Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode immersed in liquid xenon was observed with a CCD camera. With voltage across the THGEM, the appearance of bubbles was correlated with that of electroluminescence signals induced by ionization electrons from alpha-particle tracks. This confirms recent indirect evidence that the observed photons are due to electroluminescence within a xenon vapor layer trapped under the electrode. The bubbles seem to emerge spontaneously due to heat flow from 300K into the liquid, or in a controlled manner, by locally boiling the liquid with resistive wires. Controlled bubble formation resulted in energy resolution of {\sigma}/E~7.5% for ~6,000 ionization electrons. The phenomenon could pave ways towards the conception of large-volume 'local dual-phase' noble-liquid TPCs.
@article{arxiv.1509.02354,
title = {Direct observation of bubble-assisted electroluminescence in liquid xenon},
author = {E. Erdal and L. Arazi and V. Chepel and M. L. Rappaport and D. Vartsky and A. Breskin},
journal= {arXiv preprint arXiv:1509.02354},
year = {2015}
}