Tip-enhanced nanocavities — In order to facilitate the integration of our nanostructures into conventional near-field optical microscope, I decided to combine the scanning metallic tip of the microscope with a gold nanoparticle-on-mirror (NPoM) structure. According to the proposal, I could expect that for well selected tip apex versus nanoparticle diameters the tip would act as a second illumination source and condense the IR field inside the NPoM gap. I selected the NPoM geometry in order to support plasmonic resonances in the visible so that my hybrid tip-antenna design would provide strong IR and VIS field concentration inside the NPoM gap where molecules are located.
The numerical simulations of my tip-antenna design demonstrates not only the advantageous role of the tip in the infrared but also in the visible, achieving higher VIS field enhancement values than the NPoM alone. Additional numerical calculations of the tip-enhanced nanocavities confirmed that this design could reach 14 orders of magnitude enhancements of upconversion signals without the need of an IR resonant antenna.
Combined IR + Raman near-field optical microscope — In our dual IR + Raman s-SNOM setup, the sample and tip are illuminated from the side by a combination of mIR and VIS laser sources. After combination on an ITO plate both beams are sent to a customized high-NA parabolic mirror (PM), improving both the in-coupling of beams to the tip and NPOM structures and the collection of minute backscattered vibrational signals. To demonstrate the near-field aspect of my upconversion signal, or in other words to witness the contribution of the tip, I also developed in collaboration with the company attocube systems AG a new mode of measurements where the sample is maintained into focus while the tip is retracted in a controlled way via a piezo actuator.
I continue these instrumentation efforts nowadays to enable tip-enhanced Raman scattering (TERS) operations with this microscope. To this end, I improved the AFM path of my microscope to enable tapping mode measurements with tapping amplitude < 5 nm and added an external laser scanner (two-axis galvanomotors) to finely adjust while measuring the coupling between the incoming laser field and the tip apex without damaging sample or tip.
Near-field signature of molecular upconversion — The development on the cavity and on the instrumentation presented in the last sections enabled me to observe molecular upconversion signals on two vibrational modes separated by 500 cm−1 wavenumbers. The typical vibrational Raman signals arising inside the NPoM gap in the absence of IR laser (black curve) are now selectively and substantially modified by the addition of an IR laser resonant with one or the other vibrational mode of the molecular single-layer contained inside the NPoM gap.
I evidenced the role of the tip and demonstrated the near-field nature of the upconverted (sum-frequency) signal I observe, as the upconverted vanishes when the tip is retracted to a separation of 200 nm. I also confirmed the crucial role of the tip in other vibrational processes, including Raman and difference-frequency signals. Notably, the far-field contribution to the molecular upconversion signal is negligible, making this approach an ideal candidate for background-free and chemically selective near-field imaging. We are currently exploring several additional single- and few-layer samples to further demonstrate the versatility of our nanospectroscopy platform in probing upconversion, enabling a clearer distinction between electronic and vibrational contributions, as well as other nonlinear optical signals.
Engineering couplings involved in molecular upconversion — The nanomechanically controlled structures presented in the previous section provide a versatile platform for nonlinear optical spectroscopy at the nanoscale and in the few-molecule regime. However, accessing particularly interesting physical regimes—such as multi-excited photon spectroscopy or vibrational strong coupling—may require refined tip-enhanced designs. For this reason, during the project we explored different strategies to integrate infrared (IR) resonators into our nanoscale platforms.
In particular, I investigated plasmonic coaxial nanoapertures with gap dimensions between 10 and 50 nm and explored molecular layer deposition (MLD) as a method to control the molecular filling of these gaps. This approach allowed me to fabricate specific molecular chains within the cavities and characterize their IR absorption spectra via FTIR measurements. I also developed an alternative platform combining phonon-polariton IR resonances supported by polar dielectrics such as quartz with gold nanoparticles. We demonstrated that such nanostructures host strongly confined cavity modes in the infrared, akin to NPoM structures in the visible. Current work focuses on functionalizing these structures to exploit these modes and explore the vibrational strong coupling regime.
In parallel, I extensively revisited the optomechanical framework I previously introduced [Roelli et al., Nat. Nano. (2016)] to provide a more accurate description of the various couplings involved in plasmonic cavities—namely between the vibrational mode and the cavity field, as well as between the incoming laser field and the cavity mode. This work aims to clarify the current limitations in upconversion efficiency and to better identify the different coupling regimes that can be accessed in these nanoscale systems.
