The development of metasurfaces has offered unprecedented capabilities for the advance of planar photonics. Among various metadevices, the metalens has attracted widespread attentions due to their practical applications in imaging and spectroscopy. Recently, they have been developed for multifunctional wavefront manipulations, replacing the traditional refractive lenses made of polished crystals or polymers and embracing the trend of miniaturization and integration of photonic systems. Nevertheless, their functions remain static once they are fabricated. Thereby, many researchers are focusing on realizing active metalenses via introducing functional materials. The majority of them possess either switchable bifocal properties or discrete focal lengths. Till now, dynamic function, especially the tunable chromatic aberration, still remains a formidable challenge.
In many cases, metalenses suffer from broadband chromatic aberrations due to their resonances or diffraction properties, which severely degrade the resolution of full-color and hyperspectral imaging. To solve this problem, mechanical scanning along the optical path is commonly required, making the detection and corresponding analysis complicated and time consuming. More recently, via introducing the design of achromatic metalens, broadband achromatic focusing is realized in the visible, near-IR and even terahertz (THz) range. On the other hand, for spectrographic analysis and tomographic applications, large chromatism is favored to separate focal spots for different frequencies spatially without crosstalk. If dynamic manipulation of chromatic dispersion can be realized, achromatic and chromatic focusing can be accomplished using a single metalens. It would greatly promote the system integration and function versatility, facilitating practical applications of spectroscopy and imaging systems.
Structural and THz far-field characterizations of the LC integrated metalens: (a) SEM micrograph of the partial dielectric metasurface. (b) Micrograph of the partial photo-patterned LCs under crossed polarizers. Scale bars in (a) and (b) indicate 100 and 500 μm, respectively. (c) Simulated and measured focal lengths of the metalens from 0.9 to 1.4 THz with/without a saturated bias (75 Vrms). (d) Imaging of a "smiling face" mask using this metalens at bias OFF state. The image is clearly revealed within the designed broadband due to the achromatic focusing. (e) Imaging of the same mask with a saturated bias on LCs. The distortion at lower frequencies is due to the deviation of focal length from the achromatic one.
Compared with the optical or near-IR metalenses, the chromatic aberration of their THz counterparts is more significant due to the broadband of THz spectrum. Thus, it is meaningful to realize the active manipulation of chromatic dispersion in this band. Recently, researchers from Nanjing University have integrated a photo-patterned liquid crystal (LC) into the dielectric metasurface, and demonstrated the active manipulation of chromatic dispersion for the first time. The related research results are published on Advanced Photonics, Vol. 2, Issue 3, 2020 (Zhixiong Shen, Shenghang Zhou, Xinan Li, et al. Liquid crystal integrated metalens with tunable chromatic aberration[J]. Advanced Photonics, 2020, 2(3): 036002).
In their design, the metasurface can generate a linear resonant phase dispersion. LCs are responsible for the generation of frequency-independent geometric phase modulation. Via elaborately designing the structural parameters, the combination of both can achieve achromatic focusing within a designed broadband. When the transparent electrodes on both sides of the LC layer are biased, LCs are realigned and the geometric phase modulation vanishes. Correspondingly, the focusing effect exhibits a large dispersion, corresponding to the intrinsic dispersion of the metasurface. In this design, the chromatic aberration control of the LC integrated metalens is verified, and a significant dynamic broadband imaging contrast effect is exhibited. The design can also be extended to other active metadevices. A beam deflector with controllable dispersion is further presented as a demonstration.
The design flexibility of metadevices combined with the broadband electro-optical characteristics of LCs make the design competent for the wavefront control from the visible to the THz and microwaves. This is expected to give birth to a variety of active planar photonic elements based on LC integrated metadevices, enhancing the functional flexibility of the optical system.
