Tunable photonic crystal based on capillary attraction and repulsion was demonstrated in this work. A porous silicon-based photonic crystal featured periodic porosity was fabricated by electrochemical etching of 6" silicon wafer followed by hydrophobic modification of the porous silicon surface. The reciprocal capillary actions were achieved by varying the ratio of infiltrated liquid mixture which composed of high surface tension and low surface tension liquids. By adjusting the surface tension of liquid mixture in this way, surface tension of liquid mixture yielded either capillary attraction or capillary repulsion in the nanoscale voids of the porous silicon-based photonic crystal.
Capillary attraction or capillary repulsion delivered the liquid mixture into and out of the voids of the porous silicon-based photonic crystal, the volume percentage of liquid in the voids of the porous silicon-based photonic crystal as well as the reflective color can be dynamically tuned. The demonstrated tunable photonic crystal showed 70 nm-wide spectral tuning from 535 nm to 605 nm and millisecond response time, which had been proven by high speed CCD imaging and response time measurement. In addition, by wetting the porous silicon-based photonic crystal surface with thin layers of ethanol or water, the porous silicon-based photonic crystal can be switched respectively between bistable states: liquid-filled state (orange color) and vapor-filled state (yellow color). Owing to an energy barrier between the two wetting states, the tunable photonic crystal can remain at either of the two states with no external power consumption.

本論文首創利用奈米尺度下之毛細現象、產生流體雙向的移動，得以調變光子晶體顏色。其晶體折射率之變化係以大幅改變內部液體與氣體之比例而達到，突破傳統只調變固體或液體的概念，研究過程中，將單分子疏水層塗佈於微小的多孔矽內部，使光子晶體內部形成十奈米直徑且深達數百奈米之多孔流道，並以控制液體表面張力的方式而達成奈米尺度下的液體雙向流動，是目前唯一兼具快速反應時間與大幅度顏色調變能力的可變光子晶體技術。
本研究首先利用電化學蝕刻方式，製作以多孔矽為材料之光子晶體，再以氣相沉積方式，將多孔矽內部孔洞表面鍍上僅有單一分子厚度的自組裝疏水層，使得光子晶體內部孔洞形成具有低流體阻力的奈米尺度流道，而後於孔洞外部快速調變不同液體之混和比例，使其具有不同表面張力，孔洞因液體表面張力不同而產生毛細吸引或毛細排斥現象，而毛細吸引與毛細排斥則分別造成液體的進入或排出於孔洞內，進而改變光子晶體內部液體與氣體的體積比例，使其折射率產生變化，故得以快速與大幅改變光子晶體的顏色，且由於液體進入或排出於光子晶體孔洞具有雙穩態的特性，調變後的光子晶體不需消耗額外能量以維持狀態。本論文研究成果之重點特色如下：

1. 以液體與氣體置換調變光子晶體折射率，突破單純只調變固體或液體的概念。
2. 以奈米尺度下的毛細壓力作為驅動力(百倍於大氣壓力)。
3. 成功將單分子疏水層塗佈於多孔矽之內部孔洞表面。
4. 調控液體雙向流動於十奈米直徑、深達數百奈米的多孔矽孔洞。
5. 利用流體特性產生雙穩態的光子晶體調變。
6. 目前唯一兼具快速反應時間與大幅度顏色調變的光子晶體調變技術。

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