Polymer bad whispering gallery mode lar for bionsing applications Alexandre François, Nicolas Rien, Hong Ji, Shahraam Afshar V., and Tanya M. Monro
Citation: Applied Physics Letters 106, 031104 (2015); doi: 10.1063/1.4905931
View online: dx.doi/10.1063/1.4905931
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Polymer bad whispering gallery mode lar for bionsing applications
Alexandre Franc ¸ois,1,a)Nicolas Rien,1Hong Ji,1Shahraam Afshar V .,1,2and Tanya M.Monro 1,2
1
Institute for Photonics and Advanced Sensing (IPAS)and ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP),The University of Adelaide,Adelaide,South Australia 5005,Australia 2
University of South Australia,Adelaide,South Australia 5000,Australia
(Received 17December 2014;accepted 4January 2015;published online 20January 2015)Whispering gallery mode lars are of interest for a wide range of applications and especially biological nsing,exploiting the dependence of the resonance wavelengths on the surrounding refractive index.Upon lasing,the Q factors of the resonances are greatly improved,enabling measurements of wavelength shifts with incread accuracy.A way forward to improve the performance of the refractive index nsing mechanism is to reduce the size of the optical resonator,as the refractive index nsitivity is inverly proportional to the resonator dimensions.However,as the lasing threshold is believed to depend on the Q factor among other parameters,and the reduction of the mi
croresonator size results in lower Q,this pos additional challenges for reaching the lasing threshold.In this letter,we demonstrate lasing in 10l m diameter dye doped polystyrene microspheres in aqueous solution,the smallest polystyrene microsphere lars ever reported in the conditions.We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere,highlighting a much stronger dependency than initially
reported.V
C 2015AIP Publishing LLC .[dx.doi/10.1063/1.4905931]Since the discovery of Whispering Gallery Modes手机中毒
(WGM)by Lord Rayleigh in the late 19th century with the obrvation of sound wave propagation inside the whispering gallery of St Paul’s Cathedral in London,a wealth of work has been pursued to unlock the potential of WGMs at optical frequencies for a large range of applications.Whispering gal-lery modes occur when light is trapped inside a resonator by total internal reflection,circulating along the inner surface and returning in pha after single or multiple round trips to satisfy the resonance conditions.1WGMs are especially suited to refractive index nsing,since the spectral position of the resonances is dictated not only by the resonator geom-etry (e.g.,diameter and sphericity)and optical
properties but also by the surrounding refractive index.Two distinct approaches have been developed to exploit WGMs,in a wide range of resonator geometries ranging from rings/tor-oids 2and spheres 3to cylinders and capillaries.4The first approach is bad on using evanescent field coupling between a pha-matched tapered optical fiber (or prism)and the resonator,and scanning across a narrow wavelength range with a distributed feedback lar to identify the reso-nance positions and linewidths.5While this approach has allowed for unprecedented nsing performance and Q fac-tors,5it is limited in practice becau any variation of the dis-tance between the tapered fiber and resonator results not only in changes in the coupling efficiency but also in fluctua-tion of the resonance positions.6,7Furthermore,using small optical resonators,which would enable improved perform-ance,since the refractive index nsitivity is inverly pro-portional to the resonator diameter,8is challenging from a practical point of view using this approach.The alternative approach involves using resonators that contain a gain
medium.Upon illumination of the active resonators by a remote light source,light is emitted by the gain medium.At the resonance wavelengths of the microcavity,the Purcell effect increas the emission rate of the gain medium.9As a conquence,the resulting WGM spectrum obrvable remotely in the far field shows up as a fluorescent signal modulated by sharp peaks corresponding t
武汉教育云o the microcavity resonances.10While this approach offers advantages in terms of practicality such as robustness of the coupling scheme,easy excitation and collection of the WGM modulated fluo-rescence,the Q factors obrved for the radiation modes are often 3or 4orders of magnitude lower than tho obrved using the evanescent field coupling approach,eventually lim-iting the u of such a WGM excitation strategy in terms of resolution for nsing applications.11A way forward to address this issue is to operate the microresonator beyond its fluorescence regime,inducing stimulated emission,resulting in a higher Q factor.10Examples of such lasing WGMs have been published by Kuwata-Gonokami and Takeda,who ud dye doped polymer microspheres.12The smallest lasing polymer microspheres ud in aqueous solution are 15l m in diameter,and no work has been reported on investigating the size limitations of polymer microspheres for inducing lasing in aqueous media where all refractive index nsing applica-tions and especially biological nsing are performed.
Here,we prent a strategy to produce WGM lars con-sisting of 10l m diameter polystyrene microspheres,which are the smallest polystyrene microspheres ever reported to la in water for refractive index nsing applications.
Polystyrene microspheres with a nominal diameter (Ø)of 10.52l m with a standard deviation (D Ø)of
0.25l m and a refractive index of 1.591(Polysciences Inc.,USA)were doped with a fluorescent lar dye (Nile Red,k abs $532nm,k em $590nm,Sigma Aldrich)using a liquid two pha sys-tem.10The fluorescent dye was first dissolved into xylene
a)
Author to whom correspondence should be addresd.Electronic mail:alexandre.francois@adelaide.edu.au
0003-6951/2015/106(3)/031104/4/$30.00V
C 2015AIP Publishing LLC 106,031104-1
APPLIED PHYSICS LETTERS 106,031104
(2015)
柳的古诗
until the solubility limit was reached.The resulting solution was poured on top of an aqueous solution
of diluted micro-spheres(5ml H2Oþ100l l microsphere solution 2.5% solid)and left on a magnetic stirrer plate until the xylene had completely evaporated.As xylene and water are immiscible and thefluorescent lar dye ud hydrophobic,when the xy-lene evaporates,thefluorescent dye is transferred into the microspheres that come into contact with the dye solution. This method,compared with other approaches previously described consisting of coating the microresonator surface with either quantum dots or organic dye molecules,13,14ena-bles a variation of the dye content to be loaded within the polymer sphere by simply changing the volume of the liquid pha containing the dye.After the doping procedure,the microsphere solution was heated at95 C for1h to facilitate the removal of the solvent from the microspheres.The microspheres were then washed by centrifugation,the super-natant removed and the lost volume replaced by Millipore water.Several microsphere samples were prepared following the same procedure but with increasing volume of dye satu-rated xylene solution,increasing the amount of dye diffusing into the polystyrene microspheres.
A frequency doubled YAG lar(k¼532nm,$800ps pul duration,10kHz repetition rate,Alphalas GmbH, Germany)was ud for the excitation of the active micro-spheres.The beam emerging from the lar wasfirst spatially filtered using a single modefiber(SMF28,Øcore¼8l m) before being couple
d into the back port of an inverted micro-scope(IX71,Olympus,Japan)equipped with a532nm dichroicfilter,effectively using the microscope as a confocal tup.The WGM modulatedfluorescence spectra from iso-lated microspheres deposited onto a microscope glass cover slip and obrved through the microscope were spectrally resolved using a monochromator(iHR550,Horiba,Japan) equipped with three different gratings;600,1200,and 2400mmÀ1and a cooled CCD(Synap2048pixels,
Horiba,Japan).
A typical ries of WGM spectra measured from a 10l m diameter polystyrene microsphere in air with increas-ing pump power are shown in Figure1(a).As the pump power increas,some modes become more inten,espe-cially around the600nm region of the spectra.Figure1(b) shows the ratio between the most inten WGM resonance and the background level as a function of the pump power. The transition between thefluorescence and the stimulated emission regimes can be clearly en around7l W. Similarly,the measured Q factor,defined as k/D k also increas beyond the lasing threshold,from4Â103to almost 8Â103.As the Q factor describes the stored energy in the resonator,a higher gain in the resonator,especially upon las-ing will increa the stored energy and therefore the Q factor. This increa in Q factor is highly beneficial for nsing pur-pos as it increas the resolu
tion of the nsor,enabling the detection of smaller changes in the resonance wavelength positions.11To determine the optimum doping conditions, variations of the doping procedure described previously were followed which esntially incread the volume of dye solution.Similar measurements were performed for each microsphere sample,and the lasing threshold in air as a func-tion of the volume of dye solution ud was extracted,as is shown in Figure1(c).Thisfigure illustrates clearly that upon reaching the optimum dye concentration within the micro-sphere,the lasing threshold can be significantly reduced,in this ca down to2.4l W.However,once the optimum dye concentration is reached,any subquent increa in the dye content within the microsphere results in lf-quenching of the organic dye,15and hence a higher lasing threshold.
骨碌读音Similar measurements were performed with the same microsphere samples in aqueous solution,drastically decreasing the refractive index contrast between the microre-sonator and its surrounding environment,resulting in higher confinement loss of the propagating modes as the reflectiv-ity at the interface is reduced and a higher portion of the evanescentfield leaks out of the resonator.As a con-quence,all the higher order modes are quenched and only thefirst order TE and TM modes can be en in Figure2(a), which shows a typical WGM spectrum of a10l m dye doped polystyrene microsphere in water.16,17The Q factor of the first order mode is strongly affecte
d,dropping from4Â103 to2Â103.This decrea of Q factor,which defines the stored energy within the resonator,as function of the sur-rounding refractive index is consistent with the results reported in Ref.18for microsphere below10l m in
diameter FIG.1.(a)Normalized WGM spectra of a10l m diameter dye doped poly-styrene microsphere in air with increasing pump power(1l W–15l W).(b) Ratio between the highest intensity WGM resonance andfluorescence back-ground,and measured Q factor as function of the pump power for the same polystyrene microsphere in air.(c)Measured lasing threshold of the WGM in air as a function of the volume of saturated dye solution ud in the dop-ing process.
as function of surrounding refractive index.For all the differ-ent batches of the microspheres prepared with different dye concentration,lasing was obrved repeatedly in water for only one sample,shown in Figures 2(a)and 2(b),which ini-tially exhibited the lowest lasing threshold in air as shown in Figure 1(c)for 1ml of saturated dye solution.For all the other samples,lasing was not achievable before damaging the microsphere with the pump source.The lasing threshold of the 10l m dye doped polystyrene microsphere measured in water (24562l W)is shown in Figure 2(b).For the entire population of 10l m polystyrene microspheres measured,the lasing threshold in water is about 2orders of magnitude higher than the lasing threshold obrved for the same sam-ple in air.
The lasing threshold is a function of the gain dictated by the dye concentration,its quantum efficiency,and the Purcell enhancement effect that occurs at the resonance wavelengths,and also the optical loss of the microsphere,which are described by the Q factor.Only a limited number of public
ations have ever investigated the relationship between lasing threshold and the parameters listed above,and only Spillane et al.19provide an analytical expression for the lasing threshold as a function of the Q factor,mode volume (V),and gain coefficient (specifically for a WGM
Raman lar).One of the conclusions that can be drawn
from that paper is that the lasing threshold should depend on V/Q 2.In earlier work by Sandoghdar et al.,20the lasing threshold of neodymium doped silica microspheres was found to have a linear dependency of Q À1.In an attempt to understand the difference in lasing threshold obrved between the sphere in air and in water,we postulated that the gain factor and especially the dye concentration and quantum efficiency remain the same despite the modification of the local environment surrounding the sphere.The assump-tions are justified by the fact that the dye is embedded into the polymer matrix constituting the microsphere and is there-fore not affected by the change of solvent.Furthermore,as the organic dye ud in this experiment is not soluble in water,there is no reason for the concentration to change due to diffusion out of the polymer matrix.Furthermore,within the timeframe of the measurement and the pump power ud for the excitation,no significant photobleaching of the dye was obrved.Conquently,the only parameters affected are the mode volume and the Q factor.While the Q factor is directly extracted from the exper
imental data,we calculated the mode volume of a 10l m diameter polystyrene micro-sphere in both air and water using the standard definition 21
V ¼ð
e r ðÞj w r ;h ;/ðÞj 2r 2sin h drd h d /
max e r ðÞj w r ;h ;/ðÞj 2
;(1)
where e (r)is the radial dependence of the relative permittiv-ity and w (r,h ,/)is the electric field of a given whispering gallery mode in spherical coordinates.The mode fields were calculated using the tried and tested finite element method (FEM).From this calculation,it was found that the surround-ing environment has only a limited impact on the mode vol-ume,which varies for the TE mode located around 590nm from 10.1l m 3in air to 10.4l m 3in water.The proportion of energy outside the cavity increas from <1=2%to around 6%.The polar mode number l clost to 590nm was chon (l ¼77,78for air and water clad spheres,respectively)for the simulations as determined by Refs.22and 23.Similarly,the TM mode at the same wavelength increas from 10.7l m 3in air to 11.9l m 3in water,
and the increa in energy outside the cavity is again approximately tenfold.The relatively low dependency of mode volume on the spe-cific surrounding index (provided the index contrast remains significant)can also be inferred from Figure 3,in which changing the surrounding medium from air to water only marginally lowers the peak intensity as the mode fields shift outwards slightly.However,as previously mentioned,the immersion of the microsphere in water has a significant impact on its Q factor—it is reduced by a factor two from $4Â103in air to 2Â103in water.However,neither the V/Q 2nor the Q À1reported dependencies of the lasing thresh-old explained why the lasing threshold is incread by two orders of magnitude when the measurements are performed in water.Our results therefore suggest that the Q factor has a much stronger influence on the lasing threshold than initially anticipated,although further investigations could be pursued beyond the work reported here to understand fully
the
FIG.2.(a)Normalized WGM spectra of a 10l m diameter dye doped poly-styrene microsphere in water with increasing pump power (20l W–2.5mW).(b)Ratio between the highest intensity WGM resonance and fluores-cence background,and measured Q factor as function of the pump power for the polystyrene microspheres in water.
dependency of the lasing threshold as function of the Q fac-tor.Nevertheless,as measurement of the Q factor has been ud in previous work on WGMs to characterize changes of surrounding refractive index and biomolecular binding onto high Q factor resonator,24one might appreciate that meas-uring the lasing threshold might be an interesting alternative to the standard resonance wavelength characterization,espe-cially if the dependency of the lasing threshold varies as a power function of the Q factor.
We have shown that a 10l m diameter dye doped poly-styrene microsphere can support WGM lasing when operated in water.This is the smallest polystyrene microsphere ever demonstrated to la in an aqueous environment.This was made possible by investigating the lasing threshold first in air as a function of the dye concentration within the micro-sphere and then lecting the optimum dye conc
entration resulting in the lowest possible lasing threshold.With regards to the application of refractive index nsing,a reported refractive index nsitivity of 50nm/RIU (Ref.16)and a Q factor of 7Â103upon lasing (FWHM $80pm at 600nm),means that the resolution is now only limited by
the monochromator ud for the characterisation of the
WGM signal (resolution of 4pm),which enables a detection limited 8Â10À5RIU.
闪吧
This experiment has also shed light on the dependence of the lasing threshold on Q factor,which is likely to have a far stronger influence than the quadratic dependency reported in the literature.16While this would require further investigation,this letter has provided insight into the possi-bility of inducing lasing in even smaller microresonators.Since the lasing threshold reported here for 10l m diameter polystyrene microspheres is already very clo to the damage threshold at 532nm,inducing lasing in even smaller polysty-rene microspheres would require relying on alternative exci-tation schemes,using either microstructured optical fiber,10plasmonic effects,or a combination of both.
We acknowledge the support of an ARC Georgina Sweet Laureate Fellowship and the ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP).
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FIG.3.The radial dependence of the (a)TE and (b)TM mode intensities of the 10l m diameter polystyrene microsphere.Both air and water clad micro-spheres are simulated.Intensity distributions in the equatorial plane are shown in the ints with arrows showing the magnetic field directions.
