Picosecond thermoelectric dynamics in layered cobaltite thin films probed by terahertz emission spectroscopy

Abstract

The operation speed of thermoelectric devices is generally limited to microsecond or millisecond time scales. This is due to the difficulty in manipulating and measuring the thermoelectric transients at high speeds. Although recent advances in optical characterization techniques have succeeded to detect thermoelectric signals in picosecond time scales in a few special nanomaterials such as graphene, thermoelectric dynamics in most of the standard thermoelectric materials still remain unknown. Here we investigate the picosecond carrier dynamics of standard thermoelectric ${\mathrm{Ca}}_{x}{\mathrm{CoO}}_{2}$ thin films by means of terahertz emission spectroscopy. The terahertz radiation signals generated from the ${\mathrm{Ca}}_{x}{\mathrm{CoO}}_{2}$ films by femtosecond laser absorption are found to be strongly dependent on crystal orientation. We discuss that the terahertz emission properties of ${\mathrm{Ca}}_{x}{\mathrm{CoO}}_{2}$ films can be described well in terms of the unique tensorial properties of the Seebeck coefficient. The unordinary terahertz emission property associated with the tensorial property of a Seebeck coefficient provides convincing evidence of picosecond evolution of the thermoelectric current even in this standard material. Our results not only promote general understanding of the ultrafast charge dynamics in solids but may also pave the way to develop optoelectronic devices including bias-free terahertz sources and high-speed infrared sensors.