Distinct amplitude mode dynamics upon resonant and off-resonant excitation across the charge density wave energy gap in LaTe3 investigated by time- and angle-resolved photoemission spectroscopy

TL;DR

This study uses time- and angle-resolved photoemission spectroscopy to compare how resonant and off-resonant laser excitation affect the amplitude mode dynamics in LaTe3, revealing energy-dependent control over charge density wave responses.

Contribution

It demonstrates that resonant excitation selectively excites the amplitude mode without softening, unlike off-resonant excitation which causes lattice heating and mode softening.

Findings

Resonant excitation maintains a constant amplitude mode frequency.

Off-resonant excitation leads to lattice heating and mode softening from 3 to 2 THz.

Energy-dependent control of charge density wave dynamics achieved.

Abstract

Non-equilibrium states generated by ultrafast laser pulses are characterized by specific phenomena that are not accessible in static measurements. Previous time- and angle-resolved photoemission spectroscopy (TARPES) studies on rare-earth tritelluride materials have revealed the laser-driven melting of the charge density wave order as well as its collective amplitude mode excitation. Variation of the excess energy deposited by optical pumping in the material promises pathways to control the dynamic material response. To this end, we use an optical parametric amplifier to generate a tunable pump photon energy. Studying LaTe3 we compare the dynamics driven by pumping resonantly across the charge density wave energy gap with the effect of pumping at a twice higher photon energy in a TARPES pump-probe experiment. We clearly identify a pump photon energy dependent behavior. At the larger pump photon energy, the excess electronic energy generates lattice heating mediated by e-ph coupling and softening of the amplitude mode frequency from 3 to 2 THz. Remarkably, the resonant pumping across the CDW gap results in a time-independent amplitude mode frequency. We conclude that the resonant excitation across the energy gap excites the amplitude mode selectively while additional electronic excess energy deposited at higher pump photon energy modifies the crystal properties transiently by incoherent dissipative processes.