Recently, Professor Yuzhi Shi from the team of Professors Zhanshan Wang and Xinbin Cheng at the School of Physical Science and Engineering, Tongji University, has studied the chiral optical forces on chiral particles in a complex spin-curl field and experimentally demonstrated the bidirectional movement of chiral particles with different sizes and handedness, and under different light polarizations. This research aids in enantioselective separation and sensing applications. The research findings, titled " Observation of Intricate Chiral Optical Force in a Spin-Curl Light Field", were published in "Physical Review Letters."
Chirality is pervasive across numerous disciplines. In biomedicine, a rich diversity of biomolecules exhibit chirality. In chemistry, molecules with different chirality can exhibit varying properties, potentially acting as either medications or toxins. Chiral optical forces represent a universal, non-invasive method for manipulating and detecting chiral particles.
Light carries momentum, which can be transferred to objects to generate optical forces. Optical lateral force (OLF), also known as optical transverse force, refers to the optical force perpendicular to the direction of light propagation and independent of light intensity or phase gradient. The study of OLF on chiral particles, especially experimental evidence, is crucial for realizing its applications. So far, experimental demonstrations of chiral optical forces primarily involve a single light property, such as the transverse spin, lateral momentum, and energy-helicity gradient. The interplay of multiple light properties and their roles in generating anomalous chiral OLFs remain elusive.
Fig. 1 Optical forces on chiral particles in a spin-curl light field
The study employed elliptically polarized light beams focused into a line-shaped, acting on chiral particles on a surface to generate OLF. The direction of light propagation is along the -z direction, and the study examined OLF along the y direction (Fig. 1a). The chiral particle is a type of polymerized liquid crystal microparticle that remains a spherical shape in water (Fig. 1b). The constitutive relationship of chiral particles is of the electro-magnetic coupling type, with previous studies typically considering only a single optical force. Individual forces increase with particle size, whereas the combined force may decrease or change sign with particle size. (Fig. 1d). By decomposing forces, the effects of multiple forces can be studied simultaneously, as shown in Figs. 1e and 1f.
Fig. 2 Experimental demonstration of the size-dependent chiral OLF
The chiral particles in the experiment possess the feature of spiral structures (Figs. 2a and 2b). We observed the stable lateral displacement of particles in the experiment to determine the OLFs on chiral particles. For chiral particles of different sizes under the same circularly polarized light illumination, the OLFs on these particles are different (Fig. 2c). This study experimentally verified the theoretical prediction that the dominant term of the optical force varies for particles of different sizes (Fig. 2d).
Fig. 3 Experimental observation of inverse OLFs with particle handedness, size and light polarization
For particles with the same chirality but different sizes (Fig.3a), and particles with similar size but opposite chirality (Fig. 3b), opposite OLFs can be obtained under circularly polarized light illumination, which aids in the size and chirality sorting of chiral particles. Switching the polarization state of light for the same chiral particles can change the magnitude and direction of OLF (Fig. 3c).
This research reveals the complex interactions between various light properties in spin-curl fields and experimentally demonstrates the significant impact of a series of parameters on chiral optical forces. The experimental architecture tests the intricate theorem of chiral optical force, and serves as an efficient paradigm for the investigation of chirality- or spin- correlated optical forces, having marvelous potential in force probing, enantioselective sorting, and chiral sensing.
Professor Xinbin Cheng, Professor Yuzhi Shi from Tongji University, and Professor Cheng-wei Qiu from the National University of Singapore are the corresponding authors of the paper. Ph.D. student Chengxing Lai from Tongji University is the first author of the paper. Other authors who made significant contributions include Professor Zhanshan Wang, Ph.D. students Haiyang Huang and Weicheng Yi from Tongji University, Researcher Alfredo Mazzulla from the Institute of Nanotechnology CNR-NANOTEC in Italian, Professor Gabriella Cipparrone from the University of Calabria, Researcher Yan Zu from the University of Chinese Academy of Sciences in Wenzhou, Professor Weiqiang Ding and Dr. Hang Li from Harbin Institute of Technology, Associate Professor Qinghua Song from Tsinghua University, and Associate Professor Pin Chieh Wu from National Cheng Kung University in Taiwan.
Link to the paper：
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.233802
