Table of contents

The prospects of two classes of diamond devices are reviewed, namely electronic devices on single-crystal substrates and microsystems devices on Si substrate. The transistor structures on single-crystal diamond represent still proof-of-concept experiments; however, they already allow us to extract their potential. The microsystems actuator and sensor devices already reflect the materials properties in their characteristics. Two of the most complex structures and future trends are discussed.

The idea of pulling together a Cluster of papers in celebration of the 60th birthday of Professor David Hanna FRS was inspired by his award of the European Physical Society Quantum Electronics Prize 2000 for applied aspects. The prize citation recognises David's `outstanding and numerous contributions to solid-state laser physics and non-linear optics', an evaluation warmly endorsed by those of us who have been lucky enough to work with him.

Over the years, David Hanna has made immense personal contributions to the field of quantum electronics and has inspired many students and collaborators to go on to make their own contributions. His influence can be seen in many aspects of the field and those who have worked closely with him have gone on to make significant impacts in the fields of business and academia throughout the world.

David Hanna, who was the 1993 recipient of the Max Born Medal and Prize, has served as Deputy Director of the Optoelectronics Research Centre at the University of Southampton since its foundation in 1989. His achievements over this period include major advances in the high-power operation of bulk, fibre and planar guided-wave lasers, and the refinement of the optical parametric oscillator into a versatile practical source. All of these areas are represented in this Cluster, in which many of the contributors are David's students and colleagues, past and present. In Some thoughts on 38 years of lasing David shares his unique perspective on the evolution of this important field of contemporary optics.

We hope that the readers of Journal of Physics D: Applied Physics will enjoy sampling the range and depth of topics covered by this tribute to David Hanna. The publication of the Cluster coincides with developments in the board of the Journal where we intend to give greater prominence to developments in the field of what has become known as Photonics.

Professor A I Ferguson, Honorary Editor, University of Strathclyde

Dr A C Tropper, member of the Editorial Board, University of Southampton

July 2001

This paper, serving as an introduction to a special issue of Journal of Physics D: Applied Physics, marks the author's 60th birthday. It gives an account of the author's 38 years of continuing involvement in research on lasers and nonlinear optics.

Degradation in beam quality due to aberrated thermal lensing and depolarization loss due to stress induced birefringence in end-pumped edge-cooled solid-state lasers are investigated. A simple model for the dependence of thermal lensing and degradation in beam quality on the transverse intensity profile of the pump beam is presented. Experimental measurements of thermal lensing, degradation in beam quality and depolarization loss in a diode-bar-end-pumped Nd:YAG rod support the main predictions of the model and, in addition, show that there can be significant extra heating under non-lasing conditions compared to lasing conditions. The role of energy transfer upconversion as a mechanism for additional heat loading under non-lasing conditions is considered. Finally, various strategies for limiting the impact of thermal lensing and thermally induced birefringence on laser performance in simple end-pumped cavity configurations are reviewed.

In this article we present a comprehensive review of the work done by our group on the amplitude and frequency stabilization of diode-pumped near-infrared solid-state lasers. In particular, we describe experiments based on single-mode Nd:YAG (1064 nm), Er-Yb:glass (1530-1560 nm), and Tm-Ho:YAG (2097 nm) lasers, end-pumped by semiconductor laser diodes. Amplitude stabilization is achieved by means of optoelectronic control loops sensing the laser intensity fluctuations and feeding back the error signal to the current of the pump diodes. Frequency stabilization is pursued using rovibrational molecular lines as absolute frequency references by means of various frequency locking techniques. The most interesting stability results are described in some detail whereas the wide literature cited through the paper provides for a useful reference list of related topics and experiments.

We present a model and experimental investigation of a singly-resonant optical cavity to enhance the nonlinear conversion efficiency of a continuous wave mode-locked all-solid-state laser source to produce an efficient source of ultraviolet radiation. For input pulses of approximately 33 ps duration at 4.4 ns intervals, our model predicts greater than 30% conversion from fundamental to third harmonic which is particularly attractive for fundamental sources of modest average power. Experimentally, we have achieved overall optical conversion efficiencies from fundamental to third harmonic wavelength typically greater than 11%, compared with less than 0.4% in a single pass geometry. We have measured an average power of 320 mW at λ = 355 nm at picosecond pulse duration, which corresponds to a generated third harmonic average power of 0.5 W.

It is often claimed and accepted that the times-diffraction-limit number M² can be determined by the use of Shack-Hartmann sensors. In this paper the case of a laser beam consisting of a superposition of two transversal laser modes TEM₀₀ and TEM₁₀ is examined as an example. It is shown that the M² of such a beam cannot be determined from one Shack-Hartmann measurement. The common Shack-Hartmann analysis of mixed modes yields large systematic deviations from the real M² values.

The advantages and potential hazards of using a planar waveguide as the host in a high-power diode-pumped laser system are described. The techniques discussed include the use of proximity-coupled diodes, double-clad waveguides, unstable resonators, tapers, and integrated passive Q switches. Laser devices are described based on Yb³⁺-, Nd³⁺-, and Tm³⁺-doped YAG, and monolithic and highly compact waveguide lasers with outputs greater than 10 W are demonstrated. The prospects for scaling to the 100 W level and for further integration of devices for added functionality in a monolithic laser system are discussed.

We describe two novel parametric systems for wavelength-tunable ultrashort pulse generation in the spectral range around 1.5 µm. We demonstrate the first single-pass optical parametric generator (OPG) that is directly pumped by a mode-locked laser at the full laser repetition rate of 35 MHz, obtaining up to 500 mW of average signal power with a pulse duration of 300 fs. We also report on a novel type of synchronously pumped high-gain optical parametric oscillator (OPO) with feedback through a single-mode fibre. We compare two fibre-feedback OPO systems, generating multi-watt average signal powers in 10 ps and 800 fs pulses. Both the OPG and fibre-feedback OPO system require a high parametric gain, which is achieved in periodically poled nonlinear crystals of LiNbO₃ and LiTaO₃, pumped with recently developed passively mode-locked all-solid-state lasers with very high average output power.

The operating characteristics of a synchronously pumped optical parametric oscillator, based on periodically poled lithium niobate (PPLN), with a diffraction grating as the tuning element, are described and compared with design expectations. Operation at average signal output power levels of up to 2.0 W has been achieved and signal/idler tuning in the range 1.466-3.663 µm has been investigated using the tilt of the diffraction grating, while keeping the temperature of the PPLN fixed. A detailed investigation has been made of the operating characteristics that might be influenced by the presence of pulse tilt caused by the grating. It is shown that any adverse effects of tilt on beam quality can be small, with the main observed effect being an excess loss. An observed tendency to oscillate in higher-order transverse modes on the long-wavelength side of the tuning range is ascribed to non-collinear phase-matching. This higher-order-mode oscillation is easily prevented by the insertion of an aperture. The analysis presented here includes a grating loss calculation and the calculation of the tuning range achievable with a diffraction grating. The effects of cavity-length change on oscillator performance are also examined and it is found that the grating can give a valuable stabilizing influence by suppressing cavity-length-induced frequency-pulling effects. Extension of the use of gratings from the picosecond to the femtosecond regime is also considered.

We demonstrate that ion implantation can be used for response time tailoring to create high-performance indium gallium arsenide (InGaAs) quantum well semiconductor saturable absorber mirrors (SESAMs). The design and manufacture of the SESAMs are described, and their nonlinear optical and temporal responses, relevant to the mode-locking of picosecond type pulses, are given. The implanted devices shown here have response times as short as 11 ps, compared with several hundred picoseconds without implantation. They have small non-bleachable losses and low saturation fluence ({<}20µJcm^-2), allowing for operation far below the damage threshold. An implantation- and annealing-induced quantum well intermixing effect is shown to be present in the SESAMs, resulting in an energy shift in the excitonic feature. This effect can be taken into account in the SESAM design or minimized, if necessary. Using the SESAMs to mode-lock a low-power Nd:YVO₄ laser at 1064 nm, we have generated pulsewidths between 9 and 23 ps, depending on the cavity configuration and the SESAM used.

Yttrium ion implantation was conducted to synthesize Y-disilicide films on silicon wafers, using a metal vapour vacuum arc ion source and the continuous and stable YSi₂ films were directly obtained with neither external heating nor post-annealing. The formation mechanism of the YSi₂ phase is also discussed, in terms of the temperature rise caused by ion beam heating and the ion dose in the process of implantation.

The geometry of the multiple diffraction of a (001) silicon wafer in an x-ray powder diffractometer is elucidated. In particular, multiple diffraction occurs only when the sample is rotated (along the sample normal) to one of the specific azimuthal angles. A derivation based on analytical geometry is performed to obtain a general but closed formula for these azimuthal angles of multiple diffraction. Subsequent calculation shows that multiple diffraction occurs at 128 different angles in a full 360° scan to form a (002) `forbidden' reflection. Among these 128 multiple diffractions, 64 are double diffractions and the others are combined double and triple diffractions. The results of measurement obtained by a powder diffractometer show that only 96 of the 128 multiple diffractions are observed in the x-ray diffraction pattern due to the overlapping of some closely spaced diffraction peaks.

Commonly, the germane fraction (f = flowrateofGeH₄/flowrateofSiH₄ + GeH₄) is changed to vary the optical gap (E_opt) of amorphous silicon germanium alloy (a-SiGe:H) films. We report that for a particular f, the change of deposition conditions, the flow rate of diluent gas (H₂) and the radiofrequency (rf) power density can vary the optical gap (1.67-1.40 eV), the germanium content (41.3-22.5 at%) and the microstructural defect density (0.92-0.42), the mobility lifetime product (ηµτ; 6.81×10^-6-1.46×10^-8 cm² V^-1) in a wide range. Initially, with the increase of the H₂ flow rate up to 20 SCCM for the rf power density of 60 mW cm^-2 and up to 30 SCCM for the rf power density of 30 mW cm^-2, the microstructural defects decreases, although, the Ge content of a-SiGe:H films increases. The microstructural defects of a-SiGe:H films becomes a direct function of the midgap defect density and a indirect function of ηµτ. Moreover, the nature of diluent gas is also important. We present that in a wide range of optical gap (1.74-1.36 eV) the defect density is lower, and ηµτ is higher for the optimized He diluted film compared to those of the optimized H₂ diluted films.

The influence of the corona-charging process on the piezoelectric transducer coefficient d₃₃ of a cellular electret film has been investigated. An increased corona voltage can be considered as a way to enhance the charge density and thus also the resulting piezoelectric effect. Higher corona-charging voltages are possible with increased ambient pressure or in suitable dielectric gases. The effect of the gas inside the voids has also been studied. Enhanced transducer coefficients were obtained by corona charging in N₂ or N₂O gas atmospheres at 100-450 or 100-140 kPa pressures, respectively. The highest transducer coefficients of about 790 pCN^-1 were obtained when N₂ gas was filled into the voids of a cellular polymer film by means of consecutive vacuum and high-pressure treatments at 295 or 313 K.

Thickness inhomogeneities in thin films have a large influence on their optical transmission and reflection spectra. If not taken into account, this may lead to rather large calculated values for the absorption coefficient or the erroneous presence of an absorption-band tail, as well as to significant errors in the calculated values of the refractive index and the film thickness. The effect of thickness variation on the optical reflection spectrum of a thin dielectric film covering a thick non-absorbing substrate, is analysed in detail in this paper, and analytical expressions are presented for such a reflection spectrum and its upper and lower envelopes. A method is suggested for determining the refractive index n(λ) and the extinction coefficient k(λ), as well as the average thickness and the thickness variation, of a thin dielectric film with variable thickness, by using only the two envelopes of the corresponding shrunk reflection spectrum. This method is used for the geometrical and optical characterization of thermally-evaporated amorphous chalcogenide films, deposited on glass substrates.

Thin (~16 µm) base-layer monocrystalline silicon solar cells have been investigated with microcrystalline or epitaxial n-type emitters grown at low temperatures ({<}550 °C) by electron cyclotron resonance (ECR) plasma-enhanced chemical vapour deposition (PECVD). The p-type, 1-2 Ωcm, base layers were epitaxially deposited by conventional thermal CVD onto monocrystalline Si p⁺ substrates. An efficiency of 13.72% was achieved in the best epitaxial emitter cell after ECR hydrogen passivation and the application of a SiN_x anti-reflection coating deposited by ECR PECVD. Cells with microcrystalline Si emitters processed in a similar fashion gave a maximum efficiency of 10.73%. The cell performance was analysed using the two-diode model and the solar cell modelling programme PC-1D. The results are presented. It was necessary to invoke a three-layer PC-1D model to obtain self-consistent fits to the light and dark I-V characteristics and spectral response data.

The gas temperature of reactive microdischarges has been extensively investigated in a methane-fed dielectric barrier discharge (DBD) configured by a two parallel plate reactor with 0.5 mm gap spacing. Emission spectroscopy of the rotational bands of CH(²Δ→²Π) coupled with heat transfer experiments have been employed for this purpose. Stationary and space-averaged gas temperature between the discharge gap was estimated from the heat transfer experiment; thus heat capacity and enthalpy gained by the feed gas stream resulted in a ≈20 K temperature increase. The rotational temperature showed fairly good sensitivity to the inlet gas temperature variation in the range 370-670 K. However, the local gas temperature increase inside the microdischarges represented an additional 100 K above the average gas temperature, indicating one order of magnitude higher value than the theoretically expected gas temperature increase (5-10 K) for a single microdischarge. High-frequency operation (80 kHz) is responsible for memory effect, thus such a high gas temperature increase was the result of multiple microdischarges rather than a single one.

The emission of xenon excited in a capillary discharge in the region around 13 nm has been investigated as a function of current, pressure and tube radius. A semi-empirical model is proposed which reproduces a large number of the experimental data.

Effective electromechanical properties, figures of merit and related parameters are firstly studied within the framework of proposed models of piezoactive composites `ferroelectric ceramic-polymer' (connectivity 1-1) and `ferroelectric ceramic-porous polymer' (connectivity 1-0-1). The effect of the shape of the pores on the piezoelectric coefficients, their anisotropy, electromechanical coupling factors, figures of merit, etc in 1-0-1 composites is analysed. The calculated concentration dependences of the piezoelectric coefficients and other parameters as well as the piezoelectric anisotropy of 1-1 and 1-0-1 composites have no analogy to known data on two- and three-component composites on the basis of ferroelectric ceramics.

The ferroelectric relaxation of unirradiated and irradiated P(VDF/TrFE) copolymers has been studied through measurement of their dielectric and ferroelectric properties. A series of irradiations with different doses were carried out on the samples in order to study the irradiation effect. The gel content and hysteresis loops were measured to observe the results of irradiation. It is shown that ferroelectric relaxation becomes more obvious after electron irradiation. The Curie points of the copolymers are shifted to low temperatures and they decrease largely with increasing irradiation dose. The relaxation mechanism is discussed and the relationships between ferroelectric relaxation and electron irradiation doses are investigated.

The dielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis with which to understand the nature of the excited states and their involvement in the electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which act as trapping centres. The excited states involved in photoluminescence also seem to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress.

The crystallization kinetics of the chalcogenide Se_72.5Te₂₀Sb_7.5 glass were obtained using differential scanning calorimetry (DSC). The results of DSC curves, performed at different heating rates (α), have been reported and discussed. The activation energy for the crystallization of the investigated glass was found to be equal to 84.2±3.5 kJ mol^-1. A non-integer value of the Avrami index (n = 3.5) indicated that two crystallization mechanisms, two-dimensional and three-dimensional growth, are working simultaneously with an equal share through the amorphous-crystalline transformations.

The infiltration of liquid and the propagation of the moisture front in non-saturated porous media are generally described by a diffusion equation that predicts a scaling law of the type x/(t)^1/2 in one dimension. This model, generally referred to as the theory of unsaturated flow, was systematically applied to account for water movements in porous building materials. In this paper, two sets of nuclear magnetic resonance (NMR) one-dimensional water absorption profiles, respectively measured in a fired-clay brick and a limestone specimen, are re-examined according to this model. The reinterpretation of the NMR absorption data provides evidence that the infiltration front does not propagate as t^1/2 neither in brick nor in limestone, i.e. the absorption process does not conform to the predictions of the unsaturated flow theory in these materials. A new theoretical model for infiltration, based on the assumption of a non-Fickian diffusion mechanism, is thus introduced. The water transfer in partially saturated materials is assumed to follow the general nonlinear diffusion equation (∂θ/∂t)-(∂/∂x)[D(θ)(∂θ/∂x)ⁿ] = 0, with n real. For one-dimensional infiltration, the water content θ can be expressed in terms of the single variable ϕ* = xt^-α, with α = 1/(n + 1) and the cumulative water infiltration I is given at any time by I = ∫_θ0^θ₁x dθ = t^α∫_θ0^θ₁ϕ* dθ = S*t^α. The NMR absorption data are shown to be compatible with a non-Fickian diffusion process scaling as t^0.58 in brick and as t^0.61 in the limestone specimen. The application of the new anomalous diffusion model to brick indicates that the previous t^1/2 relation may underestimate the volume of absorbed water by about 30% after only 100 hours. This result has particular relevance for evaluating the durability of building structures.

A semi-empirical formula for the full energy peak efficiency of high-purity germanium (HPGe) detectors in the range from 4 to 3000 keV is presented. The formula is based on physically sound premises and features a very low number of free parameters, all of which are physically founded and exhibit physically viable values. The model furnishes very good fits and was successfully tested against a number of experimental efficiency curves measured with point sources on n- and p-type HPGe detectors.

Up to now, no technique was available to perform inventories and surveys of immersed macroalgae. This article presents a method making it possible to discriminate between macroalgae and seafloor and to identify the various macroalgae groups (green, brown or red). To our knowledge, this is the first excitation-emission matrix (EEM) of macroalgae ever carried out allowing the identification of the spectral characteristics of each macroalgae group. Fluorescence imaging provides the spatial dimension besides spectral properties. An optical parametric oscillator (OPO) laser excites a test scene of macroalgae in a seawater aquarium. Laser-induced fluorescence images from 400 to 640 nm displayed by steps of 10 nm are filtered at 680 nm. Their analysis, combined with the water depth parameter, leads to an identification method for macroalgae groups in a false-colour image: each of the three macroalgae groups is enhanced by one defined threshold on this false-colour image.