Optical tweezers use beams of lights to hold and manipulate tiny particles such as living cells, nanoparticles, or biomolecules. Researchers from Purdue University and Pennsylvania State University have recently developed tiny chip-based optical tweezers that levitate nanoparticles using a metalens in a vacuum. Researchers have integrated metamaterial technology into thin dielectric metalens that offers the high numerical aperture required for a beam to manipulate tiny particles.
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Optical levitation of nanoparticles in a vacuum is a powerful technology used in quantum science and fundamental physics. The newly designed optical tweezers can then be used to accurately measure nanoparticles, cells, and other materials. Scientists consider optical tweezers as a versatile tool for trapping and manipulating neutral particles and it has been applied in many disciplines of science such as molecular biology, nanotechnology, and physics. Arthur Ashkin is known as the father of optical tweezers since he pioneered the field in the early 1970s.
Chip-Based Optical Tweezers Design and Principle
The chip-based optical tweezer is more advanced than the previously developed tool used in the creation of the highly sensitive detector of torque and the fastest rotor. The research group envisioned designing more practical and portable levitation technology using optics.
In this context, scientists used a metalens that reduced the focusing lens size compared to the conventional lens. To focus lights, the metalens utilizes nanostructures. Metasurfaces and metamaterials typically comprise perfectly designed structures that control the properties of light passing between them. Metalenses are filled with a fluid associated with optical trapping. However, metalenses require air or vacuum manipulation for certain applications.
The newly designed metalens consists of numerous nanopillars made up of silicon, whose diameter is significantly smaller than the conventional lens used in the previous design. Tongcang Li, the research team leader at Purdue University, explained that the use of ultrathin metalens enabled the reduction of the diameter of the focusing lens from 25 mm to 0.4 mm. These metalenses can optically levitate nanoparticles in a vacuum. The optical traps were typically produced using bulky optical components.
The use of metalenses in optical trapping is not new. Another research group used this technique but the main difference is that they exhibited optical trapping of the metalens in liquids. However, optical trapping in a vacuum has greater advantages. Li explained that vacuum helps increase the sensitivity of the system.
This optical design was tested by employing a laser beam of intense force onto the metalens so that it can produce trapping forces. Subsequently, a nanoparticle solution, at a diluted concentration, was sprayed into the trapping area. Researchers explained that a trapped nanoparticle appears as an illuminated spot that can be easily visualized through a camera. The movement of these nanoparticles can be monitored instantaneously via photon detectors.
Scientists also reported that, in a vacuum, the metalens could levitate a nanoparticle without needing any feedback stabilization, at a pressure of 2×10-4 Torr and about 1/4,000,000 atmospheric pressure. Importantly, it can also move a soaring nanoparticle from one optical trap to another.
According to Xingjie Ni, the research team leader at the Pennsylvania State University, the metalens that they developed comprises a layer silicon nanostructure (thickness of 500 nm) possessing a numerical aperture of 0.9 and focal length of 100 μm. It performs functions similar to that of a conventional lens that is much larger in size. Additionally, this metalens is completely operative in vacuum conditions.
Another important feature highlighted by Ni is the highly flexible design of their metalens-based optical tweezer. For instance, the design can be modified to suit many applications that include filtering out components whose spatial frequency is lower than the focusing light. This feature is extremely important for optical levitation technology.
Application of Optically Levitated Particles
The optically levitated particles can be applied to develop accelerometers and gyroscopes typically used for navigation. The chip-based optical tweezer design can be used to develop an integrated and flexible optical system that could help investigate near-surface forces by trapping an object, which is present less than 1 micrometer away from a surface.
Therefore, scientists believe that optically levitated particles could help study gravity at short distances. This tool can also trap cold atoms in a vacuum to study quantum processes. Therefore, these particles are also used to investigate the dark matter and dark energy. Researchers stated that their metalens tweezers are compatible with cells, organelles and biological molecules, and inorganic particles.
The Next Step
Scientists claimed that their chip-based optical tweezers, using ultrathin metalenses, are the first to promote levitation of a nanoparticle in vacuum. The biggest challenge of this study was the optimization of both numerical aperture and focusing ability of the metalens.
At present, scientists are working towards improving the levitation devices by increasing the metalen’s focusing and transmission proficiency. They intend to develop metalens with a smaller diameter than their recent design so that the optical levitation would be relevant for everyday applications.
References and Further Reading
Shen, K. et al. (2021) On-chip optical levitation with a metalens in vacuum. Optica. 8 (11): 1359. https://doi.org/10.1364/OPTICA.438410
Optica. (2021) Chip-based optical tweezers levitate nanoparticles in a vacuum: Metalens-based design shrinks footprint, making optical traps practical for precision sensing and measurements. ScienceDaily. [Online] Available at: www.sciencedaily.com/releases/2021/10/211021120927.htm
Daukantas, P. (2021) Optical Tweezers on a Chip. [Online] Available at: https://www.optica-opn.org/home/newsroom/2021/october/optical_tweezers_on_a_chip/
Li, N. et al. (2019) Review of optical tweezers in vacuum. Frontiers of Information Technology and Electronic Engineering. 20(5). pp. 655-673. https://doi.org/10.1631/FITEE.190009
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