Maria Bayard Dühring - Solid Mechanics

2.1 Elastic and optical waves 5
to scalar equations in order to treat the simpler in-plane transverse electric and
magnetic waves, respectively. Since it was discovered that light is electromagnetic
waves, optics has been considered as a subfield of electromagnetism and covers the
behavior of infrared, visible and ultraviolet light.
The intense study of materials and structures to control and guide optical waves,
that started in the 20th century, has led to many applications in everyday life such
as the radar, laser, lens design and fabrication of optical components [13]. Already
from the middle of the 19th century it was known that light can be guided by
refraction, which has been used in the study of slab and channel waveguides where
light can be guided, split and bent. These components are the building blocks in
modern integrated optics, where a number of guided wave devices are combined
on a common substrate to build complex optical circuits with small dimensions.
This offers a way to guide and process information carried by optical waves and has
applications as interferometers and sensors. The guidance of waves by refraction has
also been applied in optical fibers where light is guided in a core material surrounded
by a cladding with lower refractive index. The research started in the mid 1960s and
the propagation loss in the fibers has since then been reduced significantly. Optical
fibers are now employed to carry information over long distances with low loss and
dispersion, which has revolutionized the field of telecommunication.
An alternative way to guide and control optical waves is by photonic crystals.
They are periodically structured dielectric materials in one, two or three dimensions
that prohibit the propagation of electromagnetic waves at certain frequencies such
that band gaps are created. Line and point defects can be introduced to guide and
localize optical waves. The concept gained importance after the two papers [14, 15]
were published in 1987. Since then, photonic crystals have been studied theoretically
and experimentally in an increasing number of publications, and the first 2D
photonic crystal was fabricated for optical wavelengths in 1996 [16]. Applications
are in filters, splitters and resonant cavities and an introduction to the field is found
in [17]. An important application of the 2D photonic crystal is the photonic-crystal
fiber developed in the beginning of the 1990s. In contrast to the conventional optical
fiber the optical wave is here guided in a core region surrounded by a periodic
structure of air holes, which confine the wave by the band-gap effect, see [18, 17] for
an overview. The first photonic-crystal fibers were produced for commercial purposes
in 2000 and are fabricated by a drawing process. Compared to conventional
optical fibers, photonic-crystal fibers have advantages as higher power flow, better
confinement and a possibility to enhance or avoid dispersion and nonlinear effects.
2.1.3 Acousto-optical interaction
The properties of an optical wave can be changed in different ways by applying
external fields to the material in which the wave travels, see [13]. This will change
the optical properties of the material and hence the properties of the optical wave.
One example is the electro-optic effect where an external electric field applied to the