Nanoscale chiral light matter interactions: Detecting the chirality of molecules with augmented precision

Chiral objects - those with mirror images that cannot be superimposed - abound in nature. Examples range from spiral galaxies to human hands. Most importantly, biologically relevant molecules are predominantly chiral (proteins, amino acids, etc.). Handedness is fundamental to determine how molecules will interact with its environment, and there are numerous examples of biologically inert or beneficial molecules with an “evil twin” [1] that can be toxic for the human body. Especially infamous are the horrendous side effects produced by one enantiomer of the Thalidomide molecule, sold in the 60’s as a drug to avoid nauseas and different kinds of pains in pregnant women. Distinguishing between different enantiomers is consequently of utmost importance, for example, for safe and efficient drug production.

Electromagnetic fields can also be chiral, as in the case of circularly polarized light. Circularly polarized light is used to probe the geometric and electromagnetic chiral properties of molecules. Enantiomers of a chiral molecule exhibit preferential absorption of right- or left-handed circularly polarized light. Circular dichroism (CD) spectroscopy measures this differential absorption in the ultraviolet and visible spectrum.

CD spectroscopy is a widespread experimental technique that allows determining the chiral properties of molecular samples. It is widely used in pharmacology to determine the chiral purity of drugs, and in molecular biology, since the chiral properties of biological molecules are strongly correlated with their function. Nonetheless, due to the weak nature of the chiral interaction between light and the molecules, the technique requires large concentrations of molecules and long integration times. Increasing the sensitivity of this spectroscopy technique would improve many pharmaceutical and molecular biology applications.

In the past, we have developed methods to augment the capabilities of this technique using single optical antennas. Up to date, we designed and patented some preliminary systems presenting modest (yet substantial) efficiencies; both for surface enhanced CD spectroscopy [2] and for antenna enhanced enantio-separation techniques [3].

In this internship the student will develop a new theoretical formalism to predict accurately the capabilities of nanoparticles of different materials and shapes to augment the CD spectral signals of chiral molecules. This formalism will combine analytic calculations with numerical simulations using state of the art commercial software packages that the student will run in high-performance computing facilities (Hyperion).

[1] Francotte, E., & Lindner, W. Chirality in drug research (Vol. 33). Wiley-VCH. (2006).
[2] A. Garcia-Etxarri and J. A. Dionne, Phys. Rev. B 87, 235409 (2013).
[3] C. S. Ho, Aitzol Garcia-Etxarri, Yang Zhao, Jennifer Dionne, ACS Photonics 4, 2, 197-203 (2017)