A groundbreaking study led by Prof. Yao Baoli and Dr. Xu Xiaohao from the Xi’an Institute of Optics and Precision Mechanics (XIOPM), part of the Chinese Academy of Sciences, has revealed an innovative optical trapping technique that challenges conventional understanding. The researchers have demonstrated the existence of a “full-gray optical trap” in structured light, capable of capturing nanoparticles at regions where the optical intensity is neither maximized nor minimized. The findings, published in Physical Review A, open new possibilities for the manipulation and control of nanoparticles using light.
The optical trap is a fundamental technology in the field of optics and photonics. Since Arthur Ashkin’s pioneering work in the 1970s, optical traps have become a critical tool for a wide array of applications in life sciences, physics, and engineering. Traditionally, optical traps are classified into two types: bright traps, which are located at the intensity maxima of light fields, and dark traps, which occur at the intensity minima. Both types rely on gradient forces exerted by the light’s electric field to capture and manipulate small particles, typically at regions of high or low light intensity.
However, the discovery of the full-gray optical trap by Prof. Yao and Dr. Xu’s team represents a significant departure from this established understanding. Their work reveals that it is possible to create an optical trap at an intermediate optical intensity, where the light is neither at its maximum nor minimum. This phenomenon is made possible by the unique properties of structured light, a form of light that is intentionally shaped or patterned to exhibit specific spatial characteristics.
To achieve this, the researchers developed a high-order multipole model to describe the gradient forces acting on nanoparticles. Using multipole expansion theory, they were able to accurately calculate the forces that arise from the interaction between light and matter at higher-order moments, going beyond the dipole approximation that has traditionally dominated the study of optical traps.
The key to their discovery was the use of a petal-shaped field structure. By immersing silicon (Si) particles in this structured light field, the team observed that high-order multipole gradient forces could trap the Si particles in regions where the light intensity was neither at its peak nor at its trough. This revealed the existence of an intermediate trapping state, which the researchers coined as the “full-gray optical trap.”
The full-gray optical trap is a novel phenomenon that underscores the importance of Mie resonances in optomechanics. Mie resonances refer to the scattering of light by spherical particles, where the light’s interaction with the nanoparticle leads to the excitation of various multipole moments (e.g., dipole, quadrupole, etc.). In this case, the high-order Mie resonances play a pivotal role in creating the nonlocal pondermotive effects that give rise to the full-gray trapping state. These effects are due to the light’s intensity gradient, which can exert forces on the nanoparticles in a way that was not previously understood.
The implications of this discovery are profound, as the full-gray optical trap could enable a range of new applications in nanoparticle manipulation. For instance, the ability to trap nanoparticles at intermediate intensities could pave the way for highly controlled nanoparticle cooling, patterning, and sorting. These applications are particularly important in fields such as nanotechnology, materials science, and biomedical engineering, where precise control over particle behavior is essential.
One potential application is the use of optical traps for cooling nanoparticles. Cooling techniques are critical for reducing thermal motion in nanoparticles, which can enhance their stability and allow for more precise manipulation in experiments. The full-gray optical trap, with its ability to control the gradient forces in structured light fields, offers a promising approach to cooling nanoparticles in a way that was not previously possible.
Additionally, the full-gray optical trap could contribute to advancements in nanoparticle patterning, where particles are arranged in specific configurations for various technological applications. The precision and flexibility provided by this new optical trapping method could lead to more efficient and versatile techniques for creating nanoparticle-based structures.
Another exciting possibility is the use of this trap for ultra-sensitive sorting of nanoparticles. In biological and medical research, sorting nanoparticles based on their size, shape, or other physical properties is a critical task. The full-gray optical trap’s ability to manipulate nanoparticles with high sensitivity could make it an invaluable tool in this area, enabling more efficient sorting and analysis.
This research also highlights the broader impact of Mie responses in the field of optomechanics. The ability to manipulate nanoparticles using the intricate interactions between light and matter opens up new avenues for exploring the fundamental principles of light-matter interactions. As optical trapping techniques continue to evolve, the study of Mie resonances and their role in nanoparticle dynamics will be crucial for developing even more advanced technologies.
Reference: Yanan Zhang et al, Full-gray optical trapping by high-order multipole-resonant gradient forces in structured light, Physical Review A (2024). DOI: 10.1103/PhysRevA.110.063517