Near‐Field Nano‐Optical Imaging of van der Waals Materials

Abstract

Abstract2D van der Waals (vdW) materials are emerging as the next generation platform for optical and electronic devices with their wide coverage of the energy bandgaps. The strong light–matter interactions in 2D vdW layers allow for exploring novel optical and electronic phenomena such as 2D polaritons exhibiting ultrahigh field confinement, defects‐induced new quantum states, and strain‐modulated quantum confinement of 2D excitons. Far‐field optical imaging techniques are extensively used to characterize the 2D vdW materials so far, however, subdiffraction spatial resolution is required for comprehensive investigations of 2D vdW materials of which physical properties are greatly influenced by local defects and strain. This article aims to cover historical advances, fundamental principles, and distinct features of emerging near‐field optical imaging techniques: scattering‐type scanning near‐field optical microscopy, tip‐enhanced Raman spectroscopy, tip‐enhanced photoluminescence techniques, and photo‐induced force microscopy. The recent developments toward spectroscopic analysis of near‐field imaging and applications for unveiling unique properties of 2D polaritons, nanoscale defects, and mechanical strains in 2D vdW materials, are also discussed. This review article provides an understanding of emerging near‐field imaging techniques and suggests prospective applications for exploring 2D vdW materials.

Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers

“…In the first set of experiments, we investigate the outcoupling behavior of the gratings using near-field imaging, which has proven to be a powerful tool for interrogating the nanoscale optical properties − of vdW materials. To this end, the tunable output of a continuous-wave near-infrared (NIR) laser is focused onto the tip of a near-field probe, which is raster-scanned across a 3R-MoS 2 sample (Figure a).…”

Optical Time-Resolved Microscopy on Few-Atoms-Thick Materials

“…The fiber-coupled nanowire scanning probe, the photothermal Atomic Force Microscopy, or the scanning near-field spectroscopies based on radiation scattering as the scattering-type scanning near-field optical microscopy (s-SNOM) or the tip-enhanced Raman spectroscopy (TERS), based on a plasmonic probe tip, are examples of techniques based on near-field interactions. In this cases, a lateral resolution of the order of 10 nm is achieved in s-SNOM and photothermal AFM techniques, , around 1 nm for the nanometer-sized hotspot carried by the plasmonic probe tip . These methods rely on scattering devices with nanometric dimensions, almost invariably in the form of a point of a few nanometer radii of curvature at the end of an AFM tip or a tapered fiber, often metalized to enhance local plasmons, creating an electric field hot-spot.…”

“…To reveal nanometric-scale defects or study optical confinement in small samples, nearfield techniques such as scanning near−field optical microscopy (SNOM), scattering-type scanning near-field optical microscopy (s-SNOM), tip-enhanced Raman spectroscopy (TERS), tip-enhanced photoluminescence (TEPL), and photoinduced force microscopy (PiFM) are available. 15 As a complement to these near-field techniques, here we discuss a recent optical method based on transparent dielectric microspheres that can be adapted to pump−probe optical techniques and deployed to a standard optical microscope, featuring a subwavelength lateral resolution and a high surface sensitivity, enhancing the optical signal from the very first layers deposited on a substrate. 16,17 These techniques are important to optimize the performance of devices such as solar cells and transistors, as well as to develop new materials for applications in areas such as energy storage and catalysis.…”

“…However, due to the strong light–matter interaction in TMD materials, novel optical and electronic states may be revealed if sufficient spatial resolution is available. To reveal nanometric-scale defects or study optical confinement in small samples, near-field techniques such as scanning near–field optical microscopy (SNOM), scattering-type scanning near-field optical microscopy (s-SNOM), tip-enhanced Raman spectroscopy (TERS), tip-enhanced photoluminescence (TEPL), and photoinduced force microscopy (PiFM) are available . As a complement to these near-field techniques, here we discuss a recent optical method based on transparent dielectric microspheres that can be adapted to pump–probe optical techniques and deployed to a standard optical microscope, featuring a subwavelength lateral resolution and a high surface sensitivity, enhancing the optical signal from the very first layers deposited on a substrate. , …”

Simultaneous mapping of the magnetic field components using near-field microscopy

“…Near-field microscopy (NFM) is a mature research area about the study of light-matter interactions with resolutions that overcome the classical resolution limit [1][2][3]. This research area has allowed the discovery of exciting phenomena in the last decade, particularly in the realm of plasmonics and twodimensional materials [4][5][6]. Interestingly, the light intensity detected in near-field microscope, while raster scanning the sample surface, can be approximated as the total electric field strength of the interactions at the near-field region [7].…”