Thermodynamical Analysis of the Formation of α-Si Ring Structures on Silicon Surface.

Superficial modifications on silicon wafers produced by single-shot focused femtosecond laser irradiation having a 1030 nm wavelength and 300 fs pulse duration were experimentally and theoretically analyzed. The laser fluence window when the amorphous silicon phase develops, resulting in a ring-like modification shape, was experimentally estimated to be between 0.26 J/cm2 and 0.40 J/cm2 and was independent of the silicon dopant type and laser focusing conditions; however, the window was narrower when compared to results reported for shorter pulse durations. In addition, we present a simplified numerical model that can explain and predict the formation of these patterns based on the caloric coefficients of silicon and the energy distribution of the deposited material.

Performance investigation of high-efficiency widely tunable subnanosecond optical parametric generator and amplifier based on MgO:PPLN.

We report on experimental and theoretical studies of widely tunable high-efficiency subnanosecond optical parametric generator (OPG) and amplifier (OPA) based on a 2 cm long multigrating MgO-doped periodically-poled lithium niobate (MgO:PPLN) crystal pumped by a passively Q-switched Nd:YAG micro-laser. Our OPG can be continuously tuned from 1442 nm to 4040 nm with signal wave energies ranging from 33 µJ to 265 µJ and total OPG conversion efficiency up to 46 % that depended on the pump focusing conditions. Characterization of spatial properties of the OPG determine Lorentzian spatial profile of the signal beam with M 2≈2 that was also dependent on the pump focusing conditions. High OPG gain and subsequent pump depletion led to the adjustment of the output signal pulse duration in the range of 242 - 405 ps by varying the incident pump power. By using a distributed feedback (DFB) continuous-wave (CW) 1550 nm wavelength seed laser for the OPA operation we reduced the generation threshold up to 1.6 times, increased maximum conversion efficiency by 4 - 20%, and achieved nearly transform-limited output signal pulses. Experimentally measured characteristics were supplemented by numerical simulations based on the quantum mechanical model for the OPG, and classical three-wave interaction model for the OPA operation.

Measurement of the phase refractive index of a photonic crystal fiber mode.

We present a new experimental technique for measurement of the refractive index of a photonic crystal fiber (PCF) fundamental mode. We demonstrate that the phase refractive index of the PCF mode can be estimated by analyzing a phase shift of interfering adjacent longitudinal laser modes of a continuous-wave laser corresponding to a shift from constructive to destructive interference. The experimental results are in very good agreement with numerically simulated phase refractive index values.

Estimation of photonic crystal fiber dispersion by means of supercontinuum generation.

We present a technique for photonic crystal fiber dispersion measurement. We demonstrate that investigating supercontinuum using cross-correlation frequency resolved optical gating (XFROG) technique can be used for quantitatively characterizing dispersion and observing orthogonal polarization modes in polarization maintaining photonic crystal fibers. In addition, an XFROG trace of supercontinuum generated in a polarization maintaining photonic crystal fiber reveals complex behavior of orthogonal polarization modes that is different in normal and anomalous dispersion regions of the photonic crystal fiber.

Generation and control of the spiraling zero-order Bessel beam.

We report on the generation of a spiraling zero order Bessel beam by means of conventional axicon and the phase hologram. Obtained results are in a fairly good agreement with the theoretical predictions.

Spiraling zero-order Bessel beam.

The question that we are addressing concerns the possibility of creating a zeroth-order Bessel-like beam that spirals around the axis of propagation. The analytical features of the beam propagation are studied theoretically. Approximations to such a light field can be experimentally realized by using an axicon and a hologram. The beam potentially can attract interest in microfabrication applications.

Conical third-harmonic generation in normally dispersive media.

It is shown, both theoretically and experimentally, that in normally dispersive media under tight-focusing conditions third harmonic is generated by six-wave mixing rather than via common third-order frequency tripling. Though far-field pattern of third-harmonic signal was an axially symmetric ring for a wide range of the material wave vector mismatch and laser beam focusing conditions, in some cases the generation of more complex beams has been found possible. Results of simulations of the proposed model qualitatively correspond well with experimental data for the calculated values of third- and fifth-order nonlinear susceptibilities of sodium.

Discrete damage traces from filamentation of Gauss-Bessel pulses.

Filamentation of Bessel-Gauss pulses propagating in borosilicate glass is found to produce damage lines extending over hundreds of micrometers and consisting of discrete, equidistant damage spots. These discrete damage traces are explained by self-regeneration of Gauss-Bessel beams during propagation and are potentially applicable in laser microfabrication of transparent materials.

Ultrabright femtosecond source of biphotons based on a spatial mode inverter.

A method of enhancing the efficiency of entangled biphoton sources based on a type II femtosecond spontaneous parametric downconversion (SPDC) process is proposed and implemented experimentally. Enhancement is obtained by mode inversion of one of the SPDC output beams, which allows the beams to overlap completely, thus maximizing the number of SPDC photon pairs with optimum spatiotemporal overlap. By use of this method, biphoton count rates as high as 16 kHz from a single 0.5-mm-long beta-barium borate crystal pumped by second-harmonic radiation from a Ti:sapphire laser were obtained.