Anisotropic Charge Carrier and Coherent Acoustic Phonon Dynamics of Black Phosphorus Studied by Transient Absorption Microscopy

Abstract

Due to its corrugated hexagonal lattice structure, Black phosphorus (BP) has unique anisotropic physical properties, which provides an additional freedom for designing devices. Many body interactions, including interactions with phonon, is crucial for heat dissipation and charge carrier mobility in device. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation and generation were not fully interrogated. In this paper, the polarization-resolved transient absorption microscopy was conducted on BP flakes to study the dynamics of photoexcited charge carriers and coherent acoustic phonon. Polarization-resolved transient absorption images and traces were recorded and showed anisotropic and thickness-dependent charge carriers decay dynamics. The damping of the coherent acoustic phonon oscillation was found to be anisotropic, which was attributed to the polarization-dependent absorption length of the probe pulse. From the analysis of initial oscillation amplitude and phase of coherent acoustic phonon oscillation, we proposed that the direct deformation potential mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we obtained the sound velocity of the coherent acoustic phonon from the oscillation frequency and the "acoustic echo", respectively, which agreed well with each other. These findings provide significant insights into the rich acoustic phonon properties of BP, and promise important application for BP in polarization-sensitive optical and optoelectronic devices. direction in Figure 3. AUTHOR INFORMATION

Polarization‐Dependent and Wavelength‐Tunable Optical Limiting and Transparency of Multilayer Selenium‐Doped Black Phosphorus

“…It can be seen that the PA peak gradually grows, eventually reaching a maximum in the order of ≈10 2 ps. According to a previous report, [12] we identified that this phenomenon is caused by the absorption of excited states, which highlights a feature of positive absorption change. At the same time, we found an overlaying PA peak near 2.06 eV, which indicates a resonance position conducive to ESA absorption in the I-scan.…”

“…Similar to other 2D materials such as graphene, [1][2][3] transition metal chalcogenides (TMDCs), [4][5][6][7] and topological insulators, [8,9] black phosphorus (BP) [10][11][12][13][14] have aroused extensive research enthusiasm in various fields. Possessing excellent properties, BP exhibits excellent potential for applications in electronics, [15] optoelectronics, [16][17][18][19] thermoelectrics, [20] photo biology, [21] etc.…”

Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

“…The frequency depends linearly on the inverse of the probe wavelength and lies in the tens of GHz range, as shown in Figure 7 b, in strong contrast to the previously reviewed COPs. Such phenomenon has been observed in thick exfoliated Re 6 Se 8 Cl 2 (see Figure 6 ) [ 142 ], BP [ 125 ], MoS 2 (see Figure 7 ) [ 143 ], Bi 2 Se 3 [ 39 , 235 ], and Bi 2 Te 3 [ 138 ], which all are semiconducting vdW materials with interestingly different 2D lattice structures. In these examples, samples are pumped above their respective bandgap yielding pump penetration depths evaluated in the 100-nanometers range or below, and significantly smaller than the slab thicknesses.…”

“…In this context, due to incorrect assignments of echo instead of single trip delays in the oscillation (see below) and of the corresponding resonant acoustic mode (see Section 4.4 ), a value twice higher than expected for v and thus one twice lower for n pr , are reported in MoS 2 in Reference [ 143 ]. Interestingly, the optical anisotropy in BP leads to slightly different optical indices for orthogonal probe polarizations, which induces different Brillouin frequencies even though the same propagating CAP with a single given velocity is probed [ 125 ].…”

“…In this regime, different effective quality factors are reported for different probe polarizations in BP due to its optical absorption anisotropy [ 125 ]. Moreover, echoes are observed when the propagating CAPs bounce back and forth inside the slab, at time delays corresponding to twice the trip over the film thickness [ 125 , 138 ]. When comparing amplitude reduction of such successive echoes, stronger damping in natural superlattices of QLs is found compared to bulk stacks [ 138 ].…”