Computational spectropolarimetry with a tunable liquid crystal metasurface

While conventional photodetectors can only measure light intensity,the vectorial light field contains much richer information,including polarization and spectrum,that are essential for numerous applications ranging from imaging to telecommunication.However,the simultaneous measurement of multi-dimensional light field information typically requires the multiplexing of dispersive or polarization-selective elements,leading to excessive system complexity.Here,we demonstrate a near-infrared spectropolarimeter based on an electrically-tunable liquid crystal metasurface.The tunable metasurface,which acts as an encoder of the vectorial light field,is tailored to support high-quality-factor guided-mode resonances with diverse and anisotropic spectral features,thus allowing the full Stokes parameters and the spectrum of the incident light to be computationally reconstructed with high fidelity.The concept of using a tunable metasurface for multi-dimensional light field encoding may open up new horizons for developing vectorial light field sensors with minimized size,weight,cost,and complexity.

Synthesis and Characterization of a Novel Liquid Crystalline Chitin Quaternary Ammonium Chitosan Salt

N -Butyl chitosan(NBCS) derivatives were prepared by introducing butyl groups into the amine groups of chitosan via Schiff base intermediates. The quaternization of NBCS was carried out by using ethyl iodide to produce water-soluble cationic polyelectrolytes. The degree of the substitution of the products was measured by means of the electron spectroscopy for a successful chemical analysis. The quaternary ammonium chitosan salt(QACS) was proved to be a novel cholesteric liquid crystalline chitin, by means of the polarized optical microscopy and the circular dichroism spectropolarimetry. The critical mass fraction of the QACS/formic acid solution forming a lyotropic liquid crystal phase was 50%, which was almost the same as that of the NBCS/formic acid solution, but much higher than that of the chitosan in the same solvent.

Multifunctional interleaved geometric-phase dielectric metasurfaces

Shared-aperture technology for multifunctional planar systems,performing several simultaneous tasks,was first introduced in the field of radar antennas.In photonics,effective control of the electromagnetic response can be achieved by a geometric-phase mechanism implemented within a metasurface,enabling spin-controlled phase modulation.The synthesis of the shared-aperture and geometric-phase concepts facilitates the generation of multifunctional metasurfaces.Here shared-aperture geometric-phase metasurfaces were realized via the interleaving of sparse antenna sub-arrays,forming Si-based devices consisting of multiplexed geometric-phase profiles.We study the performance limitations of interleaved nanoantenna arrays by means of a Wigner phasespace distribution to establish the ultimate information capacity of a metasurface-based photonic system.Within these limitations,we present multifunctional spin-dependent dielectric metasurfaces,and demonstrate multiple-beam technology for optical rotation sensing.We also demonstrate the possibility of achieving complete real-time control and measurement of the fundamental,intrinsic properties of light,including frequency,polarization and orbital angular momentum.

Models and observations of sunspot penumbrae

The mysteries of sunspot penumbrae have been under an intense scrutiny for the past 10 years. During this time, some models have been proposed and refuted, while the surviving ones had to be modified, adapted and evolved to explain the ever-increasing array of observational constraints. In this contribution I will review two of the present models, emphasizing their contributions to this field, but also pinpointing some of their inadequacies to explain a number of recent observations at very high spatial resolution (0.32 ). To help explaining these new observations I propose some modifications to each of those models. These modifications bring those two seemingly opposite models closer together into a general picture that agrees well with recent 3D magneto-hydrodynamic simulations.