氧化鋅薄膜在共振波段的高階非線性吸收研究

Abstract

在本論文中，我們藉由Z軸(傳播軸)掃描的方法來研究laser-MBE磊晶成長之氧化鋅薄膜在接近共振吸收波段的高階非線性吸收特性。我們使用倍頻後的高重複頻率飛秒脈衝鈦藍寶石雷射當作量測光源，操作波長為390 nm到420 nm。實驗上發現雙光子吸收係數因為和激子產生共振現象而有明顯增加的趨勢。當量測光的能量接近激子的躍遷能量時，雙光子吸收係數約為3000 cm/GW，大約是先前氧化鋅塊材在波長為532 nm量測結果的710倍，此倍數甚至比先前在近紅外光波段的210倍還高許多。另外，自由載子吸收所產生的高階非線性吸收項也因為雙光子吸收係數的增加而變得更明顯。考慮高階非線性項後，我們利用修正後的方法成功地對實驗數據進行分析，並得到雙光子吸收仍為此波段主要的非線性機制。同時，我們也利用低強度的Z軸掃描觀察到能帶尾伸態(band tailing state)會對非線性吸收造成減小的現象。

Near resonant high-order optical nonlinearities of ZnO thin film made by the laser MBE were investigated by the Z-scan method using the high repetition rate femtosecond Ti:sapphire laser with doubled-frequency wavelength operated between 390 nm to 420 nm. We observed an enormous enhancement of two photon absorption (TPA) coefficient due to near-exciton resonance. When photoexcitation energy is close to exciton transition energy, the TPA coefficient is about 3000 cm/GW, which is 710 times larger than that of ZnO bulk measured at 532 nm. The order of enhancement is even greater than that in near-IR regime. Besides, the higher order nonlinearity, free-carrier absorption, becomes more evident in the near band edge region than that in the off-resonant regime due to the enhanced TPA coefficients. A modified method, which considered higher nonlinearity, was then used and the calculated values were sensible. After considering the contributions of every possible mechanism, we found nonlinear absorption mainly resulted from TPA. Furthermore, the effects of band tailing states on nonlinear absorption were also observed under low input intensity through z-scan method.