Ultrafast transient absorption measurements of photocarrier dynamics in monolayer and bulk ReSe 2

Recently, transition metal dichalcogenides have been extensively studied as new functional materials for electronic and optoelectronic applications. Most initial efforts have been focused on several members of this material family with 2H lattice structure. ReSe2 as a less-study transition metal dichalcogenide has gained significant momentum due to its 1T lattice structure and in-plane anisotropic electronic and optical properties. Extensive efforts have shown its promising future as a novel material for optoelectronics. However, little was known about the photocarrier dynamics in this material. Here we report the first transient absorption measurement of photocarrier dynamics in ReSe2 bulk and monolayer sample in reflection geometry. We observed ultrafast thermalization and relaxation of hot carriers in bulk ReSe2, and obtained a photocarrier lifetime on the order of 80 ps and decreases slightly with increasing the carrier density. Pronounced anisotropic response of differential reflection was observed. We also studied monolayer ReSe2 samples obtained by chemical vapor deposition and deduced a photocarrier lifetime on the order of 10 ps. These results provide fundamental information for using this material in various optoelectronic devices.