Giant laser pulses

There is another way to obtain giant laser pulses of a few ns duration, known as active Q-switching. The shutter is an electro-optical cell which is triggered at some preset time after the pump flash. These electro-optical shutters are Kerr cells or Pockels cells. 

If the exciting radiation has a very high intensity, as in giant laser pulses , the induced dipole moment (Eq. 2.4-1) is larger than in the linear approximation ... 

Mizushima and Nishiyama (77) examined the action of laser and found that compressed explosives can be brought to decomposition by a giant laser pulse. Loose explosives cannot detonate. They examined initiating explosives, PFTN, RDX, TNT and Tctryl. 

In his article mainly mode-locked tunable dye lasers are discussed. Giant pulse ruby lasers (3 nsec pulse halfwidth) have been successfully used to probe electron densities as a function of time in a rapidly expanding plasma 22). The electron lifetime in the conduction band can be determined with nanosecond semiconductor lasers. By absorption of the laser pulse the electrons in the semiconductor probe are excited into the conduction band, resulting in a definite conductivity. The mean lifetime is obtained by measuring the decrease of conductivity with time 26). 

From photolysis of methylene blue by ruby-laser giant pulses, Danzinger et al. found that a 0.5 Joule, 30 nsec laser pulse causes almost total conversion of the original molecules into transients, but that the photochemical change is completely reversible. The lifetimes of three transients have been measured as 2, 30 and 140 jusec resp. at a 5.5 x 10 M dye solution. 

Fig. 6. Relative laser pulse intensity versus time of a ruby giant pulse laser that was used to bleach a solution of metal-free phthalocyanine and transmission of the dye solution at the wavelength of the He—Ne—laser. (From Ref. 14>)...

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

Since the absorption bands of many laser dyes reach from the blue-green region down to the near ultraviolet, the N2 laser at x = 337 nm is well suited as a pumping source. Sometimes frequency-doubled neodymium YAG or ruby lasers are employed. In order to avoid triplet losses, the pumping time Tp should be shorter than Tj = 1/R(S - T ), where R is the intersystem crossing rate. This demand is readily met by N2 laser pulses (Tp = 10 - 10 s) or by giant pulses from ruby and Nd-YAG lasers. Sometimes ultrashort pulses from mode-locked lasers (see Chap.11) are used. 

In this 0-switching technique, one of the cavity mirrors is effectively removed during pumping and then suddenly replaced. The build-up time of the giant pulse is determined by the switching speed and the initial gain of the pumped laser. 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

A description of a fast laser photolysis experimental arrangement has been given by Porter and Topp who used a 1.5 Joule, 20nsec ruby giant pulse, frequency doubled in ADP, to measure singlet lifetimes in phenantrene, pyrene and other organic molecules. 

Fig. 9. Experimental arrangement for laser photolysis, using the frequency-doubled output from a giant-pulse ruby laser as pump pulse and the wavelength continuum from a laser-induced high-temperature gas plasma as analysing pulse. (From Novak, J.R., Windsor, M.W., ref. 15 ))...

The output from a ruby giant-pulse laser (2 Joule, 30 nsec half-width, = 6943A) passes a KH2PO4 crystal where, due to the nonlinear characteristics of this material, the second harmonic at X = 3471 A is generated with an efficiency of 3 %.The two wavelengths are separated by means of a water filled quartz prism. The ultraviolet light pulse serves as pump pulse. 

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... 

Using as the background continuum the short-lived spontaneous fluorescence of rhodamine B or 6 G, McLaren and Stoicheff 233) developed this method further to obtain inverse Raman spectra over the range of frequency shifts 300-3500 cm" in liquids and solids in a time of 40 nsec The stimulating monochromatic radiation at 6940 A is provided by a giant-pulse ruby laser. A small part of the main laser beam is frequency-doubled in a KDP-crystal and serves to excite the rhodamine fluorescence, thus ensuring simultaneous irradiation of the sample by both beams. 

Because of the relatively large dispersion from the electrons compared with the almost constant refractivity of the neutrals and the negligible contribution of the ions, it is possible, with simultaneous measurements at two different wavelength, to determine independent values of the density of electrons and of the nonelectronic components in the plasma 274). Alcock and Ramsden 275) used the light from a giant-pulse ruby laser and its second harmonic generated in an ADP-crystal (ammonium dihydrogen phosphate) to probe a pulsed plasma and its time-dependent density in a Mach-Zehnder interferometer. 

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