An All-Optical Excitonic Switch Templated on a DNA Scaffold Operated in the Liquid and Solid Phases
Authors: Kellis Donald L. 
Published: June 2020

The natural excitonic circuitry of photosynthetic organisms, including light
harvesting antennas, provides a distinctive example of a highly attractive bio-inspired
alternative to electronic circuits. Excitonics, which capitalizes on spatially arranged
optically active molecules ability to capture and transfer light energy below the
diffraction limit of light has garnered recognition as a potential disruptive replacement
for electronic circuits. However, assembly of optically active molecules to construct even
simple excitonic devices has been impeded by the limited maturity of suitable molecular
scale assembly technologies.

An example of nanophotonic circuitry, natural light harvesting antennas employ proteins
as scaffolds to organize and self-assemble light-active molecules into excitonic networks
capable of capturing and converting light to excitonic energy, and transferring that energy
at ambient temperature. Protein self-assembly is extremely complex due to the over 20 amino
acids building blocks used in the self-assembly process and the difficulty of predicting how
proteins actually fold. An alternative method for organization and self-assembly may be found
in the field DNA nanotechnology.

DNA nanotechnology provides the most viable means to organize optically active molecules as
there are only four nucleic acid building blocks and well-established simple design rules.
Leveraging DNA nanotechnology will meet the requirements of precise proximity (selectivity)
and appropriate number (specificity) needed to create larger arrays of multifunctional optically
active molecules. Employing the design rules of DNA self-assembly, we have designed, engineered
and operated an all-optical excitonic switch consisting of donor and acceptor chromophores and
diarylethene photochromic modulating units assembled with nanometer scale precision.

This work demonstrates the first integration of three diarylethene photochromic units into a
single DNA oligonucleotide. Photoisomerization of diarylethenes has been shown to be one of the
fastest photochemical reactions thereby affording potential switching speeds in the 10's
of picoseconds. Adopting diarylethenes as optically reversible switching units provided the
ability to operate the all-optical excitonic switch through nearly 200 cycles without overt
cyclic fatigue and excellent ON/OFF stability in both the liquid and solid phases.

Assessing the static and dynamic cycling behavior of the all-optical excitonic switch allowed
for the development of a model to predict characteristic switching times (τ) of 17.0 and 23.3
seconds for the liquid and solid phases, respectively which align well with the experimental data
thereby validating the model. While these times are much faster than that of other non-optically
based DNA-templated excitonic switches (τ ~ 10's of minutes), the times noted here are
limited by the steady-state optical instrumentation, (i.e., photon flux, detector integration time,
and slit cycling speed), used to characterize the all-optical excitonic switches. Our model predicts
switching times in the picosecond range could be achieved with the use of a high peak power ultrafast
laser. First-order calculations estimate the all-optical excitonic switch has a footprint 37X smaller,
a smaller volume by over 3 orders of magnitude and over an order of magnitude less energy per cycle
than a state-of-the-art MOSFET.

These findings, combined with no production of waste products and the potential ability to switch at
speeds in the 10's of picoseconds, establishes a prospective pathway toward all-optical excitonic
circuits.