  Optical control of electronic properties is essential for future electric
devices. Manipulating such properties has been limited to the microscale in
spatial volume due to the wave nature of light; however, scaling down the
volume is in extremely high demand. In this study, we demonstrate optical
switching within a nanometric spatial volume in an organic electric material.
Photochromic materials such as diarylethene derivatives exhibit semiconducting
and insulating properties on ultraviolet (UV) and visible light, respectively,
which are promising for optical switching and memory. To control the wavelength
between visible and UV light at the nanoscale, we employed plasmon
nanofocusing, which allows the creation of a nanolight source at the apex of a
metallic tapered structure over a broad frequency range by focusing of
propagating plasmons. We utilized an aluminum tapered structure and realized
in-situ wavelength control between visible and UV light at the nanoscale. Using
this method, nanoscale optical switching between the two states of diarylethene
was demonstrated. The switching performance was confirmed for at least nine
cycles without degradation. This demonstration would make a significant step
forward toward next-generation nanoscale optoelectronic devices and stimulate
diverse scientific fields owing to the unique concept of broadband plasmon
nanofocusing.
