TDDFT study of excitation of water molecules with short laser pulses

We explore the excitation of water molecules subject to short and intense laser pulses in the frame of time-dependent density function theory (TDDFT) at the level of the time-dependent local-density approximation (TDLDA), applied to valence electrons, coupled non-adiabatically to molecular dynamics (MD) of ions. We first study the optical absorption spectra of the water molecule as an observable in the "linear" domain and results are in good agreement with experiments. We then explore the influence of the laser frequency on the excitation. It is found that when the laser frequency is off-resonant or highly above the resonant region, the excitations are weak whereas for the resonant frequency case, the ionization is enhanced and bond lengths are enlarged. Furthermore, a direct coupling of ions with the laser pulse with the off-resonant frequency is found when investigating the OH bond lengths. We finally study the effect of laser intensity on the excitation of H 2 O and it is found that ionization increases when the laser intensity varies from low to high and we observe stable vibrations to Coulomb fragmentation when the ionization is up to typically two more charge units.     

Spatial photorefractive solitons with picosecond laser pulses

We report on the formation of spatial optical solitons in photorefractive media using picosecond laser pulses. Solitons are generated with laser pulses in the visible and infrared wavelength region using photorefractive strontium barium niobate. The dynamics of the soliton formation and the region of existence is studied in detail.     

Generator of picosecond laser pulses

The design of the generator of picosecond laser pulses and the results on semiconductor target (ZnSe, CdS, and others) excitation by electric field and electron beam pulses are presented. The maximum power of laser radiation reached 10 kW at pulse durations of 100–200 ps.     

Nonequilibrium phonons in SiGe quantum-well nanostructures upon excitation by picosecond laser pulses

The phonon pulses initiated by photoexcitation of structures containing Si_0.8Ge_0.2 double quantum wells under picosecond radiation of a MIRA 900P titanium-sapphire laser (λ = 760 nm) are studied. The propagation of nonequilibrium acoustic phonons is detected with a superconducting bolometer. The recorded bolometer response is found to differ substantially from that observed in photoexcitation of the same structure by nanosecond pulses of a nitrogen laser (λ = 337 nm). The generation of coherent acoustic phonons is suggested as an explanation.     

Symmetries and asymmetries in coherent atomic excitation by chirped laser pulses

We analyze the effects of laser-induced Stark shift and irreversible population loss on the technique of chirped-frequency adiabatic passage, and the ensuing symmetries and asymmetries in the ionization and fluorescence signals. We find that the properties of the detection signal depend critically on the fashion in which it is collected: for example, the post-pulse populations of the ground and excited states, and the ionization signal collected during the excitation, possess different symmetry properties with respect to the frequency chirp rate and the static frequency detuning. We illustrate these features with two exactly soluble analytic models, which describe simultaneous excitation and ionization of a two-state quantum system, as it typically occurs in atomic excitation with femtosecond laser pulses. We find that the ionization signal may exhibit unexpected oscillations and derive the conditions for maximizing their contrast.     

Amplification of picosecond dye laser pulses

Small signal and saturated gains of picosecond pulses in Rhodamine dye amplifiers have been measured. A 17 cm amplifier produces a maximum small signal gain of ≈20, and with three amplifiers in series, peak pulse powers of > 3 GW are obtained. Mode-locking of Rhodamine 6G in the spectral range 575–600 nm is improved by employing DQOCI as saturable absorber.     

Interaction of picosecond laser pulses with a thin target

Interaction between a picosecond laser pulse of nonrelativistic intensity with a thin target is studied in terms of the kinetic theory of laser plasma, which is based on constructing propagators for the plasma particle distribution functions. Allowance is made for both the self-consistent plasma field and plasma particle correlations (collisions). The interaction causes charge separation in the target and strong heating of electrons.     

Ultrahigh-resolution coherence spectroscopy by means of periodic excitation with picosecond pulses

The feasibility of ultrahigh-resolution coherence spectroscopy with a train of picosecond light pulses from a synchronously pumped mode-locked and cavity-dumped dye laser is demonstrated. Narrow resonances of periodic pulse excitation are observed whenever the pulse repetition rate of the laser of a higher harmonic coincides with the frequency splitting of coherently excited levels. The zero-field hyperfine splitting of the Na-ground state (1771.6 MHz) could be measured in the 2100th order of the excitation rate with a resonance halfwidth of 800 Hz.     

Plasma-mediated ablation of brain tissue with picosecond laser pulses

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm². Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.     

Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

A tunable picosecond IR laser generating multi-megawatt pulses in the range 3–8 m

We describe a versatile infrared laser system capable of generating picosecond pulses continuously tunable over the wavelength range 3.3–8.4 μm, with peak powers of 1–20 MW. The IR pulse duration is ～1.3 ps with a time-bandwidth product of ～0.3. The system uses stimulated electronic Raman scattering (SERS) in caesium vapour to shift the frequency of a picosecond dye laser into the infrared. The quantum efficiency of the Raman conversion process exceeds 40% over almost the entire tuning range. This technique can be extended to other tuning ranges in the IR by using different atomic vapours and SERS transitions.     

Reduction of jitter in streak-camera synchronization with picosecond laser pulses

A laser activated silicon switch generates the sweeping voltage for a picosecond streak camera. With a Photochron streak tube shot-to-shot jitter has been reduced to ?±15 ps for writting speeds in excess of 2×10⁸ ms^-1.     

Direct ninth harmonic conversion of picosecond laser pulses

Direct ninth harmonic conversion of picosecond laser pulses is obtained for the first time. Using sodium vapor, phase- matched ten-photon process by Nd³⁺: glass laser pumping is realized. Effective ninth-order nonlinear susceptibility of the order of x⁽⁹⁾_eff ～ 10^-64 ESU is estimated.