Topological photonics has sparked substantial interest due to its introduction of a new photonic degree of freedom, namely, the valley, which refers to a pair of non-equal degenerate energy extrema at the corners of the Brillouin zone^[1–3]. By breaking the spatial-inversion symmetry, the degeneracy would lift up, giving rise to a nontrivial bandgap. As a result, the valley pseudospin and photonic valley Hall effect were utilized for manipulating electromagnetic waves on demand^[4–8]. An ingenious property of valley topological photonic crystals is that they support valley kink states at the interface of two distinct topological regions, which transmit robustly without backscattering around sharp corners^[9–12]. Owing to the robust transport between topologically distinct domains, topological photonics has acquired much attention in wired and wireless communication fields, including slow-light topological photonics^[13], valley-conserved topological integrated antennas^[14], high-Q resonators^[15–18], photonic diplexers^[19], terahertz biosensors^[20], and on-chip waveguides^[21–26].

Recently, a novel edge state arising from quantum anomalies, named the chiral edge state (CES), came into view by tuning the on-site potentials at the external boundary of the valley photonic crystals (VPCs)^[27–30]. It is worth noting that the CESs share a common origin with the valley kink state and exhibit similar robustness, backscattering immunity against disorders, and unidirectional transmission capabilities^[13–15], making them exceptional candidates for compact terahertz (THz) communication systems^[31,32]. Compared to the kink states, which rely on domain walls, the CESs exist at the interface between the VPC and air. Therefore, devices based on CESs occupy smaller volumes, thus enabling more compact dimensions. To date, CESs have been reported in various areas, including photonics^[33–35], acoustics^[36,37], and mechanics^[32], which have the potential to play a role in THz wireless communications such as the pioneering valley-locked topologically protected insulators^[13–26].

In this work, we demonstrate the THz CESs with all-silicon wafers by tuning the on-site edge potentials. More specifically, we realize the gapless, single-mode, and linear dispersion of CESs by adjusting the external dielectric triangle holes of VPCs. The THz waves supported by the CES can be transmitted only along the interface between silicon and air, without the need for internal boundaries or cladding layers, even in the presence of scatterers near the interface, which will be of great value for THz integration and miniaturization. Furthermore, we achieve highly directional topological photonic routing by utilizing unidirectional excitation of valley pseudospin-locked CESs. Besides the robust transmission across a 120°-turning sharp corner, we also demonstrate smooth transition between CESs and the acknowledged kink states for the THz inner-chip connection. Lastly, an interchip wireless interconnection system, i.e., a near-field communication (NFC) system between two mobile phones, is built to meet the demands of advanced THz communications. The layer robustness and the far-field confidentiality of the system are demonstrated. And the minimum size of our proposed NFC chip is about $12\mathrm{mm}\times 2.6\mathrm{mm}$, which is over 10 times smaller than the low-frequency NFC antennas (e.g.,  about $40\mathrm{mm}\times 50\mathrm{mm}$^[38,39]), making it more suitable for integrated applications and fast communications on mobile phones.

The all-silicon VPC slab (${ϵ}_{\text{silicon}}=11.7$) that supports the CESs is illustrated schematically in Fig. 1(a). The lattices are separated into two regions, the bulk region [i.e., the purple part of Fig. 1(a)] and the VPC–air boundary region [i.e., the magenta part of Fig. 1(a)]. In the bulk region, an equilateral triangle air hole with a side length of $l_1=l_0+\mathrm{\Delta }l$ and another inverted hole with a side length of $l_2=l_0-\mathrm{\Delta }l$ are both contained in the unit cells, with $l_0=375\mathrm{\mu m}$ and $\mathrm{\Delta }l=a/8$, as shown in Fig. 1(b). When the bulk parameter $\mathrm{\Delta }l=0$, a pair of Dirac points occurs at the K and ${\mathrm{K}}^{\prime }$ valleys of the Brillouin zone at 109 GHz, preserved by the inversion symmetry and time-reversal symmetry. When $\mathrm{\Delta }l$ increases to $a/8$, the inversion symmetry breaks, and the Dirac points open to form a complete bandgap from 106.4 to 114.1 GHz, as depicted by the red solid curves in Fig. 1(c). The simulated $H_z$ fields, Poynting vectors, and phase profiles of the split eigenstates [${\mathrm{K}}_1$ and ${\mathrm{K}}_2$ in Fig. 1(c)] at K valley are plotted in Fig. 1(e). Here, the CES is the transverse-electric (TE) polarization of the electric field confined within the plane.

The bulk Hamiltonian of the built VPC is defined by $\widehat{H}=v_D·(-j{\partial }_y{\sigma }_x+\delta k_x{\sigma }_y)+\mathrm{\Delta }{\tau }_z{\sigma }_z$, where $v_D$ is the group velocity, $\mathrm{\Delta }$ is the effective mass induced by inversion-symmetry breaking of the air holes with different side lengths, and ${\sigma }_i$ and ${\tau }_i$ ($i=x,y,z$) are the Pauli matrices. According to Refs. [23–25], the bulk Hamiltonian is a required though inadequate condition to obtain CESs with linear dispersion relation ${\epsilon }_k=\pm v_{D}k$. The boundary matrix $\widehat{M}$ describing the boundary conditions at the edge should satisfy $\widehat{M}={\tau }_z{\sigma }_y$. However, the boundary matrix of conventional zigzag edge is $\widehat{M}={\tau }_z{\sigma }_z$, which cannot support CESs. Thus, it is essential to adjust the on-site edge potentials of the zigzag edge. When ${\theta }_v$ is introduced, which is controlled by the on-site edge potentials, the effective boundary matrix could be written as ${\widehat{M}}_{\text{zigzag}}={\tau }_z({\sigma }_y\sin {\theta }_v+{\sigma }_z\cos {\theta }_v)$. With ${\theta }_v=\pi /2$, ${\widehat{M}}_{\text{zigzag}}=\widehat{M}$, which enables ${\epsilon }_k=\pm v_{D}k$, the CESs are realized. Based on the above theoretical analysis, we tune the on-site potentials by adjusting the side length of the outmost air holes in the VPC–air boundary region to acquire the required linear dispersion relations. In Supplementary Material, the simulations confirmed that increasing $l_3$ can result in an upward bending of the band curves, making it highly promising to achieve linear dispersion by adjusting the side length of the outermost air holes. Especially, when $l_3=550\mathrm{\mu m}$, it perfectly matches the situation ${\theta }_v=\pi /2$, exhibiting the desired linear dispersion within the complete bulk band gap, as plotted in Fig. 1(d).

Next, the robust broadband transport of CESs is explored. The schematic views of the straight and 120°-turning interface between VPC and air are illustrated in Figs. 2(a) and 2(b). When Port 1 is excited, CESs support wave propagation along the straight and 120°-turning interface without backscattering, as shown in Figs. 2(c) and 2(d). To confirm the robustness and compact integration of the CESs for the potential chip application, a cuboid scatterer made of copper is placed near the interface in the air. The $|H_z|$ distribution shown in Fig. 2(e) is nearly indistinguishable from that without the scatterer, exhibiting the exceptional confinement capability of CESs. In addition, Fig. 2(f) plots the scattering parameter S21 between the output and input ports. The transmission efficiencies [Figs. 2(f), 4(e), 5(c), and 5(d)] are calculated by the ratio of power integrations^[40,41]. See details in the Supplementary Material Section S4. It can be seen that, an about 30-dB drop occurs in the transmission of bulk states, while the CESs keep a high throughput (i.e., $-3\mathrm{dB}$) over the frequency region from 110 to 113.9 GHz along straight, 120°-turning interfaces, and near a copper scatterer, which demonstrates the efficient transport of CESs. The feasibility of the experiment of the chip is discussed in Supplementary Material Section S5.

Moreover, CESs possess the characteristic of unidirectional transmission, wherein electromagnetic energy cannot flow along directions that are not valley-locked. This property is known as chirality in the field of condensed matter physics^[42,43]. The characteristics of unidirectional transmission can be demonstrated by a chiral source. We utilized a clockwise-rotating array of four dipole sources to generate right-handed circular polarization (RCP) vortices and counterclockwise rotation to generate left-handed circular polarization (LCP). The distributions of the four-dipole sources and their related magnetic field phases are depicted on the bottom side of Figs. 3(a) and 3(b), respectively. The addition of a source with an RCP phase vortex at the outer boundary of the VPC enables selective excitation of the ${\mathrm{K}}^{\prime }$ valley, and the corresponding magnetic field amplitude distribution $|H_z|$ at 110 GHz is illustrated in Fig. 3(a). With near-zero energy flowing in the opposite direction, the wave travels backward at a negative group velocity, which can be the solid evidence of unidirectional propagation. For the source with the opposite chirality, the wave propagates in the K direction, as shown in Fig. 3(b). The energy transfer of the THz wave for each chirality is also confirmed by the corresponding Poynting-vector distribution shown in Figs. 3(c) and 3(d), respectively.

It is noteworthy that the kink states existing between two topologically nontrivial regions exhibit similar characteristics to CESs. Previous studies have demonstrated that CESs and kink states have the same topological origin and can achieve a smooth transition^[31,32]. In our work, this type of state transition can achieve inner-chip connections between different components within the chip. We speculatively envision the NFC chip based on CESs in mobile phones and the inner-chip connection in the mobile phone, as shown in Fig. 4(a). The VPC with both states is designed in Fig. 4(b) to show the smooth transition, where the zigzag boundaries and the domain walls converge at 60° acute turning angles. The ‘kink’-type domain wall [the red line in Fig. 4(b)] is constructed between two VPCs with opposite $\mathrm{\Delta }l$ values (the upper domain VPC 1 with $\mathrm{\Delta }l=a/8$ and the lower domain VPC 2 with $\mathrm{\Delta }l=-a/8$). The energy band diagram of kink states is plotted in Fig. 4(c), exhibiting linear dispersion within a bandgap comparable to that of the CESs. When the VPC is excited from Port 1, the field amplitude distribution $|H_z|$ at 110 GHz is displayed in Fig. 4(d). The smooth transition of electromagnetic waves from kink states to CESs makes them propagate along the intricate zigzag boundaries without encountering any undesired backscattering. The transmission spectrum can be obtained by analyzing the S parameters, e.g.,  S21 and S31 in Fig. 4(e), where Port 2 and Port 3 receive about half of the energy from Port 1, respectively. Because of the smooth transition between kink states and CESs, the results will be of great value for inner-chip mode conversion and beam splitting.

Enabling topological-insulator-based wireless interconnection in the THz frequency spectrum remains a noteworthy concern for sixth-generation (6G) communication. The smooth transition between kink states and CESs can efficiently transport the information from the internal region of the THz silicon-based chips to the external boundaries, which brings strong potential for interchip and intersystem communications. To validate this concept, we speculatively envision our NFC chip being placed in a mobile phone, as depicted in Fig. 5(a), where the VPC–air boundaries of the two NFC chips are aligned and separated by a distance of $d$. In the simulation, two sheets of polyethylene (PP) film (${ϵ}_{\mathrm{PP}}=2.1$) are added between the chips to represent the phone casing. The NFC chip has $N$ layers in the $y$ direction, as shown in the inset of Fig. 5(a). When Port 1 of Chip 1 is activated, the $|H_z|$ field distribution of the two chips for $N=10$, $d=0.7\mathrm{mm}$ in Fig. 5(b), demonstrates that virtually all of the energy is transferred from NFC Chip 1 to NFC Chip 2. To verify the layer robustness of the NFC chip, we simulated the scattering parameters S21 and S31 of the two chips with $N=4$, 7, and 10 at $d=0.7\mathrm{mm}$ in Fig. 5(c). It can be observed that the number of layers has virtually no impact on interchip energy transfer, enabling more compact integration on mobile phones. In addition, an about 20-dB drop between S21 and S31 shows a high degree of energy transfer efficiency and isolation. Because the wireless module is based on evanescent wave coupling and the dispersion of the CESs below the light line, the wireless interconnection system exhibits high transmission speed, excellent security, stability, and immunity from far-field interference. Figure 5(d) shows the transmission lines with $d=0.9$ and 2 mm at $N=10$. At $d=0.9\mathrm{mm}$, the NFC system initiates decoupling, and at $d=2\mathrm{mm}$, the system achieves complete separation, demonstrating excellent far-field confidentiality and security. Our chiral-edge-state module, with a minimum size of about $12\mathrm{mm}\times 2.6\mathrm{mm}$ NPC chip, effectively meets the demand for miniaturization and integration of mobile terminals in the coming 6G communication.

In conclusion, we proposed a novel CES in all-silicon topological valley photonic crystals at the THz regime—the next-generation communication band. It differs from the well-known kink states that rely on domain walls on both sides. Instead, it exists at the external boundary through the tuning of the on-site potentials. Supported by different topological interfaces, both CESs and kink states possess the characteristics of chiral transport and robust transmission^[13–22]. We demonstrated the robust transport capability of CESs in the THz band through its backscattering immunity against sharp bends and unidirectional transmission. Moreover, we demonstrated smooth transition between kink states and CESs due to their same topological origins to realize inner-chip connection. The proposed CES does not depend on the internal boundaries and external claddings and maintains robust transport even in the presence of scatterers, which indicates it allows a smaller physical footprint than traditional kink states for THz silicon chips. Finally, we envision that our NFC chips could be used to build a wireless interconnection system between mobile phones and demonstrate its layer robustness and far-field confidentiality. Our NFC chip employed in mobile phones, with a minimum size of $12\mathrm{mm}\times 2.6\mathrm{mm}$, is over 10 times smaller than the previous low-frequency NFC chips^[38,39]. Additionally, our NFC chip has a broad 3-dB bandwidth of approximately 5 GB around 111 GHz, indicating promising prospects for near-field communications. And the working frequency of NFC chips could be extended to higher frequencies. We expect that our work has great potential for future integrated applications in the THz regime, including 6G wireless communication^[44–50], frequency division multiplexers^[51–54], and topological lasers^[17–19].