Researchers reveal perfect gray light trap in structured light

A research group led by Professor Yao Baoli and Dr. Xu Xiaohao from the Xi’an Institute of Optics and Precision Mechanics (XIOPM), Chinese Academy of Sciences, has revealed a completely gray light trap in structured light. Although they are nanoparticles, they appear in regions where the intensity is neither maximum nor minimum. The study is published in Physical Review A.

Optical traps are one of the greatest discoveries in optics and photonics. Since Arthur Ashkin’s pioneering work in the 1970s, optical traps have been employed in a wide range of applications in life sciences, physics, and engineering. Like thermal and acoustic traps, this trap is typically located at a maximum or minimum electric field strength and can be either bright or dark.

In this study, the researchers developed a higher-order multipole model of gradient forces based on multipole expansion theory. By immersing Si particles in structured light with a petal-shaped field, we found that higher-order multipole gradient forces can trap Si particles at neither maximum nor minimum light intensities.

Therefore, the researchers demonstrated that an intermediate trapping state called full-gray optical trapping may exist. The origin of this new trap can be traced to nonlocal speculative effects of light intensity gradients achieved by excitation of higher-order multipole Mie resonances in nanoparticles.

The completely gray trap highlights the impact of the Mie response on optomechanics and will facilitate the development of nanoparticle cooling, patterning, and ultrasensitive sorting in the future.