Measurement continuum generation

Figure 3.21. Experimental setup for degenerate TPA spectral measurement by using a single intense continuum-generation beam. (From Ref. [375] with permission of the American Chemical Society.)...

Differential absorption spectra (DAS) under excitation of high-energy transitions are studied. The pump is done by 15-ps pulses from passively mode-locked Nd YA103 laser (/l=1.08 pm). White-light continuum generated from D2O by a part of 15-ps laser pulse is used as a probe. Fig. 4 demonstrates DAS registered for sample p8 (see Fig. 1). The dots present values of differential absorption obtained from intensity-dependent transmission measurements for this glass at 1.08 pm and 1.54 pm. 

The power spectrum can be recorded only as a function of the absolute value of Aft). Since the cosine function is symmetric, the intensities at coq + Acu and coq — Aco are indistinguishable from each other and both are summed in the detected spectrum at the same difference frequency, Aco, generating a double side band spectrum. This folding of the spectrum with respect to the laser frequency, coq, (LO in Fig. 5.9.1) causes an ambiguity in the position of detected spectral lines, and care must be taken to determine whether a line belongs to the upper or lower side band. Also, the spectrum will usually have a continuum, which changes only gradually and which is nearly identical in both side bands. Because both side bands are added in the heterodyne spectrum, the measured continuum level is twice the true level. 

For 2PA or ESA spectral measurements, it is necessary to use tunable laser sources where optical parametric oscillators/amplifiers (OPOs/OPAs) are extensively used for nonlinear optical measurements. An alternative approach, which overcomes the need of expensive and misalignment prone OPO/OPA sources, is the use of an intense femtosecond white-light continuum (WLC) for Z-scan measurements [71,72]. Balu et al. have developed the WLC Z-scan technique by generating a strong WLC in krypton gas, allowing for a rapid characterization of the nonlinear absorption and refraction spectra in the range of 400-800 nm [72]. 

An interesting method for generation of a broad wavelength continuum with a time duration of some picoseconds has been deseribed by Busch et al. I61e) By focussing the intense mode locked laser beam from a frequency-doubled neodynium laser into various liquids (H2O, P2O, etc.) a light continuum can be generated which spans several thousand wave numbers and yet has a picosecond pulse duration. This enables absorption spectroscopy measurements to be made in the picosecond time scale. 

In this method, the respective positions of about 100 ions (inner zone) are calculated under various assumptions for the inter-ionic potentials, assuming the rest of the lattice as a quasi-continuum. A polarization per unit cell is attributed from measured values of the static dielectric constant. The configuration within the iimer zone (which may generate a cluster ) is varied until the total energy of the lattice is minimized ... 

Transient spectra of solvated indole are measured in a 120 Jim liquid jet with a crosscorrelation of 80 fs by means of a white light continuum (450 - 740 nm) generated in a sapphire disc. The molecules are excited at 270 nm with pump pulses generated by frequency doubling the output of a noncollinearly phase matched optical parametric amplifier [2], Due to the short pump pulses there is a small yet finite probability for two-photon ionization in pure solvents. This allows us to study the spectral properties of the generated solvated electrons by measurements in pure solvents. The transient spectra of the indole solution are corrected for these solvent contributions. 

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

Because the picosecond continuum is generated through a highly nonlinear process, its detailed spatial, spectral, and intensity characteristics vary from shot to shot more severely than do the laser pulses used to generate it. In order to achieve a high degree of reliability in our spectral measurements, it is therefore necessary to obtain double beam spectra in which the data are corrected for continuum fluctuations for every shot. 

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