Spectroscopic four-wave mixing

Passive processes that are of spectroscopic interest of course require material resonances. For example, nondegenerate four-wave mixing pro-... 

Four-wave mixing (4WM) processes1-6 and spontaneous Raman and fluorescence (SRF) lineshapes7-12 provide a sensitive spectroscopic probe for polyatomic molecules in condensed phases. A 4WM process involves the interaction of three laser fields with wavevectors k, k2, and k3 and frequencies co, co2, and a>3, respectively, with a nonlinear medium. A coherently generated signal with wavevector ks and frequency cos is then detected (Fig. 1), where... 

Michele Marrocco, PhD, is a researcher in laser spectroscopy at ENEA (Rome, Italy) (1999 to present). He received his degree in physics from the University of Rome in 1994. He was employed as a postdoctorate at the Max-Planck Institute for Quantum Optics (Munich, Germany), as a researcher at the Quantum Optics Labs at the University of Rome (Rome, Italy), and as an optics researcher by the army. His research activities include traditional and innovative spectroscopic techniques for diagnosis of combustion and nanoscopic systems studied by means of optical microscopy. The techniques used include adsorption, laser induced fluorescence, spontaneous Raman, stimulated Raman gain, stimulated Raman loss, coherent anti-Stokes Raman, degenerate four wave mixing, polarization spectroscopy, laser induced breakdown, laser induced incandescence, laser induced thermal gratings. He has over 30 technical publications. 

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

Four-wave mixing is of Importance both fundamentally and for spectroscopic diagnostics. Extensive perturbational investigations of the process have been done both directly and diagramatically. >3 Exact solutions for various particular physical processes have been previously obtained.In this paper, we develop a technique for the semlclasslcal exact solution of a four-wave mixing process. For the sake of brevity, we will specialize to the dominant path, but the technique is applicable to other paths as well. 

The signal arises from the four-wave mixing signal enhancement when the laser frequency difference matches the frequency of the probed vibrational mode. CARS is at the same time a coherent and a resonant process, meaning that it differs in nature from other nonlinear processes such as two-photon excited fluorescence (i.e., only resonant) and SHG (i.e., only coherent). Both aspects determine the peculiar features in a CARS spectroscopic image. 

Another way to make a four-wave mixing process surface-sensitive is to use the evanescent part of the electromagnetic field at interfaces for spectroscopic purposes. Here one takes advantage of the field enhancement that takes place at the surface of a prism, which is coated by a thin metallic film. 

The collision-induced resonances in four-wave mixing provide striking confirmation of very subtle features of damping in nonlinear situations, where simultaneously two, three, or four optical transitions are at or near resonance. In addition, they may become a versatile spectroscopic tool for gaining new information in situations such as those described below. 

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