🔬 applied physics

DC-to-GHz Modulation In Microring Modulators Using Ferroelectric Nematic Liquid Crystal-on-Silicon in a Foundry Photonic Process

This study demonstrates the first ferroelectric nematic liquid crystal-coated microring modulator in a foundry photonic process, achieving a record-breaking 7.8 GHz electro-optic bandwidth and high power efficiency by utilizing a poling-free, CMOS-compatible platform that overcomes previous scalability and speed limitations of silicon-organic hybrid devices.

Original authors: Iman Taghavi, Alexander Tofini, Edward K. Leung, Cory Pecinovsky, Nicolas A. F. Jaeger, Lukas Chrostowski, Sudip Shekhar

Published 2026-02-25

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CC BY 4.0

Original authors: Iman Taghavi, Alexander Tofini, Edward K. Leung, Cory Pecinovsky, Nicolas A. F. Jaeger, Lukas Chrostowski, Sudip Shekhar

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 massive, ultra-fast city of light (a photonic chip) where data travels as beams of light instead of electricity. To make this city work, you need tiny traffic lights called modulators that can turn the light on and off incredibly fast to send messages.

The problem is, the current "traffic lights" have some major flaws:

The "Paint" Problem: Some high-speed materials (like organic polymers) need a special, high-temperature "poling" process to work. It's like trying to paint a delicate house while the paint is still drying; if you try to paint one room, you accidentally ruin the paint in the next room. This makes it hard to build large-scale chips.
The "Slow Motion" Problem: Other materials (like standard liquid crystals) are easy to use but move too slowly, like a snail trying to keep up with a race car. They can't handle the high-speed data needed for modern internet.

The Breakthrough:
This paper introduces a new "super-material" called Ferroelectric Nematic Liquid Crystal (FN-LC). Think of this material as a smart, self-aligning gel.

Here is how the researchers used this gel to build a better traffic light (a Microring Modulator):

1. The "Self-Aligning" Gel (No Poling Needed)

Usually, liquid crystals are like a jar of marbles; you have to shake them or heat them up to get them to line up in the same direction. This new FN-LC is different. It's like magnetic marbles that naturally want to line up in a specific direction on their own.

The Analogy: Instead of needing a giant, messy construction crew (high-temperature poling) to align the molecules, you just give them a tiny, gentle nudge (a small voltage), and they snap into place instantly. This means you can build these chips in standard factories without breaking the rules of the assembly line.

2. The "Two-Speed" Engine

The magic of this material is that it has two different gears:

Gear 1 (DC/Slow): It can shift the color of the light very precisely and efficiently using a tiny amount of power. This is like a precision tuner for a guitar. It doesn't need to be fast; it just needs to be accurate and use almost no energy. This solves the problem of needing huge heaters to tune the chip, which usually wastes a lot of power and creates heat that messes up neighboring components.
Gear 2 (AC/Fast): When you hit it with a high-speed electrical signal, it snaps into action at 7.8 GHz. That's incredibly fast—fast enough to handle the data speeds of future 6G networks. It's like switching from a precision tuner to a lightning-fast strobe light.

3. The "Wide Door" Design

Usually, to get these materials to work, engineers have to squeeze them into tiny, hair-thin gaps (slots) in the silicon. It's like trying to pour thick honey through a needle; it often clogs or doesn't fill the space.

The Innovation: The researchers built a "semi-ridge" waveguide. Instead of a needle, they made a wide, open doorway (5 micrometers wide). Because the FN-LC is thick and viscous (like honey), it stays exactly where you put it and doesn't run away. This made it much easier to coat the chip and ensured the material filled the space perfectly.

4. The Result: A Super-Efficient Chip

By combining this self-aligning gel with a clever wide-door design, they created a chip that:

Uses almost no power to stay tuned (saving energy).
Switches incredibly fast (handling high-speed data).
Is easy to build in standard factories (no special high-heat steps needed).
Is robust: They tested it with heat, humidity, and high power, and it didn't degrade. It's like building a car engine that runs on water and never overheats.

Why Does This Matter?

Currently, building complex optical computers or massive data centers is hard because the components are too big, use too much power, or are too difficult to manufacture.

This paper proves that we can now make tiny, energy-efficient, high-speed optical switches that are compatible with the same factories that make your smartphone chips. It's a major step toward a future where our computers and internet are faster, cooler, and use a fraction of the energy they do today.

In a nutshell: They found a liquid crystal that aligns itself, works at two speeds (slow & precise, fast & powerful), and is easy to pour onto a chip, solving the biggest headaches in building the next generation of light-based computers.

1. Problem Statement

The integration of Silicon-Organic Hybrid (SOH) platforms into commercial photonic foundries faces two primary hurdles:

Electro-Optic (EO) Polymers (EOPs): While offering high speed, they require a complex, high-temperature electro-thermal poling process to align molecules. This process is difficult to scale, incompatible with many back-end-of-line (BEOL) steps, and suffers from thermal instability and leakage currents. Furthermore, selective poling of specific devices on a chip has not been demonstrated.
Paraelectric Nematic Liquid Crystals (PN-LCs): These are poling-free and suitable for large-scale integration but suffer from slow switching speeds (limited to the tens of MHz range) due to reliance on molecular reorientation, making them unsuitable for high-speed data transmission.
Microring Modulators (MRMs): In native silicon photonics, MRRs often require power-hungry thermal tuners for resonance stabilization, leading to thermal crosstalk and increased power consumption, which limits Very-Large-Scale Integration (VLSI).

The authors aim to bridge this gap by utilizing Ferroelectric Nematic Liquid Crystals (FN-LCs), which possess spontaneous polarization and a non-zero second-order susceptibility ( $\chi^{(2)}$ ), offering a poling-free, high-speed, and energy-efficient alternative.

2. Methodology

The study involved the design, fabrication, and characterization of a custom Microring Modulator (MRM) integrated with FN-LC material in a commercial foundry process.

Material: The team used pm-158, a proprietary FN-LC developed by Polaris Electro-Optics. This material exhibits high viscosity (allowing localized deposition) and spontaneous polarization at room temperature.
Device Architecture:
- Waveguide: A semi-ridge, non-slotted waveguide was designed on a standard Silicon-on-Insulator (SOI) platform (220 nm thick core). Unlike traditional slot waveguides (which are hard to infiltrate and require complex doping), this design features a 5 $\mu$ m-wide oxide opening.
- Electrode Configuration: The design places a metal electrode over a lightly doped slab layer adjacent to the undoped core. This geometry optimizes the electric field overlap ( $\Gamma$ ) with the optical mode while minimizing doping-induced optical loss.
- Fabrication: Devices were fabricated at Advanced Micro Foundry. A critical step involved a surface-cleaning protocol (oxygen plasma) and a controlled thermal process to infiltrate the FN-LC into the nanogaps without degrading the material.
Packaging: To demonstrate scalability, a Photonic Integrated Circuit (PIC) containing 58 modulators was packaged using Photonic Wire Bonds (PWB). This technique uses free-form 3D waveguides to connect fiber arrays to on-chip edge couplers, bypassing the limitations of edge coupling or grating couplers.
Characterization: The device was tested under DC bias and RF signals (up to 11 GHz) to evaluate both the slow (birefringence) and fast (Pockels effect) phase-shifting mechanisms.

3. Key Contributions

First FN-LC MRM: This is the first demonstration of an MRM coated with Ferroelectric Nematic Liquid Crystal.
Poling-Free Operation: The device leverages the spontaneous polarization of FN-LC, eliminating the need for the complex electro-thermal poling required by EOPs. Alignment is achieved simply by applying a low DC electric field.
Dual-Mode Modulation: The study successfully exploits two distinct phase-shifting mechanisms within the same device:
1. DC/Slow Mode: Driven by molecular orientation (birefringence), providing high efficiency for resonance tuning.
2. AC/Fast Mode: Driven by the Pockels effect ( $\chi^{(2)}$ ), enabling GHz-speed modulation.
CMOS-Compatible Foundry Process: The work demonstrates a viable path for integrating advanced organic materials into standard foundry processes using simplified deposition and packaging techniques (PWB).

4. Key Results

Bandwidth: The device achieved an electro-optic (EO) bandwidth of $f_{-6dB} \approx 7.8$ GHz. This is the highest reported value for an SOH MRM to date (surpassing the previous record of ~1.34 GHz for Pockels-based organic materials).
Modulation Efficiency:
- DC Detuning: Achieved a linear resonance shift of $\approx 150$ pm/V across the full width at half maximum (FWHM).
- Static Power: The static power efficiency is approximately 4.5 nW/ $\pi$ (calculated as $V_\pi \times I_{stat}$ ), with a $V_\pi L$ product of 3.33 V $\cdot$ mm. This eliminates the need for power-intensive thermal heaters.
Voltage: The device operates at a low driving voltage of $V_{pp} \approx 2.83$ V for RF modulation.
Dual-Bandwidth Observation: Frequency response analysis revealed two distinct roll-offs:
- A low-frequency cutoff at $\approx 119$ kHz (attributed to dipolar orientation/PN-LC-like behavior).
- A high-frequency cutoff at $\approx 7.8$ GHz (attributed to the Pockels effect).
Reliability: The device showed no degradation under elevated optical power ( $\approx 8$ dBm) and electrical power ($13$ dBm) or after thermal cycling (5°C to 55°C). The high viscosity of the FN-LC provided inherent protection against oxidation.
Footprint: The total device footprint is $\approx 0.084$ mm $^2$ with an on-chip optical insertion loss of $\approx 0.78$ dB.

5. Significance

This work represents a significant breakthrough in hybrid silicon photonics by solving the "poling vs. speed" trade-off that has hindered SOH adoption.

Scalability: By removing the need for electro-thermal poling, the FN-LC platform allows for the selective coating of individual components on a chip, a prerequisite for VLSI photonic circuits.
Energy Efficiency: The ability to tune resonance using DC bias without thermal heaters drastically reduces power consumption and eliminates thermal crosstalk, enabling dense integration of switch banks and weight banks for optical computing and communications.
Performance Benchmark: Achieving 7.8 GHz bandwidth in an MRM proves that FN-LCs are viable for high-speed data transmission, moving beyond the limitations of traditional liquid crystals.
Future Outlook: The authors suggest that further optimization (e.g., reducing electrode spacing, adding charge barrier layers, or using inkjet printing) could push the bandwidth beyond 40 GHz, making this platform a strong candidate for next-generation coherent optical transceivers and reconfigurable photonic systems.