When levitated using laser light, glass nanoparticles exhibit surprising coupling.
The interaction between optically levitated nanoparticles is fundamentally altered by a novel mechanism discovered by researchers at the University of Vienna, the Austrian Academy of Sciences, and the University of Duisburg-Essen.
By demonstrating previously unachievable levels of control over the coupling in arrays of particles, they have established a new platform to investigate the intricacies of physics. This week's edition of Science reveals the findings. Picture specks of dust in the air, drifting aimlessly.
Light pressures are exerted on particles when a laser is activated, and if a particle gets too close to the beam's focal point, it gets caught. Optical tweezers were developed on this principle by Nobel laureate Arthur Ashkin.
When two or more particles are near one another, standing waves of light can be created between them, causing the particles to align in a crystalline pattern similar to that formed when light is bonded to a crystal of particles. There have been numerous studies on this for over 30 years.
When investigating the interactions between two glass nanoparticles, researchers in Vienna were taken aback to observe behavior that exceeded their expectations. They were able to alter the magnitude and direction of the binding force and even observe one particle (let's say the left) acting on another (the right) without the right particle reacting.
Non-reciprocal behavior happens when a system loses energy to its surroundings, such as the laser, and appears to go against Newton's third rule (anything acted upon acts back with the same strength but opposite sign). Our existing theory of optical binding was lacking.
This unexpected pattern of activity can be attributed to a phenomenon called "coherent scattering," which scientists in Vienna have been studying for years. When a nanoparticle is exposed to laser light, the atoms and molecules inside the particle become polarised and move in sync with the laser's electromagnetic wave.
As a result, the dispersed light from the particle has the same frequency as the incident laser. Interference can be created between in-phase waves. For the first time, scientists in Vienna have harnessed the interference effect offered by coherent scattering to cool a single nanoparticle to its quantum ground state of motion while keeping it at ambient temperature.
A further interference effect was discovered by Uro Deli, a senior researcher in the laboratory of Markus Aspelmeyer at the University of Vienna and the initial author of the previous cooling work when he began applying coherent scattering to two particles. "Light dispersed from one particle can interfere with the light that traps the other," explains Deli. The strength and nature of the forces in between the particles can be adjusted if the phase between these light fields can be altered.
It is possible to re-establish the classic optical binding for a particular configuration of phases. However, for some phases, never-before-seen effects like non-reciprocal forces exist. "Unfortunately, coherent scattering and the fact that photons are lost were not accounted for in earlier theories.
It turns out that when you combine these two processes, you get more complex interactions than you ever imagined. "A German team member working on the enhanced theoretical description, Benjamin Stickler, explains their findings.
"...and neither were previous trials responsive to these effects."
The team in Vienna sought to change that, so they experimented with investigating these novel light-induced forces. They did this by splitting the light from a single laser into two optical beams, trapping a single glass nanoparticle roughly 200 nm (about 1,000 times smaller than a typical grain of sand). All three variables (distance, intensity, and relative phase) were modified in their experiment.
The position of each particle, which is being tracked with exquisite precision, oscillates at a rate determined by the trap. Since every force on the trapped particle affects this frequency, it is possible to monitor the forces between them while phase and distance are being adjusted.The experiment was conducted in a vacuum to rule out the possibility that gas between the particles induced the forces instead of light.
This allowed them to verify that the novel light-induced forces between the confined particles were present. Ph.D. student and primary author Jakob Rieser states, "The couplings that we find are more than 10 times bigger than expected from conventional optical binding." And as predicted by our new model, when we alter the laser phases, non-reciprocal forces leave telltale traces. Hopefully, these findings will inspire novel approaches to investigating complicated processes in multiparticle systems.
Research leader Uro Deli believes studying model systems with well-controlled interactions are the norm when trying to comprehend what is happening in truly complex systems. This is so interesting because it gives us a whole new set of tools for manipulating interactions amongst levitated particle arrays.
The capacity to manipulate atomic interactions in optical lattices was the impetus for developing quantum simulators, which the researchers acknowledge as an important source of inspiration.
Similarly, "being able to use this immediately on the level of solid-state systems might be a similar game changer."
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