Thermo-optical properties and nonlinear optical response of smectic liquid crystals containing gold nanoparticles

The present work is devoted to the study of the thermo-optical and nonlinear optical properties of smectic samples containing gold nanoparticles with different shapes. By using the time-resolved Z-scan technique, we determine the effects of nanoparticle addition on the critical behavior of the thermal diffusivity and thermo-optical coefficient at the vicinity of the smectic-$A$–nematic phase transition. Our results reveal that introduction of gold nanoparticles affects the temperature dependence of thermo-optical parameters, due to the local distortions in the orientational order and heat generation provided by guest particles during the laser exposure. Further, we show that a nonlinear optical response may take place at temperatures where the smectic order is well established. We provide a detailed discussion of the effects associated with the introduction gold nanoparticles on the mechanisms behind the thermal transport and optical nonlinearity in liquid-crystal samples.

Figure 1

Schematic representation of the experimental setup: L, lens; BS, beam splitter; I, iris; M, mirror; Ch, chopper; D, detector.

Figure 2

TEM images of liquid crystals samples containing gold nanoparticles of different shapes: (a) nanorods and (b) nanospheres. Gold nanorods exhibit an average length of 40 nm and an aspect ratio of 2.5, while the nanospheres present an average diameter of 4 nm. (c) Extinction spectra of nanorods (solid line) and nanospheres (dashed line) of gold dispersed in the 8CB host, confined in a homeotropic cell. Notice the transversal plasmon band $({\lambda }_t=507$ nm) has a higher extinction than the longitudinal plasmon band $({\lambda }_l=669$ nm), indicating that the gold nanorods are aligned parallel to the far-field nematic director.

Figure 3

Transient intensities of 8CB liquid crystal containing gold nanorods (black circles) and nanospheres (red squares) at distinct reduced temperatures: (a) $t=0.004$ (nematic phase) and (b) $t=-0.004$ (smectic phase). In all cases, the nanoparticles concentration was $c=0.02$ wt%. One can note that the thermal lens phenomenon is less pronounced in the smectic phase, with the thermal-induced distortion in the beam wave front and the characteristic time to the formation of the thermal lens being sensitive to the nanoparticles shape.

Figure 4

Beam phase shift in units of the incident laser power as a function of the reduced temperature for 8CB samples containing (a) gold nanorods and (b) gold nanospheres at a concentration of 0.02 wt%. One can observe a sharp drop in the thermal-induced phase shift at the nematic–smectic-$A$ transition temperature, which is more pronounced in the 8CB sample doped with gold nanorods.

Figure 5

Temperature dependence of the thermal diffusivity of 8CB samples doped with (a) gold nanorods and (b) gold nanospheres. The concentration of gold nanoparticles in 8CB samples was $c=0.02$ wt%. Notice that the thermal diffusivity exhibits a vanishing critical behavior at the nematic–smectic-$A$ phase transition, with the heat diffusion being governed by short-range processes that depend on the nanoparticle shape.

Figure 6

Normalized Z-scan transmittance of 8CB doped gold nanorods at different reduced temperatures: (a) $t=0.003$, (b) $t=-0.003$, (c) $t=-0.013$, and (d) $t=-0.014$. The measurements were performed in the configurations of closed (black circles) and open (red squares) apertures. Here we observe that a nonlinear absorption takes place as the temperature is diminished well below to the nematic–smectic-$A$ transition temperature.