We use scattering-type apertureless near-field optical microscopy to demonstrate distance-controlled spectral resonance and phase shifts of a single near-field coupled system. In our experiment a ...metal tip optically interacts with a SiC surface at continuously varied nanometric distances. For illumination we use mid-infrared wavelengths (10 μm) where phonon polaritons in SiC resonantly enhance the near-field interaction. The phase of the scattered light increases and the resonance peak red shifts with decreasing gap width, in accordance with theoretical modeling.
We have enhanced the apertureless scattering‐type scanning near‐field optical microscope by two improvements which together achieve a recording of the true near field without any height‐induced ...artefact. These are the use of interferometric detection of the scattered light on one hand, and the use of higher‐harmonic dither demodulation of the scattered signal on the other. Here we present the basic rationale for these techniques, and give examples measured with two different experiments, one in the infrared (10 µm wavelength), the other in the visible (633 nm). The latter operates in a fully heterodyne mode and displays simultaneous images of optical near‐field phase and amplitude, at below 10 nm resolution.
We demonstrate a scattering-type scanning near-field optical microscope (s-SNOM) with broadband THz illumination. A cantilevered W tip is used in tapping AFM mode. The direct scattering spectrum is ...obtained and optimized by asynchronous optical sampling (ASOPS), while near-field scattering is observed by using a space-domain delay stage and harmonic demodulation of the detector signal. True near-field interaction is determined from the approach behavior of the tip to Au samples. Scattering spectra of differently doped Si are presented.
Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (<10 ...nm) and with simultaneous amplitude and phase contrast. The images depict the optical oscillation patterns of single plasmon particles. Examples are presented of particles excited in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for non-linear and single-molecule spectroscopy applications.
We evaluate the efficiency of back-scattering, eta(B), from a standard cantilevered AFM probe contacting a flat sample, and also the back-scattering phase. Both quantities are spectroscopically ...determined over a broad 9-12 mum wavelength range by coherent frequency-comb Fourier-transform spectroscopy (c-FTIR). While Fresnel reflectivity contributes a key factor with the SiC Reststrahlen edge at 975 cm(-1)as previously documented, we observe spectral effects ascribable to antenna resonances involving the shaft, cantilever, and sample. Most conspicuous is strong (eta(B) = 13%), resonant back-scattering at 955 cm(-1), a frequency that suggests the involvement of surface-phonon-polariton excitation, when the tip probes the area near a SiC/Au boundary. The probe's antenna properties are elucidated by numerically simulating the near fields, the fields in the radiation zone, and the far-field scattering distributions. The simulations are performed for a realistic tip/sample configuration with a three-orders-of-magnitude scale variation. The results suggest a standing-surface-plasmon-polariton pattern along the shaft, as well as far-field antenna lobes that change with the sample's dielectric properties.
Nonlinear probing of the fundamental lattice vibration of polar crystals is shown to reveal insight into higher-order cohesive lattice forces. With a free-electron laser tunable in the far infrared ...we experimentally investigate the dispersion of the second-order susceptibility due to the phonon resonance in GaAs. We observe a strong resonance enhancement of second harmonic light generation at half the optical phonon frequency, and in addition a minimum at a higher frequency below the phonon frequency. Measuring this frequency and comparison to a theoretical model allows the determination of competing higher-order lattice forces.
Surface plasmon‐, phonon‐ and exciton‐polaritons exist on specific materials in specific spectral regions. We assess the properties of such travelling surface‐bound electromagnetic waves relevant for ...scanning near‐field optical microscopy applications, i.e. the tightness of surface binding, the attenuation, the phase velocity and the coupling with free‐space electromagnetic waves. These quantities can be directly determined by photographic imaging of surface plasmon‐ and surface phonon‐polaritons, in both the visible and mid‐infared regions. Focusing of mid‐infrared surface plasmons is demonstrated. Surface waveguides to transport and focus photons to the tip of a scanning near‐field probe are outlined.
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