Pixelated Photodetectors Research Articles

Reducing non-effective pixel rate and nonuniformity in AlGaN solar-blind UV focal plane detectors by controlled curvature

A 1600 × 1280 small pixel photodetector was successfully fabricated to verify the controlled curvature of the focal plane array improving the non-effective pixel rate and non-uniformity of the detector. The stress strain generated during epitaxial growth was reduced by growing a 2 μm thick AlN stress correction layer on the backside of the AlGaN/sapphire system substrate. Compared with the material without an AlN stress correction layer on the backside, material curvature reduced from 7.252 μm to 2.740 μm over a length of 0.5 cm. The non-effective pixel rate and the non-uniformity of a focal plane array have been reduced. A process was proposed to reduce the impact of misalignment generated by the flip-chip interconnection process. A solar-blind UV focal plane array was hybridized with a readout integrated circuit using an unsymmetrical flip-chip interconnection process. There is no significant change in peak spectral response, nonuniformity, non-effective pixel rate, or responsivity.

Quantitative determination of fractional topological charge based on the rotational Doppler effect

The utilization of fractional-order vortex beams extends the diversity of optical field manipulation, permits for more flexible control over beam propagation, and provides novel applications in optical communications, edge enhancement imaging, and particle manipulation. However, compared with the integer-order vortex beams, the topological charge measurement techniques for fractional-order vortex beams are not well developed, impeding the further exploration of its applications. In this paper, the frequency signal of rotational Doppler effect and corresponding broadening behavior under the fractional-order vortex beam illumination were analyzed. When the fractional topological charge approaches a half integer, the broadening is minimized. Leveraging this relationship, we designed a phase-compensated scheme coupled with signal-to-noise ratio detection to realize the real-time fractional topological charge measurement. The single pixel photodetector was used and eliminated the need for two-dimensional image acquisition and analysis, ensuring efficient acquisition and quantitative analysis. Both theoretical and experimental results confirm the feasibility of this method, thereby advancing the comprehension of the optical Doppler effect and potentially paving the way for future investigations into fractional vortex beams.

High-performance single crystal diamond pixel photodetector with nanosecond rise time for solar-blind imaging

Solar-blind ultraviolet imaging detector is widely applied in many commercial and military fields, such as missile warning, guidance, ultraviolet communication, biomedical analysis, and police reconnaissance. High-performance pixel photodetector is the core of imaging technology. Therefore, its construction attracts intensive attentions in recent years. In this work, 8 × 8 planar pixels were constructed on high quality single crystal diamond using ultraviolet direct writing lithography. A typical pixel photodetector in metal-semiconductor-metal (MSM) structure achieved outstanding photo-response properties of a low dark current of 10−13–10−12 A at 50 V, a high light-dark current switching ratio over 105), a high responsivity of 22.6 A/W, and a decent specific detectivity of 4.2 × 1014 Jones. Moreover, the detector has a fast response time of 13 ns. Additional optoelectronic studies demonstrate the strong homogeneity and repeatability of the photodetectors array, enabling it to serve as a reliable solar-blind imaging photodetector with high spatial resolution.

Predictive analysis of the power spectral irradiance from blackbody radiation source using single pixel detector

Accurate spectral irradiance measurement in the near-infrared range is significant for the design and characterization of photodetector and photovoltaic cells. Approximation method is commonly used to solve for the input power using estimated spectral irradiance, where the dependency on wavelength and temperature remains uncertain. This study aims to determine the power spectrum at different radiation temperatures using a single pixel photodetector, taking into consideration factors such as transmission spectra of alumina radiator, CaF2 collimating lens, responsivity, and measured photocurrent information of photodetectors. Utilizing predictive mathematical model, five commercial photodetectors, including Silicon, Germanium, In0.53Ga0.47As, In0.73Ga0.27As, and In0.83Ga0.17As were used to solve for the power densities as a function of wavelengths at radiation temperatures of 1000 °C and 1500 °C. The spectral irradiance of photodetectors was determined with a percentage difference of <4.9 %, presenting an accurate power density estimation for the spectrum at a wide range of radiation temperatures. Power irradiance data obtained were validated in the narrow wavelength range with 1000 nm, 1400 nm, 1500 nm, and 2000 nm bandpass filters. The reported work demonstrates a simple and efficient way which could contribute to develop a cost-effective method of measuring and determining the spectrum irradiances of objects at different radiation temperatures. This predictive analysis method hopefully intensifies the progress of efforts to reduce the reliance on complex optoelectronic instruments in accurately solving power irradiance information.

3D Printed Lattice Template by Material Extrusion Technique for Fabrication of Pixelated Photodetector.

Rigid and flexible, pixelated ultraviolet photodetectors (PD) based on ZnO have been fabricated by material extrusion 3D printing technique. The photoresponse is studied in an out-of-plane configuration. An open lattice structure is printed using PLA over ITO/Glass substrate for rigid, and TPU over ITO/PET substrate for flexible PDs. ZnO slurry is filled selectively into the columnar matrix by the microdispensing technique. The optical detector printed on ITO/Glass substrate shows a sensitivity of 25 and responsivity of 1.55 nA/mW with a rise and decay time of 1.6 and 0.6 s, respectively. Similarly, the flexible PD printed using TPU lattice shows a sensitivity of 9.5 and responsivity of 0.38 nA/mW with a rise and decay time of 1.8 and 0.6 s, respectively. The charge transport mechanism is studied using band diagram analysis. 3D printed open lattice structure is found to be a potential template for sensor fabrication. This work demonstrates the capability of material extrusion 3D printing with an open lattice structure for the fabrication of high-resolution pixelated PDs.

Subwavelength Bayer RGB color routers with perfect optical efficiency

Abstract We introduce subwavelength color routers with perfect optical efficiency in a red-green-green-blue (RGGB) Bayer layout for solid state image sensors. This is the first demonstration of a subwavelength device concept that shows the full potential of color routing, i.e., perfect routing without loss of photons, with a broadband, polarization-independent, and angular robust response. As an example, we show a color router for 320 nm wide image sensor pixels, which are two times smaller than the smallest state-of-the-art pixels, that features perfect optical efficiency, i.e., no crosstalk between color channels, no reflections, and no leakage into non-photodetector regions, even though the pixel photodetectors are 2–3 times smaller than the wavelength of the incident light. Our color router outperforms all other color separation approaches and can replace the entire optical stack in solid state image sensors.

A Review of Single-Photon Avalanche Diode Time-of-Flight Imaging Sensor Arrays

Time-of-flight (ToF) sensors using light pulses or continuous waves allow accurate distance measurements. Three-dimensional imagers can be based on an array of timing or time-gated integration pixels. Single-photon avalanche diodes (SPADs) have been increasingly chosen as the pixel's photodetector device to develop fast, long-range ToF sensors. Solid-state ToF cameras are replacing other alternatives, showing attractive characteristics and bringing up new potential applications. This paper presents the technical evolution of SPAD ToF 3D imaging sensors, and provides insight into their development over the last few decades. Starting with the first SPAD imagers reported in the early 2000's, various direct and indirect arrays up to present day state-of-the-art prototypes are referenced. The existing methods, options and possible implementations are described, addressing their advantages and drawbacks, and showing the progress yet to be made. The performance of the different presented approaches are given and compared.

Mid-wavelength infrared InAsSb/InAs nBn detectors and FPAs with very low dark current density

There was a significant improvement in the performance of infrared nBn detectors utilizing InAs/InAsSb absorbers culminating in the development of infrared focal plane arrays (FPAs) with excellent operability (99.7%) and operating at temperature significantly higher than InSb FPAs. In this work, we demonstrated that these detectors can operate with very low dark current densities enabling their use in applications with a low-to-medium level of background illumination. We showed that single pixel photodetectors with a cut-off wavelength of 4.8 μm and a quantum efficiency of QEmax = 35% under backside-illumination have a dark current density of 1 × 10−10 A/cm2 at an operating bias of −0.1 V and temperature T = 100 K. Additionally, we compared the single pixel dark current results with measurements of the dark current in FPAs. The FPA showed excellent performance with an operability of 99.7% and was able to reach a mode dark current density of 5 × 10−11 A/cm2 at 80 K.

SCEMA: a high channel density electronics module for fast waveform capture

The development of fast, highly pixelated photodetectors with single-photon sensitivity has the potential to enable a variety of new radiation detection concepts. Systems that desire to employ these detectors without loss of information demand waveform digitization with high sampling rates. Switched capacitor arrays provide a low-cost, low-power, compact solution to fast readout with high channel density. The Sandia Laboratories Compact Electronics for Modular Acquisition (SCEMA) was developed to meet these demands. A single module employs two domino ring sampling switched capacitor arrays (DRS4) [1] to provide 16 channels of up to 5 GS/s waveform digitization. This paper presents an overview of the board design and function. Calibration procedures for the module are discussed. Finally, temporal resolution tests are presented demonstrating the module's viability as readout for high fidelity temporal measurements of single photons in suitable photodetectors.

Active pixel photodetector for star orientation devices and the results of its experimental study

The results of this study confirm the possible application of the developed photodetector in promising domestic star orientation devices. Field tests of a prototype photodetector are conducted as part of the layout of a star orientation device. The threshold stellar magnitude is estimated and the accuracy characteristics of the layout are determined.

Plasmonic metasurface for simultaneous detection of polarization and spectrum.

We present a new plasmonic metasurface for simultaneous detection of polarization and spectrum of incident light. The demonstrated metasurface is a rationally designed cluster of artificial atoms that are engineered to exhibit polarization and wavelength-selective optical transmission. The fundamental building block of this structure is periodically coupled subwavelength aperture arrays with different orientations and lattice constants. When integrated with pixelated photodetectors, the metasurface can be used to measure the polarization and spectral information of an optical input. In this Letter, simultaneous detection of the polarization and spectrum of polarized light was experimentally demonstrated by analyzing the transmitted intensity distribution through the metasurface. The demonstrated metasurface offers great potential for many applications, such as polarimetric multispectral imaging and polarization-division multiplexing in optical communications.

Time-based position estimation in monolithic scintillator detectors

Gamma-ray detectors based on bright monolithic scintillation crystals coupled to pixelated photodetectors are currently being considered for several applications in the medical imaging field. In a typical monolithic detector, both the light intensity and the time of arrival of the earliest scintillation photons can be recorded by each of the photosensor pixels every time a gamma interaction occurs. Generally, the time stamps are used to determine the gamma interaction time while the light intensities are used to estimate the 3D position of the interaction point. In this work we show that the spatio-temporal distribution of the time stamps also carries information on the location of the gamma interaction point and thus the time stamps can be used as explanatory variables for position estimation. We present a model for the spatial resolution obtainable when the interaction position is estimated using exclusively the time stamp of the first photon detected on each of the photosensor pixels. The model is shown to be in agreement with experimental measurements on a 16 mm × 16 mm × 10 mm LSO : Ce,0.2%Ca crystal coupled to a digital photon counter (DPC) array where a spatial resolution of 3 mm (root mean squared error) is obtained. Finally we discuss the effects of the main parameters such as scintillator rise and decay time, light output and photosensor single photon time resolution and pixel size.

Atomic layer deposited Al2O3 passivation of type II InAs/GaSb superlattice photodetectors

Taking advantage of the favorable Gibbs free energies, atomic layer deposited (ALD) aluminum oxide (Al2O3) was used as a novel approach for passivation of type II InAs/GaSb superlattice (SL) midwave infrared (MWIR) single pixel photodetectors in a self cleaning process (λcut-off ∼ 5.1 μm). Al2O3 passivated and unpassivated diodes were compared for their electrical and optical performances. For passivated diodes, the dark current density was improved by an order of magnitude at 77 K. The zero bias responsivity and detectivity was 1.33 A/W and 1.9 × 1013 Jones, respectively at 4 μm and 77 K. Quantum efficiency (QE) was determined as %41 for these detectors. This conformal passivation technique is promising for focal plane array (FPA) applications.

A study of timing properties of Silicon Photomultipliers

Silicon Photomultipliers (SiPMs) are solid-state pixelated photodetectors. Lately these sensors have been investigated for Time of Flight Positron Emission Tomography (ToF-PET) applications, where very good coincidence time resolution of the order of hundreds of picoseconds imply spatial resolution of the order of cm in the image reconstruction. The very fast rise time typical of the avalanche discharge improves the time resolution, but can be limited by the readout electronics and the technology used to construct the device. In this work the parameters of the equivalent circuit of the device that directly affect the pulse shape, namely the quenching resistance and capacitance and the diode and parasitic capacitances, were calculated. The mean rise time obtained with different preamplifiers was also measured.

Integrated color pixels in 0.18-microm complementary metal oxide semiconductor technology.

Following the trend of increased integration in complementary metal oxide semiconductor (CMOS) image sensors, we have explored the potential of implementing light filters by using patterned metal layers placed on top of each pixel's photodetector. To demonstrate wavelength selectivity, we designed and prototyped integrated color pixels in a standard 0.18-microm CMOS technology. Transmittance of several one-dimensional (1D) and two-dimensional (2D) patterned metal layers was measured under various illumination conditions and found to exhibit wavelength selectivity in the visible range. We performed (a) wave optics simulations to predict the spectral responsivity of an uncovered reference pixel and (b) numerical electromagnetic simulations with a 2D finite-difference time-domain method to predict transmittances through 1D patterned metal layers. We found good agreement in both cases. Finally, we used simulations to predict the transmittance for more elaborate designs.

Design and Implementation of an Associative Memory with Smart Pixels

A unipolar binary bidirectional associative memory (UBBAM) has been designed to associate input and output vectors in a manner of a content addressable memory. The neural network thus formed does not require inhibitory neurons and is well suited for optoelectronic implementa tion. The performance of the UBBAM has been shown to match that of the original BAM. The key device in the implementation of the UBBAM is a Smart Advanced Spatial Light Modulator (SASLM1), a ferroelectric liquid crystal (FELC) SLM driven by a silicon CMOS backplane. The smart pixel of the SASLM serves as a local processing unit which sets the state of its FELC modula tor according to some function of the optical signals incident upon the pixel's photodetector. The ac tion of each smart pixel is hence akin to a neuron's soma, and a matrix of such smart pixels therefore forms a powerful parallel processing hardware, significant in representing neuron fields, in real time, with minimal off-chip control and processing. The UBBAM model, the SASLM1 smart pixels and a proposed optical system will be described in this paper.