Z掃描系統量測大面積異質結構二維材料之非線性光學特性

Abstract

本研究利用Z掃描系統研究石墨烯轉印到氧化銦錫薄膜上的異質結構飽和吸收體(saturable absorber)之非線性光學效應，過去已有諸多文獻表明，飽和吸收體對於超快脈衝雷射的鎖模與脈衝塑形皆有很大的幫助，但也對飽和吸收器樣品表面的品質有極高的要求，所以我們將單點的Z掃描量測系統改良，透過電控元件將Z掃描系統架設成可自動量測大面積二維材料或薄膜的自動Z掃描系統。實驗使用飛秒級超快脈衝雷射、波長為800 nm的Ti-sapphire雷射入射至氧化銦錫薄膜及轉印上單層石墨烯的飽和吸收體，並透過固定式的衰減片改變雷射脈衝的能量來進行實驗，觀察樣品在不同光強度下的光學非線性效應。由實驗結果分析上述樣品的飽和強度(I_s)、雙光子吸收(β)與非線性折射率(n_2)等非線性參數值，尖鋒功率在1到26 GW/cm2的光強度下輸入至樣品，氧化銦錫薄膜的飽和強度的範圍在3到60 GW/cm2，在轉印上單層石墨烯後減少至0.3到7 GW/cm2，相較於氧化銦錫薄膜的飽和強度小了約一個級距，並發現雙光子吸收的影響極小，範圍約在10-14 cm/W左右。因此，達到飽和吸收大部份是由單光子吸收主導，而非線性折射率會隨著入射能量的增強而逐漸減小，範圍在10-12 到10-14 cm2/W。確認單點的非線性效應後，我們對石墨烯/氧化銦錫進行二維的大面積抽測(4*4 mm)及每點間隔75 um的二維掃描實驗，在尖峰強度7.84 GW/cm2的光強度下，飽和強度約落在1.2到50 GW/cm2，之所以會比單點量測的飽和強度大，是因為石墨烯在轉印過程中出現的破損或皺褶所導致的，非線性折射率在10-13 到10-14 cm2/W，與單點量測的係數在相同範圍內，顯示出樣品的均勻度對折射率的影響較小，藉由此方法能夠精確的定位掃描的範圍，有助於大面積且系統性地研究其光學特性。

This study utilized a Z-scan system to investigate the nonlinear optical effects of heterostructure saturable absorbers when graphene is transferred onto indium tin oxide (ITO) thin films. Previous literature has indicated that saturable absorbers play a significant role in mode-locking and pulse shaping for ultrafast pulse lasers. However, they also demand high-quality surfaces for the absorber samples. To address this, we improved the single-point Z-scan measurement system and developed an automated Z-scan system with a large-area 2D material or thin film scanning capability through electronic control.In the experiment, femtosecond-level ultrafast laser pulses with a wavelength of 800 nm from a Ti-sapphire laser were directed onto the ITO thin film and a monolayer graphene transferred onto it as the saturable absorber. A fixed attenuator was employed to modulate the laser pulse energy for varying experimental conditions, thereby observing the optical nonlinear effects of the samples at different light intensities. Analysis of experimental results yielded nonlinear parameters such as saturation intensity (Is), two-photon absorption (β), and nonlinear refractive index (n2). The peak power input to the samples ranged from 1 to 26 GW/cm². The saturation intensity of the ITO thin film was found to be between 3 and 60 GW/cm², which reduced to 0.3 to 7 GW/cm² after transferring a monolayer graphene, demonstrating a reduction of about one order of magnitude. The impact of two-photon absorption was minimal, around 10-14 cm/W. Hence, the saturable absorption was mainly attributed to single-photon absorption, and the nonlinear refractive index gradually decreased with increasing incident energy, ranging from 10-12 to 10-14 cm2/W.After confirming the single-point nonlinear effects, we conducted a 2D large-area measurement (4x4 mm) and a 2D scanning experiment with a point spacing of 75 µm on the graphene/ITO sample. At a peak intensity of 7.84 GW/cm², the saturation intensity ranged from 1.2 to 50 GW/cm². The higher saturation intensity compared to the single-point measurement was attributed to graphene damage or folding during the transfer process. The nonlinear refractive index ranged from 10-13 to 10-14 cm2/W, consistent with the single-point measurement, suggesting that sample uniformity had a minor impact on the refractive index. This method allows precise positioning of the scanning range, facilitating systematic investigation of optical characteristics over large areas.