| Abstract |
The main objective of the present thesis involves the study of laser emission (random laser action) that
originates from Zinc Oxide (ZnO) nanoparticles and in particular the dependence of the laser action
features as a function of material temperature. The purpose of this study is the investigation of the
potential exploitation of materials containing ZnO nanoparticles for the development of optical
thermometry devices.
Two types of hybrid materials were investigated, both containing the same optical medium (ΖnO
nanoparticles) embedded in a binder being either an organic polymer (poly-dimethylsiloxane, PDMS) or
silica (sol-gel, SiO2). Excitation of the optical medium with ultraviolet electromagnetic radiation results in
photoluminescence emission in the range of 375-410 nm. In the case of systems that exhibit strong
scattering, photoluminescence emission can be amplified producing radiation with laser emission
characteristics. The term random laser or random lasing is attributed to the effect of amplification in the
presence of scattering particles rather than conventional laser cavity mirrors. In previous studies
involving these materials, it was determined that the emission of photoluminescence produced by ZnO
nanoparticles exhibits a profound temperature dependence. With increasing temperature the emission
intensity decreases while its spectral profile broadens and the emission maximum shows a red shift over
a wide range of temperature values (20-90 oC). The research performed in the context of the present
thesis concentrates on studying how temperature changes might influence the characteristic random
laser emission in ZnO nanohybrids.
The experiments were conducted by use of an excited dimer (excimer) laser (KrF, emission wavelength λ
= 248 nm, pulse temporal width τ = 0.5 ps) as an excitation source capable of inducing random lasing
action on the samples. This choice is justified on the basis of the band gap energy value of ZnO (Eg≈ 3.3
eV) and its photoluminescence emission lifetime (τPL≈200 ps), since it guarantees efficient single-photon
excitation of ZnO at a pump rate faster than the characteristic spontaneous emission time. Variable
temperature conditions are achieved by using a thermostated stage holding the samples. The sample
temperature is measured by means of a digital thermometer with its probe in contact with the irradiated
surface of the nanohybrid material. The emitted photoluminescence is collected through an optical fiber
into a grating spectrograph and detected on an Intensified Charge-Coupled Device (ICCD) detector.
The analysis of the emission spectra reveals that the random lasing action is very sensitive to
temperature increase. It drastically decreases in intensity upon heating of the sample at just a few
degrees over room temperature. It is noted that random lasing is not restored by increasing the fluence
of the excitation radiation, in an attempt to compensate for the temperature effect. Instead, only when
samples are brought back at room temperature random lasing action is restored. This study proves that
random lasing based on Zno nanoparticles can be a very sensitive probe of temperature changes, albeit
at a very limited temperature range.
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Μελέτη της επίδρασης της θερμοκρασίας στην εκπομπή ακτινοβολίας λέιζερ από νανοϋβριδκά υλικά του οξειδίου του ψευδαργύρου (ZnO)
