We report lasing in airborne, rhodamine B-doped glycerol–water droplets with diameters ranging between 7.7 and 11.0 µm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of the intensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by changing the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.© 2013 Optical Society of AmericaThis paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-38-10-1669. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law

A. Jonáš

A. Kiraz

D. McGloin

M. Aas

S. Anand

Y. Karadag

English

Karadag, Y.

Aas, M.

Anand, S.

McGloin, D.

Kiraz, A.

Discovery Research Portal

                                                              University of DundeeDye lasing in optically manipulated liquid aerosolsKaradag, Y.; Aas, M.; Anand, S.; McGloin, David; Kiraz, A.Published in:Optics LettersDOI:10.1364/OL.38.001669Publication date:2013Document VersionPublisher's PDF, also known as Version of recordLink to publication in Discovery Research PortalCitation for published version (APA):Karadag, Y., Aas, M., Anand, S., McGloin, D., & Kiraz, A. (2013). Dye lasing in optically manipulated liquidaerosols. Optics Letters, 38(10), 1669-1671. 10.1364/OL.38.001669General rightsCopyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or othercopyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated withthese rights. • Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain. • You may freely distribute the URL identifying the publication in the public portal.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 17. Mar. 2016Dye lasing in optically manipulated liquid aerosolsY. Karadag,1 M. Aas,1 A. Jonáš,1,4 S. Anand,2 D. McGloin,2,5 and A. Kiraz1,3,*1Department of Physics, Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey2Electronic Engineering and Physics Division, University of Dundee, Nethergate, Dundee DD1 4HN, UK3Koç University TÜPRAŞ Energy Center (KÜTEM), Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey4e-mail: ajonas@ku.edu.tr5e-mail: d.mcgloin@dundee.ac.uk*Corresponding author: akiraz@ku.edu.trReceived March 11, 2013; revised April 5, 2013; accepted April 5, 2013;posted April 9, 2013 (Doc. ID 186719); published May 10, 2013We report lasing in airborne, rhodamine B-doped glycerol–water droplets with diameters ranging between 7.7 and11.0 μm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser,the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of theintensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. Theaveragewavelength of the lasingWGMs could be tuned between 600 and 630 nm by changing the droplet size. Theseresults may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission tosmall perturbations in the droplet laser cavity and the gain medium. © 2013 Optical Society of AmericaOCIS codes: (140.3945) Microcavities; (140.2050) Dye lasers; (010.1110) Aerosols; (350.4855) Optical tweezers oroptical manipulation; (300.2530) Fluorescence, laser-induced.http://dx.doi.org/10.1364/OL.38.001669Owing to the liquid surface tension, micron-sized drop-lets assume the shape of perfect spheres that can serveas natural optical resonant cavities hosting high-qualitywhispering gallery modes (WGMs). Enhanced opticalfields and small modal volumes of such WGMs then facili-tate the observation of nonlinear optical phenomena atincident power thresholds that are significantly lowerthan those required for bulk samples. Easy access tothe regime of nonlinear optical response in the dropletsenables precise characterization of the droplet size andmaterial properties via cavity-enhanced Raman spectros-copy [1] and building of microscopic lasers based onliquid cavities containing a gain medium [2]. Such drop-let-based microlasers have already found applications inbiological and chemical sensing [3,4]. In addition, thepossibility of readily adjusting the shape, size, and com-position of liquid microcavities opens up the way towardcontrolled tuning of the emission spectral characteristicsof liquid microlasers [5,6].Lasing in airborne microdroplets was first successfullydemonstrated almost 30 years ago in freely flowingstreams of droplets [7]. However, freely moving liquidaerosols have only a limited use in practical applicationsin sensing and microlaser design. In order to fully exploitthe potential of lasing WGMs in microdroplets, it is nec-essary to stabilize the droplet position over extendedtime periods. The position stabilization of droplets ofwater and other polar liquids can be effectively realizedby depositing them on a superhydrophobic surface [8].Active, noninvasive control of the droplet position by ex-ternal micromanipulation then represents the next stepin the practical exploitation of lasing aerosols. To date,manipulation of lasing airborne microdroplets has beendemonstrated in electrodynamic [9] and acoustic [10]trapping configurations. Optical tweezing is another mi-cromanipulation technique that is especially well suitedfor stable confinement of very small droplets and readilyadaptable for simultaneous manipulation of large num-bers of particles [11]. Various studies employed opticaltweezing to localize liquid aerosols over long periodsof time for applications in a large variety of fields includ-ing atmospheric chemistry and physics, and healthscience [12,13]. Optical tweezing has also been used toconfine and move solid lasing microspheres [14] or lasingemulsion droplets [15]. However, up to now lasing hasnot been demonstrated in liquid aerosols manipulatedusing optical tweezers.In this Letter, we report lasing from individual opticallytrapped liquid aerosol particles doped with a fluorescentdye. We characterize the droplet emission spectrum as afunction of the droplet size and show that the dropletlasing wavelength can be tuned over an interval largerthan 30 nm by adjusting the droplet radius.Optical setup used for the experiments with trappedaerosol lasing was described previously [15]. A continu-ous wave solid state infrared laser (λ  1064 nm, 300 mWmaximum power; Crystalaser) was used for aerosol trap-ping. The trapping laser beam was sent through a beamexpander and focused into the sample chamber by awater immersion microscope objective (NA  1.2, 60×;Nikon) in the inverted microscope geometry. Based onthe previous work [13], the power of the trapping beamwas set to 3.5 mW at the focus of the microscope objec-tive for stable on-axis trapping of aerosols with diametersranging between 5 and 10 μm. Pulsed green beam(λ  532 nm, 20 ns pulse width and 33 kHz repetitionrate) obtained after frequency-doubling the output of ahome-built, passively Q-switched Nd-YVO4 laser wasused for pumping the trapped microdroplets. The pumpbeam was delivered to the setup through a multimodefiber, combined with the trapping beam by a dichroic mir-ror, and focused by the same microscope objective thatwas employed for optical trapping to a 9.8 μm diameterspot. In our experiments, the trapping and pump beamswere focused at the same plane approximately 20 μmabove the upper surface of the cover glass that formedthe bottom part of the sample chamber. For efficient ex-citation of WGMs in the trapped droplet by on-edgeMay 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 16690146-9592/13/101669-03$15.00/0 © 2013 Optical Society of Americaoptical pumping, the focal spot of the pump beam wasapproximately centered at the rim of the droplet. Thefluorescence emission from the droplets was collectedusing the same microscope objective and dispersed bya monochromator (focal length 500 mm; Acton Re-search) on the chip of a cooled CCD detector (Pixis100; Princeton Instruments). In order to prevent fastphotobleaching of the dye molecules in the droplets, ashutter was added to the pump laser beam path. Thisshutter was synchronized with the CCD detector and trig-gered the spectrum acquisition during an exposure timeof 10 ms.A compact ultrasonic nebulizer (JIH50; Beuer) wasused for generating liquid aerosols out of 39% w/w glyc-erol/water solution (refractive index nint  1.38, densityρ  1.099 g∕cm3) doped with 1 mM rhodamine B dye.The nebulizer was attached to a closed glass samplechamber that prevented external air flow from disturbingthe stability of the trapped aerosol particles and main-tained approximately constant ambient humidity duringthe experiments. A cover glass treated with Ar∕O2plasma was attached at the bottom of the sample cham-ber. Plasma cleaning of cover glasses ensured a goodhydrophilicity of their surface, thus preventing accumu-lation of large sessile droplets that otherwise disturb thetrapping and pump beams focused into the chamber.During the experiments, initial diameters of the trappeddroplets were generally around 5 μm. Larger dropletswere then obtained by fusion of the trapped droplets withothers that were attracted to the trapping region.Trapped droplet diameters were estimated from micro-scope images and independently determined from the ob-served WGM free-spectral range (FSR), mean lasingwavelength λ¯, and known refractive indices of the droplet(nint  1.38) and air (next  1.00), using the asymptoticformula of Chylek et al. [16].Figure 1 shows fluorescence emission spectra re-corded from a trapped 9.4 μm diameter glycerol/wateraerosol droplet at varying excitation fluences. NonlasingWGMs of the aerosol particle were observed in Fig. 1(a)when the excitation fluence was lower than the lasingthreshold. These WGMs are superimposed on a broadnonresonant background emission from the dye-dopedthin solution layer formed on the cover glass duringthe experiment. In general, droplet WGMs can be char-acterized using radial, azimuthal, and angular modenumbers, and polarization (TE or TM) [6]. The WGMsthat belong to the same mode family with identical radialmode number and polarization are indicated in Fig. 1(a).Spectral separation between two such consecutiveWGMs gives the FSR, which is equal to 10.7 nm. In orderto observe lasing, the cavity losses must be smaller thanthe net cavity gain. Excitation fluence and the amount ofdye in the aerosol particle are critical to satisfy this con-dition. As shown in Figs. 1(b) and 1(c), two WGMs de-noted as A and B exhibit lasing around λ  608 nmand λ  618 nm upon crossing the threshold excitationfluences of 0.76 mJ∕cm2 and 0.84 mJ∕cm2. Owing tothe very low volatility of glycerol and approximately sta-bilized relative humidity in the sample chamber, the spec-tral positions of lasing WGMs display only a minimal driftΔλ  0.06 nm over the total duration of the experiment(2 min). For a droplet with the mean radius a  4.7 μm,this translates into the drift of the droplet radiusby Δa  Δλa∕λ  0.46 nm.Figure 2 displays the intensities of the lasing WGMs Aand B indicated in Fig. 1 and nonresonant spectral back-ground in the vicinity of these WGMs as a function of theexcitation fluence. Both lasing and background inten-sities were determined from Lorentzian fits to WGMsin a series of emission spectra acquired with graduallyincreasing excitation fluence. Subsequently, two lineswere fitted to the respective lasing and background inten-sity series and the intersection of the fitted lines providedthe threshold excitation fluences of 0.76 mJ∕cm2 and0.84 mJ∕cm2 for WGM A and WGM B, respectively[15]. As expected, the background intensity increased lin-early with the excitation fluence; this also indicated thatthe photobleaching effects during the acquisition of theexcitation fluence-dependent spectral series could beFig. 1. Excitation fluence-dependent emission spectrarecorded from a 9.4 μm diameter glycerol/water dropletat three different excitation fluences: (a) 0.49 mJ∕cm2,(b) 1.70 mJ∕cm2, and (c) 3.06 mJ∕cm2. WGM A and WGM B de-note lasing WGMs. Inset: optical microscope image of thetrapped lasing droplet.Fig. 2. Excitation fluence-dependent intensities of the lasingWGMs and background emissions in a 9.4 μm diameterglycerol/water aerosol droplet.1670 OPTICS LETTERS / Vol. 38, No. 10 / May 15, 2013neglected owing to the short exposure time required forrecording individual spectra. When the excitation fluenceexceeded the threshold, increase of the intensity of thelasing peak with a higher slope relative to the back-ground emission indicated the onset of lasing. Thethreshold excitation fluence depends on the size of thetrapped aerosol particle and the Q-factor of its WGMs[17]. Large radiative losses in smaller droplets result inan exponential decrease of their WGM Q-factors [18].This sets a lower limit to the diameter of droplets thatdisplay lasing at a given maximal excitation fluence. Inour experimental conditions, this lower limit wasdetermined as 7.7 μm.With increasing droplet size, spectral position of theemission maximum of lasing aerosols containing con-stant dye concentration was observed to shift to longerwavelengths. This trend is illustrated in Fig. 3(a), whichshows emission spectra of six droplets with differentradii recorded at fixed 2.41 mJ∕cm2 excitation fluence.Here, the increasing droplet radius corresponds to thedecreasing FSR of the droplet [16]. For the given droplet,spectral location of its overall maximal lasing gain canbe quantitatively characterized by the mean lasing wave-length λ¯ calculated as the intensity-weighted average ofthe wavelengths of all lasing WGMs from the same modefamily observed in the droplet’s spectrum. Figure 3(b)summarizes the relationship between λ¯ and FSR for 34lasing droplets of varied size. Within the studied rangeof droplet sizes (7.7–11.0 μm), FSR decreased with λ¯ fol-lowing an almost linear dependence. A similar spectralshift of the overall lasing gain profile was previouslyobserved in lasing emulsion droplets [6,15] and explainedby the size-dependence of the efficiency of light out-coupling from the droplets and the cross section ofthe lasing WGMs.We have demonstrated lasing in the WGMs of dye-doped glycerol/water microdroplets manipulated in airusing optical tweezers. By changing the trapped dropletsize, we have shown that the average lasing wavelengthcould be tuned between 600–630 nm. Controlled fusion ofmultiple droplets confined in dynamically positioned in-dependent optical traps, manipulation of relative humid-ity and temperature in the sample chamber, or localheating with the trapping laser can be further used forprecise control of the lasing droplet size. Our resultsare readily adaptable to simultaneous manipulation oflarge numbers of lasing aerosol particles. They also bringup the possibility of using the high sensitivity of stimu-lated emission to small perturbations in the shape, size,and material properties of the droplet cavity for sensitivechemical and biological analysis in airborne particles.This work was partially supported by TÜBİTAK (GrantNo. 111T059). S.A. thanks the Schlumberger Faculty forthe Future program for support.References1. J. P. Reid, H. Meresman, L. Mitchem, and R. Symes, Int. Rev.Phys. Chem. 26, 139 (2007).2. K. Mølhave, A. Kristensen, and N. A. Mortensen, AdvancedPhotonic Structures for Biological and Chemical Detection(Springer, 2009), pp. 471–486.3. M. Tanyeri and I. M. Kennedy, Sensor Lett. 6, 326 (2008).4. M. Humar and I. Musevic, Opt. Express 19, 19836 (2011).5. M. Humar, M. Ravnik, S. Pajk, and I. Musevic, Nat. Photon-ics 3, 595 (2009).6. S. K. Y. Tang, R. Derda, Q. Quan, M. Loncar, and G. M.Whitesides, Opt. Express 19, 2204 (2011).7. H.-M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, Opt.Lett. 9, 499 (1984).8. A. Kiraz, A. Sennaroglu, S. Doganay, M. A. Dundar, A. Kurt,H. Kalaycoglu, and A. L. Demirel, Opt. Commun. 276, 145(2007).9. M. Tona and M. Kimura, J. Phys. Soc. Jpn. 69, 3533 (2000).10. H. Azzouz, L. Alkhafadiji, S. Balslev, J. Johansson, N. A.Mortensen, S. Nilsson, and A. Kristensen, Opt. Express14, 4374 (2006).11. D. R. Burnham and D. McGloin, Opt. Express 14, 4176(2006).12. R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, Phys.Chem. Chem. Phys. 6, 4924 (2004).13. D. McGloin, D. R. Burnham, M. D. Summers, D. Rudd, N.Dewar, and S. Anand, Faraday Discuss. 137, 335 (2008).14. K. Sasaki, H. Fujiwara, and H. Masuhara, Appl. Phys. Lett.70, 2647 (1997).15. M. Aas, A. Jonáš, and A. Kiraz, Opt. Commun. 290, 183(2013).16. P. Chylek, J. T. Kiehl, and M. K. W. Ko, Phys. Rev. A 18, 2229(1978).17. A. J. Campillo, J. D. Eversole, and H. B. Lin, Phys. Rev. Lett.67, 437 (1991).18. A. Serpengüzel, J. C. Swindal, R. K. Chang, and W. P. Acker,Appl. Opt. 31, 3543 (1992).Fig. 3. (a) Lasing spectra recorded from six different aerosolswith diameters 7.7, 8.4, 9.0, 9.6, 10.1, and 11.0 μm (bottom totop) at constant excitation fluence of 2.41 mJ∕cm2. (b) Freespectral ranges of 34 lasing aerosols as a function of the meanlasing wavelength, λ¯.May 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 1671

(a) Lasing spectra recorded from six different aerosols

Faraday Discuss.

Dye lasing in optically manipulated liquid aerosols

                                                                    University of DundeeDye lasing in optically manipulated liquid aerosolsKaradag, Y.; Aas, M.; Anand, S.; McGloin, D.; Kiraz, A.Published in:Optics LettersDOI:10.1364/OL.38.001669Publication date:2013Document VersionPublisher's PDF, also known as Version of recordLink to publication in Discovery Research PortalCitation for published version (APA):Karadag, Y., Aas, M., Anand, S., McGloin, D., & Kiraz, A. (2013). Dye lasing in optically manipulated liquidaerosols. Optics Letters, 38(10), 1669-1671. https://doi.org/10.1364/OL.38.001669General rightsCopyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or othercopyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated withthese rights. • Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain. • You may freely distribute the URL identifying the publication in the public portal.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 18. Apr. 2023Dye lasing in optically manipulated liquid aerosolsY. Karadag,1 M. Aas,1 A. Jonáš,1,4 S. Anand,2 D. McGloin,2,5 and A. Kiraz1,3,*1Department of Physics, Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey2Electronic Engineering and Physics Division, University of Dundee, Nethergate, Dundee DD1 4HN, UK3Koç University TÜPRAŞ Energy Center (KÜTEM), Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey4e-mail: ajonas@ku.edu.tr5e-mail: d.mcgloin@dundee.ac.uk*Corresponding author: akiraz@ku.edu.trReceived March 11, 2013; revised April 5, 2013; accepted April 5, 2013;posted April 9, 2013 (Doc. ID 186719); published May 10, 2013We report lasing in airborne, rhodamine B-doped glycerol–water droplets with diameters ranging between 7.7 and11.0 μm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser,the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of theintensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. Theaveragewavelength of the lasingWGMs could be tuned between 600 and 630 nm by changing the droplet size. Theseresults may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission tosmall perturbations in the droplet laser cavity and the gain medium. © 2013 Optical Society of AmericaOCIS codes: (140.3945) Microcavities; (140.2050) Dye lasers; (010.1110) Aerosols; (350.4855) Optical tweezers oroptical manipulation; (300.2530) Fluorescence, laser-induced.http://dx.doi.org/10.1364/OL.38.001669Owing to the liquid surface tension, micron-sized drop-lets assume the shape of perfect spheres that can serveas natural optical resonant cavities hosting high-qualitywhispering gallery modes (WGMs). Enhanced opticalfields and small modal volumes of such WGMs then facili-tate the observation of nonlinear optical phenomena atincident power thresholds that are significantly lowerthan those required for bulk samples. Easy access tothe regime of nonlinear optical response in the dropletsenables precise characterization of the droplet size andmaterial properties via cavity-enhanced Raman spectros-copy [1] and building of microscopic lasers based onliquid cavities containing a gain medium [2]. Such drop-let-based microlasers have already found applications inbiological and chemical sensing [3,4]. In addition, thepossibility of readily adjusting the shape, size, and com-position of liquid microcavities opens up the way towardcontrolled tuning of the emission spectral characteristicsof liquid microlasers [5,6].Lasing in airborne microdroplets was first successfullydemonstrated almost 30 years ago in freely flowingstreams of droplets [7]. However, freely moving liquidaerosols have only a limited use in practical applicationsin sensing and microlaser design. In order to fully exploitthe potential of lasing WGMs in microdroplets, it is nec-essary to stabilize the droplet position over extendedtime periods. The position stabilization of droplets ofwater and other polar liquids can be effectively realizedby depositing them on a superhydrophobic surface [8].Active, noninvasive control of the droplet position by ex-ternal micromanipulation then represents the next stepin the practical exploitation of lasing aerosols. To date,manipulation of lasing airborne microdroplets has beendemonstrated in electrodynamic [9] and acoustic [10]trapping configurations. Optical tweezing is another mi-cromanipulation technique that is especially well suitedfor stable confinement of very small droplets and readilyadaptable for simultaneous manipulation of large num-bers of particles [11]. Various studies employed opticaltweezing to localize liquid aerosols over long periodsof time for applications in a large variety of fields includ-ing atmospheric chemistry and physics, and healthscience [12,13]. Optical tweezing has also been used toconfine and move solid lasing microspheres [14] or lasingemulsion droplets [15]. However, up to now lasing hasnot been demonstrated in liquid aerosols manipulatedusing optical tweezers.In this Letter, we report lasing from individual opticallytrapped liquid aerosol particles doped with a fluorescentdye. We characterize the droplet emission spectrum as afunction of the droplet size and show that the dropletlasing wavelength can be tuned over an interval largerthan 30 nm by adjusting the droplet radius.Optical setup used for the experiments with trappedaerosol lasing was described previously [15]. A continu-ous wave solid state infrared laser (λ  1064 nm, 300 mWmaximum power; Crystalaser) was used for aerosol trap-ping. The trapping laser beam was sent through a beamexpander and focused into the sample chamber by awater immersion microscope objective (NA  1.2, 60×;Nikon) in the inverted microscope geometry. Based onthe previous work [13], the power of the trapping beamwas set to 3.5 mW at the focus of the microscope objec-tive for stable on-axis trapping of aerosols with diametersranging between 5 and 10 μm. Pulsed green beam(λ  532 nm, 20 ns pulse width and 33 kHz repetitionrate) obtained after frequency-doubling the output of ahome-built, passively Q-switched Nd-YVO4 laser wasused for pumping the trapped microdroplets. The pumpbeam was delivered to the setup through a multimodefiber, combined with the trapping beam by a dichroic mir-ror, and focused by the same microscope objective thatwas employed for optical trapping to a 9.8 μm diameterspot. In our experiments, the trapping and pump beamswere focused at the same plane approximately 20 μmabove the upper surface of the cover glass that formedthe bottom part of the sample chamber. For efficient ex-citation of WGMs in the trapped droplet by on-edgeMay 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 16690146-9592/13/101669-03$15.00/0 © 2013 Optical Society of Americaoptical pumping, the focal spot of the pump beam wasapproximately centered at the rim of the droplet. Thefluorescence emission from the droplets was collectedusing the same microscope objective and dispersed bya monochromator (focal length 500 mm; Acton Re-search) on the chip of a cooled CCD detector (Pixis100; Princeton Instruments). In order to prevent fastphotobleaching of the dye molecules in the droplets, ashutter was added to the pump laser beam path. Thisshutter was synchronized with the CCD detector and trig-gered the spectrum acquisition during an exposure timeof 10 ms.A compact ultrasonic nebulizer (JIH50; Beuer) wasused for generating liquid aerosols out of 39% w/w glyc-erol/water solution (refractive index nint  1.38, densityρ  1.099 g∕cm3) doped with 1 mM rhodamine B dye.The nebulizer was attached to a closed glass samplechamber that prevented external air flow from disturbingthe stability of the trapped aerosol particles and main-tained approximately constant ambient humidity duringthe experiments. A cover glass treated with Ar∕O2plasma was attached at the bottom of the sample cham-ber. Plasma cleaning of cover glasses ensured a goodhydrophilicity of their surface, thus preventing accumu-lation of large sessile droplets that otherwise disturb thetrapping and pump beams focused into the chamber.During the experiments, initial diameters of the trappeddroplets were generally around 5 μm. Larger dropletswere then obtained by fusion of the trapped droplets withothers that were attracted to the trapping region.Trapped droplet diameters were estimated from micro-scope images and independently determined from the ob-served WGM free-spectral range (FSR), mean lasingwavelength λ̄, and known refractive indices of the droplet(nint  1.38) and air (next  1.00), using the asymptoticformula of Chylek et al. [16].Figure 1 shows fluorescence emission spectra re-corded from a trapped 9.4 μm diameter glycerol/wateraerosol droplet at varying excitation fluences. NonlasingWGMs of the aerosol particle were observed in Fig. 1(a)when the excitation fluence was lower than the lasingthreshold. These WGMs are superimposed on a broadnonresonant background emission from the dye-dopedthin solution layer formed on the cover glass duringthe experiment. In general, droplet WGMs can be char-acterized using radial, azimuthal, and angular modenumbers, and polarization (TE or TM) [6]. The WGMsthat belong to the same mode family with identical radialmode number and polarization are indicated in Fig. 1(a).Spectral separation between two such consecutiveWGMs gives the FSR, which is equal to 10.7 nm. In orderto observe lasing, the cavity losses must be smaller thanthe net cavity gain. Excitation fluence and the amount ofdye in the aerosol particle are critical to satisfy this con-dition. As shown in Figs. 1(b) and 1(c), two WGMs de-noted as A and B exhibit lasing around λ  608 nmand λ  618 nm upon crossing the threshold excitationfluences of 0.76 mJ∕cm2 and 0.84 mJ∕cm2. Owing tothe very low volatility of glycerol and approximately sta-bilized relative humidity in the sample chamber, the spec-tral positions of lasing WGMs display only a minimal driftΔλ  0.06 nm over the total duration of the experiment(2 min). For a droplet with the mean radius a  4.7 μm,this translates into the drift of the droplet radiusby Δa  Δλa∕λ  0.46 nm.Figure 2 displays the intensities of the lasing WGMs Aand B indicated in Fig. 1 and nonresonant spectral back-ground in the vicinity of these WGMs as a function of theexcitation fluence. Both lasing and background inten-sities were determined from Lorentzian fits to WGMsin a series of emission spectra acquired with graduallyincreasing excitation fluence. Subsequently, two lineswere fitted to the respective lasing and background inten-sity series and the intersection of the fitted lines providedthe threshold excitation fluences of 0.76 mJ∕cm2 and0.84 mJ∕cm2 for WGM A and WGM B, respectively[15]. As expected, the background intensity increased lin-early with the excitation fluence; this also indicated thatthe photobleaching effects during the acquisition of theexcitation fluence-dependent spectral series could beFig. 1. Excitation fluence-dependent emission spectrarecorded from a 9.4 μm diameter glycerol/water dropletat three different excitation fluences: (a) 0.49 mJ∕cm2,(b) 1.70 mJ∕cm2, and (c) 3.06 mJ∕cm2. WGM A and WGM B de-note lasing WGMs. Inset: optical microscope image of thetrapped lasing droplet.Fig. 2. Excitation fluence-dependent intensities of the lasingWGMs and background emissions in a 9.4 μm diameterglycerol/water aerosol droplet.1670 OPTICS LETTERS / Vol. 38, No. 10 / May 15, 2013neglected owing to the short exposure time required forrecording individual spectra. When the excitation fluenceexceeded the threshold, increase of the intensity of thelasing peak with a higher slope relative to the back-ground emission indicated the onset of lasing. Thethreshold excitation fluence depends on the size of thetrapped aerosol particle and the Q-factor of its WGMs[17]. Large radiative losses in smaller droplets result inan exponential decrease of their WGM Q-factors [18].This sets a lower limit to the diameter of droplets thatdisplay lasing at a given maximal excitation fluence. Inour experimental conditions, this lower limit wasdetermined as 7.7 μm.With increasing droplet size, spectral position of theemission maximum of lasing aerosols containing con-stant dye concentration was observed to shift to longerwavelengths. This trend is illustrated in Fig. 3(a), whichshows emission spectra of six droplets with differentradii recorded at fixed 2.41 mJ∕cm2 excitation fluence.Here, the increasing droplet radius corresponds to thedecreasing FSR of the droplet [16]. For the given droplet,spectral location of its overall maximal lasing gain canbe quantitatively characterized by the mean lasing wave-length λ̄ calculated as the intensity-weighted average ofthe wavelengths of all lasing WGMs from the same modefamily observed in the droplet’s spectrum. Figure 3(b)summarizes the relationship between λ̄ and FSR for 34lasing droplets of varied size. Within the studied rangeof droplet sizes (7.7–11.0 μm), FSR decreased with λ̄ fol-lowing an almost linear dependence. A similar spectralshift of the overall lasing gain profile was previouslyobserved in lasing emulsion droplets [6,15] and explainedby the size-dependence of the efficiency of light out-coupling from the droplets and the cross section ofthe lasing WGMs.We have demonstrated lasing in the WGMs of dye-doped glycerol/water microdroplets manipulated in airusing optical tweezers. By changing the trapped dropletsize, we have shown that the average lasing wavelengthcould be tuned between 600–630 nm. Controlled fusion ofmultiple droplets confined in dynamically positioned in-dependent optical traps, manipulation of relative humid-ity and temperature in the sample chamber, or localheating with the trapping laser can be further used forprecise control of the lasing droplet size. Our resultsare readily adaptable to simultaneous manipulation oflarge numbers of lasing aerosol particles. They also bringup the possibility of using the high sensitivity of stimu-lated emission to small perturbations in the shape, size,and material properties of the droplet cavity for sensitivechemical and biological analysis in airborne particles.This work was partially supported by TÜBİTAK (GrantNo. 111T059). S.A. thanks the Schlumberger Faculty forthe Future program for support.References1. J. P. Reid, H. Meresman, L. Mitchem, and R. Symes, Int. Rev.Phys. Chem. 26, 139 (2007).2. K. Mølhave, A. Kristensen, and N. A. Mortensen, AdvancedPhotonic Structures for Biological and Chemical Detection(Springer, 2009), pp. 471–486.3. M. Tanyeri and I. M. Kennedy, Sensor Lett. 6, 326 (2008).4. M. Humar and I. Musevic, Opt. Express 19, 19836 (2011).5. M. Humar, M. Ravnik, S. Pajk, and I. Musevic, Nat. Photon-ics 3, 595 (2009).6. S. K. Y. Tang, R. Derda, Q. Quan, M. Loncar, and G. M.Whitesides, Opt. Express 19, 2204 (2011).7. H.-M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, Opt.Lett. 9, 499 (1984).8. A. Kiraz, A. Sennaroglu, S. Doganay, M. A. Dundar, A. Kurt,H. Kalaycoglu, and A. L. Demirel, Opt. Commun. 276, 145(2007).9. M. Tona and M. Kimura, J. Phys. Soc. Jpn. 69, 3533 (2000).10. H. Azzouz, L. Alkhafadiji, S. Balslev, J. Johansson, N. A.Mortensen, S. Nilsson, and A. Kristensen, Opt. Express14, 4374 (2006).11. D. R. Burnham and D. McGloin, Opt. Express 14, 4176(2006).12. R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, Phys.Chem. Chem. Phys. 6, 4924 (2004).13. D. McGloin, D. R. Burnham, M. D. Summers, D. Rudd, N.Dewar, and S. Anand, Faraday Discuss. 137, 335 (2008).14. K. Sasaki, H. Fujiwara, and H. Masuhara, Appl. Phys. Lett.70, 2647 (1997).15. M. Aas, A. Jonáš, and A. Kiraz, Opt. Commun. 290, 183(2013).16. P. Chylek, J. T. Kiehl, and M. K. W. Ko, Phys. Rev. A 18, 2229(1978).17. A. J. Campillo, J. D. Eversole, and H. B. Lin, Phys. Rev. Lett.67, 437 (1991).18. A. Serpengüzel, J. C. Swindal, R. K. Chang, and W. P. Acker,Appl. Opt. 31, 3543 (1992).Fig. 3. (a) Lasing spectra recorded from six different aerosolswith diameters 7.7, 8.4, 9.0, 9.6, 10.1, and 11.0 μm (bottom totop) at constant excitation fluence of 2.41 mJ∕cm2. (b) Freespectral ranges of 34 lasing aerosols as a function of the meanlasing wavelength, λ̄.May 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 1671

                                                                    University of DundeeDye lasing in optically manipulated liquid aerosolsKaradag, Y.; Aas, M.; Anand, S.; McGloin, D.; Kiraz, A.Published in:Optics LettersDOI:10.1364/OL.38.001669Publication date:2013Document VersionPublisher's PDF, also known as Version of recordLink to publication in Discovery Research PortalCitation for published version (APA):Karadag, Y., Aas, M., Anand, S., McGloin, D., & Kiraz, A. (2013). Dye lasing in optically manipulated liquidaerosols. Optics Letters, 38(10), 1669-1671. https://doi.org/10.1364/OL.38.001669General rightsCopyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or othercopyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated withthese rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 30. Oct. 2025Dye lasing in optically manipulated liquid aerosolsY. Karadag,1 M. Aas,1 A. Jonáš,1,4 S. Anand,2 D. McGloin,2,5 and A. Kiraz1,3,*1Department of Physics, Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey2Electronic Engineering and Physics Division, University of Dundee, Nethergate, Dundee DD1 4HN, UK3Koç University TÜPRAŞ Energy Center (KÜTEM), Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey4e-mail: ajonas@ku.edu.tr5e-mail: d.mcgloin@dundee.ac.uk*Corresponding author: akiraz@ku.edu.trReceived March 11, 2013; revised April 5, 2013; accepted April 5, 2013;posted April 9, 2013 (Doc. ID 186719); published May 10, 2013We report lasing in airborne, rhodamine B-doped glycerol–water droplets with diameters ranging between 7.7 and11.0 μm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser,the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of theintensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. Theaveragewavelength of the lasingWGMs could be tuned between 600 and 630 nm by changing the droplet size. Theseresults may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission tosmall perturbations in the droplet laser cavity and the gain medium. © 2013 Optical Society of AmericaOCIS codes: (140.3945) Microcavities; (140.2050) Dye lasers; (010.1110) Aerosols; (350.4855) Optical tweezers oroptical manipulation; (300.2530) Fluorescence, laser-induced.http://dx.doi.org/10.1364/OL.38.001669Owing to the liquid surface tension, micron-sized drop-lets assume the shape of perfect spheres that can serveas natural optical resonant cavities hosting high-qualitywhispering gallery modes (WGMs). Enhanced opticalfields and small modal volumes of such WGMs then facili-tate the observation of nonlinear optical phenomena atincident power thresholds that are significantly lowerthan those required for bulk samples. Easy access tothe regime of nonlinear optical response in the dropletsenables precise characterization of the droplet size andmaterial properties via cavity-enhanced Raman spectros-copy [1] and building of microscopic lasers based onliquid cavities containing a gain medium [2]. Such drop-let-based microlasers have already found applications inbiological and chemical sensing [3,4]. In addition, thepossibility of readily adjusting the shape, size, and com-position of liquid microcavities opens up the way towardcontrolled tuning of the emission spectral characteristicsof liquid microlasers [5,6].Lasing in airborne microdroplets was first successfullydemonstrated almost 30 years ago in freely flowingstreams of droplets [7]. However, freely moving liquidaerosols have only a limited use in practical applicationsin sensing and microlaser design. In order to fully exploitthe potential of lasing WGMs in microdroplets, it is nec-essary to stabilize the droplet position over extendedtime periods. The position stabilization of droplets ofwater and other polar liquids can be effectively realizedby depositing them on a superhydrophobic surface [8].Active, noninvasive control of the droplet position by ex-ternal micromanipulation then represents the next stepin the practical exploitation of lasing aerosols. To date,manipulation of lasing airborne microdroplets has beendemonstrated in electrodynamic [9] and acoustic [10]trapping configurations. Optical tweezing is another mi-cromanipulation technique that is especially well suitedfor stable confinement of very small droplets and readilyadaptable for simultaneous manipulation of large num-bers of particles [11]. Various studies employed opticaltweezing to localize liquid aerosols over long periodsof time for applications in a large variety of fields includ-ing atmospheric chemistry and physics, and healthscience [12,13]. Optical tweezing has also been used toconfine and move solid lasing microspheres [14] or lasingemulsion droplets [15]. However, up to now lasing hasnot been demonstrated in liquid aerosols manipulatedusing optical tweezers.In this Letter, we report lasing from individual opticallytrapped liquid aerosol particles doped with a fluorescentdye. We characterize the droplet emission spectrum as afunction of the droplet size and show that the dropletlasing wavelength can be tuned over an interval largerthan 30 nm by adjusting the droplet radius.Optical setup used for the experiments with trappedaerosol lasing was described previously [15]. A continu-ous wave solid state infrared laser (λ  1064 nm, 300 mWmaximum power; Crystalaser) was used for aerosol trap-ping. The trapping laser beam was sent through a beamexpander and focused into the sample chamber by awater immersion microscope objective (NA  1.2, 60×;Nikon) in the inverted microscope geometry. Based onthe previous work [13], the power of the trapping beamwas set to 3.5 mW at the focus of the microscope objec-tive for stable on-axis trapping of aerosols with diametersranging between 5 and 10 μm. Pulsed green beam(λ  532 nm, 20 ns pulse width and 33 kHz repetitionrate) obtained after frequency-doubling the output of ahome-built, passively Q-switched Nd-YVO4 laser wasused for pumping the trapped microdroplets. The pumpbeam was delivered to the setup through a multimodefiber, combined with the trapping beam by a dichroic mir-ror, and focused by the same microscope objective thatwas employed for optical trapping to a 9.8 μm diameterspot. In our experiments, the trapping and pump beamswere focused at the same plane approximately 20 μmabove the upper surface of the cover glass that formedthe bottom part of the sample chamber. For efficient ex-citation of WGMs in the trapped droplet by on-edgeMay 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 16690146-9592/13/101669-03$15.00/0 © 2013 Optical Society of Americaoptical pumping, the focal spot of the pump beam wasapproximately centered at the rim of the droplet. Thefluorescence emission from the droplets was collectedusing the same microscope objective and dispersed bya monochromator (focal length 500 mm; Acton Re-search) on the chip of a cooled CCD detector (Pixis100; Princeton Instruments). In order to prevent fastphotobleaching of the dye molecules in the droplets, ashutter was added to the pump laser beam path. Thisshutter was synchronized with the CCD detector and trig-gered the spectrum acquisition during an exposure timeof 10 ms.A compact ultrasonic nebulizer (JIH50; Beuer) wasused for generating liquid aerosols out of 39% w/w glyc-erol/water solution (refractive index nint  1.38, densityρ  1.099 g∕cm3) doped with 1 mM rhodamine B dye.The nebulizer was attached to a closed glass samplechamber that prevented external air flow from disturbingthe stability of the trapped aerosol particles and main-tained approximately constant ambient humidity duringthe experiments. A cover glass treated with Ar∕O2plasma was attached at the bottom of the sample cham-ber. Plasma cleaning of cover glasses ensured a goodhydrophilicity of their surface, thus preventing accumu-lation of large sessile droplets that otherwise disturb thetrapping and pump beams focused into the chamber.During the experiments, initial diameters of the trappeddroplets were generally around 5 μm. Larger dropletswere then obtained by fusion of the trapped droplets withothers that were attracted to the trapping region.Trapped droplet diameters were estimated from micro-scope images and independently determined from the ob-served WGM free-spectral range (FSR), mean lasingwavelength λ̄, and known refractive indices of the droplet(nint  1.38) and air (next  1.00), using the asymptoticformula of Chylek et al. [16].Figure 1 shows fluorescence emission spectra re-corded from a trapped 9.4 μm diameter glycerol/wateraerosol droplet at varying excitation fluences. NonlasingWGMs of the aerosol particle were observed in Fig. 1(a)when the excitation fluence was lower than the lasingthreshold. These WGMs are superimposed on a broadnonresonant background emission from the dye-dopedthin solution layer formed on the cover glass duringthe experiment. In general, droplet WGMs can be char-acterized using radial, azimuthal, and angular modenumbers, and polarization (TE or TM) [6]. The WGMsthat belong to the same mode family with identical radialmode number and polarization are indicated in Fig. 1(a).Spectral separation between two such consecutiveWGMs gives the FSR, which is equal to 10.7 nm. In orderto observe lasing, the cavity losses must be smaller thanthe net cavity gain. Excitation fluence and the amount ofdye in the aerosol particle are critical to satisfy this con-dition. As shown in Figs. 1(b) and 1(c), two WGMs de-noted as A and B exhibit lasing around λ  608 nmand λ  618 nm upon crossing the threshold excitationfluences of 0.76 mJ∕cm2 and 0.84 mJ∕cm2. Owing tothe very low volatility of glycerol and approximately sta-bilized relative humidity in the sample chamber, the spec-tral positions of lasing WGMs display only a minimal driftΔλ  0.06 nm over the total duration of the experiment(2 min). For a droplet with the mean radius a  4.7 μm,this translates into the drift of the droplet radiusby Δa  Δλa∕λ  0.46 nm.Figure 2 displays the intensities of the lasing WGMs Aand B indicated in Fig. 1 and nonresonant spectral back-ground in the vicinity of these WGMs as a function of theexcitation fluence. Both lasing and background inten-sities were determined from Lorentzian fits to WGMsin a series of emission spectra acquired with graduallyincreasing excitation fluence. Subsequently, two lineswere fitted to the respective lasing and background inten-sity series and the intersection of the fitted lines providedthe threshold excitation fluences of 0.76 mJ∕cm2 and0.84 mJ∕cm2 for WGM A and WGM B, respectively[15]. As expected, the background intensity increased lin-early with the excitation fluence; this also indicated thatthe photobleaching effects during the acquisition of theexcitation fluence-dependent spectral series could beFig. 1. Excitation fluence-dependent emission spectrarecorded from a 9.4 μm diameter glycerol/water dropletat three different excitation fluences: (a) 0.49 mJ∕cm2,(b) 1.70 mJ∕cm2, and (c) 3.06 mJ∕cm2. WGM A and WGM B de-note lasing WGMs. Inset: optical microscope image of thetrapped lasing droplet.Fig. 2. Excitation fluence-dependent intensities of the lasingWGMs and background emissions in a 9.4 μm diameterglycerol/water aerosol droplet.1670 OPTICS LETTERS / Vol. 38, No. 10 / May 15, 2013neglected owing to the short exposure time required forrecording individual spectra. When the excitation fluenceexceeded the threshold, increase of the intensity of thelasing peak with a higher slope relative to the back-ground emission indicated the onset of lasing. Thethreshold excitation fluence depends on the size of thetrapped aerosol particle and the Q-factor of its WGMs[17]. Large radiative losses in smaller droplets result inan exponential decrease of their WGM Q-factors [18].This sets a lower limit to the diameter of droplets thatdisplay lasing at a given maximal excitation fluence. Inour experimental conditions, this lower limit wasdetermined as 7.7 μm.With increasing droplet size, spectral position of theemission maximum of lasing aerosols containing con-stant dye concentration was observed to shift to longerwavelengths. This trend is illustrated in Fig. 3(a), whichshows emission spectra of six droplets with differentradii recorded at fixed 2.41 mJ∕cm2 excitation fluence.Here, the increasing droplet radius corresponds to thedecreasing FSR of the droplet [16]. For the given droplet,spectral location of its overall maximal lasing gain canbe quantitatively characterized by the mean lasing wave-length λ̄ calculated as the intensity-weighted average ofthe wavelengths of all lasing WGMs from the same modefamily observed in the droplet’s spectrum. Figure 3(b)summarizes the relationship between λ̄ and FSR for 34lasing droplets of varied size. Within the studied rangeof droplet sizes (7.7–11.0 μm), FSR decreased with λ̄ fol-lowing an almost linear dependence. A similar spectralshift of the overall lasing gain profile was previouslyobserved in lasing emulsion droplets [6,15] and explainedby the size-dependence of the efficiency of light out-coupling from the droplets and the cross section ofthe lasing WGMs.We have demonstrated lasing in the WGMs of dye-doped glycerol/water microdroplets manipulated in airusing optical tweezers. By changing the trapped dropletsize, we have shown that the average lasing wavelengthcould be tuned between 600–630 nm. Controlled fusion ofmultiple droplets confined in dynamically positioned in-dependent optical traps, manipulation of relative humid-ity and temperature in the sample chamber, or localheating with the trapping laser can be further used forprecise control of the lasing droplet size. Our resultsare readily adaptable to simultaneous manipulation oflarge numbers of lasing aerosol particles. They also bringup the possibility of using the high sensitivity of stimu-lated emission to small perturbations in the shape, size,and material properties of the droplet cavity for sensitivechemical and biological analysis in airborne particles.This work was partially supported by TÜBİTAK (GrantNo. 111T059). S.A. thanks the Schlumberger Faculty forthe Future program for support.References1. J. P. Reid, H. Meresman, L. Mitchem, and R. Symes, Int. Rev.Phys. Chem. 26, 139 (2007).2. K. Mølhave, A. Kristensen, and N. A. Mortensen, AdvancedPhotonic Structures for Biological and Chemical Detection(Springer, 2009), pp. 471–486.3. M. Tanyeri and I. M. Kennedy, Sensor Lett. 6, 326 (2008).4. M. Humar and I. Musevic, Opt. Express 19, 19836 (2011).5. M. Humar, M. Ravnik, S. Pajk, and I. Musevic, Nat. Photon-ics 3, 595 (2009).6. S. K. Y. Tang, R. Derda, Q. Quan, M. Loncar, and G. M.Whitesides, Opt. Express 19, 2204 (2011).7. H.-M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, Opt.Lett. 9, 499 (1984).8. A. Kiraz, A. Sennaroglu, S. Doganay, M. A. Dundar, A. Kurt,H. Kalaycoglu, and A. L. Demirel, Opt. Commun. 276, 145(2007).9. M. Tona and M. Kimura, J. Phys. Soc. Jpn. 69, 3533 (2000).10. H. Azzouz, L. Alkhafadiji, S. Balslev, J. Johansson, N. A.Mortensen, S. Nilsson, and A. Kristensen, Opt. Express14, 4374 (2006).11. D. R. Burnham and D. McGloin, Opt. Express 14, 4176(2006).12. R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, Phys.Chem. Chem. Phys. 6, 4924 (2004).13. D. McGloin, D. R. Burnham, M. D. Summers, D. Rudd, N.Dewar, and S. Anand, Faraday Discuss. 137, 335 (2008).14. K. Sasaki, H. Fujiwara, and H. Masuhara, Appl. Phys. Lett.70, 2647 (1997).15. M. Aas, A. Jonáš, and A. Kiraz, Opt. Commun. 290, 183(2013).16. P. Chylek, J. T. Kiehl, and M. K. W. Ko, Phys. Rev. A 18, 2229(1978).17. A. J. Campillo, J. D. Eversole, and H. B. Lin, Phys. Rev. Lett.67, 437 (1991).18. A. Serpengüzel, J. C. Swindal, R. K. Chang, and W. P. Acker,Appl. Opt. 31, 3543 (1992).Fig. 3. (a) Lasing spectra recorded from six different aerosolswith diameters 7.7, 8.4, 9.0, 9.6, 10.1, and 11.0 μm (bottom totop) at constant excitation fluence of 2.41 mJ∕cm2. (b) Freespectral ranges of 34 lasing aerosols as a function of the meanlasing wavelength, λ̄.May 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 1671

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Dye lasing in optically manipulated liquid aerosols

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