Temperature Dependence of the Optical Response

The spectra of the clusters from n = 4 to 16 show rather sharp features at low temperatures, which broaden with increasing temperature. However, this broadening is not accompanied by an overall shift the mean of the spectrum is nearly temperature-independent, as discussed below in Section 5.4.3.4. Also, there is no qualitative change between 35 and 105 K, i.e. the number of peaks in the spectra remains constant. Contrary to this, large changes are seen between 105 K and the highest possible temperature. For NaQ , the low-temperature double peak becomes one broad peak at higher temperature, and for Nan six well-resolved lines transform into two broad humps. 

No special feature can be seen in the optical spectra which could be interpreted as a melting transition. There are many changes of the spectra, but all transitions happen very gradually with temperature. This does not imply that no melting occurs. The transition might be just too broad in these small clusters to be observable, as discussed below in Section 5.7.2. 

---

Figure 9.41. (a) The optical transmittance measured at 40 C and at different frequencies for polyimide SP550 (600 A) with and without the insulating layer, (b) The temperature-dependence of the optical response measured at 1 Hz frequency for the cells with and without the insulating layer [103]. 

Fig. 17 Temperature dependence of electro-optical response time for the polymer-stabilized blue phases (polymer fraction a = 6.3, 10.5, 15.0 mol %) in the rise process (A) and decay process (B) [46]...

E. Jakeman and E. P. Raynes, Electro-optic response times of liquid crystals, Phys. Lett. A39, 69 (1972). H. Imura and K. Okano, Temperature dependence of the viscosity coefficients of liquid crystals, Jpn. J. Appl Phys. 11, 1440 (1972). 

Recently it has been observed that the thermal grating response of a nematic is unusually large. The property exploited is the temperature dependence of the refractive index. This effect has considerable history, where prior experiments used a solvent containing a dye, e.g., rhodamine 6G in ethanol. The optical energy is absorbed by the dye and then transferred to the solvent, resulting in a thermal grating and consequent index grating. The time constant of this process in ordinary liquids is of nanosecond order. 

The scattering effects observed during the deformation of the ferroelectric helix have not yet been satisfactorily investigated [115]. For instance, one should explain the correlation between temperature dependence of the helix pitch and intensity of the scattered light [113], as well as the effect of FLC physical parameters on the response times and hysteresis behavior of transmission-voltage characteristics. Moreover, these effects have not been studied in commercial FLC mixtures operating at room temperature. Nevertheless, these electrooptical modes might be useful for applications in nonpolaroid FLC displays for realization of the optical memory, etc. 

Figure 1.30 shows temperature T dependence of electro-optical response time t (the time in which a 10-90 % change takes place for the electro-optical response when a square wave electric field = 10 MV/m is applied) for three different low-molar-mass FLCs that consist of three different chiral end groups respectively, and their corresponding FLCPs that consist of a common polyoxyethylene main chain. 

The responsivity and g-r noise may be analyzed to obtain background photon flux and temperature dependence of responsivity, noise, and detectivity. Typically, n > p, and both ate determined by shallow impurity levels. The minority carrier density is the sum of thermal and optical contributions. 

Figure 10 shows the dependency of the modulation index on the measurement gas cell concentration (%v/v), assuming dilution by nitrogen gas, at a pressure of 1 Bar and a temperature of 20 °C. This shows that there is a significant non-linearity in the modulation index response, particularly at higher CO2 gas concentrations in the measurement cell. As before, an optical filter bandwidth of 100 nm was assumed. 

---