Femtosecond filament amplification in liquids

We study the femtosecond filamentation in a liquid amplifying medium, sulphorhodamine 640 in a methanol solvent. In the presence of population inversion, the energy of the filament increases while its diameter expands because of peak intensity clamping. PACS 42.25.Bs; 42.65.Jx; 42.65.Re; 42.65.Sf     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

Spatial optimization of filaments

We demonstrate the spatial homogenization of intense laser pulses by adaptive minimization of spatial chirp of the spectrally broadened output pulses of a filament. A liquid-crystal-based two-dimensional spatial light modulator is used to control the spatial phase of the driver pulse. An evolutionary algorithm finds the optimal spatial laser phase distribution that introduces minimal distortions during filamentation and enhances the beam quality of the output pulse. A homogeneous intensity distribution favours efficient temporal compression close to the bandwidth limit without the need for spatial filtering after the filament. PACS 42.65.-k; 42.65.Re; 41.85.Ct     

Self-stabilization of third-harmonic pulse during two-color filamentation in gases

Self-stabilization of the laser pulse parameters is demonstrated during the two-color filamentation of ultrashort and intense laser pulses in gases. Experimental data and results of numerical simulations show, in good qualitative agreement, that the root-mean-square values of the intensity fluctuations decrease below the initial value for the near-infrared pump pulse and the perturbative limit for the third-harmonic pulse in the filament. It is found that the stabilization of the third-harmonic intensity and energy are due to intensity clamping of the pump pulse and a constant ‘volume’ of the laser pulse during the nonlinear propagation inside the filament. PACS 42.65.Ky; 42.65.Jx; 52.35.Mw     

Enhanced harmonic conversion efficiency in the self-guided propagation of femtosecond ultraviolet laser pulses in argon

Harmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse. PACS 42.25.Bs; 42.65.Jx; 42.65.Re; 42.65.Ky     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Theory and simulation of supercontinuum generation in transparent bulk media

Comprehensive model and computer simulations of the supercontinuum generation in bulk media are presented. Using water and air as representative condensed and gaseous media, the standard model for white-light generation is put to test. It is shown that besides the intensity clamping and plasma-induced frequency blue shift, the linear chromatic dispersion plays an important role. It is the major factor that determines the achievable spectral extent of the supercontinuum. PACS 42.65.-k; 42.65.Hw; 42.65.Ky; 42.65.Sf     

The influence of divergence on the filament length during the propagation of intense ultra-short laser pulses

We have questioned the conventional criterion, which is the critical power for self-focusing, to predict the stopping position of a filament. Our experimental result suggests that during the filamentation process the plasma induces an additional diffraction of the laser beam. This is another crucial factor that may lead to the termination of the filament even if the power is still much higher than the critical power. This should be taken into account in order to estimate the end of the filament as well as its length. PACS 52.38.Hb; 42.65.Jx; 52.35.Mw     

Computer simulation of laser pulse filament generation in rain

Initiation of femtosecond laser pulse filamentation in rain conditions is investigated numerically. It is shown that coherent scattering on rain water particles generates filament initiation sites. Results of computer simulation are consistent with full-scale experimental data. PACS 42.65.Jx; 42.68.Jg; 42.60.Jf     

Multi-parameter characterization of the longitudinal plasma profile of a filament: a comparative study

The longitudinal plasma profile of a laboratory scale filament induced by the propagation of an intense femtosecond laser pulse in air was measured simultaneously by three different methods. Each of them is based on a specific property of the filament, including electromagnetic pulse emission, backward emitted nitrogen fluorescence, and acoustic wave generation. Although each of above methods has been separately reported, it is the first time that a simultaneous multi-parameter measurement on a laser filament is presented. Such multiparameter measurement allows a direct comparison between the different methods, and a better understating of the dynamics of a filament from its generation through multi-photon/tunnel ionization (MPI/TI) to its damping through acoustic wave emission. The advantages as well as drawbacks of each method are analyzed in a comparative way. PACS 42.65.Jx; 42.68.Ay; 42.68.Wt     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Filamentation nonlinear optics

A filamenting femtosecond laser pulse self-stabilizes the intensity fluctuation inside the filament core due to intensity clamping and generates an excellent spatial beam quality inside the core due to self-spatial filtering. The high quality of the core can be sampled by nonlinear processes. A few experimental examples are shown: self-phase modulation, four-wave mixing, third-harmonic generation and waveguide writing in glass. PACS 42.65.Jx; 52.35.Mw     

Control of lasing filament arrays in nonlinear liquid media

Multiple filamentation in a high concentration solution of coumarin 153 in ethanol is studied. It is shown that the output filament pattern may be controlled by placing diffractive elements (circular aperture, edge) in the input beam path. These filaments are formed in highly reproducible arrays along diffraction maxima corresponding to the element used. Experimental results are supported by numerical simulations. They confirm that diffraction-induced intensity gradients swamp modulational instability on the wavefront, forcing filaments to form along diffraction maxima. The effect of two-photon absorption by coumarin molecules on filament patterns is also investigated over a range of dye concentrations. Control results are finally exploited in the production of arrays of localized lasing filaments, which should open novel applications. The resultant lasing sources are mutually coherent and highly repeatable from shot-to-shot, as is shown by their far-field interference patterns. PACS 42.65.Tg; 52.38.Hb; 42.68.Ay     

A new instability of counter-propagating waves in BaTiO3?

The self-development of a strong reflection grating in a BaTiO₃ sample illuminated with two mutually incoherent counter-propagating light waves is reported. The two transmitted waves diffracted from this grating become partially mutually coherent. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

Single-shot dynamics of pulses from a gas-filled hollow fiber

We present measurements of the performance characteristics of few-cycle laser pulses generated by propagation through a gas-filled hollow fiber. The pulses going into the fiber and the compressed pulses after the fiber were simultaneously fully characterized shot-by-shot by using two kHz SPIDER setups and kHz pulse energy measurements. Output-pulse properties were found to be exceptionally stable and pulse characteristics relevant for non-linear applications like high-harmonic generation are discussed. PACS 42.65.Re; 42.65.Ky; 42.65.Sf; 42.65.Jx     

Generation of 15-nJ bunched noise-like pulses with 93-nm bandwidth in an erbium-doped fiber ring laser

We report on the generation of high power superbroad spectrum bunched noise-like pulses from a passively mode-locked erbium-doped fiber ring laser without using the stretched-pulse technique. The maximum 3-dB spectral bandwidth of the noise-like pulses is about 93 nm with an energy of about 15 nJ. We further show numerically that the superbroad spectrum of the pulses is caused by the transform-limited feature of the pulses together with the Raman self-frequency shift effect. PACS 42.65.Dr; 42.55.Wd; 42.60.Fc; 42.81.Dp     

Raman effects in the infrared supercontinuum generation in soft-glass PCFs

Measurements of the Raman gain spectra in the SF6 and SF57 highly nonlinear glasses demonstrated twice as high Raman shift in comparison with the fused silica. Numerical simulation predicted that a large Raman shift in combination with high nonlinearity can significantly reduce the required input pulse intensity for supercontinuum in these glasses, retaining the necessary degree of coherence. We found that the degradation of the SC coherence due to Raman soliton jitter can be effectively controlled by a correct choice of input intensity and fiber length. Also it was found that a high degree of coherence correlates with the spectrum shape in the vicinity of the Raman threshold, providing an convenient experimental observable. PACS 42.65.Tg; 42.81.Dp; 42.72.Ai     

Plasma effects and the modulation of white light spectra in the propagation of ultrashort, high-power laser pulses in barium fluoride

We report the results of a study of white light generation in a 7.5 cm long crystal of a high band-gap material, barium fluoride, using ultrashort (<42 fs) laser pulses over a range of values of incident laser power that extend up to more than three orders of magnitude larger than the critical power for self-focusing (P_cr). We explore white light generation and the intensity and spectral distributions within filaments that are formed as a result of the interplay of self-focusing and plasma-induced defocusing. The onset of plasma effects occurs at power levels in excess of 7 GW for ultrashort pulses in BaF₂. For incident power levels that are three orders of magnitude larger than the critical power, blue-shifting of the incident laser wavelength is observed in addition to asymmetric continuum generation. The blue shift enables us to estimate the temporal variation of the electron density in the plasma that is generated within the BaF₂ crystal. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw     

Optical switching in metallic photonic crystal slabs with photoaddressable polymers

We report on a metal-polymer compound material with optical properties that can be reversibly switched all-optically. The key element is a metallic photonic crystal slab with an additional layer of photoaddressable material that provides a large variable birefringence and sharp resonances. Pump-probe experiments show a shift of the photonic crystal resonances that depends on the pump polarization and on the exposure. Comparison of the results with calculations from a scattering-matrix theory allows one to determine the refractive index changes for different polarization geometries and to model our compound material quantitatively. PACS 42.65.Pc; 42.70.Jk; 42.70.Qs; 78.67.-n     

Optimal spectral broadening in hollow-fiber compressor systems

Supercontinuum generation in gas-filled hollow fibers is investigated using numerical simulation of the nonlinear propagation of light pulses in hollow waveguides. The use of the cascading hollow-fiber configuration allows one to significantly enhance the achievable spectral broadening, particularly in the high energy regime. General design criteria for a single- and a double-fiber configuration are presented, which allow the generation of high-energy supercontinua. PACS 42.65.Re; 42.65.-k; 42.65.Jx