Advances in mid-infrared detection and imaging: a key issues review

Wide-field mid-infrared hyperspectral imaging beyond video rate

Mid-infrared hyperspectral imaging has become an indispensable tool to spatially resolve chemical information in a wide variety of samples. However, acquiring three-dimensional data cubes is typically time-consuming due to the limited speed of raster scanning or wavelength tuning, which impedes real-time visualization with high spatial definition across broad spectral bands. Here, we devise and implement a high-speed, wide-field mid-infrared hyperspectral imaging system relying on broadband parametric upconversion of high-brightness supercontinuum illumination at the Fourier plane. The upconverted replica is spectrally decomposed by a rapid acousto-optic tunable filter, which records high-definition monochromatic images at a frame rate of 10 kHz based on a megapixel silicon camera. Consequently, the hyperspectral imager allows us to acquire 100 spectral bands over 2600-4085 cm-1 in 10 ms, corresponding to a refreshing rate of 100 Hz. Moreover, the angular dependence of phase matching in the image upconversion is leveraged to realize snapshot operation with spatial multiplexing for multiple spectral channels, which may further boost the spectral imaging rate. The high acquisition rate, wide-field operation, and broadband spectral coverage could open new possibilities for high-throughput characterization of transient processes in material and life sciences.

Raman amplification at 2.2 μm in silicon core fibers with prospects for extended mid-infrared source generation

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 μm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 μm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 μm) over a length of 6 cm, with losses as low as 0.2 dB cm-1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 μm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2-5 μm range.

Mid-infrared single-photon 3D imaging

Active mid-infrared (MIR) imagers capable of retrieving three-dimensional (3D) structure and reflectivity information are highly attractive in a wide range of biomedical and industrial applications. However, infrared 3D imaging at low-light levels is still challenging due to the deficiency of sensitive and fast MIR sensors. Here we propose and implement a MIR time-of-flight imaging system that operates at single-photon sensitivity and femtosecond timing resolution. Specifically, back-scattered infrared photons from a scene are optically gated by delay-controlled ultrashort pump pulses through nonlinear frequency upconversion. The upconverted images with time stamps are then recorded by a silicon camera to facilitate the 3D reconstruction with high lateral and depth resolutions. Moreover, an effective numerical denoiser based on spatiotemporal correlation allows us to reveal the object profile and reflectivity under photon-starving conditions with a detected flux below 0.05 photons/pixel/second. The presented MIR 3D imager features high detection sensitivity, precise timing resolution, and wide-field operation, which may open new possibilities in life and material sciences.

Mid-infrared single-pixel imaging at the single-photon level

Single-pixel cameras have recently emerged as promising alternatives to multi-pixel sensors due to reduced costs and superior durability, which are particularly attractive for mid-infrared (MIR) imaging pertinent to applications including industry inspection and biomedical diagnosis. To date, MIR single-pixel photon-sparse imaging has yet been realized, which urgently calls for high-sensitivity optical detectors and high-fidelity spatial modulators. Here, we demonstrate a MIR single-photon computational imaging with a single-element silicon detector. The underlying methodology relies on nonlinear structured detection, where encoded time-varying pump patterns are optically imprinted onto a MIR object image through sum-frequency generation. Simultaneously, the MIR radiation is spectrally translated into the visible region, thus permitting infrared single-photon upconversion detection. Then, the use of advanced algorithms of compressed sensing and deep learning allows us to reconstruct MIR images under sub-Nyquist sampling and photon-starving illumination. The presented paradigm of single-pixel upconversion imaging is featured with single-pixel simplicity, single-photon sensitivity, and room-temperature operation, which would establish a new path for sensitive imaging at longer infrared wavelengths or terahertz frequencies, where high-sensitivity photon counters and high-fidelity spatial modulators are typically hard to access.

Narrow bandgap Al0.15In0.85As0.77Sb0.23 for mid-infrared photodetectors

Mid-IR is a useful wavelength range for both science and military applications due to its low atmospheric attenuation and ability to be used for passive detection. However, many solutions for detecting light in this spectral region need to be operated at cryogenic temperatures as their required narrow bandgaps suffer from carrier recombination and band-to-band tunneling at room temperature leading to high dark currents. These problems can be alleviated by using a separate absorption, charge, and multiplication avalanche photodiode. We have recently demonstrated such a device with a 3-µm cutoff using Al_0.15In_0.85As_0.77Sb_0.23, as the absorber, grown on GaSb. Here we investigate Al_0.15In_0.85As_0.77Sb_0.23 as a simple PIN homojunction and provide metrics to aid in future designs using this material. PL spectrum measurements indicate a bandgap of 2.94 µm at 300 K. External quantum efficiencies of 39% and 33% are achieved at 1.55 µm and 2 µm respectively. Between 180 K and 280 K the activation energy is ∼0.22 eV, roughly half the bandgap of Al_0.15In_0.85As_0.77Sb_0.23, indicating thermal generation is dominant.

Wide-field mid-infrared single-photon upconversion imaging

Frequency upconversion technique, where the infrared signal is nonlinearly translated into the visible band to leverage the silicon sensors, offers a promising alternation for the mid-infrared (MIR) imaging. However, the intrinsic field of view (FOV) is typically limited by the phase-matching condition, thus imposing a remaining challenge to promote subsequent applications. Here, we demonstrate a wide-field upconversion imaging based on the aperiodic quasi-phase-matching configuration. The acceptance angle is significantly expanded to about 30°, over tenfold larger than that with the periodical poling crystal. The extended FOV is realized in one shot without the need of parameter scanning or post-processing. Consequently, a fast snapshot allows to facilitate high-speed imaging at a frame rate up to 216 kHz. Alternatively, single-photon imaging at room temperature is permitted due to the substantially suppressed background noise by the spectro-temporal filtering. Furthermore, we have implemented high-resolution time-of-flight 3D imaging based on the picosecond optical gating. These presented MIR imaging features with wide field, fast speed, and high sensitivity might stimulate immediate applications, such as non-destructive defect inspection, in-vivo biomedical examination, and high-speed volumetric tomography.

The authors present a simple yet effective solution to dramatically boost the performances of an upconversion imaging system, which leads to unprecedented mid-infrared imaging features with large field of view, single-photon sensitivity and a MHz-level frame rate.

Recent Progress in Improving the Performance of Infrared Photodetectors via Optical Field Manipulations

Benefiting from the inherent capacity for detecting longer wavelengths inaccessible to human eyes, infrared photodetectors have found numerous applications in both military and daily life, such as individual combat weapons, automatic driving sensors and night-vision devices. However, the imperfect material growth and incomplete device manufacturing impose an inevitable restriction on the further improvement of infrared photodetectors. The advent of artificial microstructures, especially metasurfaces, featuring with strong light field enhancement and multifunctional properties in manipulating the light-matter interactions on subwavelength scale, have promised great potential in overcoming the bottlenecks faced by conventional infrared detectors. Additionally, metasurfaces exhibit versatile and flexible integration with existing detection semiconductors. In this paper, we start with a review of conventionally bulky and recently emerging two-dimensional material-based infrared photodetectors, i.e., InGaAs, HgCdTe, graphene, transition metal dichalcogenides and black phosphorus devices. As to the challenges the detectors are facing, we further discuss the recent progress on the metasurfaces integrated on the photodetectors and demonstrate their role in improving device performance. All information provided in this paper aims to open a new way to boost high-performance infrared photodetectors.

Age Discrimination of Chinese Baijiu Based on Midinfrared Spectroscopy and Chemometrics

Baijiu is a traditional and popular Chinese liquor which is affected by the storage time. The longer the storage time of Baijiu is, the better its quality is. In this paper, the raw and mellow Baijiu samples from different storage time are discriminated accurately throughout midinfrared (MIR) spectroscopy and chemometrics. Firstly, changing regularities of the substances in Chinese Baijiu are discussed by gas chromatography-mass spectrometry (GC-MS) during the aging process. Then, infrared spectrums of Baijiu samples are processed by smoothing, multivariate baseline correction, and the first and second derivative processing, but no significant variation can be observed. Next, the spectral date pretreatment methods are constructively introduced, and principal component analysis (PCA) and discriminant analysis (DA) are developed for data analyses. The results show that the accuracy rates of samples by the DA method in calibration and validation sets are 91.7% and 100%, respectively. Consequently, an identification model based on support vector machine (SVM) and PCA is established combined with the grid search strategy and cross-validation methods to discriminate the age of Chinese Baijiu validly, where 100% classification accuracy rate is obtained in both training and test sets.

High-speed interband cascade infrared photodetectors: photo-response saturation by a femtosecond oscillator

Interband cascade infrared photodetectors (ICIPs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature. Here we study the bias-dependent electronic impulse response of ICIPs with a mid-infrared synchronously pumped optical parametric oscillator (OPO). Since the OPO produces ultrashort 104-fs pulses, it is possible to probe the impulse response of the ICIP. From this impulse response, we identify two characteristic decay times, indicating the contribution of electron as well as hole carriers. A reverse bias voltage applied to the ICIP reduces both time scales and leads to an increased electrical cut-off frequency. The OPO emits up to 500 mW average power, of which up to 10 mW is directed to the ICIP in order to test its saturation characteristics under short-pulse illumination. The peak of the impulse response profile as well as the average photocurrent experience a gradual saturation behavior, and we determine the corresponding saturation powers by measuring the photo-response as a function of average power directed to the ICIP. We demonstrate that an increasing reverse bias increases the saturation power as well as the responsivity of the ICIP.

Hole array enhanced dual-band infrared photodetection

Photonic structures have been attracting more attention due to their ability to capture, concentrate and propagate optical energy. In this work, we propose a photon-trapping hole-array structure integrated in a nip InAsSb-GaSb heterostructure for the enhancement of the photoresponse in both near- and mid-infrared regions. The proposed symmetrical hole array can increase the photon lifetime inside the absorption layer and reduce reflection without polarization dependence. Significant enhancements in absorption and photoelectric conversion efficiency are demonstrated in dual bands for unpolarized incidence. The enhancement factors of responsivity at room temperature under zero-bias are 1.12 and 1.33 for the near- and mid-infrared, respectively, and they are increased to 1.71 and 1.79 when temperature drops to the thermoelectric cooling temperature of 220 K. Besides, such an integrated hole array also slightly improves working frequency bandwidth and response speed. This work provides a promising way for high-efficiency polarization-independent photoelectric conversion in different electromagnetic wave ranges.

Infrared Thermography as an Operando Tool for the Analysis of Catalytic Processes: How to Use it?

Infrared (IR) thermography is a powerful tool to measure temperature with high space and time resolution. A particularly interesting application of this technology is in the field of catalysis, where the method can provide new insights into dynamic surface reactions. This paper presents guidelines for the development of a reactor cell that can aid in the efficient exploitation of infrared thermography for the investigation of catalytic and other surface reactions. Firstly, the necessary properties of the catalytic reactor are described. Secondly, we analyze the requirements towards the catalytic system to be directly observable by IR thermography. This includes the need for a catalyst that provides a sufficiently high heat production (or absorption) rate. To achieve true operando investigation conditions, some dedicated equipment must be developed. Here, we provide the guidelines to assemble a chemical reactor with an IR transmitting window through which the reaction can be studied with the infrared camera along with other best practice tips to achieve results. Furthermore, we present selected examples of catalytic reactions that can be monitored by IR thermography, showing the potential of the technology in revealing transient and steady state chemical phenomena.

Stray light correction for medium wave infrared focal plane array-based compressive imaging

With focal plane array-based (FPA) compressive imaging (CI), high-resolution medium wave infrared (MWIR) images can be reconstructed by a low-resolution FPA sensor. However, in MWIR FPA CI system, the stray light is inevitable, which reduces the image contrast and increases the blocky structural artifacts of the reconstructed images. In this work, we focus on the stray light in MWIR FPA CI system. This paper investigates the sources of stray light in MWIR FPA CI system and modifies the systematic radiation model. According to the systematic computation model, we illustrate that stray light impedes the accurate sampling of compressive measurements in the MWIR FPA CI system, which may increase the blocky structural artifacts in the reconstructed high-resolution images. With the help of digital micro-mirror device modulation, we propose an operational method to substantially correct the effect of the stray light in MWIR FPA CI system, which can improve the image contrast and reduce the blocky structural artifacts of the reconstructed images, while not significantly increasing the cost of image acquisition and computation. Based on the experimental results obtained from the actual MWIR FPA CI system, we have verified the effectiveness and practicability of the proposed stray light correction method.

Non-uniformity correction for medium wave infrared focal plane array-based compressive imaging

As a super-resolution imaging method, high-resolution medium wave infrared (MWIR) images can be obtained from a low-resolution focal plane array-based (FPA) sensor using compressive imaging (CI) technology. As a common problem in MWIR FPA imaging, the non-uniformity reduces image quality, which is turning worse in MWIR FPA CI. This paper investigates the source of the non-uniformity of MWIR FPA CI, both in the captured low-resolution MWIR images and in the reconstructed high-resolution ones. According to the system model and the image super-resolution computation process of FPA CI, we propose a calibration-based non-uniformity correction (NUC) method for MWIR FPA CI. Based on the actual MWIR FPA CI system, the effectiveness and practicability of the proposed NUC method are verified, obtaining better results than the traditional method. According to the theoretical analysis and experimental results, the particularities of the non-uniformity in MWIR FPA CI are discovered and discussed, which have certain great guiding significance and practical value.

Plasmonic Resonance Enhanced Polarization-Sensitive Photodetection by Black Phosphorus in Near Infrared

Black phosphorus, a recently intensely investigated two-dimensional material, is promising for electronic and optoelectronic applications due to its higher mobility and thickness-dependent direct band gap. With its low direct band gap and anisotropic properties in nature, black phosphorus is also suitable for near-infrared polarization-sensitive photodetection. To enhance photoresponsivity of a black phosphorus based photodetector, we demonstrate two designs of plasmonic structures. In the first design, plasmonic bowtie antennas are used to increase the photocurrent, particularly in the armchair direction, where the optical absorption is higher than that in the zigzag direction. The simulated electric field distribution with bowtie structures shows enhanced optical absorption by localized surface plasmons. In the second design, bowtie apertures are used to enhance the inherent polarization selectivity of black phosphorus. A high photocurrent ratio (armchair to zigzag) of 8.7 is obtained. We choose a near-infrared wavelength of 1550 nm to demonstrate the photosensitivity enhancement and polarization selectivity, as it is useful for applications including telecommunication, remote sensing, biological imaging, and infrared polarimetry imaging.

Quantitative visualization of lignocellulose components in transverse sections of moso bamboo based on FTIR macro- and micro-spectroscopy coupled with chemometrics

BACKGROUND

Due to the increasing demands of energy and depletion of fossil fuel, bamboo is considered to be one of the most important renewable biological resources on the basis of its advantages of rapid growth ability and rich reserves. Cellulose, hemicellulose, and lignin are the three most important constituents in moso bamboo. Their concentrations and, especially, their microscopic distributions greatly affect their utilization efficiency and other physical properties as a biomass resource. However, no studies have achieved a quantitative visualization of the distribution of lignocellulose concentrations in transverse sections of bamboo. Therefore, this study proposed the use of quantitative multivariate spectral analysis to reveal the micro-chemical distribution of lignocelluloses in bamboo based on an integration of FTIR macro- and micro-spectroscopic imaging techniques.

RESULTS

Multivariate calibration models for the quantitative determination of lignocelluloses of bamboo were developed based on FTIR macro-spectroscopy, and the quantitative calibration models based on the FTIR characteristic bands showed an excellent performance with determination coefficients of 0.933, 0.878, and 0.912 for cellulose, hemicellulose, and lignin, respectively. These quantitative models were then utilized to the FTIR micro-spectroscopy of bamboo transverse sections which were corrected using a direct standardization algorithm. Subsequently, the micro-chemical distributions of cellulose, hemicellulose, and lignin were obtained based on the integration of the multivariate calibration models and corrected FTIR micro-spectroscopy. The combination of the multivariate calibration models and calibration transfer algorithm resulted in a final quantitative visualization of the chemical distributions of lignocelluloses in moso bamboos.

CONCLUSIONS

Integration of the FTIR macro- and micro-spectroscopic imaging techniques can provide comprehensive information that can be used to exploit the resource of moso bamboo to develop biofuels and biosynthetic materials.

Mid-Wave Infrared Photoconductors Based on Black Phosphorus-Arsenic Alloys

Black phosphorus (b-P) and more recently black phosphorus-arsenic alloys (b-PAs) are candidate 2D materials for the detection of mid-wave and potentially long-wave infrared radiation. However, studies to date have utilized laser-based measurements to extract device performance and the responsivity of these detectors. As such, their performance under thermal radiation and spectral response has not been fully characterized. Here, we perform a systematic investigation of gated-photoconductors based on b-PAs alloys as a function of thickness over the composition range of 0-91% As. Infrared transmission and reflection measurements are performed to determine the bandgap of the various compositions. The spectrally resolved photoresponse for various compositions in this material system is investigated to confirm absorption measurements, and we find that the cutoff wavelength can be tuned from 3.9 to 4.6 μm over the studied compositional range. In addition, we investigated the temperature-dependent photoresponse and performed calibrated responsivity measurements using blackbody flood illumination. Notably, we find that the specific detectivity (D*) can be optimized by adjusting the thickness of the b-P/b-PAs layer to maximize absorption and minimize dark current. We obtain a peak D* of 6 × 10¹⁰ cm Hz^1/2 W^-1 and 2.4 × 10¹⁰ cm Hz^1/2 W^-1 for pure b-P and b-PAs (91% As), respectively, at room temperature, which is an order of magnitude higher than commercially available mid-wave infrared detectors operating at room temperature.

Infrared Black Phosphorus Phototransistor with Tunable Responsivity and Low Noise Equivalent Power

The narrow band gap property of black phosphorus (BP) that bridges the energy gap between graphene and transition metal dichalcogenides holds great promise for enabling broadband optical detection from ultraviolet to infrared wavelengths. Despite its rich potential as an intriguing building block for optoelectronic applications, however, very little progress has been made in realizing BP-based infrared photodetectors. Here, we demonstrate a high sensitivity BP phototransistor that operates at a short-wavelength infrared (SWIR) of 2 μm under room temperature. Excellent tunability of responsivity and photoconductive gain are acquired by utilizing the electrostatic gating effect, which controls the dominant photocurrent generation mechanism via adjusting the band alignment in the phototransistor. Under a nanowatt-level illumination, a peak responsivity of 8.5 A/W and a low noise equivalent power (NEP) of less than 1 pW/Hz^1/2 are achieved at a small operating source-drain bias of -1 V. Our phototransistor demonstrates a simple and effective approach to continuously tune the detection capability of BP photodetectors, paving the way to exploit BP to numerous low-light-level detection applications such as biomolecular sensing, meteorological data collection, and thermal imaging.

Mid-wave infrared narrow bandwidth guided mode resonance notch filter

We have designed, fabricated, and characterized a guided mode resonance notch filter operating in the technologically vital mid-wave infrared (MWIR) region of the electromagnetic spectrum. The filter provides a bandstop at λ≈4.1  μm, with a 12 dB extinction on resonance. In addition, we demonstrate a high transmission background (>80%), less than 6% transmission on resonance, and an ultra-narrow bandwidth transmission notch (10  cm^-1). Our filter is optically characterized using angle- and polarization-dependent Fourier transform infrared spectroscopy, and simulated using rigorous coupled-wave analysis (RCWA) with excellent agreement between simulations and our experimental results. Using our RCWA simulations, we are able to identify the optical modes associated with the transmission dips of our filter. The presented structure offers a potential route toward narrow-band laser filters in the MWIR.

Black Phosphorus Mid-Infrared Photodetectors with High Gain

Recently, black phosphorus (BP) has joined the two-dimensional material family as a promising candidate for photonic applications due to its moderate bandgap, high carrier mobility, and compatibility with a diverse range of substrates. Photodetectors are probably the most explored BP photonic devices, however, their unique potential compared with other layered materials in the mid-infrared wavelength range has not been revealed. Here, we demonstrate BP mid-infrared detectors at 3.39 μm with high internal gain, resulting in an external responsivity of 82 A/W. Noise measurements show that such BP photodetectors are capable of sensing mid-infrared light in the picowatt range. Moreover, the high photoresponse remains effective at kilohertz modulation frequencies, because of the fast carrier dynamics arising from BP's moderate bandgap. The high photoresponse at mid-infrared wavelengths and the large dynamic bandwidth, together with its unique polarization dependent response induced by low crystalline symmetry, can be coalesced to promise photonic applications such as chip-scale mid-infrared sensing and imaging at low light levels.

Infrared photoreflectance investigation of resonant levels and band edge structure in InSb

Temperature-dependent infrared photoreflectance (PR) is employed on InSb for clarifying resonant levels (RLs) and band edge structure. Abundant PR features are well resolved around the bandgap and are verified to be of electronic inter-level transitions rather than the Franz-Keldysh oscillations. The evolution of the critical energies with temperature reveals the nature of the PR processes, from which one acceptor RL, two donor RLs, and a shallow acceptor level are quantitatively identified, and a detailed band edge structure is derived. The results show that temperature-dependent infrared PR analysis can serve as an efficient vehicle for clarifying both bound and resonant levels in semiconductors.