Multimode radiation pressure actuation of a mechanical resonator using a spatial mode sorter

In the quiet world of optical physics, light is often treated as a messenger that carries information about the objects it touches. When a beam of light strikes a vibrating object, the scattered light changes in ways that reveal the object's motion, a principle used in everything from medical imaging to detecting distant planets. However, the interaction works both ways. Just as motion changes the light, the light exerts a tiny physical push on the object, a force known as radiation pressure. While scientists have long been able to measure how motion alters light, using light to precisely push and control the motion of larger, extended objects has remained a difficult challenge. The force is incredibly weak, and for complex objects, it is hard to direct that push to specific parts of the surface without affecting the whole thing.

A team of researchers at the University of Arizona has now demonstrated a new way to harness this subtle force. They successfully used a specially shaped beam of light to push and control the vibrations of a tiny, square membrane made of silicon nitride. By shaping the light into a complex pattern and then rapidly shifting that pattern, they were able to excite specific vibrations in the membrane and even cool its motion down. This work bridges a gap between measuring mechanical motion and actively controlling it, opening a path toward more advanced technologies that rely on the delicate dance between light and matter.

The experiment centered on a microscopic membrane, a square sheet of silicon nitride measuring 2.5 millimeters on each side and only 90 nanometers thick. To the naked eye, this sheet is solid, but under a microscope, it behaves like a drumhead that can vibrate in many different patterns. The researchers wanted to see if they could use light to push this drumhead into specific vibrations. To do this, they used a device called a spatial mode demultiplexer, which can take a laser beam and split it into different spatial shapes, or patterns. They chose to work with a specific set of light patterns known as Hermite-Gauss modes, which look like grids of bright and dark spots.

The core of the discovery lies in how these light patterns interact with the membrane. When the researchers shone a single, simple spot of light onto the membrane, the resulting push was uniform across that spot. However, when they combined two different light patterns and shifted the phase between them, they created a moving interference pattern. Imagine two ripples in a pond overlapping; where they meet, they create a new, shifting pattern of peaks and valleys. The researchers did something similar with light. By modulating the relative phase between two light modes, they created a radiation pressure pattern that moved back and forth across the membrane.

This moving pressure acted like a hand pushing a swing. If the light pattern moved at the right speed, it could push the membrane into a specific vibration. The team tested this by combining a basic light spot with more complex shapes. They found that the resulting force depended entirely on where the light hit the membrane. For instance, combining two specific light modes created a pressure pattern that pushed the membrane sideways, while another combination pushed it up and down. By scanning these light patterns across the surface of the membrane, they were able to map out exactly how the membrane responded. The result was a detailed picture showing that the light could selectively excite different vibrational modes, such as those that look like a checkerboard or a figure-eight, depending on the shape of the light used.

The most significant achievement of the study was using this control to cool the membrane's motion. All objects at room temperature vibrate due to heat, and these tiny vibrations are a source of noise that can interfere with sensitive measurements. The researchers wanted to stop this thermal noise. They set up a feedback loop where they measured the membrane's movement and then instantly adjusted the phase of the light to push against that movement, effectively acting as a brake. Because they could shape the light to target specific vibrational patterns, they were able to cool two different, nearly identical vibrations at the same time. These two vibrations, which occur at frequencies near 263 kHz, are so close in pitch that they usually interfere with each other, making it hard to control them individually. The modes have a detuning of approximately 30 Hz.

By using two different pairs of light modes, the team could distinguish between the two vibrations. One pair of light modes was sensitive to the first vibration, while the other pair was sensitive to the second. They applied a feedback signal to each pair, slowing down the motion of each vibration independently. The data showed that they could reduce the energy of these vibrations by about 10 decibels, a significant reduction in the thermal noise. This simultaneous cooling of two distinct modes is a difficult task in the field of optomechanics, and achieving it with a single beam of light shaped in two ways demonstrates a new level of precision.

The implications of this work extend beyond just cooling a single membrane. The ability to use structured light to push and pull on specific parts of an object suggests new ways to manipulate mechanical systems. The researchers noted that this technique could be applied to other mechanical systems, including those that are levitated in mid-air. If the same principles can be scaled up, it might be possible to cool multiple objects to their lowest possible energy state, a condition known as the ground state, which is essential for quantum computing and ultra-sensitive force detection. The study also hints at the possibility of using different shapes of light, such as those that twist like a corkscrew, to create rotating forces that could spin mechanical objects.

The success of the experiment relied on the high quality of the silicon nitride membrane and the precision of the light-shaping device. The membrane was placed in a high-vacuum chamber to remove air resistance, allowing it to vibrate freely. The light source was a laser with a wavelength of 1550 nanometers, and the intensity of the light was kept low to avoid heating the membrane. The researchers used a motorized stage to move the light beam across the membrane, creating a map of the radiation pressure response. This map confirmed that the force exerted by the light was directly related to the shape of the light and the position on the membrane.

This work represents a step forward in the field of quantum optomechanics, where the goal is to control mechanical motion using the quantum properties of light. By demonstrating that radiation pressure can be used to selectively actuate and cool specific modes of a mechanical resonator, the researchers have provided a new tool for scientists. The ability to control multiple modes simultaneously is a crucial requirement for preparing complex quantum states, which are necessary for advanced technologies. While the current experiment was conducted on a relatively large membrane, the principles demonstrated here are applicable to smaller, more complex systems. The researchers suggest that future developments in light-shaping technology could allow for even more precise control, potentially leading to new applications in force microscopy and the manipulation of microscopic particles.

The study was supported by the National Science Foundation and the Office of Naval Research, highlighting the importance of this research for future technological advancements. The team acknowledged the contributions of their colleagues in fabricating the membrane and providing engineering support. Their work stands as a clear demonstration that by shaping light in the right way, it is possible to push and pull on the physical world with a precision that was previously out of reach. As the technology matures, it may become a standard tool for scientists working at the intersection of light and matter, enabling new discoveries in the quantum realm.