Thermo-optic effect

The sign and magnitude of the resulting thermal nonlinear refraction coefficient (which is, actually, a pure linear effect [219]) depend on the thermo-optical coefficient 9 /9r of the material. This coefficient has sometimes been assimilated to the one of the surrounding host only [132, 218], but we have recently shown that, due to local field enhancement at the SPR, they can be very different - even for weakly concentrated media exactly as for the pure electronic nonlinear properties as demonstrated in Section 3.2.4. Moreover, an absorptive thermo-optical effect, which is always disregarded in the literature, can occur parallel to the refractive one. These conclusions will be published soon. 

Falconieri, M. Thermo-optical effects in Z-scan measurements using high-repetition-rate lasers. J. Opt. A Pure Appl. Opt. 1, 662-667 (1999)... 

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

E P Raynes, Electro-Optic and Thermo-Optic Effects in Liquid Crystals. Phi1.Trans.R.Soc.Lond. A309 167 (1983). 

Thermo-optic effects in liquid crystals were investigated extensively from the point of view of device applications. Nematics are particularly suitable for such purposes because in this phase the refractive indices have an unusually large temperature dependence, especially near the nematic-isotropic phase transition. This strong temperature dependence is connected to the corresponding variation of the order parameter, 5. The order parameter is a measure of the degree to which the molecules are oriented along the director. The birefringence, is... 

In samples prepared later on with better optical quality W. Schmid observed a different behaviour [120]. The damping became stronger with lower repetition rates, i.e. higher peak intensity. No thermo-optic effect was observed. This can be well explained by the advances in crystal preparation solvent inclusions were reduced and the crystal quality improved, resulting in a lower absorption coefficient at the same, near-resonant wavelength (e.g. 720 nm). So, the absorbed heat load and the thermo-optical coefficients were reduced concurrently. 

After successful growth of thin (a few micrometers) p-TS6 crystals, it was possible to study signals at near-resonant wavelengths without an interfering thermo-optical effect... 

R. A. Soref, Thermo-Optic Effects in Nematic-Cholesteric Mixtures, J. Appl. Phys., 41, p. 3022(1970). 

Thermo-optic and Electro-thermo-optic Effects... 

Fig. 6.20 The thermo-optic effect of MBBA. The temperature range for the nematic (liquid crystal) phase is 21-48 °C. The light transmission variations are shown for the temperature change of a part of the liquid crystal (LC) cell. The temperature is changed by the laser beam irradiatioiL The (SS) and (TS) do not occur to the cell for the temperature change of a whole area [84,85]. As for the transmission reference, the transmission through a cell in an isotropic phase is taken as 100 %. This reference is applied for results hereafter. 05 opaque solid state, M meso (nematic)-phase, 55 semitransparent solid state, I isotropic (liquid) phase, TS transparent solid state...

Fig. 6.21 The thermo-optic effect of nematic-cholesteric LC mixture MBBAiEBBA ...

Fig. 6.23 The thermo-optic effect of smectic-cholesteric LC mixture. The transmission variations are caused by mixing of cholesteric LC. By the laser beam irradiation on to a portion of the cell, the light scattering is observed in the smectic phase, and it rranains in the honeycomb-H focal conic texture. It can be applied to the display of static figures. Sm smectic phase, Ch cholesteric phase. Is isotropic phase...

Fig. 6.24 The electro-thenno-optic effect of nematic-cholesteric LC mixture MBBAiEBBA CN = 45 45 10. The material here is same with the thermo-optic effect [90]. The transmission variations are enhanced by the electric field application. The scattering centers in the nematic phase are turned to a clear state with the electric field application and then the irradiated portion becomes clear. Thus, the dynamic figures can be displayed...

Fig. 6.25 The electro-thermo-optic effect of smectic-cholesteric mixture COB CN = 90 10. The light transmission is greatly decreased by the electric field application [91]. The light scattering centers are created in the smectic phase by the irradiation on to a part of the LC cell, and they are erased by the electric field application and become clear. This characteristic can be applied to a dynamic display. M mesophase, I isotropic phase...

Fig. 6.26 The operation principles of laser addressed display used the thermo-optic and the electro-thermo-optic effects [92], In the figure, the operation by a nematic LC can be expected same by a smectic LC. The LC cell used as a light valve can be applied to a large-scale projection display...

Fig. 6.28 The simultaneous display of static and dynamic figures by the thermo-optic and the electro-thermo-optic effects [94], The liquid crystal used is COB CN = 90 10. The static figures are displayed by the light scattering centers, and, holding them, the dynamic figures are displayed with the electric field application. Two photographs show dynamic display, as the right photograph was taken at 1/4 s after the left photograph...

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