Electrical and thermal tuning of quality factor and free spectral range of optical resonance of nematic liquid crystal microdroplets

We experimentally study the effect of temperature and electric field on the quality $\left(Q\right)$ factor and free spectral range (FSR) of whispering-gallery-mode optical resonance of dye-doped nematic liquid crystal microdroplets. Both the $Q$ factor and the FSR are highly sensitive to the temperature and electric field and are tunable. The $Q$ factor decreases, whereas the FSR increases substantially, with increasing temperature and electric field. The variation of the $Q$ factor and FSR is understood based on the change in the effective refractive index and the dynamic size of the microdroplets.

Figure 1

Schematic of the scanning near-field optical microscope setup.

Figure 2

Polarizing optical micrograph showing suspended microdroplets of 5CB liquid crystal in PDMS at room temperature.

Figure 3

(a) Polarizing optical micrograph of a focused 5CB microdroplet. (b) Schematic showing the molecular orientation inside the microdroplet. (c) A DCM dye–doped micro-droplet illuminated with a focused laser beam $(\lambda =532$ nm). The red arrow indicates the irradiation point.

Figure 4

(a) Representative WGM spectra recorded at two temperatures, namely, at $27.5^{\circ }\mathrm{C}$ (black line) and $30.5^{\circ }\mathrm{C}$ (red line). (b) Variation of $Q$ factor with temperature. The solid line is the best fit to Eq. (1). Droplet diameter: $9\mu \mathrm{m}$.

Figure 5

Schematic showing the director field and resonance modes in a microdroplet at (a) zero, (b) small, and (c) large electric fields. Blue and yellow ellipsoids represent modes; red lines, the director. ITO, indium-tin-oxide.

Figure 6

(a) WGM resonance spectra at two voltages: 0.35 V (black line) and 6.19 V (red line). (b) Variation of the $Q$ factor with the applied voltage. The solid line is a guide for the eye. Droplet diameter: $8\mu \mathrm{m}$.

Figure 7

(a) Variation of free spectral range (FSR) with applied voltage. The straight line is the best fit (linear) with a slope of 0.17 nm/V. (b) Variation of FSR with temperature.

Figure 8

(a) Variation of the diameters of microdroplets with respect to (a) time and (b) temperature. Lines in (a) are the linear best fits. The slope of the smaller droplet 33.$5\left(\mu \mathrm{m}\right)$ is 9 nm/min. Dotted lines in (b) show the deviation from the linear variation.