Electrically-Excited Chiral Plasmonic NanoCavities – ChiC

Chiral structures, whose initial and mirror structural images cannot be superimposed, interact differently with left-handed and right-handed circularly polarized light. Thus a structure of a certain “handedness” preferentially scatters or absorbs circularly polarized light of the same handedness, leading to a specific “chiroptical response”. Chirality is a crucial property for many essential molecules in biology, such as proteins and nucleic and amino acids. Most often, however, the corresponding chiroptical response is very weak. A method for increasing the chiroptical response would thus be a major breakthrough for many applications in biology, chemistry and physics. This is a major goal of this project.

A plasmonic nanocavity (or “nanoparticle-on-a-mirror” structure) is formed when a nanoscale gap separates a noble metal nanoparticle from a metallic substrate. The electromagnetic field in such a nanocavity is dramatically enhanced, thanks to the plasmonic resonances which “concentrate” the electromagnetic field. The main idea of this project is thus to apply this principle to chiral plasmonic cavities and thus enhance the chiral interactions between light and matter. Such a chiral nanocavity will be made by depositing a chemically-synthesized gold nanoparticle that has a chiral structure on a thin (0.5 - 2 nm) insulating spacer layer on top of a metallic film. This will be the first time that such a chiral nanocavity is studied.

The chiroptical response of materials and structures is most often studied by optical means, yet in a future optoelectronic nanodevice, a local electronic excitation is necessary. Working with this long-term goal in mind, we will use inelastic tunneling electrons to locally excite the nanoparticle-on-a-mirror samples. The exciting tunneling junction will be the one between the chiral nanoparticle and the metallic substrate. In order to polarize the junction, the electrical circuit will be completed using the conducting tip of an atomic force microscope.

In order to demonstrate a possible application of the expected enhanced chiral response in the “chiral nanoparticle-on-a-mirror” geometry, a monolayer of a transition metal dichalcogenide (TMDC) material will be placed in the chiral plasmonic cavity. TMDCs are two-dimensional (2D) semiconductors which are being considered for a new computational paradigm: “valleytronics”. In valleytronics, it is the valley state (i.e., crystal momentum) of the electrons that may be used to store and transport information. In TMDCs, electrons from different valleys emit light with different circular polarizations when recombining with holes—valleytronics is thus intimately related to chirality. In a “handed” chiral plasmonic nanocavity, the emission from a particular valley is expected to be enhanced.

The main objectives of this project are thus as follows:
i. To develop chemical methods for the scalable synthesis of high-quality colloidal chiral plasmonic metal nanoparticles of different shapes and sizes. The chiral response will be optimized via the shape and the resonance wavelength (from the visible to the near infrared) via the size of the nanoparticles.
ii. To construct optimized “chiral nanoparticle-on-a-mirror” (CNoM) structures, i.e., a complex functional nano-object, in order to enhance the chiral response.
iii. To investigate the local electrical excitation of chiral nanoparticles with inelastic tunneling electrons. Both chiral nanoparticles on transparent substrates, and CNoM structures will be studied.
iv. To preferentially and electrically excite the circularly polarized luminescence of a particular handedness (i.e., from a particular “valley) from 2D TMDCs via chiral nanoparticles and CNoM structures.