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Long-range evanescent coupling through photonic molecules

This paper demonstrates efficient long-range evanescent coupling between distant waveguides via resonant excitation of higher-order photonic molecular states in femtosecond laser-written structures, enabling applications in photonic links and topological physics.

Original authors: Romina Abarca-Ramírez, Diego Román-Cortés, Maxim Mazanov, Vlad Simonyan, Konstantin Rodionenko, Maxim A. Gorlach, Rodrigo A. Vicencio

Published 2026-06-18

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Romina Abarca-Ramírez, Diego Román-Cortés, Maxim Mazanov, Vlad Simonyan, Konstantin Rodionenko, Maxim A. Gorlach, Rodrigo A. Vicencio

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

The Big Idea: Connecting Distant Islands with a "Bridge"

Imagine you have two islands (light-carrying channels called waveguides) that are very far apart. In the world of light, these islands usually cannot talk to each other because the "signal" (light) fades away exponentially the further it travels through empty space. It's like trying to whisper to someone standing 100 meters away; the sound just dies out before it reaches them.

Usually, to connect them, you would have to build a long, winding road (a curved waveguide) between them. But building these roads is tricky, expensive, and causes the signal to get lost (attenuation) along the way.

The Solution: The researchers in this paper found a way to build a "magic bridge" without physically connecting the two islands with a road. They used a special structure called a Photonic Molecule.

What is a "Photonic Molecule"?

Think of a single waveguide as a single musical instrument, like a flute. It can only play one specific note (a basic light state).

Now, imagine putting many flutes very close together to form a giant, complex instrument. This group acts like a single, massive "Photonic Molecule." Because it is so large and complex, it can play many different, complex notes (called higher-order states). Some of these notes are like simple tunes, while others are complex, swirling patterns that stretch across the whole group.

The Magic Trick: Tuning the Radio

The researchers discovered a way to make the single flute (the isolated waveguide) talk to the giant instrument (the molecule) without touching it.

The Setup: They placed a single waveguide next to a long line of many waveguides (the molecule), but left a small gap between them.
The Tuning: Just like tuning a radio to find a specific station, they changed the color (wavelength) of the light they sent into the single waveguide.
The Resonance: When they hit the "right" color, the light in the single waveguide matched the "frequency" of one of the complex notes in the giant molecule.
The Jump: Suddenly, the light didn't just fade away; it jumped across the gap and excited a specific, long-standing pattern inside the molecule. It was as if the single flute whispered a note, and the giant instrument instantly started playing a complex symphony in perfect harmony.

What They Actually Did

The Experiment: They built these structures inside a piece of glass using a super-fast laser (like a microscopic 3D printer).
The Distance: They successfully made light jump across a gap of 127 micrometers (about the width of a human hair). In the world of light chips, this is a huge distance. Without this trick, the light would never make it across.
The Efficiency: By picking the exact right color of light and the exact right length of the glass, they managed to transfer about 85% of the light energy from one side to the other.

A New Application: The "Topological Edge"

The researchers also used this "bridge" idea to create a special kind of traffic jam for light.

Imagine a row of houses (a lattice). Usually, if you knock on the door of the first house, the sound travels down the line and fades out. However, by using these "molecule bridges" to connect houses that are far apart (specifically, connecting a house to the third house over, skipping the middle one), they changed the rules of the neighborhood.

They found that when they knocked on the first house (the edge), the sound stayed there. It didn't travel down the line; it got "trapped" at the edge. This is called a Topological Edge State. It's like a ball rolling on a track that suddenly hits a wall it can't pass, so it just bounces back and stays right where it started. This proves that by using these long-range connections, they can force light to behave in ways that are normally impossible.

Summary of Claims

Long-Range Connection: They proved that light can jump large gaps between waveguides by using a "photonic molecule" as a middleman.
Resonance is Key: This only works if you tune the light to a specific color that matches the "song" of the molecule.
Practical Use: This could be a better way to connect optical fibers to computer chips without needing messy, curved wires that lose signal.
New Physics: They used this method to create a "trapped" light state at the edge of a grid, proving that long-range connections can create new, exotic behaviors in light systems.

What they did NOT claim: They did not claim this works for clinical medical uses, quantum computing (though they mentioned it as a future possibility for research), or that it solves all fiber-optic problems immediately. They strictly demonstrated the physics of the connection and the trapping of light in a controlled lab setting.

Technical Summary: Long-range evanescent coupling through photonic molecules

Problem Statement
In photonic systems utilizing evanescent coupling, such as artificial lattices of optical waveguides, interactions are inherently limited to nearest neighbors due to the exponential decay of wavefunctions. This "tight-binding" nature imposes a cutoff on long-range interactions and restricts the propagation speed of excitations (Lieb-Robinson bound), thereby preventing the realization of certain topological and transport phenomena that rely on long-range coupling. While numerical proposals and experiments in acoustic or specific photonic lattice configurations (e.g., moiré lattices, epsilon-near-zero materials) have explored distant coupling, achieving efficient long-range evanescent coupling between standard single-mode optical waveguides—the fundamental components of photonic chips—has remained elusive. This limitation complicates practical applications, such as coupling optical fiber signals (typically spaced at 127 µm) into photonic chips, which currently requires lossy fan-in/fan-out mechanisms involving curved waveguides.

Methodology
The authors propose and demonstrate a method to bridge this gap by utilizing "photonic molecules"—long, one-dimensional structures composed of multiple single-mode waveguides—to mediate coupling between isolated waveguides.

Theoretical Framework: The study employs numerical simulations to model the guided modes of an isolated elliptical waveguide and a wider 1D photonic molecule (composed of $N=31$ waveguides). The researchers analyze the propagation constants ( $k_z$ ) of both structures, identifying regions where the propagation constant of an isolated waveguide mode matches that of a higher-order molecular state ( $\Delta k_z \approx 0$ ). This resonance condition is tunable via the excitation wavelength ( $\lambda$ ) and structural parameters.
Experimental Implementation: Photonic structures were fabricated inside borosilicate glass wafers using the femtosecond (fs) laser writing technique. The experimental setup involved a single-mode waveguide positioned at a lateral distance ( $d$ ) from a long photonic molecule.
Characterization: A supercontinuum laser source was used to sweep the excitation wavelength across a broad range ( $\lambda \in \{600, 840\}$ nm). The system was characterized by measuring output intensity profiles and normalized power transfer at various propagation distances ( $z$ ) and separation distances ( $d$ ).

Key Contributions and Results

Resonant Excitation of Higher-Order States: The authors successfully demonstrated the resonant excitation of spatially extended, higher-orbital photonic molecular states (e.g., 9th through 12th order modes) using a single-mode waveguide. By tuning the wavelength, they achieved a match between the isolated waveguide's fundamental mode and specific molecular states, resulting in efficient energy transfer.
Long-Range Coupling Link: The study validated the concept of using photonic molecules as "long-distance photonic links." The researchers demonstrated strong coupling between two single-mode waveguides separated by 127 µm (a standard fiber spacing) by inserting a photonic molecule between them.
- At specific resonant wavelengths (e.g., $\lambda = 770$ nm), the system achieved a transfer efficiency of approximately 85% from the input waveguide to the output waveguide via the molecular link.
- The coupling efficiency was shown to be dependent on both the excitation wavelength and the propagation distance, with optimal transfer occurring at specific $z$ values corresponding to the coupling strength.
Topological Edge State Excitation: The concept was extended to a one-dimensional lattice where photonic molecules acted as third-order next-neighbour couplers ( $V_{lr}$ $V_{l r}$ ).
- In a "trivial" lattice (without long-range links), the system exhibited standard discrete diffraction.
- In a "non-trivial" lattice (with long-range links activated), the system supported topological edge states. Excitation at the lattice edge resulted in the strong localization of energy at the edge, a hallmark of topological phases, which was observed experimentally at specific wavelengths (e.g., $\lambda = 725$ nm).

Significance and Claims
The paper claims to resolve the challenge of coupling distant optical waveguides without the need for complex, lossy fan-in/fan-out geometries. By matching propagation constants between isolated waveguides and extended molecular states, the authors provide a concrete solution for:

Fiber-based photonic chips: Enabling direct coupling of optical fiber signals (spaced at 127 µm) into photonic circuits.
Topological Physics: Facilitating the study of topological phases emerging from long-range interactions in otherwise trivial 1D lattices.
Fundamental Studies: Offering a platform for investigating the interaction between initially uncoupled systems.

The authors emphasize that their findings demonstrate effective long-range evanescent coupling, which could serve as a practical tool for future photonic devices and the exploration of long-range interaction physics. They suggest that this approach could be extended to study quantum information extraction between distant, isolated systems.