Ultrafast Hot-Carrier Dynamics at Chemically Modified Ge Interfaces Probed by SHG

Slow Hot-Carrier Cooling Enabled by Uniformly Arranging Different-n-Value Nanoplates in Quasi-2D Perovskites through Long-Range Orbit-Orbit Interaction toward Enhancing Photovoltaic Actions

This paper reports a slow hot-carriers cooling process enabled by uniformly arranging different-n-value nanoplates in quasi-2D perovskites (PEA₂MA_n-1Pb_nI_3n+1). Here, the energetic carriers are slowly relaxing from 1.743 to 1.688 eV within the time window of 91.7 ps, shown in transient absorption (TA) dynamics. It verifies an unusual long-order electron-lattice coupling to slow down the relaxation of hot carriers, generating long-lived energetic carriers in quasi-2D perovskites. Furthermore, the long-range orbit-orbit interaction is established through orbital polarizations between excited carriers, identified by polarization-dependent photocurrent. This indicates that the long-range orbit-orbit interaction provides the necessary condition to enable the unusual long-order electron-lattice coupling, causing a slow hot-carrier cooling effect through the energy-momentum dispersion curvature within the crystalline lattice. Interestingly, with the slow hot-carrier cooling and long-range orbit-orbit interaction, the quasi-2D perovskite demonstrates superior carrier transport shown by high fill-factor values (>80%) with reduced photovoltaic loss in solar cells at all different carrier densities.

Development of interface-/surface-specific two-dimensional electronic spectroscopy

Structures, kinetics, and chemical reactivities at interfaces and surfaces are key to understanding many of the fundamental scientific problems related to chemical, material, biological, and physical systems. These steady-state and dynamical properties at interfaces and surfaces require even-order techniques with time-resolution and spectral-resolution. Here, we develop fourth-order interface-/surface-specific two-dimensional electronic spectroscopy, including both two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy and two-dimensional electronic second harmonic generation (2D-ESHG) spectroscopy, for structural and dynamics studies of interfaces and surfaces. The 2D-ESFG and 2D-ESHG techniques were based on a unique laser source of broadband short-wave IR from 1200 nm to 2200 nm from a home-built optical parametric amplifier. With the broadband short-wave IR source, surface spectra cover most of the visible light region from 480 nm to 760 nm. A translating wedge-based identical pulses encoding system (TWINs) was introduced to generate a phase-locked pulse pair for coherent excitation in the 2D-ESFG and 2D-ESHG. As an example, we demonstrated surface dark states and their interactions of the surface states at p-type GaAs (001) surfaces with the 2D-ESFG and 2D-ESHG techniques. These newly developed time-resolved and interface-/surface-specific 2D spectroscopies would bring new information for structure and dynamics at interfaces and surfaces in the fields of the environment, materials, catalysis, and biology.

Ultrafast electronic and structural response of monolayer MoS2 under intense photoexcitation conditions

We report on the dynamical response of single layer transition metal dichalcogenide MoS2 to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron-hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions.