tailieunhanh - composite 2012 Part 2

Tham khảo tài liệu 'composite 2012 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Nematic Liquid Crystal Confined in Electrochemically Etched Porous Silicon Optical Characterization and Applications in Photonics 13 a b Fig. 6. a Temperature dependence of the microcavity resonance shift. b Resonant wavelength of the microcavity versus the surface anchoring strength W The tuning range is defined by the difference between the nLC values at the room temperature and at the clearing point. The latter value is well known Li et al. 2005 while to predict the former one we have to simulate the effective refractive index of E7 taking into account the nematic director configuration inside the pores. Comparing the spectra simulated for the different director fields with the experimental one helps to define the actual LC director configuration in the investigated PSi film. The director field in the ER configuration was calculated using the Frank s free energy approach. We have used the following constants for E7 Crawford et al 1992 Leonard et al. 2000 Tkachenko et al 2008 K11 pN K33 pN K24 pN and the dispersion curves for ordinary and extraordinary indices from Abbate et al. 2007 . According to Crawford et al 1992 Leonard et al. 2000 the surface anchoring strength W for the E7 in supramicrometer silicon pores is estimated to be 10-5 J m2. Because the magnitude of W in mesopores is unknown we took it variable in our computations. The dependence of the microcavity resonance wavelength on the molecular anchoring strength is shown in Fig. 6 b . The curves computed for different values of the pore radius are shown by the thick solid lines. The value of 25 nm is the averaged pore radius while the values of 5 and 40 nm are the minimum and maximum pore radii occurred in our experimental PSi films. Experimental position of the resonance peak at 27 C and the error bar of the measurements are presented by the horizontal thin solid and dashed lines respectively. As may be seen from the figure the simulated curves approach the experimental resonant .