Extraordinary three-state terahertz transient conductivity switching at room temperature in multi-band nickelates for high-speed communication

Abstract

The realization of multiple electronic states on ultrafast sub-picosecond timescales can be leveraged to develop energy-efficient terahertz (THz) photonic devices. Such explorations are pursued in the framework of femtosecond optical excitation with THz probes wherein either negative or positive photoconductivity manifests in metallic and insulating states, respectively. Co-existence of both states is highly desired but rarely observed. Here, a strategy is devised exploiting crystalline transport anisotropy and hetero-epitaxy in a multi-band nickelate to demonstrate switching of the positive to the negative sign of photoconductivity on a sub-picosecond timescale. This combines with a profound anisotropy of THz photoconductivity in unconventional (111) epitaxial films having preferential arrangement of oxygen vacancies along selective crystal axes—a framework that reversibly modulates the anisotropy of negative THz photoconductivity using oxygen content as a control parameter, and corroborated by theoretical calculations. Thus, ultrafast switching between negative, positive, and zero photoconductive states in timescales of a few picoseconds at room temperature, as demonstrated in this study, holds a remarkable prospect for creating multiple channels of information processing. A proof-of-concept experiment to utilize these photo-controlled states is proposed for a three-state THz communication system, which would transmit a larger volume of information vis-a-vis that of a binary communication system.