Second-harmonic generation probes

Optical second-harmonic generation (SHG) has recently emerged as a powerful surface probe [95, 96]. Second harmonic generation has long been used to produce frequency doublers from noncentrosymmetric crystals. As a surface probe, SHG can be caused by the break in symmetry at the interface between two centrosymmetric media. A high-powered pulsed laser is focused at an angle of incidence from 30 to 70° onto the sample at a power density of 10 to 10 W/cm. The harmonic is observed in reflection or transmission at twice the incident frequency with a photomultiplier tube. 

Corn R M and Higgins D A 1994 Optical second-harmonic generation as a probe of surface-chemistry Chem. Rev. 94 107-25... 

Ohihoff C, Lupke G, Meyer C and Kurz H 1997 Static and high-frequency electric fields in silicon MOS and MS structures probed by optical second-harmonic generation Phys. Rev. B 55 4596-606... 

In addition to the fourth-order response field Tfourth, the probe light generates two SH fields of the same frequency 211, the pump-free SH field Eq(2 Q), and the pump-induced non-modulated SH field non(td> 211). The ground-state population is reduced by the pump irradiation and the SH field is thereby weakened. The latter term non(td, 211) is a virtual electric field to represent the weakened SH field. Time-resolved second harmonic generation (TRSHG) has been applied to observe E on (td, 211) with a picosecond time resolution [20-25]. The fourth-order field interferes with the two SH fields to be detected in a heterodyned form. 

Liquid Membrane Ion-Selective Electrodes Response Mechanisms Studied by Optical Second Harmonic Generation and Photoswitchable lonophores as a Molecular Probe... 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

Optical second harmonic generation (SHG), which is the conversion of two photons of frequency u to a single photon of frequency 2co, is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetrical medium. At the interface between two centrosymmetrical media, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG [18]. SHG has been used as an in-situ probe of chemisorption, molecular orientation, and... 

Because of the relatively large dispersion from the electrons compared with the almost constant refractivity of the neutrals and the negligible contribution of the ions, it is possible, with simultaneous measurements at two different wavelength, to determine independent values of the density of electrons and of the nonelectronic components in the plasma 274). Alcock and Ramsden 275) used the light from a giant-pulse ruby laser and its second harmonic generated in an ADP-crystal (ammonium dihydrogen phosphate) to probe a pulsed plasma and its time-dependent density in a Mach-Zehnder interferometer. 

Mahieu-Williame, L., Gresillon, S., Cuniot-Ponsard, M., and Boccara, C. 2007. Second harmonic generation in the near field and far field A sensitive tool to probe crystalline homogeneity. J. Appl. Phys. 101 083111. 

Nonlinear optical spectroscopies such as second harmonic generation (SHG) and sum frequency generation (SFG) are finding increasing use in probing species at interfaces (e.g., Eisenthal, 1996). For example, SHG was used by Donaldson et al. (1995) to detect a surface-bound S02 species, and SFG has been applied to elucidate the structure of dimethyl sulfoxide at liquid interfaces (Allen et al., 1999). These techiques are... 

Second Harmonic Generation as an In-situ Probe of Single Crystal Electrode Surfaces... 

In the first study of its kind, second harmonic generation has been used to study potential induced reconstruction on Au(lll) and Au(100) by Kolb and coworkers [156]. These surfaces have been known to reconstruct in UHY when they are clean [153, 157], Surface reconstruction occurs when the surface atoms of a solid rearrange themselves in a structure different from that expected from simple termination of the bulk lattice. Various studies by cyclic voltammetry, electroreflectance spectroscopy and ex situ electron diffraction have suggested that flame-treated crystals form stable reconstructions in solution. Unfortunately, due to the lack of in situ probes, very little direct evidence for this reconstruction has been available. 

Figure 3.23 Schematic illustration of the femtosecond pump-probe technique used to initiate and follow the reactions of adsorbed species SHG, second harmonic generation SFG, sum frequency generation IRAS, infrared reflection-adsorption spectroscopy PPE, photons per event A R, change in reflectivity...

The OHD-RIKES signal is quadratic in the laser intensity, so any laser fluctuations are magnified in the data. To compensate for such fluctuations, the probe beam is sent into a doubling crystal after the sample, and the resultant second-harmonic signal is detected by a separate lock-in amplifier. Second-harmonic generation has the same quadratic intensity dependence as the OHD-RIKES signal, so dividing the RIKES data by the second-harmonic data acts to normalize any intensity fluctuations. 

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