Driving materials out of equilibrium by ultra-short laser pulses offers unprecedented access to new chemistry and physics. Layered tin selenide (SnSe) has recently emerged as a promising candidate for high-performance thermoelectrics (TE) with the cu
rrent record figure of merit (ZT) observed in high temperature Cmcm phase. However, traditional investigations on Cmcm phase are carried out in thermal equilibrium condition, therefore restricted in a compromised version due to electron-phonon coupling. In this study, we demonstrate that, through femtosecond-resolved coherent phonon spectroscopy, SnSe displays a transient photoinduced switch of point-group symmetry from Pnma to Cmcm at room temperature. This non-equilibrium Cmcm phase exists in SnSe with the status of cold lattice with hot electrons, which allows us to noninvasively characterize physical properties in a selective manner. By independently control the electronic and lattice degree of freedom in SnSe, a giant anharmonicity and the non-thermal softening nature are demonstrated in the transient Cmcm phase. Our results, which provide a nonequilibrium platform to investigate thermal properties with a cold lattice, may reform the current thermal equilibrium-based thinking in TE research community.
Biexcitons have been considered as one of the fundamental building blocks for quantum technology because of its overwhelming advantages in generating entangled photon pairs. Although many-body complexes have been demonstrated recently in mono-layer t
ransition metal dichalcogenides (TMDs), the low emission efficiency and scale up capability hinder their applications. Colloidal nanomaterials, with high quantum efficiency and ease of synthesis/processing, are regarded to be an appealing complement to TMDs for biexciton sources. However, a progress towards biexciton emission in colloidal nanomaterials has been challenging largely by small binding energy and ultrafast non-radiative multiexciton recombination. Here, we demonstrate room-temperature biexciton emission in Cu-doped CdSe colloidal quantum wells (CQWs) under continuous-wave excitation with intensity as low as ~10 W/cm2. The characteristics of radiative biexciton states are investigated by their super linear emission with respect to excitation power, thermal stability and transient photophysics. The interaction between the quantum confined host carriers and the dopant ions increases biexciton binding energy by two folds compared to the undoped CQWs. Such strong binding energy together with suppressed Auger recombination and efficient, spectrally narrow photoluminescence in a quasi-2D semiconductor enables sustained biexciton emission at room temperature, providing a potential solution for efficient, scalable and stand-alone quantum devices.
