Laser mode-locked picosecond

Argon ion lasers, mode-locked to produce pulses in the picosecond domain, are in widespread use, producing power levels generally higher than diode lasers up to ca. 

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

Although 0-switching produces shortened pulses, typically 10-200 ns long, if we require pulses in the picosecond (10 s) or femtosecond (10 s) range the technique of mode locking may be used. This technique is applicable only to multimode operation of a laser and involves exciting many axial cavity modes but with the correct amplitude and phase relationship. The amplitudes and phases of the various modes are normally quite random. 

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.

The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. This laser system produces tunable light pulses, each pulse with a time duration of about 10 picoseconds, and with pulse repetition rates up to 80 million laser pulses/second. The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube (or in some cases a streak camera (h.)) ... 

By the late 1960s the development of mode locking (Chapter 1) allowed the study of picosecond laser techniques. Excited-state processes carried out in the picosecond domain allow such processes as intersystem crossing, energy transfer, electron transfer and many pho-toinduced unimolecular reactions to be investigated. 

Compact and stable devices are available that take advantage of the improved quality of the crystal lasers, as well as increased pump efficiencies. Hundreds of different models of Nd + based lasers have demonstrated laser action (Kaminskii, 1981). It is possible to operate these Nd + solid state lasers in the continuous regime, with output powers ranging from 1 W to 1000 W. Pulsed operation is also possible, with a pulse length from the picosecond range, via mode-locking, to tens of nanoseconds by Q-switch operation. 

An interesting method for generation of a broad wavelength continuum with a time duration of some picoseconds has been deseribed by Busch et al. I61e) By focussing the intense mode locked laser beam from a frequency-doubled neodynium laser into various liquids (H2O, P2O, etc.) a light continuum can be generated which spans several thousand wave numbers and yet has a picosecond pulse duration. This enables absorption spectroscopy measurements to be made in the picosecond time scale. 

The experimental setup is shown in Figure 5.1. Six picosecond (ps)-long pulses at 532 nm and 80 MHz repetition rate were delivered by a frequency-doubled, passively mode-locked NdYV04 laser (Hi-Q Laser Production, Austria). The maximum available average power of the laser was reduced by an external variable attenuator to about a few hundred milliwatts. The OPO gain material is a flux-grown KT10P04 crystal,... 

Figure 10.10 shows the experimental system of TE-CARS microscopy (Ichimura et al. 2004a). As similar to the TERS system (Hayazawa et al. 2000), the system mainly consists of an excitation laser, an inverted microscope, an AFM using a silver-coated probe, and a monochromator. Two mode-locked Ti sapphire lasers (pulse duration 5 picoseconds [ps] spectral band width 4 cm- repetition rate 80 MHz) are used for the excitation of CARS. The (o and (O2 beams are collinearly combined in time and space, and introduced into the microscope with an oil-immersion objective lens (NA = 1.4) focused onto the sample surface. As the z-polarized component of the... 

The rotational relaxation times of these nitrocompounds have not been measured. Comparison with the studies of perylene by Klein and Haar [253] suggests that most of these nitrocompounds have rotational times 10—20 ps in cyclohexane. For rotational effects to modify chemical reaction rates, significant reaction must occur during 10ps. This requires that electron oxidant separations should be <(6 x 10-7x 10-11)J/2 2 nm. Admittedly, with the electron—dipole interaction, both the rotational relaxation and translational diffusion will be enhanced, but to approximately comparable degrees. If electrons and oxidant have to be separated by < 2 nm, this requires a concentration of > 0.1 mol dm-3 of the nitrocompound. With rate coefficients 5 x 1012 dm3 mol-1 s 1, this implies solvated electron decay times of a few picoseconds. Certainly, rotational effects could be important on chemical reaction rates, but extremely fast resolution would be required and only mode-locked lasers currently provide < 10 ps resolution. Alternatively, careful selection of a much more viscous solvent could enable reactions to show both translational and rotational diffusion sufficiently to allow the use of more conventional techniques. 

Laser III A picosecond mode-locked and cavity-dumped dye laser (Spectra-Physics, 375B and 344S) synchronously pumped using a cw mode-locked argon ion laser (Spectra-Physics, 2030-18), generating tunable (530-830 nm) pulses in 4-MHz repetition rate and 10-ps fwhm. 

It is also conceivable that one laser pulse could be used as both the photolysis and the probe laser. By studying the RR spectrum as a function of power, the Raman spectrum of the species formed during the pulse duration and enhanced at the laser frequency used for photolysis will be observed. It is conceivable that the amplified picosecond pulses produced from the mode locked-cavity dumped techniques (16) could be used to determine the RR spectra of species formed in the pico- and hopefully in the sub-picosecond time scale. 

The barriers to this approach have been technical in nature. Mode-locked Nd glass lasers remain a common light source for picosecond spectroscopic studies, but they suffer from poor reproducibility and very low repetition rates. These features combine to make wavelength scanning techniques unsuitable with such lasers. The alternative approach is to employ multichannel optical detection and thereby obtain full spectral coverage with each laser shot. It is also necessary to eliminate the effects of shot-to-shot variations of the laser output. 

Figure 14. Experimental apparatus for picosecond, time-resolved CD measurements using a mode-locked, Q-switched, cavity dumped pump laser. P, polarizer PC, Pockels cell Q, quarter-wave plate RHP, rotating half-wave plate S, sample cell PMT, photomultiplier tube. From ref. [42].

Picosecond pulses can be produced in a number of different types of laser systems. As an example, a brief description is first given of a synchronously pumped c.w. dye laser such as can be readily assembled from commercially available units. Generation of repetitive subnanosecond pulses in a c.w. laser by mode-locked synchronous pumping was first described by Harris et al. [12]. The essential features of such a system are shown in Fig. 3. In this system, an acousto-optically mode-locked ion laser is used to pump the dye laser. In order to achieve synchronous pumping, the length of the dye cavity must be adjusted so that the dye laser intermode spacing is an integral multiple of the pump mode-locker frequency. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

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