Nonlinear optical techniques

Koroteev N I 1995 BioOARS—a novel nonlinear optical technique to study vibrational spectra of chiral biological molecules in solution Biospectroscopy 1 341-50... 

An optical detector with appropriate electronics and readout. Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range. Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients. 

Nonlinear optical techniques are extremely useful in characterizing the chiral properties of materials, as is pointed out by Verbiest and Persoons in Chapter 9. These authors give an in-depth discussion of this tool, both from an experimental and theoretical point of view, paying special attention to the characterization of chiral surfaces and thin films. In the second part of their contribution they highlight the role chiral materials can play in the field of nonlinear optics and photonics, which opens the way for a variety of applications. 

Motivating the research is the need for systematic, quantitative information about how different surfaces and solvents affect the structure, orientation, and reactivity of adsorbed solutes. In particular, the question of how the anisotropy imposed by surfaces alters solvent-solute interactions from their bulk solution limit will be explored. Answers to this question promise to affect our understanding of broad classes of interfacial phenomena including electron transfer, molecular recognition, and macromolecular self assembly. By combining surface sensitive, nonlinear optical techniques with methods developed for bulk solution studies, experiments will examine how the interfacial environment experienced by a solute changes as a function of solvent properties and surface composition. 

In the next section, we describe two pump-probe approaches and review their use in experiments with MbCO. This places the nonlinear optical techniques of transient phase grating introduced in the subsequent section in their proper context. 

A general objective in any in situ spectroscopic technique is to maximize the signal that arises specifically from the electrode surface. Nonlinear optical techniques such as second-harmonic generation (SHG) and sum frequency generation (SFG) are of interest because they involve optical signals that by definition can only arise at the electrode-solution interface [65],... 

At high concentration, when molecules are no longer isolated in space, a conventional optical microscope is unable to resolve them within the diffraction limit. Efforts have been made to circumvent the diffraction limit by engineering the point spread function using nonlinear optical techniques. Spatial resolution of 20 nm in a cell has been demonstrated without using a proximal probe.67... 

Nonlinear optical techniques (SHG, SFG) Adsorption kinetics, interfacial coverage, reactioii kinetics, phase transitions, orientational order (average tilt angle), surface chirality. Intensity of the signal reflects the combined effect of interfacial coverage and orientational order. Tilt angles only obtainable if all non-zero elements of the hyperpolarizability tensor can be determined. 

A number of spectroscopic techniques have already been discussed in Volume 1 (secs. 7.11-13). Here, we will focus on the application of vibrational (infrared and Raman) and UV/visible spectroscopy, fluorescence and second-order nonlinear optical techniques for the study of monolayers. The type of information obtained with these techniques refers to the chemical composition, surface coverage, molecular conformation and orientation, and dynamics of monolayers. 

Time-resolved nonlinear optical techniques and high-resolution spectroscopic methods have recently been extended to the selective investigation of the dynamics of bound multiphonon states in solids. These states,... 

As already mentioned, the only techniques sensitive to the polar order are even order nonlinear optical techniques such as the already-described second harmonic generation and linear electro-optic effect (cf. Chapter 2). The hrst technique offers a high sensitivity to the fast electronic contributions to susceptibility and is widely used. As already mentioned, it also gives the opportunity to study the kinetics of the poling by in situ measurements [152]. 

The most recent publications on the S -Sq transition of TRN(OH) are by Braca-monte and Vacarro ]37, 38], who applied their polarization resolved DFWM spectroscopy method [37] to TRN vapor at its room temperature sublimation pressure. This remarkable nonlinear spectroscopic method simplifies the spectrum by allowing Q branch (A/ = 0) transitions to be observed with greatly diminished R and P branch (AJ = 1) signal and, with rotation of a polarization element, vice versa. This selective nonlinear optical technique also produces a large reduction in the signal generated by hot bands, as noted in the above discussion for MA. How-... 

Figure 5 shows the SFG vibrational spectra of carbon monoxide obtained at 10 -700 Torr of CO and at 295 K. When the clean Pt(lll) surface was exposed to 10 L (1 L=10 Torr sec) of CO in UHV, two peaks at 1845 cm and 2095 cm were observed which are characteristic of CO adsorbed at bridge and atop sites. LEED revealed that a c(4 X 2) structure was formed in which an equal number of carbon monoxide molecules occupied atop and bridge sites [15]. Such results are in agreement with previous HREELS [16] and reflection-absoiption infrared spectroscopy (RAIRS) [17] studies. ITie much higher relative intensity of atop bonded CO to bridge bonded CO in the SFG spectra is due to the specific selection rule for the SFG process [18]. As mentioned earlier, SFG is a second order, nonlinear optical technique and requires the vibrational mode under investigation to be both IR and Raman active, so that the SFG intensity includes contributions from the Raman polarizability as well as the IR selection mle for the normal mode. 

Sum-fretiuencv generation (SI Ci) is a nonlinear optical technique basoil on the interaction of two plu tons at a surface. The result of the wave-mixing interaction is the production of a single photon whose frequency is the sum of the incident frequencies. If the two incident photons are of Ihe same frequency, the technique is called second-harmonic generation because the exiting photon has a frequency Iwice iha of the incident photons. Because this is a weak second-order process, intense lasers must be used. 

Sum and difference frequency generation at electrode suifaces [130] The investigation of vibrational and electronic properties of the electrochemical interface by using nonlinear optical techniques of visible-infrared sum (SFG) and difference (DFG) frequency generation constitutes an interesting aspect of the study of adsorbed species. The vibrational behavior of H-Pt(hkl) system, both in the underpotential and overpotential regime, adsorption of CN on Au(hkl) electrode (using DFG), electrode surface electronic properties were studied and measurements of the PZC were carried out [131]. 

One of the principal analytical techniques used for surface-sensitive and orientational analysis is SFG [1, 22-26]. This nonlinear optical technique is based on a coherent photon emission process. It utilizes a combination of two laser beams, one in the visible and one in the IR range frequency, set at two possible polarizations, p - for light incident parallel to the plane of incidence, and s - for incidence perpendicular to it. The resulting SFG signals emitted from the surface are proportional to the interaction of the molecular vibrational modes with the incoming electric fields and their hyperpolarizabiHties. The emitted signals from the surface are analyzed and interpreted according to their intensities and intensity ratio at the four relevant polarization combinations [24, 27, 28]. 

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