Polarization-dependent near-field phonon nanoscopy of oxides: SrTiO3, LiNbO3 , and PbZr0.2Ti0.8

Abstract

Resonant infrared near-field optical spectroscopy provides a highly material-specific response with sub-wavelength lateral resolution of ∼ 10 nm. Here, we report on the study of the near-field response of selected paraelectric and ferroelectric materials, i.e. SrTiO 3 , LiNbO 3 , and PbZr 0.2 Ti 0.8 O 3 , showing resonances in the wavelength range from 13.0 to 15.8 µm. We investigate these materials using scattering scanning near-field optical microscopy (s-SNOM) in combination with a tunable mid-infrared free-electron laser (FEL). Fundamentally, we demonstrate that phonon-induced resonant near-field excitation is possible for both p-and s-polarized incident light, a fact that is of particular interest for the nanoscopic investigation of anisotropic and hyperbolic materials. Moreover, we exploit that near-field spectroscopy, as compared to far-field techniques, bears substantial advantages such as lower penetration depths, stronger confinement, and a high spatial resolution.The latter permits the investigation of minute material volumes, e.g. with nanoscale changes in crystallographic structure, which we prove here via near-field imaging of ferroelectric domain structures in PbZr 0.2 Ti 0.8 O 3 thin films.

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

“…Note that close to an absorption maximum (i.e., for large e 00 ), no observable near-field resonance is expected due to pronounced damping, even for suitable e 0 . Calculations also predict that the spectral position of a near-field resonance must blue shift with the increased tip-sample distance, 29,51 which has been experimentally confirmed for mid-IR wavelengths, 27,[29][30][31][32][33] but so far it has not been explored at larger wavelengths.…”

“…As a rule of thumb for sufficiently low absorption, a near-field resonance may be observed whenever the real part of a material's permittivity e 0 falls in the range of À10 e 0 res À1, with e 0 res depending on the imaginary part of the permittivity e 00 . 12,27,35 In the calculated wavelength region, BFO shows three spectral positions that fulfill this condition [marked with boxes in Fig. 1(a) and enlarged in Figs.…”

“…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). 46 Particularly, sample-resonant s-SNOM provides enhanced sensitivity to the smallest material variations such as doping level and charge carrier concentration, 22,36,39,47 optical anisotropy tensor orientation in ferroelectric materials, 30,31,40,48 polymorphism, 36 and local stress distribution.…”

“…A characteristic blueshift of the enhanced near-field response with increasing h 27,29-33 is clearly observable, leading to a typically lobe-like feature in the false color plots. 27,30,31,33 As a guide to the eye, in (d4), the approximate position of the first lobe is marked with a white shaded area.…”

“…The wavelength regions of interest are investigated with a homebuilt s-SNOM that implements demodulation at higher harmonics of the mechanical cantilever oscillation frequency 53,54 and a selfhomodyne detection scheme, with the latter leading to a combined response of near-field amplitude and phase. 17,53 For illumination, we use the tunable narrow-band free-electron laser (FEL) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, 27,31,40,41,55 which is a linearly-polarized, pulsed laser source at 13 MHz repetition rate covering the wavelength range of 5-250 lm, i.e., 1.2-60.0 THz. 56 The experimental setup is described in detail elsewhere.…”