Chiral Flat-Band Optical Cavity

Introduction

In the rapidly evolving field of photonic technology, the ability to control and manipulate light at the nanoscale has remained a fundamental challenge. Recent research has leveraged atomically thin materials to create innovative optical cavities, marking progress in light confinement and manipulation techniques. This paper explores the scientific principles and applications of a chiral flat-band optical cavity that utilizes transition metal dichalcogenides (TMDs) as mirrors [1].

Basic Concepts and Design

The foundation of this technology lies in the unique properties of TMD materials, particularly MoSe₂, when used as atomically thin mirrors. Unlike traditional metallic or distributed Bragg reflectors, these TMD-based mirrors exploit the high optical quality of excitons—electron-hole pairs that interact efficiently with light.

Figures 1A and 1B illustrate the fundamental principles of the nano-cavity device, demonstrating how a monolayer MoSe₂ acts as a mirror at the resonant wavelength and how the overall device structure is formed using two TMD mirrors embedded within hexagonal boron nitride (hBN).

The device structure consists of two monolayer MoSe₂ mirrors encapsulated in hBN layers, with a total thickness of 240 nm. These monolayers are precisely positioned 60 nm from the top and bottom of the van der Waals heterostructure. This precise placement is crucial for achieving optimal light confinement and enables independent electrical control of each layer via a silicon substrate serving as a back gate.

Device Characterization and Performance

The optical performance of these optical cavities exhibits several remarkable features that distinguish them from conventional optical cavities. One of the most striking features is the formation of a flat band in the optical mode dispersion, which remains constant at different incident angles.

Figures 2A–E present the experimental characterization of the cavity samples, including microscopy images, reflection spectra, photoluminescence spectra, and theoretical and experimental dispersion measurements.

The cavity mode appears as a narrow-band dip in the reflection spectrum, confirming the presence of confined electromagnetic modes. The photoluminescence spectrum shows that the central emission (X₀) aligns with the cavity's resonance wavelength, while secondary peaks corresponding to charged excitonic states (X±) appear at longer wavelengths.

Magnetic Field-Induced Chirality

One of the most innovative aspects of this cavity design is its chiral behavior under an external magnetic field. This characteristic originates from the valley-dependent optical selection rules and the valley Zeeman effect inherent in TMD monolayers.

Figures 3A–C illustrate the magnetic-field-induced chiral behavior, showing reflection spectra for different circular polarization states and the energy splitting between modes.

When a magnetic field is applied in the Faraday configuration, the cavity exhibits different responses for opposite circular polarizations (σ⁺ and σ⁻). The field induces energy splitting in the excitons of the K and -K valleys, leading to distinct reflection peaks for σ⁺ and σ⁻ polarized light. This magnetically tunable chirality achieves a reflection circular dichroism value of 0.41 at 10 Tesla.

Electrical and Thermal Tunability

The versatility of these optical cavities extends to electrical and thermal control mechanisms, allowing tunability through different external factors. Independent electrical contacts on each MoSe₂ mirror enable precise tuning of the cavity resonance.

Figures 4A–D demonstrate various tuning mechanisms, including the gate voltage effect, pump power dependence, and temperature response.

The cavity mode exhibits continuous tunability of approximately 0.5 nm via gate voltage control. The mode remains stable below a certain power threshold, but at higher power levels, it redshifts and broadens. The cavity maintains stable performance below ~100 K, with a tuning range of ~10 nm observed between 4 K and 100 K.

Applications and Future Prospects

This technology has far-reaching implications beyond fundamental research, opening new directions in the following areas:

• Quantum emitter control without the need for complex photonic structures

• Chiral light-matter coupling applications

• Development of optical isolators and polarization-dependent transducers

• Investigation of collective effects in condensed matter systems

• Creation of novel topological effects in photonic systems

The combination of atomic-scale thickness and unique optical properties makes these cavities highly advantageous for nanoscale applications requiring precise light control. The presence of multiple tuning mechanisms, while maintaining high optical quality, makes these devices particularly valuable for research in quantum optics, communication, and sensing technologies.

References

[1] D. G. Suárez-Forero et al., "Chiral flat-band optical cavity with atomically thin mirrors," Science Advances, vol. 10, no. 1, p. eadr5904, Dec. 2024.