Second harmonic interface

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. 

SHG Optical second-harmonic generation [95, 96] A high-powered pulsed laser generates frequency-doubled response due to the asymmetry of the interface Adsorption and surface coverage rapid surface changes... 

Shen Y R 1989 Optical second harmonic-generation at interfaces Ann. Rev. Phys. Chem. 40 327-50... 

Eisenthal K B 1996 Liquid interfaces probed by second-harmonic and sum-frequency spectroscopy Chem. Rev. 96 1343-60... 

Ghahramani E, Moss D J and Sipe J E 1990 Second-harmonic generation in odd-period, strained, (Si)y(Ge) Si superlattices and at Si/Ge interfaces Phys. Rev. Lett. 64 2815-18... 

Dadap J I, Hu X F, Russell N M, Ekerdt J G, Lowell J Kand Downer M C 1995 Analysis of second-harmonic generation by unamplified, high-repetition-rate, ultrashort laser pulses at Si(OOI) interfaces/ J. Selected Topics Quantum Electron 1 1145-55... 

Petraiii-Maiiow T, Wong T M, Byers J D, Yee H i and Hicks J M 1993 Circuiar dichroism spectroscopy at interfaces—a surface second harmonic-generation study J. Phys. Chem. 97 1383-8... 

Rasing Th, Shen Y R, Kim M W, Valint P Jr and Bock J 1985 Orientation of surfactant molecules at a liquid-air interface measured by optical second-harmonic generation Phys. Rev. A 31 537-9... 

Sitzmann E V and Eisenthal K B 1988 Picosecond dynamics of a chemical-reaction at the air-water interface studied by surface second-harmonic generation J. Phys. Chem. 92 4579-80... 

Fischer P R, Daschbach J L and Richmond G L 1994 Surface second harmonic studies of Si(111 )/electroltye and Si(111 )/Si02/electrolyte interfaces Chem. Phys. Lett. 218 200-5... 

Zhao X L, Ong S W and Eisenthal K B 1993 Polarization of water-molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface Chem. Phys. Lett. 202 513-20... 

Spierings G, Koutsos V, Wierenga H A, Prins M W J, Abraham D and Rasing Th 1993 Optical second harmonic generation study of interface magnetism Surf. Sc/. 287-8 747-9... 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

Because the second harmonic response is sensitive to the polarizability of the interface, it is sensitive to the adsorption and desorption of surface species and is capable of quantifying surface species concentrations. Furthermore, SHG can be used to quantify surface order and determine surface symmetry by measuring the anisotropic polarization dependence of the second harmonic response. SHG can also be used to determine important molecular-level and electrochemical quantities such as molecular orientation and surface charge density. 

To ensure interface-selective detection of the Raman-pumped vibrational coherence, one more incident electric field is required. A fourth-order optical response is thereby generated. The requirement is fulfilled by observing the second harmonic (SH) light generated at the interface, instead of the transmitted fundamental light. 

Antoine, R., Tamburello-Luca, A. A., Hebert, P., Brevet, P. F. and Girault, H. H. (1998) Picosecond dynamics of Eosin B at the air/water interface by time-resolved second harmonic generation orientational randomization and rotational relaxation. Chem. Phys. Lett., 288, 138-146. 

Castro, A., Sitzmann, V., Zhang, D. and Eisenthal, K. B. (1991) Rotational relaxation at the air/water interface by time-resolved second harmonic generation./. Phys. Chem., 95, 6752-6753. 

Shi, X., Borguet, B., Tarnovsky, A. N. and Eisenthal, K. B. (1996) Ultrafast dynamics and structure at aqueous interfaces by second harmonic generation. Chem. Phys., 205, 167-178. 

Steinhurst, D. A. and Owrutsky, J. C. (2001) Second harmonic generation from oxazine dyes at the air/water interface. /. Phys. Chem., 105, 3062-3072. 

Hirose, Y, Yui, H. and Sawada, T. (2005) Second harmonic generation-based coherent vibrational spectroscopy for a liquid interface under the nonresonant pump condition. J. Phys. Chem. B, 109, 13063-13066. 

Akemann W, Friedrich KA, Linke U, Stimming U. 1998. The catal)4ic oxidation of carbon monoxide at the platinum/electrolyte interface investigated by optical second harmonic generation (SHG) Comparison of Pt(l 11) and Pt(997) electrode surfaces. Surf Sci 404 571-575. 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

FIG. 3 Solvation dynamics dependence of coumarin 314 probe molecule orientation at the air-water interface. Signals are generated with a 420 nm pump photon and probed by surface second harmonic signal with 840 nm (SH at 420), x Sx element. The normalized change in SH field is plotted vs. pump delay, r is derived from a single exponential fit to the data, (a) Pump polarization S (inplane), (b) Pump polarization P (out-of-plane). (Reprinted from Ref 24 with permission from the American Chemical Society.)... 

As discussed in Section II.A, Eisenthal and coworkers have studied the related problem of isomerization at liquid-solid interfaces. They used time-resolved second harmonic generation to investigate the barrierless photoisomerization of malachite green at the silica-aqueous interface using femtosecond time-resolved second harmonic generation [26]. They found that the photoisomerization reaction proceeded but was an order of magnitude slower at the water-silica interface than in bulk solution. 

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... 

To further corroborate these potentiometric results, we again used optical second harmonic generation (SHG). The SHG measurement system was essentially the same as used in the study described in Section II, except the laser beam was first reflected by a mirror and then totally reflected by the liquid-liquid interface. 

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