Diagnostics of Filamentation in Laser Materials with Fluorescent Methods

Transcription

1 Diagnostics of Filamentation in Laser Materials with Fluorescent Methods A.V. Kuznetsov, E.F. Martynovich Irkutsk Branch of Institute of Laser Physics SB RAS Lermontov st. 130a, Irkutsk, , Russia Phone/Fax: , Introduction In our work we develop methods that allow to capture 3Dpattern of intensity distribution of fs-laser light in fluorescent photosensitive materials. Fluorescent analytical methods are known to be highly sensitive. Sensitivity of our materials is high enough to obtain visible traces of single pulses without destruction of material. In particular, diagnostics of beam quality and study of filamentation are possible. From one hand, filamentation of laser beams is harmful phenomenon besause it leads to damage of transparent materials. From other hand, the breakup of laser beam into multiple filaments carries useful information about small spatial intensity and phase perturbations in the beam profile. Thus, one can use filamentation pattern to make conclusions about beam quality. 1

2 Experimental setup for femtosecond laser irradiation of samples (ILP SB RAS, Novosibirsk): Scanning fluorescence time-resolved confocal microscope PicoQuant MicroTime 200 (IB ILP SB RAS, Irkutsk): Excitation: 5 picosecond diode lasers with wavelengths 375 to 640 nm. Repetition rate up to 40 MHz. Systems of time-correlation single photon counting. Scanning: piezo-positioner, 80 µm x 80 µm range, 10 nm resolution. Detectors: 2 single-photon avalanche diodes, spectral range nm, dark counts cps. Overall time resolution is ns. XY and Z - spatial resolution is ~0.2 and ~1 mm respectively. 4 2

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5 Интенсивность Fluorescent studies of single impurity molecules in transparent materials are possible with MicroTime 200: imaging, spectra, decay of fluoresscence, blinking, photophysical transformation: Время, с Time, s 2 mm Time, ns 5

6 Beam diagnostics Photosensitivity in our work is based on generation of fluorescent color centers. Convenient material is, for example, LiF crystal. Fluorescent channel in LiF created under the action of 10 4 femtosecond laser pulses (0.5 mj, 30 fs). Here fluorescence of the channel is observed with 450nm-optical excitation after femtosecond laser irradiation: Different color centers are generated in LiF under the action of femtosecond laser pulses. Most important of them in our work are F 2 and F 3 + color centers due to their high fluorescence yield in visible spectral region. These color centers are aggregates of 2 and 3 anion vacancies with trapped electrons. Initial stage of formation of color centers is multiphoton absorption of laser light. It leads to generation of excitons and electron-hole pairs. About 8 photons with l=800 nm are necessary to create an excitons or electron-hole pair in LiF with bandgap width ev. Consequent stages of color centers creation are similar to the case of the action of traditional ionizing radiations. Concentration of F 2 color centers is quadratically dependent on concentration of initial defects. So, from theorethical consideration we expected high degree ( 16) of nonlinearity of photosensitivity. It was confirmed with direct experiment. 12 mm Scheme of F 2 color center in alkali-halide crystal. Fluorescence of single F 2 color centers in LiF (image from microscope MicroTime 200). 12 6

7 Spectrum of fluorescence of F 2 and F 3 + color centers in LiF (l ex =470 нм): 13 High photosensitivity of LiF gives opportunity to obtain visible trace of propogation of single laser pulse. The colored channel created with single laser pulse (0.5 mj, 30 fs): 7

8 Comparison of colored channels in LiF Single laser pulse: pulses: 80 mm Crossection of trace of single filament of a single laser pulse in LiF (l ex =470 нм): Diameter is about 1.8 micron 8

9 Longitudial profile of trace of single filament of a single laser pulse The problem of theoretical approaches to prediction of multiple self-focusing distance (calculation of B-integral or direct simulation of self-focusing with nonlinear Schrodinger equation) is that the distance depends on form and scale of perturbations of initial beam crossection. Our experimental dependence of multiple self-focusing distance differs from theoretically predicted law z msf ~1/P [G. Fibich, S. Eisenmann, B. Ilan, Erlich Y., M. Fraenkel, Z. Henis, A. Gaeta, A. Zigler, Selffocusing Distance of Very High Power Laser Pulses, Opt. Express 13 (2005)]. Our experimental dependence is between z msf ~1/P and z sf ~1/ P. Channels created with pulses of different energy (0,04-0,33 mj): Most likely, the reason of this disagreement is specific spatial spectrum of perturbations in initial beam profile. 18 9

10 Lower pulse energy Higher pulse energy Beginning of channels Middle part We have carried out numerical computer simulation of multiple self-focusing of a collimated Gaussian laser beam with small initial perturbations. Mathematical model is based on nonlinear Schrödinger equation with account of Kerr self-focusing, diffraction and multiphoton absorption E E E 2k a0 n2e0 2 2 K 1 2i E E im E z x y n0 Here E is complex amplitude of electric field of light wave, c= m/s is velocity of light, n 0 = 1.39 is regular refraction index of LiF, n 2 = 7, cm 2 /W is Kerr refraction index, k=wn 0 /c is wave number (w=2 πc/l, l= m), a 0 is initial radius of laser beam, m is a coefficient of nonlinear absorption and K=8 is a degree of multiphoton nonlinear absorption. Initial conditions corresponds to collimated Gaussian initial beam profile with small intensity perturbation: E p x, y 0 exp x y 2 a z 0 Simulation shown different self-focusing distance for different spatial frequencies of perturbation

11 Conclusion Using of nonlinear fluorescent photosensitive materials gives opportunity to study spatial pattern of light-matter interaction with microscopic resolution. 3D capturing of intensity distribution of single fs-pulse in media is possible without meterial damage. In particular, this phenomena can be applied to definition of small perturbations in laser beams (diiagnostics of beams). Diversified study of low-concentration impurities in optical materials is possible with microsopic fluorescent methods. 21 Thank you! 22 11