Douglas C. Allan

Viscosity of glass-forming liquids

The low-temperature dynamics of ultraviscous liquids hold the key to understanding the nature of glass transition and relaxation phenomena, including the potential existence of an ideal thermodynamic glass transition. Unfortunately, existing viscosity models, such as the Vogel–Fulcher–Tammann (VFT) and Avramov–Milchev (AM) equations, exhibit systematic error when extrapolating to low temperatures. We present a model offering an improved description of the viscosity–temperature relationship for both inorganic and organic liquids using the same number of parameters as VFT and AM. The model has a clear physical foundation based on the temperature dependence of configurational entropy, and it offers an accurate prediction of low-temperature isokoms without any singularity at finite temperature. Our results cast doubt on the existence of a Kauzmann entropy catastrophe and associated ideal glass transition.

Low-loss hollow-core silica/air photonic bandgap fibre

Photonic bandgap structures use the principle of interference to reflect radiation. Reflection from photonic bandgap structures has been demonstrated in one, two and three dimensions and various applications have been proposed. Early work in hollow-core photonic bandgap fibre technology used a hexagonal structure surrounding the air core; this fibre was the first demonstration of light guided inside an air core of a photonic bandgap fibre. The potential benefits of guiding light in air derive from lower Rayleigh scattering, lower nonlinearity and lower transmission loss compared to conventional waveguides. In addition, these fibres offer a new platform for studying nonlinear optics in gases. Owing largely to challenges in fabrication, the early air-core fibres were only available in short lengths, and so systematic studies of loss were not possible. More recently, longer lengths of fibre have become available with reported losses of 1,000 dB km-1. We report here the fabrication and characterization of long lengths of low attenuation photonic bandgap fibre. Attenuation of less than 30 dB km-1 over a wide transmission window is observed with minimum loss of 13 dB km-1 at 1,500 nm, measured on 100 m of fibre. Coupling between surface and core modes of the structure is identified as an important contributor to transmission loss in hollow-core photonic bandgap fibres.

Surface modes in air-core photonic band-gap fibers

We present a detailed description of the role of surface modes in photonic band-gap fibers (PBGFs). A model is developed that connects the experimental observations of high losses in the middle of the transmission spectrum to the presence of surface modes supported at the core-cladding interface. Furthermore, a new PBGF design is proposed that avoids these surface modes and produces single-mode operation.

Experimental and theoretical confirmation of Bloch-mode light propagation in planar photonic crystal waveguides

The dispersion diagram of the leaky modes in the planar photonic crystal waveguide is experimentally obtained for the wavelengths from 1440 to 1590 nm. A small stop band, around wavelength 1500 nm, is detected. The experimentally obtained results are in very good agreement with our three-dimensional finite difference time domain calculations. Propagation losses of the leaky modes are estimated and we have found that they decrease as we approach the ministop band.

193-nm excimer-laser-induced densification of fused silica

We report the densification of fused silica as a function of exposure to pulsed 193-nm excimer-laser irradiation. Defining a dose as the number of pulses N times the square of fluence I per pulse, we find that densification follows a universal function of dose, a x (NI(2))(b), where a and b can vary somewhat according to glass preparation. Densification is measured with interferometry and birefringence, interpreted with a finiteelement elastic model. Wave-front distortion for a typical photolithographic lens element in typical use conditions is described.

3D localization in a channel waveguide in a photonic crystal with 2D periodicity

A high refractive index plate with a periodic system of holes (a planar photonic crystal, PPC) is considered. The thickness of the plate as well as the distance between holes is comparable with the light wavelength. Three-dimensional band structure calculations show that a PPC supporting only a few guided modes (by keeping the thickness of the PPC small enough) can have a photonic band gap (PBG). A missing row of holes in a PPC with a PBG acts as a channel waveguide and can support modes of two types: refractive modes and diffractive ones. In-plane confinement of the latter is due to the PBG. The dispersion curves of diffractive modes have points where the group velocity is equal to zero, enabling localization of light in the direction of the waveguide. Thus, 3D localization of light takes place, localization in the direction perpendicular to the plane of the slab being due to the complete internal reflection and localization in the plane of the slab being due to the diffraction on the photonic crystal lattice.

Excimer laser-induced expansion in hydrogen-loaded silica

High-purity silica that contains a high concentration of dissolved molecular hydrogen is found to undergo expansion, rather than densification, as a consequence of exposure to deep ultraviolet irradiation. Concurrent with the decreased density, we suggest that a positive photorefractive effect occurs, which increases the index. The contributions of the density change and the photorefractive effect can be observed by comparison of the birefringence and the wave front change obtained through interferometry. Expansion and the photorefractive effect are interpreted in terms of the photolytic formation of SiOH.

Communication: Resolving the vibrational and configurational contributions to thermal expansion in isobaric glass-forming systems

A fundamental understanding of isobaric thermal expansion behavior is critical in all areas of glass science and technology. Current models of glass transition and relaxation behavior implicitly assume that the thermal expansion coefficient of glass-forming systems can be expressed as a sum of vibrational and configurational contributions. However, this assumption is made without rigorous theoretical or experimental justification. Here we present a detailed statistical mechanical analysis resolving the vibrational and configurational contributions to isobaric thermal expansion and show experimental proof of the separability of thermal expansion into vibrational and configurational components for Corning Jade glass.

Ion exchange equilibria between glass and molten salts

Abstract It is shown that interaction between the ions undergoing ion exchange leads to a sigmoidal equilibrium curve. A negative interaction in the glass causes the ratio of concentration of the two ions to be more nearly equal to unity in the glass than it is in the molten salt. It is further shown that the Rothmund–Kornfeld parameter is a measure of the non-ideality of the system. A value of the parameter greater than unity indicates a negative interaction energy.

Polarized excimer laser-induced birefringence in silica

We have used linearly polarized 193-nm-excimer laser irradiation to create polarization-induced birefringence in silica. We have observed the polarization-induced birefringence irrespective of whether the sample undergoes net compaction or expansion. The sign of the birefringence is dependent on whether the glass is expanding or compacting. It is suggested that the birefringence derives from an anisotropic density change effected by the linearly polarized exposure. This effect can be expressed formally as an optical frequency-induced dc electrostriction.