Optically-tunable Surface Plasmon Resonance on dye-doped liquid crystal layer under Kretschmann geometry and its applications

• Main Authors: Chih-Han Chi, 齊志翰
• Other Authors: Tsung-Hsien Lin
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/99463085415683990570

碩士 === 國立中山大學 === 光電工程學系研究所 === 101 === Surface plasmon resonance (SPR) is very sensitive to the refractive index of the layer on top of its metal surface. Only TM polarized light can excite surface plasmon mode. The resonance angle of surface plasmon polariton (SPP) is very strict and only when the k-vector of incident light matches the k-vector of SPP mode can SPP waves be excited. With measurements of the coupling angle of SPP wave, we can measure very slight refractive index change on top of the metal film. A 50 nm thin Au film was evaporated on the flat surface of the prism which is suitable for Kretschmann geometry experiment. On top of Au film we put a liquid crystal (LC) layer. When the orientation of liquid crystal changes, the refractive index on the Au surface also change, which can modulate the resonance angle of SPR. In the reserch, a little azo-dye was doped in liquid crystal material to be photo-tunable. This study explored the mixture of various nematic liquid crystal and different photo-induced dye concentrates. We used two methods of changing the refractive index to modulate the resonance angle of SPR. The first one was to add a photo alignment layer. The polarization of pumping light affects the orientation of Methyl Red molecules and then the material changes the orientation of liquid crystal molecules which make the refractive index n_o into n_eff. Due to the attachment effect of Methyl Red on the Au surface, it is difficult to repeat the LC rotation. The second method was to use 4MAB and azobenzene liquid crystal 1205 with photoisomerization. Pumping light made the clear point of LC lower and the liquid crystal alignment became random. The refractive index could be adjusted between n_o and n_iso. However, the concentration of 4MAB would influence the birefringence of liquid crystal. It lowered the difference between the two resonance angles. In contrast, 1205 has refractive anisotropy, and the smear-out effect on the LC refractive index is cleared out. By using different nematic liquid crystals and different concentrations of azo molecules, we found that when pure 1205 was used, the max displacement of resonance angle was 2.31° and the variation of wavelength was 49nm (633 nm → 682 nm) in terms of photo modulate. In addition, azobezene liquid crystal also had the advantage of bistable and fast switch, which could be applied to optical modulations and optical filter design.