Near-field imaging of optical antenna modes in the mid-infrared

Abstract

Optical antennas can enhance the coupling between free-space propagating light and the localized excitation of nanoscopic light emitters or receivers, thus forming the basis of many nanophotonic applications. Their functionality relies on an understanding of the relationship between the geometric parameters and the resulting near-field antenna modes. Using scattering-type scanning near-field optical microscopy (s-SNOM) with interferometric homodyne detection, we investigate the resonances of linear Au wire antennas designed for the mid-IR by probing specific vector near-field components. A simple effective wavelength scaling is observed for single wires with lambda(eff) = lambda /(2.0+/- 0.2), specific to the geometric and material parameters used. The disruption of the coherent current oscillation by introducing a gap gives rise to an effective multipolar mode for the two near-field coupled segments. Using antenna theory and numerical electrodynamics simulations two distinct coupling regimes are considered that scale with gap width or reactive near-field decay length, respectively. The results emphasize the distinct antenna behavior at optical frequencies compared to impedance matched radio frequency (RF) antennas and provide experimental confirmation of theoretically predicted scaling laws at optical frequencies.

Near- and far-field measurements of phase-ramped frequency selective surfaces at infrared wavelengths

“…5 where realized gain is defined as the ratio of the radiated power in each direction to the power of the incident beam. For the phase-ramped loops, the measured deflected angle is 23 while the phase-ramped patches have a deflected angle of 24 , which is relatively close to the calculated values of 25 for both structures. One difference between the calculated and measured beam deflection angle is that the calculation is based on values of reflected phase obtained from simulations of uniform arrays of elements while the measurement is of the phase-ramped structures having different size elements.…”

“…This scattered radiation emanating from the tip is collected using the same optics for the incident beam and is detected interferometrically with the mercurycadmium-telluride (MCT) detector. 25,26 Polarization selective optics allow for predominantly p-polarized radiation to reach the detector, which restricts the measurement to mainly the E z field. 25 AFM height images are collected simultaneously with the s-SNOM signal (S d ), which is proportional to…”

“…25,26 Polarization selective optics allow for predominantly p-polarized radiation to reach the detector, which restricts the measurement to mainly the E z field. 25 AFM height images are collected simultaneously with the s-SNOM signal (S d ), which is proportional to…”

Frequency addressing of nano-objects by electrical tuning of optical antennas

“…The possibility of achieving a strong field enhancement on a nanometer scale in the antenna feed gap is certainly one of the main reasons that triggered research in the area of optical antennas in the last years [4,7,8,[27][28][29]. Here we will demonstrate that the unique properties offered by anisotropic media and in particular by nematic LCs can be used in this framework to electrically control the resonance frequency and hence as a tuning mechanism to address nano-objects at different frequencies.…”

“…Recent years have witnessed an impressive amount of research on optical devices conceived to efficiently couple free-space propagating light with localized light emitters or receivers, for a wide variety of potential applications, including nonlinear optics, efficient solar cells, near-field optical microscopy, and spectroscopy [1][2][3][4][5][6][7][8][9][10][11][12]. Beyond confining light of a given frequency at fixed locations, imposed by the structure geometry, there is a need for dynamical control of such hot-spots, for instance, to achieve selective frequency addressing of different nearby nanoobjects [13,14].…”