Difference-frequency generation infrared pulses

A typical experimental set-up consists of a pulse Nd YAG laser which is used to pump an optical parametric/difference frequency generation system to produce tunable infrared radiation with a wavelength up to 9 pm. The infrared beam is then overlapped with a frequency-doubled output beam from the laser (532 nm) at the sample surface. A photo-multiplier is used for detection. Sum frequency vibrational spectra are generated by scanning the IR frequencies through the energy range of vibrational excitations. 

A broader application range is opened by a system of two independently tunable mode-locked dye lasers, which have to be pumped by the same pump laser in order to synchronize the pump and probe pulses [808]. For studies of vibrational levels in the electronic ground states of molecules the difference frequency generation of these two dye lasers can be used as a tunable infrared source for direct excitation of selected levels on infrared-active transitions. Raman-active vibrations can be excited by spontaneous or stimulated Raman transitions (Chap. 3). Another useful short-pulse source for these experiments is a three-wavelength Ti sapphire laser, where two of the wavelengths can be indepently tuned [811]. 

The SNR of the spectra shown in Figure 12.13 is quite low as a result of the low power of the thermal source that was used. Clearly what is needed is a source with the power of an intense laser and the wavelength coverage of a thermal source. A reasonable compromise was reported by Huth et al. who demonstrated the use of a coherent broadband mid-infrared beam that was generated by a difference frequency generator (DFG), where two near-infrared 200 fs pulse trains from an erbium fiber laser system are superimposed in a GaSe crystal [30]. The laser emits two pulse trains, one at 1.55 pm and the other that is broadened and red-shifted. 

The transient infrared absorption spectra of molecules in excited electronic states were measured by Elsaesser and Kaiser [18]. The probe pulses used were (i) picosecond infrared pulses in the wavenumber region of 3500-3000 cm generated by an OPA in LiNb03 excited by the Nd YAG laser line, and (ii) picosecond infrared pulses in the region of 3000-1400 cm i obtained by the difference-frequency generation in AgGaS2 between the Nd YAG laser line and near-infrared dye-laser lines excited by the same Nd YAG laser line. 

Snee et al. [26] obtained broadband femtosecond infrared pulses (FWHM about 200 cm ) by the difference-frequency generation in LiI03 between the fundamental... 

Pulsed difference frequency generation from the outputs of a ruby laser and a dye laser mixed in LiNbO has achieved 6 KW of infrared power tunable between 3.1 and 4.5 ym [7.89], Spectral narrowing of the dye laser output reduces the bandwidth to less than 1 cm , and peak infrared powers of several hundred watts with repetition rates up to 30 s have been generated with a long-term frequency stability of better than 1 GHz [7.90]. 

Infrared pulses of 200 fs duration with 150 of bandwidth centred at 2000 were used in this study. They were generated in a two-step procedure [46]. First, a p-BaB204 (BBO) OPO was used to convert the 800 mn photons from the Ti sapphire amplifier system into signal and idler beams at 1379 and 1905 mn, respectively. These two pulses were sent tlirough a difference frequency crystal (AgGaS2) to yield pulses... 

The experiments are performed with a modelocked Nd YLF laser system of 10 Hz repetition rate. Tunable infrared pulses are generated by difference frequency mixing between... 

Infrared pulses at frequency Vp, are generated as the difference frequency between two ultrashort pulses at frequencies vj and V2 (i.e., vp, = vj - vj vj > V2>. Examples of nonlinear crystals used for the generation of a difference frequency are LiI03 and AgGaS2. 

Fig. 15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used at Nottingham. The UV pulse laser generates transient species the continuous IR laser monitors the change in transmission at a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated at different IR frequencies so that a complete IR spectrum of the transient can be built up [reproduced with permission from (97), p. 103],...

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