Robert G. Denning

Fabrication of photonic crystals for the visible spectrum by holographic lithography

The term ‘photonics’ describes a technology whereby data transmission and processing occurs largely or entirely by means of photons. Photonic crystals are microstructured materials in which the dielectric constant is periodically modulated on a length scale comparable to the desired wavelength of operation. Multiple interference between waves scattered from each unit cell of the structure may open a ‘photonic bandgap’—a range of frequencies, analogous to the electronic bandgap of a semiconductor, within which no propagating electromagnetic modes exist. Numerous device principles that exploit this property have been identified. Considerable progress has now been made in constructing two-dimensional structures using conventional lithography, but the fabrication of three-dimensional photonic crystal structures for the visible spectrum remains a considerable challenge. Here we describe a technique—three-dimensional holographic lithography—that is well suited to the production of three-dimensional structures with sub-micrometre periodicity. With this technique we have made microperiodic polymeric structures, and we have used these as templates to create complementary structures with higher refractive-index contrast.

Two‐Photon Absorption and the Design of Two‐Photon Dyes

Two-photon absorption has important advantages over conventional one-photon absorption, which has led to applications in microscopy, microfabrication, three-dimensional data storage, optical power limiting, up-converted lasing, photodynamic therapy, and for the localized release of bio-active species. These applications have generated a demand for new dyes with high two-photon absorption cross-sections. This Review introduces the theory of two-photon absorption, surveys the wide range of potential applications, and highlights emerging structure-property correlations that can serve as guidelines for the development of efficient two-photon dyes.

The electronic structure of the uranyl ion

Polarized single crystal absorption spectra at 4·2 K are reported for CsUO2(NO3)3 and NaUO2(CH3COO)3. Natural and magnetic circular dichroism and Zeeman effect measurements have been made. Measurements have also been made on crystals containing U18O2 2+. In the portion of the spectrum between 21 000 and 30 000 cm-1 seven electronic states have been identified in both CsUO2(NO3)3 and NaUO2(CH3COO)3. In the majority of cases the symmetry of the excited states has been assigned.

Chromophores for second-order non-linear optic materials

The theory of the electro-optic effect and second harmonic generation is developed in the context of the linear Stark effect. This leads to an emphasis both on the optical bandwidth of the charge-transfer transitions responsible for the hyperpolarisability, and on the relationship between the transition energies and optical frequency of an electro-optic device. A simple treatment of vibronic coupling illustrates the connection between the bond-length alternation in conjugated polyenes and the sign and magnitude of the non-linearity. A brief survey is made of current chemical strategies for optimising the non-linearity.

Observation of a pure electric quadrupole mechanism for a solid-state electronic transition

Polarized high resolution absorption spectra at 4·2 K are reported for single crystals of Cs2UO2Cl4. An analysis of the polarization of the electronic origin bands and the vibronic structure provides evidence for a transition which is allowed by a pure electric-quadrupole mechanism.

Degenerate four-wave mixing studies of butadiyne-linked conjugated porphyrin oligomers

Abstract The third-order electronic susceptibilities have been measured in solution for a series of conjugated porphyrin oligomers, and for a conjugated polymer of this type. At 1064 nm, the non-resonant susceptibilities of the oligomers and the polymer are large (|χ(3)|≈10−17–10−16 m2 V−2), have comparable real and imaginary parts, and the real part is negative in sign. The molecular susceptibility per ring chromophore, (γ/N), shows a linear dependence upon chain length, N. We propose a general mechanism for the enhancement of the third-order susceptibilities in conjugated porphyrin based materials due to two-photon resonance with charge transfer states.

New optics—new materials

The nature and applications of optical materials have evolved rapidly in recent years. Their role as passive optical elements in free space has been augmented by so-called photonic systems. These can have many active components—oscillators, amplifiers, frequency converters, modulators, switches, routers and so on—most of which rely, to some degree, on optical field confinement. The design of appropriate materials for this new technology involves progress on two separate levels. There is a need both for the optimisation of microscopic electronic properties and for the separate control of bulk optical parameters on the scale of optical wavelengths. This article uses several examples to illustrate selected areas of current activity under both these headings. It also emphasises the need for more general methods for creating optical scale microstructure.

Photonic crystals for the visible spectrum by holographic lithography

We describe a general and flexible technique for the fabrication of three-dimensional photonic crystals that is particularly well adapted to the production of structures with the sub-micron periodicity required for applications in the visible optical spectrum. Three-dimensional microstructure is generated by using a four-beam laser interference pattern to expose a thick layer of photoresist. Exposed areas are rendered insoluble; unexposed areas are dissolved away leaving a three-dimensional photonic crystal formed of cross-linked polymer with air-filled voids. The polymeric structure may be used as a template for the production of photonic crystals with higher refractive index contrast. Photonic crystals made of polymer and of TiO2 have been characterized by scanning electron microscopy and by optical diffraction measurements.

Electronic energy transfer in actinyl crystals: I. Non-resonant transfer in Cs2UO2Cl4

Using time resolved laser spectroscopy at 4·2 K we show that nearest neighbour species in single crystal caesium uranyl nitrate, whose resonant electronic energies have a difference comparable to the excitonic bandwidth, are effectively localized. Transport through the crystal must proceed through a phonon-assisted ‘hopping’ mechanism with a time constant of about 10 μs.

Magnetic circular dichroism spectrum of the permanganate ion in Ba(ClO4)2·3H2O at 4.2°K

Abstract Magnetic circular dichroism (MCD) spectra of MnO 4 − in a crystal of Ba(ClO 4 ) 2 ·3H 2 O have been measured at 4.2°K in the region of the weak near infrared transition and the lowest energy intense transition in the visible.