Applied Physics Letters

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Electrically switchable whispering gallery mode
lasing from ferroelectric liquid crystal
microdroplets
Cite as: Appl. Phys. Lett. 114, 091106 (2019); doi: 10.1063/1.5088863
Submitted: 15 January 2019 . Accepted: 17 February 2019 .
Published Online: 5 March 2019
Junaid Ahmad Sofi

and Surajit Dharaa)

AFFILIATIONSSchool of Physics, University of Hyderabad, Hyderabad 500046, Indiaa)Electronic mail: sdsp@uohyd.ernet.in

ABSTRACT
Liquid crystal microdroplets have received considerable attention over recent years owing to their potential applications in chemical,
biomedical sensing and lasing. We report experimental studies on whispering gallery mode lasing from dye-doped ferroelectric liquid crystal
microdroplets suspended in a low refractive index and highly transparent perﬂuoropolymer at ambient temperature. We show that the lasing
threshold pump energy of ferroelectric microdroplets is much lower than that of the nematic and cholesteric microdroplets. With theincreasing electric ﬁeld, the linewidth increases, while the lasing intensity decreases and eventually switches off beyond a particular ﬁeld.
Since the switching response time is fast (350 ls), ferroelectric liquid crystal based microlasers are useful for applications as electricallyswitchable sources in miniaturised devices and in soft photonic circuits.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5088863

Transparent microspheres or microdroplets could behave as optical microcavities if their refractive indices are greater than those of dispersing media. In such systems, the light circulates inside the cavity
due to the total internal reﬂection and the optical resonance occurs
when the circulating light meets in-phase. In larger spheres, under asimple approximation, this can be achieved when 2pans  lk, where ais the radius, ns is the refractive index of the sphere, and l is an integer.1
This type of resonance is known as the whispering gallery mode
(WGM) or morphology dependent resonance. They have small mode
volumes and high quality factors (Q). The light source is usually a ﬂuorescent dye, dispersed in the cavity and pumped by an external lightsource.1,2 If the pumping energy exceeds a certain threshold, the dye
gives stimulated emission and the resonator emits multimode laser
light. Liquid crystal microdroplets have drawn considerable attention
across various scientiﬁc disciplines owing to their potential applications in tunable optical microcavities2–15 and sensors.16–22 They could
well ﬁt into the integrable photonics as essential optical components inminiaturised devices and hence serve widespread applications such as
electrically or magnetically tunable lasers, ﬁlters, and sensors.
Humar et al. ﬁrst demonstrated electrically tunable WGM resonance of nematic liquid crystal microdroplets.2 Subsequently, theyreported 3D lasing and chemical sensing as well.3,4 Later, Kumar et al.
showed sensitive detection of nematic to smectic-A phase transition in

Appl. Phys. Lett. 114, 091106 (2019); doi: 10.1063/1.5088863
Published under license by AIP Publishing

the microcavity.5 With the aim of exploring and expanding the versatile usage of liquid crystal droplets, we studied the electrical and thermal tuning of the quality factor (Q) and the free spectral range (FSR)of WGM resonance.6 We also reported on the magnetic ﬁeld tuning of
lasing in ferromagnetic nematic droplets.7 Among the liquid crystals,
the ferroelectric liquid crystals (FLCs) show outstanding performancefor fast electro-optical applications due to the spontaneous polarisationof the smectic layers. FLCs are especially attractive for low voltage
operability, better optical contrast, bistability, and faster electro-opticalresponse.23–32 At the backdrop of efforts on producing tunable andminiaturised lasers for diverse applications, we report experimental
studies on electrically switchable lasing from room temperature ferroelectric liquid crystal microdroplets. It is expected that the switchable
lasing adds diversity to the domain of optical components and could
eventually make an important contribution in the race of fabricationof soft photonic chips.
We used a room temperature ferroelectric liquid crystal(KCFLC7S) procured from Kingston Chemicals Limited. It exhibits
the following phase transitions: Cry. 5  C SmC 73  C SmA 100.5  C
114.5  C Iso. Its helical pitch at room temperature is about 2.8 lm.
The details of the mixture can be found in Refs. 25 and 26. We used atransparent perﬂuoropolymer CTX-809A commonly known as
CYTOP diluted by a ﬂuorinated solvent CT-Sol.180 at a 2:1 volumetric

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mixing ratio for dispersing microdroplets of FLC.8 The polymer andthe solvent were obtained from Asahi Glass Co. Ltd, Japan. CYTOP
has a very low refractive index (n ¼ 1.34) and provides shock freehomeotropic alignment of smectic liquid crystals in cells.33–39
Recently, we have shown that CYTOP is a superior dispersing medium
for forming spherical microdroplets with homeotropic alignment of
the nematic director.8 Using a micropipette, FLC microdroplets withvarying sizes were dispersed in CYTOP solution which was injected incells made of two indium-tin-oxide coated glass plates separated by aspacer of thickness 240 lm. Before the dispersion, about 0.7 wt. %ﬂuorescent dye (Nile red) was mixed with the FLC. An isolated microdroplet of desired size was chosen in the cell mounted on the sample
stage of an inverted microscope (Nikon, Eclipse Ti-U) and was excitedwith the help of a Q-switched laser of wavelength 532 nm (Teem
Photonics, Model: PNG-M02010–130) with a pulse length of 400 ps,
through a 60 objective with NA ¼ 1.1. The absorption band of Nile
Red coincides with the pulsed laser. The light emitted from theselected microdroplet is collected by the same objective and sent to thehigh resolution spectrometer (Andor 500i). A schematic diagram of
the cell is shown in Fig. 1(a). The details of the experimental setup arepresented in our previous reports.5,6 For electric ﬁeld studies, the dc

FIG. 1. (a) Schematic diagram of the cell made of two indium-tin-oxide (ITO) coatedglass plates. Textures of FLC microdroplets of different diameters. (b)–(g) Images
in the top row show polarising optical micrographs of droplets, while the bottom row
shows the images taken using an additional k-plate. The orientation of the slow
axis of the k-plate with respect to the crossed polarisers is shown on the left side.
(h) Schematic diagram of the cross-section of a smaller droplet, showing concentricsmectic layers with the radial helical axis. The red ellipsoids represent the molecules within the cone rotating along the helical axis.

Appl. Phys. Lett. 114, 091106 (2019); doi: 10.1063/1.5088863
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ﬁeld is provided by a high voltage supply. The switching response timeof lasing was measured with the help of a photomultiplier tube connected to a digital storage oscilloscope. A k-plate retarder (530 nm)
was used to study the optical textures of the microdroplets. All theexperiments were performed at room temperature.
Our study begins with the observation of microdroplets under apolarising optical microscope (POM) in the absence of any electricﬁeld. Images on the top row of Fig. 1 show POM micrographs of afew FLC microdroplets dispersed in CYTOP solution. The images
underneath are taken with an additional k-plate inserted at 45 withrespect to the crossed polarisers. In smaller microdroplets (diameter/5:4 lm), four clear brushes are observed, suggesting that smectic
layers are concentric and the layer normal is perpendicular to theinterface. Thus CYTOP solution provides homeotropic surface
anchoring to smaller FLC droplets. For larger droplets (diameter’5:4 lm), the radial structure is not stable and a numeral 8 like defectline is formed near the central region [see k-plate images in Figs. 1(d)and 1(e)]. With the increasing size, the defect appears like a hair pin
and evolves from the centre towards the boundary [Figs. 1(f) and
1(g)]. The defect looks somewhat similar to that reported in cholestericmicrodroplets, where two cores of þ1 disclination lines forming adouble helix structure fuse together in the center of the microdroplets
and appear as a hair pin.40–45 The k-plate images suggest that the textures of the larger FLC microdroplets are very complex. The equilibrium microdroplet structure results from the competition between theelastic forces which determines the layer orientation of the FLC in thebulk and the surface anchoring, which becomes dominant when the
FLC is conﬁned. For smaller microdroplets, the surface energy dominates and the radial structure is stable [Figs. 1(b) and 1(c)]. A schematic diagram of the cross-section of a microdroplet with concentricsmectic layers is shown in Fig. 1(h). The molecules rotate in the layers
along the helical axis that is orientated radially. The defect in the bigger
FLC microdroplets is due to the geometric frustration of chiral ordering and similar to that is observed in cholesteric microdroplets, which
essentially depends on the ratio of the pitch and the diameters of the
microdroplets.
In what follows, we study the WGM resonance and lasing of the
microdroplets under the excitation of the pulsed laser. Althoughsmaller microdroplets appear radial and uniform, no WGM resonanceis observed when the microdroplet diameter is lower than about 8 lm.
This is presumably due to the dominant curvature losses that result
from the unfulﬁlled requirement of the critical angle for light to travel
through total internal reﬂection. Bigger microdroplets exhibit many
modes and are often indistinguishable. Hence, in order to avoid thecrowding of many modes, we conﬁned our studies on isolated microdroplets with diameters in the range of 20–30 lm. Figure 2(a) shows
the ﬂuorescence image of an isolated droplet of diameter 22.7 lm
when pumped by the pulsed laser of energy 5 nJ. A faint light ring isobserved surrounding the microdroplet, suggesting the excitation of
WGMs. The corresponding spectra are presented in Fig. 2(c). It isobserved that the WGM spectrum of FLC is signiﬁcantly different
from that usually observed in the case of nematic or smectic-A microdroplets.2,6,8 The modes have a lower linewidth and are not equally
spaced. This is attributed to the inhomogeneity in the refractive index
of the cavity that occurs due to the complex elastic deformation of thesmectic layers. When the pump energy is increased sufﬁciently, weobserve a clear bright ring with speckle formation [Fig. 2(b)]. The

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FIG. 2. WGM lasing of a Nile red dye doped FLC microdroplet of diameter
D ¼ 22.7 lm. Fluorescence images of the emission pattern at the droplet edge (a)at input energies of 5 nJ (below lasing threshold energy, discussed later) and (b)60 nJ (above lasing threshold energy). (c) Typical WGM spectrum below thresholdenergy. (d) Corresponding spectrum above the lasing threshold energy. The insets
show the images of the whole droplets.

corresponding spectrum is also shown in Fig. 2(d). It is noted that thepeak intensities of the modes are increased signiﬁcantly and some newmodes are also visible. For example, the intensity of the dominantmode appearing at k ¼ 626.27 nm is increased from 7.8 to 44 (a.u.)when the pump energy is increased to 60 nJ. The speckle formation at
the edge of the microdroplets is an indication of coherent emission in
the microcavity.7
To characterise the spectrum, we measured the output intensity
as well as the line-width of the modes as a function of input energy ofa droplet of diameter D ¼ 23.8 lm. Figure 3(a) shows that there is aclear threshold at 22 nJ, beyond which the output intensity of themode (k ¼ 626.3 nm) increases linearly. Figure 3(b) shows that the
linewidth of the same mode decreases and becomes constant at
the same threshold input energy. The threshold pump energy andline-width narrowing are important characteristics of laser emission. It is noticed that the threshold energy of the FLC is muchsmaller than that of the nematic and cholesteric liquid crystal
microdroplets of comparable size and dye concentration. For example, the threshold pump energies for ferromagnetic nematic andcholesteric droplets are about 7 to 8 times higher than that of the
FLC used in this experiment.4,7 The linewidth of the lasing in FLC
microdroplets is close to 0.06 nm, and this is smaller compared tothat reported (about 0.1 nm) in the two referred systems. These are
attributed to the absence of strong director ﬂuctuations in the FLC
and also the low absorption of light by the FLC compared tonematic and cholesteric liquid crystals. Threshold pump energy also
depends on the refractive index of the liquid crystals and the quantum yield of the dye, which depends on the medium, and hence, it
may be more appropriate to compare the threshold values in liquid
crystals having the same refractive index.
As a next step, we studied the effect of the dc electric ﬁeld on themorphology and lasing of the FLC microdroplets. Figure 4 shows aAppl. Phys. Lett. 114, 091106 (2019); doi: 10.1063/1.5088863
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FIG. 3. (a) Intensity of a WGM peak (k ¼ 634.5 nm) as a function of the input
energy. A typical 2-slope lasing curve is observed. The dotted lines are added as
guides to the eye, showing the lasing threshold at 22 nJ (b) Linewidth of the corresponding spectral peak as a function of input-pulse energy, showing reduction to
the linewidth below the threshold energy. Droplet diameter D ¼ 23.8 lm.

sequence of POM micrographs and the corresponding k-plate images
with the increasing electric ﬁeld. The ﬁeld is applied between two
conﬁning electrodes along the viewing direction. The textures of the
microdroplets due to the applied ﬁeld are very complex and harder toﬁgure out. However, it is evident that the spontaneous electric polarisation of the layer is coupled to the external electric ﬁeld, and consequently, the helix should unwind. The unwinding of the helix in the
conﬁned droplets is expected to deform the smectic layer orientation,
which can increase the inhomogeneity of the refractive index. Figure5(a) shows the effect of the electric ﬁeld on the lasing spectrum froma microdroplet of diameter, D ¼ 20.1 lm. The experiments were conducted at a ﬁxed pumping energy of 60 nJ, which is much above the

FIG. 4. Textures of an FLC microdroplet of diameter D ¼ 15.3 lm at various
applied dc electric ﬁeld strengths. The corresponding k-plate images are shown
underneath. The ﬁeld direction is out of the plane.

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the stated phenomenon, we looked at the corresponding effect on theline-width of the lasing of the same mode. It is noticed that the linewidth increases almost linearly with the applied ﬁeld [Fig. 5(c)]. This
conﬁrms that the energy loss in the microcavity increases with theapplied ﬁeld strength.
In conclusion, we studied WGM lasing from ferroelectric liquid
crystal microdroplets at ambient temperature. The equilibrium structure of the microdroplets is size dependent. In smaller microdroplets,
the smectic layers are concentric and almost free from elastic distortions but do not show any WGM resonance. The bigger microdroplets
possess defect loops, and the overall structure is complex. As a result,
the refractive index becomes inhomogeneous. The input thresholdpump energy for the WGM lasing of FLC microdroplets is 7 to 8 times
lower than that of the nematic and cholesteric microdroplets. The
external electric ﬁeld enhances the inhomogeneity of the refractive
index further, and consequently, the optical energy loss in the cavity
increases and eventually switches off the lasing. Because of fast switching speed, FLC could be used as an electrically switchable microlaser
source in photonic soft-chips. FLC microdroplets could also be used as
three-dimensional switchable Bragg type microresonators for omnidirectional lasing, but it is challenging to obtain perfectly radial orientation of the helical axis. The work in this direction is on progress andwill be reported elsewhere.
We gratefully acknowledge the support from SERB (EMR/2015/
001566) and DST (DST/SJF/PSA-02/2014-2015) and DST-PURSE(II).
J.A.S. acknowledges UGC for the BSR fellowship.

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FIG. 5. (a) Effect of the dc electric ﬁeld on the spectrum from an FLC droplet ofdiameter D ¼ 20.1 lm. The intensity of the modes decreases with the increasingﬁeld. (b) Variation of emitted intensity corresponding to the dominant mode at
k ¼ 615.8 nm and (c) linewidth as a function of ﬁeld. The dotted line is drawn as a
guide to the eye.

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Published under license by AIP Publishing

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