Spectroscopy nonlinear optics and laser

NONLINEAR OPTICS AND LASER SPECTROSCOPY IN THE VACUUM ULTRAVIOLET... 

Nonlinear Optics and Laser Spectroscopy IN the Vacuum Ultraviolet... 

Gagik G. Gurzadyan, Ph.D., Dr. Sci., has extensive experience in nonlinear optics and crystals, laser photophysics and spectroscopy. He has authored several books including the Handbook of Nonlinear Optical Crystals published by Springer-Verlag. He worked in the Institute of Spectroscopy (USSR), CEA/Saclay (France), Max-Planck-Institute of Radiation Qiemistry (Germany). At present he works at the Technical University of Munich with ultrafast lasers in the fields of nonlinear photochemistry of biomolecules and femtosecond spectroscopy. 

The connection of this nonlinear optical effect with spectroscopy lies in the fact that and hence the signal strength, will be enhanced whenever a linear combination of a subset oi mcomponents approaches the energy difference of two molecular states of proper symmetry. This wi-photon resonant enhancement — which can in principle be observed even in the absence of real transitions, as will be the case for states with no thermal population — has been exploited to develop a number of nonlinear spectroscopic techniques that differ in order n, order of the used resonance m, number of colours (i.e. different actual laser fields that provide the frequency components), spectral and temporal resolution and the actual method used to detect the resonances by monitoring either the power generated at (o or the change in amphtude, polarization or phase at some of the input frequencies. 

In this chapter we review some of the most important developments in recent years in connection with the use of optical teclmiques for the characterization of surfaces. We start with an overview of the different approaches available to tire use of IR spectroscopy. Next, we briefly introduce some new optical characterization methods that rely on the use of lasers, including nonlinear spectroscopies. The following section addresses the use of x-rays for diffraction studies aimed at structural detenninations. Lastly, passing reference is made to other optical teclmiques such as ellipsometry and NMR, and to spectroscopies that only partly depend on photons. 

In the previous Maxwelhan description of X-ray diffraction, the electron number density n(r, t) was considered to be a known function of r,t. In reality, this density is modulated by the laser excitation and is not known a priori. However, it can be determined using methods of statistical mechanics of nonlinear optical processes, similar to those used in time-resolved optical spectroscopy [4]. The laser-generated electric field can be expressed as E(r, t) = Eoo(0 exp(/(qQr ot)), where flo is the optical frequency and q the corresponding wavevector. The calculation can be sketched as follows. 

M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy, Optics and Photonics Series, Academic Press Revised edition (March 1989), New York, 1988. 

To demonstrate the versatility of nonlinear microspectroscopy with shaped broadband laser pulses, TPF is chosen as a second example. Remember that TPF can be implemented only by programming the pulse shaper differently and sampling a different wavelength range of the signal spectrum in the same experimental setup. TPF is so far probably the most widely applied nonlinear optical spectroscopy... 

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. 

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