arXiv subjects

A. V. Rode

Publications and source records attributed to A. V. Rode.

9 recordsLinked to original sources

arXiv2020

All femtosecond optical pump and X-ray probe: holey-axicon for free electron laser

We put forward a co-axial pump(optical)-probe(X-rays) experimental concept and show performance of the optical component. A Bessel beam generator with a central 100 micrometers-diameter hole (on the optical axis) was fabricated using femtosecond (fs) laser structuring inside a silica plate. This flat-axicon optical element produces a needle-like axial intensity distribution which can be used for the optical pump pulse. The fs-X-ray free electron laser (X-FEL) beam of sub-1 micrometer diameter can be introduced through the central hole along the optical axis onto a target as a probe. Different realisations of optical pump are discussed. Such optical elements facilitate alignment of ultra-short fs-pulses in space and time and can be used in light-matter interaction experiments at extreme energy densities on the surface and in the volume of targets. Full advantage of ultra-short 10 fs X-FEL probe pulses with fs-pump(optical) opens an unexplored temporal dimension of phase transitions and the fastest laser-induced rates of material heating and quenching. A wider field of applications of fs-laser-enabled structuring of materials and design of specific optical elements for astrophotonics is presented.

physics.optics↗

arXiv2020

Micro-Raman spectroscopy of ultrashort laser induced microexplosion sites in silicon

Confined microexplosions induced in silicon by powerful ultrashort laser pulses can lead to new Si phases. Some of these have not previously been observed via near-equilibrium compression of silicon. In this study, confocal Raman micro-spectroscopy and Raman imaging of arrays of microexplosions have been conducted to search for Raman signatures of these novel allotropes of silicon. A microexplosion is generated at the interface between a thick silicon dioxide confinement layer and underlying silicon. It is characterised by a void at the interface above a region of compressed silicon. Raman data show a rich assembly of silicon phases within the modified silicon. Residual stresses up to 4.5 GPa in the modifications have been determined from the shift in the main diamond-cubic Si Raman peak. The computed Raman spectra for a number of Si allotropes show reasonable agreement with the experimental spectra. Two structurally similar tetragonal phases of silicon (the rhombohedral r8 and the body-centred bc8) phases as well as recently identified bt8-Si are all highly likely to be contained in Raman spectra from many laser-modified sites. Although the st12-Si phase, previously observed in our electron diffraction studies of the highly compressively stressed laser-modified regions, was not reliably identified from Raman data, we suggest this could be due to the possible difference in residual stress level in the sites analysed by the electron diffraction and Raman spectra. Several other unidentified Raman peaks were observed, suggesting the presence of other unknown silicon phases. All of these silicon phases are expected to have attractive semiconducting properties including narrow band gap that open up novel applications.

cond-mat.mtrl-sci↗

arXiv2018

Extreme energy density confined inside a transparent crystal: Solid - plasma - solid transformations, status and perspectives

It was demonstrated during the past decade that ultrashort intense laser pulse tightly focused deep inside a transparent dielectric generates the energy density in excess of several MJ/cm3. Such energy concentration with extremely high heating and quenching rates leads to unusual solid-plasma-solid transformation paths overcoming kinetic barriers to formation of previously unknown high-pressure material phases, which are preserved in the surrounding pristine crystal. These results were obtained with the pulse of Gaussian shape in space and in time. Recently it was shown that the Bessel-shaped pulse could transform much larger amount of a material and allegedly create even higher energy density than that was achieved with the Gaussian (GB) pulses. Here we present a succinct review of previous results and discuss the possible routes for achieving higher energy density employing the Bessel beams (BB) and take advantage of its unique properties.

physics.optics↗

arXiv2017

Ultra-fast re-structuring the electronic landscape of transparent dielectrics: new material states (Die-Met)

The swift excitation of the transparent dielectrics by intense short laser pulse produces ultra-fast re-structuring of the electronic landscape generating a wealth of material states continuously changing in space and in time in accord with the variations of the intensity. These unconventional transient material states combine simultaneously dielectric and metal properties (Die-Met). The laser excitation transforms a transparent dielectric into electrically inhomogeneous state early in the pulse time [1]. The permittivity of excited material varies in time and in space changing from positive to negative values that strongly affects the interaction process. The interplay between the transient permittivity gradient and polarisation of the incident laser becomes the major process of the new interaction mode. In a particular relation between the polarization and the permittivity gradient the incident field amplitude grows up while the wave is approaching to the surface where the real part of permittivity turns to zero. That results in the local increase in the absorbed energy density. The complex 3D structure of the permittivity makes a transparent part of excited dielectric (at $ε0 > εre > 0$) optically active. The electro-magnetic wave passing through such a medium gets a twisted trajectory and accrues the geometric phase [2]. The plane of polarisation rotation and phase depends on the 3D permittivity structure [3]. Measuring the polarisation and phase of the probe beam allows quantitatively identify this new transient state. We discuss the revelations of this effect in different experimental situations and possible applications.

physics.optics↗

arXiv2015

Photoluminescence from voids created by femtosecond laser pulses inside cubic-BN

Photoluminescence (PL) from femtosecond laser modified regions inside cubic-boron nitride (c-BN) was measured under UV and visible light excitation. Bright PL at the red spectral range was observed, with a typical excited state lifetime of $\sim 4$~ns. Sharp emission lines are consistent with PL of intrinsic vibronic defects linked to the nitrogen vacancy formation (via Frenkel pair) observed earlier in high energy electron irradiated and ion-implanted c-BN. These, formerly known as the radiation centers, RC1, RC2, and RC3 have been identified at the locus of the voids formed by single fs-laser pulse. The method is promising to engineer color centers in c-BN for photonic applications.

cond-mat.mes-hall↗

arXiv2009

Is the ultra-fast transformation of bismuth non-thermal?

Transient state of femtosecond laser excited bismuth has been studied by various groups with time-resolved optical, x-ray, and electron probes at the deposited energy density from below through up to several times the equilibrium enthalpy of melting. However, the interpretations of the experimental results are controversial: the optical probes reveal the absence of transition to the melting phase while the authors of x-ray and electron diffraction experiments claim the observation of ultrafast non-thermal melting. The presented analysis, based on temperature dependence of bismuth optical properties, unequivocally shows a purely thermal nature of all the observed fs-laser induced transformations in bismuth.

physics.optics↗

arXiv2003

Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting

The reversible phase transition induced by femtosecond laser excitation of Gallium has been studied by measuring the dielectric function at 775 nm with ~ 200 fs temporal resolution. The real and imaginary parts of the transient dielectric function were calculated from absolute reflectivity of Gallium layer measured at two different angles of incidence, using Fresnel formulas. The time-dependent electron-phonon effective collision frequency, the heat conduction coefficient and the volume fraction of a new phase were restored directly from the experimental data, and the time and space dependent electron and lattice temperatures in the layer undergoing phase transition were reconstructed without ad hoc assumptions. We converted the temporal dependence of the electron-phonon collision rate into the temperature dependence, and demonstrated, for the first time, that the electron-phonon collision rate has a non-linear character. This temperature dependence converges into the known equilibrium function during the cooling stage. The maximum fraction of a new phase in the laser-excited Gallium layer reached only 60% even when the deposited energy was two times the equilibrium enthalpy of melting. We have also demonstrated that the phase transition pace and a fraction of the transformed material depended strongly on the thickness of the laser-excited Gallium layer, which was of the order of several tens of nanometers for the whole range of the pump laser fluencies up to the damage threshold. The kinetics of the phase transformation after the laser excitation can be understood on the basis of the classical theory of the first-order phase transition while the duration of non-thermal stage appears to be comparable to the sub-picosecond pulse length.

physics.optics↗

arXiv2003

Unconventional magnetism in all-carbon nanofoam

We report production of nanostructured carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp2 and sp3 bonded carbon atoms. The material is a novel form of carbon in which graphite-like sheets fill space at very low density due to strong hyperbolic curvature, as proposed for ?schwarzite?. The foam exhibits ferromagnetic-like behaviour up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.

cond-mat↗

arXiv2001

Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulae for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.

physics.optics↗