Pulse shape absorption

The foregoing equation can be used to give the evolution of (E). For N2, exponential decay of (E) was seen, indicating efficient thermalization but this was not observed for He. For highly efficient moderators such as ethane, the absorption signal essentially follows the pulse shape. 

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. 

As described in the section on nonlinear absorption, the transmission of a pulse which is short compared to the various molecular relaxation times is determined by its energy content. A measurement of the energy transmission ratio will then give the peak intensity of the pulse when its pulse shape is known 44>. In fact, the temporal and spatial pulse shape is of relatively little importance. Fig. 11 gives the energy transmission as a function of the peak intensity I [W/cm2] for the saturable dye Kodak 9860 with the pulse halfwidth as a parameter. It is seen that this method is useful in the intensity region between 10 and 1010 MW/cm2 for pulses with halfwidths greater than 5 to 10 psec. Since one can easily manipulate the cross-section and hence the intensity of a laser beam with a telescope, this method is almost universally applicable. 

To model the experimental data we used a global-fit procedure to simulate EPS, integrated TG, heterodyne-detected TG, and the linear absorption spectrum simultaneously. The pulse shape and phase were explicitly taken into account, which is of paramount importance for the adequate description of the experimental data. We applied a stochastic modulation model with a bi-exponential frequency fluctuation correlation function of the following form ... 

There are now direct experimental confirmations [41-43], to be discussed in Section 13.3, of the effect of the pulse shaping. In addition, the role of the phase j-1 between two pulses, predicted in both OCT [104,119] and in CC studies [94,96,1R>J. has been confirmed experimentally by Fleming et al. [137,138], Girard et al. [JO- j 142], Kinrot et al, [143], and Warmuth et al. [144] in the so-called wave paiket interferometry experiments [145], For example, Fleming et al, [137,138] [and Warmuth et al. [144] describe experiments where the fluorescence from I2 in the F t state is influenced by constructive or destructive interference between two wave pad.-, j ets induced by two-phase related excitation pulses. This study relates to a large voliuiiejy of work on wave packet interferometry in atoms [146-148], as well as to various." femtochemistry experiments, where similar effects were seen in absorption [106] . jj... 

The factor k depends on the two-photon absorption cross section of the dye at the laser wavelength, on the pulse shape, and on the spatial energy distribution in the focus. Because the effective excitation depends on the reciprocal pulse width, not on its square, the prospects are not too bad for detecting diode-laser-excited two-photon fluorescence. It has in fact been demonstrated that two-photon excitation can be achieved even with CW lasers [228]. An experiment to demonstrate two-photon excitation by diode lasers is shown in Fig. 5.134. 

An aqueous sample may be added to the cocktail directly, after minor prior processing, or at the end of a radiochemical separation procedure. Direct addition is the equivalent of gross activity counting discussed in Section 7.2.4 except that some spectral analysis may be possible. Alpha particles can be differentiated from beta particles by deposited energy, pulse shape, and decay time. Self-absorption is of no concern. Quenching and luminescence, discussed in Section 8.3.2, often occur. Identification by maximum beta-particle energy is approximate, and requires comparison to radionuclide standards. 

To set up the lifetime experiment, fluorescence excitation spectra were recorded using pulses of 90 ns duration corresponding to a spectral bandwidth of 15 MHz at a repetition rate of 1 MHz. The bandwidth was limited by the pulses rise and fall times, the pulse shapes and the frequency jitter of the laser. Fig. 8(a) shows a typical spectrum with four individual molecular resonances A, B, C and D. On average, the lines are about 25 MHz wide as compared to a homogeneous linewidth of about 8 MHz measured by earlier experiments using cw radiation and lower excitation energies. The best fit of the absorption profile of molecule C was obtained using a Lorentzian profile with a FWHM of 27 MHz. 

In a dispersive wave propagation system, a change in pulse shape occurs as an ultrasonic wave travels in a structure that is independent of any absorption of ultrasonic energy. In... 

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