Interface Enhancement of Second Harmonic Generation in WS2/MoS2 Hetero-Bilayer Nanoantennas Driven by Excitonic and Anapole Resonances

Core Concepts

This research demonstrates enhanced second harmonic generation (SHG) at the interface of a WS2/MoS2 hetero-bilayer, achieved by engineering the interplay of excitonic resonances and anapole states in nanostructured antennas.

Abstract

Bibliographic Information:

Tognazzi, A., Franceschini, P., Biechteler, J., Ba`u, E., Cino, A.C., Tittl, A., De Angelis, C., & Sortino, L. (2024). Interface second harmonic generation enhancement in hetero-bilayer van der Waals nanoantennas. arXiv:2411.06156v1 [physics.optics].

Research Objective:

This study investigates the potential of van der Waals (vdW) heterostructures, specifically WS2/MoS2 bilayers, for enhancing second harmonic generation (SHG) by exploiting the combined effects of excitonic resonances and anapole states in engineered nanoantennas.

Methodology:

• WS2/MoS2 hetero-bilayer thin films were fabricated using mechanical exfoliation and deterministic stacking.
• Hexagonal nanoantennas were patterned from the heterostructure using electron beam lithography and dry etching.
• Linear optical reflectance spectroscopy was used to characterize the fabricated nanoantennas and identify the anapole state.
• Nonlinear optical spectroscopy, using a tunable optical parametric amplifier, was employed to measure the SHG signal from the nanoantennas at different excitation wavelengths.

Key Findings:

• The fabricated WS2/MoS2 nanoantennas exhibit a clear anapole state, confirmed by a dip in the reflectance spectra, with a spectral position tunable by varying the nanoantenna size.
• A significant enhancement of the SHG signal is observed from the nanoantennas compared to the unpatterned hetero-bilayer.
• The SHG enhancement is maximized when the excitation wavelength is tuned to match the excitonic resonances of the constituent TMDCs and the anapole resonance of the nanoantenna.
• Up to two orders of magnitude enhancement in SHG is achieved in nanoantennas where both the excitonic and anapole resonances are strategically aligned.

Main Conclusions:

• This work demonstrates the first successful implementation of vdW heterostructures, specifically WS2/MoS2 bilayers, for fabricating optical nanoantennas capable of enhancing nonlinear optical processes like SHG.
• The synergistic interplay between excitonic and anapole resonances in these nanoantennas provides a powerful mechanism for amplifying SHG signals.
• This research highlights the potential of vdW materials and their heterostructures for developing novel nanophotonic devices with tailored nonlinear optical properties.

Significance:

This study significantly advances the field of nonlinear optics by demonstrating a novel approach for enhancing SHG using engineered vdW heterostructures. This opens up new possibilities for developing compact and efficient nonlinear optical devices for applications in sensing, imaging, and optical communication.

Limitations and Future Research:

• The study focuses on a specific vdW heterostructure (WS2/MoS2). Exploring other vdW materials and their combinations could lead to further SHG enhancement and functionalities.
• Investigating the influence of twist angle between the TMDC layers on the SHG response could reveal additional control mechanisms for tailoring the nonlinear optical properties.
• Further research is needed to optimize the design and fabrication of these nanoantennas to achieve even higher SHG conversion efficiencies.

Stats

The anapole scattering dip shifts from approximately 1300 nm to 1600 nm as the radial size of the hexagonal antenna increases from 260 nm to 360 nm.
When the fundamental laser is resonant with the MoS2 A exciton, a two orders of magnitude enhancement in SHG is observed.
The SHG enhancement ratio reaches up to two orders of magnitude in smaller radii nanoantennas where the anapole state is resonant with the exciton energy.

Quotes

"Our results highlights vdW materials as a platform for designing unique multilayer optical nanostructures and metamaterial, paving the way for advanced applications in nanophotonics and nonlinear optics."
"Our findings highlight the unique potential of vdW materials for designing unprecedented vertically stacked nanophotonic structures with arbitrary materials, opening to precise control over crystal thickness and orientation."

Key Insights Distilled From

Interface second harmonic generation enhancement in hetero-bilayer van der Waals nanoantennas

by Andr... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2411.06156.pdf

Deeper Inquiries

How could the integration of these vdW nanoantennas with other photonic components, such as waveguides or cavities, further enhance their nonlinear optical performance?

Integrating vdW nanoantennas with other photonic components like waveguides and cavities presents a compelling pathway to further amplify their nonlinear optical performance. This integration can be strategically implemented through several approaches:

Enhanced Light-Matter Interactions:

Waveguide Integration: By positioning the vdW nanoantennas within the evanescent field of a waveguide, light can be efficiently channeled and confined within the nanoantenna's vicinity. This intensified light-matter interaction can significantly boost the nonlinear optical processes, including SHG.
Cavity Enhancement: Enclosing the nanoantennas within an optical cavity can create a resonant environment. When the cavity's resonant frequency aligns with either the fundamental or the second harmonic frequency of the nanoantenna, the optical field within the cavity becomes significantly amplified. This concentrated field can dramatically enhance the SHG efficiency.

Phase Matching:

Quasi-phase Matching: Efficient SHG relies on phase matching between the fundamental and second harmonic waves. Integrating vdW nanoantennas with periodically poled nonlinear materials or structuring the antennas themselves can achieve quasi-phase matching. This technique compensates for the phase mismatch accumulated during propagation, leading to a substantial increase in SHG output.

Directional Emission Control:

Waveguide Coupling: Integrating nanoantennas with waveguides allows for directing the generated SHG signal into specific spatial modes. This controlled emission is crucial for efficiently collecting and utilizing the nonlinear output in integrated photonic circuits.

Hybrid Material Platforms:

Synergistic Effects: Combining vdW nanoantennas with other nonlinear materials, such as lithium niobate (LN) or barium titanate (BTO), can lead to synergistic effects. For instance, the strong second-order nonlinearity of LN or BTO can be combined with the unique excitonic and anapole resonances of vdW nanoantennas, potentially leading to unprecedented SHG efficiencies.

By carefully engineering the integration of vdW nanoantennas with waveguides, cavities, and other photonic components, researchers can unlock new possibilities for highly efficient and controllable nonlinear optical devices. These advancements hold promise for applications in areas like on-chip optical frequency conversion, nonlinear microscopy, and quantum information processing.

Could the observed SHG enhancement be attributed to other factors beyond excitonic and anapole resonances, such as local field enhancement at the edges of the nanoantennas?

While the study primarily attributes the SHG enhancement to excitonic and anapole resonances, it's prudent to acknowledge that other contributing factors might be at play, particularly the local field enhancement at the edges of the nanoantennas, often referred to as the "lightning rod effect."
Here's a breakdown of how edge effects could contribute:

Geometric Singularities: The sharp edges and corners of the hexagonal nanoantennas act as geometric singularities. These singularities can cause a concentration of the electric field in their vicinity, leading to a localized enhancement of the electromagnetic field intensity.

Enhanced Nonlinear Polarization: The increased field strength at the edges can amplify the nonlinear polarization within the material. Since SHG is a second-order nonlinear process, even a modest increase in the local field can result in a disproportionately larger SHG signal.

Surface Effects: The edges and surfaces of nanostructures often exhibit different dielectric properties compared to the bulk material. These surface effects can modify the local electromagnetic field distribution, potentially contributing to the observed SHG enhancement.

Distinguishing Edge Effects:

Simulations: Sophisticated numerical simulations, such as finite-element method (FEM) or finite-difference time-domain (FDTD) simulations, can help disentangle the contributions from different enhancement mechanisms. By carefully analyzing the spatial distribution of the electric field and the SHG generation, researchers can assess the relative importance of edge effects compared to excitonic and anapole resonances.

Experimental Validation: Experimentally, varying the geometry of the nanoantennas, such as rounding the edges or changing the aspect ratio, can provide insights into the role of edge effects. If the SHG enhancement is highly sensitive to these geometric modifications, it suggests a significant contribution from edge effects.
In conclusion, while excitonic and anapole resonances are likely the dominant factors driving the observed SHG enhancement, the potential role of local field enhancement at the edges of the nanoantennas should not be discounted. Further investigations, combining advanced simulations and carefully designed experiments, are necessary to fully elucidate the interplay of these different enhancement mechanisms.

What are the potential implications of this research for developing novel quantum optical devices based on 2D materials?

This research, demonstrating enhanced SHG from vdW heterostructures, carries significant implications for advancing the development of novel quantum optical devices based on 2D materials. Here's a glimpse into the potential avenues:

Single-Photon Sources:

On-Demand Generation: Efficient SHG can be leveraged to develop compact and bright sources of single photons. By integrating vdW nanoantennas with suitable quantum emitters, such as defect centers in 2D materials or quantum dots, the enhanced nonlinearity can enable the generation of single photons on demand.
Wavelength Conversion: SHG facilitates the conversion of single photons from one wavelength to another. This capability is crucial for interfacing different quantum systems operating at distinct wavelengths, paving the way for scalable quantum networks.

Entangled Photon Pair Generation:

Spontaneous Parametric Down-Conversion (SPDC): While the study focuses on SHG, the underlying principles can be extended to other nonlinear processes like SPDC. By carefully engineering the phase matching conditions in vdW heterostructures, it might be possible to generate entangled photon pairs, a fundamental resource for quantum communication and computation.

Integrated Quantum Photonics:

On-Chip Quantum Circuits: The compatibility of vdW materials with existing semiconductor fabrication techniques makes them promising candidates for integrated quantum photonics. The development of efficient nonlinear optical components, like SHG sources and SPDC sources, based on vdW nanoantennas could lead to the realization of compact and scalable on-chip quantum circuits.

Novel Quantum Phenomena Exploration:

Strong Light-Matter Coupling: The ability to confine light at the nanoscale using vdW nanoantennas can lead to strong light-matter coupling regimes. This interaction can give rise to novel quantum phenomena, such as the formation of polaritons (hybrid light-matter particles) with intriguing properties for quantum information processing.

Quantum Sensing:

Enhanced Sensitivity: The enhanced SHG from vdW nanoantennas can be exploited for ultrasensitive detection schemes. By functionalizing the nanoantennas with molecules or biomolecules of interest, changes in the SHG signal can be used to detect and quantify these targets with high sensitivity.

In summary, this research not only advances our understanding of nonlinear optics in vdW heterostructures but also opens up exciting possibilities for leveraging these materials in the burgeoning field of quantum technologies. The development of efficient single-photon sources, entangled photon pair generators, and other quantum optical components based on vdW nanoantennas could pave the way for transformative advancements in quantum communication, computation, and sensing.