Daniel Ramos1
CSIC1
We have developed a high throughput spectrometric technique addressing single biological entity (bacteria) resolution. This novel technique will be used to develop a novel imaging technique based on mechanical frequency shift of a nanomechanical resonator to generate a mechanical image of single particles and bacteria. The physical principle behind this technique is the modulation of the light absorption by the particle, which is translated into a thermo-mechanical effect on the nanomechanical resonator. This idea was recently demonstrated by using plasmonic gold particles of 100 nm in diameter [1] and to mechanically image viruses and bacteria cells (in press) of 700 nm in diameter. We will show not only the optomechanical coupling that emerges in the cavity formed by a plasmonic nanoparticle onto a free-standing silicon nitride membrane, but also the thermomechanical coupling by using dielectric particles and biological entities. The optical absorption depends on the scatterer material; therefore, it is possible to unambiguously discern in between different dielectric particles and bacteria cells of the same size by simply analyzing the mechanical frequency shift while shinning with a laser [2]. The optical absorption can also be tuned by playing around with the optical resonances of a photonic crystal, which allows to actively tune the mechanical resonance frequency.<br/>We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. The key effect to explain the coupling in these systems is the extinction cross-section enhancement due to the excitation of the LSPR at selected wavelengths. The extinction is the combination of the absorption and the scattering of the particle; therefore, it becomes a hot-spot on the membrane consequently tuning the mechanical resonance frequency through thermomechanical effects. Despite the low-quality factor exhibited by the broad optical resonances, these systems represent an attractive alternative when compared with other optical cavities in literature because they are easily excited by free-space light beams. This issue is of crucial importance because the free-space optomechanical coupling largely decays when the size of the mechanical system is below the wavelength[3,4], which, is needed to achieve high mechanical frequency regime. In order to validate this coupling, we have developed an interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs, dielectric nanoparticles and biological entities onto the membrane, allowing the differentiation in between single bacteria cells and particles by the frequency shift and polarization sensitivity.<br/>[1] Ramos, D., et al, “Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles”, Nano Lett., 18, 7165-7170 (2018).<br/>[2] Ferreiro-Vila, E., et al, “Micro-Kelvin Resolution at Room Temperature Using Nanomechanical Thermometry”, <i>ACS Omega</i> 6 (36), 23052-23058 (2021).<br/>[3] Ramos, D., et al, “Silicon nanowires: where mechanics and optics meet at the nanoscale”, Sci. Rep., 3, 3445, (2013).<br/>[4] Ramos, D., et al, “Optomechanics with Silicon Nanowires by Harnessing Confined Electromagnetic Modes”, Nano Lett., 12, 932-937 (2012).