Hold still
Optical trapping is used in many branches of science, for example in physics to cool things down, in nanochemistry to let reactions between single molecules take place, and in biology and medicine to trap and study complex biomolecules.
Traditional optical trapping happens with optical tweezers, i.e. in a laser light field in free space.
The field of nanophotonic trapping tries to miniaturize this trapping on chip. Plasmonic structures are most commonly used but these are made of lossy materials. Since trapping requires shining light on the materials – in the case of plasmonic structures, for example made of noble metals – heat is generated, which may worsen the performance.
Compared to metals, the heat generation in dielectric cavities will usually be negligible.
Trapping in 3D
Plasmonic and photonic nanostructures promise trapping by near-field optical effects on the nanoscale, as an alternative to standard macroscopic setups, which are inherently bound by the diffraction limit.
However, the practical design of lossless waveguide-coupled nanostructures capable of trapping subwavelength-sized particles in all spatial directions has until now proven insurmountable.
In an article recently published in ACS Photonics based on a collaboration between DTU Construct and DTU Electro, the researchers present designs of omnidirectional optical traps, realized by inverse-designing fabrication-ready integrated dielectric nanocavities.
Why is this desirable? Well, if the particle moves about due to thermal fluctuations, then it’s harder to study it. Also, there are vacuum forces that make particles and molecules stick to cavity walls. A deeper, more strongly localized trap in the middle of the cavity makes it easier to avoid hitting the walls and getting stuck there.
Sensitive particles
The trap designs proposed by the DTU researchers are promising for carefully controlling and manipulating the positions and movements of particles and molecules. Applications include:
- Trapping nanoparticles for use and study in the field of optomechanics
- Or perhaps the traps could be used to trap quantum emitters such as quantum dots or cold atoms or even arrays of them, where levitated emitters experience less decoherence.
- For biomolecules, one could prefer to study them surrounded by water rather than by a metal wall, because the wall may change their natural shape and function, like a jellyfish stranded on the beach.
- And surely, there will be further applications that have yet to be thought of.
Perspectives of the field
The results open a new regime of levitated optical trapping by achieving a deep trapping potential capable of trapping single subwavelength particles in all directions using optical gradient forces.
The team anticipates potentially groundbreaking applications of the optimized optical trapping system for biomolecular analysis in aqueous environments, levitated cavity optomechanics, and cold atom physics, constituting an important step toward realizing integrated bio-nanophotonics and mesoscopic quantum mechanical experiments.
