🔬 optics

Topological Valley-Reshaped Device: Bifunctional Waveguiding and Single-Beam Leaky-Wave Radiation for Terahertz Communication

This paper presents a bifunctional topological valley-reshaped device that simultaneously enables low-loss, high-speed terahertz waveguiding and directional single-beam leaky-wave radiation, overcoming the challenge of extracting topological edge states into free space without auxiliary couplers.

Original authors: Yulun Wu, Ziwei Wang, Faqian Chong, Hua Shao, Bingtao Gao, Shilong Li, Jin Tao, Hongsheng Chen, Song Han

Published 2026-03-31

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CC BY 4.0

Original authors: Yulun Wu, Ziwei Wang, Faqian Chong, Hua Shao, Bingtao Gao, Shilong Li, Jin Tao, Hongsheng Chen, Song 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 you are trying to build a super-fast highway for data, but instead of cars, you are sending invisible beams of light (specifically, Terahertz waves) that can carry massive amounts of information.

The problem is that building these highways on a tiny computer chip is like trying to drive a race car on a bumpy dirt road. The signal gets lost, slows down, or crashes into walls. Scientists have been trying to fix this using "Topological Photonics," which is like building a magical road where the cars (signals) are so protected by physics that they can't fall off the edge, no matter how sharp the turn is.

However, there was one big missing piece: How do you get the car off the highway and onto the open road (free space) without building a giant, clunky ramp?

This paper introduces a brilliant new invention: a "Shape-Shifting Highway" that does two jobs at once.

The Two Jobs of the Magic Device

Think of this device as a Swiss Army Knife for light. It has two distinct modes:

Mode A: The Invisible Highway (Waveguiding)
- The Analogy: Imagine a train running on a track that is invisible to the outside world. The train moves incredibly fast and doesn't lose any energy, even if the track bends sharply.
- What it does: This part of the device keeps the data signal locked inside the chip, moving it from Point A to Point B with almost no loss. It's perfect for moving data inside your computer.
Mode B: The Spotlight (Radiation)
- The Analogy: Now, imagine that same train track suddenly ends, but instead of crashing, the train gently lifts off the tracks and turns into a focused spotlight beam that shoots straight out into the air.
- What it does: This part takes the signal and beams it out into the air to talk to other devices wirelessly. It does this without needing any extra "ramps" or "tapers" (which usually cause signal loss).

The Secret Sauce: "Valley Reshaping"

How did they make one piece of silicon do both jobs? They used a concept called "Valley Photonics."

The Metaphor: Imagine the surface of the chip is a mountain range with deep valleys. The data signal is a ball rolling in one of these valleys.
The Trick: Usually, if you want the ball to fly out of the valley, you have to build a ramp. But this team realized that if you cut the valley at a specific angle (like slicing a piece of cake), the ball naturally rolls out in a straight line.
The "Valley Reshaping": To make sure the ball flies out cleanly (like a laser beam) and doesn't scatter everywhere (like a messy spray of water), they slightly changed the shape of the other side of the valley. This acts like a guardrail that stops the ball from bouncing back or going the wrong way.

Why is this a Big Deal?

No Extra Parts: Previous methods needed extra, bulky parts to get the signal out. This device is just one piece of silicon. It's like having a car that can drive on the highway and then instantly turn into a plane without needing a separate runway.
Super Fast: They tested it and it worked perfectly at speeds up to 60 Gbps (that's downloading a whole movie in seconds) while the signal was on the chip, and 24 Gbps when beaming wirelessly.
Stable Beam: Usually, when you beam light out, the direction changes as the speed changes (like a wobbly flashlight). This device is so stable that the beam stays pointed in the exact same direction, even as the speed changes. It's like a laser pointer that never wobbles, no matter how fast you move it.

The Real-World Impact

Think about the future of 6G and super-fast wireless internet. Right now, connecting a chip to the air is messy and loses a lot of power.

This new device is like a universal translator between the world inside your computer chip and the world outside. It allows chips to talk to each other wirelessly with the same efficiency as if they were connected by a wire.

In short: They built a tiny, magical silicon chip that acts as a perfect highway for data inside a computer, and then seamlessly transforms into a powerful, focused spotlight to beam that data out into the world—all without needing any extra tools or losing speed. It's a major step toward making our future devices smaller, faster, and more connected.

1. Problem Statement

Terahertz (THz) communication is a promising technology for future 6G systems due to its broad spectrum and high data rates. However, integrating THz systems on-chip faces two major bottlenecks:

High Loss and Fabrication Challenges: Conventional metal-based waveguides suffer from severe ohmic losses at THz frequencies. While topological valley photonics offers robust, low-loss waveguiding, extracting these "valley-locked" edge states into directional free-space radiation remains difficult.
Lack of Bifunctionality: Existing solutions for radiation often require auxiliary couplers (e.g., tapers, graded refractive index buffers) which introduce extra loss, complexity, and fabrication errors. Furthermore, previous topological antennas cannot seamlessly revert to waveguiding mode, and existing waveguides cannot radiate efficiently without losing their guided-wave properties.
Performance Limitations: Current topological antennas often suffer from poor directionality, high side lobes, or weak gain, and many operate only in the microwave regime due to fabrication limits at THz frequencies.

2. Methodology

The authors propose a Topological Valley-Reshaped Device that achieves bifunctionality (waveguiding and radiation) from a single topological platform without auxiliary couplers.

Design Principle:
- Base Structure: A silicon-based topological photonic crystal waveguide (TW) with an AB-type interface (formed by joining mirror-symmetric domains at larger triangular air holes). This supports robust valley-locked edge states.
- Truncation Strategy: The waveguide is truncated at a specific 60° angle. This truncation breaks the symmetry and allows the valley-locked edge state to transition into a leaky mode by satisfying the momentum-matching condition with free space (the light line).
- Valley Reshaping: To suppress reflections and side lobes, the device incorporates a BA-type interface (formed at smaller triangular air holes) as the input section. The transition from BA-type to AB-type edge states is engineered to:
  - Match impedance to reduce reflection (exploiting the opposite group velocities of AB and BA states).
  - Reshape the phase gradient to suppress parasitic side lobes.
- Parameter Tuning: Different lattice parameters ( $\delta = l_1 - l_2$ ) are assigned to the guided section ( $\delta = 0.25a$ ) and the leaky section ( $\delta = 0.3a$ ) to maximize the overlapping effective bandwidth.
Fabrication:
- Devices were fabricated on 200-µm-thick high-resistivity silicon wafers using deep reactive ion etching (DRIE).
- The structure consists of rhombic unit cells with varying triangular air hole sizes.
Experimental Setup:
- Waveguiding Test: Measured using a Vector Network Analyzer (VNA) with WR-2.8 waveguides.
- Radiation Test: Far-field patterns and gain were measured using a horn antenna and a mechanical rail system.
- Communication Demo: A THz communication link was established using a 16QAM/60 Gbps system for the waveguide and a QPSK/24 Gbps system for the wireless link.

3. Key Contributions

First Bifunctional Topological Device: Demonstrated a single device that functions as a low-loss topological waveguide (TW) and, when truncated, as a high-gain topological leaky-wave antenna (TLWA) without any external couplers.
Coupler-Free Directional Radiation: Achieved directional single-beam radiation directly from the valley-locked edge state via angular truncation and valley reshaping, eliminating the need for complex tapering or buffer layers.
Simultaneous Suppression of Reflection and Side Lobes: By utilizing the unique coupling characteristics between BA-type and AB-type edge states, the device achieves a reflection coefficient below -10 dB and a Side-Lobe Suppression Ratio (SLSR) exceeding 15 dB.
Low-Dispersion Radiation: Leveraged the K-valley radiation property to achieve a main-lobe direction that varies by only 2° across the entire operating band, a significant improvement over conventional leaky-wave antennas.

4. Key Results

Waveguiding Performance (TW):
- Operating bandwidth: 18 GHz (0.333–0.351 THz).
- Average insertion loss: -1.3 dB.
- Communication: Error-free transmission up to 60 Gbps (16QAM) with BER below the SD-FEC threshold.
Radiation Performance (TLWA):
- Operating bandwidth: 19 GHz (0.331–0.35 THz).
- Peak realized gain: 12.5 dBi.
- Beamwidth: 3-dB beamwidth of 7°.
- SLSR: >15 dB across the band.
- Dispersion: Main-lobe direction stability of 2° across the band.
- Communication: Error-free free-space reception at 24 Gbps (QPSK) over 2.5 mm distance.
Comparison: The device outperforms previous silicon-based THz antennas in terms of bandwidth, gain, and the ability to function as both a waveguide and an antenna without auxiliary structures.

5. Significance

This work represents a critical step toward highly integrated and modular terahertz on-chip systems. By solving the long-standing challenge of extracting topological edge states into free space while preserving waveguiding capabilities, the proposed device enables:

Seamless Integration: The ability to switch between on-chip routing and off-chip communication within the same physical component.
Simplified Fabrication: Elimination of complex couplers reduces fabrication complexity and improves yield.
6G Enabler: The demonstrated high data rates (60 Gbps guided, 24 Gbps wireless) and robust performance make this technology a strong candidate for future 6G transceivers and flexible photonic circuits.

In summary, the paper introduces a novel "valley-reshaping" strategy that transforms a standard topological waveguide into a high-performance, bifunctional THz transceiver component, bridging the gap between robust on-chip signal routing and efficient free-space communication.