軟鉍礦晶體於近紅外光之光折變特性研究與應用

Investigations and applications on the photorefractive effect of doped sillenite crystals in near infrared spectral range

劉仁崇(Ren-Chung Liu)

指導教授 ： 許根玉

國立交通大學/電機學院/光電工程系所/博士(2014年)

https://doi.org/10.6842/NCTU.2014.00741

摘要

本論文探討軟鉍礦晶體在近紅外光的光折變效應及其應用。我們在軟鉍礦晶體的生長過程中摻雜過渡金屬元素使得在能隙中靠近傳導帶的位置引入一個淺能階；淺能階做為光折變效應的施體能階中心，來增進晶體在近紅外光的光折變效應。我們測量摻雜釕元素的矽酸鉍晶體、摻雜釕的鈦酸鉍晶體、摻雜銠元素的鈦酸鉍晶體以及未摻雜質的矽酸鉍、鈦酸鉍等五種晶體之吸收光譜，發現摻雜過渡金屬元素確實可使吸收光譜往長波長移動，因此驗證了過渡金屬元素的摻雜可以提高軟鉍礦晶體在近紅外敏感度之構想。
摻雜過渡金屬元素引入的淺能階與軟鉍礦晶體本身的本質能階構成雙能階能帶結構，因而對於長、短波長的入射光分別產生不同之光反應，我們研究如何藉由不同波長的光及其相對強度來改變晶體光折變特性。我們以532nm作為激發光先將晶體敏感化，再以1064nm近紅外光來記錄全像，發現摻雜釕元素的矽酸鉍晶體反應時間從3秒縮短為60毫秒，加速了50倍，摻雜銠元素的鈦酸鉍的反應時間從100毫秒縮短為20毫秒，加速了5倍。針對此現象，我們以光折變理論模型，推導激發光與紅外光之間的相對強度對於光折變反應時間之關係，跟實驗相互驗證，並求優化參數。
再者，實驗發現，摻雜釕元素的矽酸鉍晶體之全像光柵的儲存時間從5秒延長至50秒，摻雜釕的鈦酸鉍晶體之全像光柵的儲存時間從10分鐘延長至一天。針對此現象，我們根據光折變晶體的雙能帶構造推導光定影的全像模型，探討光折變全像的非破壞性讀取特性，模擬結果跟實驗相符，因此，這個模型可以應用於研發可擦拭之全像記憶材料。
最後，我們結合近紅外光敏感的軟鉍礦晶體薄片與液晶板組成光閥，成為近紅外光之影像處理元件。首先研究其電光調變特性，我們發現摻雜銠元素的鈦酸鉍晶體薄片與反向對準型液晶板組成之光閥，在外加4.2V，頻率1KHz交流電壓時，曝照光強250 mW/cm2的1064nm近紅外光，可以達到2π的相位延遲，這個特性將在紅外線影像處理方面有許多應用。其次，我們以摻雜釕元素的矽酸鉍晶體與分散式高分子聚合物液晶組成之光閥進行1064nm近紅外光波之雙波混合實驗，測得增益係數為44cm-1，比起一般摻雜釕元素的矽酸鉍晶體的雙波混合之增益係數只有1cm-1，我們的光閥則高了44倍。我們也以摻雜銠元素的鈦酸鉍晶體與反向對準型液晶板組成的光閥進行雙波混合實驗，其增益係數為8.8 cm-1，比摻雜銠元素的鈦酸鉍晶體高了將近9倍。根據我們研究結果，這些增益係數之提高，乃是受惠於液晶分子的高雙折射係數，以及跟它貼近之光折變晶體薄片之空間電場的穿透性。以上的理論計算結果跟實驗結果十分相符，提供將來發展光折變晶體薄片與有機液晶分子組合之全光操作的光閥之基礎。

關鍵字

全像 ； 光折變 ； 軟鉍礦 ； 金屬過渡元素 ； 光定影 ； 液晶光閥 ； 反向對準型液晶板 ； 分散式高分子聚合物液晶

並列摘要

In this thesis, we investigate the photorefractive effect of transition metal doped sillenite crystals in near infrared(NIR) spectral range. The transition metal elements, we doped into sillenite crystal during crystal growth, introduce shallow traps near the conduction band. The traps act as the donor levels in photorefractive effect. We measure the absorption spectrum of ruthenium-doped Bi12SiO20(BSO:Ru), ruthenium-doped Bi12TiO20(BTO:Ru), rhodium-doped Bi12TiO20 (BTO:Rh), non-doped Bi12SiO20, and non-doped Bi12TiO20. The absorption spectrums show the absorption is shifted to longer wavelength region. The idea to improve the sensitivity of sillenite crystals in near infrared spectral range by doping transition metal element is proved by the shift of absorption spectrum.
The shallow traps introduced by the transition metal dopants and the intrinsic levels of sillenite crystals form two-level band structure, which has different absorptions to long and short wavelengths. We investigate how to modify the photorefractive effect by light exposure with the combination of long and short wavelengths and intensity ratio. We adopt two-level band transport model to simulate the holographic characteristics of doped sillenite crystals, which are sensitized by pre-exposure with 532nm green light, by using 1064nm NIR recording light. The response time of grating build-up is accelerated by fifty times – 3s to 60ms for ruthenium-doped BSO and five times – 100ms to 20ms for rhodium-doped BTO after pre-exposure.
Furthermore, based on two-level band transport mode, we simulate the holographic behavior by optical fixing method. The life-time of index grating recorded in ruthenium-doped BSO by optical fixing technique is extended from 5s to 20s during read-out process. The life-time of index grating recorded in ruthenium-doped BTO by optical fixing technique is extended from 10 minutes to 1 day during reading stage. The result is important for development of rewritable holographic memory.
In the end part, we combine the NIR sensitive sillenite crystal plate and liquid crystal plate to form a light valve as a NIR image processing device. First, we investigate the optical and electrical modulation characteristics of the light valve. The phase retardation of the light valve composed by rhodium-doped BTO crystal plate and anti-parallel alignment liquid crystal plate can reach 2π under the exposure of 1064nm light intensity 250 mW/cm2 with external 4.2V, 1KHz ac voltage. Then, we measure the gain coefficient of another kind of light valve composed with ruthenium-doped BSO plate and polymer-dispersed liquid crystal (PDLC) plate in two-wave mixing experiment. The gain coefficient is 44cm-1, which is 44 times larger than ruthenium-doped BSO crystal. The gain coefficient of the light valve composed with ruthenium-doped BTO plate and AP liquid crystal plate is 9 times larger than ruthenium-doped BTO crystal. The enhancement of the gain coefficient in hybrid device is resulted from the high birefringence of liquid crystal molecules and the space charge field induced in photorefractive penetrating into liquid crystal plate to drive the orientation of liquid crystal molecules. The measured value accords with theoretical estimation. The feature of the hybrid device shows the prospects in the applications of NIR image processing.

並列關鍵字

hologram ； photorefractive ； sillenite ； transition metal ； optical fixing ； liquid crystal light valve ； anti-parallel alignment ； polymer-dispersed liquid crystal

參考文獻

1. A. Ashkin, G.D. Boyd, J.M. Dziedzic, R.G. Smith, A.A. Ballman, ”Optically induced refractive index inhomogeneities in LiNbO3 and LiTaO3,” Appl. Phys. Lett., 9, pp.72-74, 1966.

2. N. V. Kukhtarve, V. B. Markov, S. G. Odoulov, M. S. Soskin, and V. Vinetskii, “Holographic Storage in Electropic Crystals. I. Steady State”, Ferroelectrics, 22, pp. 949-960, 1979.

3. F. P. Strohkendl, J. M. C. Jonathan, R. W. Hellwarth, “Hole–electron competition in photorefractive gratings”, Opt. Let., 11, pp.312-314, 1986.

4. P. Tayebati, “Effect of shallow traps on electron—hole competition in semi-insulating photorefractive materials”, JOSA. B, 9, pp. 415-419, 1992.

5. K. Buse, E. Krätzig, “Three-valence charge-transport model for explanation of the photorefractive effect”, Appl. Phys. B, 61, pp. 27-32, 1995.