We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-II Dirac semimetal platinum ditelluride (PtTe2) thin films via femtosecond (fs) pump-probe spectroscopy at room temperature. Quantitative analysis revealed that the incoherent electronic relaxation consists of two components: a subpicosecond fast relaxation process and a slow component with a time constant of hundreds of picoseconds (ps). Furthermore, the launch of a coherent acoustic phonon (CAP) in the 20 nm film but absence in the 6.8 nm film uncovers the dominant role of temperature gradient in producing a strain wave. The sound velocity and Young’s modulus in the thick PtTe2 are determined to be 1.736 km/s and 29.5 GPa, respectively. In addition, the coherent optical phonon (COP) with a frequency of 4.7 THz corresponding to Te atoms out-of-plane A1g vibration has been well resolved in all films, which is ascribed to displacive excitation of coherent phonon (DECP). The observation of a strong probe-wavelength dependent COP amplitude reveals the resonant feature of the optical excitation-induced atomic displacement in PtTe2. Our findings provide deep insight into the excitation and dynamics of CAP and COP as well as the photocarriers’ recovery pathway and lifetimes in PtTe2. Our study also demonstrates that the COP spectroscopy is a powerful tool to reveal the modulation of frequency-dependent optical constants induced by atomic vibrations, which may find applications in the fields of optoelectronics and ultrafast photonics.

We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-II Dirac semimetal platinum ditelluride (PtTe2) thin films via femtosecond (fs) pump-probe spectroscopy at room temperature. Quantitative analysis revealed that the incoherent electronic relaxation consists of two components: a subpicosecond fast relaxation process and a slow component with a time constant of hundreds of picoseconds (ps). Furthermore, the launch of a coherent acoustic phonon (CAP) in the 20 nm film but absence in the 6.8 nm film uncovers the dominant role of temperature gradient in producing a strain wave. The sound velocity and Young’s modulus in the thick PtTe2 are determined to be 1.736 km/s and 29.5 GPa, respectively. In addition, the coherent optical phonon (COP) with a frequency of 4.7 THz corresponding to Te atoms out-of-plane A1g vibration has been well resolved in all films, which is ascribed to displacive excitation of coherent phonon (DECP). The observation of a strong probe-wavelength dependent COP amplitude reveals the resonant feature of the optical excitation-induced atomic displacement in PtTe2. Our findings provide deep insight into the excitation and dynamics of CAP and COP as well as the photocarriers’ recovery pathway and lifetimes in PtTe2. Our study also demonstrates that the COP spectroscopy is a powerful tool to reveal the modulation of frequency-dependent optical constants induced by atomic vibrations, which may find applications in the fields of optoelectronics and ultrafast photonics.

碳纳米点由于具有独特的发光特性、良好的生物相容性、低毒性、良好的光稳定性等特性在近年来被广泛关注。这些特性使其在光电器件、可见光通讯、肿瘤治疗、生物成像等领域拥有潜在的应用价值。受到原料和合成方法的影响，碳纳米点材料体系多种多样。本文将系统地综述本课题组近年来以柠檬酸和尿素为主要原料合成的氮掺杂碳纳米点及其物理化学性质，探讨碳纳米点能带调控的方法及原理，并介绍碳纳米点的应用研究进展。

In this paper, an integrated compact four-channel directly modulated analog optical transceiver is proposed and fabricated. The 3 dB bandwidth of this optical transceiver exceeds 20 GHz, and the measured spurious-free dynamic range is up to 91.2dB·Hz2/3. The optical coupling efficiency (CE) is improved by using a precise submicron alignment technique for lens coupling in a transmitter optical subassembly, and the highest CE is achieved when the oblique angle of the arrayed waveguide grating using a silica-based planar lightwave circuit (AWG-PLC) in receiver optical sub assembly is set to 42°. Based on the proposed optical transceiver, we have experimentally demonstrated a 6.624 Gbit/s 4×4 multi-input multioutput (MIMO) 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16QAM-OFDM) radio signal over 15.5 km standard single mode fiber, together with 1.2 m wireless transmission in both an uplink and a downlink. To cope with the channel interference and noise of the fiber-wireless transmission system, a low-complexity MIMO demodulation algorithm based on lattice reduction zero-forcing (LR-ZF) is designed. In our experiment, 1.6 dB power penalty is achieved by using the proposed LR-ZF algorithm, compared to the commonly used zero-forcing algorithm. Moreover, this LR-ZF algorithm has much less complexity than the optimal maximum-likelihood sequence estimation (MLSE) at a given transmission performance. These results not only demonstrate the feasibility of the integrated optical transceiver for MIMO fiber-wireless application but also validate that the proposed LR-ZF algorithm is effective to eliminate the interference for hybrid fiber-wireless transmission.