Second harmonic generation DC-induced

The value of 3 and its dispersion for a molecule, or polymer chain, can be experimentally determined by DC induced second harmonic generation (DCSHG) measurements of liquid solutions -1 2). The experimental arrangement requiring an applied DC field E° to remove the natural center of inversion symmetry of the solution is described in Figure 4. The second harmonic polarization of the solution is expressed as... 

Singer, K.D. and Garito, A.F. (1981) Measurements of molecular second order optical susceptibilities using dc induced second harmonic generation. 

As was proven later by Bishop [19], the coefficient A in the expansion (73) is the same for all optical processes. If the expansion (73) is extended to fourth-order [4,19] by adding the term the coefficient B is the same for the dc-Kerr effect and for electric field induced second-harmonic generation, but other fourth powers of the frequencies than are in general needed to represent the frequency-dependence of 7 with process-independent dispersion coefficients [19]. Bishop and De Kee [20] proposed recently for the all-diagonal components yaaaa the expansion... 

The Electric-Field-Induced Second-Harmonic Generation (EFISHG) technique makes it possible to measure the molecular hyperpolarizability, p, on liquids or molecular solutions. The centrosymmetry of tire solution is broken by applying a DC electric field to induce an average orientation of the molecules due to interactions of the permanent dipoles of the molecules and the electric field. The energy of a dipole with a permanent dipole fi in an electric field E is given by ... 

Y(-2 , , 0) dc-Second Harmonic Generation dc-SHG Electric Field Induced Second Harmonic Generation (EFISH)... 

It is possible to measure p in solution using DC electric field-induced second harmonic generation (EFISH). This gives direct information on the propensity for nonlinear polarisation of a given molecule, and permits reliable comparisons between molecular structure and electronic effects in solution. The determination of large values of p, however, does not mean that the compound, when crystalline, will exhibit significant values of... 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2 > = > - - third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect co = co + 0), or the quadratic electrooptic (Kerr) effect ( > = > - - 0 -I- 0). These electrooptic effects are discussed extensively in the article Electrooptical Applications (qv). EFISH is... 

The static and dynamic linear responses, a(0 0) and a( co co), correspond to the so-called static and dynamic polarizabilities, respectively. At second order in the fields, the responses are named first hyperpolarizabilities whereas second hyperpolarizabilities correspond to the third-order responses. Different phenomena can be distinguished as a function of the combination of optical frequencies. So, p(0 0,0), p(—co co,0), p(0 o), — ea), and p(— 2co co,co) are associated with the static, dc-Pockels (dc-P), optical rectification (OR), and second harmonic generation (SHG) processes whereas y(0 0,0,0), y(- ( ( ,0,0), y( 2co co,( ,0), y( co co, — ca, ), and y(— 3 , , ) describe the static, dc-Kerr, electric-field-induced second harmonic generation (EFISHG), degenerate four-wave mixing (DFWM),... 

Bishop, D. M. and Sauer, S. P. A. (1997). Calculation, with the inclusion of vibrational corrections, of the DC-electric-field induced second-harmonic-generation hyperpolarizability of methane. J. Chem. Phys., 107, 8502-8509. 

Together with PAP, photo-induced depoling (PID) is another interesting phenomenon at the interface of photochemistry and organic nonlinear optics. Indeed, PID of poled polymers occurs when NLO chromophores, which are oriented in a polar manner, undergo photoisomerization without applied dc field. The chromophores lose their initial polar orientation after photoisomerization and reorientation in azimuthal directions around the initial polar axis, thereby erasing FID has been observed both by photo-induced destruction of EO Pockels and by second harmonic generation. The first published PID experiments have been reported for DRl in PMMA, and the theory of PID is discussed in detail in reference 25. 

S. Kielich. DC electric field-induced optical second harmonic generation by interacting multipolar molecules. Chem. Phys. Lett., 2 569-572 (1968). 

P( P(-o> w,0) P(0 -fa>,w) Y( - Y(-2(i) (i>,tD,0) Y(-o) (i>,0,0) Second harmonic generation (SHG) Electrooptic Pockels effect Optical rectification Third harmonic generation DC electric-field-induced SHG Intensity-dependent refractive index Optical Kerr effect Coherent anti-Stokes Raman pSHG pEOPE pOR. yTHG. EFISH oj DC-SHG. JlDRI or. yOKE. yCARS... 

Second harmonic generation from surfaces has been shown to be a powerful probe of surface interactions [97], which can yield information about the structure and alignment of liquid crystal molecules on surfaces and at interfaces [98], The role of the interface is similar to that of the dc electric field in electric-field-induced SHG, that is, to provide a symmetry-breaking field the direction of which is defined by the surface normal. 

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