Douglas J. Little

Femtosecond laser modification of fused silica: the effect of writing polarization on Si-O ring structure

A femtosecond laser with a 1 kHz repetition rate and two different polarization states was used to fabricate low-loss waveguides in fused silica. Investigations of chemically-mechanically polished waveguide regions using near-field scanning optical microscopy revealed the presence of modifications outside the glass regions directly exposed to a circularly polarized writing laser. These waveguides also exhibited refractive index contrast up to twice as large as that of waveguides written with linearly polarized radiation. The observed differences in refractive index were shown by Raman spectroscopy to correlate to an increased concentration of 3-member silicon-oxygen ring structures. We propose that the observed differences in material properties are due to the polarization dependence of photo-ionization rates in fused silica.

Mechanism of femtosecond-laser induced refractive index change in phosphate glass under a low repetition-rate regime

Raman microscopy and refractive near-field profilometry were used to analyze waveguides written in Yb-doped Kigre QX glass under the low repetition-rate (noncumulative-heating) regime. It was found that femtosecond-laser induced refractive index change was due to an increase in the proportion of Q1 P-tetrahedra and the associated increase in the polarizability of the glass. The role of color center formation and removal in this process is clearly defined, phosphorous–oxygen hole centers (POHCs) and PO3− ions form as a result of P–O bonds being broken during the modification process, and the subsequent removal of POHCs give rise to the increased proportion of Q1 P-tetrahedra. This result, when compared to other studies undertaken in the cumulative-heating regime, show conclusively that the mechanism of refractive index change in a particular type of glass can be very different, depending on the irradiation conditions.

Annealing dynamics of waveguide Bragg gratings: evidence of femtosecond laser induced colour centres

There is still significant speculation regarding the nature of femtosecond laser induced index change in bulk glasses with colour centre formation and densification the main candidates. In the work presented here, we fabricated waveguide Bragg gratings in doped and undoped phosphate glasses and use these as a diagnostic for monitoring subtle changes in the induced refractive index during photo- and thermal annealing experiments. Reductions in grating strengths during such experiments were attributed to the annihilation of colour centres.

Image contrast immersion method for measuring refractive index applied to spider silks.

A technique for measuring the refractive index of micron sized fibers using a series of immersion index matching oils, and image contrast measurements is proposed and demonstrated. It has been applied to radial silks of the orb web weaving spider Plebs eburnus. These have widths of ~1-2 microns. Values about 1.5500 are obtained, with birefringence values between 0.0000 and 0.0133 for individual silks. An uncertainty in the range ± 5 × 10(-4) to ± 2 × 10(-3) is achieved for these challenging samples. This accuracy is about a twenty times improvement on previously reported measurements for spider silks using other techniques. The technique is used to obtain measurements of the refractive index of spider silks as a function of wavelength, for the first time. An Abbe number for the radial silks of Plebs eburnus of ~32 is found.

Influence of bandgap and polarization on photo-ionization: guidelines for ultrafast laser inscription [Invited]

An ultrafast laser was used to fabricate waveguides in Yb:QX phosphate glass and BK7 borosilicate glass using linearly polarized and circularly polarized beams. Circularly polarized pulses were found to induce a higher refractive index change in Yb:QX phosphate glass, while in BK7 borosilicate glass circularly and linearly polarized pulses were found to induce the same refractive index change. An explanation for these contrasting results is proposed based on the fundamental polarization-dependence of photo-ionization. This explanation reconciles observations made in this study and also in a previous study in fused silica glass.

Phase-stepping interferometry of GaAs nanowires: Determining nano-wire radius

Phase stepping interferometry is used to measure the size of near-cylindrical nanowires. Nanowires with nominal radii of 25 nm and 50 nm were used to test this by comparing specific measured optical phase profile values with theoretical values calculated using a wave-optic model of the Phase stepping interferometry (PSI) system. Agreement within 10% was found, which enabled nanowire radii to be predicted within 4% of the nominal value. This demonstration highlights the potential capability for phase stepping interferometry to characterize single nanoparticles of known geometry in the optical far-field.

Hybrid Immersion-Polarization Method for Measuring Birefringence Applied to Spider Silks

A technique for accurate measurement of the principle refractive indices and birefringence for silklike samples is presented. It is based on rotating the linear polarization of the illuminating light on a silk immersed in reference liquid to achieve index matching at the silk/liquid interface. The technique was used to measure the principal refractive indices of a P. eburnus radial silk at different strains. This in turn allows the calculation of strain-optic coefficients. The first measurement of the strain-optic coefficients of spider silk is presented. The technique is more generally applicable to strain-optic study of birefringent micro-optic samples.

Measuring nanoparticle size using optical surface profilers

Optical surface profilers are state-of-the-art instruments for measuring surface height profiles. They are not conventionally applied to nanoparticle measurements due to the presence of diffraction artifacts. Here we use a theoretical model based on wave-optics to account for diffraction-based artifacts in optical surface profilers. This then enables accurate measurement of nanoparticles size of a known geometry. The model is developed for both phase shifting interferometry and vertical scanning interferometry modes of operation. It is demonstrated that nanosphere radii as small as 12 nm, and nano-cylinder radii as small as 10-15 nm can be measured from a standard profile measurement using phase shifted interferometry interpreted using the wave-optics approach.

Optics of spider "sticky" orb webs

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.

Permutation entropy of finite-length white-noise time series.

Permutation entropy (PE) is commonly used to discriminate complex structure from white noise in a time series. While the PE of white noise is well understood in the long time-series limit, analysis in the general case is currently lacking. Here the expectation value and variance of white-noise PE are derived as functions of the number of ordinal pattern trials, N, and the embedding dimension, D. It is demonstrated that the probability distribution of the white-noise PE converges to a χ^{2} distribution with D!-1 degrees of freedom as N becomes large. It is further demonstrated that the PE variance for an arbitrary time series can be estimated as the variance of a related metric, the Kullback-Leibler entropy (KLE), allowing the qualitative N≫D! condition to be recast as a quantitative estimate of the N required to achieve a desired PE calculation precision. Application of this theory to statistical inference is demonstrated in the case of an experimentally obtained noise series, where the probability of obtaining the observed PE value was calculated assuming a white-noise time series. Standard statistical inference can be used to draw conclusions whether the white-noise null hypothesis can be accepted or rejected. This methodology can be applied to other null hypotheses, such as discriminating whether two time series are generated from different complex system states.