Van der Waals (vdW) materials are promising building blocks for next-generation nanophotonics because of their atomically smooth surfaces, diverse material families, and outstanding optical properties. However, a key challenge has limited their development as standalone photonic platforms: fabricating them into low-loss, high-performance photonic structures in a general and reliable way.
collaborative team led by Professor Xiao Yunfeng of Peking University and Professor Sun Zhipei of Aalto University has now addressed this challenge by developing a versatile nanofabrication strategy for vdW materials and demonstrating efficient continuous-wave nonlinear optical processes in low-loss vdW microcavities.
A General Nanofabrication Strategy for vdW Photonic Structures
To overcome the long-standing fabrication bottleneck, the researchers developed a focused-ion-beam nanofabrication method assisted by aluminum passivation. This strategy enables high-resolution patterning across a broad range of vdW materials and their heterostructures while suppressing processing-induced damage and optical loss. Using this method, the team fabricated multiple classes of photonic structures, including microdisks, photonic crystals, and metasurfaces, in representative vdW materials such as h-BN, MoS₂, GaSe, and NbOCl₂. They also demonstrated precise processing of vertically stacked heterostructures, highlighting the flexibility of the approach for future all-vdW photonic integration.
"These results came after two years of exploration and optimization," said Professor Sun Zhipei, a corresponding author of this work. "By refining many details in the fabrication process, we developed a vdW nanofabrication strategy that combines high precision with low optical loss."
Ultrahigh-Q van der Waals Microcavities
Using this fabrication route, the team realized vdW microdisk resonators with intrinsic quality factors exceeding 10^6. Such ultrahigh-Q performance is critical for strengthening light–matter interactions in compact photonic devices. In the experiments, the cavity Q factor increased with wavelength, in agreement with theoretical analysis based on scattering loss from surface roughness, indicating that the new fabrication strategy effectively suppresses scattering caused by sidewall and surface imperfections.
The demonstrated performance surpasses previously reported vdW resonant systems by about three orders of magnitude, establishing vdW microcavities as a practical low-loss platform for integrated photonics.
Efficient Continuous-Wave Nonlinear Optics
By combining this ultralow-loss resonant platform with the strong intrinsic optical nonlinearity of vdW materials, the researchers demonstrated highly efficient continuous-wave second-harmonic generation in GaSe microdisk cavities. The normalized conversion efficiency reached about 30%/W, representing an improvement of four orders of magnitude over previously reported vdW systems. The second-harmonic signal could also be thermally tuned across a full free spectral range.
The team further demonstrated continuous-wave sum-frequency generation in GaSe microdisks and continuous-wave optical parametric amplification based on third-order nonlinearity in h-BN microcavities. Together, these results show that all-vdW microcavities can support multiple nonlinear optical functionalities under continuous-wave excitation.
"This work marks an important step toward turning van der Waals materials into practical photonic devices," said Professor Xiao Yunfeng, the other corresponding author. "It shows that van der Waals materials can be engineered into stable, controllable, and high-performance photonic structures, opening a reliable route toward functional on-chip applications."
The study opens a new avenue toward highly integrated and multifunctional photonic chips based on emerging materials, with potential applications in reconfigurable photonics, nonlinear signal processing, quantum light sources, quantum logic devices, and ultrasensitive sensing. It also creates new opportunities for exploring the coupling of vdW photonic structures with plasmons, phonon polaritons, excitons, and other resonant excitations.
The work, entitled "All-van der Waals microcavities for low-loss nonlinear photonics," has been published in Nature Materials.
