Laser ablation deposition of Ga₂S₃-La₂S₃ glass films

Gallium - lanthanum sulphide glasses (GLS) show wide range transparency and low non radiative relaxation rates for dopant ions such as Ho3+, Er3+ etc. They also show permanent photomodification of the refractive index under visible illumination. We report laser ablation deposition of these glasses and preliminary results on film stoichiometry and deposition rate as a function of excimer laser fluence. The sulphur to metal and Ga/La ratios are found to have marked fluence dependencies. The films show considerably more Urbach tail absorption than bulk material. A novel method has been developed for mapping the permanent photomodifled index

Q-switched laser damage of infrared nonlinear materials

Q-switched laser-damage thresholds have been determined for six materials (proustite – Ag3AsS3, pyrargyrite – Ag3SbS3, cinnabar – HgS, silver thiogallate – AgGaS2, tellurium – Te, and gallium arsenide – GaAs) of interest for nonlinear optics in the medium infrared. Four TEM00 mode lasers were employed with outputs at wavelengths of 694 nm, 1.06, 2.098, and 10.6 µm. Damage has been found to be confined to the surface of the crystals and occurs for radiation intensities between 3 and 75 MW/cm2. Particular care is needed in the cutting and polishing of tellurium crystals if a high-damage threshold is to be achieved

Electrically driven 8-14 micron band solid-state modulator

Simulation results for an electrically driven 8-14µm band, solid-state modulator based on a high purity germanium p-i-n diode are presented. Known carrier recombination mechanisms detrimental to device performance along with ways to reduce their effect are detailed. Device simulations using 'ATLAS', indicate that modulation depths of up to 99.5% attenuation for incident infrared radiation centred at 10.6µm can be achieved by using ion-implanted device construction and optimisation of the design parameters. Preliminary transient analysis attained higher modulation frequency when transient large bias voltage (2V) is used momentarily during switching

Germanium infrared absorption chopper

The use of Pyroelectric array detectors in Infrared cameras in the 8 to 14µm region necessitates a means for area-modulation, or chopping of the infrared scene, thus generating an element of change for measurement by the detector. Currently this is achieved by mechanically chopping the image using a rotating blade, with drawbacks of high electric motor power, blurring as camera motion changes the rotational speed, and the chopper blade largely determining the camera diameter. Adequate modulation using a novel method has been achieved in the 8 to 14µm region by introducing moderate levels of excess carriers to suitably prepared Germanium. These were introduced via excitation from a diode laser source. The process described uses inter valence band transitions from the light-hole to heavy-hole band, requiring power densities in the order of Watts cm from an AlGaAs 809nm laser is easily achievable. The on state transmission will then be then increased to essentially 100% by using anti-reflection coatings. We will describe the relative importance of the bulk and surface properties; high bulk purity and low surface recombination velocities being required to make an effective device, although operation is possible with lower quality material. The most suitable material for the device is low carrier density n-Germanium. The modulator does not operate in the much-publicised intra-band transition region which involves generating a free carrier density greater than the critical density, typically requiring laser powers in the order of MW cm

Optoelectronics and the internet

The talk introduces the concept of Optoelectronics, the fusion of optics and electronics which is now absolutely vital to modern communications, in particular the Internet. The basic ideas of the scale of information which the modern world needs to transmit, and the speeds at which we must transmit it, are discussed to introduce the requirements of modern high-bandwidth links. The physical basis of those links, the fibre optics, optical amplifiers and dispersion compensation systems essential to its operation, are introduced at a basic, non-mathematical level. Some of the new developments required by the ever growing thirst for data communications, and approaches to them underway at the University of Southampton, are included in the talk. It is intended to be suitable for students typically in their last two years or schooling pre University, and to non-specialist year University students. It may also be of interest to persons with a general interest in modern IT, and assumes only a general technical background with no specialist knowledge of the subject

Optics and the internet

The talk introduces the concept of Optoelectronics, the fusion of optics and electronics which is now absolutely vital to modern communications, in particular the Internet. The basic ideas of the scale of information which the modern world needs to transmit, and the speeds at which we must transmit it, are discussed to introduce the requirements of modern high-bandwidth links. The physical basis of those links, the fibre optics, optical amplifiers and dispersion compensation systems essential to its operation, are introduced at a basic, non-mathematical level. Some of the new developments required by the ever growing thirst for data communications, and approaches to them underway at the University of Southampton, are included in the talk. It is intended to be suitable for students typically in their last two years or schooling pre University, and to non-specialist year University students. It may also be of interest to persons with a general interest in modern IT, and assumes only a general technical background with no specialist knowledge of the subject

On the evaluation of modified Bessel functions of the second kind and fractional order for synchrotron radiation calculations

The modified Bessel functions of the second kind and fractional order K1/3(x) and K2/3(x) are of importance in the calculation of the frequency spectrum of synchrotron radiation. The parameter range of interest is typically 10-6<x<10. Recently, there has been particular interest in the generation of ‘terahertz’ radiation, which can be coherently enhanced by many orders of magnitude when the electron bunch length is shorter than the terahertz wavelength. This requires evaluation of the Bessel functions for small values of the argument. It is shown that the series commonly used to evaluate these functions has poor convergence properties under these conditions. An alternative series is derived which has much better convergence for x<1