Coherent phase control of internal conversion in pyrazine.

Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S2 and S1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulation was used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signal was largely suppressed for t < T, while for t ≫ T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t ≫ T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S2 potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S0 → S2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser.

Closed loop coherent control of electronic transitions in gallium arsenide.

A genetic algorithm was used to control the photoluminesce-nce (PL) from GaAs(100). A spatial light modulator (SLM) used feedback from the emission to optimize the spectral phase profile of an ultrashort laser pulse. Most of the experiments were performed using a sine phase function to optimize the integrated PL spectrum over a specified wavelength range, with the amplitude and period of the phase function treated as genetic parameters. An order of magnitude increase in signal was achieved after only one generation, and an optimized waveform, consisting of three equally spaced pulses approximately 0.8 ps apart, was obtained after 15 generations. The effects of fluence, polarization, relative phase of the subpulses, and spectral range of the optimized PL were investigated. In addition, preliminary experiments were performed using the phases of individual pixels of the SLM as genetic variables. The PL spectrum is identified with recombination of electron-hole pairs in the L-valley of the Brillouin zone. Control is achieved by coherent manipulation of plasma electrons. It is proposed that hot electrons excite lattice phonons, which in turn scatter carriers into the L-valley.

3D femtosecond laser patterning of collagen for directed cell attachment.

Three-dimensional micropatterned collagen scaffolds were fabricated by femtosecond laser ablation. An 800 nm, 45 fs Ti:Sapphire laser was used to create various 3D patterns in a collagen gel, including holes, lines and grids. An optimal collagen concentration was found for both substrate patterning and cell compatibility. The threshold fluence for ablation of the scaffold was found to be 0.0 6 J/cm(2), and the morphology of the ablation craters was measured as a function of fluence. Mesenchymal stem cells from rat bone marrow and human fibroblasts were seeded within the ablated patterns and were shown to be viable for at least 10 days.

Polarization-resolved laser-induced breakdown spectroscopy.

It is shown that plasma polarization measurements can be used to enhance the sensitivity of laser-induced breakdown spectroscopy (LIBS). The polarization of the plasma emission is used to suppress the continuum with only slight attenuation of the discrete atomic and ionic spectra. The method is demonstrated for LIBS detection of copper and carbon samples ablated by pairs of femtosecond laser pulses.

Femtosecond laser photodisruption of human trabecular meshwork: an in vitro study.

The goal of this in vitro study was to test the feasibility of using femtosecond (fsec) laser pulses to fistulize the human trabecular meshwork (TM), and to determine the minimum exposure time and energy dosage needed to create an ablation channel. Corneo-scleral rims were obtained from tissue used for penetrating keratoplasty. Four millimeter tissue strips hydrated in Optisol-GS were used to create partial thickness fistulas in the human TM by focusing a Ti:Sapphire laser beam (45 fsec, 1 kHz, 800 nm) with various pulse energies (7.2 and 14.4 microJ) and exposure times (0.25, 0.5, 1, and 2 sec) on the inner surface of the TM. Two-photon images of the lesions were obtained with a multiphoton microscope, using an ultrafast Ti:Sapphire light source. In addition, sections of fixed tissue were examined by light microscopy. Diameters and lengths of the lesions were determined from the hematoxylin and eosin (H&E) stained sections, and the collagen structure surrounding the lesion was evaluated from the two-photon images. All selected time points (except for 0.25 sec) and energies achieved the desired photodisruption of the TM. Incisions created with 0.5 sec/14.4 microJ irradiation appeared to be the most suitable because they were able to achieve consistent full thickness trabecular ablation. Incisions created at 1 sec /14.4 microJ/pulse and 2 sec/14.4 microJ/pulse were deeper than those at shorter time points with the same pulse energy. Longer exposure times and higher pulse energies were usually more variable and associated with deeper and larger incisions and slight collateral damage. Our results indicate that, with appropriate exposure time and pulse energy, fsec photodisruption can be employed to create lesions in the human TM without damaging the surrounding tissues. This study demonstrates that fsec laser treatments may have future potential for the surgical treatment of glaucoma.