Far Infrared Synchrotron Near-Field Nanoimaging and Nanospectroscopy

Abstract

Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful imaging and spectroscopic tool for investigating nanoscale heterogeneities in biology, quantum matter, and electronic and photonic devices. However, many materials are defined by a wide range of fundamental molecular and quantum states at far-infrared (FIR) resonant frequencies currently not accessible by s-SNOM. Here we show ultrabroadband FIR s-SNOM nanoimaging and spectroscopy by combining synchrotron infrared radiation with a novel fast and low-noise copper-doped germanium (Ge:Cu) photoconductive detector. This approach of FIR synchrotron infrared nanospectroscopy (SINS) extends the wavelength range of s-SNOM to 31 μm (320 cm −1 , 9.7 THz), exceeding conventional limits by an octave to lower energies. We demonstrate this new nanospectroscopic window by measuring elementary excitations of exemplary functional materials, including surface phonon polariton waves and optical phonons in oxides and layered ultrathin van der Waals materials, skeletal and conformational vibrations in molecular systems, and the highly tunable plasmonic response of graphene.

Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

“…Finally, a demonstration of the extreme spectral reach of SINS to probe the far‐IR plasmonics of graphene is revisited, exploiting the long wavelengths sensitivity of a Ge:Cu detector. In this case, continuous nanoscale‐resolved spectroscopy from the mid‐ to the far‐IR range (10–30 µm) was achieved for the first time …”

“…Conventional far‐field far‐IR detectors, such as pyroelectric deuterated triglycine sulfate (DTGS) detectors or liquid helium cooled Si bolometers, only operate at <few kHz frequencies and have response times too slow to respond to AFM tip tapping frequency (typically ≈250 kHz) or to any higher harmonics. There are a few liquid helium cooled detectors, however, that have been used successfully in the far‐IR region for s‐SNOM and SINS, including Ge:Cu (330–2000 cm −1 ), Ge:Ga (200–400 cm −1 ), InSb (40–200 cm −1 ), and super‐conducting bolometers …”

“…SINS has been used to directly probe weak excitations in layered materials, including the van der Waals material MoS 2 , with its A 2u out‐of‐plane mode at 468 cm −1 and E 1u in‐plane mode at 384 cm −1 . Despite the small interlayer coupling, the phonon features show unique symmetry sensitivity even down to 7 nm thickness.…”

“…The SPP dispersion as a function of energy can be observed directly near the graphene edge, where reflected surface waves constructively interfere with those launched from the AFM tip. Reproduced with permission . Copyright 2018, The American Chemical Society.…”

Nanoimaging and Nanospectroscopy of Polaritons with Time Resolved s‐SNOM

“…With a broadband probe light source, such as synchrotron light, high temperature plasma light source, or laser driven plasma, such setup, so‐called nano‐FTIR, can measure IR spectra with a spatial resolution of ≈20 nm . The technique has produced flourish results in phase transition materials, polariton dispersion, catalytic chemistry, biology, and geoscience, among other fields …”