Laser fluctuating

Even in the nominal absence of laser fluctuations or other imagedegrading aberrations, the number of photons that hit the detector during the data collection period of the image (i.e., the exposure time for a CCD image or the pixel dwell time for a confocal image) will contain considerable noise. The photon count x follows a Poisson distribution (Fig. 7.7A) with mean value fi as... 

Note that in this equation the laser energy is normalized with the reference signal (r), thus canceling eventual laser fluctuations between measurements for the blank and first data point measurements. 

All TOF spectra are recorded with a 500-MHz digital oscilloscope (LeCroy) and averaged over several thousand laser shots. Boxcar averagers (Stanford Research) are used to integrate the signal of individual ion mass or electron eneigy peaks in the pump-probe experiments. The data are later corrected for laser fluctuations that are monitored by a photodiode. 

The OHD-RIKES signal is quadratic in the laser intensity, so any laser fluctuations are magnified in the data. To compensate for such fluctuations, the probe beam is sent into a doubling crystal after the sample, and the resultant second-harmonic signal is detected by a separate lock-in amplifier. Second-harmonic generation has the same quadratic intensity dependence as the OHD-RIKES signal, so dividing the RIKES data by the second-harmonic data acts to normalize any intensity fluctuations. 

Fig. 5.111 GFP solution excited by Ti Sapphire laser. Fluctuation of the photon counts in 1 ms intervals over 1 s of the data stream...

Using the model of Eq. (15) for phofoionization, Greenland and Lane [115] studied the effect of laser flucfuafions (i.e., fluctuations in discrete to continuum matrix elements). They obtained for the decay rate the usual result, InW-, averaged over the bandwidth and also argued, based on arguments similar to those in Ref. [13], fhat laser fluctuations eliminate the post-exponential region unless the transition is fast on the timescale of the fluctuations. 

In the regime where there is considerable temporal excitation pulse overlap we obtain the results shown in Fig. 2. For three different offset delays, stepped displacements of M generated delays of several tens of picoseconds around 0, 230, and -300 picoseconds. The three traces are essentially the same any differences can be ascribed to laser fluctuations and other instabilities. The common important feature is the 1.9 psec modulation corresponding to the inverse of the 3P fine structure splitting. These results... 

The third-order nonlinear susceptibility of the polymerized nanocomposite thin films was evaluated by a CA Z-scan technique [22], Figure 22.22 shows the Z-scan results of high-quality PbS QDs uniformly dispersed in an organic-inorganic hybrid photosensitive polymer [24]. Figure 22.22a presents the setup used for the Z-scan experiment A femtosecond laser beam operating at 800 nm, with a pulse width of 100 fs and a repetition rate of 1 kHz, was used to excite the sample. The signal was collected by detector 1 (Dl) and the reference beam was collected by detector 2 (D2), to eliminate laser fluctuations. The sample was... 

As far as the quantitative evaluation of vibrational spectra is concerned, IR and NIR spectroscopy follow Beer s law, whereas the Raman intensity JRaman is directly proportional to the concentration of the compound to be determined (Figure i),i iS Si To compensate laser fluctuations, in many cases, quantitative Raman spectroscopy is performed with an internal reference signal in the vicinity of the analytical band. For Raman and IR spectroscopy, quantitative analysis can be performed by either univariate evaluation of band heights/ areas or multivariate evaluation (e.g., partial least-squares (PLS) regression) of large spectral regions. Due to the overlap of many absorption bands for the quantitative analysis of NIR spectra, predominantly multivariate chemometric procedures are applied. For an in-depth study of the precautions, pitfalls, and limitations, which have to be observed or may be encountered in the measurement of vibrational spectra, the reader is referred to the pertinent literature. " ... 

---