Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field Microscopy

Abstract

Combined with terahertz (THz) time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides subsurface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the subsurface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ∼ λ/3300, at 1 THz, while the AFM only provides a flat surface topography.

Artifact free time resolved near-field spectroscopy

“…5 we briefly outline selected materials and phenomena that may be explored with pulsed laser sources in time-resolved and spectroscopic near-field measurements. At the longest wavelengths, THz s-SNOM is ideally suited to control and interrogate electronic properties of complex materials [4,51], Josephson plasmon resonances in layered superconductors [52], hyperbolic polaritons in topological insulators [53], spin precession in ferromagnets [54] and anti-ferromagnets [55], as well as vibrational [56] and rotational [57] motions in a wide range of systems. The mid-IR spectral range is sensitive to the plasmonic modes in graphene [26,27,47,58,59], hyperbolicity in Hexagonal Boron Nitride [60], phonon resonances [35,41] as well as the electronic properties of many materials [17][18][19]41].…”

“…In the mid-IR range the second or third harmonic provides nearly background free data. Interestingly, in the THz range even demodulation to linear order may provide adequate background suppression in many cases [51]. Under these conditions Tr-pHD can be used for reliable artifact-free timeresolved near-field imaging.…”

“…When Tr-HD (II) is combined with two-phase detection [38,47] the results are free from the influence of multiplicative background radiation and can be used to extract the amplitude and phase of the near-field signal, although this has not yet been demonstrated with pulsed laser sources. Other techniques, such as EoS or detection with a photo-conductive antenna are also immune from the multiplicative background contribution, and are compatible with rapid nano-imaging in the mid-infrared and terahertz frequency ranges [28,51]. Thus there is a whole arsenal of nano-optics methods that offer means and ways for multiplicative background radiation suppression.…”

Terahertz near-field microscopy using the self-mixing effect in a quantum cascade laser

“…A separate type of scattering near-field probe has also been demonstrated to enable the highest spatial resolution, in the sub-100nm range. [4,5]. These probes use a sharp conductive tip to strongly localize the THz field.…”

Visualizing near-field coupling in terahertz dolmens

“…21 In contrast, at THz frequencies, it is possible to directly measure the time-dependent electric near-fields of resonant metallic structures. [22][23][24] Over recent years, a number of innovative advances have emerged in near-field THz microscopy resulting in diverse implementations, ranging from techniques based on sharp tips 25,26 or electron pulses 27 for ultrahigh spatial resolution, to others harnessing electro-optic sampling in nonlinear crystals, 28,29 to schemes based on miniaturized photoconductive probes. 13,30,31 We employ the latter approach to demonstrate the hybridization of modes that follows the near-field coupling between two resonant structures, as well as the excitation of the dark resonance which is responsible for the transparency window in the far-field.…”