Nonlinear optics electric-field-induced technique

Electric Field Induced Second-Harmonic Generation. An essential aspect of the development of materials for second-order nonlinear optics is the determination of the p tensor components. The technique that has been developed to accomplish this is called electric field induced second harmonic generation (EFISH) (13,14). 

The most widely employed material characterization techniques in third-order nonlinear optics are third-harmonic generation (THG) [21], degenerate four wave-mixing (DFWM) [22], Z-scan [6], and optical limiting by direct two-photon absorption (TPA) and fluorescence spectroscopy induced by TPA [23]. All of them will be discussed in the following. Further measurement techniques such as electric-field induced second-harmonic generation (EFISH) [24], optical Kerr... 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

The principles of nonlinear optics and the main techniques used to evaluate the second-order NLO properties are briefly presented here. Major details can be found in more specialised reviews and books. At the molecular level, the interaction between polarisable electron density and the alternating electric field of the laser light beam (E) induces a polarisation response (Afi) that can be expressed following Equation 1.1 ... 

P -Values can be measured by several techniques, including electric field induced second harmonic generation and solvatochromic measurements of absorption and fluorescence Moreover, quantum mechanical calculations can be used to estimate these nonlinearities For a more detailed description of the physical backgrounds of nonlinear optics we like to refer to excellent reviews and books< ° >. 

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, 2o) = co + co + 0) in a quasi-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 -I- 0), or the quadratic electrooptic (Kerr) effect ( = -I- 0 -I- 0). EFISH is discussed in this article, however, for the important role that it has played in the characterization of nonlinear optical materials for other applications. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

The nonlinear interaction of light with matter is useful both as an optical method for generating new radiation fields and as a spectroscopic means for probing the quantum-mechanical structure of molecules [1-5]. Light-matter interactions can be formally classified [5,6] as either active or passive processes and for electric field based interactions with ordinary molecules (electric dipole approximation), both may be described in terms of the familiar nonlinear electrical susceptibilities. The nonlinear electrical susceptibility represents the material response to incident CW radiation and its microscopic quantum-mechanical formalism can be found directly by diagrammatic techniques based on the perturbative density matrix approach including dephasing effects in their fast-modulation limit [7]. Since time-independent (DC) fields can only induce a... 

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