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Liquid laser cavities and waveguides. II. Bubbles, menisci, and pendant drops

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1997 Quantum Electron. 27 1015

(http://iopscience.iop.org/1063-7818/27/11/A21)

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Abstract. A continuation is reported of an investigation ofnew forms of liquid laser cavities and waveguides such asbubbles, menisci, and pendant drops. Lasing with a definitemode structure occurs in pendant drops pumped by laser andincoherent radiations. The main features of the observedoutput radiation of totally reflecting closed cavities are con-sidered.

We described in Part I [1] the methods for constructing liquidforms of laser cavities and some features of lasing in sessiledrops and in thin-walled stable and unstable rings made oflaser dye solutions. The variety of the resultant liquid cavitiesand waveguides makes it possible to employ simple methodsto investigate complex (and difficult to construct in the formof solid-state structures) optical cavities with smoothly var-iable dimensions. We continued this investigation andconsidered lasing in bubbles, menisci, and pendant dropsof a solution of the dye 9-diethylaminobenzo[a]phenoxazi-none-5 [2] in dibutyl phthalate (DBP) when pump pulses ofenergy up to 40 mJ and of about 10 ns duration wereabsorbed in a process characterised by the absorption coef-ficient 224 cmÿ1 at l � 532 nm. In all the figures reproducedbelow, the pump radiation was incident on a liquid cavityfrom the left and observations were made in a horizontalplane perpendicular to the pump direction.

Lasing in several simultaneously excited DBP drops, 0.5 ^1.0 mm in diameter, dissolved in water (Fig. 1) failed to revealany interaction between the drops. Each drop acted as a self-contained cavity andoperated practically independently, sincethe coupling between such closed cavities was very weak [1].Somedrops, formedby stirringofa solutionofDBP inmisciblewith water, proved to be hollow bubbles consisting of a thinspherical film of DBP filled with water. The addition of a salt(NaCl or NaNO3) to water made it possible to equalise thedensitiesofwaterandDBP.Thin-walledDBPbubblesofpracti-cally perfectly spherical shape, 5 ^15 mm indiameter andwithwalls 50 ^100 mm thick, were observed on a vertical tubularbrass nozzle in a cell with such a solution . In contrast to airbubbles ofDBP,whichwereunstable inair andbrokeup imme-diately,wefoundthat `weightless'DBPbubbles filledwithasaltsolution andsurroundedby the same solution could survive fordays without a change in their shape.

When the pump radiation was directed perpendicular tothe surface of a spherical bubble and the energy of this radi-ation exceeded 1.5 mJ (in an excitation zone of 0:25 mm�5 mm dimensions), lasing at wavelengths l � 630 ^ 640 nmwas observed in a sphere-like waveguide DBP film. Whenviewed from one side (perpendicular to the pump direction),such lasing was concentrated at two diametrically oppositepoints. For nonperpendicular incidence of the pump radia-tion we saw that some of this radiation passed practicallywithout refraction across a thin DBP film and water, andexcited a second zone on the opposite side of the sphere.In this case we found that the lasing region observed fromone side was in the form of four symmetric points (Fig. 2)

AYuBelonogov,AVStartsev,YuYuSto|̄lov,ChoSung-Joo P N LebedevPhysics Institute, Russian Academy of Sciences, Leninski|̄ prospekt 53,117924 Moscow

Received 30 July 1997Kvantovaya Elektronika 24 (11) 1045 ^1048 (1997)Translated by A Tybulewicz

PACSnumbers: 42.55.Mv; 42.60.Da; 42.79.GnLASER APPLICATIONS AND OTHER TOPICS IN QUANTUM ELECTRONICS

Liquid laser cavities and waveguides.II. Bubbles, menisci, and pendant drops

AYu Belonogov, AV Startsev, Yu Yu Sto|̄lov, Cho Sung-Joo

Quantum Electronics 27 (11) 1015 ^1018 (1997) ß1997 Kvantovaya Elektronika and Turpion Ltd

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Figure 1. Lasing in manyDBFdrops with diameters 0.1 ^1 mm in water,pumped by a wide laser beam (a), distributions of the output radiationintensities along horizontal (b) and vertical (c) axes passing through thepoint identified by the arrow, and also some of the drops shown on anenlarged scale (d).

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Figure 2. Lasing in a DBP drop 7 mm in diameter (wall thickness 50 mm)filled with a salt solution in water and surrounded by this solution, pumpedwith radiation shifted upwards relative to the drop equator (a), distribu-tions of the output radiation intensities along horizontal (b) and vertical (c)axes passing through the points identified by the arrow, and two brightlasing spots on the right, shown on an enlarged scale (d).

and that the distance between these points depended on theregion on which the pump radiation was incident. Horizontalstretching of the image shown in Fig. 2 was the result of opti-cal distortion when a sphere 7 mm in diameter was viewedthrough a side wall of a water-filled cylindrical cell. Typicalhorizontal and vertical distributions of the radiation intensityin the observed bright lasing regions are shown in Fig. 2. Asfound earlier for drops [1], we noted that in the case of bub-bles (and menisci) the usually smooth Gaussian emissionspectra with a maximum in the region 635 nm had sometimes(under apparently identical excitation conditions) bands withthe brightness modulation depth 20% ^ 50% and a period0.5 ^1.0 nm. The origin of these bands was not clear.

In an ideal spherical layer the structure of the `whisperingmodes' and internal waveguide properties were simple, andthey ensured focusing of the resultant laser radiation atjust one point diametrically opposite to the centre of theregion on which the pump radiation was incident. A morecomplex pattern of the mode structure was observed forliquid laser cavities in the form of concave or convex menisci(half-moons) formed by a thin DBP drop spread on a concavesurface of water in a narrow tube or under water on top of afluorocarbon (of the type C8F18) drop inmiscible with waterand lying at the bottom. When lasing was viewed from oneside, we found that the acute corners of the menisci werethe brightest and the distribution of the radiation intensityemerging from these corner zones sometimes had severalmaxima (Fig. 3) because of the complex structure of theinternal modes. We achieved lasing in the region of625 nm in a DBP meniscus lying on a drop of mercury underwater. The external shape of the meniscus was the same asthat lying on top of the C8F18 drop, but the emission spectrumwas shifted significantly by 10 ^15 nm towards shorter wave-lengths in the direction of the dye fluorescence maximum atl � 625 nm, evidently because of the presence of mercury.

A complex mode structure of the laser radiation was also

observed when lasing occurred in a DBP drop hanging in airat the end of a capillary (Fig. 4). The excitation threshold oflasing in the region of 646 nm was less than 0.2 mJ (thedimensions of the excitation zone were 0:2 mm� 0:2 mm).The distribution of the intensity of the output radiationhad the form shown in Fig. 4. The number and positionsof the points with the maximum lasing intensity depended

on the location of the region on which the pump radiationwas incident; an increase in the pump power increased thenumber of such points from two (for 0.5 mJ) to eight (for6 mJ). The neck of such a drop at the point of its contactwith the capillary was usually a zone of high intensity ofthe output radiation.

The measured threshold energy corresponded to anexcitation flux of 1021 photons cmÿ2 sÿ1, fully attainablealso by means of other pump sources. The low excitationthreshold of such a cavity made it possible to induce lasingin the region of 644 nm by illumination of the drop withNd laser radiation characterised by l � 1:06 mm and of8 mJ energy incident on an area of 0:5 mm� 2 mm dimen-sions. We initially assumed that excitation of the dyemolecules with the fundamental-frequency radiation wascaused by nonlinearities induced by two-photon absorptionor by generation of the second harmonic at inhomogeneitiesof the surface layer. However, we found that pumping wasprovided by light from a barely detectable miniature sparkgenerated by the Nd laser radiation transmitted by a dropand focused by this drop acting as a lens. The spark appearedin air in the direct vicinity of the rear boundary of the drop.At pump energies in excess of 120 mJ a shock wave from thisspark was sufficiently strong to blow away the drop from thecapillary in each experiment, but this did not hinder record-ing the lasing effect. Similar lasing was observed whenradiation with l � 1:06 mm and of 4.5 mJ energy was focuseddeliberately by a spherical lens placed in front of a drop, sothat the spark appeared in air near the front boundary ofthe drop. We thus found that lasing appeared when such adrop was illuminated with an incoherent spark light source,which could in principle be generated by nonlaser energy.

We pointed out in Part I [1] that, because of total internalreflection, the laser radiation (considered in the geometric-layer approximation) should not escape from the zones whereit was observed. Although the internal laser field in high-quality cavities was sufficiently strong and could induce non-linear effects, capable of frustrating total internal reflection,the observed profiles of the output radiation could beexplained by a simple deflection of this radiation at smallinternal scattering centres.

When such centres had an elongated highly-directionalscattering diagram, only a small fraction of the scattered-radiation energy travelled at angles less than the criticaland escaped outside. The mechanism of coupling out of

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Figure 3. Lasing in a DBP meniscus with the largest diameter 6.5 mm,lying under water on a C8F18 drop (a), distributions of the output radia-tion intensities along horizontal (b) and vertical (c) axes passing throughthe point identified by the arrow, and also the right-hand lasing spotshown on an enlarged scale (d).

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Figure 4. Lasing with a mode structure in a pendant DBP drop, 2 mmin diameter, pumped at one point near the drop equator (a), distributionsof the output radiation intensities along horizontal (b) and vertical (c)axes passing through the point identified by the arrow, and also the dropshown on an enlarged scale (d).

1016 A Yu Belonogov, A V Startsev, Yu Yu Sto|̄lov, Cho Sung-Joo

the radiation by scattering accounted for the considerablylower lasing threshold for drops suspended in air (Fig. 4),and for the observed smaller half-width of the distributionof the output radiation and its lower intensity, because thecritical angle for these drops and the corresponding losseswere lower than for the drops and menisci in water. The frontand rear edges of the recorded output radiation profiles cor-responded to the internal scattering diagram of the particlesin the lasing solution and the observed spreading of theseedges beyond the laser-active medium was evidence of addi-tional scattering of the radiation emerging from the cavitiesby other centres in the surrounding medium. In air, the num-ber of such scattering centres was less and, therefore, theouter edges of the distribution of the output radiationobtained from a drop in air were steeper than the correspond-ing edges for a drop in water.

An additional scattering centre in the form of a tiny airbubble on the wall of a drop in water, located in a liquid cavityin the path of a lasing beam, resulted in significant couplingout of the radiation at this point (Fig. 5). The angular distri-bution of the output radiation was narrower than thatobtained from the usual spots at the sides. In principle, ananalysis of the recorded output intensity profile outside thelaser-active medium and the measured angular distributionof the output radiation from a spherical liquid cavity shouldmake it possible to determine in a single pulse the angulardiagram of the scattering by particles in the surroundingmedium and to monitor the concentration of such particles.

In the case of this mechanism of coupling out the radia-

tion by scattering we could not understand why, in practicallyall the investigated cavities, the front lasing spot coincidingwith the pumping zone appeared several times weakerwhen viewed from one side than the rear spot more distantfrom the pumping zone (see, for example, Figs 2 and 3).The ration of the intensities of the radiation from the rearand front spots changed when observations were made at dif-ferent angles, so that we could not explain this change simplyby unidirectional lasing in a spherical cavity. The observedbehaviour could be possibly due to special features of themode structure of the field caused by accumulated thermalinhomogeneities or by some difference between the angular

distributions of the radiation emerging from different points.However, this would require further studies. Some help inthis respect could come from the possibility of observingbright spots of the mode structure of the investigated liquidcavities not only under pulsed pumping conditions, butalsoöas we proved ourselvesöwhen such cavities wereexcited with radiation from cw He ^Ne (l � 632:8 nm) orCd (l � 422 nm) lasers.

The formation of additional small droplets under intensepumping conditions, reported in Ref. [1], was caused (asestablished by us) by heating and boiling of surface layersof the laser solution at high excitation energies. However,the surface tension forces of the liquids rapidly restoredthe previous configuration, so that there were no significantchanges in the properties of a cavity at the moment of arrivalof the next excitation pulse.

A bulk DBP drop in water lying at the bottom of a cell wascharacterised by a lasing threshold approximately an order ofmagnitude higher when the dye concentration in the solutionwas reduced 32-fold; this was accompanied by a significantincrease in the depth of the excitation zone.

The instability of ring-shaped cavities, described inRef. [1] and demonstrated by the ejection of DBP waveson the surface of water in an open cell, was associatedwith a considerable influence on the surface tension of theliquid exerted by C8F18-type fluorocarbon molecules locatedabove the liquid. The concentration of these molecules fluc-tuated at low pressures (about 20 Torr). Our estimatesindicated that at room temperature the diffusion coefficientof these molecules in air was D � 0:20� 0:05 cm2 sÿ1.This effect was of interest because usually any gases whosepressures are less than several hundred bars do not influencethe surface tension of liquids [3], whereas fluorocarbon gasesare found to reduce considerably the surface tension of liquidsat pressures 10 000 times lower and they act on the liquid sur-faces as a `volatile soap' or a `gaseous lubricant' [4]. Ananalysis of this interaction would be undoubtedly of theoret-ical and practical interest, but it would require very powerfulcomputers because the seemingly modest task of calculatingthe influence of surface-active substances on the surface ten-sion of liquids is comparable, in respect of its complexity andvolume, with full calculations relating to launching of spacerockets or to thermonuclear reactor models [5].

We are planning to extend our investigation to othershapes of liquid cavities and waveguides, to increase the rangeof dyes and solvents, to widen our studies to lasing excited byshort and long pulses.We also want to narrow down the emis-sion spectrum by distributed feedback, to reduce thescattering and the lasing threshold, to test the sensitivity ofsuch cavities to external agencies which could make them use-ful as sensors.We need to investigate the angular distributionof the output radiation by modifying the shape of the cavity(as described in Ref. [6]), to make comparisons with theresults of mathematical simulation of the output radiation,and also to use cavities as objects for the observation of non-linear phenomena.

The small volume of the active medium makes it possibleto excite all the dye molecules by pump radiation of extremelyhigh intensity (for example, in tasks mentioned in Ref. [7])without damaging expensive optical surfaces. The practicaldamage immunity of liquid cavities and waveguides, theease of creating smoothly variable complex and not readilyaccessible (for other laser media) shapes, the sensitivity tothermal and mechanical deformations of the surfaces, as

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Figure 5. Lasing in a continuous DBP drop 3 mm in diameter, lying atthe bottom of a water-filled cell with a small air bubble about 0.2 mm indiameter, located on the drop equator and scattering inside the lasingradiation (a), distributions of the output radiation intensities along hori-zontal (b) and vertical (c) axes passing through the point identified by thearrow, and also the drop shown on an enlarged scale (d).

Liquid laser cavities and waveguides. II. 1017

well as to the chemical composition of the laser-active andsurrounding media, the low cost and the high optical qualityof smooth surfaces, the miniature size and the sphericalangular distribution of the radiation, and the ability toaccommodate laser and incoherent pumping are all attractivefeatures of these objects for further studies and because theyare the most inexpensive laser-active elements. If necessary,such laser cavities and waveguides can be prepared from sol-idifying solutions or resins [8] similar to those used forexample in flexible optics [9], and by their solidificationwith dyes to obtain solid copies of the liquid forms.

We are grateful to the Ministry of Defence of the RussianFederation and to the Department of Defence of the USA fortheir support and interest.

References

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2. Alekseeva V I, Volkov V M, Kokin V N, Luk'yanets E A,Marinina L E, Reznichenko A V, Sosunov G N Tezisy Dokl. IIVsesoyuz. Konf. `Lazery na Osnove Slozhnykh OrganicheskikhSoedineni|̄ i Ikh Primenenie', Dushanbe, 1977 (Abstracts ofPapers presented at the Second All-Union Conference on LasersBased on Complex Organic Compounds and Their Applications,Dushanbe, 1977) p. 52

3. Kikoin A K, Kikoin I K Molekulyarnaya Fizika (MolecularPhysics) (Moscow: Nauka, 1976) p. 329

4. Sto|̄lov Yu Yu Sposob Snizheniya Poverkhnostnogo NatyazheniyaZhikkoste|̄ (Method for Lowering the Surface Tension ofLiquids), Patent Application 96119 426 dated 27 September 1996

5. Gibbs W W ``Computer bombs'' (``In focus'') Sci. Am. 276 (3) 10(1997)

6. Nockel J U, Stone A D Nature (London) 385 45 (1997)7. Belonogov A Yu, Startsev A V, Sto|̄lov Yu Yu, Cho Sung-Joo

Kvantovaya Elektron. (Moscow) 23 571 (1996) [ Quantum Elec-tron. 26 556 (1996)]

8. Sto|̄lov Yu Yu Sposob Polucheniya Slozhnykh ZhidkikhOpticheskikh Poverkhnoste|̄ (Method for Forming ComplexLiquid Optical Surfaces), Patent Application 96119 425 dated27 September 1996

9. Younan Xia, Enoch Kim, Xiao-Mei Zhao, Rogers J A,Prentiss M, Whitesides G M Science 273 347 (1996)

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Liquid laser cavities and waveguides. II. 1019