6 research outputs found
Nano-FTIR Absorption Spectroscopy of Molecular Fingerprints at 20Â nm Spatial Resolution
We demonstrate Fourier transform infrared nanospectroscopy
(nano-FTIR)
based on a scattering-type scanning near-field optical microscope
(s-SNOM) equipped with a coherent-continuum infrared light source.
We show that the method can straightforwardly determine the infrared
absorption spectrum of organic samples with a spatial resolution of
20 nm, corresponding to a probed volume as small as 10 zeptoliter
(10<sup>–20</sup> L). Corroborated by theory, the nano-FTIR
absorption spectra correlate well with conventional FTIR absorption
spectra, as experimentally demonstrated with polyÂ(methyl methacrylate)
(PMMA) samples. Nano-FTIR can thus make use of standard infrared databases
of molecular vibrations to identify organic materials in ultrasmall
quantities and at ultrahigh spatial resolution. As an application
example we demonstrate the identification of a nanoscale PDMS contamination
on a PMMA sample
Ultrafast Dynamics of Surface Plasmons in InAs by Time-Resolved Infrared Nanospectroscopy
We report on time-resolved mid-infrared
(mid-IR) near-field spectroscopy
of the narrow bandgap semiconductor InAs. The dominant effect we observed
pertains to the dynamics of photoexcited carriers and associated surface
plasmons. A novel combination of pump–probe techniques and
near-field nanospectroscopy accesses high momentum plasmons and demonstrates
efficient, subpicosecond photomodulation of the surface plasmon dispersion
with subsequent tens of picoseconds decay under ambient conditions.
The photoinduced change of the probe intensity due to plasmons in
InAs is found to exceed that of other mid-IR or near-IR media by 1–2
orders of magnitude. Remarkably, the required control pulse fluence
is as low as 60 μJ/cm<sup>2</sup>, much smaller than fluences
of ∼1–10 mJ/cm<sup>2</sup> previously utilized in ultrafast
control of near-IR plasmonics. These low excitation densities are
easily attained with a standard 1.56 μm fiber laser. Thus, InAsa
common semiconductor with favorable plasmonic properties such as a
low effective massî—¸has the potential to become an important
building block of optically controlled plasmonic devices operating
at infrared frequencies
Graphene-Based Platform for Infrared Near-Field Nanospectroscopy of Water and Biological Materials in an Aqueous Environment
Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful nanoscale spectroscopic tool capable of characterizing individual biomacromolecules and molecular materials. However, applications of scattering-based near-field techniques in the infrared (IR) to native biosystems still await a solution of how to implement the required aqueous environment. In this work, we demonstrate an IR-compatible liquid cell architecture that enables near-field imaging and nanospectroscopy by taking advantage of the unique properties of graphene. Large-area graphene acts as an impermeable monolayer barrier that allows for nano-IR inspection of underlying molecular materials in liquid. Here, we use s-SNOM to investigate the tobacco mosaic virus (TMV) in water underneath graphene. We resolve individual virus particles and register the amide I and II bands of TMV at <i>ca</i>. 1520 and 1660 cm<sup>–1</sup>, respectively, using nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We verify the presence of water in the graphene liquid cell by identifying a spectral feature associated with water absorption at 1610 cm<sup>–1</sup>
Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic Material
We
investigated phonon–polaritons in hexagonal boron nitridea
naturally hyperbolic van der Waals materialî—¸by means of the
scattering-type scanning near-field optical microscopy. Real-space
nanoimages we have obtained detail how the polaritons are launched
when the light incident on a thin hexagonal boron nitride slab is
scattered by various intrinsic and extrinsic inhomogeneities, including
sample edges, metallic nanodisks deposited on its top surface, random
defects, and surface impurities. The scanned tip of the near-field
microscope is itself a polariton launcher whose efficiency proves
to be superior to all the other types of polariton launchers we studied.
Our work may inform future development of polaritonic nanodevices
as well as fundamental studies of collective modes in van der Waals
materials
Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy
Pump–probe spectroscopy is
central for exploring ultrafast
dynamics of fundamental excitations, collective modes, and energy
transfer processes. Typically carried out using conventional diffraction-limited
optics, pump–probe experiments inherently average over local
chemical, compositional, and electronic inhomogeneities. Here, we
circumvent this deficiency and introduce pump–probe infrared
spectroscopy with ∼20 nm spatial resolution, far below the
diffraction limit, which is accomplished using a scattering scanning
near-field optical microscope (s-SNOM). This technique allows us to
investigate exfoliated graphene single-layers on SiO<sub>2</sub> at
technologically significant mid-infrared (MIR) frequencies where the
local optical conductivity becomes experimentally accessible through
the excitation of surface plasmons via the s-SNOM tip. Optical pumping
at near-infrared (NIR) frequencies prompts distinct changes in the
plasmonic behavior on 200 fs time scales. The origin of the pump-induced,
enhanced plasmonic response is identified as an increase in the effective
electron temperature up to several thousand Kelvin, as deduced directly
from the Drude weight associated with the plasmonic resonances