Picosecond Dye Lasers

The presence of the AO modulator within the laser cavity causes a loss of energy at times other than the round-trip time for the photons and the nulls In the AO crystal. If the cavity length matches the AO crystal frequency, then the photons accumulate in a single bunch and bounce back and forth together within the laser cavity. This is the mode-locked condition. 

A valuable aspect of cavity dumping is that it does not typically decrease the average power from the dye laser, at least within the 1-4 MHz range typical of TCSPC. To be specific, if the 80-MHz output of the dye laso is 100 mW, the output at 4 MHz will also be close to 100 mW. When optical power is not bdng dumped from the dye laser, the power builds up within the cavity. The individual cavity-dumped pulses become more intense, which turns out to be valuable for frequeiu -doubling the ou t of the tfye laso  

A convenient feature of dye lasers is the tunable wavelength. The range of useful wavdengths is typically near the emisrion maximum of the laser cfye. IXining curves of Epical dyes are shown in Hguie 4.11. Most of these dye 

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Figure 3.6-10 Schematic diagram of a femtosecond time-resolved CARS apparatus. YAG, cw mode-locked Nd YAG laser ML, mode locker PL, polarizer A s, apertures LP, laser pot DM, dichroic mirror DLl, femtosecond dye laser SA, saturable absorber CLFB, cavity-length feedback system DL2, picosecond dye laser W, tuning wedge E, etalon FD, fixed delay VD, variable delay BS, beam splitter P s, half-wave plates (when necessary) F s, filters S, sample MC, monochromator PMT, cooled photomultiplier. (Okamoto and Yoshihara, 1990).

Hirata Y, Mataga N. (1990) Solvation dynamics of electrons ejected by picosecond dye laser pulse excitation of p-phenylenediamine in several alcoholic solutions. J Phys Chem 94 8503-8505. 

Mueh of our knowledge of the frequency dependence of VER rates in polyatomic molecules stems from low-temperature studies of moleeular erystals [2] such as pentacene (PTC C22Hj4) guest molecules in a crystalline naphthalene (N C,p,H ) host. In naphthalene, the phonon cut-off frequency is 180 cm [97]. At low temperature, PTC has well resolved vibronic transitions i hn a convenient wavelength range for picosecond dye lasers... 

Table 9.2. Fluorescence decay parameters of human hemoglobin subunits obtained with the time correlated single photon counting method using a sync-pumped picosecond dye laser as the excitation source.

In the future, we can expect the ratho expensive picosecond dye lasers and Ti sapphire lasexs to be t laced by sin ler and less eiq>ensive device. A diode-punq>ed Nd YAG laser has already been used fw Ume-resolved detection in capillary zone eleftto(4ioresis, and one can purchase a streak camera with apulsed lasCT diode excitation source. Laser diodes have also been used as the excitation source for FD fluwiMnctry. The wavelengths are usually limited to 600-700 nm, but some laso diodes can be frequency-doubled to 410 nm. It is also likely that... 

Diuing the past decade, the instrumentation for time-resolved fluorescence of proteins has advanced dramatically. The flashlamp light sources have been replaced by hi -repetition-rate (MHz) picosecond dye lasers, which provide both higher excitation intensities and raon rapid data acquisition. The dynode-chain PMTs have been replaced by MCP detectors, which provide much shorter single-photoelectron pulse widths than a dynode chain PMT. In con nnation. the new light sources and detectors provide instniment r ponse functions with half-widths near 100 ps, so that picosecond resolution can now be obtained. 

Picosecond dye lasers Nd YAG fundamental output at 1064nm, 355nm (third harmonic), 266nm (fourth harmonic)... 

Time-resolved fluorescence spectroscopy is also a valuable tool in biological and medical research [10.143-147]. Since the lifetimes involved are normally short, picosecond spectroscopy techniques are frequently employed (Sect. 9.4). Examples of fluorescence decay curves for tissue recorded with delayed coincidence techniques employing a frequency-doubled picosecond dye laser are depicted in Fig. 10.42. The decay characteristics allow the discrimination between tumour and normal tissue, and atherosclerotic plaque and normal vessel wall, respectively. General surveys of the use of LIF for medical diagnostics can be found in [10.148,149]. 

Fluorescence. We have also measured fluorescence produced by two-photon excitation for thick films of polysilane. For this experiment, the laser was a Spectra-Physics sub-picosecond dye laser system, focussed onto the polymer films to produce intensities of =440 MW/cm. Emission was focussed into a 0.5 m spectrometer and spectra were collected using an optical multichannel analyzer and analyzed on an IBM PC. For poly(di-n-hexylsilane), the two-photon induced emission is broadband (AXpwHM -10 nm at room temperature), with line center at =380 nm, as shown in figure 10. The emission spectrum is identical to that observed for this compound by UV excitation, and the average degree of fluorescence anisotropy (=0.2) produced at the two-photon resonance (579 nm) is quite similar to that oteerved for on-resonance UV excitations in polysilanes [26]. 

Near-resonant pump-probe lifetime measurements were performed by W. Schmid [105, 123], using the same picosecond dye laser system as for DFWM, described below. Th. Fehn further on investigated the subject in the off-resonant wavelength region between 720 nm and 820 nm, using a commercial Titan-Sapphire laser system (Coherent) with pulse lengths of ca. 120 fs, pumped by an argon ion laser (Coherent Mira). 

It can be seen from the absorption spectra that all oligothiophenes ( T) except 2T can be excited well with the third harmonic of a Nd YLF laser (349 nm). 2T was excited with the second harmonic of a sub-picosecond dye laser system at 308 nm [7]. 

Besides excitation and probing with infrared laser pulses the CARS technique (Sect.8.4) is a promising technique to study these relaxation processes. An example is the measurement of the dephasing process of the OD stretching vibration in heavy water D2O by CARS [11.111]. The pump at w = Wl is provided by an amplified 80-fs dye-laser pulse form a CPM ring dye laser. The Stokes pulse at Wg is generated by a synchronized tunable picosecond dye laser. The CARS signal at = 2wL-Wg is detected as a function of the time delay between the pump and probe pulses. 

Carter, G.M., Thakur, M.K., Chen, Y.J., Hryniewicz, J.V. (1985) "Time and wavelength resolved nonlinear optical spectroscopy of a polydiacetylene in the solid state using picosecond dye laser pulses.", Appl. Phys. Lett. 47, 457-459. 

Huppert, D. Rentzepis, P. M. A high-efficiency funable picosecond dye laser. J. Appl. Phys. 1978, 49,543-548. 

Sundstrom, V. and Gillbro, T., Transient absorption spectra of pinacyanol and cyanine photoisomers obtained with a sync-pumped picosecond dye laser and independently tunable probe light, Chem. Phys. Lett., 94, 580,1983. 

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