Silicon photonics, with its high integration density and excellent optical properties, is emerging as a promising platform for large-scale photonic integrated circuits [1–3]. A series of on-chip active and passive devices, such as filters, wavelength division multiplexers (WDMs), beam splitters, modulators, and detectors, are currently designed based on silicon waveguides, which have significant applications in optical communications, optical sensing, and optical computing [4–10]. Since silicon has a high refractive index in the telecommunication band, light could be strongly confined within the waveguides, enabling low transmission loss and high-density integration. However, the strong two-photon absorption (TPA) of silicon limits its transmission power capacity [11]. It is possible to increase the mode field area by increasing the waveguide width or using a thick silicon waveguide to reduce the TPA of silicon. However, it will cause the waveguide to support more higher-order modes, resulting in mode crosstalk and large losses.

Topological edge states have garnered significant attention recently due to their robustness and immunity to backscattering, which arises from the gapless dispersion relation [12–24]. These states have been extensively studied in electronics, acoustics, and elastic and photonic systems. According to the bulk-edge correspondence, a valley-locked topological kink state can be generated at the domain wall of a pair of topological valley photonic crystals (VPCs) with opposite valley Chern numbers or band inversion [22,23,25–31]. To date, robust optical devices based on topological edge states, such as power splitters [6] and WDMs [30], exhibit excellent properties. In these devices, topological edge states enable light to transmit sharp bends with little scattering and suppress backscattering, even over a large bandwidth. However, the topological edge states are confined to the vicinity of the domain wall, which has a non-adjustable width. This limitation restricts the transmitted energy capacity and the ability to integrate with other conventional waveguide devices.

To address the above problems, we proposed a topological valley-locked heterostructure waveguide (TVHW) constructed by a pair of topological VPCs with opposite valley Chern numbers for a telecommunication band on the silicon-on-insulator (SOI) platform, which exhibits similar properties to valley-locked edge states, such as being gapless, single-mode, immune to backscattering, unidirectional transmission, and robustness against defects [25]. The degree of freedom (DOF) of waveguide width provides remarkable flexibility in manipulating light. Simulation and experiment results confirm that TVHWs can transmit light with ultra-low loss in waveguides with varied widths, even in the region with structural mutation. Furthermore, we demonstrate by means of enhanced third-harmonic generation (THG) imaging that these TVHWs can sharply converge the optical energy in narrower regions, significantly strengthening the light–matter interaction. These attributes promise future applications in integrated photonic chips, such as on-chip topological lasers [16,20], on-chip communication [3,29], nonlinear optical processes [7,9], and high-capacity energy transmission [25,31].

The designed TVHW consists of three domains: A, B, and C, as shown in the left panel of Fig. 1(a). Unlike the extensively studied A|C-type waveguide with a topological edge state, this design introduces the B domain of x layers between A and C. The three domains are photonic crystals composed of a triangular air-hole honeycomb lattice with the same lattice constant being a=455nm. In domain B, the two triangular air-holes of the PhCs primitive cell have equal side lengths d3=275nm. Consequently, the photonic crystal in domain B exhibits time-reversal symmetry and inversion symmetry [32], resulting in a pair of degenerate Dirac points at 200 THz in the K and K′ valleys, as depicted in Fig. 1(b). By setting different lengths of air-holes (d1=360nm and d2=190nm) in opposite ways, we can obtain VPCs with opposite valley Chern numbers in domains A and C, respectively. Since d1 and d2 are unequal, the inversion symmetry between the VPCs in domains A and C is broken, lifting the degeneracy of the K and K′ points and creating a topological bandgap from 188.6 THz to 209.4 THz, as shown in the shaded region of Fig. 1(b). After lifting the degeneracy, the VPCs in domains A and C have the same energy band structure due to their mirror symmetry, but a band inversion occurs in the K and K′ valleys—a pseudospin with opposite polarity, as marked by the “+” and “−” signs.

The projected energy band diagram of the TVHW with A|B10|C along the kx direction is shown in the left panel of Fig. 1(c), where the blue dashed line indicates the topological bandgap created by the VPCs of the domains A and C. There is a gapless band labeled in red that crosses the entire bandgap. This band has the same characteristics as the valley-locked edge states in the domain of A|C, including a gapless band locked by the K (K′)-valley momentum with positive (negative) group velocity as well as a linear dispersion. Therefore, this TVHW allows unidirectional transmission and permits light to turn through sharp corners without back scattering [22,33]. The TVHW differs from the valley-locked edge states of A|C in the mode field distribution. The edge states of A|C are concentrated in a narrow region at the boundary of the domains A and C, which results in significant TPA of the silicon. In contrast, the TVHW of A|Bx|C is distributed over the entire B region, as depicted by the magnetic field and the phase distribution in the right panel of Fig. 1(c). Additionally, new guided modes emerge within the topological bandgap, marked as 0th+, 0th−, 1st+, and 1st− in Fig. 1(c). These waveguide modes are gapped and lack topological protection, and thus do not exhibit unidirectional transmission properties and robustness. The gap region (shaded area) between 0th+ and 0th− constitutes the only pass-through window for the frequency protected by the topological property. Consequently, the TVHW supports single-mode transmission in this frequency range. As the waveguide width determined by the number of layers in domain B increases, the bandwidth of the single-mode transmission decreases gradually, and higher-order guided modes appear within the bulk gap, as shown in the shaded area of Fig. 1(d). We can speculate that the bandgap eventually closes as the layer number in domain B increases further, but light at frequencies locating within the bandgap maintains robust single-mode transmission regardless of the waveguide width changes, which is significant for on-chip optical signal transmission.

The unique width DOF endows the TVHW with the potential of transmitting high-capacity energy. We then designed three TVHWs with different widths (x=2, 6, 10) to compare their transmission capacity. As depicted in Fig. 2(a), the two-dimensional photonic crystal plate models are calculated by the equivalent refractive index method. Three TVHWs are excited by the transverse electric (TE) field excitation model with uniform power density distribution. The excitation source size is set equal to the waveguide width, ensuring consistent energy input per unit length across all waveguides. Under these conditions, the insertion loss exhibits a linear dependence on the waveguide width, thereby permitting direct comparison of transmitted power between structures. The transmission energy fluxes of the three waveguides are obtained by integrating the Poynting vectors at the output ports. The normalized transmission capacities at different wavelengths are shown in Fig. 2(b). As the number of layers in domain B increases, the transmission energy capacity also increases, demonstrating the high-capacity signal transmission capability of the proposed heterostructure waveguide. Furthermore, the number of layers in the B region can be adaptively optimized for different on-chip devices to achieve optimal overall transmission performance. Additionally, we applied the spatial Fourier transformation to the electric field within the blue dashed box in Fig. 2(a) to obtain the Fourier spectrum. As shown in Fig. 2(c), only the K-valley is excited, indicating that the TVHW is locked by the K-valley momentum and can only propagate along the K-direction, making it immune to back-scattering.

Similar to the valley-locked topological edge state [32,34], the TVHW is locked by the valley momentum, providing robustness against structural defects. To illustrate this, we designed three waveguides with different structural defects. Structures 1 and 2 have indentation and bulging inside the domain B, respectively, while Structure 3 removes some air holes at random locations in the domain B as disorder defects, as depicted in the top panel of Fig. 3(a). The TVHWs of the three structures are excited by the transverse electric field using the same configuration as described above. The electric field distribution at the wavelength of 1520 nm is shown in the bottom panel of Fig. 3(a). We can see that TVHW can transmit robustly in the domain B without scattering loss due to its single-mode operation and momentum locking. We further calculate the transmission spectra of the three waveguides, as shown in Fig. 3(b). In waveguide structures with varying configurations, the transmission window of the topological bandgap (indicated by the yellow shaded area) refers to the wavelength range where transmission spectra remain stable and efficiency is consistently high. This is because the TVHW in the shaded region is single-mode and locked by the K(K′)-valley momentum, making it immune to backscattering at structural mutations and preventing mode crosstalk, while the transmission spectra of the structures differ more outside the shaded region due to the presence of higher-order guided modes. Furthermore, it should be noted that the protection mechanism of the valley Hall effect relies exclusively on K-valley momentum locking, making it fundamentally vulnerable to symmetry-breaking perturbations. The TVHWs contraction or expansion mainly affects the window of the topological bandgap, which decreases with the increase of the waveguide width, and when the waveguide width increases to a certain level, the topological protection property disappears.

The waveguide width as a new DOF for TVHW offers more possibilities for on-chip light manipulation, such as high-capacity optical transmission and energy convergence [25]. Since the presence of the topological single-mode within the bandgap is independent of the width of domain B, we can construct an on-chip energy concentrator by combining wide and narrow waveguides. We verified the robustness of TVHW for immunity to backscattering and its capability in energy convergence through numerical simulation. As depicted in Fig. 4(a), three different types of waveguides are designed by composing A|B10|C and A|C. The waveguide widths of Structures 1 and 3 were drastically changed, and Structures 2 and 3 contain sharp corners. Then, we excited the TVHW on the left side of the waveguide and obtained the corresponding transmission spectra, as illustrated in Fig. 4(b). We can see that in the shaded region, where only the TVHW exists, the three waveguides maintain a high transmittance and good uniformity. Outside the shaded region, Structure 1 exhibits a broader high-efficiency transmission bandwidth than the Z-shaped waveguide. This is due to the presence of the trivial edge modes in both A|B10|C and A|C configurations, which leads to mutual coupling and forward propagation. However, the coupling efficiency between the trivial edge modes of these different TVHWs has yet to be thoroughly investigated.

In contrast, the Z-shaped waveguide structure exclusively supports the K-valley-momentum-locked valley-Hall topological mode through the Z-bend. As a result, wavelength components outside the transmission window undergo strong backscattering, leading to significant transmission losses. Moreover, the bandgap transmission window in Fig. 4(b) shows a slight deviation from that in Fig. 3(b). This discrepancy arises from the coexistence of topological modes and trivial bulk modes near the bandgap edges. Variations in waveguide configurations lead to differing degrees of mode interference, which in turn affect the bandwidth of the transmission window within the bandgap. It also demonstrates that the TVHW (A|B10|C) and valley-locked edge state (A|C) can be efficiently coupled to each other, making the heterostructure compatible with existing valley-locked edge state devices. This provides a novel and robust means of on-chip optical transmission compared to conventional ridge waveguides [1,2]. From the electric field strength distribution in the bottom panel in Fig. 4(a), the waveguide disperses energy in the wide region, and when the waveguide becomes sharply narrower, the energy converges, producing field enhancement. To quantify the ability of the TVHW to concentrate energy, we integrate the electric field strength along the four integration lines a, b, c, and d in Fig. 4(a), respectively, and obtain the normalized electric field strength distributions at different locations, as shown in Figs. 4(c) and 4(d). The results show that the electric field strengths along lines b and d are much larger than those along lines a and c, which indicates that the waveguide produces a strong field enhancement in the narrow region.

To further illustrate the reliability of the TVHW in practical applications, we fabricated a series of structures by electron beam lithography and inductively coupled plasma etching on a 220 nm SOI platform. Figure 5(a) shows optical micrographs of the above structures, where the waveguide channels consist of domain B with different number of layers x. Structures 1 and 2 are composed of A|B30|C and A|C, corresponding to the coupling of the TVHW and valley-locked edge state to each other, respectively. Structure 3 is a Z-type waveguide composed only of A|B30|C. Structure 4 is based on Structure 3, with the corner channel compressed into A|C. Structure 5 is composed of A|B30|C and A|B56|C, and Structure 6 is composed of A|B30|C and A|B12|C, corresponding to the contraction and expansion of the waveguide, respectively. Structures 7 and 8 show a continuous change of the waveguide from wide to narrow. Grating couplers are designed at both ends of the waveguide to assist the light coupling-in and -out with the optical fibers. We coupled a narrowband tunable laser into the input grating coupler, and the transmission spectra were collected by the optical power meter on the output grating coupler. The normalized transmission spectra [6] are shown in Fig. 5(b), and we can see that all the devices exhibit high transmittance as well as good transmission uniformity in the frequency range of the yellow shaded region (1516–1540 nm), which is the single-mode region of the TVHW. The actual tested bandwidth is smaller than the simulation results due to the fabrication tolerance. Although Fig. 5(b) does not exhibit clear Anderson localization due to backscattering within the topological bandgap transmission window, existing evidence suggests that valley topological photonic crystals cannot fully suppress backscattering [33]. This limitation stems from the retention of time reversal symmetry by the valley-Hall effect, where topological protection depends solely on valley freedom locking and remains effective only for specific disorder scales, such as lattice-scale defects. Moreover, since valley topological photonic crystals support two topological modes in opposite directions within the bandgap, disorder may still lead to inter-valley scattering.

To further verify that these TVHWs with different waveguide widths could converge energy, we used picosecond pulsed light at 1520 nm generated by an optical parametric oscillator (OPO) to excite the TVHW. The peak power reaches 4 MW, and the average power measured by the power meter at the fiber input is approximately 25 mW. The coupling efficiency of the silicon grating coupler is around −8dB, and the third-harmonic generation (THG) of the silicon is monitored using a silicon-based CMOS camera, as shown in Fig. 5(c). By adjusting the incident optical power, we can observe THG only in the narrow waveguide regions of the TVHW, and the light passes through the structural mutations without significant loss. Due to limitations in the processing and testing conditions, the transmission loss and power saturation threshold of the TVHWs have yet to be characterized. However, existing results suggest that TVHWs can modulate energy density at different locations with flexible width DOF. Compared to the edge state A|C, the proposed TVHW structure shows differences in transmission performance, particularly in its ability to support higher-power signal transmission. We can use a wide waveguide in the region where high-capacity transmission is required, which can effectively reduce the transmission loss generated by the TPA of the silicon [11], and transition to a narrow waveguide in the working region to concentrate the energy for realizing local field enhancement. This strategy eliminates the need for tapered structures in conventional waveguide width transitions, thereby enabling more compact device architectures. Traditional waveguides exhibit significant vulnerability to fabrication defects, with their transmission properties easily affected by structural perturbations. These limitations become especially pronounced as the waveguide width increases, leading to multimode operation and mode crosstalk. In contrast, TVHWs provide a robust solution for optical signal transmission. Their inherent design ensures strong resistance to defects and structural variations, maintaining backscattering immunity within specific wavelength ranges. Additionally, counter-propagating topological modes within the photonic bandgap remain isolated, preventing inter-modal crosstalk and preserving signal integrity. Consequently, TVHWs are expected to deliver superior performance in applications such as on-chip mode converters, compact photonic circuits with multiple bends, and on-chip optical isolators.

In conclusion, we composed a heterostructure waveguide by adding a Dirac layer in the middle of two VPCs. This TVHW has similar transmission properties to the topological valley-locked edge states, such as single-mode and unidirectional transmission properties with robustness under K (K′)-valley momentum locking. Additionally, the structure also has a width DOF, allowing it to be more flexibly connected to other devices [6,24,29,35–37]. It holds significant potentials for high-capacity on-chip optical transmission, energy concentration, field enhancement, laser emitting, and nonlinear optics applications.

Acknowledgment. We thank the Analytical & Testing Center of NPU for providing facilities in EBL, ICP, AFM, and SEM measurements.