Terahertz (THz) functional devices have important application value in the fields of radar, high resolution imaging, biochemical sensing. In recent years, terahertz metasurfaces have made great progress in manipulating free-space beams flexibly. To further meet the requirements of THz system miniaturization, high efficiency and low energy consumption, it is also essential to explore the mechanism of controlling surface wave propagation on the chip. Surface plasmons are highly expected in on-chip photonics and are conducive to the construction of integrated ultra-compact devices. However, in the THz region, surface plasmonic waves (SPWs) can hardly confined at flat metal surfaces, which is a challenge for the development of THz on-chip devices.

Structured metal metasurface composed of sub-wavelength elements provides a scheme for on-chip transmission of THz SPWs. Although many researches have focused on efficient generation and flexible wavefront engineering of SPWs, polarization-dependent surface wavefront control is mostly limited to narrow band. In addition, due to the difficulties in the functional material integration on-chip and the active interaction with THz waves, the functions of previous devices are mostly fixed or can only be switched passively. At present, there are few experimental reports on active THz surface plasmonic devices. Therefore, there is an urgent need to develop THz devices for broadband on-chip transmission and active manipulation.

The team demonstrated a liquid-crystal (LC) integrated surface plasmonic metadevice based on arc-arrayed pair-slit resonators, which successfully realized the broadband focusing and the active modulation of on-chip SPWs. Multiple concentric arc resonators with mirror symmetry are adopted to achieve spin-selective unidirectional focusing of SPWs. More importantly, the broadband feature is realized by selecting several slit geometric sizes and further considering the corresponding arc radius to meet the phase-matching conditions. Moreover, the integration of LC introduces an approach for the polarization conversion enabling SPW intensity modulation and dynamic energy distribution between left and right focal spots. Relevant research results were recently published in Photonics Research, Volume 12, Issue 10, 2024. [Yi-Ming Wang, Fei Fan, Hui-Jun Zhao, Jing Liu, Yun-Yun Ji, Jie-Rong Cheng, Sheng-Jiang Chang, "Active broadband unidirectional focusing of terahertz surface plasmons based on a liquid-crystal-integrated on-chip metadevice," Photonics Res. 12, 2148 (2024)]

Figure 1 (a) The function diagram of broadband SP metasurface; (b) Schematic diagram of the working principle of LC-integrated SP metadevices; (c) Near-field scanning THz time-domain spectroscopy system; (d) Transmission spectra of slits with different geometric sizes; (e) The simulated near-field (real part) distribution and (e) phase maps of SPWs excited by each arc resonators at different frequencies.

The broadband SP metasurface based on the arc-arrayed pair-slit resonators is shown in Figure 1 (a), of which the mirror symmetric structure makes the SPWs selectively focus to one side according to the incident spin state. As shown in Figure 1 (d) -1 (e), slits with different geometries correspond to different SPW excitation frequencies. By further precisely designing the arc radii where different geometric slits are located, the SPWs excited by each arc meet the phase matching condition of constructive interference at the focal point, thus broadening the operating frequency band. More subtly, Figure 1 (b) is a schematic of the LC-integrated SP device. The LC layer plays the role of a quarter-wave plate, and the electric field drives the LC rotation to realize the active modulation of SPW energy between the left and right focal spots.

The experiments are conducted by the near-field scanning THz time-domain spectroscopy system shown in Figure 1(c). Figure 2 (a) -2 (b) shows the wideband performance of the metasurface for spin-selective unidirectional on-chip focusing. As shown in Figure 2 (c), when no external electric field is applied, the LC molecules are pre-oriented along the y-axis, and most of the SPW energy is concentrated around the left focal spot. With the increase of the applied electric field, the LC molecules gradually rotate in the x-y plane towards the x-axis, and the SPW energy is distributed in a certain proportion between both sides. Once the external electric field reaches 12V/mm, the LC molecules are completely oriented along the x-axis, so that the SPWs focus on the right center. The simulation and experimental results of this process are shown in Figure 2 (d).

Figure2 (a)Near-field spectra of broadband and narrowband SP metasurface in focusing and defocusing cases; (b) The simulated near-field intensity distributions of the broadband and narrowband plasmonic metasurfaces under the LCP incidence; (c) Diagram of active energy distribution in the LC-integrated broadband on-chip metadevice; (d) At 0.45THz, the near-field intensity distribution with the dynamic modulation of SPWs at the left and right focal spots.

The results show that the operating bandwidth is 270 GHz of 0.33 ~ 0.60 THZ, which is significantly widened by 2.45 times compared to that of the narrowband devices. In terms of the on-chip focal spots, the transverse resolution is 0.32, and the horizontal relative offset is only 12%. In addition, the experimental modulation depth of the SPW focusing can reach up to 73%.

The strong dispersion of SPWs hinders the development of THz waves in broadband communication and optical information processing, so it is of grear value to achieve on-chip achromatic wavefront modulation. Tthere are few reports on active THz surface plasmonic devices. The strategy of introducing functional materials into microstructures has been widely applied to metasurface to actively manipulate free-space THz waves, which has a reference significance for the development of active THz on-chip metadevices. In particular, the LC has broadband adjustable anisotropy in the THz regime, so remarkable phase shift can be reversibly and continuously regulated by thermal, optical, electrical, and magnetic means. Therefore, the combination with LC configuration is considered a promising method to develop active THz on-chip devices.

In the next step, the team will investigate the wavefront manipulation mechanisms of SPWs. The team will also study the properties of function diversity, multi-channel multiplexing, active tunability, reconfiguration, programmability in surface plasmonic devices, which laying the foundation for the applications of integrated photonic devices in signal processing, information caching, transmission and security.