White light continuum generation

If an intense ultrashort light pulse at a given wavelength (or frequency) is focused in a liquid or solid medium (water, glass, etc.), its spectrum broadens considerably while propagating in the medium and the output beam appears as a white light beam. This is mainly due to self phase modulation [36], a non linear process that 

In the leading edge of the pulse (positive derivative of the intensity), the new frequencies are lower than coq they are higher in the trailing edge. This spectral broadening provides a polychromatic light source of short duration, called continuum, which is commonly used in pump-probe experiments on the femto-picosecond time scale. 

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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. 

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)... 

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. 

R. Huber, H. Satzger, W. Zinth, J. Wachtveitl, Noncolhnear optical parametric amplifiers with output parameters improved by the application of a white light continuum generated in CaF2. Opt. Commun. 194, 443 (2001) ... 

Figure 11.3 Schematic representation of the experimental setup. The following notations have been used BS, beam splitter of the fundamental 775 nm beam shutter 1 and shutter 2, two motorized shutters that block the probe and pump beams, respectively Ml, M2, M3,and M4, high reflection dielectric mirrors at 775 nm, 45° X,/2, aXjl plate Cap2, calcium fluoride plate, used for white light continuum generation M5,...

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]. 

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. 

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. 

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...

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