Virtues of being single: a molecule and its golden nantenna

November 20, 2015

Vartkess Apkarian

              It is possible to communicate with individual molecules via electromagnetic waves, via Raman scattering, by equipping them with plasmonic nano-antennas (nantennas). Two recent examples from our laboratory are illustrative:
             Using the nano-analog of the antenna employed by Hertz  (a golden nanosphere dimer), we recently demonstrated time-resolved coherent anti-Stokes scattering on a single molecule. We could see a single molecule breath in real time (video in supplement of ref. 1) by following its vibrational quantum beats.
More recently, we captured the flip-flop of a single molecule between its cis and trans isomers. We could see single isomerization events, of a heterogeneously photocatalized reaction, albeit on laboratory time scales of seconds. This was accomplished in a scanning tunneling microscope where the tip-substrate junction acts as the nantenna.
             To further improve space-time resolution, to build a “chemiscope”, a deeper understanding of the workings of nantennas and their finer tuning will be essential. To this end, we scrutinize the gold nanosphere dimer nantenna, as a meta-diatomic molecule. Through polarized electronic Raman scattering on the nantenna, we show that the plasmons sustain angular momentum,  states that mix to generate chiral plasmons.
             I will expand on the virtues of being single, e.g., Raman spectra of individual molecules depend on their spatial orientation, single molecules do not undergo pure dephasing; also, I will extol the virtue of interrogating single molecules as measurements that follow quantum logic per force of the single being quantized. This is most naturally illustrated in the analysis of polarization states of light scattered on a chiral meta molecule, which map out as trajectories on the Bloch sphere. A single chiral nantenna is sufficient to fully control the polarization state of scattered photons.