VOLUME87, NUMBER24 P H Y S I C A L R E V I E W L E T T E R S 10 DECEMBER2001
Comment on “Photoalignment of Liquid Crystals by Liquid Crystals”
Recently, Reznikov et al. [1] reported on the light-induced alignment of nematic 4-40-phentylcyanobiphenyl (5CB) liquid crystal (LC). From the irradiated 5CB layer with polarized UV light, they measured the in-plane opti-cal anisotropy Dn of⬃0.1 and the nearly isotropic second-harmonic generation (SHG) signals. They suggested a planar alignment of additional 5CB molecules on a nearly homeotropically aligned first 5CB layer on the substrate. However, their interpretation lacks the consideration of axial ordering in the SHG analysis.
In this Comment, we suggest a more reasonable align-ment of the 5CB molecules from the simultaneous analysis of Dn and SHG results. The irradiated 5CB layer was considered as C2y symmetry, where the nonlinear optical
coefficients at surface coordinates (x, y, z) are xzzz 苷 N具 f1典bjjj, xzyy 苷 xyzy 苷 N具 f2典bjjj, and xzxx 苷
xxzx 苷 N具 f3典bjjjwhere z is the surface normal, x is the
easy axis, and bjjj is the dominant hyperpolarizability
along the long molecular axis j. Here, f1苷 cos3u, f2苷
共cosu 2 cos3u兲 sin2f, and f
3苷 共cosu 2 cos3u兲 cos2f.
For the unrestricted analysis, we used the modified maxi-mum entropy method [2,3] with undetermined multipliers lis and the constraint functions fis together with another
axial constraint function of f4 苷 共3 sin2ucos2f 2 1兲兾2.
Dn was calculated from an axial order parameter [2] (Sx 苷 具 f4典). In the simulation using lis as variables, the
parameters used were the incident angle of 35±and layer thickness of 1 nm with the appropriate refractive index and local fields [2,3], which also depend on lis. The best
simu-lated results for Dn苷 0.10 and SHG intensities [Fig. 1(a)] were achieved at l1 苷 4.71, l2苷 21.90, l3 苷 20.86,
l4 苷 5.79, which give xzzz:xzxx:xzyy 苷 1.00:0.04:0.03,
and Sx 苷 0.76, Sy苷 20.39, Sz 苷 20.37. The
ob-tained distribution function f共u, f兲 of 5CB molecules on substrate [Fig. 1(b)] clearly indicates that the 5CB molecules orient along the easy axis with high in-plane anisotropy and small polar ordering along the surface normal direction (具cosu典 ⬃ 0.026, 具cos3u典 ⬃ 0.024).
With具cos3u典 ⬃ 0.024 and the given ratios of xs, we can deduce the absolute x values for bjjj ⬃ 1 3 10229 esu
and N ⬃ 5 3 1014 cm22; i.e., xzzz ⬃ 1.2 3 10216 esu兾
cm2and xzyy⬃ 0.4 3 10217esu兾cm2. These values are in
good agreement with the estimated values of xzzz共5CB兲 ⬃
1.0 3 10216esu兾cm2 and xzyy共5CB兲 ⬃ 1.4 3 10217 esu兾
cm2, which were obtained based on the literature value of the bare fused quartz surface (xyzy艐 2.7 3
10217 esu兾cm2 [4,5]). Thus molecules at the first layer align almost parallel to the surface along the direction per-pendicular to the irradiated UV polarization. The predom-inantly in-plane alignment of molecules is in strong con-trast to the homeotropic alignment in the first 5CB layer suggested by Reznikov et al. [1].
FIG. 1. The simulated angular dependence of the SHG intensities (a) and three-dimensional plot of f共u, f兲 (b).
This work was supported by Brain Korea 21 project and Korea Research Foundations (KRF-2001-015-DP0175). Byoungchoo Park,1J. W. Wu,1and Hideo Takezoe2
1Department of Physics
Ewha Womans University Seoul 120-750, Korea
2Department of Organic and Polymeric Materials
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 26 September 2000; published 27 November 2001 DOI: 10.1103/PhysRevLett.87.249601
PACS numbers: 61.30.Gd
[1] Y. Reznikov et al., Phys. Rev. Lett. 84,1930 (2000). [2] J.-G. Yoo et al., J. Appl. Phys. 84,4079 (1998). [3] B. Park et al., Phys. Rev. E 57,6717 (1998).
[4] P. Guyot-Sionnest and Y. R. Shen, Phys. Rev. B 35,4420 (1987).
[5] T. F. Heinz, H. W. K. Tom, and Y. R. Shen, Phys. Rev. A
28, 1883 (1983).
