🔬 optics

Observation of Janus Chirality for Coherent Thermal Emission from Metasurfaces

This paper experimentally demonstrates Janus chiral thermal emission from anisotropic metasurfaces that utilize high-Q flatband resonances to achieve opposite circular handednesses on either side of a single device, enabling efficient spin-controlled light-matter interaction.

Original authors: Kaili Sun, Yangjian Cai, Maxim V. Gorkunov, Yuri Kivshar, Zhanghua Han

Published 2026-02-02

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Kaili Sun, Yangjian Cai, Maxim V. Gorkunov, Yuri Kivshar, Zhanghua Han

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a piece of glass that acts like a magical, two-faced coin. On one side, it glows with light that spins clockwise; on the other side, it glows with light that spins counter-clockwise. This is the core discovery of the paper: a new type of "Janus" thermal emitter that can send out two different types of spinning light from a single, flat device.

Here is a breakdown of how they did it and what they found, using simple analogies:

The Problem: The "One-Way" Light Bulb

Usually, when things get hot, they glow like a standard lightbulb: the light is messy, goes in all directions, and doesn't spin in any specific way. Scientists have figured out how to make light spin (circular polarization) using special artificial surfaces called metasurfaces.

However, there's a catch. Most of these special surfaces are like a one-way street. If you make the light spin clockwise going up, it usually also spins clockwise going down. To get the opposite spin, you usually need a completely different device or a very complicated, expensive setup. It's like needing two different keys to open two different doors on the same house.

The Solution: The "Janus" Metasurface

The researchers built a single device that acts like the Roman god Janus, who has two faces looking in opposite directions.

The Top Face: Emits light spinning one way (let's say, Left-Handed).
The Bottom Face: Emits light spinning the exact opposite way (Right-Handed).

They achieved this using a flat sheet of Germanium (a material that gets hot and glows) sitting on a Calcium Fluoride base. They carved tiny, saw-tooth patterns into the Germanium. Think of these patterns like a row of tiny, connected slides or waveguides.

How It Works: The "Flat Road" Analogy

To understand why this works, imagine a highway for light.

Normal Highways: Usually, light waves travel fast or slow depending on their angle, like cars speeding up or slowing down on a hilly road. This makes it hard to control them precisely.
The Flatband Highway: The researchers designed their structure to create a "flat road" for light. On this road, light waves of different angles all travel at the same speed and stay perfectly synchronized. This is called a photonic flatband.

Because the road is so flat and smooth, the light waves can resonate (vibrate) for a long time without losing energy. This creates a very strong, high-quality signal (high "Q-factor").

The Magic Trick: Breaking the Mirror

The secret sauce is how they arranged the "saw teeth" on the waveguides.

They put identical saw teeth on both sides of the waveguide, but then they shifted the teeth on one side slightly to the left and the other to the right.
This breaks the "mirror symmetry" of the structure.
Because of this shift, the light waves bouncing inside the structure get "tricked" into spinning one way when they exit the top, and the opposite way when they exit the bottom.

It's like a spinning top that, if you look at it from above, spins clockwise, but if you look at it from below (through the floor), it appears to spin counter-clockwise.

The Experiment: Proving It Works

The team built this tiny device (about the size of a postage stamp) and heated it to 200°C. They then measured the light coming out of the top and the bottom.

The Result: The top emitted almost perfectly Left-Handed spinning light, and the bottom emitted almost perfectly Right-Handed spinning light.
The Quality: The light was very "coherent," meaning the waves were marching in perfect step, not a chaotic jumble. The "spin purity" (circular dichroism) was nearly 100% on both sides.

Why It Matters (According to the Paper)

The paper claims this is a big deal because:

Simplicity: You don't need complex 3D structures or magnetic materials to get this effect; a flat, patterned sheet is enough.
Robustness: The effect works even if you change the material underneath the device (the substrate), which is a problem for many other designs.
Efficiency: It creates a strong, clean signal without the "rainbow" effect (where different colors go in different directions), keeping the light focused.

The authors suggest this could be a powerful tool for controlling how light and matter interact, specifically for creating advanced infrared light sources that can be used in imaging, communication, and sensing. They also mention that the principles could theoretically be applied to "valleytronics" (a field dealing with electron spins), but the primary focus of this work is the thermal light emission itself.

Problem
Metasurfaces have emerged as powerful tools for controlling thermal radiation, yet achieving coherent emission with opposite circular handednesses on either side of a single device remains a significant challenge. Existing approaches to nonreciprocal and Janus metasurfaces often rely on complex geometries requiring simultaneous in-plane and out-of-plane mirror symmetry breaking, or the introduction of magneto-optic materials and anisotropic crystals. These methods necessitate complicated, high-precision fabrication processes that are difficult to control experimentally, resulting in scarce experimental reports. Furthermore, conventional devices typically emit light of a single helicity at specific frequencies and narrow directional ranges, requiring the integration of multiple devices for practical applications. This increases the footprint and hinders the scalability and compactness required for advanced technologies such as infrared imaging, optical communication, and sensing. In flat structures, circularly polarized radiation is often confined to off- $\Gamma$ -point regions, and most circularly polarized eigenstates provide homochiral output (same handedness) in both upward and downward directions.

Methodology
The authors propose and experimentally demonstrate "Janus chiral thermal emission," where a single device emits coherent, wide-angle circularly polarized thermal radiation with opposite handednesses in the upper and lower half-spaces. The design employs anisotropic metasurfaces composed of arrays of weakly coupled waveguides with lateral periodic modulation on both sides. This platform utilizes coupled-resonator optical waveguides (CROWs) to simultaneously control pairs of thermal photons carrying opposite spin angular momenta.

The structural design involves a low-loss $CaF_2$ substrate and a Germanium (Ge) waveguide array. The waveguides feature identical lateral sawtooth corrugations of depth $a$ on both sides to break translational symmetry along the $y$ -direction, enabling guided modes to interact with the radiative continuum and form high-Q quasi-bound states in the continuum (quasi-BICs). A central distance $\delta$ is introduced between adjacent sawtooth corrugations on opposite sides to break in-plane mirror symmetry while preserving $C_2$ symmetry, thereby enabling chiral guided-mode resonances (GMR).

Theoretical analysis employs temporal coupled-mode theory (TCMT) to model the absorptance and emissivity, linking the radiative decay rate to the corrugation depth and the dissipative loss rate to the material properties. The study calculates Stokes parameters to characterize polarization states and analyzes band structures to identify photonic flatband behavior. Experimental validation involves fabricating the thermal meta-emitter using standard nanofabrication techniques and characterizing the Janus chiral thermal emission using a customized setup with a heating system, quarter-wave plates, linear polarizers, and a Fourier-transform infrared spectrometer equipped with a mercury cadmium telluride detector.

Key Contributions

Janus Chiral Thermal Emission: The paper introduces and demonstrates a single-device platform capable of emitting coherent thermal radiation with opposite circular handednesses in upward and downward directions.
Photonic Flatband Resonances: The authors utilize high-Q resonances supported by photonic flatbands, enabling near-unity circular dichroism (CD) over a wide angular range. This contrasts with traditional BICs, which are typically confined to very narrow angles of incidence and emission.
Symmetry Robustness: The study demonstrates that this chiral Janus emission is immune to variations in the substrate's refractive index and persists under both intact and broken $\sigma_z$ (vertical mirror) symmetry conditions, challenging the conventional paradigm that requires $\sigma_z$ symmetry breaking for chiral optical responses.
Experimental Realization: The work provides experimental confirmation of the concept, achieving high temporal coherence ( $Q > 150$ ) and near-unity CD ( $E_{CD} > 0.8$ ) in the mid-infrared (MIR) spectral range.

Results

Simulation: Numerical simulations confirmed that at a specific corrugation offset ( $\delta = 735$ nm), the structure supports a photonic flatband (Band A) with opposite Stokes parameter $S_3$ values ( $\pm 1$ ) for upward and downward emissions across a wide wavevector range. The $Q_{rad}$ factors for this band remained stable above $10^3$ .
Fabrication: Scanning electron microscope (SEM) images verified the high structural fidelity of the fabricated Ge waveguide arrays on $CaF_2$ substrates.
Experimental Characterization:
- Upward Emission: Measured spectra showed exclusively left-handed circular polarization (LCP) with a central wavelength invariant across a broad angular range. The experimental $Q$ -factor reached 139, and the circular dichroism was approximately 0.86.
- Downward Emission: Measured spectra revealed a reversal in handedness (Right-handed Circular Polarization, RCP) with a $Q$ -factor of 192 and a CD of approximately -0.8 at normal output.
- Discrepancies: The experimental $Q$ -factors were slightly lower than theoretical predictions, attributed to steep dispersion in the $y$ -direction causing angular collection of different frequencies, and scattering losses from rough sidewalls. The slight reduction in CD was attributed to unavoidable background thermal noise.

Significance
The paper claims that this approach provides an efficient platform for spin-controlled light-matter interaction by enabling the control of chiral thermal emission through flatband resonant metasurfaces. The ability to achieve high-coherence, wide-angle, opposite-handedness emission from a single, compact device streamlines the fabrication process and offers a versatile route for multifunctional chiral light sources and sophisticated radiative control. The authors suggest that beyond thermal emission, this approach could be extended to valleytronics, where distinct spin-polarized emissions could be spatially separated and routed, potentially opening new avenues for scalable quantum information processing. The work is supported by coupled-mode theory, offering a robust foundation for these interactions.