Soft Matter
PAPER

Cite this: Soft Matter, 2019,15, 7832

Whispering gallery mode lasing in mesomorphic
liquid crystal microdroplets
Junaid Ahmad Sofi,

Abinash Barthakur and Surajit Dhara*

In recent years, investigation on the non-display applications of liquid crystals has increased considerably.
One of the emerging applications is whispering gallery mode (WGM) lasing. Here, we report experimental
studies on the morphology and WGM lasing in nematic (N), smectic-A (SmA) and smectic-C (SmC)
microdroplets dispersed in a highly transparent and low refractive index perfluopolymer. The mesomorphic
microdroplets, obtained by varying the temperature, exhibit radial director configuration. The SmA
Received 6th June 2019,
Accepted 5th September 2019

microdroplets are found to be highly stable and robust against mechanical stress compared to the N and

DOI: 10.1039/c9sm01132c

and show that overall the SmA microdroplets are superior to the N and SmC microdroplets. The

SmC microdroplets. We study lasing properties such as intensity, threshold pump energy and linewidth,
experimental results are discussed based on the orientation of the dye molecules, director fluctuations

rsc.li/soft-matter-journal

and tilting at the interface.

I. Introduction
The quest for the successful realization of flexible and integrated
photonic devices has drawn a great deal of interest in recent
years. Such devices require tunable optical microcomponents
such as filters, sensors and waveguides. In this context, liquid
crystal based optical microcomponents are prospective contenders
for future generation tunable microdevices.1–7 Liquid crystals exhibit
a variety of mesophases with dintinct optical properties and eachmesophase might offer a unique technological solution for specificdevices. Microdroplets of a dye-doped liquid crystal act as optical
microcavities and recently several studies have been reported.5,8–12
In those systems, the light coupled to the microcavity circulates
inside due to the total internal reflection and the optical resonanceoccurs when the circulating light meets in-phase, which is known as
whispering gallery mode (WGM) resonance.13 Under a simple
approximation, this can be achieved when 2pans E ll, where a is
the radius, ns is the refractive index of the sphere and l is an integer.
If the input energy exceeds a certain threshold, the dye gives off
stimulated emission and the microresonator begins to emit multimode laser light.
The first report on the WGM resonance by Humar et al.
opened up a new direction of application of liquid crystals.1
They demonstrated tunability of WGM resonance, Bragg cavity
lasing and surfactant sensing.1–3 Peddireddy et al. demonstrated lasing and waveguiding in smectic-A liquid crystal
fibers.4 Petriashvili et al. have shown omnidirectional lasing
in cholesteric microdroplets.14 We have reported electrical and
School of Physics, University of Hyderabad, Hyderabad-500046, India.
E-mail: sdsp@uohyd.ernet.in

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thermal tuning of WGM resonance of nematic microdroplets
and the magnetic field tuning of lasing from ferromagnetic
nematic microdroplets.8,9 Recently, Sofi et al. have studied thestability of nematic microdroplets in several dispersing media.12
There are also many experimental and theoretical reports on liquid
crystal microdroplets.15–22 Despite rich and diverse structures ofliquid crystals, no comparative study has been reported so far withan aim to bring out the relative merits of lasing in the mesomorphic microdroplets. In this paper, we report on the lasing
properties of nematic (N), smectic-A (SmA) and smectic-C (SmC)
microdroplets of a liquid crystal compound. We show that the
SmA microdroplets are mechanically robust and have lower lasing
threshold pump energy and higher lasing intensity than that of N
and SmC counterparts. Hence, SmA microdroplets are more
appealing microcavities for making microlasers. We discuss thepossible effects of refractive index contrast, director fluctuations
and orientation of the transition dipole moment of the dyemolecules on the lasing properties. This study provides useful
guidelines for an appropriate choice of liquid crystal mesophasefor optical microcavities in diverse photonic applications.

II. Experimental
We used a pyridine derivative liquid crystal compound thatexhibits the following phase transitions: I 68 1C N 64.5 1C SmA
57.7 1C SmC 33.2 1C Cr. It exhibits a first order I to N and asecond order SmA–SmC phase transition.23 A small amount of
Nile red fluorescent dye (0.7 wt%) was doped in the liquid crystal for
inducing fluorescence. The transition dipole moment of the dyemolecule is along the long axis.24,25 We used a perfluoropolymer

This journal is © The Royal Society of Chemistry 2019

PaperFig. 1 (a) Schematic diagram of the experimental setup built on an
inverted polarising optical microscope. A Q-switched pulsed laser ofwavelength 532 nm triggered by the function generator is used to induce
the fluorescence emission of the Nile-red dye doped microdroplets. The
spectrum emitted from the microdroplets is detected and recorded by the
Andor spectrometer, and the images are taken by the camera. (b) Sideview of the cell showing suspended droplets in Cytop solution.

(CTX-809A) commonly known as Cytop as a dispersing medium.
It is highly transparent and its refractive index is very low (n =1.34).12,26 To reduce the viscosity of the Cytop, we added afluorinated solvent (CT-Sol.180) with a volumetric ratio of 1 : 2.
Cytop is diluted by fluorinated solvent to ease the formation ofliquid crystal microdroplets. Both the materials are obtained
from Asahi Glass Co. Ltd, Japan. The diluted mixture is named
here as ‘‘Cytop-solution’’. The Cytop-solution containing microdroplets, prepared through micropipette injection, is inserted ina cell prepared by two glass plates which are separated by aspacer of thickness 240 mm (Fig. 1(b)). The liquid crystal isimmiscible in Cytop solution. The microdroplets are formed in
the nematic phase and cooled slowly to the SmA and SmC
phases respectively. The experimental setup is built on an
inverted polarising optical microscope (Nikon, Eclipse Ti-U)(Fig. 1(a)). A temperature controller and a heating stage (Instec
Inc. mk1000) mounted on the microscope are used to control
the temperature with an accuracy of 0.1 1C. An isolated microdroplet of desired diameter is selected and pumped with a
Q-switched pulsed laser (400 ps) of wavelength 532 nm (Teem
Photonics, Model: PNG-M02010-130). The absorption band of
Nile red dye matches with the wavelength of the pulsed laser
and the fluorescence light is coupled into the microdroplets. All
the experiments are performed using a Nikon S Plan Fluor 60/0.70 objective. A high resolution spectrometer (Andor, Shamrock
SR-500i) is connected to one of the ports of the microscope and thesame objective is used to collect the emission spectrum.8,12Soft Matteranchoring of the director. The radial anchoring is strong andretained over several weeks, without any observable change in the
texture. The radial anchoring of the molecules at the interface isdue to the very low surface energy of Cytop.26,27 There are several
studies reported on the alignment behaviour of liquid crystals in
Cytop-solution coated Hele-Shaw type cells. It was reported that
Cytop-solution provides perfect homeotropic alignment for SmA
liquid crystals compared to the conventional homeotropic aligning
agents.26,28–30 To check the mechanical stability, we applied mild
physical stress on the microdroplets with the help of a mechanicaltweezer. It is observed that the textures of N and SmC microdroplets are changed irreversibly whereas the SmA microdropletsremain intact after the stress is removed. We did not observe
any diffusion of liquid crystal or dye molecules across theinterface, except in the isotropic phase. In addition, there isno sedimentation of microdroplets and also no noticeablechange in the diameter of the microdroplets is observed acrossthe phase transitions. Thus, SmA microdroplets in Cytopsolution are chemically stable and mechanically robust than N
and SmC microdroplets. Fig. 2(a–c) shows the POM micrographs and corresponding l-plate (530 nm) images of the
microdroplets. Four intensity extinction brushes and a hedgehog
point defect at the centre indicates that the director is radial(homeotropic) to the interface. This also means that the smectic
layers are concentric in both the SmA and SmC microdroplets asshown schematically in Fig. 2(e and f). We noticed that the radialanchoring is partially broken close to the SmC to Cr phase transition(33.2 1C) and eventually no lasing is observed. Hence, the experiments on the SmC microdroplets are performed just 5 1C below the
SmA–SmC phase transition (which is about 20 1C above the SmC to
Cr phase transition temperature).

III. Results and discussion
Our experiment starts with the search for an appropriate
dispersing medium for liquid crystal micodroplets that provides
radial anchoring of the director (the mean orientation direction of
the elongated molecules). We tried forming microdroplets inseveral polymers such as polydimethylsiloxane (PDMS), glycerol–
lecithin mixture and the Cytop solution. From the comparative
study, it is found that the Cytop-solution provides a perfect radial

This journal is © The Royal Society of Chemistry 2019

Fig. 2 (a–c) Polarizing optical micrographs (POM) of a microdroplet ofdiameter 14.5 mm in the N, SmA and SmC phases. l-Plate images of therespective microdroplets are shown underneath. Temperatures of the
microdroplets are shown on the l-plate images. Crossed polarisers andl-plate orientation are shown on the left side. (d–f) Schematic director
profiles in N, SmA and SmC microdroplets. Yellow ellipsoids represent Nile
red dye molecules (doped).

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In SmC microdroplets (Fig. 2(c)), the extinction brushes
rotate clockwise due to the collective tilting of the moleculesin the layers. We have measured the approximate tilt angle (y)from the POM micrographs at diﬀerent temperatures below the
SmA–SmC phase transition. Fig. 3(a) shows that the tilt angle
increases gradually from y = 01 to about y = 181 as the temperature isdecreased from 58 1C to 53 1C. We also measured the birefringence(Dn) of the sample using a phase modulation technique.31–33
Fig. 3(b) shows that Dn increases with decreasing temperatureand changes slope discontinuously at the I–N and N–SmA phasetransitions. A small but measurable change in Dn is also observed
across the SmA–SmC phase transition temperature.
In what follows, we investigate the WGM lasing of themesomorphic microdroplets. For this purpose about 0.7 wt%
Nile red dye was dissolved in the isotropic phase of the liquid
crystal before dispersing it in the Cytop-solution to form
microdroplets. We choose an isolated microdroplet and illuminate
its edge with the tightly focussed laser beam to induce fluorescence. The emitted light is coupled and circulates within the
microdroplets due to the total internal reflection, and consequently the WGMs are excited. A representative spectrum ofa typical WGM resonance (below the lasing threshold pumpenergy) of a microdroplet of diameter 10 mm is shown in Fig. 4.
When the pumping energy exceeds a certain threshold value,
some of the modes start lasing. Fig. 5(a–c) show fluorescence
images of microdroplets (diameter 4 20 mm) illuminated bypumping energies much above the lasing threshold values. In
each microdroplet, we see a brighter light spot on the diametrically opposite side of the excited edge and a thin ring of

Paper

Fig. 4 A typical WGM spectrum at a pumping energy of 20 nJ (below thelasing threshold) of a SmA microdroplet of diameter 10 mm.

Fig. 5 (a–c) Fluorescence images of illuminated edges of N, SmA and
SmC microdroplets with pump energy of 115 nJ. Insets of images (a),(b) and (c) show the respective emission patterns of the whole microdroplets. The N, SmA and SmC microdroplet diameters are 21, 22 and22.4 mm respectively. (d–f) Emission spectrum from the respective microdroplets. White arrows indicate the excitation edges of the microdroplets.

Fig. 3 (a) Temperature dependent tilt angle (y) measured from the textureof a SmC microdroplet of diameter 33 mm. (b) Temperature dependent
birefringence (Dn).

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light within the microdroplet circumference, suggesting the
WGM excitation. In addition, a speckle formation is also observed
above the lasing threshold pump energy, suggesting that theemitted light is coherent. It is observed that at the illumination
point which is just inside the interface, the scattering of light fromthe SmC microdroplets is comparatively larger than that of N and
SmA microdroplets (inset to Fig. 5(c)). The emission spectrum of
the respective microdroplets in the lasing regime is also shown in
Fig. 5(d–f). It may be noted that the linewidths (FWHM) of the laserlines are much narrower than that of the WGM resonance (Fig. 4).
The cavity quality factor of a resonator is given by Q = l/Dl, where
Dl is the full-width at half maximum. The quality factor above thelasing threshold pump energy for a fixed droplet size is found to be
Q B 104. Two types of peaks with higher and lower intensities areThis journal is © The Royal Society of Chemistry 2019

Paperobserved in Fig. 5(d–f). They are considered to be TM and TE
modes corresponding to the electric field of the light being either
parallel or perpendicular to the director. The long axes of the Nile
red molecules and hence the emission transition dipole moments
are aligned along the director (yellow ellipsoids in Fig. 2(d–f)).
Consequently, the TM modes are strongly coupled to the transition
dipole moment and have higher intensities than TE modes in N
and SmA microdroplets. However, in SmC the situation is complicated because of the molecular tilt. The transition dipole
moments have a projection on to the TE modes and as a result
TE modes are enhanced and TM modes are reduced as thetemperature is decreased across the SmA to SmC transition.
To investigate the lasing properties, we analysed the mostintense modes in three mesomorphic microdroplets. Fig. 6
shows the quantitative diﬀerences of the lasing characteristics
of N, SmA and SmC microdroplets. In all microdroplets, atypical two-slope curve for intensity and linewidth as a functionof pumping energy is observed. The lasing intensity increases
and the linewidth decreases with the increasing pump energy.
The change of slope occurs at the threshold energy, whichsuggests the onset of the stimulated emission of the gain
medium. During such an emission transition, the gain of theactive medium supersedes the optical losses in the microdropletson account of the attainment of the population inversion between
the ground and the excited state energy levels of the dye molecules.
In the nematic microdroplet (diameter 21 mm), the lasing thresholdpump energy for the highest intensity mode is 75 nJ (Fig. 6(a)).
For the SmA and SmC microdroplets (diameter C 22 mm)
the threshold energies are almost the same i.e., nearly 42 nJ(Fig. 6(c and e)). The variation of the corresponding linewidths
is also shown in Fig. 6(b, d and f). A clear change of slope isFig. 6 (a, c and e) Variation of output intensity and (b, d and f) corresponding linewidths as a function of pumping energy. The wavelengths of
selected highest intensity modes are 616.4 nm, 629.8 nm and 629.2 nm for
the N, SmA and SmC microdroplets respectively. The N, SmA and SmC
microdroplets’ diameters are 21, 22 and 22.4 mm respectively. Thresholdpump energies are marked by arrows. Dotted lines are drawn as guides tothe eyes.

This journal is © The Royal Society of Chemistry 2019

Soft Matter

Fig. 7 Variation of lasing intensity of the mode corresponding to theemission line l = 629.2 nm with tilt angle y in the SmC phase for a pumping
energy of 224 nJ. A dotted line is drawn as a guide to the eye. Droplet
diameter: 25 mm.

observed at the corresponding threshold pump energies.
Fig. 6(a, c and e) also show that the lasing intensity from the
SmA microdroplet is larger than that from both the N and SmC
microdroplets. For example, at a fixed pump energy of 80 nJ(above the threshold pumping energy) the output intensity of
the SmA microdroplet is 55 (a.u.) (Fig. 6(c)), whereas in N and
SmC microdroplets it is about 10 (a.u.) and 13 (a.u.), respectively (Fig. 6(a and e)). Thus, the intensity of SmA microdropletsis almost 5 times larger than that of the N and SmC microdroplets. In the SmC phase the lasing intensity depends on thetilt angle y as shown in Fig. 7. The intensity decreases withincreasing tilt angle. We also studied the eﬀect of the droplet
size on the lasing threshold and the results are shown in
Fig. 8(a). The radiation loss due to the curvature in smaller
microdroplets is larger than that of the bigger microdroplets
as a result of which the threshold energy decreases withincreasing diameter. The lasing threshold pumping energyof the N and SmA decreases faster than for the SmC microdroplets. The threshold pump energy of larger SmA microdroplets(diameter 425 mm) is smaller compared to both the N and SmC
microdroplets.
Uniaxial liquid crystals possess extraordinary (ne) and ordinary(no) refractive indices which are temperature dependent and incalamitic liquid crystals, ne 4 no. In mesomorphic compounds,

Fig. 8 (a) Dependence of lasing threshold pump energy on microdroplet
diameter. Dotted lines are drawn as guides to the eye. (b) Fluorescenceintensity measured from a thin film of sample confined between two glass
plates at diﬀerent temperatures. The bandwidth of the excitation filter is
510–560 nm. Small oscillations overlaying the emission are due to thefinite thickness of the cell.

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usually ne increases and no decreases while decreasing thetemperature across the N–SmA–SmC phase transitions. Hence,
the refractive index difference (ne np, where np is the refractive
index of the Cytop-solution) across the SmA–SmC phase transition is increasing. But the SmC phase has monoclinic symmetry
and is thus weakly biaxial due to the tilting of the molecules in
the layer. With decreasing temperature, y increases and theintensity of the TM modes (peaks with higher intensities)
decreases. The director fluctuations in the nematic microdroplets
cause scattering losses in the microcavity. In the SmA microdroplets, the fluctuations are suppressed and hence the scattering losses are reduced. Thus, the lower threshold energy andhigher lasing intensity of SmA microdroplets could be attributed to
the larger refractive index contrast with respect to the Cytop-solution
and the lower scattering losses due to the suppression of thedirector fluctuations. The quantum yield of the fluorescent dye isusually solvent and temperature dependent and this could also
contribute to the lasing properties. To see this effect, we havemeasured the fluorescence intensity from a thin film of the sample
confined between two parallel glass plates at different temperatures.
However, no noticeable change in the spectrum is observed(Fig. 8(b)). Hence, quantum efficiency of the dye is not responsible for the difference in the lasing properties of the mesomorphic microdroplets.

IV Conclusions
In summary, we have performed a comparative study ofthe morphology and lasing properties of N, SmA and SmC
microdroplets dispersed in Cytop-solution (perfluoropolymer).
We find that the SmA microdroplets are chemically stable andmechanically robust compared to the N and SmC microdroplets. The SmA microdroplets have in general lower lasing
threshold pump energy than the N and SmC microdroplets.
The lasing intensity of the SmA microdroplets is almost fivetimes larger than that of the N and SmC microdroplets. Thus,
SmA microdroplets are superior optical microcavities for lasing.
The refractive index contrast with respect to the surrounding
medium, director fluctuations, orientation of the transition
dipole moment of the dye molecules and director tilt haveimportant contributions to the properties of WGM lasing of themesomorphic microdroplets. We hope that this study willencourage the investigation of the unexplored lasing propertiesof several liquid crystal phases.

Funding sourceThis work was supported by the Science and Engineering
Research Board of India (EMR/2015/001566), UPE-II, DST/SJF/
PSA-02/2014-15, DST-PURSE.

Conflicts of interest
The authors aﬃliated with this work declare no conflict ofinterest.

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Paper

Acknowledgements
JAS acknowledges UGC-BSR for a fellowship. We all acknowledge
Prof. R. Dabrowski for providing the liquid crystal compound.

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