Erika T. Hamden

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

Atomically precise surface engineering of silicon CCDs for enhanced UV quantum efficiency

The authors report here on a new technique, combining the atomic precision of molecular beam epitaxy and atomic layer deposition, to fabricate back illuminated silicon CCD detectors that demonstrate world record detector quantum efficiency (>50%) in the near and far ultraviolet (155–300u2009nm). This report describes in detail the unique surface engineering approaches used and demonstrates the robustness of detector performance that is obtained by achieving atomic level precision at key steps in the fabrication process. The characterization, materials, and devices produced in this effort will be presented along with comparison to other approaches.

High-efficiency UV/optical/NIR detectors for large aperture telescopes and UV explorer missions: development of and field observations with delta-doped arrays

Abstract. Exciting concepts are under development for flagship, probe class, explorer class, and suborbital class NASA missions in the ultraviolet/optical spectral range. These missions will depend on high-performance silicon detector arrays being delivered affordably and in high numbers. To that end, we have advanced delta-doping technology to high-throughput and high-yield wafer-scale processing, encompassing a multitude of state-of-the-art silicon-based detector formats and designs. We have embarked on a number of field observations, instrument integrations, and independent evaluations of delta-doped arrays. We present recent data and innovations from JPL’s Advanced Detectors and Systems Program, including two-dimensional doping technology, JPL’s end-to-end postfabrication processing of high-performance UV/optical/NIR arrays and advanced coatings for detectors. While this paper is primarily intended to provide an overview of past work, developments are identified and discussed throughout. Additionally, we present examples of past, in-progress, and planned observations and deployments of delta-doped arrays.

Charge-coupled devices detectors with high quantum efficiency at UV wavelengths

Abstract. We report on multilayer high efficiency antireflection coating (ARC) design and development for use at UV wavelengths on CCDs and other Si-based detectors. We have previously demonstrated a set of single-layer coatings, which achieve >50% quantum efficiency (QE) in four bands from 130 to 300 nm. We now present multilayer coating designs that significantly outperform our previous work between 195 and 215 nm. Using up to 11 layers, we present several model designs to reach QE above 80%. We also demonstrate the successful performance of 5 and 11 layer ARCs on silicon and fused silica substrates. Finally, we present a five-layer coating deposited onto a thinned, delta-doped CCD and demonstrate external QE greater than 60% between 202 and 208 nm, with a peak of 67.6% at 206 nm.

Noise and dark performance for FIREBall-2 EMCCD delta-doped CCD detector

The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) is an experiment designed to observe low density emission from HI, CIV, and OVI in the circum-galactic medium around low-redshift galaxies. To detect this diffuse emission, we use a high-efficiency photon-counting EMCCD as part of FIREBall-2s detector. The flight camera system includes a custom printed circuit board, a mechanical cryo-cooler, zeolite and charcoal getters, and a Nüvü controller, for fast read-out speeds and waveform shaping. Here we report on overall detector system performance, including pressure and temperature stability. We describe dark current and CIC measurements at several temperatures and substrate voltages, with the flight set-up.

Detector performance for the FIREBall-2 UV experiment

We present an overview of the detector for the upcoming Faint Intergalactic Red-shifted Emission Balloon (FIREBall-2) experiment, with a particular focus on the development of device-integrated optical coatings and detector quantum efficiency (QE). FIREBall-2 is designed to measure emission from the strong resonance lines of HI, OVI, and CIV, all red-shifted to 195-225 nm window; its detector is a delta-doped electron multiplying charge coupled device (EM-CCD). Delta-doped arrays, invented at JPL, achieve 100% internal QE from the UV through the visible. External losses due to reflection (~70% in some UV regions) can be mitigated with antireflection coatings (ARCs). Using atomic layer deposition (ALD), thin-film optical filters are incorporated with existing detector technologies. ALD offers nanometer-scale control over film thickness and interface quality, allowing for precision growth of multilayer films. Several AR coatings, including single and multi-layer designs, were tested for FIREBall-2. QE measurements match modeled transmittance behavior remarkably well, showing improved performance in the target wavelength range. Also under development are ALD coatings to enhance QE for a variety of spectral regions throughout the UV (90-320 nm) and visible (320-1000 nm) range both for space-based imaging and spectroscopy as well as for ground-based telescopes.

The faint intergalactic-medium red-shifted emission balloon: future UV observations with EMCCDs

We present the latest developments in our joint NASA/CNES suborbital project. This project is a balloon-borne UV multi-object spectrograph, which has been designed to detect faint emission from the circumgalactic medium (CGM) around low redshift galaxies. One major change from FIREBall-1 has been the use of a delta-doped Electron Multiplying CCD (EMCCD). EMCCDs can be used in photon-counting (PC) mode to achieve extremely low readout noise (¡ 1e-). Our testing initially focused on reducing clock-induced-charge (CIC) through wave shaping and well depth optimisation with the CCD Controller for Counting Photons (CCCP) from Nüvü. This optimisation also includes methods for reducing dark current, via cooling and substrate voltage adjustment. We present result of laboratory noise measurements including dark current. Furthermore, we will briefly present some initial results from our first set of on-sky observations using a delta-doped EMCCD on the 200 inch telescope at Palomar using the Palomar Cosmic Web Imager (PCWI).

Keck/Palomar Cosmic Web Imagers Reveal an Enormous Lyα Nebula in an Extremely Overdense Quasi-stellar Object Pair Field at z = 2.45

Enormous Lyα nebulae (ELANe) represent the extrema of Lyα nebulosities. They have detected extents of >200 kpc in Lyα and Lyα luminosities >10^(44) erg s^(−1). The ELAN population is an ideal laboratory to study the interactions between galaxies and the intergalactic/circumgalactic medium (IGM/CGM) given their brightness and sizes. The current sample size of ELANe is still very small, and the few z ≈ 2 ELANe discovered to date are all associated with local overdensities of active galactic nuclei (AGNs). Inspired by these results, we have initiated a survey of ELANe associated with quasi-stellar object (QSO) pairs using the Palomar and Keck Cosmic Web Imagers (PCWI/KCWI). In this Letter, we present our first result: the discovery of ELAN0101+0201 associated with a QSO pair at z = 2.45. Our PCWI discovery data shows that, above a 2σ surface brightness of 1.2 × 10^(−17) erg s^(−1) cm^(−2) arcsec^(−2), the end-to-end size of ELAN0101+0201 is ≳ 232 kpc. We have conducted follow-up observations using KCWI, resolving multiple Lyα emitting sources within the rectangular field of view of ≈ 130 × 165 projected kpc^2, and obtaining their emission line profiles at high signal-to-noise ratios (S/Ns). Combining both KCWI and PCWI, our observations confirm that ELAN0101+0201 resides in an extremely overdense environment. Our observations further support that a large amount of cool (T ~ 10^4 K) gas could exist in massive halos (M ≳ 10^(13) M_⊙) at z ≈ 2. Future observations on a larger sample of similar systems will provide statistics of how cool gas is distributed in massive overdensities at high redshift and strongly constrain the evolution of the intracluster medium.

The Faint Intergalactic Redshifted Emission Balloon-2: End-to-end ground Calibration (Conference Presentation)

The circumgalactic medium (CGM) plays a critical role in the evolution of galaxy discs, as it hosts important mechanisms regulating their replenishment through inflows and outflows. Besides absorption spectroscopy, mapping of the HI Lyα emission of z>2 CGM is bringing a new perspective with a complete 2- or 3-D mapping. Despite this benefit, data in emission are very scarce in the large time span from z∼2 to the present because of the difficulties inherent to vacuum UV observations. The FIREBall-2 (Faint Intergalactic Redshifted Emission Balloon) instrument has been developed to help fill this gap and is scheduled for launch in September 2018. It has been optimized to provide a bi-dimensional (x, λ) map of the extremely faint diffuse Ly-a HI emission in the CGM at z∼0.7 and has the capability to observe ~200 galaxies and a dozen QSOs in a single night flight. Given its wide field of view (FOV) of 20x40 arcmin2, its angular resolution of 6-10 arcsec and spectral resolution above 1000, FIREBall-2 will bring important insights about the gas distribution in the CGM, and the velocity/temperature fields, and has the potential to bring statistical constraints.u2028The instrument is a balloon-borne 1m telescope coupled to a UV multi-object spectrograph (MOS) designed to image the CGM in emission via specific spectral lines (Lya, CIV, OVI) redshifted in a narrow UV band [1990 - 2130]A for the nearby universe (0.2< z <1). The optical design relies on a 1.2-meter moving siderostat that stabilizes the beam and reflects the light on a fixed paraboloid which in turn images it at the entrance of the payload. This payload is constituted of a focal corrector followed by a slit Multi-Object Spectrograph (reflective 2400 g/mm holographic aspherical grating located between two Schmidt mirrors). The objects selection is achieved with a series of pre-installed precision mask systems that also feed the fine guidance camera. The detector is a e2v electron multiplying CCD coated and delta-doped by the Jet Propulsion Laboratory. FIREBall-2 is funded by CNES and NASA and is developed in cooperation with a Franco-American consortium composed of LAM, CALTECH, Columbia University, JPL and CST-CNES. In this presentation, we describe the final ground calibration of the instrument. We explain what technical specifications ensue from the scientific goals of the mission and we will then highlight why this optical design has been chosen. The calibration of the instrument (alignment - through focus - distortion) will be presented followed by the analysis of the instrument scientific performances. We will then describe the improvement and the calibration of the ZEMAX-coupled instrument model developed at LAM, based on these final performances. This model is finally used to make an end-to-end prediction of the observations of the emission of the CGM from a large halo in a cosmological simulation.

Fireball multi object spectrograph: as-built optic performances

Fireball (Faint Intergalactic Redshifted Emission Balloon) is a NASA/CNES balloon-borne experiment to study the faint diffuse circumgalactic medium from the line emissions in the ultraviolet (200 nm) above 37 km flight altitude. Fireball relies on a Multi Object Spectrograph (MOS) that takes full advantage of the new high QE, low noise 13 μm pixels UV EMCCD. The MOS is fed by a 1 meter diameter parabola with an extended field (1000 arcmin2) using a highly aspherized two mirror corrector. All the optical train is working at F/2.5 to maintain a high signal to noise ratio. The spectrograph (R~ 2200 and 1.5 arcsec FWHM) is based on two identical Schmidt systems acting as collimator and camera sharing a 2400 g/mm aspherized reflective Schmidt grating. This grating is manufactured from active optics methods by double replication technique of a metal deformable matrix whose active clear aperture is built-in to a rigid elliptical contour. The payload and gondola are presently under integration at LAM. We will present the alignment procedure and the as-built optic performances of the Fireball instrument.