order hyper-Rayleigh scattering in a two-dimensional random array of silver nanoparticles.

Enhanced two-photon fluorescence excitation by resonant grating waveguide structures.

Enhanced two-photon fluorescence (TPF) spectroscopy with novel high-finesse resonant polymeric grating waveguide structures (GWSs) is presented. Under resonant conditions the field enhancement at the surface of a GWS can be exploited for TPF spectroscopy without the need for highly focused laser excitation light. We compare the TPF obtained by placing a drop of tetramethylrhodamine (TMR) on top of a GWS with that obtained with TMR on top of a glass substrate. Our procedure and results indicate that the detection of TPF can be improved by a factor of 10 with resonant GWSs.

Ferroelectric ordering and electroclinic effect in chiral smectic liquid crystals.

Ferroelectric ordering, the electroclinic effect, and chiral smectic C(SmC*)-smectic A phase transitions in thin planar ferroelectric liquid crystal (FLC) cells are studied by means of linear electro-optic and second harmonic generation (SHG) techniques. The ferroelectric switching is detected in biased FLC cells by measuring azimuthal dependences of linear and nonlinear responses. The applied dc electric field rotates the FLC symmetry axis with initial and final orientations in the cell plane. Comparative studies of the SHG switching behavior in reflection and transmission geometries allows one to distinguish the contributions from the bulk and the subsurface layers of the cell. The analysis of SHG temperature dependences shows the existence of a strong surface coupling. The temperature-dependent nonlinear polarization shows a critical behavior with the exponent approximately 0.3 in SmC* phase.

Two-photon fluorescence excitation of macroscopic areas on planar waveguides.

In this paper, we report the first successful demonstration, to our knowledge, of two-photon fluorescence excitation (TPFE) using planar thin-film waveguide structures of macroscopic excitation dimensions (square millimeters to square centimeters in size). The high intensity of excitation light required for TPFE is available not only at a single focus point but along the whole trace of the beam guided in the waveguide structure. Line profiles of the fluorescence excited by TPFE show excellent correlation with the geometry of the launched laser beams. A clear second-order dependence of the fluorescence intensity on the excitation intensity confirms the two-photon character of fluorescence generation. Spectra of the emission generated by one-photon excitation and by two-photon excitation show only minor differences.

Optical second-harmonic generation as a probe of electric-field-induced perturbation of centrosymmetric media.

Polarization-selective optical second-harmonic generation (SHG) is applied to monitor nonuniform polarization distributions in centrosymmetric media modulated by an external dc electric field. Two different systems are investigated: first, optical SHG from a Au-Si(100) Schottky Barrier is shown to be a direct probe of the electric field polarity near such a metal-semiconductor interface. Second, a poled polymer exhibits a strong anisotropic SHG signal under normal incidence, indicating a nonuniform field enhancement at the edges of the poling electrode. The results demonstrate the sensitivity of SHG to small symmetry perturbations. Consequences for device applications are discussed.

Hemicyanine monolayer orientation studied by second-harmonic generation.

Using a novel scheme of polarization-dependent excitation and detection, the molecular orientation of hemicyanine monolayers has been studied. Wavelength-dependent measurements of the average tilt angle of molecules within the monolayer reveal details of aggregate formation and mobilization of these aggregates by addition of diluting agents such as arachidic acid.

Laser amplification using axicon reflectors.

The amplification behavior of a laser pumped cylindrical dye cell located within an axicon reflector has been studied. Compared with a conventional, transversely pumped rectangular amplifying dye cuvette, the axicon shows similar high gain and enhanced beam quality as a result of its radially symmetric excitation geometry. This latter property was experimentally confirmed by a 30% reduction of the divergence of a seed beam after amplification and by increased second harmonic generation conversion efficiency.

Pump-induced population changes in broadband coherent anti-Stokes Raman scattering.

Using a powerful, excimer-laser-pumped broadband coherent anti-Stokes Raman scattering (CARS) system, we have observed considerable changes in the populations of room-temperature nitrogen under atmospheric pressure. Pump-induced populations in the first, second, and third vibronic bands of N2 at room temperature resulted in an apparent vibrational temperature of 2550 K. The implications for broadband N2 CARS thermometry are discussed.

Laser gain measurements by means of amplified spontaneous emission.

Laser gain determination by means of amplified spontaneous emission is discussed in terms of a rate-equation approach. Numerical solutions show that optical gain can be deduced only for a limited regime of experimental parameters. The theoretical analysis is examined with experimental gain data for organic dyes in the vapor phase and in liquid solution. In addition, the gain of electron-beam-excited Ar-N(2) and XeF laser mixtures has been studied.

Versatile high-temperature high-pressure vapor cell design for electron beam excited laser studies.

The design of a cell suitable for studying electron beam excited laser candidates at high temperatures such as organic dye and metal vapors is described. The cell is capable of handling temperatures up to 450 degrees C and pressures from 10(-6) Torr to 15 atm. Details of both the high-temperature laser performance of Ar-N(2) and the gain characteristics of the dye POPOP [2,2' phenylenebis(5-phenyloxazole)] are presented as an example of the use of such a cell.