Doubly‐Resonant Enhancement of Second Harmonic Generation from a WS₂ Nanomesh Polymorph with a Modified Energy Landscape

Abstract

Although transition metal dichalcogenides (TMDs, e.g., WS2, WSe2, MoS2, MoSe2) have emerged as highly promising 2D materials for nonlinear optics, they are limited by intrinsically small light‐matter interaction length and (typically) flat‐lying geometries. The first hyperspectral multiphoton analysis of a tridimensional webbed network of densely‐packed stacks (of 1–5 layers) of twisted and/or fused 2D nanosheets of WS2, referred to as “nanomesh”, is presented here. The optical second harmonic generation (SHG) is mapped across the three characteristic spectral features (A, B, and C) and two‐photon luminescence and third harmonic generation signatures are established. Compared to flat‐lying WS2 layers, the nanomesh is highly efficient, broadband, and robust against degradation (with main enhancement originating from the C feature, spreading from 850 to 1100 nm), and scalable in terms of growth. The origin of these spectral differences is assigned to hotspots, whose location changes depending on the wavelength of illumination. The main SHG enhancements result from double resonances in an energy landscape modified by in‐built defects (e.g., vacancies and their passivated variants, or grain boundaries) that induce intra‐bandgap energy levels. These characteristics establish the nanomesh as a prime candidate for integration into quantum optical technologies, such as miniaturized devices on chip.

Nonlinear Exciton‐Mie Coupling in Transition Metal Dichalcogenide Nanoresonators

“…[21] As a result, providing a sophisticated optical design for the use of TMDCs in nonlinear processes is critical. Recent studies suggest using different TMDC morphologies, such as nanomesh, [22] tubular, [23] and nanoporous [24] networks. Despite the excellent findings of these studies, these nanostructures are challenging to incorporate into optical circuits.…”

Structural Symmetry-Breaking-Driven Giant Optical Second Harmonic Generation in Janus Monolayer ReSSe

“…As a result, a giant SHG response could be observed in the Janus ReSSe monolayer due to structural symmetry breaking-induced large second-order nonlinear coefficient. In general, the second-order nonlinear optical response processes mainly divided into non-resonant transition, single-resonant transition, and doubly-resonant transition in an energy landscape. , Meanwhile, in our previous work, − , it has been confirmed that the prominent peak in the second-order nonlinear dispersion relation originates mainly from the resonant transition processes at the high-symmetry point or along the high symmetry line in the band structure for a semiconductor. In the present work, the electronic band structure of the Janus ReSSe monolayer possesses a smaller curvature, which can lead to more resonant effects including both single-resonant transition and doubly-resonant transition processes.…”

Water‐Assisted Growth of Twisted 3R‐Stacked MoSe2 Spirals and Its Dramatically Enhanced Second Harmonic Generations

“…The broad available band of the SHG signal from the MoSe 2 spiral should arise from the resonance enhancement effect of C exciton, since the C exciton was reported to show broadband feature, i.e., ≈10 times broader than that of the A and B excitons in monolayer MX 2 as reported previously based on theoretical calculations and experiments. [40] Considering that the C exciton of MX 2 is associated with multiple transitions from the Γ−Κ line of the Brillouin zone (i.e., from band nesting effect), as also evidenced by the WS 2 nanomesh, [41] certain energy transitions within the whole test range should contribute to the broad available band of the SHG signal from the MoSe 2 spiral.…”