LCE metasurface: flexible light-controlled terahertz beam steerer

Recently, Professor Gu Jianqiang’s research group of the Terahertz Research Center of Tianjin University and the research group of Professor Luo Dan of Southern University of Science and Technology cooperated to realize the reconfigurable beam steering metasurface based on the principle of mechanical denaturation of liquid crystal elastomers in the terahertz band, and verified the broadband tuning of the terahertz outgoing beam by focusing infrared light and heating rise. This liquid crystal elastomer metasurface shows a good prospect of mechanically deformed metasurfaces, paving the way for the development of reconfigurable metasurfaces.

In recent years, active metasurface research has mainly focused on changing the dielectric constant, magnetic permeability and other structural parameters of the substrate, often introducing resonance effects and ohmic losses, and reconfigurable metasurfaces based on mechanical deformation have avoided these problems. However, at present, the mechanical deformation metasurface represented by MEMS and FIB-induced deformation technology is often complicated in processing technology or has limited resilience. As a new type of liquid crystal polymer material, liquid crystal elastomer (LCE) can undergo controllable and recoverable elastic deformation under the excitation of temperature rise or light irradiation, which has attracted the attention of many researchers in the fields of chemistry, materials and bionics. Its good photothermal drive deformation properties should be an excellent choice for active materials in active metasurfaces, but it is rarely used to regulate the response of metasurfaces.

In this work, the researchers prepared an LCE with a curved structure polymerized by polymerization of liquid crystal monomer (RM006), liquid crystal crosslinker (RM257) and photoinitiator (Irgacure 651). When the temperature rises above the phase change temperature, the different strains generated in the LCE layer bend the whole to the parallel orientation side of the molecule. The researchers used this LCE as a substrate to design an active metasurface with a C-shaped metal open ring as a resonator, which realized the continuous regulation of broadband terahertz wavefront steering. The linear phase gradient combination of the metasurface is composed of eight C-shaped open loops with a phase interval of π/4, and are periodically arranged on the LCE substrate, when the incident terahertz wave passes through this metasurface, the exit direction of the orthogonal polarization wave according to the generalized Snell’s law will be bent accordingly according to the phase gradient size of the C-shaped open loop, so as to realize the function of terahertz wavefront steering. The exit angle of the orthogonal polarized transmitted wave obtained by numerical simulation was in good agreement with the theoretical prediction, and the LCE metasurface was prepared by photolithography, vacuum evaporation and wet etching according to the structural geometric parameters determined by simulation (see Figure 1).

Figure 1 Deformation mechanism of liquid crystal elastomer and image of metasurface under optical microscope

The prepared LCE metasurface measured the exit direction of orthogonal polarized waves in an angle-resolved all-fiber asynchronous sampling terahertz time domain spectroscopy system, and confirmed that the metasurface can make the orthogonal polarization component of 0.48~1.1 THz orthoincident terahertz waves exit at an angle of 70°~25°. In order to achieve precise bending of the LCE substrate, the femtosecond pulse with a central wavelength of 1030 nm is single-axis focused on the edge of the sample with a cylindrical lens, and the infrared light is focused as a line on the LCE substrate. The photothermal effect bends the LCE of the irradiated part, while the unirradiated part remains flat, realizing the overall bending of the metasurface. By varying the power of the pump infrared light, the photothermal effect can be adjusted to control the bending angle of the LCE metasurface, and the modulation speed can reach the order of seconds (see Figure 2).

Figure 2 Change of terahertz beam deflection under infrared pumping

Since the incidence angle of terahertz waves changes when the metasurface is mechanically bent, according to the generalized Snell’s law, the exit angle will also change accordingly. When four pump optical powers are applied to the metasurface, bends of 0°, 8°, 15°, and 18° occur on the metasurface. With the increase of pump power, the output angle at the same frequency gradually increases, and the effect is more obvious at low frequencies. The 0.68 THz terahertz wave output angle reaches a tuning range of up to 22° under the highest power infrared laser pump (see Figure 3).

Fig. 3 Experimental results of light regulation of flexible LCE metasurfaces

In this work, the two control methods of focused light control and uniform heating are also compared. When the heating plate is used to heat the metasurface, the uniformly heated LCE substrate has an overall bend, instead of focusing the light control to achieve bending along the axis, and the phase gradient of the curved metasurface is no longer linear, the transmitted wave not only changes the direction of exit, but also the shape of the wavefront will be different from the incident wave, indicating that the regulation of the LCE substrate will become an important topic in the design of such mechanical deformation metasurface. In addition, by stitching the metasurface structure of the two deflectors in opposite directions, a light-controlled beam splitter in the terahertz band was experimentally verified. The potential demonstrated by liquid crystal elastomer metasurfaces provides considerable options for beam control, frequency modulation, and temperature sensing in the terahertz band, while also advancing the next generation of wireless communications, T-ray imaging, and terahertz intelligent sensing research. The design principles proposed in this work can be extended to multiple frequency bands, opening a novel avenue for the study of active metasurfaces.

The research results were published online in the international high-level academic journal Light: Science & Applications under the title “Active Terahertz Beam Steering Based on Mechanical Deformation of Liquid Crystal Elastomer Metasurface”.

The corresponding authors are Professor Gu Jianqiang of Tianjin University and Professor Luo Dan of Southern University of Science and Technology, co-first authors are Xiaolin Zhuang, a master student of Tianjin University, and Wei Zhang, a doctoral student of Southern University of Science and Technology, and other contributors include Wang Kemeng, Gu Yangfan, An Youwen, doctoral students of Tianjin University, and associate professor Zhang Xueqian of Tianjin University, Professor Han Jiaguang and Professor Zhang Weili of Oklahoma State University.

This work was supported by the National Natural Science Foundation of China and the Guangdong Provincial Basic and Applied Basic Research Fund. (Source: LightScience Applications WeChat public account)

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