Optical fibers with metasurfaces or plasmonic nanostructures

Twisting Light with 2D Materials: Dynamic Chirality Control from the Atomic Scale Up

Why Control Chirality?

Chirality—the “handedness” of light—describes whether its electric field rotates clockwise or counterclockwise as it propagates. This property, once considered a niche concern in optics, has become central to modern photonic technologies:

• Chiral biosensing: Many molecules are chiral and selectively detecting them requires circularly polarized light.
• Quantum information: Encoding qubits in light’s helicity opens new pathways for robust, secure communication.
• Valleytronics and spintronics: In materials like TMDCs or topological insulators, chirality is the key to unlocking valley or spin selectivity.
• Advanced imaging: Circular dichroism imaging and contrast enhancement rely on precise polarization control.

Traditionally, chirality is manipulated with large, slow components—quarter-wave plates, birefringent crystals, or mechanical rotators. But what if we could control light’s twist faster than any mechanical system and thinner than any lens?

Thanks to 2D materials like graphene and black phosphorus, we now can.

Building the Future: Chirality Control with Atom-Thin Structures

Our recent research demonstrates that chirality control can be programmable, ultrafast, and fully integrable using metasurfaces engineered from single layers of graphene and black phosphorus. These nanostructures are not just small: they’re active, responsive, and tailored for the THz and mid-IR spectrum, where traditional components often fall short.

Below, we highlight three breakthrough studies that together form a complete narrative—from fundamental thermal dynamics to tunable devices and ultrafast chiral switches.

1) Electrically Tunable Chirality with Black Phosphorus Metasurfaces

Concept

Black phosphorus (BP), a highly anisotropic 2D material, is patterned into symmetric nano-squares. Although the pattern looks symmetric, BP’s intrinsic directional properties split the plasmons along with its crystal axes. These can interfere to generate arbitrary polarization states.

By applying a gate voltage, the spectral positions of these modes are shifted independently, enabling dynamic, broadband control of light’s polarization state, including conversion between linear and circular polarization.

Why It Matters

• No mechanical movement or rotating optics
• Entire device is only a few nanometers thick
• Programmable chirality across a wide THz band

Technical Highlights

• Spectral Range: 30–80 µm (THz regime)
• Conversion: Linear ↔ Circular, Left ↔ Right
• Modulation: Controlled with simple electrostatic gating
• Enhancement: Up to 6× boost in spontaneous emission at plasmonic hotspots

Applications

• On-chip polarimetric sensors
• Dynamic circular polarizers for terahertz communication
• Enhancing quantum emitter interactions in the THz domain

2) Third-Harmonic Generation in Plasmonic Graphene Metasurfaces

Overview

This study reports the first experimental demonstration of third-harmonic generation (THG) in an actively tunable graphene plasmonic metasurface operating in the mid-infrared. By exploiting strong light confinement and resonant enhancement in graphene nanoribbon arrays, the metasurface exhibits robust nonlinear upconversion of incident light into the near-infrared, enabled by graphene’s high third-order optical nonlinearity and its strong dependence on carrier density.

Unlike conventional THG platforms relying on bulk materials or multilayer structures, this work demonstrates efficient frequency conversion using only a single layer of graphene, showing that nanoscale field enhancement and ultrathin design can enable compact, reconfigurable, and scalable nonlinear photonic devices.

Mechanism: Plasmon-Enhanced Nonlinear Frequency Conversion

1. Mid-IR Excitation at Plasmonic Resonance
   The metasurface is patterned to support a localized plasmon mode at the mid-IR pump wavelength (~11.3 µm). The incident light excites this mode, generating strong in-plane fields confined to the graphene ribbons.
2. Third-Order Nonlinear Response of Graphene
   The enhanced local fields induce a third-order polarization P(3)∝χ(3)E3P^{(3)} \propto \chi^{(3)} E^3P(3)∝χ(3)E3 in the graphene, resulting in coherent radiation at the third harmonic (~3.77 µm). The nonlinear susceptibility χ(3)\chi^{(3)}χ(3) arises from the intraband dynamics of Dirac fermions and is significantly boosted near resonance.
3. Doping-Dependent Efficiency
   By tuning the Fermi level via electrical gating, the resonance condition and carrier population are adjusted, directly influencing the amplitude and spectral shape of the THG signal.
4. Polarization and Angular Dependence
   THG emission is strongest under TM-polarized excitation and exhibits distinct angular patterns, confirming the anisotropic nature of the nonlinear response.

Experimental Observations

3) Ultrafast All-Optical Control of Light Chirality Using Nanostructured Graphene

Overview

This study demonstrates the first ultrafast, all-optical control of light chirality in a monolayer graphene metasurface by exploiting the non-equilibrium dynamics of hot carriers. Without any external gating, the experiment shows that femtosecond mid-infrared optical pumping can modulate both the ellipticity and handedness of the polarization of reflected light within picosecond timescales.

The metasurface design consists of geometrically symmetric graphene nanostructures, yet the observed chirality control arises dynamically from the nonlinear phase and amplitude evolution of orthogonal plasmonic modes—highlighting the emergence of optical chirality as a thermal, ultrafast effect in 2D materials.

 Mechanism: Hot-Electron-Induced Modulation of Mode Interference

1. Femtosecond Optical Excitation
   A mid-IR pump pulse rapidly increases the electron temperature in graphene (Te > 1500 K), initiating a transient, non-equilibrium state.
2. Differential Redshifting of Plasmon Modes
   The metasurface supports two weakly coupled plasmonic modes along orthogonal directions. Elevated Te reduces the Drude weight, resulting in non-uniform redshifting and broadening of these modes.
3. Phase & Amplitude Evolution
   The dynamic spectral reshaping alters the relative phase and amplitude of the two modes—modulating the polarization state of the reflected probe light.
4. Emergent Chirality and Switching
   This phase interference effect manifests as a transient change in ellipticity and handedness, enabling a reversible switch between left-handed and right-handed elliptical states on a sub-10 ps timescale.

 Key Observations

Integration Roadmap: A Complete Toolbox for Dynamic Polarization Control

Beyond the Chip: Metasurfaces Meet Optical Fibers

While much of the excitement around 2D metasurfaces centers on chip-based photonics, their ultrathin, flexible, and planar nature makes them ideally suited for integration with optical fiber systems—a step that bridges cutting-edge nanophotonics with widespread fiber-based infrastructure.

Why Fiber Integration Matters

•  Compact integration: 2D metasurfaces can be directly transferred onto fiber tips or embedded into connectors.
• Distributed functionality: Add polarization control or switch anywhere along the fiber.
• Remote deployment: Ideal for sensing, communications, and mid-IR diagnostics in harsh or remote environments.
• Broadband and ultrafast: Compatible with both THz and mid-IR fiber links, including hollow-core, D-shaped, or side-polished fibers.

 Practical Integration Pathways

• Direct transfer onto flat or angled fiber tips
• Embedding onto side-polished fiber sections for evanescent coupling
• Placement at fiber junctions (e.g. FC/PC connectors or spliced modules)
• Encapsulation in micro-optics or endoscope-type probes for biomedical uses

These approaches allow metasurfaces to operate not just in free space or on wafers—but as fully integrated components in fiber-optic sensors, communication nodes, and nonlinear light sources.

Looking Ahead

We envision metasurface-enabled fibers that:

• Dynamically convert linear ↔ circular polarization
• Provide ultrafast switching for quantum-secure communication over fiber
• Enable compact, fiber-based THz imaging systems
• Deliver on-chip and in-fiber chirality control in the same hybrid photonic network

Whether you’re working on fiber-integrated sensors, mid-IR spectroscopy, or reconfigurable communication backbones, these metasurfaces offer a new degree of control over light—right at the interface of silicon, graphene, and glass.