Nanomechanical Photothermal Near Infrared Spectromicroscopy of Individual Nanorods

Abstract

Understanding light-matter interaction at the nanoscale requires probing the optical properties of matter at the individual nanoabsorber level. To this end, we developed a nanomechanical photothermal sensing platform that can be used as a full spectromicroscopy tool for single molecule and single particle analysis. As a demonstration, the absorption cross-section of individual gold nanorods is resolved from a spectroscopic and polarization standpoint. By exploiting the capabilities of nanomechanical photothermal spectromicroscopy, the longitudinal localized surface plasmon resonance in the NIR range is unraveled and quantitatively characterized. The polarization features of the transversal surface plasmon resonance in the VIS range are also analyzed. The measurements are compared with the finite element method, elucidating the role played by electron surface and bulk scattering in these plasmonic nanostructures, as well as the interaction between the nanoabsorber and the nanoresonator, ultimately resulting in absorption strength modulation. Finally, a comprehensive comparison is conducted, evaluating the signal-to-noise ratio of nanomechanical photothermal spectroscopy against other cutting-edge single molecule and particle spectroscopy techniques. This analysis highlights the remarkable potential of nanomechanical photothermal spectroscopy due to its exceptional sensitivity.

Photothermal Microscopy and Spectroscopy with Nanomechanical Resonators

“…In the most recent work, Kanellopulos et al. demonstrated near-IR polarization-dependent spectromicroscopy of ∼50 nm gold nanorods, as mentioned above (see Figure a) . This effort not only yielded a high signal-to-noise ratio without a modulated heating laser, but also uncovered the potential of nanomechanical resonators for highly sensitive broadband linear, and possibly circular, dichroism spectroscopy.…”

“…Nonetheless, the same authors employed SiN trampolines (Figure b) to position 200 nm diameter spin-coated gold nanoparticles with the assistance of an atomic force microscope (AFM) tip, photothermally localized with 3 Å precision using a 633 nm laser . Progressing from these seminal studies, a very recent study utilized drumhead resonators to investigate the polarization dependence and near-IR absorption spectra of individual gold nanorods, which are ∼50 nm long with a 4:1 aspect ratio (see Figure a). , Again, trampoline nanoresonators enabled also analysis of the carbon content of direct-write plasmonic Au nanostructures, verifying the corresponding enhancement of localized surface plasmon fields between two bowtie structures …”

“…Nanomechanical photothermal absorption spectra from the most recent works demonstrating the wavelength range and variability of probing source. (a) Near-IR plasmon absorption spectrum of a 50 nm-long silica-coated Au nanorod, using a tunable-wavelength laser (TWL), reproduced from ref . Copyright 2023 under a CC BY 4.0 license.…”

“…The caveat is that, in all resonators, higher responsivity also increases sensitivity to thermal noise. Nonetheless, research shows that nanomechanical resonators can exhibit higher signal-to-noise ratios than other photothermal methods. , …”

“…Reducing the amount of optics is also advantageous for studies in the ultraviolet range, where power is generally more significantly attenuated. Nonetheless, it maintains optical versatility, accommodating focused or wide-field, coherent or incoherent light, enabling spectroscopic analysis of diverse entities including thin films, , 2D materials, , surface-adsorbed chemical species, , explosives, nanoparticle ensembles, − and individual single nanoparticles. − …”