| Abstract |
During the last two decades, the fields of nanotechnology and lasers are growing rapidly,
introducing more and more innovative ideas and techniques, both in modern science and
the industry. Specifically, materials nanostructuring has become a promising alternative
for the modification of the functionalities of conducting and insulating, as well, materials.
The manufacturing of nanostructured materials has provided a variety of applications in
electronics, medicine, optics, etc., that have already been integrated in our daily life.
One appealing aspect of nanostructuring is the controllable surface modification induced
by pulsed lasers. This technique was firstly discovered in 1965 by Milton Birnbaum,
when he noticed some periodic structures that were created after the irradiation of a
germanium wafer with multiple laser pulses. These self-organized periodic surface
structures featured a period close to the wavelength of the laser and appeared in many
sizes: from few tends of nm up to several μm. Nowadays, they are commonly referred to
as Laser Induced Periodic Surface Structures (LIPSS).
LIPSS, also called ripples, are a universal phenomenon that can be observed on almost
any material after its irradiation by short laser pulses, with durations in the picosecond to
femtosecond range. They can be classified according to their spatial periods and the
orientation to the linear laser beam polarization that is used for their generation. Low
Spatial Frequency LIPSS (LSFL) have a period that is larger than half the laser wavelength
(λ). Based on the formation mechanism, LSFL can either be oriented parallel or
perpendicular to the laser beam polarization. On the other hand, High Spatial Frequency
LIPSS (HSFL) feature a period that is smaller than half the laser wavelength. Another interesting characteristic of LIPSS, is the wide variety of morphologies in which they come
in: grooves, spikes, nanowires, etc.
There are many theories that attempt to explain the physical mechanisms behind the
formation of LIPSS on materials. All of them share one common notion: the surface of the
material that is to be nanostructured needs to feature corrugation. One approach
suggests that laser light gets scattered due to the surface’s roughness and leads to its
interference with the incident pulse. Another popular one discusses the surface plasmon
coupling theory and how the coupled surface plasmons interfere with the laser beam
during the irradiation. For both mechanisms, interference results in a spatial
redistribution of the beam intensity, which is imprinted in the surface through many
mechanisms: ablation, amorphization, and others.
The main reason for the uprising of LIPSS in the field of research and industry, is the
simplicity and robustness of their manufacturing process. The evolution of industries in
the modern world has led to the constant search of quicker, more economic and
environmental-friendly techniques, avoiding the necessity of strict environmental
conditions like clean rooms or high vaccum. Thus, the fact that the fabrication of
nanostructures with complex surface functionalities can be conducted using an industrial
short pulse laser, in ambient air, proves why they have gained more and more interest in
the last years.
Moving on to the actual applications of the fabrication of LIPSS on a material’s surface,
the properties of LIPSS can be multiple: from optical and mechanical to chemical. A very
unique idea that has risen in the past decade, is the imitation of biological surfaces.
Natural evolution has contributed to the vast complexity of the animal species, for many
thousands of years. For example, the nano-scale structure of the surface of a plant’s leaf
presents such a morphology that can provide the property of hydrophobicity or
hydrophilia. Another example is the wings of insects, which are nanostructured by
nature in such a way that their sunlight reflectivity is minimal. All these exceptional
properties can be given to materials, through the laser fabrication of their surfaces. This
is called biomimetic laser engineering.
The topic of this diplomatic thesis is the fabrication of LIPSS, in the form of nanowires, in
metal film surfaces, as well as the study of nanostructuring of thin metal films compared
to bulk film metals. Therefore, the content of this thesis is split into two main sections:
nanowires fabrication and comparative study of thin and bulk metal films.
During the last years, the morphology of nanowires has been studied in the literature
more and more. The most important property of a nanowires’ surface morphology is the
special manipulation of light that it provides. Nanowires are subwavelength parallel metal
ridges or wires, that can serve as a polarizing element due to the grating type metasurface
that they form. Research literature of the last years, indicated that thin film metals can
be structured to have a nanowires morphology, through femtosecond pulsed laser beams. This surface modification technique can be applied for the manufacturing of Wire Grid
Polarizers, that can be used for the polarization of light that is in Mid-IR spectrum.
When it comes to the study of LIPSS on thin film metals compared to bulk materials, the
modern literature lacks many information upon this topic. The interaction of light and
material has been explored only in the regime of bulk materials. Experiments and studies
show that the thinner the film of a material is, the more different the interaction between
light and material is. Therefore, the physical mechanisms that are hidden behind this
phenomenon, need to be explored in more depth.
In this work, we are going to discuss the experimental methods and results of the study
of fabrication of nanowires in thin film metals, so that they can be used in the manufacturing of Wire Grid Polarizers. Also, there will be a detailed display of the
experiments’ findings, concerning the comparative study of thin film metals. Lastly, we
are going to present the conclusions of the most important data that were received,
during this thesis.
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