Coherence, laser spectroscopy

Optical Molecular Dephasing Principles of and Probings by Coherent Laser Spectroscopy, A. H. Zewail, Acc. Chem. Res. 13, 360 (1980). 

The realization of such coherent systems requires special experimental preparations that, however, can be achieved with several techniques of coherent laser spectroscopy (Chap. 12). An elegant theoretical way of describing observable quantities of a coherently or incoherently excited system of atoms and molecules is based on the density-matrix formalism. 

J.M. Vaughan Coherent laser spectroscopy and Doppler LIDAR sensing in the atmosphere. Phys. Scr. T78, 73 (1998)... 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

Multiple Phase-Coherent Laser Pulses in Optical Spectroscopy. I. The Technique and Experimental Applications, W. S. Warren and A. H. Zewail, J. Chem. Phys. 78, 2279 (1983). 

By contrast, laser scattering methods now permit temperature composition and flow measurements that are both nonintrusive and give very high spatial resolution. These light scattering methods include laser Raman spectroscopy, laser-induced fluorescence, coherent Raman spectroscopy as well as laser velocimetry... 

Owyoung, A., "High Resolution Coherent Raman Spectroscopy of Gases", 4th International Conference on Laser Spectroscopy, Rottach-Egern, FRG, 11-15 June 1979. 

Continuous wave coherent Lyman-a radiation has recently become available [85] so that laser cooling or sensitive shelving spectroscopy of magnetically trapped hydrogen atoms is coming within reach. The ability to work with a small number of atoms is of particular interest for laser spectroscopy of antihydrogen, a goal pursued by the ATRAP and ATHENA collaborations at CERN [8]. 

An alternate approach is to perform coherent Raman spectroscopy in the time domain rather than in the frequency domain. In this case, a single laser that produces short pulses with sufficient bandwidth to excite all of the Raman modes of interest is employed. One pulse or one pair of time-coincident pulses is used to initiate coherent motion of the intermolecular modes. The time dependence of this coherence is then monitored by another laser pulse, whose timing can be varied to map out the Raman free-induction decay (FID). It should be stressed at this point that the information contained in the Raman FID is identical to that in a low-frequency Raman spectrum and that the two types of data can be interconverted by a straightforward Fourier-transform procedure (12-14). Thus, whether a frequency-domain or a time-domain coherent Raman technique should be employed to study a particular system depends only on practical experimental considerations. 

In this section we first give a survey on the most common nonlinear Raman processes, i. e. the (incoherent) hyper Raman scattering and several forms of coherent nonlinear Raman scattering. We then describe the instrumentation needed to perform several practical kinds of these nonlinear laser spectroscopies. Applications of nonlinear Raman spectroscopy will be found in Sec. 6.1. 

E. Arimondo, Coherent population trapping in laser spectroscopy. Progr. Opt. 35. p. 257 354,(1996). 

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. 

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