Ultrashort pulse laser

For CW applieations of optieal-heterodyne eonversion, two laser fields are applied to the optoeleetronie material. The non-linear nature of the eleetro-optie effeet strongly suppresses eontimious emission relative to ultrashort pulse exeitation, and so most of the CW researeh earried out to date has used photoeonduetive anteimae. The CW mixing proeess is eharaeterized by the average drift veloeity t and earrier lifetime Xq of the mixing material, typieally... 

Using a perturbative analysis of the time-dependent signal, and focusing on the interference term between the one- and two-photon processes in Fig. 14, we consider first the limit of ultrashort pulses (in practice, short with respect to all time scales of the system). Approximating the laser pulse as a delta function of time, we have... 

Kruger, J. and Kautek, W. Ultrashort Pulse Laser Interaction with Dielectrics and Polymers, Vol. 168, pp. 247-290. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

Figure 8.2 shows an example of the electron density map obtained by numerical simulation, in this case 1.5 ps after the entrance of the ultrashort pulse in the gas. The laser is coming from the right hand side. The picosecond precursor was included in the code, while the ASE was not. 

In praetiee, one nses femtosecond lasers, which necessarily feature a broader spectrum the shorter the pulses are. This provides another benefit for multiphoton imaging ultrashort pulses (sub-20 femtoseconds [fs]) possess a laser bandwidth increased in such a way that several typical flurophores with different emission wavelengths can be excited at once. Usually, fluorescence signals in microscopy are recorded in a single... 

Influence of Laser Bandwidth and Effective Pulse Duration on Nonlinear Signal Intensity, Showing the Dramatic Effect of Dispersion on Ultrashort Pulses... 

The reason two-photon excitation efficiency depends on the spectral phase of the laser pulse is beyond the scope of the peak intensity argument given above. It requires a more detailed consideration of how the pulse contributes to the nonlinear optical transitions. Let us consider, e.g., the case of two-photon excitation at wavelengths corresponding to = 400 nm. In addition to a pair of 800-nm photons, there are multiple combinations of spectral components within the bandwidth of an ultrashort pulse that can combine to cause excitation equivalent to 400 nm. Their wavelengths satisfy the relation particular, one can have photons at... 

After about 25 fs, the Pg (t) gradually decreases due to predominance of the radiationless transitions into the manifold of states over the laser excitation process. The decay rates are different in different groups of states, with some states over 4.83 eV being more slowly depopulated due to IC than states with lower energies [42]. This feature of the IC decay in pyrazine is manifest here in more pronounced way than in the case of ultrashort pulse excitation. Here, it is clear... 

Applications making use of the nonlinear absorption of dyes are passive Q-switching in solid-state lasers, pulse shaping, pulse intensity measurements of high-power ultrashort pulses, optical isolation between amplifier stages of high power solid-state lasers, and pulse width measurements of ultrashort pulses by the two-photon-fluorescence (TPF) method. 

To use the two-photon fluorescence (TPF), i. e., the fluorescence excited by two-photon absorption, for the measurement of the halfwidth of ultrashort pulses from mode-locked lasers, the laser beam containing the train of pulses is split into two beams by a beam splitter (Fig. 12) which are then redirected by two mirrors Mi and M2 so that they are collinear and completely overlap in the dye cell DC which is situated in the center between the two mirrors. In the selection of the dye, it need only be remembered that it should, as far as possible, be free from absorption at the laser wavelength, have a strong absorption band at half the wavelength, and have the highest possible fluorescence quantum yield 97. Thus a... 

Fig. 12. Experimental arrangement for the measurement of pulse widths of ultrashort pulses by the two-photon fluorescence method. B, beam splitter. Mi, 2, mirrors, DC, dye cell containing fluorescing dye solution that absorbs at half the laser wavelength, C, camera...

Pump-probe patterns are obtained by exciting the molecule of interest to an excited state, and subtracting the diffraction pattern with the pump-laser off from a pattern with the pump-laser on. The ultrashort pulsed nature of the laser and the electron pulses allows us to probe structural dynamics by measuring the pump-probe diffraction patterns as a function of the delay time between the laser pulse and the electron pulse. 

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