🔬 optics

Electrically-driven chiral emission from plasmonic tunnel junctions

By integrating tunnel junctions with chiral plasmonic nanohelicoids, researchers have achieved electrically driven, single-particle generation of chiral vortex light beams with high spin handedness selectivity, offering a promising nanoscale source for next-generation optical and quantum technologies.

Original authors: Yuanyang Xie, Alexey V. Krasavin, Anatoly V. Zayats

Published 2026-04-10

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CC BY 4.0

Original authors: Yuanyang Xie, Alexey V. Krasavin, Anatoly V. Zayats

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 you have a tiny, invisible factory inside a speck of gold. This factory doesn't make toys or cars; it makes light. But not just any light—it makes light that twists.

In the world of physics, this "twisting" is called chirality. Think of your hands: your left hand is a mirror image of your right, but you can't stack them perfectly on top of each other. They are "chiral." Light can be chiral too. It can spin like a right-handed screw or a left-handed screw. This paper describes how scientists built a tiny machine that creates this spinning light using electricity, without needing giant, complicated lenses or magnets.

Here is the story of how they did it, broken down into simple parts:

1. The Problem: Making Light Spin is Hard

Usually, to make light spin (like a corkscrew), you need special crystals or strong magnets. These are big, heavy, and hard to fit into tiny gadgets like your phone or a VR headset. Scientists have been trying to make a tiny, electric-powered "light spinner" for a long time, but it's been like trying to build a tornado in a teacup.

2. The Solution: The "Golden Helicoid"

The team at King's College London came up with a clever design. Imagine a tiny, golden screw or a spiral staircase (they call it a "nanohelicoid").

The Setup: They placed this tiny golden screw on top of a flat gold sheet.
The Gap: Between the screw and the sheet, they left a microscopic gap (thinner than a virus) filled with a special insulating material.
The Power: When they push electricity through this gap, the electrons can't just walk across; they have to "tunnel" (jump) through the gap like ghosts passing through a wall.

3. The Magic Trick: From Jumping to Twisting

When the electrons jump across this gap, they don't just disappear. They crash into the golden screw and make it vibrate.

The Analogy: Imagine the golden screw is a guitar string. When the electron hits it, it plucks the string.
The Result: Because the screw is shaped like a spiral, it doesn't just vibrate up and down; it vibrates in a twisting motion. This vibration creates a beam of light that inherits that twist.

4. The "Spin" and the "Orbit"

The light coming out of this tiny machine has two types of "spin":

Spin Angular Momentum (SAM): This is the light's own "handedness." It's spinning like a top. The team found that over 80% of the light spins in the same direction (either all left or all right). That's a very clean, strong signal!
Orbital Angular Momentum (OAM): This is the light's "orbit." Imagine the light beam isn't just a straight laser pointer; it's a vortex or a tornado of light. The wavefronts of the light swirl around the center as they travel.

The Cool Part: The laws of physics say that if the light spins one way (SAM), it must orbit the other way (OAM) to balance the books. This tiny machine naturally does exactly that, creating a perfect "twisting tornado" of light.

5. Why Does This Matter?

You might ask, "Why do we need a tiny light tornado?"

Super-Compact Screens: Imagine future Augmented Reality (AR) glasses or VR headsets that are as thin as contact lenses. This technology could generate 3D effects and high-resolution images without needing heavy, bulky lenses.
Quantum Computers: In the world of quantum computing, information is often stored in the "twist" of light. This device could be a tiny switch to turn that information on and off.
Chemistry: Some molecules are "left-handed" and some are "right-handed" (like your hands). This twisted light could be used to pick out specific molecules for making medicines, acting like a key that only fits one specific lock.

The Bottom Line

The scientists successfully built a microscopic factory that turns electricity into a swirling, twisting beam of light. They did it by using a tiny golden screw and a quantum "jump" of electrons. It's a breakthrough because it proves we can control the shape and spin of light at the smallest scale possible, opening the door to smarter, smaller, and more powerful future technologies.

1. Problem Statement

Chirality (handedness) is a fundamental property crucial for light-matter interactions in physics, chemistry, and biology. Generating chiral light (carrying both Spin Angular Momentum, SAM, and Orbital Angular Momentum, OAM) at the nanoscale is a significant challenge for next-generation technologies like AR/VR, quantum information processing, and enantioselective photochemistry.

Current Limitations: Existing methods rely on external magnetic fields, complex chiral molecular designs, or optical excitation of emitters. These approaches often suffer from weak chiral responses, lack of miniaturization, or the inability to be electrically driven.
The Gap: While quantum tunneling in metal-insulator-metal (MIM) junctions is a known mechanism for broadband electroluminescence, achieving electrically-driven chiral light emission with high handedness selectivity at the single-particle level has remained elusive.

2. Methodology

The authors developed a novel device architecture integrating chiral plasmonic nanohelicoids with MIM tunnel junctions.

Device Fabrication:
- Structure: Gold nanohelicoids (chiral plasmonic resonators) are dispersed on a 30-nm gold film.
- Barrier: A 3-nm thick aluminum oxide ( $Al_2O_3$ ) layer serves as the dielectric tunneling barrier.
- Top Electrode: A 50-nm gold layer is deposited on top, separated from the helicoids by a PMMA matrix (which acts as an insulating spacer to prevent short-circuiting while optimizing the dielectric environment).
- Operation: A bias voltage ( $V_b$ ) is applied between the top gold electrode and the bottom gold film. Electrons tunnel inelastically across the gap, exciting localized surface plasmon modes on the helicoid.
Characterization Techniques:
- Electrical: Current-Voltage (I-V) measurements to verify tunneling integrity (nonlinear, symmetric I-V curves).
- Optical: Spectral emission analysis separated into Left-Handed Circularly Polarized (LCP) and Right-Handed Circularly Polarized (RCP) components using a quarter-wave plate and calcite beam displacer.
- Simulation: Numerical modeling (FDTD/Finite-Difference Time-Domain) to analyze near-field charge distributions, far-field radiation patterns, and angular momentum conservation.

3. Key Contributions

First Electrically-Driven Nanoscale Chiral Source: Demonstrated the generation of chiral light from a single tunnel junction without external magnetic fields or optical pumping.
High Chiral Selectivity: Achieved an emission dissymmetry factor ( $g_{emit}$ ) of up to 0.8, indicating a strong preference for one circular polarization state over the other.
Generation of Vortex Beams: Proved that the emitted light carries both Spin Angular Momentum (SAM) and Orbital Angular Momentum (OAM). The OAM is equal in magnitude but opposite in sign to the SAM, satisfying angular momentum conservation.
Mechanism Elucidation: Identified that the chirality arises from the resonant excitation of chiral dipolar modes within the tunneling gap, where the helicoid's geometry imprints chirality onto the tunneling current-induced emission.

4. Key Results

Spectral Characteristics:
- The device exhibits three distinct emission peaks at approximately 620 nm, 740 nm, and 860 nm, corresponding to resonant plasmonic modes of the nanohelicoid.
- The emission spectrum is tunable with bias voltage; increasing voltage intensifies the emission and causes a slight blue shift due to changes in the spectral profile of the tunneling current source.
Chiral Performance:
- The emission dissymmetry factor ( $g_{emit} = 2(I_{LCP} - I_{RCP}) / (I_{LCP} + I_{RCP})$ ) reaches a maximum of ~0.8 around the 760 nm peak.
- The high $g_{emit}$ is attributed to the suppression of the RCP component at 760 nm. Simulations reveal that the top gold electrode acts as a mirror, reflecting RCP emission from the air side back into the substrate with a handedness flip, constructively enhancing LCP emission in the substrate.
Orbital Angular Momentum (OAM):
- Numerical simulations and Fourier microscopy confirm that the emitted light forms vortex beams.
- At the resonant peaks, the light possesses a phase variation of $0$ to $\pm 2\pi$ around the beam axis.
- Conservation Law: The tunneling event (initially an achiral electric dipole) excites chiral dipolar modes. To conserve angular momentum, the generated SAM (handedness) is compensated by an equal and opposite OAM ( $l = \pm 1$ ).
Control Experiments:
- Simulations of a non-chiral cube of identical size showed the same spectral peaks but zero chiral response, confirming that the helicoid morphology is essential for the effect.

5. Significance and Impact

Fundamental Physics: This work bridges the gap between quantum tunneling processes and chiral nanophotonics, demonstrating that inelastic electron tunneling can drive complex photonic states (SAM + OAM) at the single-particle level.
Technological Advancement:
- Miniaturization: Offers a path to ultra-compact, electrically driven chiral light sources, eliminating the need for bulky external optics or magnetic fields.
- Applications:
  - Display & AR/VR: Enables high-resolution, polarization-controlled micro-displays.
  - Quantum Technologies: Provides a platform for generating entangled photon states or manipulating quantum information via spin-orbit coupling.
  - Photochemistry: Facilitates enantioselective reactions by providing localized, high-intensity chiral light for interacting with chiral molecules.
  - Sensing: Enhances capabilities for detecting chiral molecules at the nanoscale.

In summary, the paper presents a breakthrough in nanophotonics by successfully engineering a tunnel junction that converts electrical energy directly into highly chiral, vortex-carrying light, opening new avenues for integrated quantum and optical devices.