Periodic Reporting for period 1 - PoSHGOAT (Potential-dependent Second-Harmonic Generation in Optical Antennas measured Time-resolved)
Periodo di rendicontazione: 2020-09-01 al 2022-02-28
The project addressed one relevant aspect of nonlinear optical phenomena at the nanoscale, namely the possibility to control second-harmonic generation in metal nanoantennas by electrical bias, i.e. by modulating the amount of surface charges on the nanostructures. Metal nanoantennas are widely used to boost light-matter interactions at the nanoscale because of their ability to sustain localized resonances supported by conduction electrons and, among the many applications, have been successfully applied over the last decade to boost nonlinear processes over regions smaller than the wavelength of the electromagnetic radiation, something that is not possible with conventional optics. Within this framework, the possibility to achieve an external and efficient control of the nonlinear conversion processes by electrical means would pave the road towards electrically-controlled devices that find applications e.g. in optical quantum protocols or sensing. To contribute to this goal, the PoSHGOAT project aimed at demonstrating the experimental feasibility of the concept by achieving modulation of the second harmonic generation (SHG) in electrically-connected antennas. Along this path, advanced nanofabrication techniques, numerical modeling, spectrally- and time-resolved spectroscopies were considered among the main ingredients for the realization and understanding of the modulation phenomena.
In brief, the work performed during the project included several aspects:
1 - The development of optimized nanofabrication procedures to achieve high-resolution patterning of nanoantennas with sharp tips and well-defined features down to a few nm. This was achieved by focused ion beam milling starting from chemically-synthesized Au single-crystalline flakes, also through the design of dedicated software optimization algorithms for the milling procedure.
2 - The simulations of the linear and nonlinear optical response of gold nanoantennas, which was tackled with finite-difference time-domain and finite-element numerical methods. The investigated antenna geometries pushed the necessity for accurate numerical modeling at the limit of the state of the art, requiring a complete assessment of the role of any detailed geometrical feature and of all the asymmetries in the investigated samples.
3 - The realization of a dedicated setup for the concurrent electrical and nonlinear optical characterization of the antennas, allowing for electrical I-V measurements together with SHG microscopy at the single antenna level.
The main results achieved by the project are the following:
1 - We succeeded in experimentally demonstrating that it is possible to modulate the SHG in an electrically-connected nanoantenna located in front of a counter-electrode. The modulation depth is currently limited by the occurrence of electrochemical processes induced at ambient conditions on the gold surfaces, but optimization of the environmental vacuum (allowing for larger applied biases) and/or of the antenna geometries (leading to larger efficiencies) make 100% modulations an intriguing and realistic perspective for future applications.
2 - We thoroughly studied the dependence of SHG on the presence of sharp features and local asymmetries, which could be finely controlled thanks to the advanced ion beam milling fabrication developed by the project. In doing so, we demonstrated that the control of the gap asymmetry can lead to a substantial enhancement in the SHG signal compared to a reference symmetric situation.
3 - We discovered that the SHG efficiency also impacts the third harmonic generation in nanoantennas, because of the occurrence of a cascaded process that leads to the generation of third-harmonic photons via the sum-frequency process between a second-harmonic photon and a fundamental-wavelength photon. We experimentally demonstrated this occurrence in our nanoantennas by studying the interference process between the direct third-harmonic generation and the one originating from the cascaded process.
1 - The development of optimized nanofabrication procedures to achieve high-resolution patterning of nanoantennas with sharp tips and well-defined features down to a few nm. This was achieved by focused ion beam milling starting from chemically-synthesized Au single-crystalline flakes, also through the design of dedicated software optimization algorithms for the milling procedure.
2 - The simulations of the linear and nonlinear optical response of gold nanoantennas, which was tackled with finite-difference time-domain and finite-element numerical methods. The investigated antenna geometries pushed the necessity for accurate numerical modeling at the limit of the state of the art, requiring a complete assessment of the role of any detailed geometrical feature and of all the asymmetries in the investigated samples.
3 - The realization of a dedicated setup for the concurrent electrical and nonlinear optical characterization of the antennas, allowing for electrical I-V measurements together with SHG microscopy at the single antenna level.
The main results achieved by the project are the following:
1 - We succeeded in experimentally demonstrating that it is possible to modulate the SHG in an electrically-connected nanoantenna located in front of a counter-electrode. The modulation depth is currently limited by the occurrence of electrochemical processes induced at ambient conditions on the gold surfaces, but optimization of the environmental vacuum (allowing for larger applied biases) and/or of the antenna geometries (leading to larger efficiencies) make 100% modulations an intriguing and realistic perspective for future applications.
2 - We thoroughly studied the dependence of SHG on the presence of sharp features and local asymmetries, which could be finely controlled thanks to the advanced ion beam milling fabrication developed by the project. In doing so, we demonstrated that the control of the gap asymmetry can lead to a substantial enhancement in the SHG signal compared to a reference symmetric situation.
3 - We discovered that the SHG efficiency also impacts the third harmonic generation in nanoantennas, because of the occurrence of a cascaded process that leads to the generation of third-harmonic photons via the sum-frequency process between a second-harmonic photon and a fundamental-wavelength photon. We experimentally demonstrated this occurrence in our nanoantennas by studying the interference process between the direct third-harmonic generation and the one originating from the cascaded process.
The results achieved during the project impact on the fundamental research on nanoscale nonlinear optical conversion. In particular, they highlight the importance of the local geometry in the efficiency of the process, an issue that is already general wisdom in the community but that has now been tackled systematically and most importantly quantitatively thanks to the high precision and the high level of geometrical control granted by the optimized nanofabrication methods and to the accurate numerical modeling employed for the analysis. Moreover, the demonstration that SHG can be controlled via electrical bias opens the way to hybrid devices that exploit the interplay between static charges and nonlinear optical processes for active modulation and sensing. This will impact the future research in the field by providing researchers in Europe and around the world with established procedures and geometries to further investigate these processes and apply them to practical working devices.