我們用鈦:藍寶石鎖模雷射，波長為800nm，單一脈衝功率81.25KW經過啾頻脈衝放大（chirped-pulse amplification）後，波長為800nm，單一脈衝功率為4.62GW的雷射作為激發光，在氮化鎵半導體材料上激發出螢光來，藉此來觀察氮化鎵半導體材料其多光子吸收的現象，並由螢光的頻譜特性來了解其氮化鎵半導體的能帶結構及其非線性的效率。

我們所使用的n型GaN薄膜樣品於He-Cd雷射光（325nm）激發下，所對應出的復合發光在帶隙附近，進一步也進行10K到315K的變溫實驗；理論上對於TE和TM模的激發，氮化鎵材料內部電偶極躍遷有光場極化方向之相依性，所以應會有不同的電偶極躍遷，但是我們未觀察到此一現象，可能的原因為矽摻雜所導致能帶結構不明顯；為了比較，我們也對非刻意摻雜的GaN薄膜做同樣的量測，經相互比較後，討論GaN薄膜樣品可能的發光機制。最後以800nm的脈衝雷射來激發n-GaN薄膜樣品，所看到的光譜圖與線性激發所得到的略同，但其頻譜寬度顯然較窄；在脈衝光激發下，激發光強度對應發出螢光的強度，顯示其有著約三次方的相依關係，證明這螢光是來自於三光子吸收的非線性過程。

We excite GaN compound semiconductor by using the Ti: sapphire pulse laser as the pumping source so that we could observe the nonlinear absorption phenomenon, and hence the band structure of GaN. The wavelength of the Ti: sapphire pulse laser is 800 nm, unit peak power could be 81.25 KW. After chirped-pulse amplification, its wavelength is still 800nm, but unit peak power could reach 4.62 GW.

First we use He-Cd laser (325nm) to excite n-GaN sample with variable pump power and temperature. The photoluminescence we got correspond to the band-edge structure of GaN. The PL spectrum of GaN should has the dependence of polarization of E-filed in theory, but we couldn’t observe that phenomenon. It maybe due to Si-doped induced merge of band structure. For comparing, we also do experiment of unintentially-doped GaN, and with these data we could try to understand the mechanism of the carrier recombination in these two sample, both are dominated by donor-bound excitons. Alternatively, we use the pulse laser to excite GaN sample and thus have a PL spectrum looks like the linear excited one, but with a thinner spectrum width. Besides, using pulse-laser excitation, we could extract the power dependence (cubic) between the intensity of pumping laser and PL signal, and it shows that the photoluminescence is due to the 3-photon absorption of GaN.

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