Andrei A. Ionin

Ultrafast changes in the optical properties of a titanium surface and femtosecond laser writing of one-dimensional quasi-periodic nanogratings of its relief

One-dimensional quasi-periodic nanogratings with spacings in the range from 160 to 600 nm are written on a dry or wet titanium surface exposed to linearly polarized femtosecond IR and UV laser pulses with different surface energy densities. The topological properties of the obtained surface nanostructures are studied by scanning electron microscopy. Despite the observation of many harmonics of the one-dimensional surface relief in its Fourier spectra, a weak decreasing dependence of the first-harmonic wavenumber (nanograting spacing) on the laser fluence is found. Studies of the instantaneous optical characteristics of the material during laser irradiation by measuring the reflection of laser pump pulses and their simulation based on the Drude model taking into account the dominant interband absorption allowed us to estimate the length of the excited surface electromagnetic (plasmon-polariton) wave for different excitation conditions. This wavelength is quantitatively consistent with the corresponding nanograting spacings of the first harmonic of the relief of the dry and wet titanium surfaces. It is shown that the dependence of the first-harmonic nanograting spacing on the laser fluence is determined by a change in the instantaneous optical characteristics of the material and the saturation of the interband absorption along with the increasing role of intraband transitions. Three new methods are proposed for writing separate subwave surface nanogratings or their sets by femtosecond laser pulses using the near-threshold nanostructuring, the forced adjustment of the optical characteristics of the material or selecting the spectral range of laser radiation, and also by selecting an adjacent dielectric.

Nanoscale cavitation instability of the surface melt along the grooves of one-dimensional nanorelief gratings on an aluminum surface

Femtosecond laser nanostructuring at low fluences produces a one-dimensional quasiperiodic grating of grooves on an aluminum surface with a period (≈0.5 μm) that is determined by the length of a surface electromagnetic wave. The structure of the grooves of the surface nanograting is formed by regular nanopeaks following with a period of about 200 nm. Some nanopeaks manifest craters at their tops. It is suggested that nanopeaks are formed due to the frozen nanoscale spallative ablation of a nanolayer of an aluminum melt in quasiperiodic regions corresponding to interference maxima of the laser radiation with the surface electromagnetic wave. The periodicity of the appearance of nanopeaks along grooves is due to the previously predicted mechanism of cavitation deformation of the melt surface in the process of macroscopic spallation ablation. However, in this case, cavitation is coherent (similar to a near-critical spinodal decay) rather than spontaneous.

Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (<1 PW/cm2) femtosecond laser pulses have been detected by observing the propagation of a shock wave in air from the ablated surface to a broadband piezoelectric receiver. The estimated initial pressure and velocity of the shock wave (ablation plume) agree well with data obtained earlier by various methods for shock waves propagating inside ablated targets.

Acoustic monitoring of microplasma formation and filamentation of tightly focused femtosecond laser pulses in silica glass

Contact acoustic technique has been employed to perform spatially resolved in situ detection of microplasma formation and filamentation of tightly focused femtosecond laser pulses with supercritical pulse powers in bulk dielectrics, via corresponding acoustic emission. Investigation of acoustic generation mechanisms related to the plasma formation and filamentation effects reveals the critical character of the opaque microplasma and provides estimates of its gigapascal-level pressures and energy densities of a few kJ∕cm3. The acoustic measurement enables real-time in situ monitoring and revealing of basic mechanisms of ionization and filamentation in bulk dielectrics.

Lasers on overtone transitions of carbon monoxide molecule

The experimental and theoretical study of lasers working on the first vibrational overtone of the CO molecule is reviewed. A theoretical model of the active medium of the CO laser and its verification are presented. The CO laser operation in a wide spectral range (2.5–4.2 μm) that spans the atmospheric transparency window is demonstrated in the review of experimental works. It is shown that the overtone CO laser can serve as an effective mid-IR source for atmospheric spectroscopy, long-distance transportation of laser radiation, remote laser sensing, etc.

Multiterawatt Ti:Sapphire/KrF laser GARPUN-MTW as a test bench facility for verification of combined amplification of nanosecond and subpicosecond pulses

The possibility of the same large-aperture KrF laser driver to amplify simultaneously both nanosecond pulses for thermonuclear target implosion and picosecond ones for fuel ignition is discussed relative to KrF-based Fusion Test Facility. In this way experiments were performed at hybrid Ti:Sapphire/KrF GARPUN-MTW facility on amplification of subpicosecond pulses. 2-TW, 330-fs pulses were produced with beam divergence 20 μrad in direct double-pass amplification scheme. Peak power as high as 30–40 TW can be achieved in 50-fs pulses, being combined with long pulses (of a few ns to 100 ns) of ~1 GW power.

The electro-ionization co laser: A multiwavelength ir oscillator (λ = 2.7–3.3 μm; 4.9–6.0 μm)

Abstract The development of the electro-ionization method of pumping high gas density CO lasers is detailed. Energetic, spectral and temporal characteristics are described. Pulse outputs up to 800J and efficiencies up to 30% have been achieved.

Bulk femtosecond laser marking of natural diamonds

Point-by-point multi-shot femtosecond laser writing of micro-scale linear damage tracks and symbolic logo information was performed inside single-crystal natural diamonds in transversal writing geometry at variable basic operational parameters (numerical aperture of focusing optics, depth of focusing inside samples and peak laser pulse powers). The filamentary character of femtosecond laser writing of buried damage tracks in this material at the supercritical peak laser powers was revealed.

Optical and ultrasonic signatures of femtosecond pulse filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica by single, moderately focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy Eopt=5 μJ corresponding to a few megawatt power, the pulses collapse due to self-focusing and the nonlinear focus moves upstream with increasing pulse energy. Behind the focus, elongated, gradually narrowing awl-shaped channels of electron-hole plasma and luminescent defects are produced. In the channels, whose dimensions generally depend on the pulse energy, supercontinuum emission propagating downstream the channels occurs, although its observation requires elevated pulse energies above 25 μJ in order to compensate energy dissipation in the channels. Ultrasonic side-view imaging of the channels, conducted from a few millimeters distance, reveals predominantly compressive pressure transients. The compressive signals...

Mid-IR Zeeman spectrum of nitric oxide molecules in a strong magnetic field

Nonlinear Zeeman splitting of ro-vibrational spectral lines of nitric oxide molecules in a variable magnetic field with maximum induction up to 6 T was studied both experimentally and theoretically. A single line frequency-tunable CO laser operating in the wavelength range of ~5–6 µm was employed to detect absorption of its radiation by NO molecules versus time, i.e. to detect laser magnetic resonance (LMR) spectrograms. These LMR spectrograms were simulated with a numerical model in which Zeeman shifting of ro-vibrational energetic levels in the magnetic field was calculated with the perturbation theory of different orders. The observed LMR spectrograms appeared to be in adequate agreement with the numerical data calculated in the third order of the perturbation theory.