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Sensing & Measurement
Detecting phase transitions in an opticalmicrocavity
Surajit Dhara
Changes in the whispering gallery mode resonance of liquid crystal droplets
offer insight into their internal ordering.
26 June 2015, SPIE Newsroom. DOI: 10.1117/2.1201506.005925

Liquid crystals made of organic molecules areextensively used in display devices and othertechnological applications.1 Phase transitions
between differently ordered states in these materials
occur with changes in temperature. Usually, thesetransitions are studied by techniques such aspolarizing optical microscopy, x-ray scattering,differential scanning calorimetry (DSC), or bymeasuring various physical properties as a function of temperature. We
have shown that phase transitions can also be detected by studying opticalresonance in micrometer-sized liquid crystal droplets, as the resonant
response strongly depends on temperature and is proportional to the degreeof order in the system.2
Spherical microparticles made of solid
dielectric material are widely used formaking microresonators. Light isconfined inside the microsphere bytotal internal reflection, as therefractive index of the microsphere islarger than that of the surrounding
medium. Resonance is achieved
when the circulating light remains in phase after completing one cycle,
known as a whispering gallery mode (WGM).3 Such optical microresonators
are very useful because of their small mode volumes and high quality
factors.3 Together, these enhance the interaction between light and matter inthe device. These characteristics have drawn significant attention due topotential applications as high-sensitivity biological and chemical sensors.4–8
Most sensor applications for solid dielectric microspheres use WGM
methods. Recently, liquid-crystal-based microresonators have shown great
potential for use as soft microresonators9 and for 3D lasing.10 Additionally,
the preparation of liquid-crystal-droplet-based microresonators is rather
simple, and more importantly they can be tuned with external electric or
magnetic fields.
We describe an interesting application of this resonance for detecting phasetransitions in liquid crystals. To demonstrate our concept, we chose a typicalliquid crystal, octyl cyanobiphenyl (8CB), which transitions between nematicand smectic A (SmA) phases at 33.5°C. The nematic phase possesses onlyorientational order, while the SmA phase is both orientationally ordered andadditionally formed into layers. Liquid crystal droplets were prepared using a
micropipette in a polymer matrix of polydimethylsiloxane (PDMS), which has
a refractive index (np=1.37) lower than the extraordinary refractive index of
the liquid crystal (nLC=1.67). Some typical optical microscopy images of adroplet in two different phases are shown in Figure 1(a) and (c), respectively.
Schematic molecular orientations inside droplets in the respective phases
are also shown in Figure 1(b) and (d). A small amount ( 1wt%) of

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Detecting phase transitions in an optical microcavity | SPIE New...

fluorescent dye—4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)4H-pyran (DCM)—was mixed with the liquid crystal before making thedroplets. A laser of wavelength 532nm, matched to the absorption band ofthe dye, was used to illuminate the droplet. Figure 1(e) shows the bright field
image of an SmA droplet embedded in PDMS. A thin light ring along thecircumference of the droplet and the brighter spot on the diametrically
opposite side of the droplet indicate the excitation of WGM resonance. The
characteristic resonance spectra from both the nematic and SmA phases
are shown in Figure 2. The resonance peaks are observed overlaying thefluorescence background. As the temperature is increased and the phaseshifts from SmA to nematic, the width of the resonance spectra (Δλ), which
is proportional to the quality factor (Q) of the resonance, decreases. For
example, the Q-factor in the SmA phase (26°C) is 770 and decreases to135 in the nematic phase (35°C). Thus, by measuring the characteristic
variation of the Q-factor as a function of temperature, the phase transition isdetected. Details are reported elsewhere.2

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Figure 1. Optical microscopy images and schematics of molecularorientation inside a liquid crystal droplet. (a) Smectic A (SmA) phase (28°C).
(b) Schematic molecular orientation in SmA phase. (c) Nematic (N) phase(35°C). (d) Schematic molecular orientation in N phase. (e) Bright field
image of an SmA droplet when illuminated with a laser of wavelength
532nm. The plus sign (+) indicates the illumination region. The red ellipsoids
represent fluorescent dye molecules.

Figure 2. Representative optical resonances from a droplet in the SmA (at26°C) and nematic (at 35°C) phases. Intensity is given in arbitrary units(a.u.). A dark field image of the SmA droplet is also shown.
In summary, we have shown that the phase transition in spherical droplets ofliquid crystals can be detected by studying the optical resonance as afunction of temperature. We have demonstrated the concept for a simple
phase transition. As a next step, we intend to study subtle phase transitions
such as those among the smectic subphases. We believe this idea can ingeneral be extended to study structural phase transitions in solid materials.
The author gratefully acknowledges support from the Department of Scienceand Technology, Government of India, and from various others who havehelped along the way.
Surajit Dhara
School of Physics
University of Hyderabad
Hyderabad, India
Surajit Dhara and his group research the physics of soft materials. Theirinterests include the structures and properties of unconventional liquid
crystals, liquid crystal-nanoparticle composites, rheology of complex fluids,and photonics of soft materials.
References:
1. P. G. de Gennes, The Physics of Liquid Crystals, Oxford University Press, 1993.
2. T. Arun Kumar, M. A. Mohiddon, N. Dutta, N. K. Viswanathan, S. Dhara, Detection of phasetransitions from the study of whispering gallery mode resonance in liquid crystal droplets, Appl.

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Phys. Lett. 106, p. 051101, 2015.
3. K. Vahala, Optical Microcavities, World Scientific, 2004.
4. F. Vollmer, S. Arnold, D. Keng, Single virus detection from the reactive shift of a whisperinggallery mode, Proc. Nat'l Acad. Sci. USA 105, p. 20701-20704, 2008.
5. M. Loncar, Molecular sensors: cavities lead the way, Nat. Photonics 1, p. 565, 2007.
6. H. Wang, L. Yuan, C.-W. Kim, Q. Han, T. Wei, X. Lan, H. Xiao, Optical microresonator based onhollow sphere with porous wall for chemical sensing, Opt. Lett. 37, p. 94, 2012.
7. J. J. Yang, M. Huang, X. Z. Dai, M. Y. Huang, Y. Liang, A spoof surface WGM sensor based on atextured PEC cylinder, Europhys. Lett. 103, p. 44001, 2013.
8. L. He, S. K. Qzdemir, J. Zhu, W. Kim, L. Yang, Detecting single viruses and nanoparticles usingwhispering gallery microlasers, Nat. Nanotechnol. 6, p. 428, 2011.
9. M. Humar, M. Ravnik, S. Pajk, I. Musevic, Electrically tunable liquid crystal optical
microresonators, Nat. Photonics 3, p. 595600, 2009.
10. M. Humar, I. Musevic, 3D microlasers from self-assembled cholesteric liquid-crystal
microdroplets, Opt. Express 18, p. 26995-27003, 2010.

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