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Abstract

This thesis investigates the use of single liquid-crystal droplets as optical microresonators and
lasers. We have shown that liquid-crystal droplets can support a number of different optical
modes that can be excited by introducing a fluorescent dye in the liquid crystal material and
using an external excitation source of light. These optical modes are in general different from
the ones in isotropic materials, because of birefringence of the liquid crystals and specific
configurations of the director field within the droplet. The droplets were prepared just by
mechanical mixing of a liquid crystal material and another non-miscible fluid. The director
structure in the droplets self assembles to minimize the elastic energy. To measure the
optical properties of the microresonators an optical setup was built that includes a pulsed
laser for excitation and a spectrophotometer for spectral analysis. Regarding the type of
liquid crystalline material used for the droplets and the type of optical modes supported in
the droplets, the thesis is basically divided into two parts.
In the first part, nematic liquid crystal is used to make the droplets. It has been shown
that the nematic droplets confine light by total internal reflection and therefore support
whispering-gallery modes. At higher peak intensities of the pump laser, low threshold multimode
lasing has been achieved. The optical modes were tuned by electric field, temperature
and mechanical deformation. In the case of applying electric field, the nematic director orientation
changes locally, so that the circulating light sees a change in the refractive index.
On the other hand, by changing the temperature only the order parameter is altered, which
results in the change of both ordinary and extraordinary refractive indices. In the case of
mechanical deformation, only geometrical path length is changed with the applied strain,
like it would happen also in an isotropic droplet. It has been shown that the response to
electric field and temperature is much larger than reported before in the literature for other
materials. By applying just an electric field of few V/µm or changing the temperature by
few ◦C, the modes shift by more than 10nm in visible light. By using a nematic droplet
floating in water, also a chemical sensor was demonstrated. The surfactant molecules, which
concentration we want to measure, adsorp to the surface of the liquid-crystal droplet and
change the anchoring and therefore also the director configuration in the droplet. This results
in a change of the optical properties of the droplet and therefore the frequencies of the
optical modes. The spectrum of light captured from such a droplet serves as an indicator of
the presence of the surfactant in the surrounding water.
In the second part of the thesis, cholesteric liquid crystals with selective reflection in
visible light were used to make the droplets. The cholesteric liquid crystal in the droplet
self assembles so that the helical axis is pointing from the center in all the directions to the
surface of the droplet. The structure is acting as a spherical Bragg onion microcavity with
periodic modulation of the refractive index that confines the light into the center of the
droplet. By having a fluorescent dye in the LC and illuminating the droplet by an external
pulsed laser, the droplet starts to emit laser light in all the directions. In this way we have
made a 3D laser, a coherent isotropic point source of light. The spherical cholesteric laser
is one of the first lasers emitting in all the directions and also one of the easiest lasers to
make in general. The laser is also highly tunable by changing the temperature.