This PDF file contains the front matter associated with SPIE Proceedings Volume 11831, including the Title Page, Copyright information, and Table of Contents

In this presentation, we will report our recent efforts in achieving high performance in Antimonides type-II superlattice (T2SL) based infrared photodetectors using the barrier infrared detector (BIRD) architecture. The high operating temperature (HOT) BIRD focal plane arrays (FPAs) offer the same high performance, uniformity, operability, manufacturability, and affordability advantages as InSb. However, mid-wavelength infrared (MWIR) HOT-BIRD FPAs can operate at significantly higher temperatures (<150K) than InSb FPAs (typically 80K). Moreover, while InSb has a fixed cutoff wavelength (~5.4 μm), the HOT-BIRD offers a continuous adjustable cutoff wavelength, ranging from ~4 μm to <15 μm, and is therefore also suitable for long wavelength infrared (LWIR) as well. The LWIR detectors based on the BIRD architecture has also demonstrated significant operating temperature advantages over those based on traditional p-n junction designs. Two 6U SmalSat missions CIRAS (Cubesat Infrared Atmospheric Sounder) and HyTI (Hyperspectral Thermal Imager) are based on JPL’s T2SL BIRD focal plane arrays (FPAs). Based on III-V compound semiconductors, the BIRD FPAs offer a breakthrough solution for the realization of low cost (high yield), high-performance FPAs with excellent uniformity and pixel-to-pixel operability. We have also exploring the possibilities of integrating either metasurface resonator cavity or metasurface based flatlens with individual pixels to improve the signal-to-noise ratio of the detectors. Furthermore, we will discuss the advantages of the utilization of all digital read out integrated circuits with HOT-BIRDs.

High performance detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. The graphene-based HgCdTe detector technology involves integration of graphene with HgCdTe photodetectors allowing higher performance detection over 2-5 μm compared with photodetectors using only HgCdTe material. The graphene layer functioning as a high mobility channel reduces recombination of photogenerated carriers in the detector to further enhance performance. Graphene bilayers on Si/SiO2 substrates have been doped with boron using a spin-on dopant (SOD) process. The p-doped graphene is then transferred onto HgCdTe substrates for high mobility layers in MWIR photodetectors. Various characterization techniques including Raman spectroscopy and secondary-ion mass spectroscopy (SIMS) have analyzed dopant levels and properties of the graphene throughout various stages of development to qualify and quantify the graphene doping and transfer. The objective of this work is demonstration of graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is enablement of a scalable, low cost, low power, and small footprint uncooled MWIR sensing technology capable of measuring thermal dynamics with better spatial resolution for applications such as remote sensing and earth observation.

Recent progress in plasmonic absorption devices provides a new way of photon-electron conversion through hot carrier emission if a carrier possesses sufficient energy to overcome the Schottky barrier. However, the behavior of carriers with energy lower than that of the barrier has rarely been discussed, while it could be very useful if the energy is converted efficiently. Very recently, the photothermal effect, which used to be treated as an energy loss, is used to detect low energy photons. Here, we systematically and quantitatively analyzed the mechanism of this effect, which, to the best of our knowledge, has not yet been determined stringently. A very thin layer of Ni is deposited on n-type Si (n-Si) with an electron-beam evaporator, and annealed by rapid thermal processer to form a NiSi/n-Si Schottky junction. The device was measured under intermittent light illumination with several incident power and bias. Under 0.05 V forward bias, the device generates a photothermal assisted response, which is boosted by 23% of the traditional photoelectric response at 1550 nm in 5 s. The response in different incident wavelengths is also presented in this work. The photothermal response is caused by low energy carriers, which dissipates thermally and heats the interface locally, causing the change of the electrical characteristics of the device. This effect could be used to detect signals regardless of wavelength and has a potential in future low energy photon conversion technology.

In this work, we present a photodetector based on PbS colloidal quantum dots which can be used for low-cost, high-resolution multispectral imaging in the short-wavelength infrared range. Using versatile solution-based processing of thin films, we fabricated a switchable, dual-channel, two-terminal photodetector that can be monolithically integrated with small-pitch CMOS readout arrays. Its vertically stacked structure provides higher spatial resolution compared to conventional snapshot multispectral image sensors. We show the results of the optical simulations based on the transfer matrix method, which allowed us to achieve a wavelength-tunable narrowband response. We demonstrate the operation of the photodetector and its facile tunability by showing an EQE of more than 25% at different bands in the wavelength range of 1-1.5 μm. This work demonstrates the potential of the emerging thin-film technology for multispectral imaging.

We present a pseudo-planar geometry 26µm diameter Ge-on-Si single-photon avalanche diode (SPAD) detector with temperature insensitive single photon detection efficiency of 29.4% at 1310nm wavelength for applications including free-space LIDAR. A record low dark count rate of 104 counts/s at 125K at an excess bias of 6.6% is demonstrated, with temporal jitter reaching 134ps. The noise-equivalent power is measured to be 7.7x10-17WHz-12 which is a 2 orders of magnitude reduction when compared to comparable 25µm mesa devices. This device represents the state-of-the-art for Ge-on-Si SPADs, and highlights that these Si foundry compatible devices have enormous potential for SWIR single-photon applications.

Infrared imagery, like almost any other two-dimensional (2D) imagery, have been traditionally sampled and acquired using a traditional rectangular grid. Therefore, nonuniformity correction (NUC) algorithms for infrared imaging systems which mitigate the most dominant, bias/offset portion of the nonuniformity were developed on the rectangular grid. However, it is well-known that hexagonal sampling grid captures more information in sampled data/imagery when compared to traditional rectangular sampling, and a hexagonal addressing scheme (HAS) for hexagonally-sampled imagery to convert imagery between the two different coordinate systems was developed. In this work, we build on prior work by Sakoglu et al. who developed bilinear interpolation equations between two image frames under the 2-D global motion of the scene or the camera, and apply this 2D algebraic NUC algorithm to hexagonally-sampled imagery directly in the HAS domain by utilizing simulated hexagonal sampling of real IR images.

Broadband antireflection (AR) optical coatings covering the ultraviolet (UV) to infrared (IR) spectral bands have many potential applications for various NASA systems. The performance of these systems is substantially limited by signal loss due to reflection off substrates and optical components. Tunable nanoengineered optical layers offer omnidirectional suppression of light reflection/scattering with increased optical transmission to enhance detector and system performance. Nanostructured AR coatings enable realization of optimal AR coatings with high laser damage thresholds and reliability in extreme low temperature environments and under launch conditions for various NASA applications. We are developing and advancing high-performance AR coatings on various substrates for spectral bands ranging from the UV to IR. The nanostructured AR coatings enhance transmission of light through optical components and devices by significantly minimizing reflection losses over a wide range of incidence angles, providing substantial improvements over conventional thin film AR coating technologies. In this paper, we review our latest work on high performance nanostructure-based AR coatings, including recent efforts in the development of the nanostructured AR coatings for various sensor applications over the 2-5 μm MWIR and 8-12 μm LWIR spectral bands.

The Cr/n-Si Schottky interface can effectively extend the cutoff wavelength of the silicon-based device. The estimated barrier height, ideality factor, and series resistance are obtained by characteristic curves and thermionic-emission formula. In order to improve the accuracy of the estimation, a method of adjusting the external resistance in the experimental setup was proposed in this paper. Eventually, the Cr/n-Si Schottky device was well analyzed with an estimated error of < 0.05 in the resistance value, and the results also confirmed that the detection wavelength of the silicon-based components could be extended to mid-infrared range.

In this paper, the properties of broadband absorbers for visible and near-infrared frequencies, based on multilayered metal-insulator (MI) structures have been analyzed. In our analysis, we have considered Titanium as metal and Silicon Nitride (Si3N4) as dielectric. In addition, the effect of the incident angle on the absorption properties of transverse magnetic and transverse electric polarized waves has been also investigated. Further, the influence of structural geometrical parameters has been studied in order to maximize the absorption and bandwidth. We believe that our results can be used as practical guidelines for realization of efficient broadband visible and infrared absorbers enabling applications as filters or absorbers for visible, infrared radiation and optical communications frequencies. The Finite Element Method has been used to carry the simulations.

Presentation of "Evolution of readout integrated circuits (ROICs): past, present, and future."

The HyTI (Hyperspectral Thermal Imager) mission, funded by NASA’s Earth Science Technology Office InVEST (In-Space Validation of Earth Science Technologies) program, will demonstrate how high spectral and spatial long-wave infrared image data can be acquired from a 6U CubeSat platform. The mission will use a spatially modulated interferometric imaging technique to produce spectro-radiometrically calibrated image cubes, with 25 channels between 8-10.7 microns, at 13 wavenumber resolution), at a ground sample distance of ~60 m. The HyTI performance model indicates narrow band NEdTs of <0.3 K.

We present a conceptual design for an in-situ methane gas sensor that could be deployed rapidly to suspected sites of spurious methane emission. Based on the remote detection of suspected methane leaks now possible with a class of satellites currently in orbit or soon to be launched, in-situ sensors would be deployed to the location of the detection, and accurate measurements of methane leak rates would be reported. Our design is very high level at this time, but incorporates a spectral capability that allows switching on and off the wavelengths of peak methane emission to facilitate detection. Unlike the case for the satellite-based sensing of the gas, our design will lead to the quantification of smallest levels of methane leaks. Two approaches will be considered: ground sensors detecting methane in emission against a cooler sky background, and aerial sensors detecting the gas in absorption against the ground scene as a background source. Sensitivity plays a key role, with the infrared detector working in the 2.4-micron region and operating with near-theoretical sensitivity, with the limiting noise sources set by the background levels for the ground sensor. Consideration of levels of detector dark current, based on the background signal level, and required detector operation temperatures will be derived. The paper reports on the conceptual design, with details on the electronics approach needed to realize the needed levels of sensitivity. Performance quantification will be accomplished through simulation using accurate noise models.

The high level of expertise accumulated by Teledyne in designing ROIC for different spectral imaging sensors (from X-rays to far–infrared) enables the company to develop high performance thermal imaging sensors optimized for Size, Weight, Power and Cost (SWaP-C). The specific ROIC design discussed avoids almost any trade-off between scene dynamic range and NETD which is the main issue encountered by all other FPA suppliers. In this paper, we will show how to achieve a NETD of 50mK with a dynamic range higher than 1000°C without any FPA adjustment settings. Key characteristics of the sensitive material are described to highlight the capabilities of this technology for system operation: mainly the ease of operation due to fully digital ROIC and specific design will be demonstrated. Finally, we will see how all the previously mentioned key parameters are paving the way to affordable, powerful thermal imaging modules and cameras.

Infrared imaging finds numerous applications in airborne measurements, structural monitoring, medical diagnostics, and spectroscopy. Long wave infrared (LWIR) radiation (λ= 8-14 m) enables self-illuminated thermal imaging, and can uniquely identify different chemical species. The investigation of thermal emission control using plasmonic antenna devices,¹ and the study of tunable free-form planar optics² has motivated our development of a novel high-resolution thermal imaging technique. Speckle imaging has been successfully used to image optical intensity or phase through complex inhomogeneous scattering media - particularly at visible wavelengths,³ and recently in the infrared.⁴ Single-shot high-resolution images of the scattered light capture sufficient information to reconstruct images through opaque media and around corners with diffraction-limited resolution. In this context, we have developed a high-resolution broadband speckle imaging setup in the LWIR for phase reconstruction, using a thin scattering medium in front of an uncooled microbolometric camera. Our method utilizes the large angular memory effect of a thin scattering medium. Local phase gradients within the incoming beam produce distorted speckle images after scattering by the scatterer's surface. Local translation shifts between the speckle patterns are estimated by a fast diffeomorphic image registration algorithm to obtain a phase gradient map. Integrating this gradient map in 2-D finally yields the wavefront profile. We demonstrate infrared phase image reconstruction using our broadband LWIR speckle imaging methodology, which promises future applications in imaging through visually opaque objects like semiconductor circuits, solid-state nanoelectronics, and infrared optical components, for defect monitoring.

Cardiovascular disease is a prominent cause of death. Among the markers of cardiovascular morbidity, the Augmentation Index (AIx) is the ratio between augmentation pressure and pulse pressure. AIx’s increase is associated to vascular stiffness and cardiovascular risk. Currently, AIx is measured employing pressure cuffs reaching the supra-systolic pressure. In order to avoid the use of pressure cuffs and to foster wearable technology capable of assessing vascular diseases, in this study a novel method to predict AIx from multisite photoplethysmography (PPG) through a Deep Convolutional Neural Network (DCNN) model is presented. Seventy-six volunteers (age: 20-80 years) were enrolled in the study. AIx was measured using a commercial instrument (Enverdis Vascular Explorer, VE), whereas PPG was recorded from right tibial, radial and brachial arteries, using a custom-made ECG-PPG system. A leave-one-out cross-validation procedure was performed to test DCNN generalization performances. The DCNN estimated AIx reaching a correlation coefficient between real and predicted AIx of r = 0.74 (p<0.001). Based on the cardiovascular risk provided by VE, a two-class classification (i.e. high- and low-risk) from the cross-validated output of the DCNN was performed. Since the two classes were not balanced, a bootstrap (10000 iterations) was implemented, obtaining an area under the Receiver Operating Curve of 0.93±0.04. Although further studies are necessary to provide a finer classification of the risk (i.e. high-, medium-, low-, very-low-risk) and to exploit the multisite PPG potentialities to early detect cardiovascular pathologies, these results could foster the employment of PPG and DCNN approaches for wearable device-based screenings of cardiovascular risk.

Age-related diseases such as glaucoma, diabetic retinopathy, and macular degeneration remain the leading causes of low, vision in developed countries. Early detection of such diseases can prevent the risk of progression to blindness. To this end, regular check-ups are encouraged to favor timely eye disease diagnosis. Yet, conducting routine large-scale eye screening can be difficult and time-consuming. In this study, a novel, fast and automatic approach for age-related ocular surface modifications (AR-OSM) assessment is proposed. Indeed, accurate AR-OSM detection in the healthy population may allow to establish age-matched normal ranges, valuable for the preliminary identification of age-related diseases. The task was performed combining thermal infrared (IR) imaging of the eye with artificial intelligence techniques. Thermal IR imaging enables non-invasive real-time imaging of the ocular surface temperature (OST). OST is influenced by ocular factors like the tear film, blood flow perfusion, heat conduction, and convection of the aqueous humor, thus providing significant information on eye health. Ninety-two healthy subjects participated in the experiment (age: 20-90 years-old). A Deep convolutional neural network (DCNN) model was implemented to predict the subjects’ age based on their eye IR-image. The DCNN was able to predict the participants’ age with a good level of accuracy, reporting a correlation between real and predicted age of r=0.82 and RMSE=9.9years. In conclusion, this method allows an accurate AR-OSM evaluation usable for early recognition of eyes at risk for age-related disease.

Many microbolometer arrays being used for high-accuracy applications, such as body temperature measurement, are observed to exhibit a pixel crosstalk-like artifact which is not reflected in the system specifications. The ISO/IEC standard for thermographic screening specifies a total system uncertainty incorporating a calibrated image sensor and a paired IR calibration target to linearize the scene to high accuracy. Despite this use of an IR calibration target and calibrated image sensor meeting the required specifications, this crosstalk-like artifact is not addressed and can cause readings of facial temperatures to vary by as much as 2C in some systems, due to the artifact’s dependence on the thermal structure of an image. This artifact has been described previously as the “size-of-source” artifact, and is well-known within metrology laboratories but most users are unaware of it. To our knowledge, this thermal structure-dependent artifact has not been characterized nor corrected for human body temperature applications. We discuss test methods for evaluating the artifact and its impact on any resulting application, calibration methods for determining correction parameters and an example implementation of such a correction in real-time thermographic imaging. Finally, we propose a modification to the total system uncertainty equation in the IEC performance standard to account for the effect of this artifact.

Thermographic fever-screening systems have been deployed widely for the non-contact detection of febrile temperatures during the COVID-19 pandemic. The standard for required performance (IEC 80601-2-59:2017) [1] of thermographic screening systems describe a specification-based requirement to assure the system is able to detect real febrile temperatures, but does not describe any clinical testing. Thermographic systems have been shown to be sensitive to febrile temperatures in controlled [2,3] and real-world conditions [4]. However, concerns have been raised about elements of these standards [5,6]. In a related report by the author [20], the size-of-source artifact was presented as a major confound in thermographic systems, causing surface temperatures to be altered by enough to make certain systems miss common fever thresholds, and this confound is not addressed by the standard. More concerningly is the recent observation of unacceptably strong bias-towards-normal algorithms in a selection of widely-deployed thermographic systems [7]. On the other hand, the standard covering non-contact body thermometry (IEC 80601-2-56:2017) [8] describes both laboratory and clinical testing to assure the system can detect febrile temperatures. Many single-pixel and at least one multi-pixel infrared thermometers are widely available with 510(k) marketing approval by the US FDA. In principle, clinical testing can give a greater certainty about real-world device performance. However, as we show in this report, many if not most of these devices also resort to unacceptably strong bias-to-normal algorithms, with parameters such that they would not be able to distinguish body temperatures ranging from 95F to 103F from normal, similar to the observations of [7]. A challenge for the clinical testing of thermometry and screening devices is the impracticality of finding a population of individuals having a sufficient distribution of elevated temperatures, rather than one group of normal and one group of severely febrile. In this report, we characterize the physiology of the core-to-surface skin temperature relationship, show how several approved devices deviate significantly from this relation and demonstrate a simple test protocol to assess the real-world sensitivity and specificity of an elevated body temperature system. The data we provide in this report show it is possible that many, if not most, non-contact thermometry and thermographic devices are inadequate for their intended uses and investigation is urgently needed.

Driver’s drowsiness is one of the major causes of traffic accidents worldwide. An early detection of episodes of sleepiness becomes of fundamental importance for safety purposes. Several studies demonstrated that PERCLOS, that is the percentage of eyelid closure over the pupil across time, is one of the most accurate parameters for drowsiness state assessment. However, since PERCLOS is typically computed from the visible video of the subjects, its evaluation is strictly dependent on the lighting conditions and it is not accessible if the driver wears sunglasses. The objective of this study is to overcome these limitations, evaluating drowsy states using a low-cost and high-resolution thermal infrared technology. Ten sleep-deprived subjects were recruited for the experiment, consisting in one-hour driving task on a driving static simulator. During the experiment, facial skin temperature was recorded by means of the thermal camera Device Alab SmartIr640, together with facial visible videos of the subjects. Relevant thermal features were estimated from facial regions of interest (i.e., nose tip, glabella) whereas PERCLOS was performed on visible videos. Features were extracted over a time window of 30 seconds. A data-driven multivariate machine learning approach based on a three-level Support Vector Classification of the drowsy state (AWAKE class: PERCLOS<0.15, FATIGUE class: 0.15<PERCLOS<0.23, and SLEEPY class: PERCLOS>0.15) was employed. The average classification accuracy was 0.65±0.09 (mean ± standard deviation). Although preliminary, these results indicate the possibility to assess driver's drowsiness based on facial thermal features, overcoming the limitation related to lighting condition and eyes detection, typical of standard methods.

This article mainly shows that coherent accumulation of multi-aperture receiver array based on frequency modulation continuous wave (FMCW) coherent lidar has an excellent performance for the weak signal detection of target which is far distance or moving with a high velocity. This method can improve the signal and noise ratio (SNR) and detection range accuracy by multi-aperture receiver array. In addition, the analysis done by simulation shows that phase fluctuation of atmospheric turbulence has a significant influence on the performance of coherent accumulation of multi-aperture receiver array. Stimulation result shows that while μ_x is equal to - σ²_x , the amplitude fluctuation of signal could degrade the quality of coherent accumulation based on multi-aperture receiver array and its existence leads to the worse performance before non-amplitude fluctuation. Phase fluctuation of signal deteriorates the performance of coherent accumulation while its size is big or small.