Optically induced change

Another effect sensitive to optically induced change is the electric field induced second harmonic generation (EFISH) experiment. In the presence of external electric static field, an isotropic medium becomes noncentrosymmet-rical and the second harmonic generation becomes possible ... 

An extremely useful feature of the third-order nonlinear optical response is the intensity-dependent refractive index, where the refractive index of the medium changes due to the interaction with a light beam. This optically-induced change in the refractive index is essential for all-optical switching applications. 

In Part II we discuss NMR measurements in intrinsic and doped a-Si H and include results on hydrogen, deuterium, silicon, boron, phosphorus, and fluorine. The topics discussed include local structural order, hydrogen evolution and diflusion, and spin-lattice relaxation. In Part III we present ESR results in a-Si, intrinsic and doped a-Si H, and some complicated alloy systems. The topics discussed include the characterization of the observed ESR responses, hydrogen diflusion and evolution, optically induced changes in the ESR, the influence of impurities, and spin- lattice relaxation. 

Both transient and metastable changes in the ESR are observed with optical excitation by light with energy greater than the band gap. Explanations analogous to those proposed to explain the ESR in doped samples have also been suggested to account for the optically induced changes. 

The radiation intensity 7 1 in first-order self-diffraction is uniquely connected with the optically induced change An c of refractive index, viz.,... 

Lailo, R., Lahderanta, E., Saisa, L., Kovacs, G. Y. Zsolt. G. (1990). Optically induced changes in the magnetic properties of the ceramic superconductor Lai.8Bao 2Cu04. Phys. Rev. B Condens. Mater. Phys., 42, 347-353. 

Optical detectors can routinely measure only intensities (proportional to the square of the electric field), whether of optical pulses, CW beams or quasi-CW beams the latter signifying conditions where the pulse train has an interval between pulses which is much shorter than the response time of the detector. It is clear that experiments must be designed in such a way that pump-induced changes in the sample cause changes in the intensify of the probe pulse or beam. It may happen, for example, that the absorjDtion coefficient of the sample is affected by the pump pulse. In other words, due to the pump pulse the transparency of the sample becomes larger or smaller compared with the unperturbed sample. Let us stress that even when the optical density (OD) of the sample is large, let us say OD 1, and the pump-induced change is relatively weak, say 10 , it is the latter that carries positive infonnation. 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Theoretical analyses (75-77) of the matrix-induced changes in the optical spectra of isolated, noble-metal atoms have also been made. The spectra were studied in Ar, Kr, and Xe, and showed a pronounced, reversible-energy shift of the peaks with temperature. The authors discussed the matrix influence in terms of level shift-differences, as well as spin-orbit coupling and crystal-field effects. They concluded that an increase in the matrix temperature enhances the electronic perturbation of the entrapped atom, in contrast to earlier prejudices that the temperature dilation of the surrounding cage moves the properties of the atomic guest towards those of the free atom. 

Lambeck P.V., Hoekstra H.J.W.M., VanLith J., Van Elzakker G., Two novel integrated optical sensor types for measuring chemical concentrations, based on chemically induced changes of modal field profiles, J. Nonlinear Opt Phys. Mat 2004 13 (2) 209-217. 

The Stark effect is electric-field-induced change in optical transition energy of materials, and the effect is observed as spectral change in absorption due to the energy shift. In the linear Stark effect, energy shift of optical transition Av in proportion to the electric field F is presented by... 

Since the band structure which develops upon doping induces changes not only in the conductivity but also in the optical absorption (see Fig. 9.8), conducting polymers may be exploited for electrochromic displays, which are optical devices with marked colour transitions. An example is illustrated diagramatically in Fig. 9.18. 

Evoked potentials with latencies below 100 ms are used clinically for functional testing of the optic, auditory and various somatosensory nerve pathways they are not influenced by factors such as motivation and tiredness, including drug-induced changes of alertness, and are considered to be uninteresting for psychological questions. 

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