Manipulating the Orbital Angular Momentum of Light at the Micron Scale with Nematic Disclinations in a Liquid Crystal Film

We report on the experimental manipulation of the orbital angular momentum of light by exploiting a kind of topological defects that spontaneously appear in nematics—disclinations—as microscopic optical spin-orbit interfaces whose operating wavelength can be controlled electrically. Using six different kinds of disclinations, we demonstrate the efficient generation of both scalar and vectorial singular light beams with a broad topological diversity from a fundamental Gaussian beam.

Figure 1

Upper row: Observation of disclinations with strength $s=m/2$, $-3\le m\le 3$ between crossed linear polarizers oriented along the $x$ and $y$ axes. Bottom row: Experimental reconstruction of the director field whose local orientation is represented by segments. Scale bar is $10\mu \mathrm{m}$.

Figure 2

Reduced spin-to-orbital angular momentum efficiency $\eta /{\eta }_{\max }$ versus reduced applied voltage difference, $(U-U_{\max })/U_{\max }$, for scalar optical vortex generation using six different kinds of disclinations. In all cases, $U_{\max }\approx 1V_{\mathrm{rms}}$.

Figure 3

Generation of scalar vortex beams from disclinations with strength $s=m/2$, $-3\le m\le 3$ and fixed incident circular polarization state; here, $\sigma =+1$. Upper row: Output intensity profiles. Bottom row: Interference patterns that result from the coaxial superposition of the vortex beam with a reference Gaussian beam having the same polarization state.

Figure 4

Intensity and phase characterization of the scalar optical vortices shown in Fig. 3. (a) Radius of maximal intensity of the transverse intensity profiles. Solid line refers to the Laguerre-Gaussian behavior of the form $R_{\ell }\propto |\ell {|}^{1/2}$. (b) Azimuthal behavior of the phase profiles. Solid lines refer to linear dependence of the form $\Phi =\ell \varphi$.

Figure 5

Intensity and phase analyses for the states content of a vector beam defined by state ${\Psi }_2^{\mathrm{V}}$. Left: Azimuthally averaged radial intensity profiles of the total and left- or right-handed circular components of the output light fields, $I_{\mathrm{tot}}$ and $I_{L,R}$. Right: Upper panels show the spatial light intensity distributions $I_{L,R}$ whereas bottom ones show the interferograms obtained from the coaxial superposition of the left or right circular components with a reference Gaussian beam having the same polarization state.

Figure 6

Generation of various vector beams by using six different disclinations presented in Fig. 1 for an incident fundamental Gaussian beam linearly polarized along $x$ axis. The segments superimposed to the total intensity profiles refer to polarization azimuth patterns, taken at radius of average maximal intensity.