Discrimination of Chiral and Helical Contributions to Raman Scattering of Liquid Crystals Using Vortex Beams

We use vortex photon fields with orbital and spin angular momentum to probe chiral fluctuations within liquid crystals. In the regime of iridescence with a well-defined pitch length of chirality, we find low energy Raman scattering that can be decomposed into helical and chiral components depending on the scattering vector and the topological charge of the incident photon field. Based on the observation of an anomalous dispersion we attribute quasielastic scattering to a transfer of angular momenta to rotonlike quasiparticles. The latter are due to a competition of short-range repulsive and long-range dipolar interactions. Our approach using a transfer of orbital angular momentum opens up an avenue for the advanced characterization of chiral and optically active devices and materials.

Figure 1

(a) Spectroscopically resolved reflectivity $I_{\mathrm{LH}}-I_{\mathrm{RH}}$ for different temperatures. The inset (squares) shows the intensity of a maximum fitted by a Gaussian. (b) Differential scanning calorimetry (DSC). The box corresponds to the regime of our OAM RS experiments. (c) X-ray diffraction graph taken at room temperature after multiple thermal cycling.

Figure 2

(a) Divided OAM Raman spectra with $\ell =+2/-2$ as a function of temperature, shifted for convenience. (b) Divided OAM Raman spectra comparing helical polarization (RH/LH) only and different OAM states at $T_s=84°\mathrm{C}$. Full lines show fits using a Gaussian centered at $E\approx 0{\mathrm{cm}}^{-1}$. (c) Integrated Gaussian intensity (blue circles) and FWHM linewidth (green diamonds) of (b). $\mathrm{OAM}=0$ corresponds to helical only, $\mathrm{RH}/\mathrm{LH}$, polarizations.

Figure 3

(a) Comparison of transmission RS (upper curves), backscattering RS (lower curves) with two polarizations, and fits to finite-energy Lorentzian and quasielastic, Gaussian functions, respectively. (b) Transmitted power measurements as a function of light polarization at $T_s=84°\mathrm{C}$. (c) Sketch of an acoustic dispersion branch, $T$-dependent roton minima, and suggested RS signals, see text. The dashed arrows correspond to the softening of the acoustic dispersion branch due to the roton instability at certain momenta with different temperatures.