Single-molecule optomechanics in “picocavities”

Abstract

Coinage metal nanostructures support localised surface plasmons, which confine optical fields much tighter than their wavelength (1). This extreme enhancement enables vibrational spectroscopy within small volumes, even down to single molecules (2,3). For many years lateral resolution was believed to be 10 nm (4), however recent experiments resolve the atomic structure of single molecules using tipenhanced Raman spectroscopy (3) and directly sequence RNA strands (5). Atomistic simulations also suggest plasmonic confinement to atomic scales is possible (6). Here we show that light-activated mobilisation of surface atoms in a plasmonic hotspot triggers the formation of additional 'picocavities'bounded by a single gold atom. Their ultra-small light localisation alters which vibrational modes of trapped molecules are observed, due to strong optical field gradients that switch the Raman selection rules. The resulting cascaded ultra-strong plasmonic confinement pumps specific molecular bonds, thereby creating non-thermal vibrational populations, and forms a new type of optomechanical

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

“…The nature of chemical bonds as quantum vibrational oscillators can now be very effectively manipulated by trapping light tightly enough around molecules [8]. Analogous optomechanical systems use macroscopic vibrational modes such as cantilevers [9,10], beams [11], or pillars [12][13][14][15] coupled to highfinesse optical cavities in order to demonstrate a variety of effects such as cooling to the quantum vibrational ground state [16], parametric oscillation [17], and supercontinuum comb generation [18].…”

“…Analogous optomechanical systems use macroscopic vibrational modes such as cantilevers [9,10], beams [11], or pillars [12][13][14][15] coupled to highfinesse optical cavities in order to demonstrate a variety of effects such as cooling to the quantum vibrational ground state [16], parametric oscillation [17], and supercontinuum comb generation [18]. Recent theory [19,20] and experiments [8,21] have shown that this same optomechanical Hamiltonian describes vibrating molecular bonds in nanocavities, but with single-photon coupling coefficients ℏg 0 ∼ 10-100 meV, a millionfold larger than for macroscopic resonators. Here, we explore how this coupling can be used to elicit stimulated phonon scattering within vibrating bonds, leading toward phonon lasing or "phasing," observed previously in microfabricated mechanical oscillators [22][23][24], and we suggest how such mechanical resonances can lead to chemical reactions [ Fig.…”

“…Using an optomechanical model, it is possible to extract the photon-phonon coupling [8,20]. When the nanocavities are formed by ultrathin gaps between plasmonic metal nanostructures that trap the optical field, this strongly amplified process is termed surface-enhanced Raman spectroscopy (SERS) of the molecules in the gap.…”

Classical and ab Initio Plasmonics Meet at Sub-nanometric Noble Metal Rods

“…Thus, it has been recently shown that sub-nanometric inhomogeneities in field-enhancement patterns in plasmonic gaps [24][25][26] can be described, at least qualitatively, through classical optics calculations. On the other hand, the physical mechanisms behind the failure of classical predictions for the LSPR frequencies in compact nanoparticles (with sizes in the ∼ 1-3 nm range) are well understood.…”

Nanoscale Insights into Enhanced Raman Spectroscopy

“…Duan and Luo have proposed involvement of nonlinear optical processes [47]. Creation of an "atomic-scale hot spot" has also been proposed [48]. In addition, multiple elastic scattering of light between molecular dipoles adsorbed on the surface has been proposed to explain the improved signal intensity and TERS spatial resolution [49].…”