Fig. 2: Field enhancement in the SLG CSRR gap.
a Scanning electron microscope (SEM) image of optimized CSRR array, with period p, patterned with SLG that only covers the split gap area (blue dashed area on the right), where the field enhancement is maximized. The dotted blue area marks SLG. b Experimental and (c) simulated transmission of bare CSRR (black) and SLG-CSRR (red) arrays. The transmission curves are normalized by the reference sample trace, acquired on a portion of a bare SiO2/Si substrate. d FTIR emission spectrum of QCL frequency comb with spectral bandwidth~0.7 THz centered ~2.94 THz. e Schematic near-field scattering detector-less nanoscope, employing a THz QCL frequency comb as source and detector simultaneously. The self-mixing signal, collected to retrieve the near field maps, arises from re-injection of the backscattered beam from the atomic force microscopy (AFM) tip of the scattering type scanning near field optical microscope (s-SNOM) into the laser facet. A delay-line is used to control the length L of the optical path and tune the optical feedback. OAP: off axis parabolic mirror. f Topographic maps of a single SLG-CSRR. g Near-field map of third-order self-mixing signal ΔV3 of the CSRR in panel a (right side). The white dashed area highlights the field enhancement in the slit gap. The near-field map is collected with the radiation approaching the sample at a 45° incident angle. The sample is therefore excited with two equally contributing s- and p- terms with respect to the CSRR plane, under the hypothesis of a purely collimated beam. However, the focusing of the beam onto the SNOM tip also implies a spreading of the probed incident wavevectors k (therefore of the polarizations, orthogonal to k), dictated by the beam radius and the numerical aperture of the focusing component, i.e. the SNOM parabolic mirror in front of the tip.
