Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus

Abstract

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

Optical and Optoelectronic Properties of Black Phosphorus and Recent Photonic and Optoelectronic Applications

“…In monolayer BP, the polarization‐resolved PL spectra of monolayer BP show nearly perfect linear dichroism . The reduced dimension of ultrathin BP also features a prominent excitonic effect because of the suppressed screening effect and strong Coulomb interaction, giving rise to a high exciton binding energy. Combing the PL and PL excitation (PLE) measurements, the exciton binding energy of monolayer BP is extracted to be ≈0.9 eV, consistent with the theoretical energy difference of ≈0.8 eV between the quasiparticle and excitonic absorption edges .…”

Epitaxial Growth of Topological Insulators on Semiconductors (Bi2Se3/Te@Se) toward High‐Performance Photodetectors

“…The excitation time as fast as 700 fs can be obtained at the PIB minimum of 549 nm as shown in the timedependent TA spectrum (Figure 3c). Figure 3d presents the excitation and cooling dynamics of the Bi 2 Se 3 /Te@Se at 549 nm, and the recovery time is fitted to be τ 1 = 6.99 ± 0.47 ps and τ 2 = 318 ± 45 ps through the biexponential function, which are comparable to that of β-PbO [28] and BP, [29] indicating the Bi 2 Se 3 /Te@Se are excellent candidates for constructing highspeed photodetectors and optical modulators up to gigahertz. Figure 4 exhibits the typical photoresponse behavior of the Bi 2 Se 3 /Te@Se-based PDs which were irradiated by the simulated light (SL, a mixture of light ranging from 300 to 800 nm) by using a PEC-type photodetection system with a standard three electrodes as shown in Scheme S1 of the Supporting Information.…”

“…The excitation time as fast as 700 fs can be obtained at the PIB minimum of 549 nm as shown in the time‐dependent TA spectrum (Figure c). Figure d presents the excitation and cooling dynamics of the Bi 2 Se 3 /Te@Se at 549 nm, and the recovery time is fitted to be τ 1 = 6.99 ± 0.47 ps and τ 2 = 318 ± 45 ps through the biexponential function, which are comparable to that of β‐PbO and BP, indicating the Bi 2 Se 3 /Te@Se are excellent candidates for constructing high‐speed photodetectors and optical modulators up to gigahertz.…”

Time‐Resolved Terahertz Spectroscopy Studies on 2D Van der Waals Materials

“…After the cooling of hot electrons, the nonequilibrium distributed electrons and holes merged together within around 100-200 fs to form uniform electron liquid in each layer. [113] Copyright 2018, Springer Nature. This process is schematically shown in Figure 10a.…”

“…Although numerous time-resolved pump-probe measurements have been performed on 2D TMD materials to understand their ultrafast carrier dynamics, [111][112][113] most of these studies were performed using optical pump-optical probe frameworks. Performances using optical pump-THz probe spectroscopy have only been investigated in a few reports.…”