半導體飽和吸收鏡對飛秒雷射脈衝所造成的影響

Abstract

本論文探討半導體飽和吸收體（semiconductor saturable absorber mirror, SESAM)對飛秒雷射脈衝的塑型效應與啾頻影響。首先，在三種啾頻的情況下（正啾頻、零啾頻、負啾頻），我們進行非線性反射率量測（nonlinear reflectivity measurement）。根據飽和吸收體模型，分別取得三種啾頻情況下的飽和能量密度（saturation fluence）。我們發現飽和能量密度在入射脈衝為負啾頻(-288 ps^-2)的情況下較其餘兩種啾頻情況（0 ps^-2 及 86 ps^-2）少了兩個數量級。接下來，藉由頻析式雙光數耦合技術（spectrally resolved two-bean coupling）直接觀察脈衝的塑形效應與啾頻影響。此外也觀察到exciton在發射(re-emssion)的現象。我們藉由一簡單模型，定性化解釋所觀察到的現象。當入射脈衝與量子井作用時，所激發自由電子-電洞電漿（free electron-hole plasma）的熱弛緩與excition的離子化效應因發生時序不同，而影響飽和能量密度的變化。同時exciton與對於SESAM所導致的啾頻各有不同的貢獻。因exciton所生的負啾頻變化大於自由載子所產生的正啾頻變化，故SESAM對雷射脈衝會產生正啾頻的變化。

This thesis researchs the infuences of semicondutcor saturable absorber mirror (SESAM) to femtosecond laser pulse shaping and chirp effects. The nonlinear reflectivity of SESAM are calibrated under three chirp conditions (positive chirp, negative chirp, and transform-limited) and the satruation fluences are calculated by the saturable absorption model. We find that the saturation fluence under negaticve chirp condition is 2-order smaller than those under other two conditons. Then We observe the shaping and chirp effects of laser pulses directly by spectrally two-beam coupling technique. Under transform-limited condition, SESAM have great influences to laser pulse shaping and chirp effect. Additionally, exciton re-emssion is observed. We explain these results qualitatively by a simple model. When SESAM interacted with incident pulses, the timing between the free carrier plasma and ionization of exciton vary the saturation fluence of SESAM. The negative chirp contriubuted by exciton is larger than positive chirp contributed by free carrier. So SESAM will effectly induce the positive chirp to laser pulses.