Second-Harmonic Young Interference in Atom-Thin Heterocrystals

TL;DR

This paper demonstrates how second-harmonic generation in atom-thin heterocrystals of transition metal dichalcogenides exhibits interference effects governed by optical phase delays, enabling custom nonlinear optical functionalities.

Contribution

It reveals the optical interference mechanism of SHG in TMD hetero-bilayers and quantifies phase differences using spectral phase interferometry, supported by theoretical calculations.

Findings

SHG in TMD hetero-bilayers is governed by optical interference.

Quantified phase difference between MoS2 and WS2 matches theoretical predictions.

Demonstrated a second-harmonic Young interference effect in atom-thin materials.

Abstract

Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials, it has been widely used in frequency modulation of lasers, surface scientific investigation, and label-free imaging in biological and medical sciences. Two-dimensional crystals are ideal SHG-materials not only for their strong light-matter interaction and atomic thickness defying the phase-matching requirement but also for their stackability into customized hetero-crystals with high angular precision and material diversity. Here we directly show that SHG in hetero-bilayers of transition metal dichalcogenides (TMDs) is governed by optical interference between two coherent SH fields with material-dependent phase delays using spectral phase interferometry. We also quantify the frequency-dependent phase difference between MoS2 and WS2, which also agrees with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.