Resonant SHG

Fig. 4.19 Resonant SHG spectra of the porphyrin ZnTPPC at the polarized water/DCE interface at -0.15 V (full circles),...

The NLO performances of these polymer are interesting. The off-resonance SHG activity of polyurethane PUY3 is quite fair, while the time stability can be considered very good, as shown in Fig. 2.21. 

The dominant term in the surface spectroscopy is the one of the second order, and the theory commonly used for the interpretation of SHG and SFG surface spectra is based on the electric dipole approximation. Within this approximation, the effect of optical magnetic fields and multipoles are neglected. This assumption can be at stake in some scenarios, especially for the interpretation of non-resonant SHG phenomena. Quadrupolar contributions are much weaker than the dipolar ones, but they scale with the number density and can amount to noticeable levels which may interfere with the surface term. There is no general solution to this problem, and it is often not known a priori in SHG and SFG studies whether interfacial contribution is dominant over that of the bulk in an interface system.Furthermore, it is commonly assumed that the dipoles induced in the molecules are solely due to the external laser field and that contribution from neighbouring molecules can be neglected. The local fields are hard to capture at a molecular level and require further theoretical work. In this framework, the simplest description of SFG is given by... 

Fe(CN)g ], azide (N ), and thiocyanate (SCN ) — and have been studied by resonant SHG (see Fig. 8). In the same spectral region, the water hyperpolarisability is non-resonant and contributes to the signal via its concentration-dependent non-resonant susceptibility term. 

Fig. 8. Resonant SHG of NaJ and KJ. The high concentration increase in the resonant SHG intensity is due to iodide anions at the interface forming a dense ionic double layer with the cations. Due to the relatively small hyperpolarisability of iodide, it is not possible to determine whether the iodide anions are enhanced at the interface or whether the surfece mole fraction increases linearly with the bulk. (Reprinted with permission from Ref 76. CopyTight 2006 American Physical Society.)...

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Figure Bl.22.7. Left resonant seeond-hannonie generation (SHG) speetnimfrom rhodamine 6G. The inset displays the resonant eleetronie transition indueed by tire two-photon absorption proeess at a wavelength of approximately 350 mn. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fiised quartz, mapped by reeording the SHG signal as a fiinetion of position in the film [55], SHG ean be used not only for the eharaeterization of eleetronie transitions within a given substanee, but also as a mieroseopy tool.

The hyperpolarizability tensor is obtained in a way similar to the case of SHG. However, the selection rules for an SFG resonance at the IR frequency implies that the vibrational mode is both IR and Raman active, as the SF hyperpolarizability tensor elements involve both an IR absorption and a Raman-anti-Stokes cross-section. Conversely, the DFG hyperpolarizability tensor elements involve an IR absorption and a Raman-Stokes cross-section. The hyperpolarizability tensor elements can be written in a rather compact form involving several vibrational excitations as [117] ... 

We anticipate that, regardless of the detuning from an optical resonance used, the parameters f, g, and h will always be determinable from measurements of SHG as a function of the rotation angle of a quarter waveplate used to set the polarization state of the incident fundamental light. The amount of SHG-CD can be calculated from the parameters or, of course, read directly... 

SHG can be extended to a true spectroscopy by varying the frequency of the incident laser beam. Electronic structures such as surface states and surface plasmon resonances show up in the frequency dependence of the signal [14]. 

In LB films not only the interaction of chromophores but also their orientation can be controlled at the molecular level. Molecular orientation of chromophores has been determined by several methods including polarized UV/vis or IR absorption, second harmonic generation (SHG), Electron Spin Resonance (ESR), or resonance Raman scattering. We have measured the incident angle and polarization angle dependencies of polarized UV/vis absorption to study the molecular orientation of alloxazine, porphyrin, and carbazolyl chromophores, or 4,4 -bipyridinium radical cations in LB films[3-12]. Usually in-plane components of transition dipoles of chromophores are... 

In this chapter we explore several aspects of interferometric nonlinear microscopy. Our discussion is limited to methods that employ narrowband laser excitation i.e., interferences in the spectral domain are beyond the scope of this chapter. Phase-controlled spectral interferometry has been used extensively in broadband CARS microspectroscopy (Cui et al. 2006 Dudovich et al. 2002 Kee et al. 2006 Lim et al. 2005 Marks and Boppart 2004 Oron et al. 2003 Vacano et al. 2006), in addition to several applications in SHG (Tang et al. 2006) and two-photon excited fluorescence microscopy (Ando et al. 2002 Chuntonov et al. 2008 Dudovich et al. 2001 Tang et al. 2006). Here, we focus on interferences in the temporal and spatial domains for the purpose of generating new contrast mechanisms in the nonlinear imaging microscope. Special emphasis is given to the CARS technique, because it is sensitive to the phase response of the sample caused by the presence of spectroscopic resonances. 

Following these studies were a series of works which examined the SH response from polycrystalline and crystalline silver electrode surfaces biased within the ideally polarizable region [42-44, 54-64], These studies showed the sensitivity of SHG to adsorption of ions to the electrode surface. More importantly, they showed that the sensitivity arose from the changes in the optical properties of the electrode surface itself, and not from the optical properties of the ionic and simple molecular species at the surface. Conversely, the SH response from an adsorbate, such as a dye molecule, can become an important factor if either the fundamental or SH photon energy is resonant with electronic transitions in the adsorbate [65, 66]. For more details on these SH studies of both ORC effects and adsorption phenomena on polycrystalline substrates, the reader is referred to Refs. 7 and 9. 

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