Ultrafast photocarrier and coherent phonon dynamics in type-II Dirac semimetal PtTe₂ thin films probed by optical spectroscopy

Abstract

We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-II Dirac semimetal platinum ditelluride ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>PtTe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>PtTe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>PtTe</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> . Our findings provide deep insight into the excitation and dynamics of CAP and COP as well as the photocarriers’ recovery pathway and lifetimes in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m5"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>PtTe</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> . 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.