Twinkle is a space mission designed for visible and near-IR spectroscopic observations of extrasolar planets. Twinkle’s highly stable instrument will allow the photometric and spectroscopic observation of a wide range of planetary classes around different types of stars, with a focus on bright sources close to the ecliptic. The planets will be observed through transit and eclipse photometry and spectroscopy, as well as phase curves, eclipse mapping and multiple narrow-band time-series. The targets observed by Twinkle will be composed of known exoplanets mainly discovered by existing and upcoming ground surveys in our galaxy (e.g. WASP, HATNet, NGTS and radial velocity surveys) and will also feature new discoveries by space observatories (K2, GAIA, Cheops, TESS). Twinkle is a small satellite with a payload designed to perform high-quality astrophysical observations while adapting to the design of an existing Low Earth Orbit commercial satellite platform. The SSTL-300 bus, to be launched into a low- Earth sun-synchronous polar orbit by 2019, will carry a half-meter class telescope with two instruments (visible and near-IR spectrographs - between 0.4 and 4.5μm - with resolving power R~300 at the lower end of the wavelength scale) using mostly flight proven spacecraft systems designed by Surrey Satellite Technology Ltd and a combination of high TRL instrumentation and a few lower TRL elements built by a consortium of UK institutes. The Twinkle design will enable the observation of the chemical composition and weather of at least 100 exoplanets in the Milky Way, including super-Earths (rocky planets 1-10 times the mass of Earth), Neptunes, sub-Neptunes and gas giants like Jupiter. It will also allow the follow-up photometric observations of 1000+ exoplanets in the visible and infrared, as well as observations of Solar system objects, bright stars and disks.
Selex ES produces a wide range of infrared detectors from mercury cadmium telluride (MCT) and triglycine sulfate (TGS), and has supplied both materials into space programmes spanning a period of over 40 years. Current development activities that underpin potential future space missions include large format arrays for near- and short-wave infrared (NIR and SWIR) incorporating radiation-hard designs and suppression of glow. Improved heterostructures are aimed at the reduction of dark currents and avalanche photodiodes (APDs), and parallel studies have been undertaken for low-stress MCT array mounts. Much of this development work has been supported by ESA, UK Space, and ESO, and some has been performed in collaboration with the UK Astronomy Technology Centre and E2V.<p> </p>This paper focuses on MCT heterostructure developments and novel design elements in silicon read-out chips (ROICs). The 2048 x 2048 element, 17um pitch ROIC for ESA’s SWIR array development forms the basis for the largest cooled infrared detector manufactured in Europe. Selex ES MCT is grown by metal organic vapour phase epitaxy (MOVPE), currently on 75mm diameter GaAs substrates. The MCT die size of the SWIR array is 35mm square and only a single array can be printed on the 75mm diameter wafer, utilising only 28% of the wafer area. The situation for 100mm substrates is little better, allowing only 2 arrays and 31% utilisation. However, low cost GaAs substrates are readily available in 150mm diameter and the MCT growth is scalable to this size, offering the real possibility of 6 arrays per wafer with 42% utilisation.<p> </p>A similar 2k x 2k ROIC is the goal of ESA’s NIR programme, which is currently in phase 2 with a 1k x 1k demonstrator, and a smaller 320 x 256 ROIC (SAPHIRA) has been designed for ESO for the adaptive optics application in the VLT Gravity instrument. All 3 chips have low noise source-follower architecture and are enabled for MCT APD arrays, which have been demonstrated by ESO to be capable of single photon detection. The possibility therefore exists in the near future of demonstrating a photon counting, 2k x 2k SWIR MCT detector manufactured on an affordable wafer scale of 6 arrays per wafer.
IASI (Infrared Atmospheric Sounding Interferometer), developed by CNES and launched since 2006 on the Metop satellites, is established as a major source of data for atmospheric science and weather prediction. The next generation - IASI NG - is a French national contribution to the Eumetsat Polar System Second Generation on board of the Metop second generation satellites and is under development by Airbus Defence and Space for CNES. The mission aim is to achieve twice the performance of the original IASI instrument in terms of sensitivity and spectral resolution. In turn, this places very demanding requirements on the infrared detectors for the new instrument. Selex ES in Southampton has been selected for the development of the infrared detector set for the IASI-NG instruments. The wide spectral range, 3.6 to 15.5 microns, is covered in four bands, each served by a dedicated detector design, with a common 4 x 4 array format of 1.3 mm square macropixels. Three of the bands up to 8.7 microns employ photovoltaic MCT (mercury cadmium telluride) technology and the very long wave band employs photoconductive MCT, in common with the approach taken between Airbus and Selex ES for the SEVIRI instrument on Second Generation Meteosat. For the photovoltaic detectors, the MCT crystal growth of heterojunction photodiodes is by the MOVPE technique (metal organic vapour phase epitaxy). Novel approaches have been taken to hardening the photovoltaic macropixels against localised crystal defects, and integrating transimpedance amplifiers for each macropixel into a full-custom silicon read out chip, which incorporates radiation hard design.
Detector arrays using Metal-Organic Vapour Phase Epitaxy (MOVPE) grown HgCdTe (MCT) on GaAs substrates have been in production at SELEX Galileo for over 10 years and are a mature technology for medium wave, long wave, and dual-band tactical applications. The mesa structure used in these arrays is optimised for MTF, quantum efficiency and dark currents. Further development of the technique has migrated to very long wave and short wave bands, mainly for space and astronomy applications, and for mid wave applications towards smaller pixels and higher operating temperatures. The emphasis of this paper is on recent experiments aimed at further improving HOT performance.
The Exoplanet Characterisation Observatory (EChO) is currently being studied by ESA as a medium class mis sion to be launched in the third decade of the new millennium. EChO requires exquisite detector sensitivity and stability under low- background conditions. The EChO longest wavelength instrument covers the very long wave length (VLWIR) spectral band from 10 to 16 µm and HgCdTe (MCT) photoconductors constitute a promising technology. Currently available MCT detectors have been developed for high-background and high-temperature operations and little is known about the performance achievable when the same detectors are operated at cryo genic temperatures, between 77 K and 4 K. Here we report on the optical and dark measurements obtained on VLWIR MCT photoconductors from European manufacturers at cryogenic temperatures.
Raising the operating temperature of infrared detectors has benefits in terms of reduced cooler power and increased life
and enables an overall reduction in size and weight for handheld applications. With MCT the composition can be tuned
to achieve the required wavelength range at a given temperature. Work on detectors operating in the 3-5μm atmospheric
transmission window at operating temperatures up to 210K will be described. The influence of limiting factors such as
excess noise, radiation shield emission, dark current and injection efficiency will be presented.
Packaging aspects will be discussed emphasizing the importance of achieving low cost, weight and power for handheld
applications. The impact of the detector design on overall system size and performance is considered with specific
attention to time to image, passband and f-number.
Finally images will be presented showing performance from a high operating temperature (HOT) camera.
This paper summarises measurements and calculations of HOT performance in Selex Galileo's MW
detectors and demonstrates that high quality imagery can be achieved up to 175K. The benefits of HOT
operation for cooler performance and power dissipation are also quantified.
The variable band gap of MCT provides the ability to optimise the cut-off wavelength for a wide range
of operating temperatures. In particular, it provides the means to produce a MW detector that is well
matched to the 3-5μm atmospheric transmission window at any temperature in the range from 80K up
to room temperature. Competing InSb technology is disadvantaged at higher operating temperatures by
a narrowing band gap, increasing cut-off wavelength, and inadequate EO performance.
The practical upper limit of operating temperature for near-background limited performance is
influenced by several factors, which fall into two categories: the fundamental physics of thermal dark
current generation and black body emission from the cooled radiation shield, and the technology
limitations of MCT diode leakage currents, excess noise, dark current due to defects, and injection
efficiency into the ROIC.
This paper describes progress in the development of dual-band (MW / LW) infrared detectors made from HgCdTe grown
by Metal-Organic Vapor Phase Epitaxy. The technologies of LW and MW single band detectors, which feed into dualband
capability, are discussed. The performance of single-band detectors is detailed to give an indication of the quality
that can be achieved through MOVPE processes. For single-band detectors, pixel resolution has reached 1024 x 786,
while pixel pitch has been reduced to 16μm. Operability for single-band detectors has exceeded 99.98% in both bands.
Full-TV (640 x 512 pixels) dual-band arrays on 24μm and 20μm pitches have been developed. MW median NETD
values achieved are 10mK and 14mK for the 24μm and 20μm pitch arrays respectively. The corresponding LW median
NETD values are 23mK and 27mK respectively.
This paper describes the fabrication and performance of our LW Hawk arrays. These are Full-TV (640x512) LW infrared
detectors at small pitch (16 μm) made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE).
The detectors are staring, focal planes consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out
circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between wider band-gap
cladding layers. Device processing is wafer-scale. This is an extension of the work reported in previous years with the
innovation of dry etching for mesa isolation. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology will be described. Results will be presented which show operability of 99.96% with a median NETD of
32 mK, reducing to 22 mK in binning mode. The results of various imaging trials will also be presented.
Selex Sensors and Airbourne Systems has been active in developing Very Long Wave arrays for space applications
under a contract of the European Space Agency. Arrays have been demonstrated with a 15 μm cut-off operating at 55 K.
The technology is an extension of our standard LW, described elsewhere, using MOVPE layers grown on GaAs to
provide a low cost, large area capability with state-of-the-art performance. The test vehicle for the VLW development is
a direct injection 320 x 256, 30 μm pitch ROIC with a well capacity of 20 million electrons. While it may be considered
that direct injection is not ideal for typical diode impedances expected in the VLW, and alternatives are in design, it is a
testament to our technology that the diodes have sufficient dynamic resistance to allow this approach.
Our diode design provides low diffusion currents such that at these operating temperatures the arrays are largely limited
by trap assisted tunnelling (TAT). Results of dark current as a function of voltage and temperature will be presented
along with the array electro-optical performance.
There is considerable interest in sensors which are optimised for detecting infrared radiation outside the normal thermal
bands (3-12μm). This paper presents the development of photodiode arrays in Hg<sub>1-x</sub>Cd<sub>x</sub>Te (MCT) that are sensitive in the
very long wave (VLW) band to 14μm or in the visible and SWIR band below 2.5μm wavelength.
The VLW arrays are heterostructure diodes fabricated from MCT grown by Metal Organic Vapour Phase Epitaxy
(MOVPE). These are staring, focal plane arrays of mesa-diodes bump bonded to silicon read-out circuits. Measurements
are presented demonstrating state-of-the-art performance over the temperature range 55-80K, for detectors with a cut-off
wavelength of up to 14μm (at 77K).
The SWIR/Visible detectors consist of an array of loophole photodiodes fabricated using MCT grown by Liquid Phase
Epitaxy (LPE). The technology is suited to imaging LIDAR, NIR/Visible imaging, spectroscopy or hyperspectral
applications. The diodes operate as avalanche photodiodes (APDs) which provides near-ideal gain in the pixel.
Measurements are presented demonstrating state-of-the-art performance in the range 80K-200K from arrays with a cut-off
Supporting technologies are also discussed. Silicon circuitry must be implemented in the SWIR and VLW bands that is
appropriate to avalanche operation or copes with the low photon flux or low photodiode impedance. Trade-offs between
conventional direct injection (DI), buffered direct injection (BDI), pixel capacitive transimpedance amplifier (CTIA) and
source-follower per detector (SFPD) are presented. Work is in progress to increase the MOVPE wafer size to 6" which
will enable large area arrays to be produced in the SW, MW, LW and VLW bands.
Following the development of 1<sup>st</sup> Generation systems in the 1970s, thermal imaging has been in service with the UK
armed forces for over 25 years and has proven itself to be a battle winning technology. More recently the wider
accessibility to similar technologies within opposing forces has reduced the military advantage provided by these 1st
Generation systems and a clear requirement has been identified by the UK MOD for thermal imaging sensors providing
increased detection, recognition and identification (DRI) ranges together with a simplified logistical deployment burden
and reduced through-life costs.
In late 2005, the UK MOD initiated a programme known as "Albion" to develop high performance 3<sup>rd</sup> Generation single
waveband infrared detectors to meet this requirement. At the same time, under a separate programme supporting higher
risk technology, a dual waveband infrared detector was also developed. The development phase of the Albion
programme has now been completed and prototype detectors are now available and have been integrated into
demonstration thermal imaging cameras. The Albion programme has now progressed into the second phase,
incorporating both single and dual waveband devices, focussing on low rate initial production (LRIP) and qualification
of the devices for military applications.
All of the detectors have been fabricated using cadmium mercury telluride material (CMT), grown by metal organic
vapour phase epitaxy (MOVPE) on low cost, gallium arsenide (GaAs) substrates and bump bonded to the silicon read
out circuit (ROIC). This paper discusses the design features of the 3<sup>rd</sup> Generation detectors developed in the UK together
with the results obtained from the prototype devices both in the laboratory and when integrated into field deployable
thermal imaging cameras.
This paper describes the design, fabrication and performance of dual-band MW/LW infrared detectors made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays
consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out circuits. Each mesa has one connection to the
ROIC and the bands are selected by varying the applied bias.
Arrays of 320x256 pixels on a 30 μm pitch have performed exceedingly well. For example, arrays with a cut-off
wavelength of 5 μm in the MW (mid-wave) band and 10 μm in the LW (long-wave) band have median NETDs of 10 and
17 mK and defect levels of 0.3% and 0.05%, in the MW and LW bands respectively. Interestingly the LW defect level is
often lower than the MW defect level and the defects are not correlated; i.e. a pixel that is defective in the MW band is
usually not defective in the LW band.
Arrays of 640x512 pixels on a 24 μm pitch have been developed. These use a read-out integrated circuit (ROIC) that has
two capacitors per pixel and the ability to switch bands during a frame giving quasi-simultaneous images. The
performance of these arrays has been excellent with NETDs of 14mK in the MW band and 23mK in the LW band. Dual
band-pass filters have been designed and built into a detector.
This paper describes long wavelength (LW) infra-red detectors made from HgCdTe grown by Metal Organic Vapour
Phase Epitaxy (MOVPE) and the performance in a low photon flux background compatible with a multispectral
requirement. The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to
silicon read-out circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between
wider band-gap cladding layers. Device processing is wafer-scale. Wet etching is used to define the mesas and the mesa
sidewalls are passivated with inter-diffused CdTe. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology is sufficiently advanced to enable production not only of LWIR detectors but also dual band
MWIR/LWIR detectors, as reported last year. Cameras for both types have been developed.
There is now increasing interest in using the technology for LWIR multispectral imaging. Due to the requirement for
narrow bandwidths, resulting in low radiant flux, the diode quality, in terms of dark current and resistance, must be
exceptionally good. This requirement has been difficult to achieve in many technologies, however MOVPE grown
MCT has consistently provided LWIR arrays with the necessary low dark current and high resistance. Performance from
arrays of size 640x512 with 24 μm pixels and having a cut-off of 10 μm will be described. These achieve diode
impedances of several GΩ's with less than 1 nA dark current at 90K.
This paper describes the fabrication and performance of affordable LW infrared focal plane arrays (IRFPAs) made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated
circuits (ROICs). The growth substrate is GaAs, being readily available from several sources and suitable for wafer
scale processing. Arrays of size up to 640x512 at 24 μm pixel pitch have been produced, encapsulated, and
demonstrated in a camera system. Arrays of this size are produced in n-on-p material, that is, the common layer is p-type.
This orientation is chosen from a contact technology viewpoint. It is shown that at higher biases trap-assisted
tunnelling (TAT) can limit the performance of arrays. This becomes an issue for large arrays at high infrared flux with a
p-type common layer due to its inherent higher sheet resistance compared to n-type, this can result in debiassing of the
central elements. The key is found to be the control of the MCT structure and quality to ensure good diode performance
with minimal TAT, allowing the higher biases needed to overcome debiassing.
The drive towards improved target recognition has led to an increasing interest in detection in more than one infrared band. This paper describes the design, fabrication and performance of two-colour and three-colour infrared detectors made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out integrated circuits (ROICs). Each mesa diode has one connection to the ROIC and the colours are selected by varying the applied bias. Results will be presented for both two-colour and three-colour devices.
In a two-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the doping and composition of the p-type layer are chosen to prevent transistor action. The bias polarity is used to switch the output between colours. This design has been used to make MW/LW detectors with a MW band covering 3 to 5 μm and a LW band covering 5 to 10 μm.
In a three-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the p-type absorber, which has an intermediate cut-off wavelength. The absorbers are separated from each other by electronic barriers consisting of wide band-gap material. At low applied bias these barriers prevent photo-electrons generated in the p-type absorber from escaping and the device then gives an output from one of the n-type absorbers. At high applied bias the electronic barrier is pulled down and the device gives an output from both the p-type absorber and one of the n-type absorbers. Thus by varying the polarity and magnitude of the bias it is possible to obtain three-colours from a two-terminal device. This design has been used to make a SW/MW/MW detector with cut-off wavelengths of approximately 3, 4 and 6 μm.
The first generation of high performance thermal imaging sensors in the UK was based on two axis opto-mechanical
scanning systems and small (4-16 element) arrays of the SPRITE detector, developed during the 1970s. Almost two
decades later, a 2nd Generation system, STAIRS C was introduced, based on single axis scanning and a long linear
array of approximately 3000 elements. The UK has now begun the industrialisation of 3<sup>rd</sup> Generation High Performance
Thermal Imaging under a programme known as "Albion". Three new high performance cadmium mercury telluride
arrays are being manufactured. The CMT material is grown by MOVPE on low cost substrates and bump bonded to the
silicon read out circuit (ROIC). To maintain low production costs, all three detectors are designed to fit with existing
standard Integrated Detector Cooling Assemblies (IDCAs). The two largest focal planes are conventional devices
operating in the MWIR and LWIR spectral bands. A smaller format LWIR device is also described which has a smart
ROIC, enabling much longer stare times than are feasible with conventional pixel circuits, thus achieving very high
sensitivity. A new reference surface technology for thermal imaging sensors is described, based on Negative
Luminescence (NL), which offers several advantages over conventional peltier references, improving the quality of the
Non-Uniformity Correction (NUC) algorithms.
The first generation of high performance thermal imaging sensors in the UK was based on two axis opto-mechanical scanning systems and small (4-16 element) arrays of the SPRITE detector, developed during the 1970s. Almost two decades later, a 2nd Generation system, STAIRS C was introduced, based on single axis scanning and a long linear array of approximately 3000 elements. This paper addresses the development of the UK's 3rd Generation High Performance Thermal Imaging sensor systems, under a programme known as "Albion". Three new high performance detectors, manufactured in cadmium mercury telluride, operating in both MWIR and LWIR, providing high resolution and sensitivities without need for opto-mechanical scanning systems will be described. The CMT material is grown by MOVPE on low cost substrates and bump bonded to the silicon read out circuit (ROIC). All three detectors are designed to fit with existing standard Integrated Detector Cooling Assemblies (IDCAs). The two largest detectors will be integrated with field demonstrator cameras providing MWIR and LWIR solutions that can rapidly be tailored to specific military requirements. The remaining detector will be a LWIR device with a smart ROIC, facilitating integration times much longer than can typically be achieved with focal plane arrays and consequently yield very high thermal sensitivity. This device will be demonstrated in a lab based camera system.
In recent years IR detector technology has developed from early short linear arrays. Such devices require high performance signal processing electronics to meet today's thermal imaging requirements for military and para-military applications. This paper describes BAE SYSTEMS Avionics Group's Sensor Integrated Modular Architecture thermal imager which has been developed alongside the group's Eagle 640×512 arrays to provide high performance imaging capability. The electronics architecture also supprots High Definition TV format 2D arrays for future growth capability.
Medium wavelength IR arrays have been develoepd which have 1024×768 pixels on a 26 micron pitch. The arrays are made from epitaxially grown indium antimonide, the use of which confers two advantages over conventional InSb owing to the ability to exercise atomic level control of dopants and material thicknesses. Firstly, the photodiodes can be grown on degenerately doped InSb substrates which have a high degree of transparency, so the requirement for the substrate to be thinned is much reduce dleading to simplified manufacture. Secondly, it offers the potential for an increase in operating temperature of many tens of degrees, through elimination of contact leakage currents, though we focus on 80K performance here for comparison with conventional structures. We present initail results form arrays which indicate high operability, despite the need to stitch reticles in the fabrication of the silicon read-out circuit, and temperature sensitivity close to the theoretical limit. Imaging from the arrays compares very favorably with that taken using generation II cameras and gives confidence that this technology offers a cost effective route to large format MWIR systems.
Staring InSb FPAs grown by MBE have been demonstrated. Low growth temperatures have been employed to provide p<SUP>+</SUP>-n- n<SUP>+</SUP> photodiodes with a dark, 80 K R<SUB>O</SUB>A equals 9 X 10<SUP>5</SUP>(Omega) cm<SUP>2</SUP>. A degenerately doped substrate has been used to provide transparency in the 3.5 micrometer - 5.5 micrometer spectral region. Free carrier absorption necessitates some thinning of the substrate and an anti- reflection coated external quantum efficiency of 62% has been achieved with a final thickness of approximately equals 40 micrometer. 320 X 256 FPA's operating at 90 K and looking at a 295 K scene in f/2 have a noise equivalent temperature (NE(Delta) T) at half well of 10.4 mK. FPA operability exceeds 99.7%.
Surface resonant structures can be used as spectral filtering elements. Both band-stop and band-pass designs are possible using capacitive and inductive mesh concepts respectively. With the current evolution in electron lithographic technology to allow the realisation of sub-micron features, it has become possible in recent years to produce resonant mesh arrays to high degrees of fidelity and performance over significant areas of optical substrates. These can provide, for example, responses in the infra-red spectral region using elements with a dimensional scale of a few microns. The unit cell design in the mesh can be relatively complex and can be tailored to provide specific spectral responses in the waveband range of interest. Such structures can find a wide variety of applications, especially as dichroic beam splitter elements for the separation of infra-red and RF radiation. The possibility of using such structures for spectral filtering has been known for many years, particularly for the microwave, millimeter and far infra-red wavelength regime. Some of the earlier applications in the far JR have been discussed by UJriCh [1,21 More recently, Byrne et al [31 fabricated capacitive and inductive mesh filters for the infra-red using electron beam lithography and demonstrated the bandstop and bandpass transmissive properties achievable. Byrne and coworkers used a lift-off technique to create capacitive mesh patterns having linewidths of less than O.25j.tm in O.lp.m thick aluminium and gold on calcium fluoride. Crossed capacitive dipoles of aluminium 2.6.tm in length with an aspect ratio of 10:1 produced a broad reflection band at 6.25tm. The measured bandwidth (FWHM) was about 2.5pm. For inductive designs, a two level lift-off approach was adopted, involving pattern transfer from an initial capacitive design using oxygen reactive ion etch techniques. A 1.8pm length slot with aspect ratio of 10:1 produced a resonance at 6.5.tm with peak transmittance of about 80% and bandwidth of 2.75p.m. Low levels of intrinsic absorption in the slot material of the inductive mesh are enhanced by the resonant effect of the mesh and reduce the level of transmittance achievable. It is of interest from the point of view of potential high power laser applications to explore the relationship between the level of optical absorbance and the ensuing laser damage threshold of such meshes. Mohebi [71 explored the response of wire grid polarisers to pulsed C02 laser irradiation at 10.6j.m and found that damage thresholds were largest for the case when the polarisation of the incident radiation was perpendicular to the wires. Antirerfiection coatings reduced the damage threshold. This work assesses the case of crossed dipole meshes and explores the role of absorption by incorporating weakly absorbing films in the meshes and by the addition of dielectric material between the absorber and the antenna plane. It also explores the role of resistive loss in determining the properties of capacitive meshes. The absorber used has optical constants of n = 2.9 - 0.25i which are typical of some transition metal oxides and chalcogenide materials.