Observation of Janus Chirality for Coherent Thermal Emission from Metasurfaces

Abstract

Metasurfaces emerged as a powerful tool for controlling thermal radiation, yet achieving coherent emission with opposite circular handednesses remains a highly challenging problem. Here, we demonstrate experimentally the Janus chiral thermal emission from metasurfaces with opposite circular handednesses on either side of a single device. We employ anisotropic metasurfaces supporting high-Q resonances with photonic flatbands enabling near-unity circular dichroism through in-plane symmetry control. Our experiments confirm the Janus coherent emission, and they are validated by the results of the coupled-mode theory. The flatband resonant metasurfaces enabling a control of chiral thermal emission provide an efficient platform for spin-controlled light-matter interaction.

Figures (4)

• Figure 1: (a) Schematic of the principle of Janus chirality from CWMs, where red and blue beams represent upward- and downward-directed circularly polarized emission of opposite handedness, respectively. (b–d) Evolution of the structural design from a conventional 1D waveguide array to a chiral CWM. (e) Band folding in the $k_y$ direction resulting from the transition from a 1D lattice of uncoupled waveguides to a periodic structure with period $P_y$. (f) $Q_{rad}$-factors of band A versus $k_y$ for various corrugation depths $a$ (0, 1, 10, and 100 $nm$) as depicted in (d).
• Figure 2: (a) Evolution of Stokes parameter $S_3$ for upward and downward emission as a function of $\delta$. At $\delta$ = 735 $nm$, $S_3 = \pm 1$. Left inset: top view of the structure; right inset: the trajectory of polarization states on the Poincaré sphere as $\delta$ changes. (b) Simulated far-field polarization maps in momentum space for upward and downward radiations at $\delta$ = 735 $nm$. (c) Circular polarization components in the $xz$ plane at the $\Gamma$ point. (d) Calculated band structure at $\delta$ = 735 $nm$, with the color scale indicating $S_3$ values for upward and downward radiations. (e) Q factor of the band structure at $\delta$ = 735 $nm$.
• Figure 3: (a) Optical microscopy and top-view/cross-sectional SEM images of the fabricated thermal meta-emitter ($\delta$ = 735 $nm$). (b) Schematic of the experimental setup used for ThE characterization of the structure. Ap., Aperture; QWP, Quarter-wave plate; LP, Linear polarizer; BS, Beam splitter; MM, Moving mirror.
• Figure 4: (a,b) Numerically simulated and experimentally measured spectra of thermal radiation in the normal direction for upward emission from the structure in Fig. \ref{['fig3']}, with red and blue lines denoting LCP and RCP, respectively. (c, d) Simulated and measured angular-resolved spectra of upward emission with the opposite helicities, confirming high chirality contrast in the photonic flatband. (e, f) Simulated and measured angular-resolved spectra of downward emission with the opposite helicities, showing helicities reversed relative to upward emission and thus demonstrating Janus spin-coherent tghermal emission. (g, h) Emission spectra in the normal direction extracted from (e) and (f).