Refractive index modulation, stability

Fig. 14 Typical growth and short-term stability of the refractive index modulation (writing with ss-polarization at different writing times). After 50 s, overexposure leads to a loss of contrast between illuminated and non-illuminated areas...

To compare different materials and preparation conditions, holographic gratings were always inscribed up to the first maximum of their diffraction efficiency rj which - as was shown in Sect. 3 - is connected with the refractive index modulation m. The refractive index modulation was calculated from the diffraction efficiency according to (1). Besides the value of the refractive index modulation, the stability of the grating is another important parameter which can be observed by monitoring the diffraction efficiency for a given time. A further parameter which is related to the refractive index modulation, and therefore to the diffraction efficiency, is the sensitivity. The sensitivity can be considered to be the inscribed refractive index modulation per time and is defined as... 

Besides the absolute value of the refractive index modulation, the stability is an important parameter, particularly for the development of a storage material. In Fig. 27 the temporal variation of the diffraction efficiency of volume gratings is plotted for films of a statistical copolymer 6 (azobenzene content 29.5 wt%) and... 

It was demonstrated above that both the value and the stability of the refractive index modulation are far better for block copolymers than for statistical copolymers with comparable azobenzene content. But the influence of the morphology and the domain size on the refractive index modulation remains unclear. To investigate this we chose a block copolymer series with three different morphologies lamellar, cylindrical, and spherical. In Fig. 28 the growth and decay for different morphologies of the block copolymers 10 (Afn 45,900gmoP1 azobenzene content 31 wt%) and 11 (Mn 42,400gmon1 azobenzene content 25 wt%), which are based on a PMMA-PHEMA backbone, are shown [66, 113]. Both block copolymers exhibit a similar... 

Photopolymerizable materials, typically multifiinctioiial acrylates, have mostly been studied as materials for HPDLC because of their advantages of large refractive index modulation, low cost, and easy fabrication and modification. However, these HPDLC materials still have significant drawbacks such as volume shrinkage, low reliability, and poor long-term stability. 

So far, I have outlined the deployment of our initial research with respect to the applications of liquid crystal PCs. They are described in detail in references [127-130]. Starting from there, the progress of research related to liquid crystal lasers has advanced remarkably. In the early studies, it was necessary to excite samples with large Q-switched pulse lasers, but the research results led to a cmistant improvement of materials and device structures that reduced lasing threshold and improved slope efficiency [131-136]. This made it possible to excite those PCs with semiconductor lasers. Furthermore, it has even come possible to excite with a CW laser also shown in [137]. In addition, control of the lasing wavelengths by modulation of the helical pitch (by temperature and electric field) in the early days of development led to effective refractive index modulation and fast stability control [138]. In 2000 it was quite a surprise that laser actimis occur even in a system containing fluctuation such as liquid crystals, but by now, it can be said that... 

From this example it is evident that the requirements for such stabilization are by no means trivial. Before we discuss possible experimental solutions, let us consider the causes of fluctuations or drifts in the resonator length d or the refractive index n. If we could reduce or even eliminate these causes, we would already be well on the way to achieving a stable laser frequency. We shall distinguish between long-term drifts of d and n, which are mainly caused by temperature drifts or slow pressure changes, and short-term fluctuations caused, for example, by acoustic vibrations of mirrors, by acoustic pressure waves that modulate the refractive index, or by fluctuations of the discharge in gas lasers or of the jet flow in dye lasers. 

The typical SBF texture shown in Fig. 7.28 develops when an a.c. electric field (15 V//zm, 10 Hz) is applied during cooling from the isotropic phase down to the smectic A phase and further to the ferroelectric phase. This stripe texture means a periodic modulation of the refractive index perpendicular to the lines, while the lines themselves are parallel to the rubbing direction. The spacing between lines increases with increasing cell gap. These stripes are similar to those observed by Beresnev et al. [128, 129]. They seem to suppress formation of the helix. Bistability transition is observed between two unwound states of the helix which are called, in [154], the field stabilized states. The thickness of the cell should be chosen as optimum dAn = 0.27 //) and an important requirement [103] gives... 

---