A simple piece of carefully chosen glass could be the key to cancelling out noise in extremely precise quantum sensors.

Dr Jiayi Qin, from ANU Quantum Optics Group, said this simple solution to photothermal noise – disruptions to experiments due to heat from lasers – could benefit many other industries and experiments that use compact systems or high-powered lasers, such as gravitational waves detectors and photonics

“This kind of noise cancellation has not been done before – it’s a new tool to stabilise cavities,” Dr Qin said.

“I hope this demonstration will inspire other researchers with ways to control noise in their experiments.”

Dr Qin was part of a team from the Quantum Optics Group in the Department of Quantum Science and Technology, Research School of Physics that recently published their results in OPTICA. The breakthrough is part of a quest to levitate a mirror using laser light, creating an extremely sensitive measurement device. 

Although having no physical connections holding the mirror would remove many sources of noise, the intensity of the laser beam required to suspend even a tiny mirror (in this experiment a diameter of 3 mm, thickness 50 µm and mass just over 1 mg) is enough to heat components to a level that disrupts the stability of the experiment.

The team tried numerous methods such as optical cooling and direct feedback loops but found none could achieve stable levitation in the presence of photothermal effects.

“Photothermal effects are sometimes overlooked, or considered as a parasitic effect to be gotten rid of,” said team member Dr Giovanni Guccione. 

“With our work we hoped to shine more light into how they can be harnessed to one's advantage.”

Going back to mathematical modelling they noticed the photothermal effects add new interactions to the system dynamics. 

“Holding an object steady is normally not possible in an optomechanical system subject to pure radiation pressure from light,” Dr Guccione said.

The team set about searching for an element that could invert the effect, and act as a damping element.

The team realised the required property was a rise in index of refraction with temperature increase – something that lengthens the optical path as it heats up, said Dr Qin.

“Our technique uses the photothermal backaction itself as feedback to stabilise the system, without the need for external control,” she said.

Modelling and tests showed the best material was N-BK7, a borosilicate crown glass, so the team ran the experiment with a small N-BK7 window in the cavity, and found noise was indeed reduced. 

Initially the power was affected by reflections from the glass surface, however these losses were reduced by tilting the glass to the Brewster angle, at which reflections are at a minimum.

A second challenge came from the glass absorbing energy, which the team believe they can minimise with a piece of glass with a more suitable thickness, to get the optimum cancellation of photothermal effects.

When the search to procure the right size piece of N-BK7 is successful, the team will be set to achieve their goal to levitate the tiny mirror on a tripod of laser beams.

“This demonstration gives me hope for successful levitation at last,” Dr Qin said.

“I’ve been working on this world-first project for five years – it’s a pretty exciting and challenging experiment!”

Dr Qin has recently joined the ANU Centre for Gravitational Astrophysics, where she will be applying the recent findings.

“This technique can help improve the sensitivity of future gravitational wave interferometers,” she said.