T. Shih

Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores

We report on the femtosecond-laser micromachining of poly(methyl methacrylate) (PMMA) films doped with nonlinear azoaromatic chromophores: Disperse Red 1, Disperse Red 13 and Disperse Orange 3. We study the conditions for controlling chromophore degradation during the micromachining of PMMA doped with each chromophore. Furthermore, we successfully used fs-micromachining to fabricate optical waveguides within a bulk sample of PMMA doped with these azochromophores.

Nonlinear spectra of ZnO: reverse saturable, two- and three-photon absorption

We present a broadband (460 - 980 nm) analysis of the nonlinear absorption processes in bulk ZnO, a large-bandgap material with potential blue-to-UV photonic device applications. Using an optical parametric amplifier we generated tunable 1-kHz repetition rate laser pulses and employed the Z-scan technique to investigate the nonlinear absorption spectrum of ZnO. For excitation wavelengths below 500 nm, we observed reverse saturable absorption due to one-photon excitation of the sample, agreeing with rate-equation modeling. Two- and three-photon absorption were observed from 540 to 980 nm. We also determined the spectral regions exhibiting mixture of nonlinear absorption mechanisms, which were confirmed by photoluminescence measurements.

Faraday rotation in femtosecond laser micromachined waveguides

We demonstrate magneto-optic switching in femtosecond-laser micromachined waveguides written inside bulk terbium-doped Faraday glass. By measuring the polarization phase shift of the light as a function of the applied magnetic field, we find that there is a slight reduction in the effective Verdet constant of the waveguide compared to that of bulk Faraday glass. Electron Paramagnetic Resonance (EPR) measurements confirm that the micromachining leaves the concentration of the terbium ions that are responsible for the Faraday effect virtually unchanged.

Ultrafast exciton dynamics in ZnO: Excitonic versus electron-hole plasma lasing

The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.

Ultrafast reflectivity dynamics in bis (n-butylimido) perylene thin films

Using pump-probe reflectometry, we study the ultrafast excited-state dynamics in thin films of BuPTCD, an organic semiconductor, deposited on gold nanoparticles. We observe depletion of the ground state and excited state absorption after photo-excitation.