All-optical excitation and control of lattice dynamics in layered materials

Sergio Ivan Rey Puentes

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis builds upon the groundbreaking results in the field of ultrafast physics, motivated by the ongoing pursuit of faster processing devices. We studied the control and excitation of the structural degrees of freedom in van der Waals materials using an all-optical ultrafast spectrometer. Central to our investigations was the development of the double-pump probe spectrometer, capable of measuring transient reflection and the time-resolved magneto-optical Kerr effect (TRMOKE). Despite counting with cryogenic capabilities, the need for an enhanced damping system to improve signalto-noise ratio is paramount. A significant portion of this work focuses on WTe2. Here, we successfully excited coherent phonons using a 400 nm pump wavelength, a first in this field. We successfully activated a shear mode at 0.24 THz and an intralayer mode at 2.39 THz. Our research reveals that the thickness of WTe2 samples drastically influences the dynamics, particularly in terms of heat dissipation because of laser-induced heating effects. Additionally, we uncovered a strong Fano resonance in WTe2, indicating complex energy exchange mechanisms, possibly involving electrons, multiphonon processes, or interactions with Weyl fermions. Employing a double-pump excitation scheme, we achieved control over the amplitude and phase of coherent oscillations in WTe2. This technique allowed for intricate manipulation of the phonon properties, revealing a dependence of the Fourier phase on the timing of the second pulse. Using the same experimental setup, we measured the transient reflection of a Janus alloy, WSSe, where we observed anomalous dynamics and a 27 GHz coherent acoustic phonon. We propose a multi-step mechanism involving exciton formation, phonon recycling, and thermoelastic effects augmented by an out-of-plane piezoelectric field. This hypothesis is supported by comparative studies with the symmetric counterparts, WS2 and WSe2, where such oscillations were notably reduced and decayed faster. Furthermore, we proved that the double-pump scheme can also be used to control the amplitude and direction of these propagating strain waves in WSSe coherently. Finally, our initial experiments using the TRMOKE setup for WTe2 and other materials revealed an ultrafast response fit by a Gaussian function. The dynamics suggest an all-optical effect, potentially dichroic bleaching, which could be key to understanding the optical Kerr effect and its applications in measuring the pulse duration. Additional experiments with pumps of varying helicities in WTe2 showed potential for coherent control of ultrafast spin currents, but several questions regard-ing the interplay of ultrafast dynamics and the polarization of light present the need for further research. In summary, this thesis not only sheds light on the complex ultrafast dynamics and phonon behavior in TMDCs and Janus alloys but also sets a benchmark for future applications using multipulse experiments.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages170
Publication statusPublished - 2023

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