Dmitry V. Sinitsyn

Enhancement of ultrafast electron photoemission from metallic nanoantennas excited by a femtosecond laser pulse

We have demonstrated for the first time that an array of nanoantennas (central nanotips inside sub-micrometer pits) on an aluminum surface, fabricated using a specific double-pulse femtosecond laser irradiation scheme, results in a 28-fold enhancement of the non-linear (three-photon) electron photoemission yield, driven by a third intense IR femtosecond laser pulse. The supporting numerical electrodynamic modeling indicates that the electron emission is increased not owing to a larger effective aluminum surface, but due to instant local electromagnetic field enhancement near the nanoantenna, contributed by both the tip’s ‘lightning rod’ effect and the focusing effect of the pit as a microreflector and annular edge as a plasmonic lens.

Sub-100?nanometer transverse gratings written by femtosecond laser pulses on a titanium surface

One-dimensional transverse (perpendicular to the laser polarization) gratings with periods Λ ≈ 50–60 nm were observed on a titanium surface within 150 nm wide, micrometer-long regular surface modification longitudinal stripes fabricated by multiple 744 nm Ti:sapphire femtosecond laser shots, occurring at a repetition rate of 10 Hz. In the center of the surface laser spot these stripes are oriented strictly perpendicular to the laser polarization, in accordance with the plasmon-polaritonic model, and appear as ablative longitudinal trenches centered along the main stripe axes, which are precursors of longitudinal common ripples with a 500 nm period. At the low-fluence periphery of the laser spot, the stripes appear not as ablative longitudinal trenches, but as linear arrays of sub-ablative transverse nanoripples with periods down to 50 nm. The appearance of such superfine transverse nanoripples is related to incomplete spallation of the laser–molten surface layer, periodically modulated at the nanoscale through coherent sub-surface cavitation.

Filamentation of femtosecond laser pulses governed by variable wavefront distortions via a deformable mirror

Filamentation of focused UV and IR femtosecond laser pulses and plasma channel formation governed by variable wavefront distortions was experimentally and numerically studied. A deformable mirror was used to control the plasma channel length by introducing a spherical aberration into the initial transverse spatial distribution of a femtosecond laser pulse. An at least double increase of the plasma channel length was observed with increasing deformation of the mirror. Numerical calculations show that the hat-like phase shape of the aberration ensures that the energy of the initial laser pulse remains confined for a longer distance within the limited transverse size of the filament.

Nonlinear optical dynamics during femtosecond laser nanostructuring of a silicon surface

Non-linear cumulative self-organization dynamics of femtosecond laser-induced surface relief ripples was for the first time experimentally revealed on a silicon surface as their primary appearance, degradation and revival, reflecting ultrafast non-linear dynamics of corresponding optical interference surface patterns. Such dynamics were revealed by electrodynamic modeling to be directly driven by related instantaneous surface optical patterns, which are sensitive not only to cumulative ripple deepening (steady-state feedback factor), but to laser-induced instantaneous variation of surface dielectric function, providing either positive or negative fluence-dependent optical feedbacks.

Ultrafast femtosecond laser ablation of graphite

Fluence dependences of IR and UV reflectivity of femtosecond laser pulses on a HOPG surface demonstrate their saturation in a certain fluence range, starting from 0.2 J cm−2, where single-shot non-linear plasma emission is detected by electric probe measurements. This correlation between prompt solid-state optical/electronic dynamics and electron–ion plasma emission indicates prompt freezing of surface electronic dynamics via its plasma-emission cooling and simultaneous ultrafast shallow laser ablation of the surface. Strong HOPG disordering is observed in Raman spectra for laser fluences, exceeding the plasma emission threshold.

Femtosecond laser modification of titanium surfaces: direct imprinting of hydroxylapatite nanopowder and wettability tuning via surface microstructuring

Femtosecond laser modification of titanium surfaces was performed to produce microstructured hydrophilic and biocompatible surface layers. Biocompatible nano/microcoatings were prepared for the first time by dry femtosecond laser imprinting of hydroxylapatite nano/micropowder onto VT6 titanium surfaces. In these experiments HAP was first deposited onto the titanium surfaces and then softly imprinted by multiple femtosecond laser pulses into the laser-melted surface metal layer. The surface relief was modified at the nano- and microscales depending on the incident laser fluence and sample scanning speed. Wetting tests demonstrated that the wetting properties of the pristine Ti surface can be tuned through its laser modification in both the hydrophobic and hydrophilic directions.

Repetitively Pulsed and CW Sealed-off Slab CO Laser with Cryogenic Cooling

First experiments on fundamental band CO lasing in sealed-off cryogenically cooled slab facility with RF discharge excitation were carried out. Repetitively pulsed and CW modes of RF discharge excitation were studied. The laser output characteristics for different slab geometries were compared. Average output power achieved 12 W. Lasing efficiency came up to ~14 %. The output laser spectrum was observed within wavelengths range 5.08-5.34 &mgr;m. Stable lasing was obtained for more than one hour.

The influence of the energy reservoir on the plasma channel in focused femtosecond laser beams

The influence of the energy reservoir on plasma channel formation during the filamentation of tightly focused femtosecond laser beams was studied both experimentally and numerically. It was found that for the reservoir localized near the propagation axis, its diameter is much smaller than in the case of a collimated beam and decreases in the vicinity of the focus. A small diaphragm placed in the focal area does not eliminate the plasma channel behind the focal point.

Compact sealed-off cryogenic slab RF discharge CO laser

First experiments on fundamental band CO lasing in sealed-off cryogenically cooled slab facility with RF discharge excitation were carried out. Repetitively pulsed and CW modes of RF discharge excitation were studied. Average output power achieved 12 W. Lasing efficiency exceeded 10%. The output laser spectrum was observed within wavelengths range 5.08-5.34 &mgr;m. Stable lasing was obtained for tens minutes.

Transverse gas flow RF slab discharge generator of singlet delta oxygen for oxygen-iodine laser

Results of experimental and theoretical study of singlet delta oxygen (SDO) production in transverse gas flow RF slab discharge for an electric discharge oxygen-iodine laser are presented. The electric discharge facility operating in both pulse-periodic and CW mode was manufactured: gas flow duct including multi-path cryogenic heat exchanger, dielectric slab channel, and slab electrode system incorporated in the channel for RF discharge ignition. Experiments on SDO production in transverse gas flow RF discharge were carried out. SDO production depending on gas mixture content and pressure, gas flow velocity, and RF discharge power was experimentally studied. It was shown that SDO yield increased with gas pressure decrease, gas flow deceleration and helium dilution of oxygen at the same input power. CW RF discharge was demonstrated to be the most efficient for SDO production as compared to pulse-periodic RF discharge with the same averaged input power. SDO yield was demonstrated to be not less than 10 percent. The model developed was further modified to do simulations of CW and pulse periodic RF discharges. A reasonable agreement between experimental and theoretical data on SDO production in CW and pulse-periodic RF discharges in oxygen is observed.