Optically seeded stimulated Raman scattering of aqueous sulfate microdroplets

1510 J. Opt. Soc. Am. B/Vol. 13, No. 7/July 1996 Pasternack et al.

Optically seeded stimulated Ramanscattering of aqueous sulfate microdroplets

Louise Pasternack

Code 6111, Naval Research Laboratory, Washington, D.C. 20375-5342

James W. Fleming


Jeffrey C. Owrutsky*


Received August 8, 1995

Optically seeded, stimulated Raman scattering is demonstrated for 25-mm-radius water droplets containingsulfate. Frequency-doubled Nd:YAG laser-excited Stokes radiation from the 3450-cm21 O–H-stretchingvibration of water provides the seed for morphology-dependent stimulated Raman scattering from the n1vibration of sulfate that is excited by a dye laser. Seeding enhances the otherwise weaker signal for lowconcentrations of the anion, thereby reducing the solute detection limit, by a method that does not requirechanging of the droplet composition. 1996 Optical Society of America

1. INTRODUCTION

Aerosols are of fundamental importance in marineand atmospheric environments. They are the primarysite for atmospheric heterogeneous chemistry, facilitat-ing reaction pathways that play a significant role inatmospheric chemistry, including stratospheric ozonedepletion.1,2 In addition, the uptake of gases by wa-ter droplets and their reactions occur in nonprecipitatingcloud cycles that cause a growth in cloud condensationnuclei.3 Recent evidence indicates that reaction ratesthat occur in aerosols may be different than rates in-ferred from bulk liquids. For example, from Henry’slaw4 we can predict that the uptake of SO2sgd into wa-ter should be of the order of 2.2 M atm21 at 10 ±C.5

However, the uptake of SO2sgd onto droplet surfacesis greater than predicted by Henry’s law and is depen-dent on pH (Ref. 6); this indicates that heterogeneousprocesses may be important. Also, in cloud-chamber ex-periments, both the oxidation of SO2 to sulfate in thepresence of NH3 and the reaction of SO2 with O3 aresignificantly faster than predicted from rates measuredin bulk solutions.3 To measure gas uptake directly andto monitor heterogeneous chemical reactions in dropletsin real time, a sensitive method is necessary to identifyand measure the concentration of dissolved species andtheir reaction products.

Morphology-dependent stimulated Raman scattering(MDSRS) is a sensitive spectroscopic technique for moni-toring the chemical composition of aerosols. First re-ported ten years ago,7 MDSRS is stimulated Ramanscattering8 – 10 (SRS) that occurs within the optical cavityformed by a droplet. Illumination near the edge of adroplet results in trapping of radiation in a region near

0740-3224/96/071510-07$10.00

the droplet surface because of nearly total internal reflec-tion at the liquid–air interface. Thus the droplet actsas an optical cavity. When resonance conditions are sat-isfied between the radiation wavelength and the dropletcharacteristics (size, shape, and index of refraction) astanding-wave pattern is established. These resonancesare called morphology-dependent resonances (MDR’s).They can be calculated with the Lorenz–Mie theory ofscattering by a homogeneous sphere.11 Light at an ap-propriate wavelength circulates in the droplet, increasingthe optical path length to ,1 m and the photon lifetime inthe cavity to several nanoseconds. The increased pathlength promotes nonlinear processes such as SRS, so theycan be observed by a relatively low-intensity laser.11

SRS9 is an automatically phase-matched, partially de-generate four-wave mixing process in which spontaneousRaman scattering at frequency vs, generated from inci-dent laser light at frequency v,, stimulates emission ofanother Stokes photon because of interaction with an-other laser photon. The difference v, 2 vs correspondsto the frequency of a Raman-active transition of the sam-ple, typically vibrational or electronic. SRS has beenused extensively to shift laser radiation to generate newlaser frequencies and for studies of vibrational dynamics.Sequential multistep SRS that is due to excitation bythe (n 2 1 order) Stokes-shifted incident radiation is alsoobserved (at v, 2 nvs).8,9 The relatively long opticalpath lengths in droplets facilitate SRS, so MDSRS is apromising spectroscopic technique for droplet compositionmeasurements. The effective SRS threshold is furtherreduced by the curved liquid–air interface that focusesthe laser intensity.12 The frequency spacing of theMDSRS signals can be used to determine the size andthe index of refraction of the droplets.13

1996 Optical Society of America

Pasternack et al. Vol. 13, No. 7 /July 1996/J. Opt. Soc. Am. B 1511

MDSRS has been used to measure species concen-trations in fuel14 and water droplets.12,15,16 In waterdroplets concentrations of ammonium sulfate15 and potas-sium nitrate16 as low as 0.1 M have been measured.At low laser intensity only spontaneous Raman scat-tering is detected. As the input laser intensity is in-creased beyond the threshold, the SRS signal increasesexponentially until saturation is reached, and ultimatelylaser-induced breakdown of the droplet occurs. Thesephenomena have all been observed experimentally.12,17

The presaturation exponential growth of the Stokes waveis described by the SRS gain equation8,9

I s,dSRS / IR expfsGR 2 ad,g , (1)

where I s,dSRS is the intensity of the SRS, IR is the in-tensity of the spontaneous Raman signal, , is the opticalpath length in the droplet, and a represents the absorp-tion and the scattering losses. GR , the Raman gain, canbe expressed as

GR /NI0sdsydVd

G, (2)

where N is the concentration of the Raman-active mate-rial (assuming no excited-state population), I0 is the laserintensity, sdsydVd is the spontaneous Raman cross sec-tion, and G is the spontaneous Raman linewidth.

In bulk samples and for long laser pulses, the SRS sig-nal is dominated by the mode with the maximum gain. Along pulse in this case means a time longer than the vibra-tional dephasing time, typically less than a few picosec-onds in the condensed phase. The dominant mode is theone with the largest Raman gain coefficient, so SRS favorshigh-concentration species with large cross sections andnarrow linewidths. The input radiation is depleted bythe SRS process, reducing the intensity below thresholdfor other modes.9 SRS in droplets is not as selective asin bulk, which suggests that in this case pump depletionis not as severe.18 This reduced pump depletion appearsto depend on factors specific to the droplet, including theadditional constraint of the MDR’s, which result in incom-plete spatial overlap between the pump and the strongestStokes waves. Nevertheless, the gain competition limitsthe utility of SRS for detecting low-concentration solutes.

In water droplets containing dissolved salts, Eickmanset al.16 tentatively concluded that SRS of the solute is notgenerated once its concentration is such that the sponta-neous Raman intensity of the ion is weaker than that ofthe O–H-stretching mode of water. They observed SRSsignals from the sulfate anion in water droplets at concen-trations $ 0.4 M. Zhang and Aker15 reported SRS sig-nals from ammonium sulfate at concentrations as low as0.1 M. They predicted that the MDSRS detection limitcan be lowered by use of higher-purity water and an in-cident laser with a narrower frequency bandwidth anda shorter wavelength. These predictions have not beenverified experimentally.

Currently, the applicability of the MDSRS techniqueto droplets of atmospheric interest is limited by sensi-tivity. Several techniques have been investigated to en-hance the sensitivity of MDSRS. A successful methodof enhancing weaker-gain Raman modes was reported by

Kwok and Chang19; they seeded a weaker-gain mode inethanol, using fluorescence seeding by dissolving Rho-damine 6G (R6G) in the droplet. The seeding linearlyenhanced the SRS of the weaker-gain Raman mode. Athigher dye concentrations20 lasing of the R6G occurred;the lasing gain, together with the Raman gain, enhancedthe SRS exponentially. The enhancement due to the las-ing R6G was much stronger than the spontaneous fluores-cence seeding alone.20 By adding small latex particles toethanol containing R6G that affect the output coupling ef-ficiency for the MDR’s, Xie et al.21 demonstrated that SRSemission pumped by lasing from the R6G in the dropletis a double-resonance effect.

There have been only a few reports about the use oftwo lasers simultaneously interacting with small liquiddroplets. Qian et al.22 reported coherent Raman mixingand coherent anti-Stokes Raman spectroscopy mixing aswell as SRS in both ethanol and water droplets, using thefundamental and the second harmonic of a Nd:YAG laser.They found a significant coherent Raman mixing signalconsisting of spectrally narrow peaks similar to the SRSspectrum. The coherent Raman mixing process loweredthe SRS threshold. In contrast, they observed no MDRstructure and critical phase matching for the coherentanti-Stokes Raman spectroscopy.

In this paper the enhancement of dye-laser-excited SRSsignals from water droplets containing a dilute sulfatesolute by seeding with Nd:YAG laser-excited water SRSsignals is reported. This enhancement is accomplishedby selection of the dye-laser wavelength so that the n1

sulfate SRS signal occurs at the same wavelength as theO–H-stretch Raman scattering from the Nd:YAG laser.A schematic of this process is shown in Fig. 1. We ob-serve SRS that is orders of magnitude stronger for sulfateat the Stokes wavelength of the dye laser with the useof both lasers compared with the use of only one. Thismethod does not involve additives to the droplets. It isthe seeded MDSRS (SMDSRS) enhancement that is thetopic of this paper.

2. EXPERIMENTA stream of monodispersed droplets is produced by a mod-ified Berglund–Liu23 vibrating orifice aerosol generator(VOAG) as described by Lin et al.24 The generator usesa 25-mm-diameter aperture mounted on a piezoelectrictranslator (TSI Model 1030431X). The solution deliv-ered to the aperture contains ammonium sulfate (Aldrich991%) dissolved in triply distilled deionized water. Thesolution is contained in a vinyl bag inside a pressurizedchamber (,15 lbyin.2) to avoid problems created by dis-solved gases. A filter (Millipore, 2-mm pore size) pre-vents clogging of the aperture. Droplets are formed byvibration of the aperture at 55–90 kHz by a waveformgenerator (Stanford Research System Model DS345) tosupply the driving voltage (3–10 V) to the piezoelectrictranslator. One monitors the droplet stream by observ-ing the stability of the diffraction pattern of a He–Nelaser incident upon the droplet stream, using a photo-multiplier tube.

A frequency-doubled Nd:YAG laser pumped dye-lasersystem is the excitation source for generating SRS spec-tra. Eighty percent of the 532-nm frequency-doubled


Fig. 1. Schematic showing the wavelengths for spontaneous Raman and MDSRS with frequency-doubled Nd:YAG (lower trace) anddye-laser (upper trace) excitation. The sulfate spontaneous Raman peak is narrow, so only a single MDR is excited. The waterspontaneous Raman peak at 650 nm for Nd:YAG excitation is broad, so many MDR’s (solid vertical lines) can be excited, as describedin the text. The region of overlap near 650 nm is the region in which seeded MDSRS can occur. Solid and dashed double-headedarrows indicate dye-laser and dye-laser-excited sulfate SRS tuning ranges, respectively.

Nd:YAG (YAG) laser output (Quantel Model 581C; 10 Hz,10 ns) pumps a dye laser (Quantel Model TDL50) op-erating between 606 and 625 nm with Rhodamine 610.Pulse energies, selected to be just below the energiesthat would result in droplet breakdown, are 2–5 mJ forboth the dye and the 532-nm pulses incident upon thedroplet stream. The intensity of the YAG laser is var-ied by use of neutral-density filters and a polarizer. Theintensity of the dye laser is varied with neutral-densityfilters. The Q switch of the Nd:YAG laser is synchro-nized to the droplets by means of the signal obtained froma He–Ne laser incident upon the droplet stream. Thedye and the YAG laser beams are combined both in timeand space and are focused onto the droplets with a 10-cmfocal-length lens. The beam waists of the YAG and thedye lasers are 75–100 mm in diameter. The laser inten-sities at the droplets are estimated to be approximately0.5–2 GWycm2.

The SRS signals are collected at 90± to the excitationbeams with a monochromator (Jarrell–Ash, 0.5 m) andan optical multichannel analyzer (EG&G Model 1460).Typically, 20-nm-wide spectra (for 512 channels) are col-lected for three laser shots and are stored on the opticalmultichannel analyzer and transferred to a personal com-puter for analysis and display.

3. RESULTS

A. Single-Color Stimulated Raman ScatteringWe observe strong single-color SRS signals in two spec-tral regions from aqueous ammonium sulfate droplets(0.2–2 M) when exciting with the YAG laser at 532 nm,as depicted schematically in Fig. 1. The narrow featureat 561 nm corresponds to the 981-cm21 shift that is due

to the n1 mode of the sulfate ion, similar to previouslyreported results.12,15,16 At higher sulfate concentrations(1–2 M) the second-order Stokes sulfate shift is also ob-served at 594 nm sv, 2 2vsd, similar to higher-orderStokes signals previously reported from droplets.16,25

A broad spectrum is observed near 650 nm from the3450-cm21 O–H-stretching vibration of water, as pre-viously described.7,12,16,22,26 The envelopes for sulfateand water SRS bands that we observe are narrower andbroader, respectively, than the MDR spacing. In sepa-rate experiments with dye-laser excitation (near 610 nm),single-laser SRS from sulfate is observed near 650 nm,and a dye-laser excited-water O–H stretch SRS occursnear 775 nm.

The water SRS signal near 650 nm consists of agroup of sharp, evenly spaced peaks that occur withinthe bandwidth of the spontaneous Raman water signal(,350-cm21 width16). Because the bandwidth is greaterthan the MDR mode spacing, the water SRS is observedfor a wide range of droplet sizes (or, equivalently, sizeparameters x 2pryl). This is in contrast to the sul-fate signal, as illustrated in Fig. 1. The lines observedin the water spectrum are due to MDR’s that have thesame mode order but different mode numbers.11,16 Theline spacing Dl of adjacent mode numbers at a particu-lar wavelength l is related to the droplet radius r asdescribed by the formula11,27

r l2

2pDl

arctansm2 2 1d1/2

sm2 2 1d1/2, (3)

where m is the index of refraction of the droplet atl. Using this formula, we determine from the ,2-nmobserved mode spacing that the droplets have radii of


,25 mm. The droplet size is controlled to better than0.3% by variation of the frequency of the vibrating ori-fice and observation of changes in the MDSRS spectrum.The radius r is related to the VOAG frequency f by theequation r / 1yf 1/3. Changing the droplet size causesthe water signal to become broad with discrete peakssuperimposed on it. This effect is likely due to partici-pation of more MDR modes and mode orders and a super-position of spontaneous and stimulated Raman signals.Because the water SRS bandwidth is broad, it is possi-ble to observe the size-dependent MDR changes withoutlosing the signal. The water SRS spectra provide a con-venient way to adjust the droplet size and to assess theexperimental system stability.

The sulfate is a single, sharp feature because the band-width of the sulfate spontaneous Raman peak (4.14-cm21

full width15) is less than the spacing of MDR modes. Theexperimental SRS bandwidth is unresolved, but it is prob-ably due to the MDR linewidth rather than the sulfatebandwidth, based on expectations for the quality factor Qof the (output) mode. (Q is the circulating power dividedby the power loss per round trip and is equal to lyDl .

105.) As noted above, the ,25-mm-radius droplets resultin a MDR spacing of ,2 nm (ø 60 cm21 at 561 nm). Be-cause the sulfate SRS bandwidth is less than the MDRspacing, it is possible for the Stokes wavelength to fallbetween MDR wavelengths, in which case SRS is not de-tected. The resonance between the Stokes and the MDRwavelengths can be achieved by alteration of the dropletsize. Alternatively, the dye-laser wavelength can be ad-justed so that the Stokes signal wavelength coincides withthat of a MDR. The correspondence between droplet sizeand wavelength is evident from Eq. (3). At higher con-centrations (0.5–2 M) and greater excitation intensities,MDR’s with different mode numbers (but the same modeorder) are also observed in the wings of the sulfate SRSsignal at spacings (,1.5 nm) corresponding to the dropletsize, as was previously reported.12 Also at higher con-centrations, the sulfate SRS signal is saturated, showingvery little dependence of the signal on laser intensity.

B. Two-Color, Enhanced Stimulated Raman ScatteringSRS spectra are also observed for water microdropletscontaining ammonium sulfate that are simultaneouslyexcited by both dye and YAG lasers. The dye laser is op-erated near 610 nm so that the SRS from sulfate ions(981-cm21 shift) occurs in the same spectral regionas Stokes radiation from the O–H stretch of water(3450-cm21 shift) excited by the YAG laser, as illustratedin Fig. 1. The two-color Stokes intensity is significantlyenhanced compared with that from one laser. Its depen-dence on the dye-laser wavelength, the MDR structure,and the solute concentration provides experimental in-formation that allows us to investigate the mechanism ofthe observed seeding enhancement.

Figure 2 shows the SRS spectra observed with boththe dye and the YAG lasers (upper trace) and with theYAG alone (lower trace) for 0.2 M aqueous ammoniumsulfate solution droplets. The frequency of the dropletgenerator is tuned to obtain a broad water spectrum withpoorly resolved MDR structure near 651 nm. The ob-served spectrum is probably a combination of spontaneousand SRS signals, as described above. In the upper trace

both lasers are coincident on the droplets; the dye laseris operated at 612 nm so that the Stokes signal from then1 sulfate mode occurs at 651 nm. The two-color spec-trum shows a strong, narrow feature appearing with thespectral signature of sulfate SRS that is . 10 times theintensity of the water SRS from the YAG laser alone.The water SRS spectrum is otherwise unperturbed by thedye laser. Under these conditions of laser intensity andalignment, there is no detectable signal from the sulfatewith the use of only the dye laser. This remains trueeven when the droplet size is tuned by variation of theVOAG frequency. Therefore, compared with the sulfateSRS signal generated by the dye laser alone, the enhance-ment by water Raman seeding is . 1000, determined bythe observable signal-to-noise ratio. The two-color sul-fate SRS is also stronger than that generated by the single532-nm laser (which occurs at 561 nm). It is also clearthat the two-color signal is not merely the sum of intensi-ties of the scattering from each laser alone, but rather isthe result of a nonlinear optical mixing process occurringwithin the droplet.

The spectra in Figs. 3 and 4 show seeded SRS spec-tra as a function of dye-laser wavelength. The two-colorSRS intensity depends on the MDR structure, as shownin the (single-color) water spectrum. An example of well-resolved, quasi-periodic, single-mode-order water SRS ob-tained with the YAG laser alone is shown in the lowercurve of Fig. 3. When the dye-laser beam is also incidentupon the droplets and its wavelength is scanned, sulfate

Fig. 2. Stimulated Raman spectra observed for aqueous 0.2 Msulfate droplets showing enhanced SMDSRS from using twolasers. The lower curve was recorded by means of a singlelaser at 532 nm (frequency-doubled Nd:YAG) and shows partiallyMDR-resolved SRS near 650 nm from the O–H-stretching vibra-tion of water. The upper curve, offset for clarity, was obtainedunder similar conditions, except that a dye laser operating at612 nm was also incident upon the droplets. The sulfate-shiftedStokes beam from the dye occurs at 651 nm, and the two-laserSMDSRS is considerably stronger than the surrounding waterSRS from the YAG laser. We observed no signal in this regionwhen using the dye laser alone.


Fig. 3. Optically seeded stimulated Raman spectra in the re-gion near 650 nm for aqueous 0.35 M sulfate droplets for sev-eral dye-laser wavelengths. The lower curve was obtained withonly the frequency-doubled Nd:YAG laser at 532 nm showinga spectrum with MDR structure from water. The middle andthe upper curves, offset for clarity, display two-laser SMDSRSspectra occurring when the sulfate SRS signal overlaps a waterMDSRS peak. In the middle and the upper curves the dye laseris operating at 613.8 and 612 nm, corresponding to sulfate Stokesscattering at 653.1 and 651.1 nm, respectively. The SMDSRS inthis case suppresses the water SRS from the 532-nm laser.

signal enhancement is observed only when the sulfateStokes shift is coincident with the water MDSRS peaks(upper and middle curves in Fig. 3). In this case, notonly is the sulfate SRS signal enhanced, but the watersignal is also suppressed.

At higher ammonium sulfate concentration (1 M) thesignal enhancement is observed as the dye laser isscanned, so the sulfate SRS occurs farther from the cen-tral region of the water MDSRS spectrum. In Fig. 4 weshow a series of MDSRS spectra as a function of dye-laserwavelength in which enhancement is observed at dis-crete dye-laser wavelengths. As the dye laser is scannedfrom 618 to 615.75 nm, the SMDSRS signal appears at657 nm (for lDL 617.3 nm), and after disappearing itreemerges at the next water MDSRS peak at 655 nm (forlDL 615.8 nm). Signal is observed only when there isoverlap between the sulfate Stokes shift of the dye laserand a water MDSRS resonance.

As a result of the superposition of a number of MDR’sandyor the spontaneous Raman spectrum, we also observestrong two-color sulfate signals when the water SRS is ad-justed to yield a poorly resolved spectrum. This is eas-ily arranged by means of the optical alignment and theVOAG frequency and results in spectrally broad and un-structured sulfate, two-beam SRS as the dye wavelengthis scanned. The two-color SRS signal generally followsthe contour of the water SRS spectrum.

The one-color and the two-color SRS spectra both de-pend on several experimental parameters; the most im-portant ones are sulfate concentration, laser intensities

and alignment, dye-laser wavelength, and droplet size.Even single-color MDSRS is quite sensitive to alignment.For example, when the laser intersects the edge or thecenter of the droplet, MDSRS or spontaneous Raman scat-tering is favored, respectively. The laser mode structureand alignment with the droplet strongly affects the ob-served MDR structure, i.e., how periodic it appears in thewater SRS. This becomes more complicated when twolasers are involved because their overlap also becomes afactor. We observed a nonlinear dependence of the two-color SRS on the sulfate concentration. We also observeda nonlinear dependence of the signal on the intensity ofeach laser. The dye-laser wavelength and droplet-sizedependence of the SMDSRS is similar to that for the one-color sulfate MDSRS.

4. DISCUSSIONWe observe stronger sulfate SRS with two-laser seed-ing. One way to characterize the sensitivity enhance-

Fig. 4. Two-color SRS spectra measured in the 645–660-nmregion for aqueous 1 M sulfate droplets obtained with a 532-nmlaser and a dye laser as a function of dye-laser wavelengthin the range 615.75–618 nm. The scans were collected every0.25 nm and are offset for clarity. The water SRS from the532-nm laser is partially MDR resolved and occurs near 651 nm.Because of the high solute concentration, two-color SMDSRSspectra are seen in the wings of the water band for these dye-laserwavelengths in the region near 655 nm. The SMDSRS signaldisplays MDR behavior by exhibiting an ,2-nm spacing becauseof the ,25-mm-radius droplets.


ment is to compare the SMDSRS sulfate signals withthe single-color SRS signals from the YAG laser andthe dye laser. A direct comparison between the sulfateMDSRS and the seeded signal is not straightforward be-cause the one- and the two-color signals optimize underdifferent conditions, e.g., laser alignment. Sulfate canbe detected at lower concentration with the YAG (underour experimental conditions, 0.2 M with the YAG laserversus 0.5 M with the dye laser). The relative single-color SRS intensity and sulfate detection sensitivity re-flect characteristics of the lasers, including linewidth andmode quality as well as the wavelength dependence of theSRS intensity. In this spectral region the Raman crosssection of sulfate decreases with increasing wavelength.The stronger absorption of water at 612 nm comparedwith 532 nm reduces the Q factor within the droplet, fur-ther reducing the gain. Absorption also decreases thebreakdown threshold, thereby decreasing the maximumusable pump intensity. Nevertheless, at lower concen-tration (0.2 M), it is clearly easier to detect the sulfateSMDSRS (at 650 nm) than the MDSRS with the YAGalone (at 561 nm). This is also true if the signal is in-dependently optimized for each case. Comparisons athigher concentration (0.5–1 M) also show that the two-color signal is stronger. At high concentration one- andtwo-color SRS are both easily saturated, and other fac-tors come into play, such as generation of higher-orderStokes and MDR signals. These effects render absolutesignal- and laser-power dependence measurements unre-producible. The lower-concentration studies provide theclearest evidence for the SMDSRS enhancement, in whichsingle-color MDSRS from the dye laser is orders of magni-tude weaker and in which, from the YAG laser pumpingalone, it is at least an order of magnitude weaker.

Seeding the solute SRS signal from one laser with thesolvent Raman scattering from another laser has severaladvantages that are particularly evident when comparedwith other seeding methods that we tried. Using a dyelaser operating at the sulfate Stokes wavelength (561 nm)to seed the signal from the YAG laser in a straightforwardstimulated Raman gain geometry is not feasible becauseof the excessive scatter from the dye laser at the detec-tion wavelength that is due to the droplet. Whereas it ispossible to use the second Stokes wavelength for sulfatefrom the YAG laser (at 594 nm) to seed the first Stokeswavelength from the dye laser (operating at 561 nm), thesignal is much stronger for the first Stokes wavelengthfrom the YAG, so the sensitivity is not greatly improved.Also, because in this case both sulfate MDSRS signalsare narrow, this technique suffers from MDR mode struc-ture sensitivity and the inability to monitor it indepen-dently. The current approach, employing the first Stokeswavelength from each laser (albeit for different species),provides higher detection sensitivity. Additionally, thebroad spectrum of water provides a convenient way tomonitor changes that occur in the MDR structure withthe addition of a second laser. The nearly constant wa-ter contribution at low solute concentrations provides thepotential for quantitative measurements, contingent onknowledge of the origin of the signal.

The precise description of the two-color SRS signal isnot obvious. Its MDR behavior demonstrates that someMDSRS is occurring. Because the SMDSRS has the ap-

pearance of enhanced sulfate SRS, it is tempting to as-cribe it to water SRS-seeded sulfate SRS. However, itcould also be described as sulfate SRS-seeded water SRS.Furthermore, the seeding of the SRS signal for one speciescould be due to the spontaneous Raman intensity from theother species. The SRS intensity for a particular vibra-tional mode takes the form

I s,diSRS / sIR 1 IseeddexphfGi

R sIpd 2 ag,j , (4)

which is similar to relation (1) for one-color SRS. InEq. (4), i denotes the vibrational mode, in this case fordifferent molecular species, sulfate sn1d or water (O–Hstretch). For the SRS of interest in this study, near650 nm, Ip is either the dye or the YAG laser inten-sity, and the Raman gain coefficient GR , as describedin relation (2), is a function of concentration and Ramanspontaneous cross section for the seeded SRS species. IR

and Iseed are spontaneous Raman and seed intensities, re-spectively, from which the SRS develops. For SMDSRS,Iseed from one species can be from the SRS or spontaneousRaman intensity from the other species. It is not easy todistinguish which species is seeding and which is respon-sible for the gain.

There is evidence that the SMDSRS involves both sul-fate SRS and water SRS, which would be seeded by wa-ter and sulfate, respectively. The type of seeding maydepend on experimental variables, especially the align-ment. We find a strong sulfate concentration depen-dence for the SMDSRS that is similar to what is ob-served for single-color MDSRS. This is typical of SRS,which displays an exponential concentration dependence,as indicated in relations (1) and (2). If the spontaneousRaman from sulfate were seeding water SRS, we wouldexpect the concentration dependence to be linear. (Thiswould be beneficial because it could extend detection tolower sulfate concentration). The participation of waterSRS is demonstrated by evidence of pump depletion. Insome cases the two-color signal suppresses the water spec-trum (as in Fig. 3). This situation may best be describedas sulfate (stimulated Raman or spontaneous Raman)-seeded water SRS that is strong enough to deplete theYAG pump and to inhibit the unseeded modes of thesingle-color YAG water MDSRS. The specific nature ofthe SMDSRS probably varies with experimental condi-tions. For example, it is possible to adjust the alignmentand the droplet size to detect a broad water spectrum(as in Fig. 2), which may indicate spontaneous Ramanscattering in the one-color spectrum and spontaneous Ra-man water-seeded SMDSRS. Regardless of the specificdescription of the SMDSRS, the experimental design pro-vides enhanced sensitivity for the solute compared withthe single-laser SRS.

Several approaches have been suggested for increasingthe SRS sensitivity in droplets, such as the use of nar-rower linewidth and shorter-wavelength lasers.14 Forwater, the maximum SRS gain is predicted to occurat 475 nm, at the minimum absorption of water. At475 nm, the Q factor is largest, and the spontaneousRaman cross section will be larger than at longerwavelengths. Combining the seeding with these othermeasures should provide the best sensitivity, further


improving the enhancement that we observe for the two-color SRS compared with the SRS from just the dye laser(. 1000 in signal and .2.5 in concentration).

We have demonstrated enhanced solute detection sen-sitivity by using two-color SRS. This is achieved by useof the SRS from the high-concentration species (solvent)to seed SRS from the low-concentration solute. Minorspecies are detected more easily in droplets than in thebulk because the droplet MDR structure reduces themode competition in which the mode with the strongestRaman gain depletes the pump intensity. Althoughthis still limits the sensitivity of MDSRS for minorityspecies, the technique that we have presented for seededMDSRS overcomes this limitation. For low solute con-centrations in which the solvent SRS would otherwise bedominant, SMDSRS decreases the solute detection limitcompared with using one laser. Furthermore, unlikemost other methods reported for enhancing MDSRS spec-tra, this method does not involve changing the chemicalcomposition of the droplets. Hence this method could beused under circumstances in which it is not possible tochange the droplets, such as in remote sensing applica-tions of the environment. Finally, because SMDSRS is atwo-laser process that is automatically phase matched, itis conceivable that it could be used for imaging of aerosols.

ACKNOWLEDGMENTSThe authors thank L. J. Medhurst and G. Longrie for as-sistance with the experimental setup and H.-B. Lin forthe VOAG equipment loan and valuable discussions. Wealso thank Richard Chang for his helpful discussions.Funding for this research was provided by the U.S. Of-fice of Naval Research in support of the EnvironmentalChemistry Program at the Naval Research Laboratory.

*Naval Research Laboratory–National Research Coun-cil Postdoctoral Research Associate.

REFERENCES1. S. Solomon, “The mystery of the Antarctic ozone ‘hole,’ ” Rev.

Geophys. 26, 131–148 (1988).2. S. N. Pandis, A. S. Wexler, and J. H. Seinfeld, “Dynamics

of tropospheric aerosols,” J. Phys. Chem. 99, 9646–9659(1995).

3. W. A. Hoppel, G. M. Frick, J. W. Fitzgerald, and B. J. Wat-tle, “A cloud chamber study of the effect that nonprecipi-tating water clouds have on the aerosol size distribution,”Aerosol Sci. Technol. 20, 1–30 (1994).

4. W. J. Moore, Physical Chemistry, 4th ed. (Prentice-Hall,Englewood Cliffs, N. J., 1972), pp. 239–240.

5. D. R. Worsnop, M. S. Zahniser, C. E. Kolb, J. A. Gardner,L. R. Watson, J. M. Van Doren, J. T. Jayne, and P. Davi-dovitis, “Temperature dependence of mass accommodationof SO2 and H2O2 on aqueous surfaces,” J. Phys. Chem. 93,1159–1172 (1989).

6. J. T. Jayne, P. Davidovits, D. R. Worsnop, M. S. Zahniser,and C. E. Kolb, “Uptake of SO2(g) by aqueous surfaces asa function of pH: the effect of chemical reaction at theinterface,” J. Phys. Chem. 94, 6041–6048 (1990).

7. J. B. Snow, S.-X. Qian, and R. K. Chang, “Stimulated Ra-man scattering from individual water and ethanol droplets

at morphology-dependent resonances,” Opt. Lett. 10, 37–39(1985).

8. N. Bloembergen, “The stimulated Raman effect,” Am. J.Phys. 35, 989–1023 (1967).

9. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, NewYork, 1984), Chap. 10, pp. 141–186.

10. A. Yariv, Quantum Electronics, 2nd ed. (Wiley, New York,1975), pp. 470–488.

11. S. Hill and R. E. Benner, “Morphology dependent reso-nances,” in Optical Effects Associated with Small Particles,P. W. Barber and R. K. Chang, eds. (World Scientific, Tea-neck, N.J., 1988), Chap. 1, pp. 3–61.

12. A. Serpenguzel, G. Chen, and R. K. Chang, “Stimulated Ra-man scattering of aqueous droplets containing ions: con-centration and size determination,” Particulate Sci. Technol.8, 179–189 (1990).

13. P. Chylek, V. Ramaswamy, A. Ashkin, and J. M. Dziedzic,“Simultaneous determination of refractive index and sizeof spherical dielectric particles from light scattering data,”Appl. Opt. 22, 2302–2307 (1983).

14. W. P. Acker, A. Serpenguzel, R. K. Chang, and S. C. Hill,“Stimulated Raman scattering of fuel droplets,” Appl. Phys.B 58, 9–16 (1990).

15. J.-X. Zhang and P. M. Aker, “Spectroscopic probing ofaerosol particle interfaces,” J. Chem. Phys. 99, 9366–9375(1993).

16. J. H. Eickmans, S.-X. Qian, and R. K. Chang, “Detection ofwater droplet size and anion species by nonlinear opticalscattering,” Part. Charact. 4, 85–89 (1987).

17. M. Sparks, “Stimulated Raman and Brillouin scattering:parametric instability explanation of anomalies,” Phys. Rev.Lett. 32, 450–453 (1974).

18. M. M. Mazumder, K. Schaschek, R. K. Chang, and J. B.Gillespie, “Efficient pumping of minority species stimulatedRaman scattering (SRS) by majority species SRS in a mi-crodroplet of a binary mixture,” Chem. Phys. Lett. 239,361–368 (1995).

19. A. S. Kwok and R. K. Chang, “Fluorescence seeding ofweaker-gain Raman modes in microdroplets: enhancementof stimulated Raman scattering,” Opt. Lett. 17, 1262–1264(1992).

20. A. S. Kwok and R. K. Chang, “Suppression of lasing bystimulated Raman scattering in microdroplets,” Opt. Lett.18, 1597–1599 (1993).

21. J.-G. Xie, T. E. Ruekgauer, R. L. Armstrong, and R. G. Pin-nick, “Suppression of stimulated Raman scattering from mi-crodroplets by seeding with nanometer-sized latex particles,”Opt. Lett. 18, 340–342 (1993).

22. S.-X. Qian, J. B. Snow, and R. K. Chang, “Coherent Ramanmixing and coherent anti-Stokes Raman scattering from in-dividual micrometer-size droplets,” Opt. Lett. 10, 499–501(1985).

23. R. N. Berglund and Y. H. Liu, “Generation of monodis-perse aerosol standards,” Environ. Sci. Technol. 7, 147–153(1973).

24. H.-B. Lin, J. D. Eversole, and A. J. Campillo, “Vibratingorifice droplet generator for precision optical studies,” Rev.Sci. Instrum. 61, 1018–1023 (1990).

25. S.-X. Qian and R. K. Chang, “Multiorder Stokes emis-sion from micrometer-size droplets,” Phys. Rev. Lett. 56,926–929 (1986).

26. J.-G. Xie, T. E. Ruekgauer, J. Gu, R. L. Armstrong, andR. G. Pinnick, “Random occurrence of stimulated Ramanscattering emission from liquid water microdroplets,” Appl.Opt. 33, 368–372 (1994).

27. R. G. Pinnick, A. Biswas, P. Chylek, R. L. Armstrong, H.Latifi, E. Creegan, V. Srivastava, M. Jarzembski, and G.Fernandez, “Stimulated Raman scattering in micrometer-sized droplets: time-resolved measurements,” Opt. Lett. 18,2229–2233 (1978).

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Optically seeded stimulated Raman scattering of aqueous sulfate microdroplets