A group of researchers recently published a paper in the journal ACS Nano that demonstrated the feasibility of using transition metal dichalcogenides (TMDs) for tailored light-matter interactions, demonstrating their potential for nanophotonics applications.
Study: Transition Metal Dichalcogenide Dimer Nanoantennas for Tailored Light–Matter Interactions. Image Credit: sakkmesterke/Shutterstock.com
Significance of TMDs in Nanophotonics
TMDs have attracted considerable attention for their potential application in nanophotonics. In several studies, these layered materials have been integrated with nanophotonic structures to achieve low-threshold lasing, strong and weak coupling, quantum efficiency and Purcell enhancement in single-photon emitters (SPEs). However, the application of TMDs in these studies was limited to either single- or multiple-layer samples that focused on coupling emitted light from two-dimensional (2D) semiconductors to cavity modes and resonances in various material systems.
Hexagonal boron nitride (hBN), a TMD-like layered material, is often used to fabricate photonic resonators. For instance, 2D and one-dimensional (1D) photonic crystal cavities and waveguides are fabricated using hBN through reactive ion etching (RIE) and electron-beam induced etching techniques. Still, the use of TMDs for the fabrication of photonic resonators is a more recent phenomenon, even though TMDs have several advantages over hBNs.
For instance, TMDs such as tungsten disulfide (WS2) possess a higher refractive index in the visible range than hBN or other high index dielectrics conventionally used to fabricate nanophotonic resonators. Additionally, the layered nature of TMDs offers advantages such as large optical anisotropy, and these materials maintain a substantial transparency window in the visible range.
These properties can facilitate the production of a highly contrasting refractive index boundary by the deposition of TMD crystals on low refractive index materials such as silicon dioxide (SiO2) to obtain highly confined optical resonances. Recently, TMD photonic structures such as WS2 nanoantenna resonators, photonic crystals, and gratings were used to realize strong coupling. Similarly, TMD nanodisk Mie resonators with nonradiative anapole modes were fabricated to demonstrate Raman scattering enhancement and second harmonic generation (SHG) enhancement.
Realizing photonic structures with less than 20 nanometers gaps and double-vertex geometry is necessary. These structures can lead to strong confinement of magnetic and electric fields owing to the boundary conditions on the parallel and normal electric field components at a sharp refractive index contrasting boundary. Large electric fields are a prerequisite for achieving large enhancements in emitter radiative rates, such as plasmonic bowtie antennas and stable optical trapping.
Stable optical trapping can help in the accurate positioning of nanoparticles, such as quantum dots (QDs), that closely resemble large proteins' refractive index and size. Nanoantenna optical trapping using plasmonic resonators and dielectric nanoresonators led to several drawbacks, such as loss of stability and positioning of QDs without emission quenching.
Effectively realizing optical trapping and Purcell enhancement requires large field confinement in closely spaced double-vertex structures, which can be achieved in WS2 using the weak van der Waals forces and etching anisotropy of crystallographic axes.
Fabrication and Characterization of TMD Nanoantenna Structures
In this study, researchers patterned the dimer and monomer nanoantenna structures into thin WS2 crystals in different geometries using the anisotropy in the WS2 crystal structure and characterized the fabricated samples to determine their feasibility for nanophotonics applications. 25-500 nanometers thick WS2 flakes were exfoliated from a bulk crystal on 290 nanometers thick SiO2 substrate. Nanofabrication techniques such as RIE and electron beam lithography (EBL) were utilized to imprint submicrometer nanoantennas with nanometer-scale gaps.
WSe2 monolayers were exfoliated mechanically from a bulk crystal on a polydimethylsiloxane (PDMS) stamp. Photoluminescence (PL) imaging was used to identify the large monolayers.
A Nikon LV150N microscope with a fiber-coupled output was employed to achieve optical spectroscopy in a dark-field configuration of dimer and monomer nanoantennas. The fiber output from the microscope was coupled to a charge-coupled device (CCD) and Princeton Instruments spectrometer.
Atomic force microscope (AFM) repositioning and imaging were performed using a JPK Nanowizard 3 Ultra AFM with Bruker SNL probes. Lumerical Inc. software was utilized to conduct the finite-difference time-domain (FDTD) simulations. Three FDTD simulations were performed, including Purcell factor simulations, electric field intensity simulations, and scattering simulations. The geometric Mie resonances were compared with the FDTD simulations.
The three-dimensional (3D) finite element method was utilized to measure the hexagonal dimer nanoantenna optical forces through optical trapping force simulations.
Research Findings
Dimer and monomer WS2 nanoantennas were fabricated successfully. WS2 nanoantennas were fabricated selectively in hexagonal, square, and circular geometries with atomically sharp vertices and edges. The resonances of the nanopillar resonators were tuned by varying the geometry, height, or radius to fit the structures for various applications.
Dark-field spectroscopy of double/dimer and single/monomer revealed geometric Mie resonances. The dimer resonances and monolayer WSe2 emission were coupled for the first time in the same TMD material system and a Purcell enhancement factor lower bound of almost two and PL enhancement factors of over 240 were achieved for 150 nanometers gap between dimer nanoantennas.
The polarization-dependent SHG enhancement was demonstrated by using a dimer anapole mode. However, such enhancement was not achieved in monomer nanoantennas. The polarization-dependent SNG enhancement was rotated by changing the excitation polarization.
Ultrasmall gaps of 10 ± 5 nanometers were achieved during the post-fabrication repositioning of dimer nanoantennas. The ultrasmall gaps can enable several potential applications such as optical trapping and a robust Purcell enhancement of single-photon emitters. The Purcell factors of hexagonal and square geometry of WS2 nanoantennas were 157 and 153, respectively, which demonstrated the practical utility of the nanoantennas for radiative rate enhancement for single-photon emission. Moreover, quantum emission enhancement in dimer nanoantennas was also higher than the largest enhancement currently achievable in photonic crystal cavities.
Taken together, the findings of this study demonstrated that TMDs, specifically TMD dimer nanoantennas could be effectively used for applications in nanophotonics, specifically in nanophotonic resonators.
Reference
Sortino, L., Mullin, N., Genco, A. et al. Transition Metal Dichalcogenide Dimer Nanoantennas for Tailored Light−Matter Interactions. ACS Nano 2022. https://doi.org/10.1021/acsnano.2c00802
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