Sharp, high numerical aperture (NA), nanoimprinted bare pyramid probe for optical mapping

Abstract

The ability to correlate optical hyperspectral mapping and high resolution topographic imaging is critically important to gain deep insight into the structure–function relationship of nanomaterial systems. Scanning near-field optical microscopy can achieve this goal, but at the cost of significant effort in probe fabrication and experimental expertise. To overcome these two limitations, we have developed a low-cost and high-throughput nanoimprinting technique to integrate a sharp pyramid structure on the end facet of a single-mode fiber that can be scanned with a simple tuning-fork technique. The nanoimprinted pyramid has two main features: (1) a large taper angle (∼70°), which determines the far-field confinement at the tip, resulting in a spatial resolution of 275 nm, an effective numerical aperture of 1.06, and (2) a sharp apex with a radius of curvature of ∼20 nm, which enables high resolution topographic imaging. Optical performance is demonstrated through evanescent field distribution mapping of a plasmonic nanogroove sample, followed by hyperspectral photoluminescence mapping of nanocrystals using a fiber-in-fiber-out light coupling mode. Through comparative photoluminescence mapping on 2D monolayers, we also show a threefold improvement in spatial resolution over chemically etched fibers. These results show that the bare nanoimprinted near-field probes provide simple access to spectromicroscopy correlated with high resolution topographic mapping and have the potential to advance reproducible fiber-tip-based scanning near-field microscopy.

Probing Plexciton Emission from 2D Materials on Gold Nanotrenches

“…We then investigate the nanoscale optical response of plexcitons via scanning near-field optical microscopy (SNOM) using a fiber-based pyramidal probe fabricated with a nanoimprinting procedure. 31 We start by collecting the near-field using a bare dielectric probe to minimize the tip-induced perturbation introduced in the plasmonic resonance and prevent PL quenching, which are known issues when using metallic tips. [32][33][34][35] The probe allows us to characterize plexciton emission via hyperspectral PL imaging by exciting the sample with a continuous wave (CW) laser at 633 nm wavelength coupled through a glass fiber, where the light is tightly focused by the pyramidal probe at the end of the fiber into a nearly diffraction-limited spot.…”

Near-Field Coupling with a Nanoimprinted Probe for Dark Exciton Nanoimaging in Monolayer WSe2

“…As shown in Figure a, the near-field probe is based on a sharp pyramid placed onto the end facet of a single-mode optical fiber (630HP, Thorlabs) using nanoimprint lithography . With tuning-fork feedback, the pyramid probe provides excellent topographic sensitivity and a confined light intensity profile. , To enhance out-of-plane dark excitonic emission, a gap mode plasmonic cavity was fabricated by coating the probe with a ∼20 nm gold thin film and the substrate with a 100 nm gold film to support the ML WSe 2 . The sample was separated from the substrate gold by ∼2 nm of atomic layer deposition (ALD) grown SiO 2 to reduce Ohmic contact and photoluminescence quenching (for more details, see Section S1 in the Supporting Information) .…”

“…The calculated excitation profiles presented in Figure c reveals that the gap-mode configuration provides a highly confined excitation spot, with a size of hundreds of nanometers, which is close to the diffraction limit at the excitation wavelength without the need for a high numerical aperture (NA) objective lens . This high NA property of the probe is primarily determined by the optical cavity and tapered angle of the nanoimprinted pyramidal probe . This confined excitation spot is critical for probing the dark state emission, as it minimizes the amount of bright excitons that are excited, which can act as background noise and obscure the signal from the dark states in the room-temperature spectra.…”

“…23 With tuning-fork feedback, the pyramid probe provides excellent topographic sensitivity and a confined light intensity profile. 23,24 To enhance out-of-plane dark excitonic emission, a gap mode plasmonic cavity was fabricated by coating the probe with a ∼20 nm gold thin film and the substrate with a 100 nm gold film to support the ML WSe 2 . The sample was separated from the substrate gold by ∼2 nm of atomic layer deposition (ALD) grown SiO 2 to reduce Ohmic contact and photoluminescence quenching (for more details, see Section S1 in the Supporting Information).…”

“…28 This high NA property of the probe is primarily determined by the optical cavity and tapered angle of the nanoimprinted pyramidal probe. 23 This confined excitation spot is critical for probing the dark state emission, as it minimizes the amount of bright excitons that are excited, which can act as background noise and obscure the signal from the dark states in the roomtemperature spectra. In contrast, the scattering-type near-field technique typically has a far-field spot size coupled from the side objective lens typically on the order of micrometers, 10 leading to a larger amount of bright excitons being excited.…”