KEYWORDS: Mid-IR, Near field scanning optical microscopy, Upconversion, Microscopes, Sum frequency generation, Visible radiation, Vibration, Imaging spectroscopy, Surface properties, Spectroscopy
S-SNOM has transformed into a key actor for nanoscale imaging and spectroscopy. Extension of its abilities beyond the mere infrared absorption would be interesting for numerous technological developments.
Here, we use s-SNOM to study mid-IR to VIS sum-frequency generation (SFG), in which the sample’s SFG cross-section is enhanced by orders of magnitude by the microscope’s tip. Combining mid-IR and VIS illuminations on gold nanostructures and studying several molecular vibrations, we clarify the contribution of the tip onto this nonlinear optics process.
Our work could also lead to a novel near-field microscopy modality where non-linear optical properties could be probed at the nanoscale.
As applications in fields like security or medicine require sensitive schemes in order to detect IR photons, an interesting strategy consists in converting weak IR signals into the optical domain where detectors with single photon sensitivity are readily available. We introduce here a novel platform for ultra-sensitive conversion and detection of far and mid-infrared signals, inspired by cavity optomechanics. The conversion process, relying on the intrinsic ability of specific molecular vibrations to interact both with optical and IR fields, is optimized through the use of doubly resonant nano-antennas. Our study demonstrates noise levels improving on state of the art for IR detectors operating at room temperature and opens the path to single IR photon detection devices.
When two sub-wavelength metallic nanoparticles, each of them supporting a resonant plasmon, are brought within few nanometers or less from each other, the two plasmonic resonances are strongly coupled. The new eigenmodes of the system include in particular a dipolar mode, for which the maximum electric field is localized in the nanoscale gap between the particles. The local field enhancement compared to the incoming far field can be several hundred folds.
We present the design and fabrication of such plasmonic gap cavities, created by depositing gold nanospheres on an atomically flat gold surface, which has been functionalized with a self-assembled monolayer of thiol molecules. This system enables extremely large and reproducible enhancement of the Raman signal from the molecules.
Although these nanogap cavities have been used in SERS studies for some time already, a detailed understanding of the out-of-equilibrium physics under laser irradiation is missing. What is the local temperature of the electrons in the metal? Is the molecular vibration in equilibrium with the surrounding thermal bath?
We will present our latest results in the spectroscopy of these nanocavities under broadly tunable excitation. In particular, we want to clarify if a suitable detuning of the laser from the plasmonic resonance can lead to amplification of molecular vibrations [1] well above the thermal occupancy.
[1] P. Roelli et al, Nature Nanotechnology 11, 164–169 (2016)
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