Interferometric autocorrelation

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Figure 8.1c shows the interferometric autocorrelation signal of second harmonic generation (SHG) from a BBO crystal positioned at the sample plane of the microscope. The shape of the SHG trace was symmetrical with respect to the time origin the ratio of the maxima to the background was 8 1, indicating that nearly ideal... 

Figure 8.1 (a) Block diagram of the femtosecond near-infrared laser microscope system, (b) Spectrum ofthe light pulse from the Cr F laser, (c) Interferometric autocorrelation trace of SHG signal with envelope curve calculated assuming a chirp-free Gaussian pulse with 35 fs fwhm. 

Figure 8.3 Interferometric autocorrelation traces of the fluorescence intensities of perylene (a) and anthracene (b) microcrystals irradiated by two NIR Cr F laser pulses centered at 1.26 Xm with the same intensity.

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

The Degree Angular Scale Interferometer (DASI) is a very small interferometric array that operates at 26-36 GHz and the South Pole. After measuring the angular power spectrum of the anisotropy (Halverson et al., 2002) the instrument was converted into a polarization sensitive interferometer which detected the E mode polarization at 5.5a by looking at a small patch of sky for most of a year of integration time (Kovac et ah, 2002). The level agreed well with the solid predictions for adiabatic primordial perturbations. Since the measured quantity was the EE autocorrelation, the 5.5a corresponds to a 9% accuracy in the polarization amplitude. 

Figure 13.5 Interferometric autocorrelation traces of uncompressed and compressed pulses. 1 (a) Uncompressed 80-fs pulses obtained directly from Ti sapphire laser (power spectrum is Shown in the inset), (b) Compressed pulses after 1000 iterations. Pulses were compressed to (Taken from Fig. 2, Ref. [42].)...

For the characterization of laser pulses, there exist several approaches. Up to the picosecond range, photodiodes can be applied to measure the pulse duration. For femtosecond pulses, interferometric autocorrelation techniques are applied (Demtroder 2007). 

There are two different techniques that are used to measure the time profiles and optical oscillations of ultrashort pulses noncoUinear intensity correlation and interferometric autocorrelation. While the former measures the envelope of the pulse, the latter can even measure the optical oscillations within the pulse envelope. Combined with the spectral resolution, the time profiles of the different spectral components within the optical pulse spectrum can be simultaneously measured by the FROG technique. The relative phases of these spectral components are observable using the SPIDER technique (see Sect. 6.2.4). 

In interferometric autocorrelation the coherent superposition of the two collinear partial beams is realized. The basic principle is shown in Fig. 6.65. The incoming laser pulse is split by the beamsplitter BSl into two parts, which travel through two different pathlengths and are then collinearly superimposed at BS2. When they are focused by the lens L into a nonlinear optical crystal, the output signal (6.39) is generated at 2co. Instead of the delay line arrangement in Fig. 6.65 a Michelson interferometer in Fig. 6.70 can also be used. The second harmonics are detected by a photomultiplier, while the fundamental wavelength is rejected by a filter. 

Fig. 6.71 Interferometric autocorrelation trace of a 7.5 fs pulse with upper and lower envelopes [759]...

In interferometric autocorrelation, averaging is not complete (unlike in intensity correlation), and the phases of the electric fields have to be taken into account. Here all of the terms A1-A4 in (6.40) can contribute to the signal. If a filter which rejects the fundamental frequency o) and transmits only the doubled frequency 2cp is inserted behind the frequency-doubling crystal, the third term in (6.40) with A3 is suppressed. 

For illustration. Fig. 6.72 shows a 12 fs pulse measured with intensity correlation (a) and interferometric autocorrelation (b). 

Fig. 6.72 A femtosecond pulse with time duration AT = 12.2 fs (a) measured with intensity correlation (b) measured with interferometric autocorrelation [771]...

In Fig. 6.74 the power density spectrum and the interferometric autocorrelation signal of a femtosecond laser pulse is compared. 

With interferometric autocorrelation the chirp of a pulse and the resulting change in its time profile can be determined. This is illustrated by the example of a chirped pulse with a Gaussian profile... 

Fig. 6.74 Femtosecond laser pulse, (a) Optical power spectrum (b) interferometric autocorrelation of the same pulse [828]...

Fig. 6.75 (a) Interferometric autocorrelation of a pulse with AT = 10 fs and a chirp of a = 2 dashed line). The solid line shows the pulse profile obtained by intensity correlation, (b) Upper and lower envelopes of a chirped Gaussian pulse [675] for various chirp parameters a [775]... 

There are two different techniques of measuring the time profiles of ultrashort laser pulses the interferometric autocorrelation and the noncollinear intensity correlation. 

Fig. 11.48. Interferometric autocorrelation of a pulse with AT = lOfs and a chirp of 6 = 2...

Fig. 2.8. Interferometric autocorrelation trace of the femtosecond titanium sapphire laser (taken from [203]). The pulse width is 70 fs. The trace was recorded with a step width of 1 fs...

Fig. 2.23. Interferometric autocorrelation trace directly measured at the molecular beam. The 3PI signal of Ks is detected while exciting with 90 fs laser pulses at A = 800 nm. The inset nicely depicts the fast 2.67 fs oscillation (taken from [260])...

For several excitation wavelengths the femtosecond real-time dynamics of the trimer has been observed. In Fig. 3.53, a representative time evolution of the 3PI signal for excitation of Ka with 70 fs laser pulses at a central wavelength of 798 nm is shown. In the center, which marks the zero-of-time, the interferometric autocorrelation peak is clearly observable (see also Fig. 2.23). The transient ion signal is symmetric with respect to this peak as is expected for a one-color 3PI experiment. A fast decay is clearly seen and is superimposed on a 450 fs oscillation. Two processes are involved, wave packet propagation on the PES and ultrafast dissociation. Hence, in the following discussion the ion signals are analyzed in two steps. First the ultrafast decay is discussed, followed by an analysis of the superimposed coherent oscillation. 

Fig. 3.53. Real-time evolution of the 3P1 signal for Ks excited at A = 798 nm (taken from [260]). The symmetric shape is due to the applied pump and probe pulses of similar wavelength. Therefore, at the zero of time the pulses interchange their roles. Besides the ultrafast decay a superimposed oscillation with T = 450 fs is visible. Around the zero of time the interferometric autocorrelation as shown in detail in Fig. 2.23 is seen...

The one-color experiment was performed with pump and probe pulses of the same photon energy (2.00eV). In this case a synchronously pumped femtosecond optical parametric oscillator was used (see Sect. 2.1.1). At A = 1.3 im the signal wave s maximum output reached more than 400 mW, corresponding to 20% conversion efficiency. The signal wave was frequency-doubled by a BBO crystal (60 mW). By measuring an interferometric autocorrelation... 

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