Modern Scattering‐Type Scanning Near‐Field Optical Microscopy for Advanced Material Research

Abstract

IntroductionOver the past decade, optical near-field techniques, especially scattering-type scanning near-field optical microscopy Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm −1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research. and Astronomy. His research interests cover nanoscale and ultrafast electromagnetic responses of strongly correlated electron materials, 2D materials, and metamaterials that span from the near-infrared to terahertz frequencies.tip-sample interactions. In s-SNOM, these difficulties result from at least three factors. First, the well-known antenna effect [48] causes light to be highly confined between the probe apex and the sample surface. The specific geometry of the tip shank plays a significant role in determining the intensity of the scattered signal. [49] Second, in addition to the local optical information, a strong but undesired background signal can be detected. This background signal can be mainly attributed to the light scattering from the tip shank, cantilever, and sample surface. Third, due to the broad momentum distribution of the localized radiation, in some cases, nominally "far-field trivial" Adv. Mater. 2019, 31, 1804774 Figure 1. Far-field (blue) and near-field (yellow) measurements, accessing the propagating field and the evanescent field, respectively.The authors declare no conflict of interest.

Nanoimaging and Nanospectroscopy of Polaritons with Time Resolved s‐SNOM

“…The ultrafast near‐field measurements of phonon polariton in h‐BN by Yoxall et al reveals a propagation mode with negative phase‐velocity and a group velocity down to 0.002 c using ≈100 fs pulses. In a homodyne scheme ( Figure a), E ref E nf , i.e., the multiplication of reference with the near‐field signal is measured. The reference beam amplifies the near‐field signal and provides a time‐resolved measurement, yielding the time profile of the electric field of polaritons.…”

“…Here we briefly introduce the basic concepts in near‐field optics and the s ‐SNOM technique. For a more detailed discussion of modern near‐field optical microscopy, readers are directed to several recent reviews …”

“…For a single‐wavelength laser, the focusing optics can be a lens. Otherwise, an off‐axis parabolic mirror is often used . As the AFM tip raster scans the surface of the sample, the spatial variations in the near‐field image suggests the wave vector q in the corresponding laser frequency ω, settling a data point on the dispersion.…”

Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

“…Recently, remarkable features of subdiffractional polaritons have been observed in van der Waals (vdW) crystals and in their corresponding 2D materials . Most of those observations stem from real‐space images of exciton‐polaritons in transition metal dichalcogenides, plasmon‐polaritons in graphene, phonon‐polaritons in polar vdW crystals, and hybrid plasmon‐phonon polaritons in vdW heterostructures . For these studies, the use of advanced optical methods such as scattering scanning near‐field optical microscopy (s‐SNOM), synchrotron infrared nanospectroscopy (SINS), and, more recently, photoinduced force microscopy (PiFM) have been paramount.…”

Revealing Phonon Polaritons in Hexagonal Boron Nitride by Multipulse Peak Force Infrared Microscopy

“…The charge distributions and motions due to optical excitation of the sample modify the polarizability of the metallic AFM tip, leading to resonance‐dependent scattered signals . s‐SNOM can both excite polaritons with high momenta and detect them with high spatial resolution, thus it is suitable for studying polaritons in low dimensional materials . Polaritons in BN, MoSe 2 , MoO 3 were experimentally discovered and quantified by s‐SNOM.…”