Engineering the second harmonic emission from arrays of L‐shape nanoantennas
Résumé
Second Harmonic Generation (SHG) is well known to be a powerful imaging tool for background‐free and nondamaging live tissues investigation. Incidentally, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase‐matching in confined volumes. Proper optimization of such phenomena may allow realizing biosensing platforms based on nanoscale nonlinear sources. Recently, we reported single‐crystalline gold nanoantennas working in the near‐infrared that show unprecedented SHG efficiency thanks to (i) a multi‐resonant response occurring at both the excitation and SH wavelength, (ii) a significant spatial overlap of the localized fields at the wavelengths of interest and (iii) a broken‐symmetry geometry to achieve dipole‐allowed SHG. The effective combination of these key features in a single plasmonic antenna, characterized by the absence of local defects, allowed for the optimization of SHG efficiency in a well‐controlled fashion. Here, by expanding the paradigm for SHG enhancement to extended metasurfaces of nanoantennas, we aim at realizing a nonlinear platform that can be easily integrated in standard biosensing devices, since it can be excited by a weakly‐focused light. To this purpose, using electron beam lithography (EBL), we developed a first prototypical device based on a 10x10 um2 array of closely‐packed L‐shaped antennas. We have characterized the nonlinear response of this device using a homemade confocal microscope equipped with a 0.7 numerical aperture (NA) objective (Nikon) working in epireflection, coupled to a 120 fs laser centered at 1550 nm. By comparing experiments and simulations, we found that in this device SHG is dominated by the plasmonic resonance of the antennas at the fundamental wavelength. When comparing the average SHG from the array with the signal from individual L‐shape nanoantennas, an enhancement of about 15‐20 times is found that, considering about 50 antennas under the illumination spot, implies a drop in the emission efficiency of the individual antennas. Indeed, FDTD simulations demonstrate that, while a single antenna has a directional emission peaking away from the sample normal, the square‐lattice array radiates only through the (0,0) diffraction order (i.e. normal to the sample). The effective second harmonic emission from the array is then the convolution between the single‐antenna radiation pattern and the array diffraction pattern. This effectively inhibits some of the SH radiation that would be emitted by the isolated nonlinear antennas, which gets dissipated in the array itself. The relatively high NA employed, used here to probe possible inhomogeneities in sample fabrication, allows one to efficiently collect SHG from both the array and the individual antennas. On the contrary, calculations show that using a NA as small as 0.2 all of the SHG from the individual antennas would be lost while more than 80% of the SHG emitted by the array could still be collected. This is achieved thanks to the grating effect of the array, which strongly confines the light emission into a very small solid angle. If properly optimized, this emission mode‐matching paradigm could lead to the implementation of simple filterless and objective‐free nonlinear biosensors.