White-light pulses

Concurrent with the development of suitable photodiode arrays, techniques were being developed for generating dispersion in both the time and wavelength domains. In 1970 it was observed (21) that focusing intense picosecond pulses into some solids and liquids produced superbroadened continua with band-widths of >10 cm l. It was also shown (22) that these "white light" pulses had durations on the order of the picosecond pulses which produced them. These continuum pulses have proven to be very useful for time-resolved spectroscopy. 

These high intensity femtosecond pulses are fundamental for spectroscopic purpose in that they can generate white light pulses of comparable duration. This can be done by focussing the amplified 620 nm pulse of 100 fs duration into many different transparent liquids such as water, alcohols, ethylene glycol... 

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... 

Fig. 5.9. (a) Spectra of white light obtained upon irradiation of BaF2 as a function of the position of physical focusing within the crystal. 0.3 cm indicates that the focus lies fairly close to the entrance face of the crystal. Note how the extent and flatness of the spectrum depends on focus position, (b) Corresponding variation in spectral extent and shape obtained with incident pulses of 45 and 300 fe duration... 

There are numerous applications of white light generation and filamentation that span across disciplines, such as time-resolved broadband spectroscopy, generation of few cycle pulses [37], LIDAR [31], material modification [2], and... 

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. 

Fig. 3. Pump-deplete-probe spectroscopy on lycopene in hexane, a) Experimental setup After excitation and depletion of Car S2 with a delay of r=50fs, a white-light probe pulse at delay tprob<.=2ps measures the transient absorption spectrum, b) Spectra without (solid curve) and with depletion pulse (dotted) and their difference (shaded area). Only the Car Si state is depleted the ground state bleach (S0-S2) and positive absorption feature on its low energy side (hotSo-S2) are unaffected.

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. 

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