Twist phase matching of high harmonic generation in two-dimensional van der Waals crystals

A new method has been developed using interlayer twist to boost the efficiency of high harmonic generation (HHG) in crystals, addressing a long-standing challenge in nonlinear optics. HHG arises when intense laser fields interact with a material, generating new frequencies at integer multiples of the fundamental and producing deep/extreme ultraviolet radiation. These coherent light sources are indispensable for advanced scientific and technological applications, including attosecond physics and nanoscale fabrication. However, HHG in crystalline solids has remained intrinsically inefficient due to phase mismatch, wherein the generated harmonic waves destructively interfere because of chromatic dispersion. To overcome this limitation, we introduce a "twist-phase-matching" strategy: by stacking and rotating ultrathin van der Waals crystals at a precise angle, a nonlinear geometric phase is imparted to compensate for the phase mismatch. Guided by this framework, we demonstrate a record-high conversion efficiency for the fifth HHG in twisted hexagonal boron nitride crystals within thickness of only 1 micrometer. Moreover, the designed crystal has the polarization controllability that can emit a pure circularly polarized HHG light. This work paves the way for compact, efficient, and polarization-controllable tabletop solid-state HHG sources for applications in attosecond science and strong-field physics.