Tightly focused femtosecond laser pulses can be used to alter the refractive index of virtually all optical glasses. As the
laser-induced modification is spatially limited to the focal volume of the writing beam, this technique enables the
fabrication of fully three-dimensional photonic structures and devices that are automatically embedded within the host
material. While it is well understood that the laser-material interaction process is initiated by nonlinear, typically
multiphoton absorption, the actual mechanism that results in an increase or sometimes decrease of the refractive index of
the glass strongly depends on the composition of the material and the process parameters and is still subject to scientific
In this paper, we present an overview of our recent work aimed at uncovering the physical and chemical processes that
contribute to the observed material modification. Raman microscopy and electron microprobe analysis was used to study
the induced modifications that occur within the glass matrix and the influence of atomic species migration forced by the
femtosecond laser writing beam. In particular, we concentrate on borosilicate, heavy metal fluoride and phosphate glasses.
We believe that our results represent an important step towards the development of engineered glass types that are ideally
suited for the fabrication of photonic devices via the femtosecond laser direct write technique.
The silks of Orb-Weaver spiders (family <i>Araneidae</i>) are emerging as fascinating optical materials due to their biocompatibility, ecological sustainability and mechanical robustness. Natural spider silks are mainly spun as double cylinders, with diameters ranging from 0.05 to 10 μm, depending on the species and maturity of the spider. This small size makes the silks difficult to characterize optically with traditional techniques. Here, we present a technique that is capable of measuring both the real and imaginary refractive index components of spider silks. This technique is also a new capability for characterizing micro-optics more generally. It is based on the measurement and analysis of refracted light through the spider silk, or micro-optic, while it is immersed in a liquid of known refractive index. It can be applied at any visible wavelength. Results at 540 nm are reported. Real refractive indices in the range of 1.54-1.58 were measured, consistent with previous studies of spider silks. Large silk-to-silk variability of the <i>p</i>-polarized refractive index was observed of around 0.015, while variability in the s-polarized refractive index was negligible. No discernible difference in the refractive indices of the two cylinders making up the double cylinder silk structure were observed. Measured imaginary refractive indices corresponded to an optical loss of around 14 dB/mm at 540 nm.
Certain spider webs are composed of several types of micro-optical elements made from transparent optical materials.
The silks (radial and capture) are almost exclusively protein. The nearly cylindrical silks have diameters in the range 0.1
to several microns and cross-sectional morphology that is cylindrical-multi-layered,.as studied by transmission electron
microscopy, The capture threads are coated with aqueous adhesive that also forms into nearly elliptical micro-lenses
(adhesive droplets) mounted on the near cylindrical silks. The remaining elements of the web are the cement junctions
tying the radial and the capture threads of the web together. These are irregularly shaped platelets. Progress to date on
our research characterizing the optical properties and function of these transparent orb webs has been to interpret the
reflection and transmission properties of the elements of the web, and the web as a whole, in natural lighting; to evaluate
the optical finish of the surface of the silks and capture droplets; and to measure the principal refractive indices of radial
silks using new immersion based methods developed for application to micron-sized, curved optical elements. Here we
report the principal refractive indices, birefringence, dispersion and morphology of transparent spider silk subject to
various chemical treatments. The morphology is measured using TEM. Insight into the physical origin of the refractive
index properties will be discussed.
Spider orb webs are known to produce colour displays in nature, both in reflection and transmission of sunlight, under
certain illumination conditions. The cause of these colours has been the subject of speculation since the time of Newton.
It has also been the topic of observational interpretation and some experiment which has proposed diffraction by the fine
silks, scattering from rough/structured surfaces and thin film effects as the primary causes. We report systematic studies
carried out using the silks of Australian orb web weaving spiders. Studies of both white light and laser light
scattering/propagation by natural spider silks have definitively determined the primary cause of the colour displays is
rainbows that can be understood by the application of geometric optics combined with new knowledge of the optical
properties of the spider web strands, silks, and proteins as optical materials. Additionally, a range of microscopies
(optical, AFM, optical surface profiling) show the silks to be optically flat. Overall, spider silks emerge as fascinating
optical materials with high dispersion, high birefringence and the potential for future research to show they have high
nonlinear optical coefficients. Their importance as a bioinspiration in optics is only just beginning to be realised. Their
special optical properties have been achieved by ~136 million years of evolution driven by the need for the web to evade
detection by insect prey.
We have performed measurements using a purpose-built Near-field Scanning Optical Microscope and shown that
waveguides written with a fs laser in the kHz regime have an asymmetry associated with the unidirectional nature of the
writing beam. Further, the asymmetry becomes more pronounced with increasing pulse energy. At very high pulse
energies (5-10 J) the presence of multiple guided regions was also observed, indicating that the refractive index profile
of the waveguide possesses several maxima, a result which is consistent with current studies on the filamentation process
that high-powered laser pulses experience in a dielectric medium. In this paper we will present these observations, their
subsequent analysis and implications for photonic device fabrication using this method.