Bridging macroscopic and microscopic nonlinear optics with layered semiconductors

Nonlinear frequency conversion provides essential tools for light generation, photon entanglement, and manipulation. Conventional nonlinear optical crystals display moderate second-order nonlinear susceptibilities and perform well in benchtop setups with discrete optical components. However, such crystals do not easily lend themselves to miniaturization and on-chip integration. Transition metal dichalcogenides (TMDs) possess 10-100x stronger nonlinear susceptibilities and, thanks to their deeply sub-wavelength thickness, offer a unique platform for on-chip nonlinear frequency conversion and light amplification. Recently, such giant nonlinearity has been exploited to demonstrate nonlinear light amplification at the ultimate thickness limit[1]; however, optical gain was still limited by the sub-nm propagation length.

3R-stacked TMDs naturally combine broken inversion symmetry (|χ⁽²⁾| ≠ 0) and aligned layering, representing ideal candidates to boost the nonlinear optical gain with minimal footprint. The nonlinear optical response of 3R-MoS₂ has been explored in some recent pioneering studies[2,3], so far focusing on thinner crystals, reporting the quadratic enhancement with the layer number at the 2D limit. Pushing towards general application, however, requires higher nonlinear enhancements and thus larger layer number, which in turn leads to more intricate interferences and interactions within the crystal.

Here we measure SHG from multilayer 3R-MoS₂. We report the first measurement of the coherence length at telecom wavelengths (~ 530 nm). At such thicknesses, we achieve record nonlinear optical enhancement, i.e., >10⁴ stronger than a monolayer, revealing the intrinsic single-pass upper limits of the material[4]. Further enhancement can then be achieved by engineering larger crystals or waveguides, or by exploiting birefringence.

Our results highlight the potential of 3R-stacked TMDs for integrated photonics, providing critical parameters for designing highly efficient on-chip nonlinear optical devices.

[1] Trovatello, C. et al. Nat. Photonics, 15, 6-10 (2021)

[2] Shi, J. et al., Adv. Mater. 29, 1701486 (2017)

[3] Zhao, M. et al., Light: Sci. Appl. 5, e16131 (2016)

[4] Xu, X., Trovatello, C. et al., Nature Photonics, 16, 698-706 (2022)