By Will Soutter
Researchers from the University of Rochester have managed to levitate photoluminescent nanodiamonds using only a laser beam. This achievment will allow more specific studies into how optomechanical systems behave at the nanoscale, and potentially provide a platform for advanced sensor technology or quantum computuation.
Photoluminescent nanodiamond suspended in a laser beam. Image credit: J. Adam Fenster/University of Rochester
The research, published in Optics Letters this week, was led by Nick Vamivakas, assistant professor of optics at Rochester. As he explains in the video below, beams of light are actually capable of exerting a force on objects - although the forces are much too small to be felt by humans in ordinary conditions.
A well-established technique called optical trapping harnesses these forces to hold tiny particles, like atoms, ions, or molecules, suspended in space with no other support. More recently, the technique has also been shown to be capable of trapping larger structures, like nanoparticles.
The Rochester team used just this technique to trap nanodiamonds around 100 nanometers across in a focused laser beam, and measure photoluminescence effects occuring due to defects in the diamonds.
This permits their properties to be studied in a unique way - without the influence of any larger structures that they would normally be in contact with.
The nanodiamonds were inserted into the laser chamber in the form of an aerosol spray. The diamonds are then attracted to the laser focus spot in the centre of the chamber - however, it sometimes took a while for a diamond to get close enough to the beam for the optical forces to take over, as graduate student Levi Neukirch explained:
"Sometimes it takes a couple of squirts and in a few minutes we have a trapped nanodiamond; other times I can be here for half an hour before any diamond gets caught. Once a diamond wanders into the trap we can hold it for hours."
This experiment is just the first step, proving that the technique works in practice. Vamikavas and his team are now planning a range of future experiments using this platform, which could cover a great deal of new ground - both describing the properties and testing potential applications of optomechanical resonators like the trapped nanodiamonds.
Optomechanical resonators are structures in which the physical vibrations can be affected by light. Having the ability to probe the nanodiamonds in detail could yield discoveries about the borders between the realms of classical and quantum mechanics - a fundamental area of physics which still holds many unanswered questions. Prof. Vamivakas commented:
"Levitating particles such as these could have advantages over other optomechanical oscillators that exist, as they are not attached to any large structures. This means they are easier to keep cool, and it is expected that fragile quantum coherence, essential for these systems to work, will last sufficiently long for experiments to be performed. We are yet to explore this, but for example, in theory we could encode information in the vibrations of the diamonds and extract it using the light they emit."
These planned experiments have the potential to create the much sought-after "Schrödinger's Cat" states - where a macroscopic system exists in a superposition of multiple quantum states. It should also be possible to monitor the quantum states of the internal nitrogen defects in the nanodiamonds (which cause their luminescent properties) by measuring their physical vibrations.
The additional knowledge these experiments could generate about optomechanical systems could be invaluable for future development of quantum conputing systems, MEMS-based devices (Micro Electromechanical Systems) or ultra-sensitive force sensors.