Nanoantennas, as miniature optical devices, have garnered significant attention due to their immense potential in fields such as super-resolution imaging, surface-enhanced Raman spectroscopy, and quantum sensing. However, because nanoantennas are much smaller than the wavelength of light—typically below the diffraction limit—precise positioning and manipulation of these nanoantennas remain urgent challenges. Traditional positioning techniques are constrained by the optical diffraction limit, making it impossible to achieve high-precision three-dimensional positioning at the nanoscale, particularly for accurate measurements in the longitudinal (z) direction.
To address this issue, researchers have begun exploring new theories and technologies. One promising direction is the precise positioning of nanoantennas through the control of polarization transformations in the light field. By designing light fields with special polarization characteristics, it is possible to achieve accurate measurements of nanoantenna positions on superwavelength scales, especially along the z-axis. However, existing methods predominantly focus on two-dimensional plane positioning and lack effective solutions for high-precision positioning in three-dimensional space.
In response, the research team at Zhejiang Laboratory has proposed a novel polarization transformation light field control technique for high-precision positioning of nanoantennas. This method fully leverages the variation characteristics of polarized light fields in three-dimensional space, achieving precise positioning of nanoantennas both laterally and longitudinally (in the z direction) by accurately adjusting the helical structure of the light field's polarization. This technology not only overcomes the precision limitations of traditional methods in longitudinal positioning but also significantly enhances overall accuracy, marking a new stage in nanoantenna positioning technology. The relevant research results were recently published as On the Cover in Photonics Research, Volume 13, Issue 1, 2025. [Haixiang Ma, Fu Feng, Jie Qiao, Jiaan Gan, Xiaocong Yuan, "Positioning spherical nanoantennas with picometer precision," Photonics Res. 13, 49 (2025)]
Fig. 1 Generation principle of the excitation light field. (a-c) are the superposition principles of right- and left-handed circular polarization components; (d) and (e) are the three-dimensional spatial distribution of the excitation light field; (f) is the curve of polarization ellipse degree variation during light field propagation.
In this study, the generation process of the light field comprises two core components: theoretical derivation and experimental implementation. Firstly, based on the Richards–Wolf vector diffraction theory, the researchers derived the translation phase factor under tightly focused conditions. Utilizing this translation factor, they successfully achieved the reverse translation of left- and right-circularly polarized light within the light field. Subsequently, these translated polarized light components were superimposed to form a novel light field structure, wherein the polarization of the light field gradually transitions from right-circular polarization to left-circular polarization along a specific path, as illustrated in Figure 1. By precisely modulating the translation factor, highly accurate control over the excitation light field's polarization was realized, enabling rapid and controllable changes in the polarization characteristics of the light field in space as the nanoantenna moves.
On the experimental side, the researchers ingeniously exploited the circular dichroism of liquid crystal materials by customizing dedicated liquid crystal plates to load the conjugate phases of left- and right-circularly polarized light. The liquid crystal plates adjusted the phase difference between these two circular polarizations, and combined with the focusing action of a high numerical aperture (NA) microscope objective, effectively separated the focal points of the two circularly polarized lights. This method allowed the researchers to generate a light field with specific polarization variation characteristics in the experiments.
Fig. 2 Nanoantenna displacement measurement. (a-c) are the scattering characteristics of the nanoantenna at different positions in the excitation light field; (d) and (e) are the measurement results of the nanoantenna's linear displacement signal.
In this study, the designed light field is capable of highly sensitive responses to the spatial position changes of nanoantennas through polarization variations. Specifically, when a nanoantenna moves along the z-axis within the light field, the transition in polarization induces significant changes in the polarized components of the scattered light field. By measuring the intensity variations of left- and right-circularly polarized light in the scattered light field and introducing the concept of a "component factor," the researchers were able to quantitatively analyze the changes in the light field's polarization components relative to the antenna's position, thereby achieving picometer-level positioning accuracy.
During the experimental process, by gradually altering the position of the nanoantenna and precisely tracking the polarization changes in the scattered light field, the researchers successfully controlled the positioning error to within 20 picometers, as shown in Figure 2. By adjusting the trajectory of the polarization transition in the light field, the researchers overcame the precision limitations of traditional technologies, achieving three-dimensional precise positioning of nanoantennas. This breakthrough not only significantly enhanced positioning accuracy but also provided a practical and feasible technical solution for the precise stacking of multi-layer nanoantenna arrays and the high-precision integration of optical devices.
The research team stated: "In the fields of nanophotonics and picophotonics, precise nanoantenna positioning has always been crucial for achieving high-performance optical devices and systems. The nanoantenna positioning technology based on polarization transformation light fields proposed in this work represents an important breakthrough over existing technologies. By precisely controlling the changes in the light field's polarization, we successfully achieved picometer-level positioning accuracy in three-dimensional space, particularly making significant advancements in the longitudinal (z) direction, thereby filling the precision gap that traditional technologies could not overcome. This technology not only provides an innovative solution for high-precision measurements of nano-optical devices but also lays the foundation for the further development of picophotonics. The ingenious use of the circular dichroism of liquid crystal materials in the experiments made polarization control more flexible and efficient, opening up broad prospects for applications in quantum sensing, super-resolution imaging, and other fields. Undoubtedly, this work provides important theoretical support and technical assurance for fundamental research and applications in picophotonics."
