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Abstract

This thesis presents studies of ultrafast carrier relaxation dynamics in charge density
wave (CDW) materials by means of femtosecond time-resolved optical spectroscopy. In
these experiments, a femtosecond pump pulse excites electrons via interband transitions,
thereby changing the optical properties of the material. The dynamics of relaxation processes
is then recorded by measuring the resulting photoinduced changes in the dielectric
function as a function of time after photoexcitation. Utilizing this technique, one can
therefore measure the non-equilibrium quasiparticle and phonon relaxation dynamics in
real time.
The motivation for this research was on one side the major progress that has been made
in the last couple of years in theoretical understanding of the relaxation phenomena in
systems with a narrow gap in the density of states. The Rothwarf-Taylor (RT) model,
developed for understanding the non-equilibrium dynamics in superconductors was originally
used to interpret the relaxation phenomena in CDWs as well. On the other hand,
several arguments could be made against the simple RT model interpreted in CDWs.
Using femtosecond time-resolved optical spectroscopy, we systematically measured the
temperature and excitation intensity dependence of photoinduced re
ectivity changes in
CDW materials K0:3MoO3 and (TaSe4)2I. We found that at low perturbations the quasiparticle
relaxation dynamics is excitation intensity independent, which speaks against
the validity of the RT model in CDW materials and calls for the models to be revisited.
The reason for choosing these particular two prototype CDW materials, i.e. K0:3MoO3
and (TaSe4)2I, was to compare the ultrafast dynamics of weakly and strongly coupled
CDW systems. The aim was to understand their dynamics in order to gain further
complementary knowledge about these materials and the role of interaction strengths
in CDW physics. Surprisingly, we have shown that the temperature evolution of the
electronic and phonon responses in both systems were qualitatively the same.
We have also systematically explored for the first time the high perturbation physics in
both CDW materials and found that non-thermal melting of the CDW can be achieved.
The temperature evolution of the energy Esat needed to induce CDW meltdown was also
studied. In fact, the uncharacteristically low value for Esat in (TaSe4)2I, as compared to K0:3MoO3, was the only clue suggesting any qualitative difference between the two
systems.
Systematic studies, where the excitation intensity was changed by more than four orders
of magnitude, suggest that during the process of melting and sub-ps recovery of the
electronic modulation, the electronic and lattice systems are uncoupled. This could
explain why the order parameter recovery is so extremely fast ( 200 fs) in this
entire class of low-dimensional materials, because a frozen lattice and consequent 2kF
modulation would present a strong potential well driving ultrafast reformation of the
charge density modulation.
Finally, we grew a series of thin K0:3MoO3 films using pulsed laser deposition. The films
were characterized using various experimental techniques including femtosecond time-
resolved optical spectroscopy. We observed the amplitude mode along with zone-folding
phonons characteristic for K0:3MoO3, proving existence of CDW domains. The systematic
study of temperature and excitation intensity dependence of relaxation dynamics in
films revealed that there is no major difference between the CDW physics of K0:3MoO3
in films or in bulk.