Measurement white-light continuum

Yamaguchl S and Hamaguchl H 1995 Convenient method of measuring the chirp structure of femtosecond white-light continuum pulses App/. Spectrosc. 49 1513-15... 

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]. 

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. 

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. 

Two-photon absorption spectra (650-1000 nm) of a series of stilbene chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white light continuum... 

Figure 4.4 Transient spectra after optical excitation of HBT and HAN in cyclohexane measured with a white-light continuum for a pump-probe delay of 2 and 20 ps. (Adapted from [3].)...

Method indicates the experimental technique which is used to measure the TPA cross section (NLT, nonlinear transmission WLC, white light continuum). 

---