Recently, a research group led by Prof. YAO Baoli and Dr. XU Xiaohao from Xi'an Institute of Optics and Precision Mechanics (XIOPM), Chinese Academy of Sciences, revealed a full-gray optical trap in structured light, which is able to capture nanoparticles but appears at the region where the intensity is neither maximized nor minimized.
The research results were published in Physical Review A.
The optical trap is one of the greatest findings in optics and photonics. Since the pioneering work by Arthur Ashkin in the 1970s, the optical trap has been employed in a broad range of applications in the life sciences, physics, and engineering. Akin to its thermal and acoustic counterparts, this trap is typically either bright or dark, located at the field intensity maxima or minima.
In this work, the researchers described a full grey optical trap in structured light that can capture nanoparticles but appears at the region where the intensity is neither maximized nor minimized. The origin of this novel trap can be traced back to the nonlocal pondermotive effect of the optical intensity gradient, which is achieved through the excitation of higher-order multipole Mie resonances in nanoparticles. The researchers have developed in this work a multipole model for the optical gradient force. This analytical theory not only explains the results, but also addresses a long-standing issue in our area, i.e., what is the explicit expression of gradient force for higher multipoles (e.g., quadupole and octupole). This work filled this theoretical gap, because it generalizes the theory of gradient force to multipoles of arbitrary orders.
“We expect that future research dealing with optical trapping of Mie particles would cite our this paper, especially in the current era of Mie-tronics,”said Dr. XU Xioahao from XIOPM.
（Published 16 December 2024）
Fig. Calculated focused petal-shaped field and the transverse optical force on a Si sphere at the focal plane. (Image by ZHANG Yanan)
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