Mode-matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation

Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs. Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.Comment: 14 pages, 4 figure

Plasmon-Enhanced Second Harmonic Generation: from Individual Antennas to Extended Arrays

We analyze the emission yield of the second harmonic generation (SHG) from dense ordered arrays of L-shaped Au nanoantennas within a well-defined collection angle and compare it to that of the isolated nanostructures designed with the same geometrical parameters. Thanks to the high antenna surface density, arrays display one order of magnitude higher SHG yield per unit surface with respect to isolated nanoantennas. The difference in the collected nonlinear signals becomes even more pronounced by reducing the collection angle, because of the efficient angular filtering that can be attained in dense arrays around the zero order. Albeit this key-enabling feature allows envisioning application of these platforms to nonlinear sensing, a normalization of the SHG yield to the number of excited antennas in the array reveals a reduced nonlinear emission from each individual antenna element. We explain this potential drawback in terms of resonance broadening, commonly observed in densely packed arrays, and angular filtering of the single antenna emission pattern provided by the array 0th order

Emission engineering in germanium nanoresonators

Over the last decade Ge has been proposed as one of the most promising materials for light detection, modulation, and emission for silicon-photonics architectures [1]. Its direct band-gap, which is only about 140 meV larger than the indirect band-gap, ensures excellent absorption and promising emission properties, which recently led to the realization of integrated detectors [2], electroluminescent devices [3], and to the first demonstration of optically-pumped [4] and electrically-pumped [5] Ge lasers. Here we investigate the smallest germanium-on-silicon Fabry-Pérot cavity resonators working around 1.55 m [see Panel (a)] and, by properly tuning their cavity length, demonstrate experimentally almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a Ge film of the same thickness. This can be described as the combined result of the nanoresonator effective beaming (acting as optical antennas), laser excitation enhancement, and fluorescence emission enhancement (Purcell effect). These results are in excellent agreement with finite-difference time-domain simulations [see Panel (b)] and set the basis for understanding and engineering emitting devices based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths

Emission Engineering in Germanium Nanoresonators

We experimentally investigate the smallest germanium waveguide cavity resonators on silicon that can be designed to work around 1.55 μm wavelength and observe an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a continuous germanium film of the same thickness. The enhancement is due to an effective combination of (i) excitation enhancement at the pump wavelength, (ii) emission enhancement (Purcell effect) at the emission wavelength, and (iii) effective beaming by the nanoresonators, which act as optical antennas to enhance the radiation efficiency. Our results set a basis for the understanding and engineering of light emission based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths