Colloidal particle lens arrays-assisted nano-patterning by harmonics of a femtosecond laser.

We consider nanopatterning of dielectric substrates by harmonics of single powerful femtosecond pulses from a Ti:Sapphire laser. The nanopatterning is mediated by closely packed monolayers of polystyrene microspheres that act as microlenses at the surface. Observed modification of the material proceeds via ionization. By our theory, the second harmonic is more effective in multi-photon ionization and is better focused than the fundamental frequency which is effective in multiplying of the amount of free electrons via impact ionization. Experiments show that conversion of a part of the pulse energy into the second harmonic decreases the modification threshold and improves the localization of the structures. Optimization of the time offset between the harmonics could further improve the efficiency and quality of nanostructuring.

Laser-induced photopatterning of organic-inorganic TiO2-based hybrid materials with tunable interfacial electron transfer.

Hybrid organic-inorganic materials based on TiO(2) gels demonstrate high photosensitivity. Associated with their stable photochromic behavior, these make them suitable for laser-induced photopatterning. We show that the electronic coupling along the extended interface between the inorganic, TiO(2)-based gel, and the organic, poly(hydroxyethyl methacrylate) networks allows (i) a rapid scavenging of the photo-excited holes by the polymer, (ii) an efficient trapping of the photo-exited electrons as small polarons (Ti(3+)) that develop "dark" absorption continuum covering the spectral range from 350 nm (UV) to 2.5 microm (IR), and (iii) long-term (over months) conservation of trapped charges at high number density. Furthermore, we give the proof that the electron transfer depends on the material microstructure, which can be affected by the material chemistry and processing. Undeniably, a delay between the gelation of the system and the organic polymerization step allows tuning the photochromic responses of the resulting nanocomposites. A comparison is made between the prepared gel-based samples and a reference sample, which is obtained by the organic copolymerization of functional precondensed inorganic building units, titanium oxo-clusters, Ti(16)O(16)(OEt)(24)(OEMA)(8) with hydroxyethyl methacrylate. The experiments show the highest values of quantum yield (12%) and Ti(3+) concentration (1.7 x 10(20) cm(-3) or 14% of titanium atoms) attained in samples where the organic polymerization is induced after gelation. This behavior is explained by a strong coupling between the organic and the inorganic components of the hybrid towards the hole exchange and a poor coupling towards the electron exchange.

Supercontinuum assisted trapped electron accumulation in titanium oxide gel by femtosecond laser pulses.

Long-lived trapped electrons are generated in titanium oxide gels in a convergent part of a loosely focused beam from a millijoule, femtosecond Ti:Sapphire laser. The appearance of trapped electrons is accompanied by the generation of the white supercontinuum. Considering the relevant physical mechanisms of trapped electron accumulation, we show that two-photon interband transitions with participation of photons at the fundamental frequency and from the blue part of the supercontinuum explain the experimental findings.

Chemical activity of photoinduced Ti3+ centers in titanium oxide gels.

We report on the chemical activity of trapped electrons in wet titanium oxide gels. These electrons are generated under the band gap irradiation of gels in the spectral range between 3.25 and 4.4 eV and stored as Ti3+ centers that absorb in the visible. Chemical processes in photoirradiated gels are generally similar to those earlier reported in TiO2 colloids; however, peculiarities exist. In particular, a high internal surface of gels strongly enhances interface reactions. Measurements of UV-visible absorption kinetics allow conclusions to be made about extremely high available traps concentration and the activity of all trapped electrons toward nitrate and nitrite anion reduction according to a heterogeneous photocatalytic mechanism.

Light-induced charge separation and storage in titanium oxide gels.

We report on the interaction of light with a particular class of media--wet gels, which in contrast to sols of nanoparticles possess a macroscopic bulk structure, and which differ from conventional solids by the existence of the internal liquid-solid interface. We show, taking an absorption cross section of trapped electrons from Safrany, Gao, and Rabani [J. Phys. Chem. B 104, 5848 (2000)], that a separation of charges with quantum efficiency as high as 46% appears under the band-gap irradiation of titanium oxide gels: electrons are stored in the gel network as Ti3+ centers, whereas holes are stored in the liquid phase. Under a prolonged UV-laser irradiation, more than 14% of Ti4+ centers can be converted into Ti3+ . Their lifetime can be extremely long and exceeds months at room temperature. The trapped electrons are responsible for a "dark" absorption continuum covering the spectral range from 350 nm to 2.5 mum .

Cross talk in photochemical three-dimensional optical recording.

It is shown that the cross talk in three-dimensional optical photochemical recording results in ineffectiveness of single-photon recording. For two-photon recording for a given allowed cross-talk level there are optimal distances between spots that provide the maximum memory density. Estimations show that real restrictions here are connected with the reading of information.

In situ Monitoring of Laser Modification Process in Human Cataractous Lens and Porcine Cornea Using Coherence Tomography.

We demonstrate that optical coherence tomography (OCT) is a convenient diagnostic tool to monitor pulse-to-pulse kinetics in laser interactions with biological tissue. In experiments on laser modification and ablation of the cataractous human lens and the porcine cornea we have applied this technique in situ to investigate different modes of preablation tissue swelling, crater formation and thermally affected zone development. The cataractous lens is an example of highly scattering media whereas the cornea is initially low scattering. The radiation with different wavelengths has been employed including that of a YAG:Er laser (λ=2.94 µm), a glass:Er laser (λ=1.54 µm), YAG:Nd lasers (λ=1.32 µm and λ=1.44 µm), as well as of the fifth harmonic of a Nd:YAP laser (λ=0.216 µm). Pulse-to-pulse OCT monitoring has been accompanied by the probe beam shielding diagnostics to provide the time-resolved observation of the interaction dynamics. © 1999 Society of Photo-Optical Instrumentation Engineers.