過去的(de)(de)研究(jiu)表明，飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)脈沖無法在(zai)空氣中實(shi)現遠距離(li)(li)傳(chuan)輸。1995年(nian)(nian), BRAUN等(deng)人(ren)頭一(yi)次在(zai)實(shi)驗上觀測(ce)到飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)成絲現象，強紅外飛(fei)秒(miao)(miao)(miao)脈沖的(de)(de)強度(du)(du)在(zai)幾十(shi)米后不僅沒有減弱反(fan)而增大(da)。飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)成絲是一(yi)種非線性光(guang)(guang)(guang)(guang)學現象，飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)在(zai)透明介質中傳(chuan)播(bo)時，基于克爾自聚焦(jiao)效(xiao)應(ying)和(he)等(deng)離(li)(li)子(zi)體散焦(jiao)效(xiao)應(ying)的(de)(de)動(dong)(dong)態(tai)平衡，形成一(yi)條細長的(de)(de)等(deng)離(li)(li)子(zi)體通(tong)道，即飛(fei)秒(miao)(miao)(miao)光(guang)(guang)(guang)(guang)絲。飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)成絲在(zai)遠程(cheng)探測(ce)、人(ren)工增雨(yu)、大(da)氣污染(ran)物(wu)檢(jian)測(ce)等(deng)方面都有廣泛的(de)(de)應(ying)用前景，引起了(le)(le)人(ren)們(men)的(de)(de)關注。近年(nian)(nian)來，為(wei)了(le)(le)進一(yi)步發(fa)掘其在(zai)遠程(cheng)探測(ce)方面的(de)(de)潛在(zai)應(ying)用價值，相干的(de)(de)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)發(fa)射現象成為(wei)飛(fei)秒(miao)(miao)(miao)激(ji)(ji)(ji)光(guang)(guang)(guang)(guang)成絲研究(jiu)的(de)(de)重點，特別(bie)是N2+的(de)(de)“激(ji)(ji)(ji)射”。為(wei)了(le)(le)解(jie)釋“激(ji)(ji)(ji)射”的(de)(de)產生，人(ren)們(men)先(xian)后提出了(le)(le)電子(zi)再碰撞激(ji)(ji)(ji)發(fa)理論、多電子(zi)態(tai)耦合和(he)分(fen)子(zi)轉動(dong)(dong)誘(you)導光(guang)(guang)(guang)(guang)學增益等(deng)。最近的(de)(de)研究(jiu)表明，N2+在(zai)391 nm和(he)428 nm的(de)(de)“激(ji)(ji)(ji)射”強度(du)(du)與光(guang)(guang)(guang)(guang)絲長度(du)(du)存在(zai)強烈關聯性。
研究飛秒激光(guang)(guang)(guang)成(cheng)絲(si)的(de)熒光(guang)(guang)(guang)輻(fu)射(she)以及實現對其長度(du)的(de)表(biao)征，有利于揭示飛秒激光(guang)(guang)(guang)成(cheng)絲(si)中“激射(she)”現象(xiang)的(de)本質。為此，本文作者設計了(le)實驗(yan)裝置(zhi)，測量(liang)了(le)空(kong)氣中光(guang)(guang)(guang)絲(si)不同(tong)位(wei)置(zhi)處發出的(de)熒光(guang)(guang)(guang)信號(hao)(hao)，通過改變入射(she)激光(guang)(guang)(guang)的(de)脈沖能量(liang)和偏(pian)振狀態來研究熒光(guang)(guang)(guang)信號(hao)(hao)的(de)變化; 同(tong)時(shi)，采用聲音測量(liang)法(fa)對光(guang)(guang)(guang)絲(si)做(zuo)進一(yi)步測量(liang)，通過兩種方法(fa)的(de)結合對等離子體光(guang)(guang)(guang)絲(si)進行長度(du)表(biao)征。
1. 實驗測量方(fang)法(fa)
實(shi)驗(yan)(yan)裝(zhuang)置如(ru)圖 1所示，以獲得(de)飛(fei)秒激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)成絲(si)(si)過程(cheng)(cheng)中(zhong)(zhong)的(de)熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)輻射光(guang)(guang)(guang)(guang)(guang)(guang)譜(pu)(pu)和聲音信(xin)(xin)號(hao)。實(shi)驗(yan)(yan)中(zhong)(zhong)采(cai)用的(de)飛(fei)秒激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)器型(xing)號(hao)為(wei)(wei)Astrella(Coherent, Inc.)，該系統可輸出(chu)中(zhong)(zhong)心波(bo)長(chang)為(wei)(wei)800 nm、脈(mo)沖(chong)(chong)(chong)時間(jian)寬度為(wei)(wei)35 fs、單脈(mo)沖(chong)(chong)(chong)較大(da)能(neng)量為(wei)(wei)7 mJ、重復(fu)頻率(lv)為(wei)(wei)1 Hz~1000 Hz可調的(de)線偏振(zhen)(zhen)激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)脈(mo)沖(chong)(chong)(chong)。激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)器發(fa)射出(chu)的(de)重復(fu)頻率(lv)為(wei)(wei)1000 Hz的(de)激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)經過λ/4波(bo)片、中(zhong)(zhong)性密度濾光(guang)(guang)(guang)(guang)(guang)(guang)片后，被焦距為(wei)(wei)40 cm的(de)透鏡聚焦，在(zai)(zai)空氣中(zhong)(zhong)產生光(guang)(guang)(guang)(guang)(guang)(guang)絲(si)(si)。在(zai)(zai)光(guang)(guang)(guang)(guang)(guang)(guang)絲(si)(si)的(de)垂直方(fang)向(xiang)上由光(guang)(guang)(guang)(guang)(guang)(guang)纖將熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)信(xin)(xin)號(hao)引入光(guang)(guang)(guang)(guang)(guang)(guang)譜(pu)(pu)儀(yi)(yi)，測(ce)量光(guang)(guang)(guang)(guang)(guang)(guang)絲(si)(si)的(de)熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)光(guang)(guang)(guang)(guang)(guang)(guang)譜(pu)(pu)。光(guang)(guang)(guang)(guang)(guang)(guang)譜(pu)(pu)儀(yi)(yi)型(xing)號(hao)為(wei)(wei)USB4000(Ocean Optics, Inc.)，光(guang)(guang)(guang)(guang)(guang)(guang)譜(pu)(pu)每次測(ce)量的(de)積分(fen)時間(jian)為(wei)(wei)1.5 s，平均(jun)次數為(wei)(wei)5次。控制電(dian)動位移(yi)(yi)臺使組裝(zhuang)在(zai)(zai)其上面的(de)光(guang)(guang)(guang)(guang)(guang)(guang)纖探頭沿激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)傳(chuan)播方(fang)向(xiang)移(yi)(yi)動，以獲得(de)等離子體光(guang)(guang)(guang)(guang)(guang)(guang)絲(si)(si)不(bu)同(tong)位置處的(de)熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)信(xin)(xin)號(hao)。轉動波(bo)片控制激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)的(de)偏振(zhen)(zhen)態實(shi)現(xian)由線偏振(zhen)(zhen)到(dao)圓偏振(zhen)(zhen)的(de)演化。調節中(zhong)(zhong)性密度濾光(guang)(guang)(guang)(guang)(guang)(guang)片可連(lian)續改變激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)脈(mo)沖(chong)(chong)(chong)能(neng)量，以獲得(de)不(bu)同(tong)激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)脈(mo)沖(chong)(chong)(chong)能(neng)量下的(de)熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)信(xin)(xin)號(hao)。測(ce)量過程(cheng)(cheng)中(zhong)(zhong)以熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)信(xin)(xin)號(hao)出(chu)現(xian)的(de)位置為(wei)(wei)等離子體光(guang)(guang)(guang)(guang)(guang)(guang)絲(si)(si)傳(chuan)輸的(de)零點，位移(yi)(yi)臺每移(yi)(yi)動1 mm收(shou)集一次信(xin)(xin)號(hao)，直至(zhi)熒(ying)光(guang)(guang)(guang)(guang)(guang)(guang)信(xin)(xin)號(hao)消失。
圖 1 實驗裝置示意(yi)圖
Figure 1. ; Schematic diagram of the experimental setup
采(cai)(cai)用麥(mai)克(ke)風測(ce)量等離(li)子體光絲(si)(si)不(bu)同(tong)位(wei)置(zhi)處的聲(sheng)音(yin)信(xin)(xin)號(hao)，并與同(tong)步測(ce)量的熒光信(xin)(xin)號(hao)進行對比(bi)，來反映(ying)飛秒光絲(si)(si)的變化趨(qu)勢。麥(mai)克(ke)風被放置(zhi)在距光絲(si)(si)約0.5 cm處，其收集到的聲(sheng)音(yin)信(xin)(xin)號(hao)經放大器后(hou)，接入(ru)示波(bo)器(Tektronix DPO 2024, Tektronix, Inc)。本文作者(zhe)設(she)計(ji)了聲(sheng)音(yin)的放大電路，以更好的實現聲(sheng)音(yin)信(xin)(xin)息的采(cai)(cai)集，并提高其靈敏度(du)。麥(mai)克(ke)風與光纖探頭(tou)固定在同(tong)一(yi)位(wei)移臺(tai)上，以保證其同(tong)步測(ce)量光絲(si)(si)同(tong)一(yi)位(wei)置(zhi)的聲(sheng)音(yin)信(xin)(xin)號(hao)。
2. 熒光光譜測量(liang)與分析
由(you)于多光(guang)(guang)(guang)子(zi)(zi)(zi)(zi)電離(li)和(he)隧(sui)道(dao)電離(li)，飛秒激(ji)光(guang)(guang)(guang)成絲(si)過程(cheng)會伴生大量(liang)自(zi)由(you)電子(zi)(zi)(zi)(zi)以及(ji)激(ji)發態(tai)(tai)(tai)的(de)(de)(de)(de)離(li)子(zi)(zi)(zi)(zi)和(he)分(fen)(fen)子(zi)(zi)(zi)(zi)。氮氣(qi)是空氣(qi)的(de)(de)(de)(de)主要(yao)成分(fen)(fen)，因此光(guang)(guang)(guang)絲(si)內部產生大量(liang)處(chu)于激(ji)發態(tai)(tai)(tai)的(de)(de)(de)(de)N2+和(he)N2。實驗測量(liang)表明，這些(xie)激(ji)發態(tai)(tai)(tai)的(de)(de)(de)(de)離(li)子(zi)(zi)(zi)(zi)和(he)分(fen)(fen)子(zi)(zi)(zi)(zi)在(zai)躍(yue)遷至(zhi)低能(neng)級(ji)(ji)時，會產生290 nm~440 nm波長(chang)范圍內的(de)(de)(de)(de)熒光(guang)(guang)(guang)。氮熒光(guang)(guang)(guang)的(de)(de)(de)(de)激(ji)發機(ji)理主要(yao)有兩(liang)個(ge)通(tong)(tong)道(dao)：一個(ge)通(tong)(tong)道(dao)是N2+的(de)(de)(de)(de)B2Σu+ → X2Σg+躍(yue)遷，如(ru)391 nm對(dui)(dui)(dui)應(ying)于B2Σu+(ν=0) → X2Σg+(ν′=0)躍(yue)遷及(ji)428 nm對(dui)(dui)(dui)應(ying)于B2Σu+(ν=0) → X2Σg+(ν′=1)的(de)(de)(de)(de)躍(yue)遷；另(ling)一通(tong)(tong)道(dao)是N2的(de)(de)(de)(de)C3Пu → B3Пg的(de)(de)(de)(de)躍(yue)遷，如(ru)337 nm對(dui)(dui)(dui)應(ying)于C3Пu(ν=0) → B3Пg(ν′=0)躍(yue)遷及(ji)357 nm對(dui)(dui)(dui)應(ying)于C3Пu(ν=0) → B3Пg(ν′=1)躍(yue)遷，其中(zhong)，ν代表電子(zi)(zi)(zi)(zi)態(tai)(tai)(tai)的(de)(de)(de)(de)上振(zhen)(zhen)動能(neng)級(ji)(ji)，ν′代表電子(zi)(zi)(zi)(zi)態(tai)(tai)(tai)的(de)(de)(de)(de)下(xia)振(zhen)(zhen)動能(neng)級(ji)(ji)。控制(zhi)入射(she)激(ji)光(guang)(guang)(guang)的(de)(de)(de)(de)單脈(mo)沖能(neng)量(liang)為2.0 mJ, 圖(tu) 2為在(zai)光(guang)(guang)(guang)絲(si)中(zhong)間位置(zhi)測量(liang)得(de)到(dao)的(de)(de)(de)(de)熒光(guang)(guang)(guang)光(guang)(guang)(guang)譜(pu)。其中(zhong)藍色實線(xian)(xian)和(he)紅色虛(xu)線(xian)(xian)分(fen)(fen)別(bie)表示(shi)入射(she)激(ji)光(guang)(guang)(guang)為圓偏(pian)(pian)振(zhen)(zhen)態(tai)(tai)(tai)和(he)線(xian)(xian)偏(pian)(pian)振(zhen)(zhen)態(tai)(tai)(tai)時的(de)(de)(de)(de)熒光(guang)(guang)(guang)光(guang)(guang)(guang)譜(pu)。由(you)圖(tu) 2可知，337 nm處(chu)的(de)(de)(de)(de)N2熒光(guang)(guang)(guang)信(xin)(xin)號在(zai)圓偏(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)下(xia)的(de)(de)(de)(de)強度比線(xian)(xian)偏(pian)(pian)振(zhen)(zhen)態(tai)(tai)(tai)下(xia)強，391 nm處(chu)的(de)(de)(de)(de)N2+熒光(guang)(guang)(guang)信(xin)(xin)號強度則在(zai)線(xian)(xian)偏(pian)(pian)振(zhen)(zhen)態(tai)(tai)(tai)下(xia)較(jiao)強。圖(tu)中(zhong), LP表示(shi)線(xian)(xian)偏(pian)(pian)振(zhen)(zhen)(linearly polarized), CP表示(shi)圓偏(pian)(pian)振(zhen)(zhen)(circularly pola-rized)。
圖 2 入(ru)射(she)激光(guang)脈(mo)沖能量為2.0 mJ時(shi)，測量得到的(de)熒光(guang)光(guang)譜
Figure 2. Fluorescence spectrum with an incident pulse energy of 2.0 mJ
在(zai)(zai)氣體等(deng)(deng)離(li)(li)子(zi)(zi)(zi)體產(chan)(chan)(chan)生(sheng)過(guo)程(cheng)中，線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)激光(guang)(guang)(guang)(guang)(guang)脈(mo)(mo)(mo)沖光(guang)(guang)(guang)(guang)(guang)矢量端點(dian)的(de)(de)(de)(de)(de)(de)軌(gui)跡沿著傳播(bo)方(fang)向為(wei)直線(xian)(xian)(xian)，電(dian)(dian)(dian)(dian)場的(de)(de)(de)(de)(de)(de)方(fang)向周期性正(zheng)負變化，其中的(de)(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)被(bei)交替地加速和減速；圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)激光(guang)(guang)(guang)(guang)(guang)脈(mo)(mo)(mo)沖光(guang)(guang)(guang)(guang)(guang)矢量端點(dian)的(de)(de)(de)(de)(de)(de)軌(gui)跡為(wei)圓周，電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)總是一直被(bei)加速，圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)作用下(xia)(xia)產(chan)(chan)(chan)生(sheng)的(de)(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)有更(geng)(geng)高(gao)的(de)(de)(de)(de)(de)(de)動(dong)能(neng)。對(dui)于(yu)(yu)毫焦耳(er)量級的(de)(de)(de)(de)(de)(de)入射激光(guang)(guang)(guang)(guang)(guang)脈(mo)(mo)(mo)沖，在(zai)(zai)常壓下(xia)(xia)光(guang)(guang)(guang)(guang)(guang)絲內部的(de)(de)(de)(de)(de)(de)功率密(mi)度被(bei)限(xian)制在(zai)(zai)1.0×1014 W/cm2附近。圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)場產(chan)(chan)(chan)生(sheng)的(de)(de)(de)(de)(de)(de)自由電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)的(de)(de)(de)(de)(de)(de)動(dong)能(neng)集(ji)中分布在(zai)(zai)16 eV左(zuo)右，而線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)場中自由電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)的(de)(de)(de)(de)(de)(de)動(dong)能(neng)大多處于(yu)(yu)2 eV以下(xia)(xia)。高(gao)能(neng)量電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)與氮分子(zi)(zi)(zi)碰撞(zhuang)產(chan)(chan)(chan)生(sheng)激發(fa)(fa)態(tai)N2(C3Пu)，使得圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)飛秒(miao)激光(guang)(guang)(guang)(guang)(guang)誘(you)導(dao)的(de)(de)(de)(de)(de)(de)氮分子(zi)(zi)(zi)的(de)(de)(de)(de)(de)(de)337 nm熒光(guang)(guang)(guang)(guang)(guang)增強(qiang)。線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)下(xia)(xia)產(chan)(chan)(chan)生(sheng)的(de)(de)(de)(de)(de)(de)391 nm熒光(guang)(guang)(guang)(guang)(guang)信號總是強(qiang)于(yu)(yu)圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)，該現(xian)象與氮分子(zi)(zi)(zi)內價電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)的(de)(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)離(li)(li)情況有關(guan)。對(dui)于(yu)(yu)氮分子(zi)(zi)(zi)離(li)(li)子(zi)(zi)(zi)N2+(B2Σu+)，其電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)碰撞(zhuang)激發(fa)(fa)所(suo)需的(de)(de)(de)(de)(de)(de)能(neng)量更(geng)(geng)高(gao)，且激發(fa)(fa)截(jie)面小于(yu)(yu)N2(C3Пu)的(de)(de)(de)(de)(de)(de)截(jie)面，在(zai)(zai)線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)和圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)誘(you)導(dao)的(de)(de)(de)(de)(de)(de)等(deng)(deng)離(li)(li)子(zi)(zi)(zi)體中均未明顯觀(guan)察(cha)到電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)碰撞(zhuang)激發(fa)(fa)現(xian)象。但是，激光(guang)(guang)(guang)(guang)(guang)強(qiang)度為(wei)1014 W/cm2時，線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)的(de)(de)(de)(de)(de)(de)離(li)(li)子(zi)(zi)(zi)產(chan)(chan)(chan)率是圓偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)的(de)(de)(de)(de)(de)(de)一到兩個數量級，因此線(xian)(xian)(xian)偏(pian)(pian)(pian)(pian)振(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)下(xia)(xia)N2+的(de)(de)(de)(de)(de)(de)產(chan)(chan)(chan)額更(geng)(geng)高(gao)，可產(chan)(chan)(chan)生(sheng)更(geng)(geng)強(qiang)的(de)(de)(de)(de)(de)(de)391 nm熒光(guang)(guang)(guang)(guang)(guang)信號。
沿(yan)著(zhu)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)的(de)(de)傳播方向(xiang)移動光(guang)(guang)(guang)(guang)(guang)(guang)(guang)纖探頭，可(ke)以測量飛秒(miao)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)絲(si)不同(tong)位(wei)(wei)置(zhi)處(chu)的(de)(de)熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)信號(hao)(hao)。如圖(tu) 3所示，為(wei)(wei)入(ru)射激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)能量分別為(wei)(wei)1.0 mJ，1.5 mJ和(he)2.0 mJ時，激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)的(de)(de)偏(pian)振(zhen)(zhen)(zhen)態分別為(wei)(wei)線(xian)(xian)偏(pian)振(zhen)(zhen)(zhen)和(he)圓偏(pian)振(zhen)(zhen)(zhen)情況下(xia)，測量得(de)到391 nm處(chu)的(de)(de)N2+熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)信號(hao)(hao)強(qiang)(qiang)度(du)(du)和(he)337 nm處(chu)的(de)(de)N2熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)信號(hao)(hao)強(qiang)(qiang)度(du)(du)隨光(guang)(guang)(guang)(guang)(guang)(guang)(guang)傳輸距離的(de)(de)變(bian)化。可(ke)以發現(xian)，在(zai)3種(zhong)不同(tong)的(de)(de)入(ru)射激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)能量下(xia)，光(guang)(guang)(guang)(guang)(guang)(guang)(guang)絲(si)中心位(wei)(wei)置(zhi)處(chu)337 nm的(de)(de)熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)信號(hao)(hao)在(zai)圓偏(pian)振(zhen)(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)作用下(xia)的(de)(de)強(qiang)(qiang)度(du)(du)約為(wei)(wei)線(xian)(xian)偏(pian)振(zhen)(zhen)(zhen)態下(xia)的(de)(de)2倍，而391 nm的(de)(de)熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)信號(hao)(hao)則是在(zai)線(xian)(xian)偏(pian)振(zhen)(zhen)(zhen)態下(xia)的(de)(de)強(qiang)(qiang)度(du)(du)約為(wei)(wei)圓偏(pian)振(zhen)(zhen)(zhen)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)下(xia)的(de)(de)1.3倍。隨著(zhu)入(ru)射激(ji)光(guang)(guang)(guang)(guang)(guang)(guang)(guang)單脈(mo)沖能量的(de)(de)增加(jia)，熒光(guang)(guang)(guang)(guang)(guang)(guang)(guang)強(qiang)(qiang)度(du)(du)的(de)(de)較(jiao)大(da)值在(zai)逐(zhu)漸(jian)增加(jia)。
圖 3 熒(ying)光信號強度隨光傳輸距(ju)離的變化
Figure 3. Variation of fluorescence signal intensity with light propagation distance
由于透(tou)鏡和克爾(er)自聚焦效應的(de)作(zuo)用，隨(sui)著傳播距離(li)的(de)增(zeng)加，其激光強度(du)逐(zhu)漸增(zeng)加，當(dang)達到一定強度(du)時，空氣介質(zhi)被激發、電離(li)，飛秒光絲逐(zhu)漸形成，開(kai)始產生熒(ying)光輻射并通過光致電離(li)產生等(deng)(deng)離(li)子(zi)體(ti)，等(deng)(deng)離(li)子(zi)體(ti)會對激光產生散焦作(zuo)用。中心波長為(wei)(wei)800 nm的(de)飛秒激光對應的(de)等(deng)(deng)離(li)子(zi)體(ti)臨(lin)界密(mi)度(du)為(wei)(wei)1.7×1021 cm-3。通常飛秒激光誘(you)導氣體(ti)等(deng)(deng)離(li)子(zi)體(ti)的(de)密(mi)度(du)為(wei)(wei)1016 cm-3~1017 cm-3，等(deng)(deng)離(li)子(zi)體(ti)產生后折射率np可以表示為(wei)(wei)：
np=n0?N2n0Nnp=n0?Ne2n0Nc (1)
式中(zhong)，Nc=ε0meω02/e2為臨界密度(du)，而Ne，n0，e，ε0，me和ω0分別是等(deng)離(li)子(zi)(zi)體電(dian)子(zi)(zi)密度(du)、線性折(zhe)射率(lv)、電(dian)子(zi)(zi)電(dian)荷(he)、真空介電(dian)常數、電(dian)子(zi)(zi)質量和飛(fei)秒激光(guang)中(zhong)心的(de)圓(yuan)頻(pin)率(lv)。等(deng)離(li)子(zi)(zi)體濃度(du)越(yue)(yue)高，散焦作用越(yue)(yue)強。在(zai)飛(fei)秒激光(guang)成絲過程中(zhong)，當等(deng)離(li)子(zi)(zi)體的(de)散焦作用暫(zan)居主導地(di)位后(hou)(hou)，激光(guang)開(kai)始逐漸(jian)發散，等(deng)離(li)子(zi)(zi)體密度(du)下(xia)降(jiang)，表現為熒光(guang)強度(du)的(de)降(jiang)低。因此，光(guang)傳輸距(ju)離(li)增(zeng)加(jia)，熒光(guang)信號(hao)強度(du)先增(zeng)加(jia)后(hou)(hou)減小。
從圖(tu) 3所示結果(guo)還可以發(fa)現，熒光較強位置隨著(zhu)入射激光脈沖能量的增(zeng)加而前(qian)移。該現象可以用半(ban)經驗自聚焦公(gong)式來解(jie)釋：
Zf=0.367k0w20?0.0219+Zf=0.367k0w02?0.0219+[(P0P)12?0.0852]2 (2)
式中，Zf為光(guang)束自聚焦的(de)焦距，實驗中在此位置處熒光(guang)強度較(jiao)強，k0為激光(guang)脈沖的(de)波數，w0為激光(guang)初始束腰半(ban)徑，P0為初始激光(guang)平均功(gong)率，Pc為自聚焦臨界功(gong)率。考慮聚焦透鏡的(de)影響，熒光(guang)強度較(jiao)強位置會向透鏡的(de)焦點方向移動：
1Z=1Z+1f1Zf′=1Zf+1f (3)
式中，f為透鏡焦距(ju)。同時由(you)于群速(su)度(du)色散效應所導致的散焦作用的影(ying)響(xiang)，實(shi)驗中測得(de)的Zf會與(yu)理(li)論值(zhi)存在些許(xu)偏差。由(you)此(ci)可知，光束焦點的位置隨入射(she)(she)激(ji)光平均功率的變(bian)化而變(bian)化。當入射(she)(she)激(ji)光單脈(mo)沖(chong)能量增加時，激(ji)光平均功率也相(xiang)應的增大，因此(ci)Zf的值(zhi)減小，實(shi)驗中熒(ying)光較強位置將隨著(zhu)能量的增加而前(qian)移。
取(qu)熒光(guang)(guang)(guang)(guang)(guang)信(xin)號(hao)較(jiao)大強(qiang)(qiang)度(du)(du)的(de)(de)5%為(wei)飛(fei)(fei)秒光(guang)(guang)(guang)(guang)(guang)絲(si)出現的(de)(de)基準，即認(ren)為(wei)當熒光(guang)(guang)(guang)(guang)(guang)信(xin)號(hao)強(qiang)(qiang)度(du)(du)達到其較(jiao)大強(qiang)(qiang)度(du)(du)的(de)(de)5%時飛(fei)(fei)秒光(guang)(guang)(guang)(guang)(guang)絲(si)存在。采用(yong)(yong)(yong)不同熒光(guang)(guang)(guang)(guang)(guang)信(xin)號(hao)獲得(de)光(guang)(guang)(guang)(guang)(guang)絲(si)長(chang)(chang)度(du)(du)信(xin)息如(ru)表 1所示(shi)。無(wu)論(lun)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)的(de)(de)偏(pian)振(zhen)(zhen)態如(ru)何設定，光(guang)(guang)(guang)(guang)(guang)絲(si)的(de)(de)長(chang)(chang)度(du)(du)都是隨著入射(she)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)能量的(de)(de)增加(jia)而減少。這(zhe)是因為(wei)實(shi)驗(yan)中(zhong)使(shi)用(yong)(yong)(yong)的(de)(de)透(tou)(tou)鏡焦距(ju)較(jiao)小，隨著入射(she)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)能量的(de)(de)增加(jia)，透(tou)(tou)鏡和(he)空(kong)氣的(de)(de)聚焦作(zuo)用(yong)(yong)(yong)與等離(li)子體散焦作(zuo)用(yong)(yong)(yong)形成的(de)(de)動態平衡很容(rong)易被打破導(dao)致光(guang)(guang)(guang)(guang)(guang)絲(si)長(chang)(chang)度(du)(du)變(bian)短，如(ru)(2)式(shi)所示(shi)。無(wu)論(lun)是N2+的(de)(de)391 nm熒光(guang)(guang)(guang)(guang)(guang)、N2的(de)(de)337 nm熒光(guang)(guang)(guang)(guang)(guang)還是統計一定光(guang)(guang)(guang)(guang)(guang)譜(pu)范圍內的(de)(de)熒光(guang)(guang)(guang)(guang)(guang)強(qiang)(qiang)度(du)(du)，線偏(pian)振(zhen)(zhen)態激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)作(zuo)用(yong)(yong)(yong)下(xia)等離(li)子光(guang)(guang)(guang)(guang)(guang)絲(si)的(de)(de)長(chang)(chang)度(du)(du)都略大于圓偏(pian)振(zhen)(zhen)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)作(zuo)用(yong)(yong)(yong)下(xia)的(de)(de)結果(guo)。相關測(ce)量說明，飛(fei)(fei)秒激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)光(guang)(guang)(guang)(guang)(guang)絲(si)的(de)(de)長(chang)(chang)度(du)(du)是由(you)入射(she)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)強(qiang)(qiang)度(du)(du)、透(tou)(tou)鏡焦距(ju)和(he)激(ji)(ji)光(guang)(guang)(guang)(guang)(guang)偏(pian)振(zhen)(zhen)態等諸(zhu)多(duo)因素綜合決(jue)定的(de)(de)。
圖 4 聲學測量結果
a—聲音信號 b—熒光信號與聲音信號隨等(deng)離(li)子(zi)體光絲傳輸(shu)距離(li)的關系
Figure 4. Acoustic measurement results
a—acoustic signal b—intensity of fluorescence signal and acoustic signal & propagation distance
4. 結論
本實驗中測量了不同能量和偏振態的激光入射到空氣中時產生的等離子體光絲不同位置處的熒光信號強度，在相同激光脈沖能量下，相比線偏振光，圓偏振光產生的N2熒光信號更強；而對于N2+熒光信號，線偏振光更占優勢。圓偏振光作用產生的電子具有更高動能，與氮分子碰撞產生激發態N2 (C3Пu)，使得氮分子的337 nm熒光增強。雖然在線偏振和圓偏振光下通過電子碰撞激發產生N2+(B2Σu+)的效率非常低，但線偏振光下的離子產率遠高于圓偏振光，因此線偏振光下可產生更高強度的391 nm熒光信號。光束自聚焦的焦距Zf隨著入射(she)激光(guang)功率密度(du)的增(zeng)加而(er)減少，表現(xian)為熒光(guang)較強位置隨著入射(she)激光(guang)脈(mo)沖能(neng)量(liang)的增(zeng)加而(er)前(qian)移。同時，熒光(guang)測量(liang)可(ke)以(yi)給出(chu)等(deng)離子體光(guang)絲(si)的長度(du)信息，且相較于聲學測量(liang)光(guang)絲(si)長度(du)，熒光(guang)測量(liang)法精度(du)更(geng)為準確。
注明 文章出處：激光技(ji)術網 //www.jgjs.net.cn/cn/article/doi/10.7510/jgjs.issn.1001-3806.2023.03.003
