全像光柵在液晶及高分子材料的研究 黃宣瑜 國立中山大學物理系 非線性光學實驗室 奈米科技研發中心 博士後研究

Experiment I: Temperature and polarization dependence of transient gratings in azo-dye- doped liquid crystals II: The enhancement of diffraction in liquid crystals on dye-doped polymer films

Introduction 主要特徵： 1. 分子長軸方向有序 2. 重心位置無序 (director axis) 向列相 (nematic) 液晶分子的排列圖及方位秩序 S(θ) =

液晶的正單光軸晶體光學雙折射性 e n o n e n o n )(  eff n  )(  n 

Dielectric anisotropy of the liquid crystals E LC Δε=ε ∥ -ε ⊥ 若 Δ ε >0 u E minimum 若 Δ ε <0 u E minimum

5CB 液晶折射率隨溫度的變化

5CB 液晶方位秩序隨溫度的變化

Dye-Induced Torque in Dye-Doped Nematics Janossy model (“Positive” torque ) Excited-state of dye molecules Ground-state of dye molecules n n

Gibbons model (“Negative” torque) E

Photoisomerization Effect Induced LC Reorientation Upon absorbing light, azo-dye molecules may undergo a photo-induced conformational change, which is called photoisomerization effect.

Laser induced grating  p I1I1 I2I2  x I x z y  1 k  2 k  x z y  x z y  k  k  k  k  x z y 

I 1 =I 2 =I 0 I max =4I 0 I min =0 The definition of the first order diffraction efficiency: Λ

Experimental setup MIRROR He-Ne LASER MIRROR BEAM SPLITTER SAMPLE APERTURE FILTER DETECTOR OSCILLOSCOPE POLARIZER Nd: YAG LASER

Results and discussion Experiment I-1: (Pump, Probe) A ∥ /A ⊥ =4.5 ( ⊥, ∥ ) ( ⊥, ⊥ ) ( ∥, ∥ ) ( ∥, ⊥ )

(⊥,∥)(⊥,∥) ( Pump, Probe)

31 ℃ 30.5 ℃ 30 ℃ 29.5 ℃ 31.5 ℃

(Pump, Probe)

nono nene n av ~ ~ ~31.0 ℃ n 31.5 ℃ ~32.5 ℃ 激發光源偏振 垂直導軸

n av 激發光源偏振平行導軸 n

Conclusions of Experiment Ⅰ 產生瞬態光柵的機制有兩個。 2. 寫入光的偏振平行於導軸時的一階繞射訊號較大。 3. 一階繞射訊號衰減時間會隨溫度的上升而增加。

Experiment II: The enhancement of diffraction in liquid crystals on dye-doped polymer films

DR1-PMMA 薄膜在不同的 pulse energy 作用下之一階繞射效率

27 ℃ 34 ℃ 32 ℃ 36 ℃

DDPF 及 DDPF/5CB 薄膜的 SRG DDPF DDPF/5CB

Conclusions of Experiment II : 1. 寫入光源能量足夠大時 (4.5mJ/cm2) ，作用於 DR1- PMMA 薄膜產生的繞射訊號沒有鬆弛現象。 2. 訊號凹陷 (dip) 是由於光散射的結果。 3. DR1-PMMA/5CB 樣品所產生的表面起伏光柵 (SRG) 較 深。