History-dependent nano-photoisomerization by optical near-field in photochromic single crystals

We demonstrate history-dependent or dynamic nano-photoisomerization by sequential formation of multiple memory pathways in photochromic crystals via optical near-field interactions. We observed the incident photons passing through the photoisomerization memory pathways by a double-probe optical near-field microscope, with one probe located on the front surface for local excitation and the other on the rear surface for near-field observation. By carrying out localized near-field excitation twice but at spatially different positions, we observed negatively correlated near-field output patterns between the first memory pathway and the second memory pathway. That is, the added memory pathway was formed exclusively to the previously formed memory pathway. We also confirmed that the first memory pathway was preserved after the second memory pathway was formed. This result indicates that photoisomerization by an optical near-field in diarylethene crystals has a history dependence, leading to brain-like dynamic information memorization using light-matter interactions on the nanometer-scale.