The dye lasers

A later section of this review is concerned with experimental techniques for the measurement of relaxation processes. There are many studies of these processes in the literature and, in the larger proportion of the recent studies, pulses from tunable dye lasers have been used for the initial excitation. Therefore, a brief description of some pulsed dye lasers is given below. 

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Figure B2.3.9. Schematic diagram of an apparatus for laser fluorescence detection of reaction products. The dye laser is syncln-onized to fire a short delay after the excimer laser pulse, which is used to generate one of the reagents photolytically.

Historically, the first type of laser to be tunable over an appreciable wavelength range was the dye laser, to be described in Section 9.2.10. The alexandrite laser (Section 9.2.1), a tunable solid state laser, was first demonstrated in 1978 and then, in 1982, the titanium-sapphire laser. This is also a solid state laser but tunable over a larger wavelength range, 670-1100 nm, than the alexandrite laser, which has a range of 720-800 nm. 

A pulsed dye laser may be pumped with a flashlamp surrounding the cell through which the dye is flowing. With this method of excitation pulses from the dye laser about 1 ps long and with an energy of the order of 100 mJ can be obtained. Repetition rates are typically low - up to about 30 FIz. 

In order to observe such high-resolution fluorescence excifafion spectra, the laser must have a very small line width. To achieve this a ring dye laser, a modification of the dye laser described in Section 9.2.10, is used a line width as small as 0.5 MFIz (1.5 x 10 cm ) can be obtained. 

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

LLNL AVLIS Laser. The first WFS measurements using a Na LGS were performed at LLNL (Max et al., 1994 Avicola et al., 1994). These experiments utilized an 1100 W dye laser, developed for atomic vapor laser isotope separation (AVLIS). The wavefront was better than 0.03 wave rms. The dye laser was pumped by 1500 W copper vapor lasers. They are not well suited as a pump for LGSs because of their 26 kHz pulse rate and 32 ns pulse length. The peak intensity at the Na layer, with an atmospheric transmission of 0.6 and a spot diameter of 2.0 m, is 25 W/cm, 4x the saturation. The laser linewidth and shape were tailored to match the D2 line. The power was varied from 7 to 1100 W on Na layer to study saturation. The spot size was measured to be 7 arcsec FWHM at 1100 W. It reduced to 4.6 arcsec after accounting for satura-... 

Subsequent to the advent of the dye laser, tunable lasers based upon other lasing media were developed which operate over various wavelength ranges. Nobable among these are the f-center lasers and diode lasers which are tunable in the infrared. 

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

Figure 2.19 shows, as an example, the output power of a Ti-sapphire laser pumped by a 15 W multiline Ar+ laser. The wide spectral region covered by this laser substitutes for those of the dye lasers emitting in the same spectral region. Several sets... 

Overall, the quantum yield of ring closure is reported to be low when measured in toluene [100] and this is in accordance with the transient studies. Further, in the transient study by Bohne et al. [71,72], the dye laser used to bleach the merocyanine form was rather powerful at 100-300 mJ/pulse. Even using this strong laser pulse on a merocyanine solution with an absorbance of 0.25, they only permanently bleached the mero-form absorbance by one-third in acetonitrile and one can, therefore, imagine that the bleaching yield is also rather low in acetonitrile. 

Dyes have the important role in dye lasers of allowing a fixed wavelength laser input to be tuned to a wide range from which a selection can be made appropriate to the end use of the dye laser. 

The power required to levitate an oil drop as its size parameter is varied by tuning the dye laser wavelength is shown in the lower curves of Fig. 11.11. The calculated radiation pressure efficiency (plotted as 1 /QpT) is shown in the middle curve and Qext in the upper curve the refractive index m = 1.47 + 110—6 is approximately constant over the small wavelength interval. This figure is taken from Chylek et al. (1978b), who identified the peaks in the upper curve. Curve a of the experimental results is for values of x calculated from the drop size determined microscopically with an accuracy of 5%. The ripple structure... 

It is easy then to write down the oscillation condition for a dye laser. In its simplest form a dye laser consists of a cuvette of length L [cm], with dye solution of concentration m [cm-3], and of two parallel end windows carrying a reflective layer, each of reflectivity R, which form the laser resonator. With mi molecules/ cm3 excited to the first singlet state, the dye laser will start oscillating at a wavelength A if the overall gain is equal to or greater than one ... 

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

Figure 9. Design of the dye laser amplifier. Ultrafast laser pulses are amplified roughly 10,000 times by seven passes through a dye jet pumped by a copper vapor laser.

A time response function of the apparatus can be measured by upconversion of the excitation beam. The width of such measured instrument response function is 280fs (FWHM). Comparing this result with the width of the autocorrelation function of the dye laser 110fs we observe 170fs broadening of the instrument response function due to group velocity... 

Figure 3 shows the excitation spectrum of CaO doped with Eu2+ when fluorescence from all sites in the sample was monitored. The dye laser was scanned over the possible absorption lines and each time the wavelength matched a transition on any site, the fluorescence intensity increased and gave a line. Figure 4 shows the same procedure on the same crystal except now a high resolution monochromator was used to monitor the fluorescence that occurred at a wavelength characteristic of a specific site. Now, one sees increases in the fluorescence only when the dye laser matches an absorption line of the same site that has the fluorescence line being monitored. 

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