Dr Suren Vasilyan | Measuring Microscopic Forces with Extreme Precision using Laser Beams

Even though light particles – or ‘photons’ – don’t have any mass, the laws of quantum mechanics show that they still carry a tiny amount of momentum. This suggests that when light shines on a reflective object, its photons will exert a tiny amount of force. The effect was first noticed by Kepler and partially formalised by Maxwell and Lebedev, finally entering the field of quantum mechanics in the form of the Planck–Einstein relation.

Vasilyan’s team proposed that by amplifying the forces exerted by the photons within a laser beam, these forces could be measured with extreme precision using classical mechanical measurement systems. Through a series of recent studies, the researchers explored how these forces can be amplified using a multi-reflected laser beam folded inside a setup named an ‘optical cavity’.

Essentially, an optical cavity is an arrangement of two highly reflective mirrors placed opposite each other. When trapped inside, the photons in a pulsed or continuous wave laser beam will bounce between the mirrors, amplifying the total force exerted onto the mirrors after every reflection takes place.

To measure these imparted forces, Vasilyan’s team used a technique named ‘electromagnetic force compensation’. Here, both mirrors are attached to moveable parts of a balance instrument, which use electromagnetic forces to balance out the forces exerted by the photons as they repeatedly bounce between the mirrors.

By monitoring these electromagnetic forces, the researchers could accurately measure the force exerted by photons and trace back the results to fundamental natural constants – one being Planck’s constant. The primary source of these forces is due to the momentum transfer of photons, which is quantum in its nature. As such, the team calls their system a ‘Photon Momentum setup’ or a ‘Quantum Balance’.

To test their technique, Vasilyan and his colleagues carefully aligned the cavity’s mirrors to ensure the laser beam is reflected as many times as possible before escaping. The team’s system enabled them to amplify the forces imparted to photons by a factor of thousands: from hundreds of pico-Newtons, to just under one micro-Newton.

Thanks to the research-focused environment of TU Ilmenau, they could achieve these results by combining scientific and technological advances from different disciplines. These achievements were also made possible by the enthusiasm of young students to conduct brave and ambitious scientific studies.

Through a further study, the researchers showed that their setup could measure forces of around 2 micro-Newtons – a result that quickly attracted the attention of industry and metrology institutes. This was a vast improvement on the accuracy and precision achievable with some previous techniques. Also, once the measurement technique is reversed, the optical power of lasers can be measured in a similar way, with several-fold improvements in accuracy. 

In their latest research, the team measured a broad range of forces: as high as around 10 micro-Newtons, and down to just 10 nano-Newtons, which is just about one magnitude closer and comparable to the tiny electrostatic forces exerted by single atoms. Just as they had hoped, these experimental measurements matched closely with the team’s theoretical predictions, which they based on the principles of quantum mechanics. Again, this clearly demonstrated the extreme accuracy and precision of the technique.

The team’s results could have important implications for future experiments. By replicating the technique, Vasilyan and colleagues hope that researchers could calibrate their instruments more precisely, eliminating much uncertainty from their measurements of nanoscale forces.

Ultimately, the technique’s power stems from its ability to test the limits of classical and quantum physics, and possibly bridge the gap between them. At the verge of multiple disciplines, the ability to amplify and measure the forces imparted by individual photons could soon pave the way for exciting new discoveries: from new advances in nanotechnology, to a deeper understanding of quantum mechanics.