Modulation refractive-index

The gratings can also be made in situ by holographic irradiation as was demonstrated for low molecular stilbenes in a polystyrene matrix [197]. Here, the spatial modulation of gain dominates over the refractive index modulation in its contribution to optical feedback. The principles of holographic irradiation will be described in Section VIII, which discusses photosensitive materials. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

Xr is the readout wavelength and s the sample thickness. Q < 1 indicates thin, Q > 1 thick grating conditions,which are prevalent in the present case with Q 5. An extensive treatment of the diffraction theory of phase gratings has been given by Kogelnik [48]. All experiments discussed here have been conducted within the weak modulation depth limit, where the heterodyne or electric field diffraction efficiency Chet(t) is simply proportional to the refractive index modulation depth ... 

In Sect. II, a brief review of the fundamentals of the PR effect is provided. The energy transfer and light diffraction of the wave mixings in a PR medium is introduced, and the optical gain coefficient and diffraction efficiency are defined. The process of light-induced refractive index modulation is considered, and the main results of Kukhtarev s PR model (commonly used in inorganic and organic materials) are presented. 

We note from Eq. (15) that the space-charge field is shifted in space by n/2 with respect to the intensity pattern [Eq. (11)], which corresponds to a distant shift of Ag/4 in the x direction (Fig. 3d). This space-charge field induces an index volume grating via the Pockels effect (Fig. 3e). The refractive index including the fundamental component of refractive index modulation with magnitude of An can be written as... 

D. Nonlinear Optical Chromophores and Refractive Index Modulation... 

Both the amplitude and the direction of the total poling field are functions of position x. The dipole chromophore will align in response to the field Et(x), and a periodic refractive index modulation will be formed due to both molecular anisotropy and the Pockels effect [37], The contribution from molecular anisotropy is an important, sometimes even dominant, mechanism. 

This value corresponds to a refractive index modulation of An = 2.55 x 10-3. A response time of 42 ms is obtained at an applied field of 61.6 V/pm. The fast response time constant can be attributed to the relatively higher purity and fewer defects in this material, compared to those in polymeric counterparts. 

The data in Table 1 show large differences in sensitivity, refractive index modulation, and other properties, and if all the variations were included the differences would be much larger. However, each application has its own requirements, so the strategy that is most appropriate depends on all the properties in the table as well as others not listed. Work on all of these strategies will undoubtedly go forward, and continuing improvements in properties can be expected. 

UV light induced refractive index change has been observed in organic polysilane (PS)-silica and PS-titania hybrid thin films prepared by the sol-gel method.135 The magnitude of the refractive index change was found to be 0.16 for 50 wt % PS-silica and 0.18 for 50 wt % PS-titania hybrid thin films. It was demonstrated that the refractive index change of the hybrid thin films is due to the photodecomposition of PS. These findings indicate potential usefulness of the hybrid thin films as optical devices with refractive index modulated structures. 

The response time is one of the crucial factors in the manipulation of light waves by means of refractive index modulation by light-induced change in alignment of LCs. 

Figure 9A (a) The incoming pump beam kp is scattered at the scattering center S. (b) The scattered wave ks interferes with the propagating pump beam. A sinusoidal light interference pattern /(r) occurs, which is transferred into a refractive-index modulation An(r) via the pho-torefractive effect, (c) The pump beam is diffracted at the recorded refractive index modulation. Initially scattered light in direction of the polar axis is depleted and amplified in the opposite direction. 

Figure 9.12 Seed scattering at refractive index modulations induced by localized internal random fields via the electro-optic effect. The internal fields are also responsible for the formation of a rich ferroelectric domain structure. Here, a periodic sequence of domains with lengths A d is shown. Note, that the grating period of the refractive index modulation As is equal to the lengths of the ferroelectric domains.

TABLE 2. Refractive Index Modulations Required to Achieve 100% Scattering Efficiencies in Phase Holograms of Various Thicknesses, and the Corresponding Approximate Angular and Spectral Bandwidths of the Holograms. The Angular Bandwidths are Measured in the Air Outside the Hologram. 

Figure 18. Spatial frequency response of a photopolymer (37). The response is measured in terms of the efficiencies of holographic gratings. For the solid curve each efficiency was measured with the sample immersed in an index-matching fluid, so as to eliminate the effects of any modulation of the sample thickness. For the dashed curve the sample was in air. The curves show that for spatial frequencies >100 lines/mm the response of the material is almost entirely a refractive index modulation, while for <100 lines/mm there is little index modulation but there is a substantial thickness modulation.

These photopolymer systems were found to contain both volume refractive-index modulation and thickness modulation the latter constitutes a thin hologram. The volume hologram was shown to be present by removing one glass cover and covering the surface with the same monomer mixture. Upon uniform overall exposure, the... 

C for 20 min. The scattering efficiency (633 run) increased from far less than 1% to a maximum of 36%. The resolution was at least 2000 lines/mm. Because they were unable to measure the thicknesses of the films, the authors could not determine the refractive-index modulation. 

In order for the space charge field to produce a refractive index modulation and hence a grating or a hologram within the organic photorefractive material, the... 

Figure 22. Refractive index modulation as a function of the external angle of the probe beam with respect to the sample normal for the material given in [20], when the writing beams are arranged at a constant 55° and 75° to the sample normal. A) angular variation if reorientational birefringence is dominant B) angular variation if Pockels response were the primary source of index contrast.

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