Speaker
Description
Nonlinear optical imaging with a spatial resolution well below the Abbe diffraction limit of light, such as stimulated emission depletion (STED) microscopy, has enabled many ground-breaking discoveries. In the infrared (IR) spectral range, however, no com¬parable schemes exist, and high-resolution infrared imaging has so far typically been restricted to scanning probe techniques such as scattering near-field optical microscopy (s-SNOM).
Here, we demonstrate sum-frequency generation (SFG) microscopy as an IR super-reso¬lution imaging technique [1], which can additionally provide interface-specific signals through the selection rules of the nonlinear-optical process. By combining a resonant IR and non-resonant visible light beam at an interface, a light field at the sum of the two incoming frequencies is generated. Since this signal emerges in the visible, it can be imaged with a resolution limited by the wavelength of the SFG. The imaging contrast, however, arises from the IR resonances in the sample.
To demonstrate the concept, we studied an infrared nanophotonic system based on localized phonon polaritons in sub-diffractional nanostructures made from silicon carbide. Since SiC has broken inversion symmetry, we here probe the mode volume of the nanophotonic resonators rather than the sample surface. For our experiments, we use the infrared free-electron laser (FEL) at our institute [2] as a unique high-power infrared light source. Previous SFG microscopy results [3] by scanning tightly focused beams demonstrated the principle, but also revealed laser-induced damage.
Our wide-field SFG microscope clearly resolves polariton modes in individual sub-diffractional nanostructures, providing a resolution of ~1.35 µm at resonant imaging wavelengths in the range of λIR = 10-12 µm, i.e., we achieve a spatial resolution of ~λIR/9 [1]. Full spectral mapping over the whole SiC reststrahlen band allows the spectroscopic identification of polariton resonances, while the high spatial resolution allows the microscopic identification of the origin of the SFG light. Specifically, we are able to spatially resolve the different polariton modes in SiC nanopillars, i.e., monopole and dipole modes [1], as well as edge modes in square arrays of the pillars.
Further, we study SiC nanorods were we find length dependent polariton resonances. We compare the SFG microscope images with s-SNOM and nano-Fourier transform IR measurements of the same structures, as well as with electric field simulations calcu¬lated in COMSOL.
References
[1] R. Niemann, S. Wasserroth, G. Lu, S. Gewinner, M. de Pas, W. Schöllkopf,
J.D. Caldwell, M. Wolf, and A. Paarmann, Appl. Phys. Lett. 120, 131102 (2022).
[2] W. Schoellkopf et al., Proc. SPIE 9512, 95121L (2015).
[3] R. Kiessling et al., ACS Photonics 6, 3017 (2019).
| Abstract Number (department-wise) | PC 18 |
|---|---|
| Department | PC (Wolf) |
