Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers

Abstract

Waveguides play a key role in the implementation of on-chip optical elements and, therefore, lie at the heart of integrated photonics. To add the functionalities of layered materials to existing technologies, dedicated fabrication protocols are required. Here, we build on laser writing to pattern grating structures into bulk noncentrosymmetric transition metal dichalcogenides with grooves as sharp as 250 nm. Using thin flakes of 3R-MoS 2 that act as waveguides for near-infrared light, we demonstrate the functionality of the grating couplers with two complementary experiments: first, nano-optical imaging is used to visualize transverse electric and magnetic modes, whose directional outcoupling is captured by finite element simulations. Second, waveguide second-harmonic generation is demonstrated by grating-coupling femtosecond pulses into the slabs in which the radiation partially undergoes frequency doubling throughout the propagation. Our work provides a straightforward strategy for laser patterning of van der Waals crystals, demonstrates the feasibility of compact frequency converters, and examines the tuning knobs that enable optimized coupling into layered waveguides.

Tunable Optical Nonlinearities in Layered Materials

“…Scale bar 10 μm . (e,f) In-situ patterned grating couplers . (e) AFM scan of the 3R-MoS 2 slab region with a laser-written grating coupler .…”

“…(e,f) In-situ patterned grating couplers . (e) AFM scan of the 3R-MoS 2 slab region with a laser-written grating coupler . (f) Images of the broadly tunable waveguided SHG launched from the grating coupler .…”

“…(e) AFM scan of the 3R-MoS 2 slab region with a laser-written grating coupler . (f) Images of the broadly tunable waveguided SHG launched from the grating coupler . (a–c) are reprinted and adapted with permission from ref .…”

“…(d) is reprinted and adapted with permission under the Creative Commons CC-BY 4.0 from ref . (e,f) are reprinted and adapted with permission from ref . Copyright 2024 American Chemical Society.…”

“…In particular, the difference between in-plane and out-of-plane refractive indexes and the dependence of the effective refractive index on the waveguide mode can be exploited to obtain birefringent and modal phase matching. Very recently, programmable grating couplers directly patterned by femtosecond laser writing into thin slabs of 3R-MoS 2 have enabled efficient in- and out-coupling of visible and infrared radiation into 3R-MoS 2 waveguides Figure e shows the atomic force microscopy scan of a 3R-MoS 2 slab with a patterned grating coupler.…”