Geometry of ultrasonic imaging experiments and side-view images of plasma channels at laser pulse energies of (a) and (b). Cylindrical enclosure of the ultrasonic transducer is seen as a dark, 15 mm wide rectangle mounted to slide on the top surface of the FS sample. The layer of vacuum grease of thickness between the transducer and the sample is outlined by the rectangle in (a). Side face of the FS sample is seen as the brighter and wider, horizontally oriented area at the bottom of the images. The laser beam enters FS sample through the right side wall as indicated by the arrows. The plasma channels are emphasized by the dashed circles. Dark channels in front and behind the plasma channels represent integrated contribution of damage traces left by preceding pulses (in the same horizontal plane but at different lateral positions).
Intensity distribution within luminous channels vs axial (a) and radial (b) coordinates at different laser pulse energies of and . In (a) the axial profiles measured in the central part of the cross section of filaments and radially integrated profiles are labeled by “1” and “2,” respectively. In (b), the radial profiles are taken at the axial coordinate of 2.3 mm. Laser pulse propagates from right to left, as in Fig. 1. The gray-shaded box marks the coordinate range containing a false peak due to a scratch on the sample surface.
Side-view optical images of damage traces of filaments in bulk FS after irradiation by pulses of 300 fs duration, energy, and central wavelength of 1030 nm. The pulses were focused by a lens with ; their propagation direction is indicated by the arrow.
Ultrasonic transients measured after excitation of FS by 800 nm laser pulses having the energy of 65 (a), 10 (b), and 45 (c) . Transfer function of the ultrasonic transducer with the preamplifier (d) represented by its response (dashed line) to a 10 ns wide compressive pulse (solid line). For comparison, theoretical pulse shape of a cylindrical ultrasonic pressure wave adapted from the literature (Ref. 38) is also shown (solid gray line). In (a)–(c), positive and negative pulses signify compression and rarefaction phases of the transients, respectively. In (a), the instant is marked by the vertical dashed line. In (a)–(c), positions of the major compressive pulses are emphasized by arrows, and calibration for vertical and horizontal axes is given by scale bars.
Dependencies of the full width of compression pulses (full circles) and the ratio of rarefaction pressure to compression pressure (hollow squares) on the laser pulse energy . indicates the threshold energy for ultrasonic emission at which is close to (see the main text).
Energy dependence of the channel length as measured from optical images in Fig. 1 (filled circles) and calculated from full widths of the compression pressure pulses (open squares) with the threshold . Inset: energy dependence of the calculated channels length in log-log scale and its linear fit with the slope .
Energy dependence of the ultrasonic compressive pressure with the threshold . Inset: the same dependence in log-log coordinates and its linear fit with slope .
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