You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Compositional-asymmetry influenced non-linear optical processes of plasmonic nanoparticle dimers. The influences of compositional asymmetry on the two-photon photoluminescence and the second harmonic generation processes in weakly coupled plasmonic dimers were addressed. Au-Au homodimer and Au-Ag heterodimer arrays produced using electron-beam lithography were investigated using confocal nonlinear optical imaging and spectroscopy. Compared to the Au-Au homodimers, the Au-Ag dimers showed slightly broadened two-photon photoluminescence near the X symmetry point at the first Brillouin zone of Au, whilst that from the L symmetry point stayed the same. Additionally, weakly coupled Au-Ag heterodimers generated strong second harmonic signals which were invisible in the Au-Au homodimers. The observations highlighted the importance of compositional asymmetry in the non-linear optical studies of plasmonic dimers.
{"Chemical": ["Au-Au", "Au", "Au-Ag"]}