V. Kudriašov

Beam transformation and permanent modification in fused silica induced by femtosecond filaments

Results of an investigation of lateral beam structure by propagating femtosecond pulses in fused silica are presented. Observed spatial transformations of the beam were closely related to the onset of continuum generation. Further beam propagation was in the form of an intense central spike (filament) and surrounding rings. Increasing power led to beam splitting and the formation of a regular structure of multiple filaments. The filament was responsible for the spontaneous formation of permanent modification inside the material. Experimental results are backed by numerical simulations that include self-guided channels of a permanent change of the refractive index in a bulk medium.

On the Role of Conical Waves in Self-focusing and Filamentation of Femtosecond Pulses with Nonlinear Losses

This chapter concerns with the experimental observations and theoretical investigations on the propagation of intense femtosecond pulses in water and fused silica. It emphasizes spontaneous transformation of a beam into a conical (Bessel-like) wave during the filamentary propagation in media with nonlinear losses. This transformation constitutes an interpretation of the energy reservoir surrounding the high intensity central core of the filament. The adopted model is shown as being able to explain related phenomena such as the formation of multiple filaments and that of X-waves, observed experimentally in both water and fused silica.

Transfer of orbital angular momentum of light using two-component slow light

We study the manipulation of slow light with an orbital angular momentum propagating in a cloud of cold atoms. Atoms are affected by four co-propagating control laser beams in a double tripod configuration of the atomic energy levels involved, allowing us to minimize the losses at the vortex core of the control beams. In such a situation the atomic medium is transparent for a pair of co-propagating probe fields, leading to the creation of two-component (spinor) slow light. We study the interaction between the probe fields when two control beams carry optical vortices of opposite helicity. As a result, a transfer of the optical vortex takes place from the control to the probe fields without switching off and on the control beams. This feature is missing in a single tripod scheme where the optical vortex can be transferred from the control to the probe field only during either the storage or retrieval of light.

Holographic study of ultrafast optical excitation in GaN film induced by nonlinear propagation of light

The dynamics of pulse transformation and free-carrier generation associated with the nonlinear propagation of 1030 nm and 300 fs light pulses in Gallium nitride film has been studied by use of pulsed digital holography with 25 fs temporal resolution. Domination of the Kerr effect at the leading part and electron plasma at the trailing part of the pulse has been identified from the reconstructed spatiotemporal phase-contrast images. Experimental results have been supported by the simplified numerical model capable to reproduce the main features of a dynamic process.

Superluminal two-color light in a multiple Raman gain medium

We investigate theoretically the formation of two-component light with superluminal group velocity in a medium controlled by four Raman pump fields. In such an optical scheme only a particular combination of the probe fields is coupled to the matter and exhibits superluminal propagation, the orthogonal combination is uncoupled. The individual probe fields do not have a definite group velocity in the medium. Calculations demonstrate that this superluminal component experiences an envelope advancement in the medium with respect to the propagation in vacuum.

Fabrication of the Refractive Index Gratings in Optical Glasses by Filamentary Propagation of Femtosecond Laser Pulses

We report on the refractive index grating formation by filamentary propagation of femtosecond pulses in fused silica. The relevant exposure and work cycles are considered both experimentally and through numerical study, involving a model of light filaments supported by conical wave, capable to capture permanent glass refraction index changes.

Influence of the nonlinear losses on the modifications induced by femtosecond filaments in fused silica

Nonlinear losses experienced by the self-focusing femtosecond pulse is shown to have an important effect on the refractive index modifications in fused silica. The region of the maximum induced change is found to coincide with that of the maximum nonlinear losses of the pulse. It is found as well that material densification and the formation of color centers both contribute to the index change in that zone. Experimental results are supported by numerical simulations using model that takes into account accumulation of the permanent refractive index changes and their influence back on the pulse. Both the color center- and compaction-induced changes cause the modification to develop into a waveguide and lead to the narrowing of supercontinuum spectra.

Bulk modification and damage in fused silica induced by femtosecond pulses

We report experimental observation of modification and damage in bulk of fused silica induced by intense multiple femtosecond pulses. At power levels several times exceeding critical power for self-focusing the modification is in the form of some permanent changes inside the material and these effects are closely correlated with the beam filamentation process. Longer distances of propagation and much higher pulse energies produce bulk damage in the form of scattering zones. The formation of damage in anti-reflective-coated fused silica windows is also reported. Numerical simulations involving self-focusing, multiphoton absorption and permanent change of the refractive index of the bulk material were found to be in agreement with the experimental results.

Experimental demonstration of spinor slow light

Over the last decade there has been a continuing interest in slow and stored light based on the electromagnetically induced transparency (EIT) effect, because of their potential applications in quantum information manipulation. However, previous experimental works all dealt with the single-component slow light which cannot be employed as a qubit. In this work, we report the first experimental demonstration of two-component or spinor slow light (SSL) using a double tripod (DT) atom-light coupling scheme. The oscillations between the two components, similar to the Rabi oscillation of a two-level system or a qubit, were observed. Single-photon SSL can be considered as two-color qubits. We experimentally demonstrated a possible application of the DT scheme as quantum memory and quantum rotator for the two-color qubits. This work opens up a new direction in the slow light research.

Cumulative refractive index modification and waveguide formation by femtosecond filaments

The theoretical and experimental studies of the cumulative effects on refractive index modification dynamics during filamentation from femtosecond light pulses in transparent media are presented. Residual changes of the refractive index were shown as developing both in forward and backward directions forming prolonged tracks of modified index profiles (waveguides) during repetitious laser shots.