Laser-Pumped Pulsed Dye Lasers

The pulsed dye laser, pumped by a ruby laser, was the first dye laser, realized already 1966 independently by Schafer [1.161] and Sorokin [5.162]. In these early days of dye laser development giant-pulse ruby lasers, frequency-doubled Nd glass lasers and nitrogen lasers were the main pumping sources. All these lasers have sufficiently short pulse durations Tp Tjc which are shorter than the intersystem crossing time constant Tic(Si- Tj). 

The short wavelength A = 337 nm of the nitrogen laser permits pumping of dyes with fluorescence spectra from the near UV up to the near infrared. The high pump power available from this laser source allows us to achieve sufficient inversion even in dyes with lower quantum efficiency [5.163-167]. Nowadays the most important dye-laser pumps are the eximer laser [5.168,169], the frequency-doubled or -tripled output of high-power Nd YAG or Nd glass lasers [5.170,171] or copper-vapor lasers [5.172]. 

If wavelength selection is performed with a grating, it is preferable to expand the dye-laser beam for two reasons. 

For reliable single-mode operation of the Littman laser longitudinal pumping is better than transverse pumping, because the dye cell is shorter and inhomogenities of the refractive index caused by the pump process are less severe [5.176b]. 

Assume a reflectivity of R(a=89°) = 0.05 into the wanted first order at = 0°. The attenuation factor per round trip is then (0.05) c 2.5-10 The gain factor per round trip must be larger than 4-10 in order to reach threshold. With preexpanding prisms and an angle a = 85°, the reflectivity of the grating increases to R(a=85°) = 0.25, which yields the attenuation factor 0.06. Threshold is now reached already if the gain factor exceeds 16. 

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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. 

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

To summarize the state of technology for the chemist wishing to practice laser chemistry the laser devices exist with the capability one would like, but they are expensive. We may expect that cheaper pulsed laser systems based upon excimer, Nd YAG, N2, alexandrite, etc. may be in the offing in the near future. This has already begun to happen with a new generation of N2 pumped dye lasers from two manufacturers. No such prospects presently exist for c.w. lasers in the visible and ultraviolet, but one may hope that the ion laser will be radically improved or supplanted soon. For chemical applications which can use infrared excitation, satisfactory devices presently exist and the price is right. 

Sorokin and Lankard illuminated cesium and rubidium vapors with light pulses from a dye laser pumped by a ruby giant-pulse laser, and obtained two-step excitation of Csj and Rbj molecules (which are always present in about 1 % concentration at atomic vapor pressures of 10" - 1 torr) jhe upper excited state is a repulsive one and dissociates into one excited atom and one ground-state atom. The resulting population inversion in the Ip level of Cs and the 6p level of Rb enables laser imission at 3.095 jum in helium-buffered cesium vapor and at 2.254 pm and 2.293 /zm in rubidium vapor. Measurements of line shape and frequency shift of the atomic... 

S2 - Sq fluorescence in condensed media has so far been found in several types of molecules. However, metalloporphyrins are contrasted with these compounds by another arresting feature such that the S2 fluorescence can be observed even upon photoexcitation to the state. Stelmakh and Tsvirko have first noticed the anomalous S2 - Sq fluorescence in metalloporphyrins (15,16). Figure 1(a) shows the fluorescence spectra of ZnTPP in EPA taken by the 540 nm excitation of a nitrogen pumped dye laser. The fluorescence band at around 430 nm observed by visible excitation is safely assigned to the S2 state fluorescence. The laser power dependence of the fluorescence intensity is quadratic at low power density of excitation (<5 x 10 photons cm"2 pulse ) but shows typical saturation effect with increasing the laser intensity. It should be emphasized here that the S2 fluorescence of ZnTPP can be observed without focusing of the laser beeim. 

Figure 1. Fluorescence spectra (uncorrected for the spectral response) of ZnTPP in EPA at room temperature taken by the 540 nm excitation of a nitrogen pumped dye laser, (a)normal fluorescence spectra, (b)delayed fluorescence spectra taken at 1 s after the laser pulse excitation.

Lasers and LEDs. Dye lasers pumped by Ar ion, Cu ion and frequency doubled Nd YAG solid state lasers. LEDs operating at 635-652, diode lasers at 635 (AlGalnP), 652 (InGaAlP) and 730 mn (AlGaAs). Solid state pulsed lasers, (e.g. Nd YAG, Nd YLF) operating at second, third and fourth harmonic generation. 

The first dye laser generates 10-ps, 2-pJ, 365-nm pulses that pump a microscopic distributed feedback dye laser (DFDL) producing Fourier transform hmited 0.7-ps pulses at 616 nm. These DFDL pulses are amplified in two stages to give 100-pJ, diffraction-limited pulses. 

A pumping laser satisfying these requirements is the Cu vapor laser, first advocated for the same reasons by J. G. Anderson in about 1980. Thus Stimpfle et al. (93, 94) used a Cu vapor laser pumped dye laser that was frequency-doubled to 282 nm to determine stratospheric HO during balloon-borne descent in the stratosphere. At the 17-kHz repetition rate of this laser, multiple-pulse photolytic HO accumulation appears to have been avoided by the use of a fan downstream of the excitation zone to increase the air velocity beyond that provided by balloon descent. 

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