🔬 optics

Trapping of nanofabricated SiO $_2$ high-aspect-ratio discs in an optical lattice

This paper reports the successful nanofabrication, high-efficiency loading, and optical trapping of customized high-aspect-ratio silicon dioxide discs in a vacuum, demonstrating a scalable method for creating particles suitable for high-frequency gravitational wave detection and sensitive accelerometry.

Original authors: Shelby Klomp, Shafaq Gulzar Elahi, Scott Grudichak, Chethn Galla, Zhiyuan Wang, Mark Nguyen, George Winstone, Katarina Boskovic Guy, Emma Derrick, Maddox Wroblewski, Jacob Sprague, Andrew Laeuger, Jac

Published 2026-10-19

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CC BY 4.0

Original authors: Shelby Klomp, Shafaq Gulzar Elahi, Scott Grudichak, Chethn Galla, Zhiyuan Wang, Mark Nguyen, George Winstone, Katarina Boskovic Guy, Emma Derrick, Maddox Wroblewski, Jacob Sprague, Andrew Laeuger, Jackson Larsen, Nancy Aggarwal, Shane L. Larson, Andrew A. Geraci

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

In the quiet corners of modern physics, researchers are learning to hold the smallest things in the air without touching them. This field, known as levitated optomechanics, uses the gentle push of light to suspend tiny objects in a vacuum, keeping them isolated from the vibrations and heat of the world around them. By trapping these objects with lasers, scientists can measure forces with incredible precision, searching for the faintest whispers of the universe, such as the ripples in space-time caused by distant cosmic events. For years, this work has relied on tiny spheres, like microscopic marbles, because they are easy to make and catch in a beam of light. However, to detect the highest frequencies of gravitational waves or to act as ultra-sensitive accelerometers, scientists need objects with different shapes—specifically, flat, disc-like structures that can interact with light in unique ways. The challenge has been that making these flat discs with the perfect size and shape is difficult, and catching just a few of them in a trap has proven nearly impossible with older methods.

A team of researchers has now solved this problem by combining advanced manufacturing with a delicate new way of loading particles into a trap. They created millions of tiny silicon dioxide discs, each ten micrometers wide and four hundred nanometers thick, using a top-down process similar to how computer chips are made on silicon wafers. Unlike the spheres grown from chemical solutions, which often vary in size and density, these discs are uniform and precisely engineered. The breakthrough lies not just in making them, but in getting them into the trap. Instead of spraying a cloud of particles and hoping one gets caught, the team used a specialized micropipette, a tool resembling a microscopic dropper, to pick up just a handful of these discs. They then launched these few particles, sometimes as few as five or ten, directly into a standing wave of laser light. This method proved highly efficient, allowing them to successfully trap individual discs one by one, a feat that was previously out of reach for such specific, wafer-scale manufactured objects.

Once trapped, the researchers watched how the discs moved, listening to their vibrations to understand how they behaved in the light. The discs did not just sit still; they wobbled and spun in response to the laser beams and the surrounding gas. By analyzing the sound of these movements, the team identified distinct patterns of motion: the discs sliding back and forth, side to side, and up and down, as well as rocking on their edges. They found that the flat shape of the discs made them behave differently than spheres, with some movements happening much faster than others. The team also tested how these movements changed as they adjusted the pressure of the gas inside the chamber. They discovered that the speed at which the discs slowed down due to air resistance matched the predictions for a flat disk, confirming that their theoretical models were correct. While they noticed some unexpected vibrations that did not fit the standard patterns, they determined these were not caused by the discs spinning freely, but were likely due to tiny imperfections on the edges of the discs or slight variations in their shape.

The significance of this work extends beyond the immediate success of trapping a single disc. The researchers demonstrated that it is possible to load optical traps with very small numbers of custom-made particles, opening the door for experiments that require specific geometries. This is crucial for future detectors of high-frequency gravitational waves, where the shape of the object can make the difference between hearing a signal or missing it entirely. The team also showed that they could cool the motion of the discs using feedback from the laser, a necessary step to reach the extreme sensitivity required for fundamental physics experiments. By proving that these high-aspect-ratio discs can be fabricated, loaded, and trapped with high efficiency, the researchers have provided a new tool for the scientific community. They have shown that the path to detecting the subtlest forces in the universe may lie not in the simplicity of a sphere, but in the precision of a flat, engineered disc, held in the light.

Technical Summary: Trapping of Nanofabricated SiO2 High-Aspect-Ratio Discs in an Optical Lattice

Problem Statement
Levitated optomechanics has traditionally relied on sub-wavelength spherical particles or larger microspheres loaded via bottom-up synthesis (e.g., Stöber process) or nebulization. While effective, these methods often suffer from size dispersion (typically ~10%), density inhomogeneities, and a reliance on large initial particle quantities for successful trapping. Furthermore, many proposed applications—such as high-frequency gravitational wave (GW) detection, torque sensing, and tests of gravity-mediated quantum entanglement—benefit from or require non-spherical geometries with precise dimensions and customized orientations. Existing top-down fabrication techniques (lithography) offer superior control over shape and uniformity but have historically been challenging to integrate with optical trapping due to the difficulty of loading small quantities of micron-sized, high-aspect-ratio objects into vacuum traps.

Methodology
The authors demonstrate a high-efficiency loading and trapping protocol for lithographically patterned, high-aspect-ratio silicon dioxide (SiO2) discs.

Particle Fabrication: The SiO2 discs (10 µm diameter, 400 nm thickness) were fabricated using a top-down lithographic process on a 100 mm Si wafer. The process involved depositing a Si3N4 etch-stop layer, an amorphous silicon (a-Si) sacrificial layer, and a 400 nm SiO2 device layer. The SiO2 was patterned via optical lithography and etched using inductively coupled plasma. The discs were released by selectively etching the a-Si layer with Tetramethylammonium hydroxide (TMAH). This method yields over 10^6 free-standing discs per wafer with high uniformity (10 ± 0.5 µm diameter, 400 ± 10 nm thickness).
Loading Mechanism: Instead of nebulizers or piezoelectric transducers requiring bulk quantities, the team utilized a micropipette-based launching method (previously reported in Ref. [55]). A micropipette with a 30 µm opening was loaded with a small number of discs (5–10 particles) and positioned 0.5–2 mm above the trap. The particles were launched via piezoelectric actuation into the trap region at a pressure of 6 mbar.
Trapping and Detection: The particles were trapped in a dual-beam counter-propagating optical standing wave (1560 nm wavelength) within a vacuum chamber. Motion was characterized using two detection schemes: (1) scattered light imaging onto quadrant photodiodes (QPD) positioned to the side and bottom of the vacuum chamber, and (2) interferometric detection using a balanced photodetector via a fiber-coupled circulator.
Characterization: The motional spectra were analyzed using SCUFF-EM (a Boundary-Element-Metho solver) to predict optical forces and torques. Experimental damping rates were extracted by fitting amplitude spectral density (ASD) peaks and compared against theoretical gas-damping models in the free-molecular regime. Preliminary 3D linear feedback cooling was also tested using 1064 nm light.

Key Results

Successful Trapping: The authors successfully trapped single high-aspect-ratio SiO2 discs using only 5–10 loaded particles per attempt, demonstrating high-efficiency loading compatible with wafer-scale nanofabrication. Up to two discs were trapped from a single loaded pipette in successive attempts.
Motional Spectra: The observed spectra exhibited peaks corresponding to translational motion (transverse $x,y$ and axial $z$ ) and librational motion ( $\theta_x, \theta_y$ ). The axial and librational frequencies were significantly higher than transverse frequencies due to the particle being confined to a single anti-node ( $t < \lambda/2$ ).
Mode Identification:
- Theoretical predictions (SCUFF-EM) aligned reasonably well with observed frequencies, though discrepancies were attributed to potential over-etching or damage during transfer.
- Additional peaks at 8 kHz and 10.2 kHz were observed. Pressure scaling tests (0.46–1.6 mbar) showed these frequencies were independent of pressure, ruling out free rotation or precession/nutation modes (which scale linearly with pressure). These were identified as additional translational or librational modes, potentially arising from slight asymmetries or particulates on the disc edges.
- Feedback cooling experiments confirmed mode identification: the 20 kHz peak responded best to axial cooling, while 1.6 kHz peaks responded to transverse cooling.
Damping Analysis: The axial damping rate ( $\Gamma_z$ ) showed a linear dependence on pressure consistent with theoretical predictions for a rigid disk in the free-molecular regime. The slope of the experimental data (168 ± 5 Hz/mbar) agreed with the theoretical range (177–193 Hz/mbar).

Significance and Claims
The paper establishes the experimental feasibility of loading optical traps with small quantities of dielectric objects produced by wafer-scale top-down nanofabrication. The authors claim this method is capable of high-efficiency trapping of customized geometries (specifically high-aspect-ratio discs) that are difficult to load using traditional bulk-loading techniques.

The study highlights the potential of these disc-like particles for specific applications:

Gravitational Wave Detection: The high mass and specific geometry of the discs are identified as promising candidates for detectors of high-frequency gravitational waves when suspended in an optical cavity. The authors calculate a thermal-noise-limited figure of merit ( $\eta_{thermal}$ ) of 340 Hz²kg/m for their trapped particle, representing an improvement of approximately a factor of 3 over previous work.
Fundamental Physics: The ability to control non-spherical particles with precise geometry supports protocols for testing gravity-mediated entanglement and other foundational quantum mechanics experiments.

The authors conclude that while the current work demonstrates successful trapping and spectral characterization, future work will focus on optimizing feedback cooling, refining particle handling to minimize impurities, and investigating trapping in the ultra-high vacuum regime to further enhance sensitivity for force and acceleration detection.

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