In this work the transient non-instantaneous polarization, i.e., laser-pulse injected small
polarons and self-trapped excitons, is studied in the perovskite-like ferroelectric lithium
niobate. The investigations span a time scale from femtoseconds to several hours. It is
shown that the established small polaron picture is not able to describe transient absorption
and photoluminescence of lithium niobate consistently. Several strong indications
are presented demonstrating that the photoluminescence cannot be caused by geminate
small polaron annihilation.
Instead, the idea of radiatively decaying self-trapped excitons at the origin of the
blue-green photoluminescence is revived. Excitons pinned on defect sites are proposed to
lead to the already observed long-lived transient absorption in the blue spectral range in
Mg- and Fe-doped crystals. Excitons pinned on iron-defects are studied in more detail.
Their spectral fingerprint and absorption cross section is determined. Furthermore, it is
shown that the occurrence of these pinned STEs can be tailored by chemical treatment
of the samples and the experimental parameters such as the pump pulse intensity and
photon energy. Based on the new experimental results and reviewing data published
in literature, an atomistic picture of hopping and pinning of self-trapped excitons in
lithium niobate is proposed.
The question is addressed whether small polarons and self-trapped excitons in
lithium niobate are coupled species in the sense that oppositely-charged polarons may
merge into self-trapped excitons or STEs break into small polaron pairs. Decay kinetics
of transient absorption and luminescence assigned to free small polarons and STEs indicate
that this is not the case. For a more complete picture the ultrafast time scale is
investigated as well. The formation times of small polarons and STEs are determined,
which both lie in the range of 200 fs. No indications are found on the (sub)picosecond
time scale indicating a coupling of both quasi-particle species either.
In order to gain access to the formation of self-trapped excitons a custom-built
femtosecond broadband fluorescence upconversion spectrometer is installed. Based on
an already existing scheme, it is adapted to the inspection of weakly luminescent solid
samples by changing to an all reflective geometry for luminescence collection. To avoid
the necessity for an experimentally determined photometric correction of the used setup,
an already established calculation method is extended considering the finite spectral
bandwidth of the gate pulses.
The findings presented here are important not only as fundamental research, but
also regarding the technical application of lithium niobate and other similar nonlinear
optical crystals. The simultaneous occurrence of both small polarons and self-trapped excitons is a rather rarely described phenomenon. Usually, the optical response of wide
band gap oxide dielectrics is associated with only one of these quasi-particle species. This
work may therefore be a stimulus to review the existing microscopic models for transient
phenomena in other oxide dielectrics, which may help to improve their application in
nonlinear optical and electro-optical devices. In this context the ultrafast transient
photoluminescence spectroscopy established here for weakly luminescing solid samples
may again provide valuable insight.
With respect to lithium niobate, the results do not only resolve inconsistencies
between the microscopic pictures described in literature, but also provide information
regarding the extends to which the propagation of ultrashort laser pulses may be affected
by (pinned-)STE absorption. It is shown that tailoring of the long-lived absorption center
in the blue spectral range is possible, which may be used to avoid optical damage when
high repetition rates are applied.
It is important to emphasize that the microscopic model proposed in this work is
mainly based on experimental indications. It is the task of further detailed theoretical
investigations, e.g., via time-dependent density functional theory, to test whether the
proposed model is justified. From an experimental perspective the important question
remains whether (pinned-)STEs contribute to a photorefractive effect. In the experimentally
easily accessible spectral range no absorption feature of mobile STEs is observed.
As a complementary experimental technique, ultrafast holographic spectroscopy may
reveal an excitonic contribution to photorefraction and provide further insight to STE
transport and pinning phenomena.
| Identifer | oai:union.ndltd.org:uni-osnabrueck.de/oai:repositorium.ub.uni-osnabrueck.de:urn:nbn:de:gbv:700-202008283512 |
| Date | 28 August 2020 |
| Creators | Krampf, Andreas |
| Contributors | Prof. Dr. Mirco Imlau, Prof. Dr. Simone Sanna |
| Source Sets | Universität Osnabrück |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf, application/zip |
| Rights | http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0022 seconds