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Liquid laser cavities and waveguides. III. Radiation coupling-out and fluorescence under

uniform excitation. 'Evapolators'

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1998 Quantum Electron. 28 608

(http://iopscience.iop.org/1063-7818/28/7/A09)

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Abstract. An investigation was made of laser radiationcoupling-out from totally reflecting closed liquid cavities bylocal frustration of total internal reflection with the aid ofinternal water spheres and external contact with a substrateor a prism. A rotating drop of ethanol containing rhodamineB in an envelope of liquid C8F18 generated laser radiationwith a divergence of several angular degrees and an effi-ciency up to 5% relative to the excitation energy. A trans-parent drop of ethanol with rhodamine B, suspended in air,and subjected to uniform pumping was used to determinethe depth of the zone in which lasing took place in a liquidcavity. An increase in the integral (during the pump time)fluorescence energy from a selected internal part of this drop,which did not participate in lasing, on increase in the exci-tation flux was in agreement with the expectations and itbegan to fall at high pump intensities. Characteristics ofoscillations in immiscible liquids which evaporated in anevaporator ^ oscillator (`evapolator') were discussed.

Natural laser cavities in the form of drops and rings of laserdye solutions [1, 2] are the cheapest undamageable lasersources with ideal optical surfaces. However, coupling-outis difficult to effect because of total internal reflection andthe energy of the coupled-out radiation does not exceed0.05% of the pump energy [1]. The ideal nature of the opticalsurfaces and the closed form of the liquid cavities ensure lowexcitation thresholds for laser and incoherent pumping [2],but they prevent the attainment of highly efficient laser radi-ation coupling-out.

We coupled-out the radiation by utilising local frustrationof total internal reflection, which was achieved by severalmethods. A drop of dibutyl phthalate (DBP) containingthe laser dye 9-diethylamino[a]phenoxazinone-5, 5 ^ 8 mmin diameter, was at the bottom of a cell surrounded by anaqueous solution of a salt (7.545 g NaNO3 per 100 g ofwater). At this solution concentration the DBP drop wasspherical because of equal specific weights. A small globule(0.5 ^ 2 mm in diameter) of the same solution was blowninto the sessile drop through a bent tube introduced intothe drop from above. This globule was located at a distance

of about 0.05 mm from the drop boundary, i.e. it was in thelaser radiation path. Some of the laser radiation, excited inthe 635 ^ 640 nm range by the second harmonic of a neody-mium laser at l � 532 nm, was coupled-out as a directedbeam. The intensity of the output radiation could be variedby moving the water globule along the drop radius.The diver-gence of the output radiation depended on the distance of theglobule from the drop boundary and could amount (at half-amplitude) to 78� 18 along the horizontal and 28� 18 alongthe vertical.When the pump energy was 0.06 ^ 37 mJ, the las-ing efficiency was about 0:5% � 0:1%.

In the second series of experiments the laser radiation wascoupled-out from spherical DBP drops 6 ^12 mm in diameterin an aqueous solution of NaNO3, which were in contact withthe surface of a flat glass substrate or a prism immersed in thesolution alongside the drop. Such contact could be estab-lished by bringing the glass substrate closer to the DBPdrop until a small contact area was established betweenthe drop and the glass. A drop of DBP of moderate size(with a diameter less than 1 mm) could be placed first onthe substrate and then immersed in the solution, approachingthe lasing drop. In this case the drops also coalesced, ensuringfull contact with the glass substrate necessary for radiationcoupling-out. The contact area increased slowly with time,but initially it was governed by the dimensions of the depos-ited small drop. The motion of the substrate then deformedthe lasing drop and made it nonspherical.

When the drop was spherical, the laser radiation wascoupled-out through the contact zone in the form of beamsdirected along two directions and characterised by a diver-gence of several angular degrees; the radiation nearlyfollowed the tangent to the drop surface. When this cou-pling-out method was adopted, the lasing efficiency was�0:2% � 0:03%. A displacement of the substrate, which dis-turbed the spherical symmetry, resulted in some of the laserradiation reflected by the substrate inside the drop escapingthrough its side surface in the form of an additional beam ofintensity comparable with the radiation coupled-out throughthe substrate. Similar results were obtained when a drop wasin contact with the surface of a prism in water.

A series of experiments was carried out with a liquid lasercavity of a new form, representing drops in a rotating testtube. A drop of DBP with dye 9-diethylaminobenzo[2]-phenoxazinone-5 or a drop of ethanol containing rhodamineB, with a concentration ensuring the absorption of the pumpradiation characterised by the coefficients 100 ^1000 cmÿ1,was placed together with an immiscible liquid (C8F18) anda small air bubble in a glass test tube with an internal diam-eter 6 mm and 6 ^ 7 cm long. The hermetically sealed testtube was oriented horizontally and rotated about its axis

A Yu Belonogov, AV Startsev,Yu Yu Sto|̄lov, Cho Sung-JooP N Lebedev Physics Institute, Russian Academy of Sciences,Leninski|̄ prospekt 53, 117924 Moscow,e-mail: stoilov@sci.lpi.msk.su

Received 9 April 1998Kvantovaya Elektronika 25 (7) 625 ^ 628 (1998)Translated by A Tybulewicz

PACSnumbers: 42.55.Mv; 42.60.Da; 42.79.GnCONTROL OF LASER RADIATION PARAMETERS

Liquid laser cavities and waveguides.III. Radiation coupling-out and fluorescenceunder uniform excitation. `Evapolators'

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

Quantum Electronics 28 (7) 608 ^ 611 (1998) ß1998 Kvantovaya Elektronika and Turpion Ltd

at a rate of 800 rpm. Such rotation displaced the heavy liquidC8F18 (density 1.8 g cmÿ3) to the test tube walls, whereas thelighter drop of the lasing solution in the shape of a sphere oran ellipsoid 2 ^ 3 mm in diameter and 7 ^ 25 mm long waslocated on its axis (Fig. 1). The low refractive index(n � 1:3) of C8F18 ensured the conditions necessary for totalinternal reflection at the walls of the rotating drop and for theappearance of lasing.

The elongated shape of the drop and frustration of totalinternal reflection near its end made possible partial cou-pling-out of the radiation. When the DBP drop was excitedby the second harmonic of a neodymium laser, which passedthrough a cylindrical lens, the laser radiation emergingthrough the end of the drop and through the test tube hada divergence of several angular degrees. The lasing efficiencythen reached 1:3% � 0:5%.

When a rotating drop of ethanol with rhodamine B,15 mm long, was excited, the efficiency of emission in therange 608 ^ 609 nm reached 5:1% � 1% and the outputenergy was then up to 1.3 mJ. A reduction of the drop lengthusually reduced the output radiation energy. Therefore, localfrustration of total internal reflection in an elongated rotatingdrop made it possible to couple-out, from a closed liquid cav-ity, laser radiation representing up to 5% of the pump energy.

It seemed of interest to investigate a liquid cavity sub-jected to uniform longitudinal pumping throughout itsvolume. In this case the depth of the lasing zone wasdetermined by employing a transparent drop of ethanolcontaining rhodamine B (diameter �2 mm, absorptioncoefficient 0.2 ^ 0.4 cmÿ1) hanging from the end of a capillary

in air. It is evident from Fig. 2a that the l � 532 nm pumpradiation passed practically without attenuation throughthe whole drop and excited molecules in the drop uniformly.When the threshold energy density (about 0.5 J cmÿ2) wasreached, lasing appeared in the drop near its boundaries ina narrow zone where high-Q whispering modes could exist(Figs 2b and 3b). The distribution of the pumping alongthe vertical axis was not uniform (Fig. 2c), but it had a max-imum at the beam centre. However, the longitudinal pumping(Fig. 2b) was highly uniform, so that it was possible to com-pare the observed width of the lasing zone in a drop withtheoretical calculations of the laser field in spherical cavitiescharacterised by total internal reflection.

The high threshold energy for the excitation of lasing, atwhich the pump flux exceeded considerably the saturationpower, observed in these experiments was unrelated to thelosses, but was related to the need to maintain for a longtime an almost complete population inversion of the mediumrequired for lasing growth. When the gain of the activemedium was 0.15 ^ 0.2 cmÿ1, lasing required traversing a dis-tance of about 2 ^ 3 m, which was achieved after 9 ^15 ns, i.e.only at the end of a pump pulse when the intensity of the excit-ing flux fell considerably.

The profile and the duration (usually �10 ns [1]) of theexcitation pulses were such that nearly complete populationinversion of particles in the cavity required for the evolutionof lasing was reached during the relevant time at the highexcitation energy indicated above when the pump intensityat the maximum of a pulse was several times higher thanthe saturation intensity for the dye.

The high pump intensities and the uniform excitation ofmolecules in a transparent drop made it possible to confirmearlier conclusions [3] on the nature of the fluorescence andon the lifetime of the excited dye molecules at high pumpintensities, which were questioned in Ref. [4]. As we pointedout in Ref. [3], the characteristic features of the experimentswith small cells and the danger of damage to the outputwindows at high excitation fluxes, the smallness of the fluo-rescence signals and detection of these signals throughout thewhole volume of a cell presented serious difficulties in theexperiments with uniform excitation of the active mediumand reduced their precision.

12

3

Figure 1. Test tube with liquid C8F18 ( 1 ) and with ethanol drops contai-ning rhodamine B ( 2 ), rotating at a rate of 800 rpm, and also an air bub-ble on the test tube axis ( 3 ). The view was recorded from above.

d

b

c

a

Figure 2. Pendant (in air) drop of ethanol with rhodamine B, 2 mm indiameter and with the absorption coefficient 0.4 cmÿ1, pumped byl � 532 nm radiation of I0 � 1026 cmÿ2 sÿ1 intensity (a), distribution ofthe radiation energy emitted by the drop along a horizontal axis in a sec-tion passing through the drop centre, determined at the lasing thres-hold (b), distribution of the radiation energy from the drop along avertical axis in a section passing through the drop centre (c), and a magni-fied image of the drop (d). The pump radiation focused by a cylindricallens is incident on the drop from the left.

d

a

b

c

Figure 3. Shape of an ethanol drop pumped by radiation of I0 � 1027

cmÿ2 sÿ1 intensity, causing spark breakdown in air behind the drop (a),distribution of the radiation energy emitted by the drop along a horizon-tal axis in a section passing through the drop centre (b), distribution ofthe radiation energy from the drop along a vertical axis in a section pas-sing through the drop centre (c), and a a magnified image of thedrop (d).

Liquid laser cavities and waveguides. III. 609

The use of small transparent drops avoided these difficul-ties and allowed us to carry out measurements with uniformexcitation at unlimited high pump intensities. Recording ofdrop images with a highly sensitive CCD camera made it pos-sible to determine the local changes in the total (accumulatedduring a pulse) fluorescence energy in a selected part of adrop, for example, at its centre on the axis of the incidentpump beam.

Lasing appearing at the end of a pulse in a thin wall layerof the drop (Figs 2 and 3) was of low intensity and hadpractically no influence on the dynamics of the fluorescenceof the molecules outside the lasing zone, i.e. at the centre ofthe drop.

Fig. 4 gives the experimental dependence of the fluores-cence energy on the maximum intensity of the pumppulses when a drop was excited via a cylindrical lens. Thepumped region was a strip of 2 mm� 0:1 mm dimensions.A theoretical curve ( 2 ) was calculated for the total fluores-cence energy of the excited particles in the mediumreleased during the action of a pump pulse [1]. The observeddeviation of the experimental curve from the calculations athigh excitation fluxes (when the total fluorescence energydecreased) could be the result of activation of a mechanismof nonlinear quenching of the fluorescence by high pumpfluxes.

The vertical distribution of the energy of the radiationemitted by a drop (Fig. 3c) indicated that in such cases therecorded fluorescence energy fell at the beam centre, but itwas higher (compared with the axial intensity) at the edges,i.e. at points characterised by pumping with a lower intensity.When radiation was recorded from the whole drop, this madea considerable contribution, ensuring the subsequent rise ofthe overall fluorescence signal by another factor of 2. Theexperimental results were in good agreement with the curverepresenting the fluorescence energy calculated on theassumption that the lifetime of the fluorescing dye moleculeswas constant at low and high pump intensities, and that theprofile of a pump pulse was typical. This confirmed our ear-lier conclusions [3] on conservation of the lifetime of excitedmolecules when they were in focused high-intensity excitationfluxes.

As pointed out by us earlier [3], had the radiative lifetimeof the molecules decreased at high pump fluxes, this wouldhave increased the radiation energy to the range 1025 ^1027 cmÿ2 sÿ1, which was tens of times higher than thatobserved. Consequently, the absence of such an increaseand a significant reduction of the total fluorescence energy

in focused beams of intensity exceeding 5� 1026 cm2 sÿ1

allowed us to reject the objections put forward in Ref. [4]against our conclusion of a constant lifetime of excited dyemolecules at high excitation flux intensities.

A multitude of the available solution combinations madeit possible to investigate also lasing in drop cavities withouttotal internal reflection.We could thus estimate the influenceof total internal reflection on the evolution of lasing. A dropof ethanol with rhodamine B (pump absorption coefficient12 cmÿ1) lying at the bottom of a cell with kerosene wasobserved to lase, although the refractive index of ethanolwas less than that of kerosene.When a cylindrical lens, form-ing a strip of 0:5 mm� 10ÿ2 cm dimensions was used inpumping (and the threshold density of the pump energywas 0.36 J cmÿ2), lasing appeared in the region of 578 nmas a bright spot on one side of the drop.

A thin Teflon film, not wetted by ethanol, was placed atthe bottom of the cell in order to prevent ethanol spreadingover the glass. An ethanol drop then remained at the bottomand retained its flattened form. Since ethanol was dissolvedpartly in kerosene, the dissolution process was minimisedby saturating kerosene first with ethanol and then immersinga lasing drop inside it. The drop then retained its lasing prop-erties for weeks.

For nearly identical excitation fluxes it was found thatlasing in a drop of ethanol in kerosene appeared at activeparticle densities 60 times higher than in the transparent etha-nol drop described above; this threshold was approximately1000 times higher than the threshold for an opaque DBPdrop in air for which the threshold energy density was5� 10ÿ4 J cmÿ2 [2]. A comparison of these threshold den-sities demonstrated the major influence of the surroundingspecular surface with total internal reflection on the develop-ment of lasing in liquid cavities.

The small volumes of the active medium in liquid lasercavities made it possible to investigate lasing in expensivebut interesting solvents, for example, volatile hexafluoro-2-trifluoromethyl-2 propanol (CF3)3COH (compound No. 509in the catalogue of the PiM Company). Dyes such as rhod-amine 6G, coumarine 6, and several others dissolvedreadily in this incombustible fluorocarbon alcohol. When apendant drop of (CF3)3COH with rhodamine 6G (1 ^1.5 mm in diameter) was placed in a closed test tube contain-ing saturated (CF3)3COH vapour and was pumped withl � 532 nm radiation via a cylindrical lens, lasing wasobserved at wavelengths 578� 1 nm. The absorption coeffi-cient of the pump radiation was then about 1 cmÿ1 and thethreshold excitation energy density was 0.3 J cmÿ2. Theseresults demonstrated the feasibility of using miniature liquidlaser cavities for the study of new (including nontrivial) activesolutions present in small volumes, dispensing with the expen-sive optical cells and mirrors usually needed for this purpose.

The use of inert fluoro-organic liquids such as C6F14,C8F18, C10F18, etc., with room-temperature vapour pressuresof 10 ^100 Torr, reduces the surface tension of many liquidsby 10% ^ 30% [5] and makes it possible to form in a naturalway some complex optical shapes of liquid laser cavities[1, 2, 6]. Moreover, in the case of the usual evaporation ofliquids from open cells, there is a continuous motion of thesurface layers (Marangoni effect [7]) observed in both closedcells as well as for continuous periods of many months inevaporators ^ oscillators (`evapolators') representing her-metically sealed cells with a trap cooled by evaporatingwater at room temperature.

Efl (rel. units)

300

200

100

01024 1025 1026 1027 1028 I0

�cm2 sÿ1

12

Figure 4. Experimental ( 1 ) and theoretical ( 2 ) dependences of the totalfluorescence energy Efl, obtained from the central part of a drop during apump pulse, on the pump intensity I0.

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

Such `evapolators' include a large number of variousphysical self-oscillatory systems which appear on the sharedinterface of fluoro-organic and other liquids in the course oftheir evaporation [8] when the velocity of films of one liquidon the surface of another exceeds the velocity of diffusion ofmolecules in a gaseous medium. Constant vortical motion ata linear velocity of 1 ^10 cm sÿ1 (0.5 ^ 2 rps) of rings ofliquids such as ethanol, 2-propanol, DBP, benzene, kerosene,etc. in a vertical plane above a continuous layer of a fluoro-organic liquid in an open cell under natural evaporation con-ditions (see the formula for a ring of water in Fig. 4a ofRef. [1]), as well as continuous oscillations or surface rotationin hermetically sealed cells (with a closed vapour condensa-tion cycle in a cooling trap and with return of the dropletsto the solution) occur because of film adsorption and desorp-tion of fluoro-organic molecules on a surface liquid layer.

Reduction in the surface tension and its subsequentincrease on evaporation of fluoride molecules make it possi-ble to utilise natural motion of solutions in `evapolators' inorder to set them in motion mechanically. Combined useof `evapolators' and liquid laser cavities make it possible toconstruct sealed systems of dye lasers, dispensing with theusual mechanical pumps. Such film motion during evapora-tion may occur also in biological systems, for example,synthetic `blue' blood is extracted via light fluorocarbonsin the course of their evaporation.

A theoretical description of film motion in numerous sys-tems belonging to the `evapolator' class and determination ofthe threshold conditions for the appearance of oscillatorymotion in these systems is a separate interesting task whichis not simple from the physical or mathematical points ofview. By way of formulating the task, we shall list only afew problems which have to be solved in order to accountfor details of the physics of `evapolators'. A quantum-mechanical description and explanation is needed of themechanism of spring-like penetration of fluorocarbon mole-cules into the surface of liquids which repel them underadsorption conditions, when work done in such penetrationis compensated almost completely by a reduction in the sur-face tension energy. There is also need for explanation of themechanism of the subsequent escape of fluorocarbon mole-cules from the surface during evaporation.

Moreover, in an analysis of the dynamics of motion of asurface layer we have to solve a system of the Navier ^ Stokeshydrodynamic equations for simultaneous motion of viscousliquids and of the equation describing the upward climb of avertical wall by a liquid, which should take into account theMarangoni effect in the presence of a surface tension gradient[9]. The dynamics of motion of boundary layers of liquids andof the gaseous diffusion of fluoro-organic molecules whenthey evaporate from a surface should be described takingaccount of the climb of a liquid to a height of 5 ^ 30 mmup a wall as a result of capillary forces [10]. One shouldbear in mind the characteristics of initial motion of a liquidfront along a solid and the `finger' instability which thendevelops [12, 13].

From the practical point of view the various devices inwhich liquid laser cavities and `evapolators', characterisedby a high sensitivity to the smallest changes in temperature,are employed could serve as optical sensors for monitoring ofthe ambient medium.The high velocities of motion and oscil-lations of liquid layers in `evapolators' in the presence ofsmall temperature drops (from 0.1 8C) not only can have sci-entific and practical applications, but can also be used for

wide-ranging demonstrations of self-organising oscillatorymacrostructures in open thermodynamic systems, as well asin constructing models for checking theoretical descriptionsof simple systems with complex dynamic and chaotic behav-iour. The results obtained demonstrate the usefulness of ascientific investigation and practical applications of liquidlaser cavities.

We are grateful to PiMCompany (http:==www.fluorine.ru)for supplying samples of fluoro-organic molecules and to theMinistries of Defence of Russia and of the USA for their helpand interest in this investigation.

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Liquid laser cavities and waveguides. III. 611