R. Michael Redfern

Toward integrated single-photon-counting microarrays

Silicon, shallow junction, Geiger-mode avalanche photo- diodes (APDs) can be manufactured with complementary metal-oxide semiconductor (CMOS) compatible processing steps and provide single- photon-counting sensitivity. As we move toward providing integrated de- tection of increasingly nanoscopic-sized emissions, small-area detectors and arrays that can be easily integrated into marketable systems will be required. Geiger-mode diodes with diameters of 10, 15, and 20 mm are manufactured and the dark counts measured at 10 V above breakdown are 9, 95, and 990, respectively, at room temperature. The simulated and measured optical crosstalk is found to be significantly reduced for detec- tor pixel pitches beyond 300 mm. The activation energy of the dark count with temperature is found to be 0.58 eV, representing an order of mag- nitude drop in dark count for every 27°C decrease in temperature. The responsivity of the detectors, without antireflection coatings, is found to peak between 550 and 650 nm with a photon detection probability of 43% at 10 V above the breakdown voltage. The low dark counts of the detectors and high photon detection probability highlight the potential these detectors have for fluorescence decay experiments and also in future integrated photonic detection systems.

Characterization of Geiger Mode Avalanche Photodiodes for Fluorescence Decay Measurements

Geiger mode avalanche photodiodes (APD) can be biased above the breakdown voltage to allow detection of single photons. Because of the increase in quantum efficiency, magnetic field immunity, robustness, longer operating lifetime and reduction in costs, solid-state detectors capable of operating at non-cryogenic temperatures and providing single photon detection capabilities provide attractive alternatives to the photomultiplier tube (PMT). Shallow junction Geiger mode APD detectors provide the ability to manufacture photon detectors and detector arrays with CMOS compatible processing steps and allows the use of novel Silicon-on-Insulator(SoI) technology to provide future integrated sensing solutions. Previous work on Geiger mode APD detectors has focused on increasing the active area of the detector to make it more PMT like, easing the integration of discrete reaction, detection and signal processing into laboratory experimental systems. This discrete model for single photon detection works well for laboratory sized test and measurement equipment, however the move towards microfluidics and systems on a chip requires integrated sensing solutions. As we move towards providing integrated functionality of increasingly nanoscopic sized emissions, small area detectors and detector arrays that can be easily integrated into marketable systems, with sensitive small area single photon counting detectors will be needed. This paper will demonstrate the 2-dimensional and 3-dimensional simulation of optical coupling that occurs in Geiger mode APDs. Fabricated Geiger mode APD detectors optimized for fluorescence decay measurements were characterized and preliminary results show excellent results for their integration into fluorescence decay measurement systems.

Photon-counting arrays for spatially varying low-light-level signals

The capability of combining single photon counting with information on the spatial distribution of incoming photons is of great interest to many scientific disciplines such as astronomy and spectroscopy, where an accurate assessment of this information can be critical to the success of an experiment. Photomultiplier arrays and scintillators are usually used in these applications. However these electromechanical solutions need to be operated at high voltages (approximately kV), they tend to be very fragile, bulky and extremely expensive. A solution to this problem which utilizes silicon integrated circuit technology would alleviate a lot of these undesirable effects and would be low cost, small area and more robust. This makes the concept of a solid state photon counting array very attractive for a large number of applications. To date this has not been achieved because of the difficulties involved in integrating high voltage optical detectors into an integrated circuit. This paper shows how this can be achieved and indicates areas of future research which will enable the possibility of large area and high pixel count spatially resolved photon counting devices.

Geiger mode avalanche photodiodes for microarray systems

New Geiger Mode Avalanche Photodiodes (GM-APD) have been designed and characterized specifically for use in microarray systems. Critical parameters such as excess reverse bias voltage, hold-off time and optimum operating temperature have been experimentally determined for these photon-counting devices. The photon detection probability, dark count rate and afterpulsing probability have been measured under different operating conditions. An active- quench circuit (AQC) is presented for operating these GM- APDs. This circuit is relatively simple, robust and has such benefits as reducing average power dissipation and afterpulsing. Arrays of these GM-APDs have already been designed and together with AQCs open up the possibility of having a solid-state microarray detector that enables parallel analysis on a single chip. Another advantage of these GM-APDs over current technology is their low voltage CMOS compatibility which could allow for the fabrication of an AQC on the same device. Small are detectors have already been employed in the time-resolved detection of fluorescence from labeled proteins. It is envisaged that operating these new GM-APDs with this active-quench circuit will have numerous applications for the detection of fluorescence in microarray systems.

A modular camera system for high time-resolution photometry

A new modular high time resolution imaging camera system with sub-microsecond timing accuracy has been built in the Physics Dept. of NUI, Galway. The system was designed to be mounted on large telescopes for observing the temporal, spectral and polarisation characteristics of faint astronomical objects, such as optical pulsars. The camera system developed allows simultaneous and independent observing of multiple wavebands of emission from the target objects. This is achieved using optics that split images into their different spectral or polarisation components. The system currently incorporates a multi-anode microchannel array (MAMA) photon detecting and imaging camera with a time resolution of up to 100ns. This is combined with three high quantum efficiency avalanche photodiodes (APDs) with count rates of up to 16 million photons per second. The high time resolution recording system can allow for the removal of telescope tracking inaccuracy and wind shear off-line. This yields better PSFs for bright objects such as crowded globular star clusters. This combination of different detectors allows the system to be operated as a multi purpose, high QE, high time resolution system. The modular nature of the design electronics also allows the addition and removal of detectors without limiting the performance of other elements within the system. The data path is also designed so that archiving integrity is maintained while the data path is simultaneously used for real-time analysis and display systems. Future applications in the bio-medical imaging sector are envisaged for high time resolution fluorescence imaging, and astronomical polarisation studies.

Reflow soldering of fine-pitch devices using a Nd:YAG laser.

The drive towards miniaturization in electronics assembly has produced new devices with small lead pitch and high lead count which are difficult to bond reliably to substrates using conventional mass soldering technology. Micro-soldering using lasers has the potential to solve many of the problems involved. The results of Nd:YAG laser soldering trials on a ceramic chip with Kovar leads spaced at 0.025 in. are described. The results are compared with the predictions of a detailed finite-element thermal model of the systems and also with real-time thermocouple measurements of the temperature reached at specific locations during the soldering step. Implications for future work in this area are discussed.

Laser guide star: monitoring and light pollution

This paper summarizes three years of observations of the resonant optical backscatter of laser, used to produce a mesospheric sodium-layer laser guide star for an adaptive optics system. Observations were obtained from a neighboring telescope. The aim of this work was two-fold: to study the Na plume (altitude and profile variations) and to study the Rayleigh cone in order to achieve scattering measurements relevant to the light pollution created by a sodium laser guide star. We report on the short-term characteristics of the sodium layer and stress the consequences of these variations for Laser Guide Star Adaptive Optics System operations. From the measurements of the background intensity measured while observing the laser guide star and the top of the Rayleigh cone, we can derive information on the light pollution produced by the laser as well as the resulting implications for an observatory laser management policy. Information on the laser intensity, size and shape along the Rayleigh cone are also presented.

Versatile high-resolution imaging detector system for use in x-ray, EUV, and visible wavelength regions

The Rutherford Appleton Laboratory Photon Counting Detector (RALPCD) is a highly adaptable intensified imaging system with applications in the x-ray, EUV and visible wavelength regions. The detector comprises commercially available high gain microchannel plate intensifiers fiber optically coupled to CID or CCD cameras, to form a modular detector arrangement. Frames of data from the cameras are detected and centroided in a transputer parallel processor array where correction algorithms using look up tables are used to produce pattern free images at high resolution. Data from the applications are used to illustrate the performance and future advances are discussed.

An Ultra-High-Speed Stokes Polarimeter forAstronomy

Optical polarization is a valuable diagnostic tool in astrophysics, frequently enabling asymmetries in source regions, magnetic field configurations, and magnetic field strengths to be investigated. In the context of high time resolution astrophysics polarimetry is particularly valuable in understanding the nature of the optical pulsations from pulsars, and whilst the averaged (over many cycles) linear polarization of the Crab nebula pulsar was determined many years ago, no attempts have been made to determine the Stokes vector of single, individual, pulse cycles. This may be very important information in the investigation into the nature of the enhanced optical pulses associated with random giant radio pulses. In the case of the Crab nebula this requires an instrumental response time of no more than 100 μs, and, possibly, the capability of tagging individual photons with nanosecond resolution. This imposes certain constraints on the design of the polarimeter, which rules out all of the popular designs. We describe the design of a novel polarimeter which will be able to meet these constraints.This is based upon a design by Compain and Drevillon [1] in which their design is modified, by the use of a different glass and prism angles, to work over a wide bandwidth of 400–800 nm and with a high polarimetric efficiency.

Optimised Post-Exposure Image Sharpening Code for L3-CCD Detectors

As light from celestial bodies traverses Earths atmosphere, the wavefronts are distorted by atmospheric turbulence, thereby lowering the angular resolution of ground‐based imaging. Rapid time‐series imaging enables Post‐Exposure Image Sharpening (PEIS) techniques, which employ shift‐and‐add frame registration to remove the tip‐tilt component of the wavefront error—as well as telescope wobble, thus benefiting all observations. Further resolution gains are possible by selecting only frames with the best instantaneous seeing—a technique sometimes calling “Lucky Imaging”. We implemented these techniques in the 1990s, with the TRIFFID imaging photon‐counting camera, and its associated data reduction software. The software was originally written for time‐tagged photon‐list data formats, recorded by detectors such as the MAMA. This paper describes our deep re‐structuring of the software to handle the 2‐d FITS images produced by Low Light Level CCD (L3‐CCD) cameras, which have sufficient time‐series resolution (...