Nonlinear laser spectroscopic studies

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

Thus far, we have examined vibrational spectroscopy using IR absorption spectroscopy, what we called in Ch. 3 one photon method , a general type that encompasses most experiments in spectroscopy. There exist, however, other types of spectroscopy to observe vibrations. These are for instance Raman spectroscopy, which is also of a current use in chemical physics and may be considered a routine method. Other less known methods are modem time-resolved IR spectroscopies. All these methods are two-photon or multiphoton spectroscopies. They do not involve a single photon, as in absorption, but the simultaneous absorption and emission of two photons, as in Raman and in other scattering experiments, or the successive absorption(s) and emission(s) of photons that are coherently delayed in time, as in time-resolved nonlinear spectroscopies. By coherently , we assume the optical waves that carry these two photons keep a well-defined phase difference. In this latter type of spectroscopy, we include all modem set-ups that involve time-controlled laser spectroscopic techniques. We briefly sketch the interest of these various methods for the study of H-bonds in the following subsections. 

Harmonic Generation. Nonlinear frequency conversion, especially harmonic generation, is important for the development of sources of coherent light at frequencies not otherwise accessible by efficient lasers, in particular the blue, violet, and UV frequencies needed for dense data storage. Harmonic generation is also useful for nonlinear spectroscopic studies, which probe excited states in materials (34). 

Besides the spectroscopic investigations of solids by laser-excited spontaneous Raman or Brillouin scattering already discussed in Sections III.6 and 7, much new insight into the optical properties and the structure of solids has been gained by studying nonlinear optical effects. (Surveys and more detailed information about nonlinear optics can be found in refs. 306-308))... 

Written by an international panel of experts, this volume begins with a comparison of nonlinear optical spectroscopy and x-ray crystallography. The text examines the use of multiphoton fluorescence to study chemical phenomena in the skin, the use of nonlinear optics to enhance traditional optical spectroscopy, and the multimodal approach, which incorporates several spectroscopic techniques in one instrument. Later chapters explore Raman microscopy, third-harmonic generation microscopy, and nonlinear Raman microspectroscopy. The text explores the promise of beam shaping and the use of a broadband laser pulse generated through continuum generation and an optical pulse shaper. 

One-photon spectroscopy is due to the linear term, whereas the nonlinear terms lead to the simultaneous absorption of two or more photons. Although the theory was worked out almost 50 years ago, observation of multiphoton absorption was made feasible only after the development of lasers. This chapter deals with the application of two-photon excitation (TPE) to kinetic studies in low-pressure gas-phase samples. For a systematic, extensive discussion of spectroscopic applications, one of the excellent reviews available should be consulted. ... 

The use of tuned lasers increases possibilities of the RLS method, however, the radiation intensity still remains much lower than the intensity of the exciting light. This linear spectroscopic method possesses a very low sensitivity and, therefore, it is rarely used for studies in the gas phase. The sensitivity of nonlinear scattering methods is much higher. 

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