Dr Gary Hawkins

Abstract:

The health risks associated with the inhalation or ingestion of cadmium are well documented. During the past 18 years, EU legislation has steadily been introduced to restrict its use, leaving a requirement for the development of replacement materials. This paper looks at possible alternatives to various cadmium II-VI dielectric compounds used in the deposition of optical thin-films for various optoelectronic devices. Application areas of particular interest are for infrared multilayer interference filter fabrication and solar cell industries, where cadmium-based coatings currently find widespread use. The results of single and multilayer designs comprising CdTe, CdS, CdSe and PbTe deposited onto group IV and II-VI materials as interference filters for the mid-IR region are presented. Thin films of SnN, SnO2, SnS and SnSe are fabricated by plasma assisted CVD, reactive RF sputtering and thermal evaporation. Examination of these films using FTIR spectroscopy, SEM, EDX analysis and optical characterisation methods provide details of material dispersion, absorption, composition, refractive index, energy band gap and layer thicknesses. The optimisation of deposition parameters in order to synthesise coatings with similar optical and semiconductor properties as those containing cadmium has been investigated. Results of environmental, durability and stability trials are also presented.

Abstract:

In this paper we report the observation of drifts in the responsivity of cryogenically cooled InSb detector-based infrared filter radiometers which have very strong wavelength dependence. These drifts can result in the increase or decrease of the response of the filter radiometers by over 5%. The origin of these variations was investigated and was shown to arise due to a thin film of ice formed on the multi-layer bandpass filter used to define the spectral response of the filter radiometer. The thin layer of ice interacts with the characteristics of the filter (which itself consists of a number of thin layers) and modifies the filter spectral transmission thus modifying the response of the filter radiometer of which the filter is part of. These observations are particularly relevant to space instruments which use infrared filter radiometers for earth observation. Debris from the spacecraft engines is known to accumulate on cold surfaces of instruments carried on board. The deposition of this debris on cold filters can modify the spectral response of the instruments, which use these filters to define a spectral response.

Abstract:

The spectrometer sub-system of the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) contains four channels which spectrally multiplex the incoming radiation. This incident radiation, spanning a wavelength range from 4.8 to 28.8µm is spectrally divided between the channels using sets of three dichroics combined in series along the optical trains. The four channels, with overlapping wavelengths 4.8-7.8, 7.4-11.9, 11.4-18.3 and 18.3-28.8µm, are in-turn each split into three wavelength ranges to provide the required resolving power with the available detector pixels. This splitting of the wavelengths within each channel is achieved using three separate sets of dichroics and diffraction gratings, mounted on two wheels. This paper describes the design of the dichroics together with a spectral performance model developed to simulate the system spectral throughput for each of the four channels of the MIRI instrument. Details of the spectral design, manufacture, testing and mounting of the dichroics are presented together with the optomechanical layout of the instrument.