Metasurface lenses, or metalenses, offer advantages such as miniaturization, lightweight design, and ease of integration, making them an effective alternative to traditional lenses in micro-nano optical systems. These lenses not only focus light fields but also enable high-degree-of-freedom manipulation over multiple dimensions of light field. As a result, they are highly applicable in areas such as sub-wavelength resolution imaging, virtual reality, optical information processing, and light field detection. Currently, dynamic control of light focusing remains a major scientific challenge in metalens research. Previous studies have demonstrated that dynamic focusing can be achieved by incorporating mechanically stretchable substrates, utilizing dual-layer lens designs, or integrating materials with tunable optical properties—such as phase-change materials—into the metasurface. However, enhancing the modulation depth and speed of light focusing with metasurfaces continues to be an active research area.
Lithium niobate (LN) is an ideal material for integrated photonic devices, owing to its broad transparency window, high refractive index, large electro-optic coefficient, and excellent chemical and mechanical stability. It enables refractive index modulation at extremely high speeds (up to 100 GHz), making it widely utilized in on-chip ultrafast electro-optic modulators. Recent studies have shown that integrating thin-film lithium niobate (TFLN) into metasurface designs allows dynamic control over the phase and amplitude of light fields, offering a powerful approach for optical switches and reconfigurable wavefront control. The proposed method of dynamically controlling light focusing using a TFLN-based metasurface design is crucial for achieving high-speed light focusing control.
Based on the aforementioned research background, Professor Shuqi Chen's research group at Nankai University has proposed an electro-controlled metasurface based on TFLN to realize electrically controlled light focusing. The designed metasurface features a high-quality factor guided-mode resonance with an electrically controllable resonant wavelength, resulting in a high extinction ratio of transmittance at the operational wavelength by changing the applied voltage. A reconfigurable one-dimensional Fresnel zone plate (FZP) with a focusing efficiency around 15% has been realized through spatial modulation of transmitted light intensity, whose focal position can be electrically tunable in both longitudinal and lateral directions. The research results are published in Chinese Optics Letters, Vol. 22, Issue 12, 2024: Haoyu Wang, Zhancheng Li, Wenwei Liu, Yuebian Zhang, Hua Cheng, and Shuqi Chen. Electrically controlled light focusing by a tunable metasurface using thin film lithium niobate[J]. Chinese Optics Letters, 2024, 22(12): 123601.
Fig. 1. Electro-controlled metasurface based on TFLN for transmission control and light focusing. (a) Schematic diagram of the electro-controlled metasurface based on TFLN. (b) Transmittance manipulation achieved through the electro-optic effect of lithium niobate. (c) Reconfigurable light focusing achieved by modulating the spatial distribution of transmittance.
A sharp transmission dip with quality (Q) factor equals to 1929.4 exists at 829.65 nm, indicating a strong coupling between the incident light and the designed metasurface. This sharp dip is caused by a TM mode guided-mode resonance (GMR), which is a stationary mode resulting from the interference between two counter-propagating waves along the x-direction. The variation of the refractive index of the LN influences the GMR, since Ez is enhanced and localized within the TFLN. The resonant wavelength red shifts with positive external applied voltage and blue shifts with negative external applied voltage with a tuning sensitivity value of 0.096 nm/V, while the Q factor of the resonance remains unchanged. Due to the exceptional tuning sensitivity, a high extinction ratio of transmittance with a value of 21.3 dB can be achieved at λ0 = 829.10 nm, as shown in Fig. 1.
A metasurface acting as a 1D reconfigurable FZP is designed to have 1000 unit cells in the y-direction. The longitudinal and lateral focal position of the designed FZP can be easily modulated by adjusting the spatial distributions of applied voltages V(y). The simulated results demonstrate the capability of the designed 1D FZP to effectively modulate its longitudinal focal position within a range spanning from 1 mm to 5 mm, thereby indicating an impressive tuning depth exceeding 4800×λ0 and lateral focal position shift of the focal line is ±100 µm, which is 50% of the designed metasurfaces width. It behaves similarly to an ideal FZP, in that the full width at half maximum increases with focal length while the focusing efficiency does not change with focal position.
This research highlights the significant potential of high-Q optical resonant modes in metasurfaces using TFLN for achieving electrically controlled light focusing. However, due to limitations in electrode design and the sensitivity of non-local resonances to structural periodicity, this study focuses solely on dynamic control of light focusing in the one-dimensional case. Further research is needed to explore multi-dimensional, highly efficient control of light focusing in two-dimensional scenarios, along with corresponding experimental investigations.
