Narrow-band near-field nanoscopy in the spectral range from 1.3 to 8.5 THz

Abstract

Nano-spectroscopy in the terahertz frequency range remains challenging despite recent technological progress in developing both THz emitter sources and near-field optical microscopy (SNOM). Here, we combine scattering-type SNOM with a free-electron laser light source, to tune into the 1.3–8.5 THz range. A significant portion of this range, namely, the frequencies above ∼3 THz, is not covered by previously reported near-field microscopy systems. However, it constitutes an indispensable regime where many elementary processes in solids including collective lattice excitations, charge, and spin transport occur. Our approach of nano-spectroscopy and nano-imaging provides a versatile analysis of nanostructures as small as 50 nm, hence beating the optical diffraction limit by λ/4600.

Probes for Ultrasensitive THz Nanoscopy

“…On Alumina, ℎ Q/7 increases only by a factor of 1.5 (from 11 to 16 nm) when the tip radius increases by a factor of 30 (from R = 25 nm to 750 nm). These dramatic sub-radius decay distances are in stark contrast to the widespread assumptions 6,19,27,39,43 that (i) the near-field signal´s decay distance is on the scale of the tip radius, and (ii) the lateral resolution is closely related to ℎ Q/7 . Comparison of Figure 3 and Figure 2c reveals that ℎ Q/7 can be one order of magnitude smaller than the lateral resolution Dx (e.g.…”

“…Unavoidable background scattering can be suppressed by vertical tip oscillation and demodulation of the detector signal at a higher harmonics (multiples) of the tip´s oscillation frequency. Improved background suppression is obtained in combination with interferometric detection, which additionally yields amplitude and phase of the scattered field that are related to the reflection and absorption properties of the sample, respectively 19-21 . s-SNOM at THz frequencies 22,23,6,[24][25][26][27][28][29][30][31][32][33] a tool of rapidly growing interest, as it allows, for example, for THz nanoimaging of complex electronic phases in 2D materials and exotic conductors 34. However, THz s-SNOM is still in its infancy, mainly due to two reasons. First, the power of most THz sources is rather weak compared to that of infrared and visible sources, and detectors are often bulky, expensive and require cryogenic operation.…”

Phonon-induced near-field resonances in multiferroic BiFeO3 thin films at infrared and THz wavelengths

“…6 Near-field spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM) combine the material specificity of optical and IR techniques with the wavelength-independent 12,13 nanoscale resolution of atomic force microscopy, [14][15][16][17] making s-SNOM an outstanding tool for investigations down to the THz region. 13,[17][18][19][20][21][22] In addition to the spatial resolution, the probing depth of s-SNOM of about 100 nm 21,23-26 presents a major advantage for the investigation of small volumes or thin film samples, allowing for IR thin film spectroscopy with negligible direct substrate contribution to the optical signal. 27 The signal strength of s-SNOM is greatly enhanced via polaritoninduced resonant tip-sample interaction, 12,17,27,[30][31][32][34][35][36][37][38][39][40][41][42][43][44][45] which is of special advantage when exploring technically challenging wavelength regimes, 17,45 such as the "THz gap" (30-300 lm, i.e., 1-10 THz).…”

Far Infrared Synchrotron Near-Field Nanoimaging and Nanospectroscopy

“…To date far-IR s-SNOM has only routinely been achieved using a free electron laser (FEL) [28] that provides the necessary high intensity quasi-cw radiation as needed for s-SNOM spectroscopy. Although in principle continuously tunable from 1.…”