Stimulated-emission polarization

Inserting the first-order and second-order wave functions (26) into (28) and arranging the electric fields according to the stimulated-emission process, we obtain for the stimulated-emission polarization... 

To make contact with a widely used alternative formulation of nonlinear spectroscopy, we rewrite, for example, the stimulated-emission polarization (33) as... 

Finally, it is instructive to discuss the relation of the integral pump-probe signals derived above from the electronic polarization to the corresponding expressions obtained from the calculation of electronic population probabilities. To this end, we consider the stimulated-emission polarization (33) and rewrite the the integral signal (8) in the suggestive form... 

Although straightforward in principle, this procedure is clearly too time-consuming in practice. To obtain a computationally more manageable formulation, we restrict ourselves to the case of nonoverlapping laser pulses. The stimulated-emission polarization can then be expressed in terms of a two-time (instead of a three-time) correlation function. Dropping unimportant prefactors, the polarization may be written in the instructive form... 

Buehler, C., Dong, C. Y., So, P. T. C., French, T. and Gratton, E. (2000). Time-resolved polarization imaging by pump-probe (stimulated emission) fluorescence microscopy. Biophys. J. 79, 536-49. 

All lasers have the following three fundamental components (see Figure 1). The gain medium, the business end of the laser, is the medium in which excited states are created which will produce stimulated emission. The exciter is the source of energy for production of the excited states in the gain medium. Finally, the optical resonator determines the directionality, wavelength selectivity, optical feedback, polarization, and other characteristics of the stimulated emission from the gain medium. 

Fundamentally, the properties of laser light are concomitants of its coherence, which is in turn a consequence of the nature of stimulated emission. Most of these properties, especially brightness, monochromaticity, directionality, polarization, and coherence itself, are useful (for many applications, indis-pensible) in a spectroscopic light source. The spectroscopic potential of lasers was recognized even before they were invented. Actual applications remained very specialized until tunable lasers were devised. 

In contrast to spontaneous emission, induced emission (also called stimulated emission) is coherent, i.e. all emitted photons have the same physical characteristics - they have the same direction, the same phase and the same polarization. These properties are characteristic of laser emission (L.A.S.E.R. = Light Amplification by Stimulated Emission of Radiation). The term induced emission comes from the fact that de-excitation is triggered by the interaction of an incident photon with an excited atom or molecule, which induces emission of photons having the same characteristics as those of the incident photon. 

Collision-induced absorption is a well developed science. It is also ubiquitous, a common spectroscopy of neutral, dense matter. It is of a supermolecular nature. Near the low-density limit, molecular pairs determine the processes that lead to the collision-induced interactions of electromagnetic radiation with matter. Collision-induced absorption by non-polar fluids is particularly striking, but induced absorption is to be expected universally, regardless of the nature of the interacting atoms or molecules. With increasing density, ternary absorption components exist which are important especially at the higher temperatures. Emission and stimulated emission by binary and higher complexes have also... 

A photon with an energy that exactly spans two states can be absorbed to raise a molecule to an excited state. Alternatively, that same photon can stimulate the excited molecule to emit a photon and return to the lower state. This is called stimulated emission. When a photon emitted by a molecule falling from E2 to E( strikes another molecule in E2, a second photon can be emitted with the same phase and polarization as the incident photon. If there is a population inversion (n2 > nt), one photon stimulates the emission of many photons as it travels through the laser. 

The light generated by stimulated emission is usually identical to the light that stimulates it in wavelength, phase, and polarization. This gives laser light its characteristic coherence and allows it to maintain the uniform polarization and (often) monochromaticity allowed by the optical cavity design. 

Figure 16 Absorption spectrum of the cycfo-Mamb-Abu-Arg-Gly-Asp in D2O. The dashed line shows a representative spectrum of the pump pulses (width 12 cm-1) utilized to generate the 2D-IR spectra. (b,c) 2D pump-probe spectra of the same sample measured with the polarization of the probe pulse perpendicular and parallel to the polarization of the pump pulse, respectively. The dashed contour lines mark regions where the difference signal is negative (bleach and stimulated emission), while the solid contours lines mark regions where the response is positive (excited state absorption). The most prominent off-diagonal bands are marked by arrows. (d,e,f) A global least-squares fit of the experimental data, used to refine the coupling Hamiltonian in Equation (29c). (From Ref. 42.)...

The laser scheme is modified when neodymium is placed in other host environments in that the electronic levels have slightly different energies with different stimulated emission cross sections which may vary with the polarization of the stimulating radiation. As an example, Nd in lanthanum beryllate host (Nd BeL), the major laser transition is at 1079 nm and 1070 nm for two different polarizations. YAG is the most common and versatile host for neodymium but glass has advantages for high energy applications such as fusion research. Other crystal hosts such as YAP, YLF and BeL have some unique features. 

Dynamics Determining the Band Shape of Stimulated Emission from a Polar Styryl Dye. 

Several points deserve mention. First, retention of the sum over the harmonic wavevector and polarization signifies an initial condition that assumes the absence of any harmonic radiation prior to interaction, so that no direction is favored on the basis of stimulated emission (i.e., although the emergent wavevectors are equal in magnitude, they may differ in direction). Secondly, although photon branch indices are suppressed for conciseness, they are to be... 

FIGURE 1.26 Ultrafast stimulated emission anisotropy of MEH-PPV in mesoporous silica glass with the excitation pulse polarization parallel to the pore direction (excited at 490 nm and probed at 590 nm). The solid curve in the figure is a 250-ps exponential rise and the different symbols represent scans taken with different amounts of time between points. , 0.067 ps per point O, 1.67 ps per point A, 6.67 ps per point. The inset shows the same data with a log scale for the time axis. 

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