Near-infrared Spectral Region Research Articles

Optical properties of unoxidized and oxidized titanium nitride thin films

This study reports a pulsed laser deposition-assisted synthesis of highly metallic titanium nitride (TiN) and a series of semiconducting titanium oxynitride (TiNxOy) compounds in thin film form with tunable plasmonic properties by carefully altering the nitrogen (N)-oxygen (O) ratio. The N/O ratio was controlled from 0.3 (highest oxygen doping of TiN) to ~ 1.0 (no oxygen doping of TiN) by growing the TiN films under nitrogen pressures of 50, 35, and 10 mTorr and high vacuum conditions of 2 × 10−6 Torr with no external gas introduced. The presence of nitrogen in the deposition chamber during the film growth affects the gas phase oxidation of TiN to TiNxOy by increasing the mean free path-dependent N and O inter-collisions per second by two to three orders of magnitudes. The evidence of increased oxidation of TiN to TiNxOy with an increase in nitrogen deposition pressure was obtained using X-ray photoelectron spectroscopy analysis. While the TiN samples deposited in high vacuum conditions had the highest reflectance, TiNxOy thin films were also found to possess high reflectance at low frequency with a well-defined edge around 20,000 cm−1. Furthermore, the vacuum-deposited TiN samples showed a large negative dielectric constant of -330 and the largest frequency of zero-crossing at 25,000 cm−1; the TiNxOy samples deposited in the presence of nitrogen ambient also showed promising plasmonic applications at the near-mid infrared range. A comparison of the dielectric constant and loss function data of this research with the literature values for noble metals seems to indicate that TiN and TiNxOy have the potential to replace gold and silver in the visible and near-infrared spectral regions.

Temperature Characterization of Perovskite LEDs by Electroluminescence.

Despite the rapid progress in perovskite light-emitting diodes (PeLEDs), achieving high stability remains an outstanding challenge. PeLEDs produce heat during operation, raising the temperatures, which accelerate device degradation. To determine the PeLED temperatures, a very limited number of techniques represented by infrared thermal imaging (ITI) are employed. However, ITI mainly characterizes the temperature of the exposed top surface of the device, leaving the actual temperature in the perovskite emissive layer unresolved. Here, we address this challenge by directly characterizing the temperature of the perovskite layer via an electroluminescence (EL) analysis. Model PeLEDs emitting in the near-infrared (FAPbI3), red (FA0.5Cs0.5PbI2Br) and green (FA0.6Cs0.7PbBr3) spectral regions are tested. Urbach energies related to thermal broadening are obtained from the EL spectra, enabling real-time tracking of the perovskite emissive layer temperature under working conditions. Our results demonstrate that the EL analysis is a simple and effective method for the operando temperature characterization of PeLEDs.

Luminescent Thermometer Based on a Praseodymium(iii) Cyanide-Based Metal-Organic Framework.

Trivalent lanthanide ions have emerged as promising candidates for precise and remote temperature sensing. Among them, Pr3+-based luminescent thermometers remain underexplored, particularly those operating in the near-infrared (NIR) spectral region. This work presents the synthesis and thorough characterization of a novel Pr3+-based coordination polymer, {[Pr2IIIPt3II(CN)12(4,4'-bpyO2)4(H2O)6]·4H2O}n (1), as a rare example of Pr3+ luminescent thermometry. Coordination between Pr3+ ions, cyanido-bridged Pt2+ centers, and 4,4'-bpyO2 ligands enables efficient energy transfer, producing luminescence in visible and near-infrared regions. The polymer demonstrates distinct temperature-dependent luminescence over a wide range (12-386 K), with relative thermal sensitivities of ≅1%·K-1 and a minimum temperature uncertainty of 0.2 K.

Structural Complexity Significantly Impacts Canopy Reflectance Simulations as Revealed from Reconstructed and Sentinel-2-Monitored Scenes in a Temperate Deciduous Forest

Detailed three-dimensional (3D) radiative transfer models (RTMs) enable a clear understanding of the interactions between light, biochemistry, and canopy structure, but they are rarely explicitly evaluated due to the availability of 3D canopy structure data, leading to a lack of knowledge on how canopy structure/leaf characteristics affect radiative transfer processes within forest ecosystems. In this study, the newly released 3D RTM Eradiate was extensively evaluated based on both virtual scenes reconstructed using the quantitative structure model (QSM) by adding leaves to point clouds generated from terrestrial laser scanning (TLS) data, and real scenes monitored by Sentinel-2 in a typical temperate deciduous forest. The effects of structural parameters on reflectance were investigated through sensitivity analysis, and the performance of the 3D model was compared with the 5-Scale and PROSAIL radiative transfer models. The results showed that the Eradiate-simulated reflectance achieved good agreement with the Sentinel-2 reflectance, especially in the visible and near-infrared spectral regions. Furthermore, the simulated reflectance, particularly in the blue and shortwave infrared spectral bands, was clearly shown to be influenced by canopy structure using the Eradiate model. This study demonstrated that the Eradiate RTM, based on the 3D explicit representation, is capable of providing accurate radiative transfer simulations in the temperate deciduous forest and hence provides a basis for understanding tree interactions and their effects on ecosystem structure and functions.

Albumin-Chaperoned Deep-NIR Triarylmethane Dyes for High-Contrast In Vivo Imaging and Photothermal Therapy.

Fluorophores absorbing/emitting in the deep near-infrared (deep NIR) spectral region, that is, 800 nm and beyond, hold great promise for in vivo bioimaging, diagnosis, and phototherapy due to deeper tissue penetration. The bottleneck is the lack of bright, stable, and readily synthesized deep NIR fluorophores. Here, it is reported that the albumin-chaperon strategy is a viable one-for-all strategy to address these difficulties. A focused library of deep-NIR absorbing dyes (EA5) is easily synthesized via a two-step cascade. They are neither very stable nor bright in phosphate bufferdue to a propeller-type flexible scaffold. Through screening, EA5_c3 is found to exhibit a high affinity toward bovine serum albumin (BSA). Binding-associated structural rigidification resulted in a gigantic 26-fold fluorescence enhancement. The albumin chaperone also greatly improved the stability of EA5_c3 by shielding the bisbenzannulated triarylmethane core from nucleophilic or oxidative species. The resulting EA5_c3@BSA exhibits high biocompatibility. It offered high-resolution vasculature, lymph systems, tumors, and other tissue imaging with its bright deep NIR emission. At the same time, it exhibits prominent potential in photoacoustic imaging and photothermal treatment of subcutaneous and orthotopic breast tumors. These findings provide insights into robust and high-performance fluorophores with deep NIR regions for theranostic against aggressive cancers.

Br I spectral line measurements in the range 6000–12000 cm‒1: Part II

The present study uses a high-resolution Fourier Transform Spectrometer to provide data on the spectral line measurements of atomic bromine (Br I) in the 6000 – 12,000 cm‒1 range in the near-infrared spectral region. Bromine electrodeless discharge lamps (EDL) were prepared and utilized as light sources, while light detectors comprised InGaAs and Si diodes. A total of 302 spectral lines were detected using two different resolutions of 0.008 and 0.009 cm‒1, unveiling 104 lines reported for the first time. All observed lines have been assigned new center of gravity wavenumber values. We also present the unidentified lines and blended lines that appeared in our spectra. This study provides critical insights and data that enhance the understanding of atomic bromine spectroscopy.

Tunable continuous wave Yb:CaWO4 laser operating in NIR spectral region

A tunable continuous wave (CW) Yb:CaWO4 laser operating in near infrared (NIR) spectral region is demonstrated by pumping with a diode laser. Continuously broadband tunable wavelengths are obtained in two polarizations by rotating the Lyot filter. The tuning widths of the output wavelengths in the π- and σ-polarizations are 42 nm (from 1005.2 nm to 1047.2 nm) and 41.8 nm (from 1005.1 nm to 1046.9 nm), respectively. At an absorbed pump power of 15.6 W at 976 nm, the maximum output powers in the π- and σ-polarizations are 5.2 W at 1026.2 nm and 4.7 W at 1028.1 nm, respectively. To the best of our knowledge, this is the first tunable laser operation by using Yb:CaWO4 crystal.

Two-photon upconversion photoluminescence in CsPbBr3 nanoplates

The utilization of solar energy by light conversion processes in both natural and artificial photosynthesis systems has become a pivotal means of sustaining a consistent power supply for our daily lives and production. In general, these reactions rely on the visible spectra region where the sun produces abundant energy according to the black body's emission. Further expanding the adoption of the near-infrared (NIR) spectral region through upconversion has become an intelligent strategy to improve the entire utilization rate of solar energy. In this work, the two-photon upconversion (TPUC) properties were discovered in CsPbBr3 nanoplates (NPs) emitted at 450 nm, and a sub-gap energy level was observed to serve as the center to facilitate the upconversion process. The excitation threshold for TPUC is drastically lower than that for the two-photon absorption, another nonlinear upconversion path that requires strengthened excitation power. Moreover, a significant negative correlation between the excitation threshold for TPUC in the CsPbBr3 NPs and the exciton binding energy was unveiled, which could be attributed to the enhanced quantum confinement effects. Our work paved the way for a feasible way to expand the utilization of the NIR spectral region in light conversion applications.

Structural and optoelectronic studies of BiCuOS semiconductor: A potential photoconverter

As a p-type semiconductor, Bismuth copper oxysulfide (BiCuOS) with a narrow bandgap has exceptional absorption characteristics in the visible and near-infrared spectral regions, which have good application prospects for optoelectronic devices. A simple hydrothermal technique is successfully employed for the complicated material synthesis. The desired phase formation of the material has been investigated by varying the reaction time in the procedure, and it is observed to be crucial in maximizing the yield of this material. The scanning electron microscope (SEM) and transmission electron microscope (TEM) are utilized to analyze the morphological and lattice arrangements of the prepared sample. The photodetection performance of the material is assessed through the fabrication of a self-powered p-n heterojunction photodetector with grown n-type ZnO. At self-powered conditions, the device showed a better spectral responsivity of 118 μA/W,detectivity of around 8.27 × 107 Jones, and linear dynamic range (LDR) of 5.7 Hz. The proposed interface of oxychalcogenide semiconductor is anticipated to broaden the research on its improvement and usage in various optoelectronic applications.

Optically enhanced organic phototransistors for adaptive image processing under complex light conditions

Effectively handling massive information in complex lighting conditions is crucial for security-focused machine vision, as it relies on precise image feature detection and encryption/decryption to ensure integrity and confidentiality. Traditional hardware, primarily based on complementary metal-oxide-semiconductor (CMOS) technology, struggles with the complexity of processing tasks around the clock. To address this challenge, a ternary organic heterostructure visuomorphic phototransistor was tailored for adaptive critical image processing. The light coupling induced by the porous heterostructural layer enhances the light absorption efficiency by 2.5-fold. Our phototransistor exhibits bidirectional photoresponse across the visible and near-infrared (NIR) spectral regions, enabling adaptive edge detection in varying lighting conditions with precision above 85 %. The phototransistor array facilitates image encryption/decryption with a superior accuracy of 12 %/90 % compared to existing counterparts. This ternary organic hetero-integration combines multiple response modes, enabling compact and efficient critical feature processing.

Sub-picometer level all-solid-state narrow linewidth single-frequency Pr3+:LiYF4 laser in the near-infrared spectral region.

We report an all-solid-state near-infrared single-frequency (single longitudinal mode, SLM) Pr3+:LiYF4 (Pr:YLF) laser with the spectral linewidth at the sub-picometer level. The SLM lasers with center wavelengths of 868 and 907 nm are realized in Pr:YLF crystal for the first time to the best of our knowledge. The maximum output powers of SLM lasers at 868 and 907 nm are 102 and 213mW, corresponding to the narrowest spectral linewidths of 82 MHz (0.21 pm) and 94 MHz (0.26 pm), respectively. At the maximum output power, the beam quality factors in the x and y directions are measured as 1.25 and 1.16 at 868 nm and 1.21 and 1.13 at 907 nm, respectively. The output power stabilities of the 868 and 907 nm SLM lasers are calculated as 1.39% and 0.87%, respectively. The successful realization of 868 and 907 nm all-solid-state SLM lasers makes up for the gap that the Pr:YLF SLM lasers developed in the past are focused on the visible region, enriches the types of near-infrared (NIR) SLM lasers, and can provide practical applications in biomedicine, cold atom physics, and optical atom manipulation.

Construction of double plasma MoO3-x/ZrN S-Scheme heterojunction for efficient photothermal overall water splitting

The secret to creating high-performing photothermal catalysts for photocatalytic overall water splitting was the careful design of light absorbers and photogenerated charge transfer pathways. Here, we presented a nonmetallic dual-plasmonic catalyst of MoO3-x/ZrN for photocatalytic H2 and O2 evolution that integrated the localized surface plasmon resonance (LSPR) effect and the S-scheme photogenerated charge transfer route with two different plasma semiconductors. This noble-metal-free catalyst’s dual-plasmonic action demonstrated its potential to tune the long-wavelength photon reflectance (LSPR) absorption spanning the visible to near-infrared (NIR) spectral region and convert photon energy into localized heat. The photogenerated electrons at the conduction band of MoO3-x and the photogenerated holes at the valence band of ZrN were driven to recombine with each other by the inherent electric field in the interface between MoO3-x and ZrN. This allowed the photogenerated holes at the valence band of MoO3-x and the photogenerated electrons at the conduction band of ZrN to be reserved for strong H2 and O2 evolution, respectively. Consequently, MoO3-x/ZrN exhibits remarkable photothermal catalytic performance in overall water splitting. Under light irradiation, the catalyst achieved a H2 and O2 yield rate of 204.22 μmol g−1 h−1 and 95.85 μmol g−1 h−1. This work demonstrated the viability of using two plasma semiconductors to realize wide-spectrum driven photocatalysis by connecting the S-scheme heterojunction and LSPR effect.

A practical approach for extracting the photosystem II (PSII) contribution to near-infrared solar-induced chlorophyll fluorescence

Recent studies have indicated a good potential for using solar-induced chlorophyll fluorescence (SIF) to estimate photosynthetic CO2 assimilation. SIF can be emitted by both Photosystem I (PSI) and Photosystem II (PSII), but it is the SIF signals from PSII which are related to photosynthetic carbon fixation. However, since top-of-canopy SIF observations (SIFTOC) always contain contributions from both photosystems, to mechanistically estimate gross primary productivity (GPP) from SIF, it is essential to extract PSII SIF from SIFTOC. Based on the differences in the relative contribution of PSII across different wavelengths, we propose a practical approach for extracting PSII contribution to SIFTOC at the near-infrared (NIR) band (fPSII_760) using measurements of SIFTOC in the red and NIR spectral regions. A leaf-scale concurrent instrument was developed to assess the response of fPSII_760 under varying environments. For winter-wheat leaves, as light intensity increased from 0 to 400 μmol m−2 s−1, fPSII_760 rose from 0.6 to 0.8; with further increase in light intensity to 1800 μmol m−2 s−1, fPSII_760 consistently decreased to 0.65. There was a slight decreasing trend in fPSII_760 with rising temperatures, with values dropping from 0.65 at 15 °C to 0.61 at 40 °C. We found that variations in fPSII_760 are due to changes in the fluorescence yield of PSII, with the two having a positively proportional relationship. We also estimated canopy-scale fPSII_760 for a winter-wheat study site: fPSII_760 varied from 0.61 to 0.83, with a mean value of 0.78 during the peak growing season. A comparison with eddy covariance-derived GPP reveals that GPP estimated with dynamic fPSII_760 was more accurate than that calculated using fixed fPSII_760, with R2 increasing from 0.6 to 0.84. This study contributes to a deeper understanding of the link between SIF and photosynthetic CO2 assimilation, paving the way for more effective use of SIF to estimate GPP.

Plasmon dephasing time and optical field enhancement in a plasmonic nanobowl substrate studied by scanning near-field optical microscopy.

Plasmonic substrates have been extensively investigated due to their potential applications in fluorescence microscopy, chemical sensing, and photochemical reactions. The optical properties of the substrate depend on the spatial and temporal features of the plasmon excited. Hence, the ability to directly visualize plasmon dynamics is crucial. In this study, we investigated the spatial and temporal properties of plasmon excitation in a plasmonic nanobowl substrate consisting of a periodic hexagonal array of nanoscale bowl-like structures developed with self-assembly. Near-field transmission imaging revealed that multiple plasmon resonance bands are observed from visible to near-infrared spectral region, and the optical contrast of the image is dependent on the observed band. Near-field two-photon photoluminescence microscopy revealed that the probability of excitation inside each nanoscale bowl-like structure is greater than that in the surrounding area. Near-field time-resolved imaging revealed that the nanobowl substrate exhibited a substantially long plasmon dephasing time, exceeding 12fs. Based on the spectral features of the near-field and far-field spectra, we found that optically dark plasmon mode is excited by the near-field illumination and only partly contributes to the long dephasing time observed. This fact indicates that the dephasing time is extended by some other mechanism in the periodic substrate. We revealed from this study that the enhanced optical fields induced in the nanobowl structure originate from the photosynergetic effect of the cavity mode and plasmon mode excited.

Tunable optical properties of graphene wrapped ZnO@Ag spherical core-shell nanoparticles

In this paper, we studied theoretically and numerically the material’s response to incident electromagnetic wave of graphene wrapped zinc-oxide/silver (g − ZnO@Ag) core–shell spherical nanoparticles embedded in a dielectric host matrix. As the nanoparticles size is ∼30 nm, a size much smaller than the wavelength of light, the quasi-static approximation is utilized to obtain analytical expressions for the electric polarizability and the corresponding extinction cross-section. It is found that the spectra of the extinction cross-section of g − ZnO@Ag nanoparticles exhibit two sets of localized surface resonance peaks in the visible and near infra-red (NIR) spectral regions. The first set of peaks observed below ∼900 nm are due to the coupling of the energy gap of the ZnO core with the local surface plasmon resonances of Ag shell, and the second set of graphene-assisted narrow peaks located in the NIR region (above ∼900 nm) are attributed to the plasmons excited at the Ag/graphene interface. It is found that the intensity of the extinction cross-section as well as the positions of the resonance wavelengths are interesting that the graphene-assisted narrow peaks are strongly dependent on the number of layers (N g ) and the chemical potential (μ) of graphene. It means that the response of ZnO@Ag core–shell nanoparticles to electromagnetic fields are greatly enhanced when it is wrapped with graphene and can also be tuned in the therapeutic NIR spectral region by varying N g and μ. The results may be used for possible application in the medical fields, especially for cancer detection and drug delivery.

Thermo-Optoplasmonic Single-Molecule Sensing on Optical Microcavities.

Whispering-gallery-mode (WGM) resonators are powerful instruments for single-molecule sensing in biological and biochemical investigations. WGM sensors leveraged by plasmonic nanostructures, known as optoplasmonic sensors, provide sensitivity down to single atomic ions. In this article, we describe that the response of optoplasmonic sensors upon the attachment of single protein molecules strongly depends on the intensity of WGM. At low intensity, protein binding causes red shifts of WGM resonance wavelengths, known as the reactive sensing mechanism. By contrast, blue shifts are obtained at high intensities, which we explain as thermo-optoplasmonic (TOP) sensing, where molecules transform absorbed WGM radiation into heat. To support our conclusions, we experimentally investigated seven molecules and complexes; we observed blue shifts for dye molecules, amino acids, and anomalous absorption of enzymes in the near-infrared spectral region. As an example of an application, we propose a physical model of TOP sensing that can be used for the development of single-molecule absorption spectrometers.

Emission Spectra of Uranium Particulates at High Temperature.

The emission spectrum of micron-scale uranium particulates at high temperatures in the ultraviolet, visible, and near-infrared spectral regions is investigated using a heterogeneous shock tube. Temperatures from 3000 to 9000 K are characterized in an inert argon environment and with incremental amounts of added oxygen. Atomic line spectra do not emerge above the continuum emission spectrum until between 4500 and 5000 K in pure argon, and 6100 and 6600 K in 1% oxygen. For 5% oxygen, however, the threshold for atomic emission drops below 3800 K. Uranium monoxide molecular emission in the strongest visible band at 595.4 nm is not observed at any condition. Uncertainties in particle temperature determination in high-temperature shock tube environments are discussed, and limitations to such measurements are presented, such as those from experimental factors such as the powder loading method and expected detection limits of uranium species in relevant conditions.

Tunable polarization control with self-assembled arrays of anisotropic plasmonic coaxial nanocavities

Polarization control with nanostructures having a tunable design and allowing inexpensive large-scale fabrication is important for many nanophotonic applications. For this purpose, we developed and experimentally demonstrated nanostructured plasmonic surfaces based on hexagonal arrays of anisotropic coaxial nanocavities, which can be fabricated by a low-cost self-assembled nanosphere lithography method. Their high polarization sensitivity is achieved by engineering anisotropy of the coaxial nanocavities, while the optical response is enhanced by the excitation of surface plasmon resonances. Particularly, varying the geometrical parameters of the coaxial nanocavities, namely the height and tilt angle of their central core nanoellipsoids, the plasmonic resonance wavelengths as well as the polarization-selective behavior can be individually tuned in the entire visible and near-infrared spectral regions, which makes such nanostructures good candidates for the implementation of polarization-controlled optical switches and polarization-tunable filters. Moreover, the developed nanostructures demonstrate sensitivity up to 1335 nm/RIU in refractive index sensing.

Ultrabroadband Near-Infrared Transient Absorption Spectrometer with Simultaneous 900-2350 nm Detection.

In this work, we detail an ultrafast pump-probe transient absorption (TA) spectrometer capable of probing the near-infrared (NIR) spectral region from 900 to 2350 nm simultaneously. Two key advances were required to overcome previous spectral window limitations, which typically result from constrained supercontinuum ranges (e.g., 1700 nm) and/or InGaAs detector line rates, especially those with >1700 nm range. First, we generated a broadband NIR supercontinuum using the 1980 nm idler beam of an optical parametric amplifier and implement a unique spectral filtering scheme to balance the detected spectrum. Second, we used a prism-based spectrometer system equipped with high speed InGaAs cameras having ∼2500 nm sensitivity cutoffs. To the best knowledge of the authors, such an extended probe range was previously inaccessible because the combination of two optical geometries either using different supercontinuum crystal materials for generating the NIR and shortwave infrared (SWIR) regions, or using differing pump wavelengths, were required. Finally, we demonstrate the performance and capabilities of the ultrabroadband TA spectroscopy system by presenting data showing ultrafast charge photogeneration in a polymer : fullerene blend thin-film and comparing the results to the literature with a complete agreement.

Fluoride nanocrystals and quantum dots nanocomposite for efficient luminescence nanothermometer in NIR-II biological window

Biosensors working in the second near-infrared (NIR-II) spectral region have recently gained great attention owning to their deep tissue bioimaging and high spatiotemporal resolution. However, there is still a challenge to develop efficient nanothermometers based on NIR-II luminescence. Herein, we present a pioneering advancement in the field of nanothermometry by synthesizing a novel nanothermometer, a composite of fluoride nanocrystals and quantum dots. This NaYF4:1Nd-Ag2S QDs@SiO2 nanocomposite demonstrates exceptional functionality, being excited by a single light irradiation at 808 nm for NIR-II luminescence imaging. Notably, the composite emits at two distinct wavelengths-1064 nm and 1125 nm, attributable to Nd3+ and Ag2S QDs, respectively. This unique emission profile enables deeper tissue penetration, mitigating overheating risks compared to conventional excitation sources at 980 nm. Furthermore, the nanomaterials exhibit remarkable temperature-sensing capabilities, with maximum sensitivities of 0.81%K−1 (SR) at 333 K. These superior sensitivities outperform many existing temperature sensors relying on thermally coupled Stark sublevels. The SiO2 matrix not only encapsulates and protects the fluoride nanocrystals and quantum dots but also shields them from aqueous environments, ensuring outstanding luminescence stability. Our findings highlight the potential of this nanothermometer as a highly efficient temperature sensor with promising applications in cutting-edge bioapplications. This development introduces a new frontier in temperature sensing and bioimaging, showcasing its potential for transformative contributions to the field.