Modulation of the Visible Absorption and Reflection Profiles of ITO Nanocrystal Thin Films by Plasmon Excitation

• Published in: ACS photonics Vol. 7; no. 5; pp. 1188 - 1196
• Main Authors: Blemker, Michelle A, Gibbs, Stephen L, Raulerson, Emily K, Milliron, Delia J, Roberts, Sean T
• Format: Journal Article
• Language: English
• Published: American Chemical Society 20-05-2020
• Subjects: indium tin oxide; localized surface plasmon resonance; transient reflectivity; optical gating; infrared plasmonics; metal oxide nanocrystals
• ISSN: 2330-4022; 2330-4022
• DOI: 10.1021/acsphotonics.9b01825

Heavily doped metal oxide nanocrystals (NCs) possess tunable infrared localized surface plasmon resonances (LSPRs) that give them utility for several potential applications, including photothermal therapy, smart electrochromic windows, photocatalysis, and optical gating. However, studies of the photoresponse of these materials have largely been limited to the short-wave infrared (SWIR) spectral region, where their LSPR response peaks. Little has been done to study how LSPR excitation modulates the optical properties of these materials around their band gap in the UV/visible region. Here we demonstrate changes in the optical absorption and reflectivity of Sn-doped In2O3 (ITO) nanocrystal thin films following LSPR excitation. SWIR irradiation of ITO NCs substantially heats their free charge carriers, altering the NC film’s dielectric response. This gives rise to an ultrafast increase in film reflectivity and absorptivity that relaxes within 1 ps following photoexcitation as hot charge carriers thermalize with the NC lattice. Using a model that accounts for free charge carrier behavior according to the Drude model as well as Fermi–Dirac statistics, we reproduce this behavior and predict that reflectivity changes of ∼6% can be achieved via sufficient SWIR irradiation of ITO NC films. We also apply our model to other common plasmonic materials, highlighting that plasmonic metal oxide NC films can be used to obtain large, spectrally flat reflectivity changes throughout the visible region. Our results suggest these materials hold potential for use as terahertz optical gates and further our fundamental understanding of their plasmonic behavior.