OPTICAL heterodyNE detection
PHOTONICS INSIGHTS
PSEUDO-HETERODYNE DETECTION FOR NEAR-FIELD OPTICS MICROSCOPY
Scanning near-field optical microscopy( SNOM) measures optical fields scattered by a nanometric probe positioned within the optical near field. Since the useful scattered signal is several orders of magnitude weaker than the background radiation, heterodyne-like detection is essential. Pseudo-heterodyne detection achieves this over a broad spectral range from visible to THz without shifting the laser frequency [ 11 ]. In pseudo-heterodyne scattering- SNOM, the AFM tip is vertically modulated at a frequency Ω. Because the near-field interaction depends strongly and nonlinearly on the tip – sample distance, the scattered near-field signal is modulated at Ω and its higher harmonics nΩ. By demodulating the detector signal at nΩ, typically with n ≥ 2, the strongly
localized near-field contribution can be distinguished from the more slowly varying far-field background. The resulting harmonic components therefore provide access to the amplitude and phase of the complex optical field scattered by the tipsample interaction. When several harmonics are demodulated simultaneously, they also provide the Fourier coefficients of the periodically modulated signal, allowing its temporal waveform over one modulation cycle to be reconstructed by an inverse fast Fourier transform. At the same time, a mirror in the reference arm is sinusoidally displaced at a lower frequency ω m, periodically modulating the phase of the reference field, φ( t) = φ 0 + ∆φsinω m t. Interference between this phase-modulated reference field and the tip-scattered field produces sidebands at frequencies nΩ ± mω M whose amplitudes are described by Bessel functions. Demodulation of these sidebands yields two independent quadrature components of the near-field signal, from which its amplitude and phase can be reconstructed. The AFM-tip modulation thus provides near-field selectivity, whereas the vibrating reference mirror enables phasesensitive detection of the complex near-field response. Pseudo-heterodyne detection has become the reference technique in scattering-type SNOM, enabling quantitative nanoscale imaging of dielectric function, plasmonic resonances, phonon-polaritons and phase-change materials to name just a few.
Figure 3.( a) Pseudo-heterodyne near-field detection in reflection mode. The AFM tip is vertically modulated at frequency Ω, generating near-field components at the harmonics nΩ, while the vibrating reference mirror introduces an additional phase modulation at frequency ω M.( b) Interference between the tip-scattered field and the phase-modulated reference field produces sidebands at nΩ ± mω M, from which the amplitude and phase of the complex near-field signal can be retrieved. Adapted from Ocelic et al. [ 10 ].( c) Experimental near-field images of a gold micro disk illuminated with circular polarization in transmission mode and reconstructed from 3Ω side bands. Scale bar 1 µ m. Images recorded with Attocube neaSCOPE microscope on the SMARTLIGHT platform( https:// www. smartlight. fr /) by F. Dell’ Ova.
approach compensates for chromatic dispersion, polarization-mode dispersion, and other electronic transmission impairments. Consequently, it has become a key enabling technology for high-spectral-efficiency optical networks and aggregated multi-terabit-per-second transmissions over transoceanic distances.
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