飛秒及連續波雷射對於微米至次微米粒子之光學捕捉剛度的研究

Abstract

飛秒脈衝雷射在光學捕捉技術中的應用近來已經受到廣泛的注意，我們分別對飛秒脈衝雷射捕捉以及連續波脈衝雷射捕捉進行捕捉剛度的測量，並且藉由兩者間剛度的比較，我們希望能對飛秒脈衝雷射捕捉有更深入的認識。在本研究中，單一微米至次微米之二氧化矽粒子被用來做為光學捕捉研究的目標，並分別利用非秒脈衝雷射及連續波雷射進行捕捉。藉由位置感測器紀錄被捕捉粒子於捕捉期間的熱擾動，我們測量了雷射功率介於0－40 mW間的捕捉剛度。微米粒子在連續波雷射捕捉中的結果顯示線性的功率相依性，這與我們的預測以及文獻資料相符，並且證明測量系統及方法的可靠性。藉此，我們進一步進行飛秒脈衝雷射捕捉以及連續波脈衝雷射捕捉的比較，對於1和0.5微米粒子而言，兩者均在飛秒脈衝雷射捕捉中有更高的捕捉剛度。我們認為這可能是由於高瞬時強度的脈衝所導致，而盡管飛秒脈衝雷射在脈衝間有時間間隔，但如此短暫的間隔可能沒有對光學捕捉或是對被捕捉粒子之熱擾動造成足夠的影響。本研究是我們第一次架設光學捕捉剛度測量系統並進行可靠的測量，而研究成果亦可作為日後其他光學捕捉實驗的參考。

Optical trapping utilizing femtosecond laser has received much attention recently. We propose to use a classic method for measuring the trapping stiffness in femtosecond and continuous-wave laser trapping. By comparing the measurement results, we prompt a deeper understanding of femtosecond laser trapping. Therefore, a single silica sphere with a size of one micron or half micron was set to be trapped by femtosecond laser and continuous-wave laser separately. With a position sensing device that records the thermal fluctuation of a trapped silica sphere, the trapping stiffness was investigated in a power range from 0 to 40 mW. The result of a micron silica sphere trapped by continues-wave laser showed a linear power dependency, which is consistent with our expectation and references. Hence, it can be regarded as evidence of the reliability of our measurement. Results of both micron and half micron silica spheres showed higher trapping efficiency for femtosecond laser trapping in this power regime. We deduce that the high intensity pulses, nonlinear optical effects, and temporal force may contribute to this difference. Although there is a relaxation time between pulses, it is too short to cause the significant change in sphere position. As a conclusion, we have constructed a trapping stiffness measuring system for the first time in our group, and reliability of the measurement has been checked experimentally. Also we have made a comparison between femtosecond laser trapping and continuous-wave laser trapping. This comparison can be used for standard reference information in the research of optical trapping.