Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) optically detects the scattered light from a sharp metallic tip. As the light scattering by the sharp tip depends on its surrounding materials, s-SNOM allows access to the sub-diffraction limited spatial resolution at optical and infrared frequencies. The initial concept was proposed by Edward H. Synge in 1928. Modern s-SNOM instrument was developed in the 1990s with the combination of atomic force microscopy and optical detection of scattered light. Our research group works on s-SNOM since 2015. We hope to improve s-SNOM with new instrument designs to reveal its full potential for nanoscale characterizations.
Peak force scattering-type scanning near-field optical microscopy (PF-SNOM)
Instead of using the commonly used tapping mode and lock-in detection, PF-SNOM uses the peak force tapping mode (also known as the pulsed force mode). It correlates the tip-sample distance with the near-field scattered light to obtain the vertical near-field signal dependence. After subtraction of the far-field background, PF-SNOM provides the tomographic near-field response with an explicit dependence of the tip-sample distance.
PF-SNOM also allows three-dimensional measurement of the near-field responses from nano- and microstructures, such as those made of polaritonic materials.
Ultra-broadband light source for Scattering-type scanning near-field optical microscopy
Scattering-type scanning near-field optical microscopy (s-SNOM) is a microscopy based on linear light scattering. The bandwidth of the light source determines its frequency coverage. Current broadband infrared lasers from difference frequency generations are expensive. Here we developed a laser-driven plasma source that provides wide frequency coverage, high brilliance, spatial coherence, and low cost for s-SNOM.
Scattering-type scanning near-field optical microscopy with low-repetition-rate pulsed light source through phase-domain sampling
We have discovered a way to combine low repetition rate pulsed laser with scattering-type scanning near-field optical microscopy through sampling in the phase domain. This technique opens the door to combining scattering-type scanning near-field optical microscopy with ultrafast lasers for nanoscale nonlinear and time-resolved processes at a high energy regime.
Currently, s-SNOM requires either a continuous wave laser source or a high repetition rate pulsed laser. Pulsed laser sources with a repetition rate < 100 kHz are unable to be used for scattering-type scanning near-field optical microscopy due to the limitation of the sampling rate by Nyquist-Shannon sampling theorem. Our phase-domain sampling technique bypasses this limitation by acquiring near-field interaction from a different and equivalent perspective.
Scattering-type scanning near-field optical microscopy with the reconstruction of vertical interaction
Despite the high lateral spatial resolution, conventional scattering-type scanning near-field optical microscopy does not provide simple access to the vertical characteristics of near-field interactions.
We have developed a technique to obtain the vertical interaction characteristics of tip-sample near-field interaction. It is done through the reconstruction of near-field response with metric optimization. Three-dimensional tip-sample interactions mapping can be achieved through s-SNOM with this method.