Nonresonant activation

The key requirements for ISRS excitation are the existence of Raman active phonons in the crystal, and the pulse duration shorter than the phonon period loq1 [19]. The resulting nuclear oscillation follows a sine function of time (i.e., minimum amplitude at t=0), as shown in Fig. 2.2e. ISRS occurs both under nonresonant and resonant excitations. As the Raman scattering cross section is enhanced under resonant excitation, so is the amplitude of the ISRS-generated coherent phonons. 

The nonresonant contributions pertain to electron cloud oscillations that oscillate at the anti-Stokes frequency but do not couple to the nuclear eigenfrequencies. These oscillatory motions follow the driving fields without retardation at all frequencies. The material response can, therefore, be described by a susceptibility that is purely real and does not depend on the frequencies of the driving fields. The resonant contributions, on the other hand, are induced by electron cloud oscillations that are enhanced by the presence of Raman active nuclear modes. The presence of nuclear oscillatory motion introduces retardation effects relative to the driving fields i.e., there is phase shift between the driving fields and the material oscillatory response. 

The desire to increase P values above those of molecules used in the earliest materials has led to the exploration of organic compounds as the active components of second order NLO devices. Considerable effort has been expended in the synthesis and analysis of candidate molecules, which are largely donor-acceptor substituted conjugated n systems. (2) Examples of compound classes whose members display large nonresonant P include azo dyes, stilbenes, polyenes, merocyanines, stilbazolium salts, and quinoid... 

The complex quantity, y6br = e (y(3)r) + i Im (x r), represents the nuclear response of the molecules. The induced polarization is resonantly enhanced when the Raman shift wp — ws matches the frequency Qr of a Raman-active molecular vibration (Fig. 6.1A). Therefore, y(3)r provides the intrinsic vibrational contrast mechanism in CRS-based microscopies. The nonresonant term y6bnr represents the electronic response of both the one-photon and the two-photon electronic transitions [30]. Typically, near-infrared laser pulses are used to prevent the effect of two-photon electronic resonances. With input laser pulse frequencies away from electronic resonances, y(3)nr is independent of frequency and is a real quantity. It is important to realize that the nonresonant contribution to the total nonlinear polarization is simply a source for an unspecific background signal, which provides no chemical contrast in some of the CRS microscopies. While CARS detection can be significantly effected by the nonresonant contribution y6bnr [30], SRS detection is inherently insensitive to it [27, 29]. As will be discussed in detail in Sects. 6.3 and 6.4, this has major consequences for the image contrast mechanism of CARS and SRS microscopy, respectively. 

Recently, Silberberg and coworkers have introduced a different approach for active phase control in CARS [44 47]. By tailoring the spectral phase of a single ultrashort laser pulse, phase-sensitive detection of the resonant signal has been demonstrated where the strong nonresonant CARS background of... 

Consider a time-resolved, electronically nonresonant CARS spectrum from a molecular liquid. In the CARS process, the laser pump pulses create a linear combination (that is the inteimolecular rovibrational coherence) of Raman active rovibrational transitions between molecules at position rr and r in the mixture. This stimulated Raman scattering process is carried out by two-coincident laser pulsesfl, II) with central frequenciesfwave vectors) C0i(k ) and (Oiiikii). By applying the third pulse with C0 (kni) to the liquid after time delay t, the time dependence of the inteimolecular rovibrational coherence is detected through the measurement of the intensity of the scattered photon with kj... 

Since the H-T terms are developed from the first-order terms active in the nonresonance Raman effect, they are usually the most important source of preresonance enhancement whereas the FC terms are active in proportion to the extent to which closure fails for the vibrational levels of the intermediate state. Some physical insight into these different scattering mechanisms is furnished by the following interpretation of the energy denominators of the scattering tensor. 

Such interactions include all active processes and all nonresonant nonlinear dispersions. Thus, in such spectroscopies, the signal field must always remain in-phase with the induced polarization and y(r) is determined from Eq. (2.15a) with Ay(r) = 0. 

In this section, we have shown how Ay(r) is determined according to the nature of the light-matter interactions and the presence or absence of material resonances. For interactions of the quadrature type, the active processes or the nonresonant passive dispersions, the signal field is always in-phase with respect to the induced polarization, namely A( y(r) = 0. For all remaining passive interactions, where the light-matter interactions are not directly in quadrature in the fields, the signal field must be out-of-phase, Aphase components in the presence of resonance (0< A0y(r) < jtt). 

Fig. 5.4. Temperature dependence of the Eu- -Eu transfer rate in EuMgBjOio. Line I is a fit using thermally activated migration via the F level of the Eu ion line 2 is a fit to the T temperature dependence predicted by a iwo-site nonresonant process...

Nonresonant tunneling can be identified by several features, including an exponential decay of the tunneling current as the thickness of the molecular layer increases, a very weak temperature dependence (with essentially zero activation over a fairly wide temperature range), and a somewhat distinctive... 

In our experiment, the velocity diffusion of sublevel coherence between the excitation and detection processes plays a role similar to the spatial motion of the active atoms in conventional Ramsey experiments for the case of nonresonant... 

The most specific type of detectors used in Mfissbauer spectroscopy is the so-called resonance detector. In this type of detectors, mainly conversion electrons are detected in a gas-filled chamber in which an Fe-enriched standard target is placed. As conversion electrons can only form after the recoilless absorption of the y rays in the standard target, the nonresonant radiation is very effectively filtered off, and the resultant Mhssbauer spectrum has hardly any background (baseline). Therefore, the signal-to-noise ratio becomes excellent and the required measuring time (or the source activity that is needed) drastically decreases. 

The experimental layout is presented in Fig. 14.5. A Co( Fe) isotope I with activity of lOmCi in a chromium matrix was used as a source of Mossbauer radiation. This source has a spectrum in the form of single line of natural width. The source is fixed in the Plexiglas disc and put in the center (/ = 2.5 cm) or near the edge (/ = I cm) of the resonant absorber 2, having a form of cylinder with diameter D = 2 cm and length L = 5 cm, made of Fe isotope (200 mg) in stainless steel (100 mg). The thickness of absorber 7 mg cm provides the requirement of total absorption of resonant radiation (for co - coeg < T/2) and almost full transparency for nonresonant radiation (for co - coeg > T/2). 

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