Mapping local field enhancements at nanostructured metal surfaces by second-harmonic generation induced in the near field.

We report on an aperture scanning near-field optical microscope in which femtosecond pulses are coupled to a hollow-pyramid aperture sensor. Such probe displays high throughput and preserves pulse duration and polarization, enabling the achievement of sufficiently high peak power in the near field to perform nonlinear optics on the nanoscale. We use the system to observe the nonlinear optical response of nanostructured metal surfaces with sub-100-nm spatial resolution. We study second-harmonic generation from gold nanoparticles both isolated and in high-density patterns, highlighting a strong dependence of the generation efficiency on the shape and on the fine structure of the nanoemitter. In particular, we present results on closely packed gold triangles as well as on nanoellipsoids with different local surface plasmon resonances.

Optical properties of waveguides written by a 26 MHz stretched cavity Ti:sapphire femtosecond oscillator.

We report on the fabrication, by a 26 MHz stretched-cavity femtosecond Ti:sapphire oscillator, of optical waveguides in different glass substrates, and their optical characterization. Operation of these waveguides in the telecom range at 1.55 microm is demonstrated. Digital holography microscopy is used to measure their refractive index profile. The results evidence a strong dependence of the fabrication process on the glass matrix.

Highly efficient second-harmonic nanosource for near-field optics and microscopy.

A nanometric source of second-harmonic (SH) light with unprecedented efficiency is demonstrated; it exploits the grazing-incidence illumination of a metal tip, which is conventionally used for atomic force microscopy, by 25-fs laser pulses of a high-energy Ti:sapphire oscillator. Tip scanning around the beam focus shows that the SH generation is strongly localized at its apex. The polarization dependence of the SH light complies with the model of an on-axis nonlinear oscillating dipole.

Coherent dynamics of photoexcited green fluorescent proteins.

The coherent dynamics of vibronic wave packets in the green fluorescent protein is reported. At room temperature the nonstationary dynamics following impulsive photoexcitation displays an oscillating optical transmissivity pattern with components at 67 fs (497 cm(-1)) and 59 fs (593 cm(-1)). Our results are complemented by ab initio calculations of the vibrational spectrum of the chromophore. This analysis shows the interplay between the dynamics of the aminoacidic structure and the electronic excitation in the primary optical events of green fluorescent proteins.

Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors.

We report on double-chirped mirrors with custom-tailored dispersion characteristics over a bandwidth of 170 THz in the visible. The mirrors are used in a prismless compressor for a noncollinear optical parametric amplifier in the visible. The compressed pulses, characterized for the what is believed to be first time by use of the spectral phase interferometry for direct electric field reconstruction technique, display a nearly flat phase from 510 to 710 nm and have a duration of 5.7 fs.