Pulse shaping for background free broadband CARS

Transcription

1 Pulse shang for bakground free broadband CAR Young Jong Lee and Marus T. Cerone Polymers Dvson, Natonal Insttute of tandards and Tehnology, 1 Bureau Drve, Gathersburg, MD, UA ABTRACT We demonstrate that ulse shang of a narrowband ulse an suess the nonresonant bakground (B) ontrbuton and retreve resonant Raman sgnals effently n a broadband oherent ant-tokes Raman satterng (CAR) setrum. A ulse shaer eares a obe ulse wth two setral omonents of dfferng hase. When the CAR felds generated by these two out-of-hase omonents are otally mxed, the B sgnal s greatly redued whle a resonant CAR sgnal survves wth mnmal attenuaton. We dsuss three model shemes for the nterferng ulse omonents: (1) two ulses wth dfferent bandwdths and the same enter frequeny (s- sheme); () two ulses wth the same bandwdth and shfted enter frequenes (s-s sheme); and a ulse wth dfferent hases aross the enter frequeny ((+/-) sheme). In all shemes, only the resonant sgnal from the 3-olor CAR mehansm survves. The resonant sgnal from -olor CAR mehansm vanshes along wth the B. We dsuss otmzaton ondtons for sgnal ntensty and shae of resonant CAR eaks. Keywords: ulse shang, Raman setrosoy, CAR, nonresonant bakground 1. INTRODUCTION Broadband oherent ant-tokes Raman satterng (CAR) aoahes ovde a broad Raman setrum from a sngle measurement by overlang a narrowband oseond ulse and a broadband ontnuum ulse [1-4]. Ths multlex measurement tehnques show advantages over sngle-frequeny aoahes for quanttatve and qualtatve Raman setrum analyss of omlex meda nludng bologal samles, whh generally requre setral data over a wde frequeny range from a sngle measurement. For both sngle-frequeny and broadband CAR aoahes, the nonresonant bakground (B) oses a sgnfant nterferene to the resonant Raman sgnal and often overwhelms weak resonant sgnals, leadng to sgnfant reduton n magng ontrast and senstvty. The B n a broadband CAR setrum s artularly dffult to haraterze beause the CAR setrum s dretly affeted by the ontnuum ulse, whose ntensty and hase s often unstable and may ontan multle temoral modes [5]. everal exermental aoahes have been establshed for redung the B ontrbuton, but those aoahes ether ovde sgnfantly redued sgnal ntensty or are not alable to the broadband CAR sheme. Interferometr CAR offers the ossblty of detetng B-free CAR sgnals wth mnmal attenuaton of resonant sgnal ntensty. Conventonal nterferometr deteton methods ombne the sgnal of a CAR feld from the samle of nterest and a well-ontrolled referene feld. However, when a referene feld s generated n a dfferent medum, t s generally very dffult to aount for dfferental hase shft and dfferental hr over a broad frequeny range, and to remove hase jtter. Generatng sgnal and referene felds n the same samle obvates these oblems. The general aoah s to generate nterferene between sgnal and referene CAR olarzatons ndued by two hase-ontrolled obe ulses. lberberg et al. [6] have demonstrated and Lm et al. [7] have moved sngle ulse CAR tehnques where B s redued by nterferng adjaent narrow setral omonents of a sngle ultrashort laser ulse usng a ulse shaer. Cerone et al. [8] have demonstrated a dfferent aoah to sannng nterferometr broadband CAR mrosoy by mxng sgnal and referene felds that are generated by a setrally narrow ulse and a broad ulse for um/obe lght, resetvely. However, the sngle ulse tehnque has lmted ontrol for otmzaton of the obe ulse beause a satal lght modulator ontrols the whole frequeny range of um, tokes, and obe, and the sannng nterferometr aoah requres long aquston tme to extrat Fourer amltudes of the sgnals generated. In ths aer we desrbe sngle-shot nterferometr aoahes to suess B ontrbuton to a broadband CAR setrum that do not requre sannng the relatve ulse hase and result n very lttle attenuaton of resonane sgnal ntensty. In these aoahes, two obe ulse omonents are searately ontrolled by a ulse shaer and the CAR felds they generate are nterferometrally mxed. We dsuss haratersts of the nterferene CAR setra Multhoton Mrosoy n the Bomedal enes IX, edted by Ammas Perasamy, Peter T. C. o, Pro. of PIE Vol. 7183, 7183Y 9 PIE CCC ode: /9/$18 do: /1.891 Pro. of PIE Vol Y-1 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

2 deendng on CAR mehansms ourrng n broadband CAR. We esent three model shemes and dsuss the otmzaton ondtons for hgh resonant sgnal ntensty of B-free broadband CAR setra.. THEORY The CAR sgnal s ndued by the thrd-order nonlnear olarzaton, P, whh s exessed as P ) χ,, ; ) E( ) E ) E( ) δ + d d (1) = where χ s the thrd-order nonlnear susetblty, E are the eletr feld vetors, and the subsrts,,, and ndate um, tokes, obe, and ant-tokes transtons, resetvely. Reent studes [5,9] show that a broadband CAR sgnal an be generated by two dfferent generaton mehansms as desrbed n Fg. 1: () -olor CAR, where the um and obe are ovded by a narrowband ulse, and the ontnuum ulse onsttutes the tokes lght; and () 3-olor CAR, where um and tokes are ovded by two dfferent frequeny omonents n the ontnuum ulse and the narrowband ulse serves as obe. Deendng on the generaton mehansm, the nonlnear olarzaton for CAR emsson s exessed as follows: olor ) = χ En ) E + ) En d P 3 olor ) = χ E( ) E + ) En d P where E n and E are the eletr felds of the narrowband and ontnuum ulses. The nonlnear olarzatons and eletr felds are treated as salar values for smlty. The thrd-order nonlnear susetblty, χ, an be desrbed as a funton of ( - ) and an be exessed usng arameters from a sontaneous Raman setrum [1] as follows [( ) Ω ] = + A χ, ) χ (4) + Γ where χ s the nonresonant ontrbutons, Ω R, s the frequeny of the th Raman mode, A s a onstant reesentng the sontaneous Raman ross seton, and Γ s the Raman lnewdth. From energy onservaton onsderatons, ( - ) = ( - ), and Eq. (4) an be exessed as a funton of ( - ) as follows: = + A χ, ) χ (4 ) + Γ R, [( ) Ω ] In broadband CAR setrosoy, the setral bandwdth of E s generally muh larger than that of E n. Thus, the amltude and the hase of E ( + - ) s essentally onstant wthn the ntegral over d n Eqs. () and, when E s transform-lmted and ts setral shae s smooth. Under these ondtons the ouled double ntegrals n Eqs. () and an be reasonably aoxmated as oduts of searate ntegrals. R, () -olor CAR 3-olor CAR narr (um) narr (obe) ont (tokes) CAR (ant- tokes) ont ont narr CAR CAR CAR ontnuum narrowband ontnuum narrowband Fg. 1. hemat dagrams of - and 3-olor CAR generaton mehansms n two-ulse broadband CAR setrosoy. narr and ont reesent hotons from narrowband and ontnuum ulses resetvely. Pro. of PIE Vol Y- Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

3 olor ) = En ) E + χ, ) En P 3 olor ) = E( ) E + χ, ) En P The 3-olor and -olor CAR sgnals dffer n several ways; one s that the former always dslays amltude that dereases wth nreasng Raman shft due to the frequeny-doman autoorrelaton of the ontnuum ulse, exessed n the frst ntegral of Eq. (6). Another dfferene, rual n the ontext of ths aer, s that the -olor sgnal s generated wth two hotons from the narrowband ulse, whereas 3-olor uses only one. As we wll see, ths dfferene results n vanshng of all -olor sgnal but survval of resonant 3-olor sgnal under the ondton requred for B suesson. Below we demonstrate three model ases n whh we modfy a narrowband ulse E n nto a sueroston of two ulse omonents whose amltudes and hases are ndeendently ontrolled. In ase (1) s-, two narrowband Gaussan ulses have dfferent bandwdths and are loated at the same enter frequeny. In ase () s-s, two narrowband Gaussan ulses have the same bandwdths and the enter frequenes are searated from eah other. In ase (+/-), a narrowband Gaussan ulse has hase reversal aross the enter frequeny. In all model ases, the resultng CAR setra show a strong nterferometr behavor wth the relatve hase between the two ulse omonents. Below we desrbe the model ases usng both analytal exessons and smulatons of setra. In the setral smulatons we use a Raman setrum wth two eaks entered at Ω R,1 = 1 m -1 and Ω R, = 1 m -1 wth amltudes A 1 = 1, and A =.5, and wdths Γ 1 = 1 m -1 and Γ = 5 m -1, we also assume χ =.. In all smulatons the full wdth at half maxmum (FWHM) of the ontnuum ulse s set to be 5 m -1. Both the narrowband and ontnuum ulses are assumed to be transform-lmted Gaussan funtons..1 s- nterferene sheme In the s- sheme, E n onssts of two Gaussan ulses E s and E, where the bandwdths are sgnfantly dfferent but enter frequenes are the same. The eletr felds of the two Gaussan ulses are exessed as E s ) = Es ex[ ( ln ) ) / Δs ] and E ) = E ex[ ( ln ) ) / Δ ], where E and s E are the eak feld amltudes, s the enter frequeny, and Δ s and Δ are the FWHM of the ntensty setra. The overall eletr feld of the narrowband ulse an be wrtten as E ) = E )e + E ) (7) n ( s where Δφ s the hase dfferene between the two ulses. (Note that the ulse s onsdered as a omonent of the narrowband ulse.) We obtan exessons for -olor or 3-olor CAR emsson ntensty, I ) P ), by ombnng Eq. (7) wth Eqs. (5) or (6), resetvely: I [ Es( )e + E( )] E + χ, )[ Es( )e + E( ] olor s -, Δφ) I (8) [ E )e + E ] 3 olor s -, φ) E( ) E + χ, ) s Δ (9) The B ontrbuton to the sgnal s obtaned by relang χ wth χ (a frequeny-ndeendent onstant) n Eqs. (8) and (9). In -olor and 3-olor CAR, nterferene between the sgnals generated from E s and E results n strong deendene of B ntensty on both Δφ and E / E. s Fgure 3(a) shows an examle E n omosed of E s and E wth Δ s = 5 m -1, Δ = 5 m -1 and E / E = s.1. For any gven value of E / E, the B ontrbuton s mnmzed any tme Δφ = π, when the two ulses are out of s hase. It s maxmally suessed only when Δφ = π and E ) d = E ) d. Inseton of Eq. (8) s mmedately shows that both resonant and nonresonant sgnals arsng from the -olor mehansm are elmnated under the ondtons of B suesson sne the frst ntegral term n Eq. (8) beomes zero under these ondtons ndeendent of the value of χ (, ). For 3-olor CAR sgnal, desrbed by Eq. (9), the ntegral nvolvng E s (5) (6) Pro. of PIE Vol Y-3 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

4 and E an have a non-zero value under the ondtons of B suesson due to the esene of the term χ (, ). As dsussed above, the B ontrbutons generated usng a ulse ar suh as E and E s deted n Fg. 3(a), wll be of equal amltude and wll anel beause the two ontrbutons are hase shfted by π wth reset to eah other. In suh a setrum, the 3-olor resonant Raman features generated by E are smeared out, yeldng a more-or-less setrally flat sgnal, whle the resonant Raman features generated by E s are shar and dsersve, haraterst of hgh-resoluton CAR setra. As wth the B, the resonant omonents are π out of hase, but the resonant sgnal from E only slghtly attenuates the resonant sgnal from E s due to the flat lne shae of the E -derved sgnal, and the fat that the latter s sead out over a wder frequeny range. Thus, the survvng sgnal s omosed marly of ontrbutons from the resonant CAR sgnal due to the s ulse. 1 (a) E s.6 (b) E Intensty (a.u.).4. E Raman hft (m -1 ) Intensty (a.u.) 1 () Δ = 5 m -1 Δ = 5 m -1 Normalzed Intensty 1. (d) Ω R = 1 m Raman hft (m -1 ) Δ /Δ s Fg. 3. (a) The eletr felds E s and E used n the s- sheme, where Δ s = 5 m -1 and Δ = 5 m -1, resetvely.. (b) The 3-olor CAR setra generated by ether E s or E for the narrowband ulse. The dotted lne ndates the B only ( χ R = ) CAR setrum. () B suessed 3-olor CAR setra n the s- sheme for two Δ values of 5 m -1 (blue) and 5 m -1 (red) when Δ s = 5 m -1. The dotted lne s the 3-olor CAR setrum alulated only wth resonant sgnal [ χ = ] generated wth a sngle s ulse. (d) em-log lot of alulated eak ntensty of the Raman mode at 1 m -1 as a funton of Δ when Δφ = π and E E = Δ Δ. / s s Fgure 3(b) shows 3-olor CAR setra alulated usng ths B elmnaton sheme, wth Δ s = 5 m -1, and wth Δ = 5 m -1 or 5 m -1. The eak shaes are not sgnfantly mated by the relatve wdths the s and obe omonents, but the larger value of Δ yelds an nreased reovery of resonant Raman sgnal. Ths nreased reovery wth broad obe ulse an be seen learly n Fg. 3(), where the eak ntensty of the Raman mode at 1 m -1 s lotted as a funton of nreasng Δ /Δ s, Δ s beng fxed at 5 m -1. The eak ntensty asymtotally nreases towards 1 % reovery of the ure resonant sgnal, whh s shown as the dotted lne n Fg. 3(b). The dotted lne, as a referene, s alulated wth χ = and E n omosed only of E s, wth E s havng an ntegrated ntensty equal to that of E s +E n the orresondng sold lne. Fgure 3 shows that wth Δ = 5 m -1 and 5 m -1, the eak ntensty of the Raman mode at 1 m -1 reah 59 % and 9 % ntenstes of the ure resonant CAR amltude, resetvely. However, we note that t may not be ossble to omletely suess the B for a large Δ (e.g., Δ = 5 m -1 ). In ases where there s non-neglgble varaton n amltude and hase of the ontnuum ulse wthn frequeny ntervals equal to the broad ulse bandwdth, the two ntegrals n Eqs. (5) and (6) annot be searated to a good aoxmaton, and the smlfed analyss esented here does not neessarly hold. However, deendng on the Pro. of PIE Vol Y-4 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

5 detals of the shae of the ontnuum ulse, we may stll exet a sgnfant derease n B ontrbuton for muh of the CAR sgnal bandwdth by adjustng the relatve ntensty and hase of the two narrowband ulse omonents.. s-s nterferene sheme In the s-s sheme, E n onssts of two narrowband Gaussan ulses whose bandwdths are the same but whose enter frequenes are shfted, as shown n Fg. 4(a). The eletr felds of the s ulses an be exessed as E s1( ) = Es ex[ (ln ) + Δ ) / Δs] and E s( ) = Es ex[ (ln ) Δ ) / Δs], where Δ s the frequeny searaton between the two ulses. The overall eletr feld of the narrowband ulse s thus wrtten as: E ) = E )e + E ) (1) n ( s1 s We obtan exessons for the -olor or 3-olor CAR sgnal ntensty n the s-s sheme when we ombne Eq. (1) wth Eqs. (5) or (6), resetvely: I olor s-s, Δφ) [ E )e + E )] s1 s E + (11) χ, ) [ E )e + E )] s1 s d I 3 olor s-s, Δφ) E ) E + χ, ) [ E )e + E )] s1 s d (1) When the B suesson ondton, [ E )e + E )] d s1 ( s =, s satsfed, the entre resonant -olor CAR setrum, as well as B ontrbutons from both -olor and 3-olor mehansms vansh, as wth the s- sheme. In the s-s sheme, the B suesson ondton s fulflled when Δφ = π and E s1 = E s rresetve of the value of Δ, ovded the amltude of the B does not hange sgnfantly over the nterval Δ. E (a) E s1 E s1 Intensty (a.u.).6 (b).4. Normalzed Intensty 1..5 () Γ 1 = 1 m -1 Γ = 5 m -1 Δ 35 m -1 m -1 1 m -1 5 m Raman hft (m -1 ) Raman hft (m -1 ) Fg. 4. (a) Eletr feld amltudes E s1 and E s used n the s-s sheme, where Δ s = 5 m -1. (b) The 3-olor CAR setra generated by one of E s1 and E s for the narrowband ulse. The dotted lne ndates the B only ( χ R = ) CAR setrum. The Raman shft s alulated from wth. () As a referene (the dotted lne), a 3- olor CAR setrum s alulated for χ = by a sngle s ulse for E n when the ulse energy s assumed to be equal to the sum of those of E s1 and E s for the other s-s smulatons. Fgure 4 shows 3-olor CAR setra alulated for Δφ = π and varous values of Δ. As Δ s nreased, the sgnal ntensty nreases. However, when the sang exeeds twe the FWHM of the non-nterferometr setral feature,.e. Δ >[Γ + (Δ s ) ] 1/, there s no further nrease n sgnal amltude, and a eak slttng artfat emerges, as learly demonstrated by the Raman mode at 1 m -1. The maxmum eak ntensty n the s-s sheme s lose to 1% of the ure resonant [ χ = ] CAR ntensty alulated wth one of two searate s ulses. If, for onssteny wth the s- sheme dsusson, we omare alulated sgnal reovery aganst sgnal alulated wth a sngle s ulse havng the same ulse energy as the sum of the gemnate E n ulses, we alulate a maxmum sgnal reovery of 5 %. The Pro. of PIE Vol Y-5 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

6 sgnal nrease and eak slttng wth Δ n the s-s 3-olor CAR an be understood as a result of two out-of-hase resonant CAR olarzaton setra. When Δ s small and the two equvalent eaks are lose, the resonant-only CAR olarzatons nterfere destrutvely and the resultng eak beomes weak. When Δ s larger and the two eaks are suffently searated, both eaks aear ndvdually and the eak heghts reman unaffeted. For a small Δ, destrutve nterferene n the overla regon of the two ulse setra an redue the total (tme-averaged) lght ntensty. However, ths destrutve nterferene leads to a sgnal reduton of only a fator of two between Δ = 5 m -1 and m -1, whle the CAR eak heght rato for the eaks resultng from these ondtons s 6. Ths addtonal nrease s a result of a faster-rsng eletr feld n the tme doman for Δ = m -1, and thus a more effent samlng of the resonant Raman resonse at early tmes, before t deays. Ths s shown n Fg. 6(e) below..3 (+/-) nterferene sheme The nterferometr aoah by the (+/-) sheme was frst demonstrated by lberberg et al. [11], who reversed the hase of a Gaussan-lke obe ulse by π aross the enter frequeny. They found that the 3-olor nterferometr CAR setrum generated by the hase modfed femtoseond obe ulse showed narrower bandwdth and stronger sgnal ntensty at resonane frequenes than the CAR setrum by an equvalent transform-lmted obe ulse. The smlar aoah has been aled to the sngle ulse CAR setrosoy to suess the B ontrbuton. However, sne a ulse shaer n the sngle ulse CAR ontrols the whole frequeny range, the lmted setral resoluton of the ulse shaer lmts further otmzaton of the obe ulse. Here we esent not only B suessed CAR setra but the effet of the obe ulse shae, n artular the bandwdth, on the CAR sgnal ntensty and the setral resolvng ower. The eletr feld of the narrowband ulse n the (+/-) sheme s exessed as follows: E En ) = E ( + ) ( ) ) = E ) = E ex[ (ln ) ) ex[ (ln ) ) / Δ ] e / Δ ] Δφ, for >, for < (13) E (a) E (-) E (+) Intensty (a.u.) (b) Γ 1 = 1 m -1 Δ 6 m -1 m -1 5 m -1 Γ = 5 m Raman hft (m -1 ) 1 () Γ 1 = 1 m -1 1 (d) Δ = 1 m -1 m m -1 Peak Intensty Γ = 5 m -1 Intensty (a.u.) 1 6 m -1 8 m -1 1 m Δ (m -1 ) Raman hft (m -1 ) Fg. 5. (a) Eletr feld amltudes E (+) and E (-) used n the (+/-) sheme, where Δ = 6 m -1. (b) mulated 3- olor nterferometr (Δφ = π) CAR setra for varous Δ. () The eak ntenstes at the Raman modes at 1 m - 1 and 1 m -1 as a funton of Δ. (d) Interferometr CAR setra (sold lnes, Δφ = π) for the Raman band at 1 m -1 are omared wth the non-nterferometr CAR setra (dashed lnes, Δφ = ) for resonane-only Raman susetblty ( χ = ). Pro. of PIE Vol Y-6 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

7 As wth the s- and s-s shemes, when the B suesson ondton (Δφ = π) s satsfed, the total -olor CAR sgnal and the nonresonant 3-olor CAR sgnal dsaear and only a resonant 3-olor setrum remans. Fgure 5(b) shows 3-olor nterferometr CAR setra alulated wth varyng Δ, whle the tme averaged ntensty of the obe ulse s ket onstant for all Δ. Frst, t s noted that the eak ntensty of the nterferometr (Δφ = π) CAR nreases wth nreasng Δ but after the eak heght reahes the maxmum t dereases very slowly wth Δ, whh s shown n Fg. 5(). Ths Δ deendene of the nterferometr CAR eak ntensty s sgnfantly dfferent from that of nonnterferometr (Δφ = ) CAR, whh monotonally derease wth nreasng Δ, an shown n the seres of CAR setra (the dashed urves) n Fg. 5(d). eond, the wdths of the nterferometr CAR eaks are muh narrower than that of the non-nterferometr CAR eak, whh orresonds to [Γ + (Δ ) ] 1/. Fgure 5(d) shows the learly dfferent Δ deendene of the eak wdth between the nterferometr and non-nterferometr CAR. Thrd, the ooste Δ deendene of the eak heght between the nterferometr and non-nterferometr CAR setra results n a greater sgnal ntensty at the eak of an nterferometr CAR setrum than that of a non-nterferometr CAR setrum generated by a transform-lmted obe ulse. Ths strongly suggests that the otmal bandwdth of the obe ulse (e.g., Δ 8 m -1 for Γ = 1 m -1 ) n the (+/-) sheme should be muh larger than the tyal sontaneous Raman lnewdth ( 1 m -1 ). Ths otmzaton ondton an be aled not only to broadband CAR but sngle ulse CAR setrosoy..4 Tme doman onsderatons Fgures 6(d)-6(f) show tme-doman eletr feld amltudes for obe ulses n the s-, s-s, and (+/-) shemes desrbed above. Note that the obe feld amltude s zero at t = n all the shemes. Wth obe ulses suh as these, the sngle-shot nterferene CAR shemes for B suesson an be aheved when a well-omessed, broadband ontnuum ulse, F[E ](t), arrves at t =. Under these ondtons nether the -olor resonant sgnal nor the B ontrbutons are generated sne all of these requre that eletr felds from all ulses (um, tokes, and obe) to have sgnfant amltude smultaneously. urvval of resonant sgnal n a 3-olor CAR setrum an be understood as beng due to the esene of sgnfant obe ulse feld amltude durng the non-zero lfetme the vbratonal oherene set u n the samle by the ontnuum ulse. The Raman resonse funton deays slowly, on the tme sale of the Raman dehasng tme, whh tyally ranges several hundreds femtoseonds to several oseonds [1]. The temoral overla between F[E n ](t) and the nstantaneous eletron resonse for B ontrbuton beomes neglgble omared wth that between F[E n ](t) and the Raman resonse. Ths makes the B free CAR setrum avalable n the 3-olor broadband CAR setrosoy. (a) s- sheme E s (b) E s1 s-s sheme () (+/-) sheme E (+) E s E (-) E B F[E (eletron) (d) ](t) ~ resonse (e) -1 1 (f) F[E n ](t) resonant (Raman) resonse F[E n ](t) F[E n ](t) Tme (s) Tme (s) Tme (s) Fg. 6. (a)-(), eletr feld amltudes of obe ulses n the frequeny doman for the s-, s-s, and (+/-) shemes, resetvely. (d)-(f), eletr feld amltudes for obe and broadband ulses n the tme doman for the s-, s-s, and (+/-) shemes, resetvely. The red dashed lnes n (d)-(f) reesent resonant vbratonal oulaton deay. For the s- sheme, Δ s = 5 m -1 and Δ = 5 m -1. For the s-s sheme, Δ s = 5 m -1 and Δ = m -1. For the (+/-) sheme, Δ = 6 m -1. Pro. of PIE Vol Y-7 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms

8 It s lear from the tme-doman desrton of Fg. 5 that the B s omletely suessed only when the ontnuum ulse s extremely short. A small amount of hrng s exeted to nrease temoral overla between the ontnuum and the ulse-shaed obe ulses, resultng n resdual B. We note that a transform-lmted ontnuum ulse s desrable not only for removal of the B ontrbuton, but sgnfantly nreases sgnal ntensty and setral overage of the entre 3-olor CAR setrum [5]. However, t s dffult to generate a omessble and setrally broad (>3 m -1 ) ontnuum ulse from a nonlnear fber, and these ontnua generally ontan multle modes wth hgher order dserson [13]. 3. CONCLUION We have ntrodued a new nterferometr aoah to suess nonresonant bakground n a 3-olor CAR setrum and have demonstrated t usng broadband CAR. The aoah uses an amltude-and-hase otmzed ar of obe ulses and yelds bakground-suessed setra n eah shot wthout need for hase sannng. We have demonstrated three model shemes; the s- sheme, the s-s sheme, and the (+/-) sheme. In all shemes, the resonant CAR sgnal from only the 3-olor CAR mehansm s reovered wth mnmal attenuaton whle the resonant sgnal from -olor CAR mehansm beomes neglgble when the B ontrbuton s suessed. We have dsussed otmzaton ondtons for the sgnal ntensty and shae of resonant eaks. [1] REFERENCE J. X. Cheng, A. Volkmer, L. D. Book, and X.. Xe, "Multlex oherent ant-stokes Raman satterng mrosetrosoy and study of ld vesles," J. Phys. Chem. B 16, 8493 (). [] T. W. Kee and M. T. Cerone, "mle aoah to one-laser, broadband oherent ant-tokes Raman satterng mrosoy," Ot. Lett. 9, 71 (4). [3] G. I. Petrov and V. V. Yakovlev, "Enhanng red-shfted whte-lght ontnuum generaton n otal fbers for alatons n nonlnear Raman mrosoy," Ot. Exess 13, 199 (5). [4] H. A. Rna, M. Bonn, and M. Muller, "Quanttatve multlex CAR setrosoy n ongested setral regons," J. Phys. Chem. B 11, 447 (6). [5] Y. J. Lee, Y. Lu, and M. T. Cerone, "Charaterzaton of 3-olor CAR n a -ulse broadband CAR setrum," Ot. Lett. 3, 337 (7). [6] D. Oron, N. Dudovh, and Y. lberberg, "Femtoseond hase-and-olarzaton ontrol for bakground-free oherent ant-tokes Raman setrosoy," Phys. Rev. Lett. 9, 139 (3). [7] B. C. Chen and. H. Lm, "Otmal laser ulse shang for nterferometr multlex oherent ant-stokes Raman satterng mrosoy," J. Phys. Chem. B 11, 3653 (8). [8] T. W. Kee, H. X. Zhao, and M. T. Cerone, "One-laser nterferometr broadband oherent ant-tokes Raman satterng," Ot. Exess 14, 3631 (6). [9] H. Kano and H. Hamaguh, "Dserson-omensated suerontnuum generaton for ultrabroadband multlex oherent ant-tokes Raman satterng setrosoy," J. Raman etros. 37, 411 (6). [1] G. L. Eesley, Coherent Raman etrosoy, (Pergamon Press, Oxford, 1981). [11] D. Oron, N. Dudovh, D. Yeln, and Y. lberberg, "Narrow-band oherent ant-stokes Raman sgnals from broadband ulses," Phys. Rev. Lett. 88, 731 (). [1] A. Morres, L. Maran, M. R. Dstefano, and M. G. Gorgn, "Vbratonal-Relaxaton Proesses n Isotro Moleular Lquds - A Crtal Comarson," J. Raman etros. 6, 179 (1995). [13] J. M. Dudley, G. Genty, and. Coen, "uerontnuum generaton n hoton rystal fber," Rev. Mod. Phys. 78, 1135 (6). Offal ontrbuton of the Natonal Insttute of tandards and Tehnology; not subjet to oyrght n the Unted tates Pro. of PIE Vol Y-8 Downloaded from PIE Dgtal Lbrary on 6 Ot 9 to Terms of Use: htt://sedl.org/terms