Having a good vibe: strong coupling to the electronic system creates incoherent and coherent phonons in 1L-TMDCs

Abstract The electronic dynamics in monolayer transition metal dichalcogenides (1L-TMDCs) following optical excitation crucially determine all their optoelectronic applications. The basic processes can be divided into energy redistribution leading to a thermal equilibrium of the electronic and the phononic system, and the recombination of the excited species. Furthermore, the displacive excitation of coherent phonons assigned to the A <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow/> <mml:mn>1</mml:mn> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>′</mml:mo> </mml:msup> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> Raman mode has been demonstrated employing sufficiently short pump pulses. In this study, we shed light on the ultrafast dynamics of three different 1L-TMDCs using transient absorption spectroscopy. We reveal a cascade-like cooling of the excited electronic states, owing to their differing coupling strengths to high- versus low-energy phonons. In addition, the coherently excited lattice vibrations imprint a strong oscillatory contribution onto the signal. We extract a dominating, spectrally broad signature of the A <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow/> <mml:mn>1</mml:mn> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>′</mml:mo> </mml:msup> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> mode, whose shape challenges existing models. On top of that, we identify a second frequency band matching the E <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>′</mml:mo> </mml:msup> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> Raman peak in 1L- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>WS</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> . Finally, measurements at high pump fluences demonstrate a phonon bottleneck for the cooling cascade. Simultaneously, the dominant A <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow/> <mml:mn>1</mml:mn> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>′</mml:mo> </mml:msup> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> mode softens, providing a new degree of tunability for these coherent lattice vibrations. Overall, our findings demonstrate the strong couplings between the electronic and phononic (sub)systems in 1L-TMDCs.