The dynamic properties of the laser emission are very important in studying the characteristics of the laser and may
reveal the underlying operating mechanism. Here we report a more precise measurement of the build-up time of random
laser pumped by picosecond pulse laser. The build-up time is defined as the time delay from the peak of the pumping
pulse to that of the emission. The random laser is R6G dye solutions with nanometer size TiO 2 as the scatterer. Various
dye concentrations and scatterer density are tried and measured. A specially customized fiber and a streak camera with a
spectrometer are employed to make the simultaneous measurement. The fiber has two branches and the lengths of both
branches are made equal with a difference of much less than 1 mm. The dispersion of the fiber, which introduces much
error in the results, is also measured and later compensated in the following data processing. The streak camera with
spectrometer can catch the random laser pulse and the pumping pulse signal in one shot with a resolution of less than 2
picoseconds. The results show that the build-up time changes evidently with the dye concentration, while it changes a
little along with the scatterer density. The pulse width almost remains the same in our experiment considering the errors.
We present time-resolved measurements of pulse transmission at wavelength 532 nm (60 ps pulse width, 10 Hz
repetition rate) on samples of titanium powders suspended in methanol. The average particle diameter of the powders is
80 nm. We used a streak camera with 2 ps time revolution to record the transmitted signals. When the particle density is
low, the results agreed with the diffusion theory and we obtained the time-independent diffusion constants. By adding
the titanium powders gradually in methanol, we obtained the relationship between the diffusion constant and the particle
density of TiO2 in the suspended solution. When using the TiO2 powders as the sample with a particle density of
1.36x1015 cm-3, the experimental result showed a little deviation from the diffusion theory, which may be the signature of
localization in the random media.
We studied the spectral shift of random lasing in the Rhodamine 6G dye solution with TiO2 nanoscatterers under
picosecond pulses pumping. The red shift, resulting from the re-absorption and re-emission of the dye, indicates a longer
optical path length of the emitted laser traveling inside the medium. Thus the optical paths of the random laser in the
solution can be estimated using the values of red shifts for different dye concentrations and scatterer densities. The
diffusion theory is provided and the theoretical results agree very well with that calculated from the red shifts before the
inflection points appear for increasing scatterer density. The followed increasing scatterer density results in the lights
staying longer in the medium, in contrast to that predicted by the diffusion theory. So it is clear that the inflection point
shows that the system is changing from a diffusion system to a weakly localized one in which the light stay longer
because of the localization.
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