Excite Spoof Surface Plasmons with Tailored Wavefronts Using High‐Efficiency Terahertz Metasurfaces

Abstract Spoof surface plasmons (SSPs) play crucial roles in terahertz (THz) near‐field photonics. However, both high‐efficiency excitation and wavefront engineering of SSPs remain great challenges, which hinder their wide applications in practice. Here, a scheme is proposed to simultaneously achieve these two goals efficiently using a single ultracompact device. First, it is shown that a gradient meta‐coupler constructed by high‐efficiency Pancharatnam–Berry (PB) meta‐atoms can convert circularly polarized (CP) THz beams into SSPs with absolute efficiency up to 60%. Encoding a parabolic phase profile into the meta‐coupler based on the PB mechanism, it is demonstrated that the device can covert CP beams into SSPs with focusing or defocusing wavefronts, dictated by the chirality of the incident wave. Finally, two distinct chirality‐dependent phase distributions are encoded into the meta‐coupler design by combining the PB and resonance phase mechanisms, and it is demonstrated that the resulting meta‐device can achieve SSP excitations with chirality‐delinked bifunctional wavefront engineering. THz near‐field experiments are performed to characterize all three devices, in excellent agreement with full‐wave simulations. The results pave the road to realize ultracompact devices integrating different functionalities on near‐field manipulations, which can find many applications (e.g., optical sensing, imaging, on‐chip photonics, etc.) in different frequency domains.


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A. Numerical studies on the meta-device examined in Figure 3 of the maintext Figure S1. FEM simulated Re[E x ] field patterns on (a, c) the x-z plane (with y=0) and Re [E z ] field patterns on (b, d) x-y plane (50μm below the artificial metal) of the meta-device depicted in Figure 3 of the main-text, as the center region is shined respectively by a (a, b) RCP and (c, d) LCP Gaussian beam at 0.4 THz. In our simulations, we use a source current on the upper boundary to illuminate the CP wave on the meta-device and perfectly matched layers (PMLs) along ±x direction to dissipate the guided-out SSPs on the artificial metal.
Periodic boundary conditions are applied along y direciton. We numerically integrated the powers carried by the guided-out SSP beam and the input CP beam, respectively. The ratio between them is defined as the absolute working efficiency of our meta-coupler, which is 60% at 0.4 THz. The length of the simulated PB meta-coupler is 1499.4 μm, the same as that of the sample studied in Figure 3.
Since the input CP wave contains TE and TM polarized components with equal amplitudes, it is intriguing to note that the SSP excitation efficiency achieved with our metacoupler (see Figure 3 in the main-text) can exceed 50% considering that the excited SSPs are only of the TM polarization. Such an intriguing result is caused by a polarization conversion effect in this process, as detailed in the following.
As schematically depicted in Figure S2, the conversion from incident CP propagating wave (PW) to SSP actually contains two steps: 1) a conversion from CP PW to TE and TM polarized driven surface wave (SW) bounded at the metasurface coupler; 2) a conversion from the driven SW to eigen SSP flowing on the designed artificial metals placed at two sides of the meta-coupler. In the first process, the polarization of impinging wave is well perserved, implying that the generated driven SW still possess nearly equal components of TE and TM polarizations (as schematically depicted in Figure S2a). However, in the second process (i.e., such driven SWs pass across the coupler boundary to flow as eigen SSPs on the artificial metals), we find that while the TM polarized driven SW can efficiently couple into the TM eigen-SSP, the TE-polarized driven SW, however, can convert a large portion of its energy into TM-polarized eigen-SSP on the artificial metals (as schematically depicted in Figure   S2b). It is such a polarization conversion effect that finally pushes the PW-SSP conversion efficiency to a value higher than 50% (60% in present THz devices and even higher value in microwave devices as demonstrated in Sci. Rep. 7, 1354Rep. 7, (2017). Figure S2. Two fundamental steps in the conversion process from free-space CP PW to nearfield SSP.
As the two processes are coupled together, it is difficult to directly see such a polarization conversion effect. To clearly reveal the physics, we purposely designed a THz artificial metal that supports both TE and TM SSPs at the working wavelength (see Figure S4), and put two such artificial metals on both sides of the meta-coupler (adopted in Figure 3 of the main-text) 4 to form a model system for numerical simulation (see Figure S3). We employed FEM simulations to study the transmissions through the junction of SSPs with different polarizations launched on the left-hand-sided artificial metal. As the input TE polarized SSP passes through the meta-coupler region, we find that strong TM SSP signal appears on the right-hand-sided artificial metal (see Figure S3b where the E z component is depicted) but the TE-SSP signal nearly disappears (see Figure S3c where the E y component is depicted). In contrast, as we change the polarization of input SSP to TM polarization, we obtain strong TM SSP signals and very weak TE SSP signals on the right-hand-sided artificial metal (see Figure   S3e-S3f)). These results unambiguously demonstrate the polarization conversion effect mentioned above, well explaining why the finally achieved PW-SSP conversion efficiency can exceed 50%.
Finaly, we briefly discuss the inherent physics. Here, TE SSP cannot propagate inside the region occupied by the PB meta-coupler, but TM SSP can propagate inside it (see Figure S3), caused by different boundary conditions for waves with two different polarizations.
Intriguingly, since our PB meta-atom is anisotropic, once it is rotated, it can convert some of the TE-polarized wave to TM-polarized one after transmission. Therefore, after passing through the region occupied by the meta-coupler, a considerable portion of TE-SSP has been converted to TM-SSP, as illustrated in Figure S3.  (gray region in (b)), simply because both couplers exhibit finite sizes. Here, the simulation conditions (i,e., the beam size, the boudnary conditions, the artificial metals) are totally same as those in Figure S1. We numerically integrated the powers carried by the guided-out SSP beam and the input CP beam, respectively. The ratio between them is defined as the absolute working efficiency of the grating coupler 7 D. Simulation details of the SSP focusing effect in Figure 4 of the main-text Figure S6. (a) Simulated Re[E z ] field patterns on the x-y plane (with z=0) and on the x-z plane (with y=0) in the meta-coupler as studied in Figure 4, while the meta-coupler is shined by a LCP beam at 0.4 THz. (b) Simulated SSP focusing efficiency of the PB meta-coupler as a function of frequency. Here, we integrate the power of the focused SSP within a rectagular region at the its focal plane (see (a)), and integrate the power illuminated on the meta-coupler of the size 1500  5160μm 2 . The ratio between these two values is defined as the working efficiency. Note that our device still works even at frequencies with 0 k   (gray region in (b)), simply because the meta-coupler exhibits a finite size. Here, a scouce current boundary is used at the upper boundary (along +z direction) to illuminate CP on the meta-coupler and PMLs are applied at the boundaries along ±x and ±y directions to absorbe the out-going SSPs on the artificial metal. (S1) To build such a device, we need to sort out all meta-atoms with appropriate  and  . For our meta-atoms, their total spin-dependent phases contain two parts, the resonance phase r e s  and the PB phase PB  . Therefore, we have r e s P B where re s ( , )  Figure S8(b) as a black line.
However, we note that the meta-device presented in Figure 5 of the main-text works for SW manipulations. Each row of such a device is a gradient meta-coupler to convert normally incident propagating wave into a SSP, but with different initial phase. It is such a phase, defined by the near-field (NF) phase NF r e s  , working to modulate the wave-front the whole SSP beam. Therefore, it is more accurate to use the NF r e s  relation to select our meta-atoms with different geometric structures (i.e.,  ). The calculation method is schematically shown in  The proposed meta-coupler for near-field SSP excitation and wavefront engineering can find many photonic applications in THz and other frequency domains, e.g., enhancing lightmatter interactions, coupling on-chip optical devices, bio-or chemical sensing, imaging, etc.
For example, we have numerically demonstrated that such meta-device can efficiently couple the optical waveguide with its end put at the SSP focal point of the meta-coupler proposed in Figure 4. As shown in Figure S10c and S10d, most of the input CP wave can be efficiently converted to SSP at the focal point and then further coupled into the waveguide. For comparison, the conventional coupling method (e.g., the direct illumination by the input Gaussian beam) is obviously less efficient, which is quite reasonable considering that the cross-section of the waveguide is much smaller than input beam size, as depicted in Figure   S10e and S10f.