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Optomechanical Levitation and Control of High Aspect Ratio Silicon Nanorods

This paper reports the successful optomechanical levitation and control of high-aspect-ratio silicon nanorods, demonstrating tunable oscillation frequencies and high rotation rates that pave the way for precision torque sensing and future tests of quantum physics.

Original authors: Zhenan Chen, Sophie Sanghera, Yanhui Hu, James Sabin, Maryam Nikkhou, Alex Gee, David Burt, Qiongyuan Wu, Joseph Kelly, James Millen

Published 2026-08-11

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CC BY 4.0

Original authors: Zhenan Chen, Sophie Sanghera, Yanhui Hu, James Sabin, Maryam Nikkhou, Alex Gee, David Burt, Qiongyuan Wu, Joseph Kelly, James Millen

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 a world where you can pick up a speck of dust and hold it perfectly still in mid-air, not with tweezers, but with a beam of light. This isn't magic; it's a field of physics called "optomechanics," where scientists use lasers to trap tiny objects like marbles in a invisible bowl of energy. For years, researchers have been trapping round balls, but nature is full of weird shapes like sticks, needles, and rods. The big question is: what happens if we trap something that isn't a ball? If you spin a round ball, it looks the same from every angle. But if you spin a rod, it looks different as it turns. This difference is the key. By trapping these rod-shaped particles, scientists can measure tiny twists and turns with incredible precision, and they might even be able to test the weirdest rules of the universe—quantum mechanics—by spinning these rods so fast they exist in two places at once.

In this paper, a team of scientists from King's College London and the University of Glasgow decided to build the ultimate "light trap" for silicon rods. They didn't just find some random sticks; they manufactured thousands of them in a lab, making sure they were all nearly identical, like a factory line of perfect pencils. These rods are incredibly small, with diameters as tiny as 50 nanometers (that's about 2,000 of them lined up to equal the width of a human hair) and lengths up to 1,500 nanometers. The researchers used a powerful laser to levitate these rods in a vacuum chamber, essentially holding them up against gravity with light.

What they found was exciting. They discovered that by adjusting the power of the laser, they could make these rods wiggle back and forth at frequencies ranging from 10,000 times a second (10 kHz) to over one million times a second (1 MHz). Even cooler, they figured out how to make the rods spin. By changing the laser's polarization (a property of light that makes it twist), they applied a massive amount of "optical torque," or twisting force, to the rods. This made them spin at incredible speeds, reaching rotation rates well above 1 MHz at certain pressures.

The team also tested how stable these floating rods were. They found that if the air pressure got too low, the rods would sometimes fly out of the trap, but they could keep them stable by using more laser power. They compared their real-world measurements with mathematical models and found that while the models worked well for spinning, the rods wobbled a bit more than expected when they were just sitting still, likely because they weren't perfectly aligned with the laser.

Ultimately, this work proves that we can control these high-tech silicon rods with extreme precision. The authors suggest that because these rods are so uniform and spin so fast, they are perfect candidates for future experiments. Specifically, they hope to use them to create "superposition states" in rotation—where a single rod spins in two different directions at the same time. This would be a giant leap toward testing the fundamental laws of physics, turning a tiny silicon rod into a gateway for understanding the quantum world.

Technical Summary: Optomechanical Levitation and Control of High Aspect Ratio Silicon Nanorods

Problem Statement
Optically levitated nanoparticles represent a frontier for precision sensing and tests of fundamental physics, including the exploration of macroscopic quantum superposition states. While translational motion has been extensively studied, controlling the angular degrees of freedom of anisotropic particles offers distinct advantages: higher oscillation frequencies (facilitating deep quantum cooling), enhanced torque sensitivity, and the potential to generate angular momentum superposition states without the need for diffracting elements. However, realizing these capabilities requires particles that are highly uniform, possess high refractive indices, and exhibit strong shape anisotropy to maximize optical torque. Existing spherical particles lack the necessary alignment for angular readout, and previous attempts with silicon have faced challenges regarding absorption-induced instability. This work addresses the need for a reproducible platform of high-aspect-ratio silicon nanorods to enable precise control over both translational and rotational dynamics.

Methodology
The authors fabricated silicon nanorods with aspect ratios ranging from 2 to 30 using electron beam lithography and inductively coupled plasma (ICP) etching on silicon (100) wafers. The process involved defining silica islands as masks, etching pillar structures, and introducing a base narrowing to facilitate loading via Laser-Induced Acoustic Desorption (LIAD). The resulting nanorods feature diameters as small as 50 nm and lengths up to 1500 nm, with demonstrated uniformity at the percent level.

The experimental setup utilizes a counter-propagating optical tweezer operating at 1550 nm within a vacuum chamber. The system employs high-speed fiber electro-optic polarization controllers to switch between linear and circular polarization states. Nanorods are loaded into the trap at pressures of a few mbar and subsequently characterized across a pressure range from near-atmospheric down to the high-vacuum regime. Motion is detected via balanced photodetectors and a quadrant photodiode (QPD), measuring five degrees of freedom: three translational ( $x, y, z$ ) and two librational ( $\alpha, \beta$ ). Theoretical models based on Langevin equations and Rayleigh-Gans approximations for cylindrical particles were used to predict trapping frequencies, momentum damping rates, and optical torques.

Key Contributions and Results

Fabrication and Characterization: The study successfully produced highly uniform silicon nanorods. Dimensional analysis of 30 rods (nominal $1000 \times 180$ nm) revealed a uniformity of $(1024 \pm 16) \times (174 \pm 2)$ nm, significantly better than the $\sim15\%$ dispersion typical of commercial spherical silica nanoparticles.
Trapping Dynamics and Frequency Tuning: The authors demonstrated stable levitation of nanorods from below 1 mbar to near-atmospheric pressures. They verified that trapping frequencies scale with the square root of optical power ( $f \propto \sqrt{P}$ ), as predicted by the harmonic potential model. By adjusting laser power, oscillation frequencies were tuned from 10 kHz to over 1 MHz. The high aspect ratio and refractive index of silicon ( $\epsilon_R = 12.1$ ) resulted in significantly higher trapping frequencies compared to silica.
Momentum Damping: Momentum damping rates were extracted across a wide pressure range. In the low-pressure free-molecular flow regime ( $< \sim50$ mbar), the damping rates for rotational modes ( $\alpha, \beta$ ) and the $y$ -translational mode agreed well with theoretical predictions. Discrepancies in the $x$ and $z$ translational modes were attributed to large libration angles causing mixing between the particle's body frame and the laboratory frame. At higher pressures, damping rates plateaued, deviating from the free-molecular model.
Optically Driven Rotation: By switching the trapping light to circular polarization, the authors applied large optical torques, driving the nanorods into rapid rotation. Rotation frequencies exceeding 1 MHz were achieved at 100 mbar. The maximum rotation frequency was shown to be consistent with the balance between optical torque and gas damping, provided an empirical model was used to correct for high-pressure damping effects.
Stability Limits: The study investigated trapping instability at low pressures, observing that nanorods escape the trap below a critical pressure. This loss pressure was found to increase approximately linearly with trapping power in the low-power regime, suggesting a balance between absorption-induced heating and gas-cooling. Notably, the silicon nanorods remained stable at significantly lower pressures than previously reported for silicon nanospheres under similar intensities.

Significance
The paper establishes that optically levitated silicon nanorods provide a robust platform for studying rotational dynamics and precision torque sensing. The combination of high geometric uniformity, megahertz-range librational frequencies, and strong optical torque enables the generation of high rotation rates. The authors posit that these systems are promising candidates for future experiments aiming to generate and detect angular momentum superposition states, a feat that requires precise initialization and readout of alignment—capabilities uniquely afforded by these anisotropic particles. Furthermore, the demonstrated ability to tune frequencies and exert high torque opens avenues for testing quantum physics in the rotational domain and exploring non-equilibrium rotational dynamics.

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