Z-Scan效應在皮秒雷射與飛秒雷射的擬合研究

Simulated study of Z-Scan effect at with ultrashort laser pulses

李英齊(Ying-Chi Li)

指導教授 ： 湯兆崙

國立中正大學/理學院/物理學系/碩士(2012年)

摘要

我們使用惠更斯繞射(Huygens’ diffraction)原理來處理簡單液體分子在皮秒雷射與飛秒雷射的Z-scan實驗數據模擬分析。皮秒雷射輸出波長532nm、重複率10Hz、脈衝時寬19皮秒(高斯光斑的1/e高度半寬)。飛秒雷射為中心波長817nm、重複率82MHz、脈衝時寬15飛秒(高斯光斑的1/e高度半寬)。我們對CS2、C6H6、C2H4Br2與C2H4Cl2等樣品進行分析，試圖去釐清在不同時域下的非線性的響應，如電子極化、分子轉向運動、原子飽和吸收、熱效應等機制[1]。我們在皮秒雷射下的Z-Scan觀察到樣品折射率變化呈現正透鏡效應；而在飛秒雷射下則呈現負透鏡效應。在皮秒時域擬合的非線性折射率n2的量級為10的-14次，我們可以得知皮秒雷射下主要為分子轉向運動的響應。在飛秒時域由於透明液體的吸收機制為受激拉曼散射原理，單發脈衝對樣品進行增溫的動作，由於相鄰兩脈衝間距時間遠小於純熱擴散(thermal diffusivity)時間，樣品的熱增溫有跨脈衝累積效應，樣品的z負透鏡效應最終於微秒等級趨於平緩，故推論飛秒雷射下的負透鏡效應主要為熱效應。

關鍵字

飛秒

並列摘要

We use Huygens’ diffraction principle to analyze the experimental data of simple transparent liquid molecules with picosecond and femtosecond laser pulses. Our picosecond laser is of 532nm wavelength, 10Hz repetition rate, and 19ps pulse width (Half width at 1/e intensity of Gaussian). Furthermore, our femtosecond laser is of 817nm central wavelength, 82MHz repetition rate, and 22fs pulse width (Half width at 1/e intensity of Gaussian). We have analyzed CS2, C6H6, C2H4Br2, and C2H4Cl2 and have tried to distinguish the nonlinear responses of different time scale laser pulses, for instance, the electron polarization, the molecular reorientation, the atomic saturation absorption, thermal effect, and etc.
The change of nonlinear refractive index caused by picosecond laser pulses present the positive lens effect. On the other hand, the femtosecond laser shows the negative lens effect. We successfully use the nonlinear refractive index n2 with 10-14 order to simulate the data of picosecond laser experiment. Consequently, we believe the nonlinear response in picosecond laser experiment is mostly caused by molecular reorientation. Also, we believe that the femtosecond laser pulses induce the negative lens effect which is caused by thermal effect. We reasonably speculate that the absorption mechanism of the transparent liquid is the simulated Raman scattering. Thus, the single pulse would heat the sample. Moreover, because the pulse to pulse duration time is much less than the thermal diffusivity time, the sample would be heated by the high frequency pulses train which called cumulative effect across neighboring pulses, and then, the negative thermal lens effect in the sample would be much more gradual after micro second time. That is why we believe that the thermal effect causes the negative lens effect in femtosecond laser experiment.

並列關鍵字

z-scan

參考文獻

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