White light continuum

Fork R L, Shank C V, Hirlimann C, Yen R and Tomlinson W J 1983 Femtosecond white-light continuum pulses Opt. Lett. 8 1-3... 

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]. 

Balu M, Hales J, Hagan DJ, Van Stryland EW (2005) Dispersion of nonlinear refraction and two-photon absorption using a white-light continuum Z-scan. Opt Express 13 3594-3599... 

Very recently, white light continuum pulses of duration ca. 200 fsec, pulse energy ca. 1 / J, and peak wavelength of ca. 780 nm have been generated at repetition rates up to 250 kHz by commercially available Ti sapphire regenerative amplified laser systems. Such systems are very expensive, but the expected easier use, as compared with homemade systems, should open up new research applications for time-resolved fluorescence and absorption techniques in the near-IR. 

Petrov, G. L, and Yakovlev, V. V. 2005. Enhancing red-shifted white-light continuum generation in optical fibers for apphcations in nonlinear Raman microscopy. Opt Express 13 1299-1306. 

Zeidler, D., Witte, T., Proch, D., and Motzkus, M. 2001b. Optical parametric amplification of a shaped white-light continuum. Opt. Lett. 26(23) 1921-23. 

Comprehensive experimental investigations of 2PA processes in fluorene derivatives were performed by Hales et al. [53,56-59] with open aperture Z-scan [26], two-photon induced fluorescence [60] and femtosecond white-light continuum pump-probe methods [61]. For degenerate two-photon excitation, the experimental 2PA spectra of symmetrical and asymmetrical fiuorenes are presented in Figs. 15 and 16. These spectra were obtained with the combination of open aperture Z-scan and two-photon fluorescence methods [57]. For centrosymmetric molecules, two-photon transitions from... 

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. 

Transient absorption and gain spectroscopy was performed by the pump-probe technique using a subpicosecond set-up described in detail elsewhere [10-12]. For the present experiments, the pump was set at 355 nm (pCA2- and pCM ) or 430 nm (pCT ) and the probe was a white-light continuum produced by focusing 570 nm pulses in a 1-cm water cell. The... 

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. 

Fig. 7.2 Two-beam experimental setup for femtosecond transient absorption studies using a white light continuum. A commercially available CPA 2101 laser system delivers the pulses. Ultrashort tunable visible pulses are obtained by the NOPA optical parametric converter. A chopper wheel is used to cut every second pump pulse in order to compare the signal with and without the pump. The white light continuum is generated by a sapphire disc. The time delay between the pump and probe pulses is adjusted by the optical delay rail...

Fig. 7.3 Experimental setup for the nanosecond laser Flash Photolysis with a white light continuum. A Brilland-Quantel Nd YAG laser delivers the fundamental pulses (355 and 532 nm). A pulsed XBO lamp is used as white light source. The laser signal is split in order to trigger the digital storage oscilloscope (DSO) utilizing a second photodiode (PD). Two separate detection units in different geometries—photomultiplier (PMT) in front face and a PD in side face—detect the signal in the UV/vis and NIR region, respectively. The monochromator is operated by a standard PC...

Recent Experimental Approaches Our most recent attempt to investigate the problem of ground-state heterogeneity experimentally uses tunable pump laser pulses derived from a home-made optical parametric amplifier (Fig. 1.9). A white light continuum is used to seed a two-stage optical parametric amplifier pumped by the second harmonic of a regeneratively amplified Ti-sapphire laser... 

A. Brodeur, S. L. Chin, Ultrafast white-light continuum generation and self-focusing in transparent condensed media, Journal of the Optical Society of America B 16, 637 (1999)... 

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