12 January 1994 Photoinduced phenomena of anisotropy and gyrotropy
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The elementary act of the interaction of light with a material medium is vectorial by its nature. This vectority has a double character: on the one hand the quantum of polarized radiation is anisotropic and has spin, on the other hand the material centre interacting with radiation is anisotropic and gyrotropic. Ultimately all the veriety of phenomena of interaction of light with the medium must lead to the reaction product that Is also anisotropic and gyrotropic by structure. Such state of affairs is the manifestation of initial properties of the electromagnetic field and the substance. While using polarized light fluxes of sufficient transversal length the anisotropy and gyrotropy induced by light turn out to be macroscopically observable. Light—sensitive media reacting to the action of polarized light by anisotropy appearing in them have been known since 1919 when F.Veigert discovered the phenomenon of photoinduced anisotropy (photoanisotropy) in photochemical systems of silver halo-genide . In 1928 the similar phenomenon of the appearance of gyrotropy (photo— gyrotropy) under the action of actinic radiation of circular polarization was discovered by Zocher and Coper2 . Later in 1956 Bakingem predicted the light induced analogue of Kerr effect that means as follows: under the action of the strong pulse of linearly polarized light in nonlinear—optical medium there appears anisotropy that disappears when the action stops3 . The predicted phenomenon was soon confirmed by experiment . The nonlinear optics gives every possibility for various manifestations of light induced anisotropy in photophysical processes. Anisotropy in nonlinear—optical phenomena appears as a result of several independent mechanisms making different contribution to the resulting effect. One of the main reasons is the appearance of nonlinear polarization as a result of the orientation of anisotropic molecules. Certainly to connect the anisotropy induced by light with the real turn orientation of the molecule is wrong as at present no mechanisms of turn orientation in the elementary act of absorption of quantum of polarized radiation are known. When we have enough densities of inducing polarized field the greater part of molecular dipoles having orientation parallel to the field turns out to be in an excited state whereas the orthogonally oriented dipoles are not excited. It, creates the anisotropy of the space distribution of excited dipoles. As the dipo— les parallel to the field unlike orthogonal ones turn out to be as if they were "occupied" by the pump field, the medium when it is optically probed turns out to be pronounced anisotropic. The second reason of the appearance of anisotropy under the action of polarized light is electrostriction. In very strong light fields the defor— mation of an electronic cloud is possible and then the reason of anisotropy proves to be purely an electronic effect. Finally the reason for the anisotropic change of the characteristics of the medium can serve an electrocaloric effect connected with the heating of the medium in the process of polarized pump wave propagation. The little excitation time and quick return to the initial state are characteristic of photo— chemical processes (the electronic effect develops for 1012 — 1O1 sec, the orientation effect — for 1O— 10L2 sec, electrostriction — for 1O — 1O sec, the electrocaloric effect — of order I sec). It is also true of the anisotropy induced by polarized light that quickly disappears in the course of competing processes when the inducing radiation is removed. Unlike photophysical processes, photochemical processes lead to changes that are revealed after the light excitation stops. From the most general considerations It follows that in order to have photochemical reaction the energy of the absorbing quantum must be higher than the energy of the dissociation of chemical bonds of a molecule. In this case photochemical decomposition or conformational transformations begin to prevail over the radiating deactivation of the molecule. In some cases the role of the quantum of the polarized radiation comes to the overcoming of activatory barrier and to the formation of primary products that start a chain. Both reversible and Irreversible varied photochemical reactions In principle must always take place with induction of photoanisotropy and photogyrotropy expressed in a different degree. It is evident all the more that the photochemical transformation of the molecule leads to the change of its absorbing ability and the selective action of polarized light only on molecules, which are oriented in a definite way, conditions the induc— tion of the macroanisotropy of the medium as a whole. The above mentioned equally refers to the appearance of gyrotropy in the medium when the circularly polarized actinic light causes in an Initially isotropic and nongyrotropic medium the induction of circular dichroism and circular birefringence (optical activity) as a result of the selective absorption of light by molecules according to rotary orientations
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Shermazan D. Kahkichashvily, Shermazan D. Kahkichashvily, "Photoinduced phenomena of anisotropy and gyrotropy", Proc. SPIE 2043, Holographic Imaging and Materials, (12 January 1994); doi: 10.1117/12.165580; https://doi.org/10.1117/12.165580

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