This PDF file contains the front matter associated with SPIE Proceedings Volume 8033, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Many applications require high performance Single Photon Avalanche Diodes (SPAD) either as single pixels or as small arrays of detectors. Although currently available silicon devices reached remarkable performance, nevertheless further improvements are needed in order to meet the requirements of most demanding time-resolved techniques. In this paper we present a new planar silicon technology for the fabrication of SPAD detectors, aimed at improving the Photon Detection Efficiency (PDE) of classical thin SPAD in the near infrared range while maintaining a good Temporal Resolution (TR). Experimental characterization showed a significant increase in the PDE with a remarkable value of 40% at 800nm; a photon timing jitter as low as 93ps FWHM as been also attained, while other device performances, such as Dark Count Rate (DCR) and Afterpulsing Probability (AP) are essentially unchanged, compared to thin SPAD. Being planar, the new technology is also intrinsically compatible with the fabrication of arrays of detectors.

Single Photon Avalanche Diodes (SPADs) are valuable detectors in numerous photon counting applications in the fields of quantum physics, quantum communication, astronomy, metrology and biomedical analytics. They typically feature a much higher photon detection efficiency than photomultiplier tubes, most importantly in the red to near-infrared range of the spectrum. Very often SPADs are combined with Time-Correlated Single Photon Counting (TCSPC) electronics for time-resolved data acquisition and the temporal resolution ("jitter") of a SPAD is therefore one of the key parameters for selecting a detector. We show technical data and first application results from a new type of red sensitive single photon counting module ("τ-SPAD"), which is targeted at timing applications, most prominently in the area of Single Molecule Spectroscopy (SMS). The τ-SPAD photon counting module combines Laser Components' ultra-low noise VLoK silicon avalanche photodiode with specially developed quenching and readout electronics from PicoQuant. It features an extremely high photon detection efficiency of 75% at 670 nm and can be used to detect single photons over the 400 nm to 1100 nm wavelength range. The timing jitter of the output of the τ-SPAD can be as low as 350 ps, making it suitable for time-resolved fluorescence detection applications. First photon coincidence correlation measurements also show that the typical breakdown flash of SPADs is of comparably low intensity for these new SPADs.

Sub-band absorption at 1550 nm has been demonstrated and characterized on silicon Geiger mode detectors, which normally would be expected to have no response at this wavelength. We compare responsivity measurements to single-photon absorption for wavelengths slightly above the band gap wavelength of silicon (~1100 μm). One application for this low efficiency sub-band absorption is in deep space optical communication systems where it is desirable to track a 1030 nm uplink beacon on the same flight terminal detector array that monitors a 1550 nm downlink signal for pointing control. The currently observed absorption at 1550 nm provides 60-70 dB of isolation compared to the response at 1064 nm, which is desirable to avoid saturation of the detector by scattered light from the downlink laser.

Frequency up-conversion technology can be used to increase detection efficiency for near infrared photons, as has been demonstrated in fiber-based quantum communication systems. In a continuous wave pumped up-conversion detector, the temporal resolution is limited by the timing jitter of the detector in the visible range, which limits the maximum clock rate of a quantum communication system. In this paper we describe a scheme to improve the temporal resolution of an up-conversion single-photon detector using multi-wavelength optical-sampling techniques, allowing for increased transmission rates in single-photon communications systems. We experimentally demonstrate our approach with an up-conversion detector using two spectrally and temporally distinct pump pulses, and show that it allows for high-fidelity single-photon detection at twice the rate supported by a conventional single-pump up-conversion detector.

We present a "smart-pixel" suitable for implementation of monolithic single-photon imaging arrays aimed at 3D ranging applications by means of the direct time-of-flight detection (like LIDAR systems), but also for photon timing applications (like FLIM, FCS, FRET). The pixel includes a Single-Photon Avalanche Diode (SPAD) and a Time-to-Digital Converter (TDC) monolithically designed and manufactured in the same chip, and it is able to detect single photons and to measure in-pixel the time delay between a START signal (e.g. laser excitation, LIDAR flash) and a photon detection (e.g. back reflection from a target object). In order to provide both wide dynamic range, high time resolution and very high linearity, we devised a TDC architecture based on an interpolation technique. A "coarse" counter counts the number of reference-clock rising-edges between START and STOP, while high resolution is achieved by means of two interpolators, which measure the time elapsed between START (and STOP) signal and a successive clock edge. In an array with many pixels, multiple STOP channels are needed while just one START channel is necessary if the START event is common to all channels. We report on the design and characterization of prototype circuits, fabricated in a 0.35 μm standard CMOS technology containing complete conversion channels (i.e. 20-μm active-area diameter SPAD, quenching circuitry, and TDC). With a 100 MHz reference clock, the TDC provides a time resolution of 10 ps, a dynamic range of 160 ns and DNL < 1% LSB rms.

A family of scaleable single photon avalanche diode (SPAD) structures in 130nm and 90nm CMOS is presented. Performance trends such as dark count rate (DCR), jitter and breakdown voltage are studied versus active diameter for devices ranging from 32μm down to 2μm. To address pixel pitch we introduce a shared buried n-well approach allowing compact arrays containing both NMOS-transistor readout circuitry and SPAD devices. A pixel pitch of 5μm has been achieved in 90nm CMOS technology, offering the potential for future megapixel single photon image sensors.

We have demonstrated a wafer-scale back-illumination process for silicon Geiger-mode avalanche photodiode arrays using Molecular Beam Epitaxy (MBE) for backside passivation. Critical to this fabrication process is support of the thin (< 10 μm) detector during the MBE growth by oxide-bonding to a full-thickness silicon wafer. This back-illumination process makes it possible to build low-dark-count-rate single-photon detectors with high quantum efficiency extending to deep ultraviolet wavelengths. This paper reviews our process for fabricating MBE back-illuminated silicon Geigermode avalanche photodiode arrays and presents characterization of initial test devices.

Direct-detection three-dimensional imaging laser radar system using Geiger-mode av.alanche photodiode (GAPD) is investigated in order to acquire three-dimensional images of objects at a long distance (more than 100m). Due to extremely high sensitivity of the GAPD, a laser radar system using GAPD is not only advantageous in terms of ranging a distant object but also in detecting a target screened by a sparse obstacle located in front of it. Both laser radar systems using a single-pixel GAPD and 1x8-pixel GAPD focal plane array as detectors are built up and analyzed. Passively Q-switched microchip laser is used as a laser source and a compact peripheral component interconnect system, which includes a time-to-digital converter (TDC), is set up. With both the GAPD having short dead-time (45ns) and the TDC functioning multi-stop acquisition, the system operates in a multi-hit mode. Three-dimensional images taken by the laser radar systems are shown. Both the single-shot precision and the dependence of the precision on the effective number of laser pulses are shown. Range walk reduction and autofocus techniques are proposed and demonstrated experimentally; they improve the accuracy and transverse spatial resolution of the laser radar system, respectively.

Typical commercially available airborne lidar systems utilise analogue avalanche photodiodes to detect the return pulses but can be restricted to operation at low altitudes because of the weak pulse energies associated with returns from Lambertian surfaces. The ultra high sensitivity of single photon avalanche diodes have been demonstrated in many applications such as time-resolved photoluminescence and quantum key distribution and with their ability to detect single photons with high efficiency these devices are potential candidates for use in high altitude lidar systems. However, the long hold-off times of these devices has been a cause for concern in an application where rapid data collection is necessary. This paper discusses the use of single photon avalanche diodes in high altitude lidar systems. The general requirements for a lidar system are described and the performance of single photon avalanche diode devices is predicted for both static and scanned lidar systems.

We present results obtained from 3D imaging focal plane arrays (FPAs) employing planar-geometry InGaAsP/InP Geiger-mode avalanche photodiodes (GmAPDs) with high-efficiency single photon sensitivity at 1.06 μm. We report results obtained for new 32 x 128 format FPAs with 50 μm pitch and compare these results to those obtained for 32 x 32 format FPAs with 100 μm pitch. We show excellent pixel-level yield-including 100% pixel operability-for both formats. The dark count rate (DCR) and photon detection efficiency (PDE) performance is found to be similar for both types of arrays, including the fundamental DCR vs. PDE tradeoff. The optical crosstalk due to photon emission induced by pixel-level avalanche detection events is found to be qualitatively similar for both formats, with some crosstalk metrics for the 32 x 128 format found to be moderately elevated relative to the 32 x 32 FPA results. Timing jitter measurements are also reported for the 32 x 128 FPAs.

Multi-dimensional Time Correlated Single Photon Counting has nowadays reached a prominent position among analytical techniques employed in the medical and biological fields. The development of instruments able to perform simultaneously temporal and spectral fluorescence analysis (sFLIM) is limited by the performances of single-photon detectors; in fact currently available arrays cannot satisfy simultaneously all the requirements. To face this rising quest, a fully-parallel eight channel module, based on a monolithic Single Photon Avalanche Diode (SPAD) array with great temporal resolution, high Photon Detection Efficiency (PDE) and low Dark Counting Rate (DCR), has been designed and fabricated. The system relies on a novel architecture of the single pixel, based on the integration of the timing pick-up circuit next to the photodetector, making the negative effects of electrical and optical crosstalk on photon timing performance negligible. To this aim, the custom technological process used to fabricate the SPAD has been modified, allowing the integration of MOS transistors without impairing the structure and the performance of the detector. The single channel is complemented by an external Active Quenching Circuit, fabricated in a standard CMOS technology, that ensures high maximum counting rate (> 5MHz) and low afterpulsing (< 2%). Finally, the output timing signals are read and conditioned by a proper CMOS electronics. The complete pixel shows a very good temporal resolution of about 45 ps (FWHM).

We adapt a previously-demonstrated gating technique for InGaAs SPADs to enable double-bias-pulse measurements of afterpulsing at nanosecond time scales with gate durations down to 500 ps. We present preliminary results for afterpulse probabilities below 10 ns, time scales comparable to those in the self-differencing technique, and show that afterpulse probabilities low enough to support reliable counting above 100 MHz can be observed.

In recent years substantial effort has been made in material growth, device design and fabrication, and driving circuitry to improve the performance of InGaAs/InP single photon avalanche diodes (SPADs) operated in Geiger mode. Despite these efforts, InGaAs/InP SPADs are constrained by certain performance limitations due to the inherent positive feedback involved in the avalanche process. With the goal of overcoming some of these performance limitations, we have successfully designed and implemented thin film resistors monolithically integrated with InGaAs/InP SPADs to provide a negative feedback mechanism to regulate the avalanche sizes. The monolithic integration scheme ensures very small parasitic effects, results in fast quenching of avalanches, and allows for wafer-level integration which facilitates the fabrication of array structures. We will discuss the design and operation of NFAD devices and performance characterization of these devices. Basic characteristics of NFADs such as pulse response, quenching and recovery dynamics will be described. We will also present device performance parameters such as photon detection efficiency (PDE), dark count rate (DCR) and afterpulsing probability (P_ap). InGaAs/InP negative feedback avalanche diodes with different device sizes and quenching resistances have been designed and fabricated. Devices with ~10% PDE and acceptable P_ap has been realized, which provides a simple, practical solution for certain photon-counting applications.

Linear mode photon counting (LMPC) provides significant advantages in comparison with Geiger Mode (GM) Photon Counting including absence of after-pulsing, nanosecond pulse to pulse temporal resolution and robust operation in the present of high density obscurants or variable reflectivity objects. For this reason Raytheon has developed and previously reported on unique linear mode photon counting components and modules based on combining advanced APDs and advanced high gain circuits. By using HgCdTe APDs we enable Poisson number preserving photon counting. A metric of photon counting technology is dark count rate and detection probability. In this paper we report on a performance breakthrough resulting from improvement in design, process and readout operation enabling >10x reduction in dark counts rate to ~10,000 cps and >10⁴x reduction in surface dark current enabling long 10 ms integration times. Our analysis of key dark current contributors suggest that substantial further reduction in DCR to ~ 1/sec or less can be achieved by optimizing wavelength, operating voltage and temperature.

A linear mode photon counting FPA using HgCdTe MWIR cutoff e-APDs has been designed, fabricated, and characterized. The broad spectral range (0.4 μm to 4.3 μm) is unique among photon counters, making this a "first of its kind" system spanning the visible to the MWIR. The low excess noise ((F(M) ≈ 1) of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500 to 1000. The ROIC design included a very high gain-bandwidth product RTIA (3x10¹¹ Ohm-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high bandwidth analog and 16 LVDS digital outputs. The 2x8 array was integrated into an LN2 Dewar with a custom LCC and daughter board design that preserved high bandwidth analog and digital signal integrity. The 2x8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured APD gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse SNR of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a background limited false event rate (FER) of about 1 MHz. The measured jitter was in the range of 550 ps to 800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.

Linear-Mode Photon Counting (LMPC) detection requires a combined system consisting of a semiconductor avalanche photodiode (APD), a high-gain low-noise amplifier, and a comparator circuit. Modeling these aspects of the system requires a combination of semiconductor detector theory, electronics circuit modeling, and classic decision theory. Because of the disparate skills involved, it is difficult to both model and build such devices. In this paper, we present an end-to-end model of the LMPC detector that contains all the required theory. As part of the decision theory aspect of LMPC technology, we present a three-dimensional Receiver Optimization Characteristic (ROC) curve that contains the key performance aspects of the LMPC as a function of the comparator threshold setting. We present nomenclature and specification methods that provide for unambiguous definitions of the combined-system detector performance for both the fabricators and users of LMPC technology. Finally, we apply the model to a noiseless-gain HgCdTe APD, ROIC, and comparator device being developed by DRS and GEOST in order to demonstrate the photon counting end result, as well as several key intermediate values in the signal chain.

Visible light photon counters (VLPCs) are solid-state devices providing high quantum efficiency (QE) photon detection (>88%) with photon number resolving capability and low timing jitter (~250 ps). VLPC features high QE in the 0.4-1.0μm wavelength range, as the main photon absorption mechanism is provided by electron-hole pair generation across the silicon bandgap. In this paper, we will discuss the optical and electrical operating principles of VLPCs, and propose a range of device optimization paths that improves various aspects of VLPC for advanced quantum optics and quantum information processing experiments, both in the UV and the telecom wavelength range.

Detection of single photons is crucial for a number of applications. Geiger photodiodes (GPD) provide large gains with an insignificant amount of multiplication noise exclusively from the diode. When the GPD is operated above the reverse bias breakdown voltage, the diode can avalanche due to charged pairs generated from random noise (typically thermal) or incident photons. The GPD is a binary device, as only one photon is needed to trigger an avalanche, regardless of the number of incident photons. A solid-state photomultiplier (SSPM) is an array of GPDs, and the output of the SSPM is proportional to the incident light intensity, providing a replacement for photomultiplier tubes. We have developed CMOS SSPMs using a commercial fabrication process for a myriad of applications. We present results on the operation of these devices for low intensity light pulses. The data analysis provides a measured of the junction capacitance (~150 fF), which affects the rise time (~2 ns), the fall time (~32 ns), and gain (>10⁶). Multipliers for the cross talk and after pulsing are given, and a consistent picture within the theory of operation of the expected dark current and photodetection efficiency is demonstrate. Enhancement of the detection efficiency with respect to the quantum efficiency at unity gain for shallow UV photons is measured, indicating an effect due to fringe fields within the diode structure. The signal and noise terms have been deconvolved from each other, providing the fundamental model for characterizing the behavior at low-light intensities.

We have developed a new Hybrid Photo-Detector (HPD) with relatively large effective area, fast time response, high timing resolution, high quantum efficiency and extremely low probability of afterpulse. A GaAsP (Gallium Arsenide Phosphide) photocathode type has high quantum efficiency approximately 45% around the wavelength of 500 nm, and the size of the effective area is 3 mm in diameter. The pulse height for single photon is approximately +2 mV with 50 ohm load impedance, and it can be easily observed with a fast response oscilloscope. The rise and fall times for impulse light are approximately 0.4 ns. The timing resolution for single photon for the GaAsP photocathode type was estimated to be approximately 80 ps.

An overview of the Intensified Photodiode (IPD) is presented with an emphasis on IPDs optimized for use in the 950nm to 1350nm spectral range for single photon detection applications. The theory of operation of the IPD, two different electron optics designs, and device performance for a multichannel, 4x4 pixel array, low jitter IPD optimized for operation at 1060nm are presented in this paper. Key results include greater than 15% quantum efficiency, large active area, and less than 550ps impulse response.

We report on a cross-disciplined, multi-institutional effort to develop large-scale 'frugal' photo-detectors capable of mm-scale space resolution and pico-second time resolution. This new R&D effort is being led by the High Energy Physics branch within DOE. The large-area fast photodetectors (LAPPD) being developed would have applications in many fields, including particle physics, astrophysics, nuclear sciences, and medical imaging. The basic approach uses novel inexpensive micro channel pores which have been functionalized using a technique called atomic layer deposition. A custom anode and fast electronics are used to readout the photodetector. High quantum efficiency photocathodes are also being explored. The R&D program includes detailed testing and end to end simulations.

Cross strip and cross delay line readout microchannel plate detectors in 18 mm, 25 mm and 40 mm active area formats including open face (UV/particle) and sealed tube (optical) configurations have been constructed. These have been tested with a field programmable gate array based electronics for single event encoding. Using small pore MCPs (6 μm) operated in a pair, we achieve gains of >1 x 10⁶ which is sufficient to provide spatial resolution of ~17 μm FHWM with the 18 mm and 40 mm cross strip readouts. New cross strip electronics can process high output event rates (> 4 MHz) with high spatial resolution, and self triggered event timing accuracy of ~1.5 ns for sealed tube XS optical sensors. A peak quantum efficiency of between 13% and 19% at 500 nm has been achieved with SuperGenII photocathodes with response from 400 nm to 900 nm for both cross strip and cross delay line sealed tubes. Local area counting rates of up to 40 kHz (100μm spot) have been attained with XS sealed tubes, along with image linearity and stability to better than 50 μm. 25mm cross delay line tubes achieve ~50 μm resolution and > 2 MHz output event rates.

We describe a superconducting transition edge sensor based on a nanoscale niobium detector element. This device is predicted to be capable of energy-resolved near-IR single-photon detection with a GHz count rate. The increased speed and sensitivity of this device compared to traditional transition edge sensors result from the very small electronic heat capacity of the nanoscale detector element. In the present work, we calculate the predicted thermal response time and energy resolution. We also discuss approaches for achieving efficient optical coupling to the sub-wavelength detector element using a resonant near-IR antenna.

We present our implementation of an optical homodyne detector used to measure optical Schrödinger cat states. We show how we minimized the losses and mode-mismatches associated with homodyne detection, which become important when measuring non-classical states of light. We present a pulse-shaping scheme applied to the local oscillator to improve its temporal mode overlap with the non-classical state.

To image astronomical objects, the Hanbury Brown Twiss (HBT) technique involves measuring intensity correlation for an array of telescopes. The correlation of the intensity fluctuations is a measure of the magnitude of the coherence and can be used to retrieve the intensity distribution of the source using the Van Cittert-Zernike theorem. For low spectral irradiance sources, coincidence counting using modern techniques can drastically reduce data storage/processing requirements as well as allowing for optimization of the effective SNR bandwidth. In counting Intensity Interferometry (II), count fluctuations are measured instead of intensity fluctuations as with an analog II. Those are the two II techniques currently reported in the literature. Since the successful width measurements of bright stars by HBT in the 70's, advances in detectors promise opportunities to apply II to dimmer non-point source objects. To improve SNRs, we propose a new data processing technique for measuring correlation in the low light regime that ensures maximum bandwidth allowed by the reproducibility of photon pulses.

On the basis of the theory of light scattering, photon correlation spectroscopy has been used for more than three decades to study ocular tissues. From first in-vitro experiments to study cataractogenesis, this approach has been extended to characterize semi-quantitatively in-vivo all the ocular tissues from cornea to retina and choroids. In order to acquire high quality measurement data from the experiments, serious attention has to be paid to the detector and processing system performance. Detector noise, sensitivity, dead time and afterpulsing lead to a direct or indirect corruption of the acquired correlation function whereas counting range and resolution should be optimized to take into account the wide variability of the ocular tissue optical characteristics.

A hybrid system, composed of a SPAD fabricated in a dedicated detector technology coupled to a CMOS readout ASIC, is presented. The SPADs under test have an active area of 380 μm², while the ASIC is built in a 0.35 μm CMOS technology and has 16 readout channels, each one featuring an active quenching circuit and four time-gated 8-bit counters, with programmable gate duration. In the paper we will discuss the Dark Count Rate, Gain and Afterpulsing Probability performances with respect to relevant system parameters, such as Overvoltage, Off Time and Precharge Time, as well as FLIM measurements with the system.

Solution-based single-molecule fluorescence spectroscopy is a powerful new experimental approach with applications in all fields of natural sciences. Two typical geometries can be used for these experiments: point-like and widefield excitation and detection. In point-like geometries, the basic concept is to excite and collect light from a very small volume (typically femtoliter) and work in a concentration regime resulting in rare burst-like events corresponding to the transit of a single-molecule. Those events are accumulated over time to achieve proper statistical accuracy. Therefore the advantage of extreme sensitivity is somewhat counterbalanced by a very long acquisition time. One way to speed up data acquisition is parallelization. Here we will discuss a general approach to address this issue, using a multispot excitation and detection geometry that can accommodate different types of novel highly-parallel detector arrays. We will illustrate the potential of this approach with fluorescence correlation spectroscopy (FCS) and single-molecule fluorescence measurements. In widefield geometries, the same issues of background reduction and single-molecule concentration apply, but the duration of the experiment is fixed by the time scale of the process studied and the survival time of the fluorescent probe. Temporal resolution on the other hand, is limited by signal-to-noise and/or detector resolution, which calls for new detector concepts. We will briefly present our recent results in this domain.

Using Time Resolved Emission (TRE) to measure electrical signals inside VLSI CMOS circuits in a non-invasive fashion is a very powerful technique. However, node scaling and the related supply voltage reduction have created significant challenges. In this paper, we investigate the limits of established and prototype single photon detectors for future low voltage applications. In particular the performances of a state of the art InGaAs Single Photon Avalanche Photodiode (SPAD) and Superconducting Single-Photon Detector (SSPD) are reported and compared for low voltage applications using test vehicles fabricated in IBM 65 nm and 45 nm SOI technologies.