# Compact Second-Harmonic Generation in the C‑Exciton Band of 3R-MoS2 for Integrated Quantum Photonics

## TL;DR

Researchers developed a compact device using MoS2 to efficiently generate visible light through a nonlinear optical process, suitable for quantum photonics.

## Contribution

A novel design for efficient second-harmonic generation using 3R-MoS2 integrated on a waveguide with resonant coupling and counter-propagating pump configuration.

## Key findings

- Conversion efficiencies up to 0.004% are achieved with a device footprint of ~1 μm².
- The design enables resonant second-harmonic emission near 445 nm via strong field localization and leaky-mode resonance.
- The platform is compatible with spontaneous parametric down-conversion for integrated photon-pair generation.

## Abstract

We present a novel scheme for highly efficient second-harmonic
generation in the visible spectrum using minimal volumes of 3R-phase
molybdenum disulfide (MoS2) integrated on a silicon nitride
(Si3N4) ridge waveguide. The device is designed
to operate near the C-exciton resonance of MoS2, where
the material exhibits its strongest second-order nonlinear response.
A periodic array of nanometric MoS2 stripes is patterned
on the waveguide surface to form a one-dimensional photonic crystal,
enabling strong field localization at the pump wavelength (λFF ≈ 890 nm) near the photonic band edge. This leads
to resonant second-harmonic emission around 445 nm, close to the highly
absorbing C-exciton. To mitigate the absorption while maintaining
phase matching, we implement a counter-propagating pump configuration
that ensures zero in-plane momentum mismatch and enables vertical
out-coupling of the second harmonic signal via a leaky-mode resonance.
The combination of strong nonlinear susceptibility, mode engineering,
and resonant coupling allows for efficient frequency conversion within
an interaction region only tens of nanometers thick, with an overall
device footprint on the order of 1 μm2. Full-wave
3D simulations predict conversion efficiencies up to 0.004% at peak
pump intensities of 300 MW/cm2. Moreover, the same architecture
is compatible with spontaneous parametric down-conversion (SPDC),
enabling integrated photon-pair generation with intrinsic modal and
directional filtering. These features make the platform well-suited
for compact, scalable nonlinear, and quantum photonic applications
in the visible and near-infrared range.