Near-infrared Light-emitting Diodes Research Articles

Bright and stable near-infrared lead-free perovskite light-emitting diodes

Long-wavelength near-infrared light-emitting diodes (NIR LEDs) with peak emission wavelengths beyond 900 nm are of critical importance for various applications including night vision, biomedical imaging, sensing and optical communications. However, the low radiance and poor operational stability of state-of-the-art long-wavelength NIR LEDs based on soft materials remain the most critical factors limiting their practical applications. Here we develop NIR LEDs emitting beyond 900 nm with improved performance through the rational manipulation of p doping in all-inorganic tin perovskites (CsSnI3) by retarding and controlling the crystallization process of perovskite precursors in tin-rich conditions. The resulting NIR LEDs exhibit a peak emission wavelength at 948 nm, high radiance of 226 W sr−1 m−2 and long operational half-lifetime of 39.5 h at a high constant current density of 100 mA cm−2. Our demonstration of efficient and stable NIR LEDs operating at high current densities may also open up new opportunities towards electrically pumped lasers.

Plasmonically-boosted exciton-photon coupling strength in a near-infrared LED based on a ZnO:Ga microwire/GaAs heterojunction with surface-coated Au&Ag alloy nanorods.

The development of electrically-driven low-dimensional coherent light sources via highly-polarized polariton emission behavior has been extensively researched, but suffers from limited modulation of the exciton-photon coupling strengths. Herein, an electrically-biased near-infrared exciton-polariton light-emitting diode (LED), which includes a Ga-doped ZnO microwire (ZnO:Ga MW) and p-type GaAs substrate, is demonstrated. The well-designed LED structure is conducive to producing strong coupling between excitons and cavity photons, thus yielding highly-polarized light-emissions due to the optical birefringence in the ZnO:Ga MW microcavity. In particular, when the LED device is modified using Au&Ag alloy nanorods (AuAgNRs) with desired plasmonic properties, the electroluminescence (EL) performance is significantly boosted, especially the Rabi-splitting energy, which increases from 96 to 285 meV. The current-injection exciton-polariton emission from the LED undergoing a strong coupling regime is confirmed through angle-resolved EL measurements. This study exhibits a performance-boosted near-infrared exciton-polariton LED at room temperature, which provides a new scheme toward the realization of highly energy-efficient polariton coherent light sources. Further, the significantly lower density of polariton states induced by the incorporated metal nanostructures highlights a bright future of realizing ultralow-threshold polariton lasers much more feasibly, in comparison to conventional lasers based on narrow bandgap semiconductors.

An efficient and thermally stable source with Cr3+ near-infrared luminescence for non-destructive testing applications

The quest for efficient and heat-resistant near-infrared (NIR) phosphors to create advanced smart NIR lighting sources continues to pose a significant challenge. This study introduces a new fluoride phosphor, Cr3+-doped Cs2NaScF6, wherein the Cs2NaScF6 host provides a weak crystal field suitable for Cr3+ doping. This arrangement allows the production of a broad NIR emission peaking at 797 nm, coupled with a notable internal quantum efficiency of 90.8 % when excited by 446 nm blue light. Meanwhile, due to the relatively mild electron-phonon coupling effect and a high activation energy within this phosphor, the overall NIR emission intensity at 150 °C sustains 81.8 % of its level at room temperature. This highlights exceptional thermal stability in photoluminescence performance. Combining the Cs2NaScF6:Cr3+ phosphor with a commercially available blue InGaN chip to construct a NIR light-emitting diode (LED) device, which exhibits efficient and stable NIR emission, making it suitable for non-destructive testing applications. These findings affirm that the Cs2NaScF6:Cr3+ phosphor can function as a promising candidate to fabricate high-performance device for NIR spectroscopy application.

Thickness-variation-insensitive near-infrared quantum dot LEDs

In terms of tunable luminescence and high quantum efficiency, colloidal quantum dots (CQDs) are promising semiconductors for constructing near-infrared light-emitting diodes (NIR-LEDs). However, currently available NIR-LEDs are susceptible to variations in the emission layer thickness (EMLT), the highest external quantum efficiency (EQE) decreases to below 50% (relative to peak EQE) when the EMLT varies out of a narrow range of (±30 nm). This is due to the thickness-dependent carrier recombination rate and current density variation, resulting in batch-to-batch EQE fluctuations that limit LED reproducibility. Here we report efficient NIR-LEDs that exhibit EQE variations of less than 15% (relative to the champion EQE) over an EMLT range of 40–220 nm; the highest achievable EQE of ∼11.5% was obtained by encapsulating a 212 nm-thick CQD within a type-I inorganic shell to enhance the radiative recombination in the dots, resulting in a high photoluminescence quantum yield of 80%, and by post-treating the films with a bifunctional linking agent to improve and balance the hole and electron mobilities in the entire film (electron mobility: 8.23 × 10−3 cm2 V−1 s−1; hole mobility: 7.0 × 10−3 cm2 V−1 s−1). This work presents the first NIR-LEDs that exhibit EMLT-invariant EQE over an EMLT range of 40–220 nm, which represents the highest EQE among reported CQD NIR-LEDs with a QD thickness exceeding 100 nm.

Room-Temperature, Multigram-Scale Synthesis and Conversion Mechanism of Highly Luminescent Lead Sulfide Quantum Dots.

PbS quantum dots (QDs) are attractive near-infrared (NIR) materials, but traditional synthetic methods require inert atmosphere and/or high temperature. Herein we develop a facile, room-temperature synthetic route for in situ halide passivated PbS QDs through controllable reactions between lead halide, N,N'-diphenyl thiourea, and oleyamine (OLA) in toluene. Contrast experiments and theoretical calculations reveal that the OLA plays a bifunctional role as a mild base to initiate the formation of PbS monomers and as a dynamic ligand to control the crystallization of PbS QDs and further ligand exchange. The oleic acid-capped PbS QDs exhibit high photoluminescence quantum yields up to 45%. The scaled-up synthesis on multigram scales shows great batch-to-batch consistency. We further demonstrate high-power NIR light-emitting diodes using the PbS QDs as color converters, delivering NIR optical power of 9.2 mW at 160 mA. This work provides a simple and versatile synthetic route for high-quality PbS QDs and boosts the applications of NIR materials.

Investigation of Narrow-band NIR LED Spectral Response Towards a Non-invasive Glucose Sensor.

Spectroscopy is utilised extensively in medical sensing technology. Typically, hand-held spectroscopy equipment uses miniature narrow-band light emitting diodes (LEDs) and photodiodes to emit and detect light, respectively. Photodiodes typically absorb light across a wide spectra so measurements can be corrupted by surrounding light. LEDs in the visible spectrum have a narrower spectral response and can be used in place of a traditional photodiode. However, the absorption characteristics of near infrared (NIR) spectrum LEDs is unknown. A discrete, low-cost spectrophotometer was designed to assess spectral response for 8 narrow band NIR LEDs. The normalised and raw spectral response determined the optimum detector for 1050 nm - 1300 nm is the 1450 nm LED, and the optimum detector for 1450 nm - 1650 nm emissions is the 1650 nm LED.Clinical relevance - Understanding the spectral response of narrow-band LEDs in the NIR spectrum will aid development of NIR hand-held spectroscopy medical devices.

Numerical Modeling and Simulation of Non-Invasive Acupuncture Therapy Utilizing Near-Infrared Light-Emitting Diode.

Acupuncture is one of the most extensively used complementary and alternative medicine therapies worldwide. In this study, we explore the use of near-infrared light-emitting diodes (LEDs) to provide acupuncture-like physical stimulus to the skin tissue, but in a completely non-invasive way. A computational modeling framework has been developed to investigate the light-tissue interaction within a three-dimensional multi-layer model of skin tissue. Finite element-based analysis has been conducted, to obtain the spatiotemporal temperature distribution within the skin tissue, by solving Pennes' bioheat transfer equation, coupled with the Beer-Lambert law. The irradiation profile of the LED has been experimentally characterized and imposed in the numerical model. The experimental validation of the developed model has been conducted through comparing the numerical model predictions with those obtained experimentally on the agar phantom. The effects of the LED power, treatment duration, LED distance from the skin surface, and usage of multiple LEDs on the temperature distribution attained within the skin tissue have been systematically investigated, highlighting the safe operating power of the selected LEDs. The presented information about the spatiotemporal temperature distribution, and critical factors affecting it, would assist in better optimizing the desired thermal dosage, thereby enabling a safe and effective LED-based photothermal therapy.

Male facial rejuvenation using a combination 633, 830, and 1072nm LED face mask.

Home-based photobiomodulation is a popular treatment modality for patients seeking non-invasive aesthetic treatment. Studies demonstrate that photobiomodulation is effective for skin rejuvenation, which is aimed at improving the overall appearance of the skin by reducing fine lines and wrinkles and improving skin texture, skin tone, and dyspigmentation. Most current skin rejuvenation research focuses on treatments in women. However, men's aesthetics remains an underserved market. A combined red light (RL) and near-infrared (NIR) light-emitting diode (LED) has been designed specifically to target male skin, which may have different physiological and biophysical properties compared to female skin. Herein, the safety and efficacy of a commercially available RL and NIR (633, 830, and 1072nm) LED array designed to be worn as a face mask was assessed. Primary outcomes included adverse events and facial rejuvenation as determined by participant-reported satisfaction scales and quantitative digital skin photography and computer analysis after 6weeks of treatment. The participants reported overall favorable results and improvements in all individual categories, were satisfied with the treatment, and would recommend the product to others. The participants perceived the greatest improvement in fine lines and wrinkles, skin texture, and youthful appearance. Photographic digital analysis demonstrated favorable improvements in wrinkles, UV spots, brown spots, pores, and porphyrins. These results support the use of RL and NIR to treat male skin. Advantages of the LED facemask include its safety, efficacy, convenient home-based use, minimal associated downtime, simple operation, non-invasiveness, and appreciable results in as few as 6weeks.

Monovalent Charge Compensation Enables Efficient Lanthanide‐Based Near‐Infrared Perovskite LEDs

AbstractLanthanide ions (Yb3+ or Er3+) alloying of CsPb(Cl1‐xBrx)3 quantum dots (QDs) to emit approaching 1000 nm show promise in near‐infrared light‐emitting diodes (NIR‐LEDs). High Yb3+ alloying ratio increases the electroluminance efficiency of emission at 990 nm and enables high external quantum efficiency (EQE) of NIR‐LEDs, however, the high alloying ratio also results in inferior material stability and PLQY drop because of Yb3+‐induced nanocrystal precipitation. This study finds that the heavy alloying of Yb3+ ions causes lattice distortion and coherent energy reduction of Yb3+: CsPb(Cl1‐xBrx)3 QDs, induced by two Yb3+ ions replacing three Pb2+, which leads to the collapse of the octahedral structure in ambient conditions. It posits that spontaneous monovalent ion (Na+) alloying can address the trade‐off between material stability and emission intensity. The Na+ occupies the vacancy of Pb2+ ions, relaxing the distortion in the lattice and improving the phase stability of octahedral structure, and this optimized structure in turn allows a higher Yb3+ alloying ratio. Stability measurements show that the Na+/Yb3+ co‐alloyed films show ten‐fold higher material stability and 2.0‐fold emission efficiency related to controls. It reports that as a result Na+/Yb3+ co‐alloyed NIR‐LEDs have an EQE of 6.4% at 990 nm, which is among the highest perovskite NIR‐LEDs beyond 950 nm.

Integrated Dose-Effect Relationship of Near-Infrared Light-Emitting Diode Light on Bone Regeneration in Disuse Osteoporosis Rats.

Objective: To examine the integrated dose-effect relationship of near-infrared (NIR) light-emitting diode (LED) light therapy in promoting bone defect repair in the rat model for osteoporosis (OP). Background: Low-intensity laser therapy has been shown to promote bone regeneration in OP rats. However, its integrated dose-effect relationship is not clear. Methods: Twenty-week-old male Sprague-Dawley rats were randomly assigned to 11 groups: (1) no-treatment control group (C group), (2) tail suspension (TS)-induced disuse OP experimental group (TS-OP group), and (3) OP rats with LED light treatment at nine dosages (L1-L9 groups). The tail of the rat was tied and suspended on the beam of the cage to suspend their hind limbs to induce bone loss for 4 or 7 weeks. The rats were then released and returned to their regular positions. An NIR LED at 810 nm was used on the bilateral hind limbs daily for 4 weeks. The C group rats were not given any treatment. The TS-OP group rats were subjected to identical procedures with L groups, with the exception that the light power was not turned on. After the experiment, the dual-energy X-rays or the microcomputed tomography scan analysis was performed to evaluate bone tissue status. Data analysis was done using SPSS and the health scale. Results: The trabecular thickness, trabecular number, bone volume/total volume, and connectivity density of cancellous bone and the biomechanical properties of femur in light groups were significantly increased compared with the TS-OP group, while the trabecular separation and structure model index were significantly decreased. Conclusions: NIR LED light therapy may promote trabecular bone repair of TS-OP rats. Light intensity influences photobiomodulation. In our dose levels, the greater the light intensity, usually the more effective.

Different rare earth ions activated NaBiF4 microcrystals for adjustable multicolor luminescence and white light-emitting diodes

The Er3+, Tm3+, Ho3+ activated, and Tm3+, Ho3+ co-doped NaBiF4: Yb3+ microcrystals are achieved by a hydrothermal method, and the optimum synthesis condition is explored. The phase purity, crystal structure, morphology, upconversion luminescence (UCL) performance and thermal stability of the target products are investigated. By doping different rare earth ions and changing the doping concentration, the UCL color can be adjusted, and the color purity is high under the 980 nm irradiation. Meanwhile, The NaBiF4:20%Yb3+, 0.5%Tm3+, 1.5%Ho3+ microcrystals synthesized under the optimal conditions realize the upconversion white emission. Furthermore, the light-emitting diode (LED) devices are fabricated by integrating a near-infrared LED chip with different microcrystals, displaying bright white and other colors emitted light. These achieved features implied that the NaBiF4 microcrystals doped with rare earth ions are promising functional materials for lighting and display devices.

Broadband near infrared luminescent properties of Cr3+ activated SrGa2Si2O8 phosphor

There has been a lot of interest in trivalent chromium ion-doped phosphors for near-infrared (NIR) luminescence in recent years. Here, trivalent chromium-doped NIR emitting SrGa2Si2O8 phosphors were synthesized. The electronic structure was calculated via density functional theory. Although only [SrO7], [SiO4], and [GaO4] polyhedra were found in the host, we still observe broadband luminescence of Cr3+ in the region of 650–950 nm from octahedra site. Due to the doping of Cr3+, a transformation from [GaO4] tetrahedron rearrange into [CrO6] octahedron was produced. The experimental results of X-ray photoelectron spectroscopy and electron paramagnetic resonance spectroscopy demonstrated the presence of Cr3+. A systematic study of the photoluminescence properties of SrGa2Si2O8:Cr3+ was conducted. The luminescent intensity of SrGa2Si2O8:0.005Cr3+ at 420 K can be retained at 83.42% compared to 300 K, indicating its high thermal stability. The electroluminescence of the prepared light emitting diode (LED) based on SrGa2Si2O8:Cr3+ and blue LED chip was measured under various currents. The application as a NIR LED light source was demonstrated in human hands.

Negative phototaxis of jumping cocooned parasitoid wasp larvae against short wavelengths and physicochemical properties of the cocoon shell

The parasitoid wasp Bathyplectes anurus (Hymenoptera: Ichneumonidae: Campopleginae) is a successful biocontrol agent against the alfalfa weevil, Hypera postica. This weevil is a serious pest of beneficial fabaceous plants such as alfalfa and Chinese milk vetch. One of the possible reasons for the success of this wasp in hot climates may be the ability of its cocooned larvae to repeatedly jump and roll until they relocate themselves away from detrimental sunlight and heat. It is not yet known which wavelengths of light trigger this avoidance behavior or the microstructure of the cocoon shell that might allow light transmission. Here, the response of the cocooned larvae to different wavelengths, and the microstructure, hardness, and elemental components of the cocoon shell were studied. A population of cocooned larvae were introduced on the boundary line between illuminated and shaded areas with blue, green, red, or near-infrared light-emitting diodes. The cocoons moved away from the blue and green light. The distance from the boundary to the cocoons in the shaded area was longer under these long wavelengths, followed by the red light and shortest under the near-infrared light and nil under darkness. No difference was found in mortality between different wavelengths after three days of illumination. Scanning electron microscope observations of the surface of the cocoon shell revealed that the belt-like central ridge was porous, which likely allows ventilation and light transmission. The surface of the cocoon shell showed a uniform distribution of sulfur, potentially aiding in the capture of green wavelengths. The ridge had twice the thickness of the main body and was 1.9 times harder than the main body. These results may be applied to better understand the individual responses of this biological control agent to modifications to their environment, including light pollution.

리간드가 광가교된 CuInS2/ZnS 양자점 기반 근적외선 발광다이오드

Near-infrared (NIR) light-emitting components have gained attention for applications in sensing, communication, lighting, healthcare, and security. Colloidal CuInS2/ZnS quantum dots, composed of environmentally-friendly elements, are promising as active materials for NIR light-emitting devices. While the solution processibility of colloidal quantum dots allows for cost-effective film formation, it can also result in film dissolution when subsequent solution processes are performed to deposit upper functional layers during device fabrication. In this study, we demonstrate the effectiveness of ligand crosslinking in CuInS2/ZnS quantum dots, providing chemical tolerance to the film during subsequent solution processes. This enables the development of solution-based NIR light-emitting diodes, overcoming the film dissolution issue. Our findings open new possibilities for cost-effective fabrication of NIR devices using colloidal quantum dots.

Non-Toxic CuInS2 /ZnS Colloidal Quantum Dots for Near-Infrared Light-Emitting Diodes.

Ternary CuInS2 quantum dots (QDs) with photoluminescence that is tunable from the visible to the near-infrared (NIR) region are promising light-emitters for consumer electronics due to the absence of toxic elements such as Pb, Cd, or As. Despite the compelling performance of visible-light-emitting CuInS2 QDs, reports on NIR emission remain limited, with modest efficiencies at wavelengths beyond 900nm. In this work, the facile synthesis of NIR-emitting CuInS2 /ZnS QDs is reported. A combination of twosulfur precursors w as used in the synthesis, comprising 1-dodecanethiol (DDT) and hexamethyldisilathiane (HMDS). The reactive HMDS facilitates faster nucleation and leads to a higher density of emissive Cu-deficiency sites. The resulting QDs exhibit high photoluminescence quantum efficiency (PLQE) of 65% at a long emission wavelength of 920nm. Using these QDs, NIR light-emitting diodes (LED) are fabricated, which attain an external quantum efficiency (EQE) of 8.2%. This efficiency is comparable to the best reported PbS and InAs QD LEDs, and the emission wavelength exceeds that of lead iodide perovskites. This work thus marks one of the first reports of efficient NIR LEDs based on environmentally benign CuInS2 QDs and may open up promising new applications in consumer electronic products.

Tunable and Efficient Photoluminescence of Lanthanide-Doped Cs2NaScCl6 Double Perovskite Single Crystals toward Multifunctional Light-Emitting Diode Applications.

Lead-free halide double perovskite, as one of the promising candidates for lead halide perovskite materials, shows great potential in light-emitting diodes (LEDs), benefiting from its environmental friendliness and high chemical stability. However, the poor regulation of the emission spectra severely limits its application range. Herein, various lanthanide ions were successfully doped in Cs2NaScCl6 double perovskite single crystals (DPSCs) to yield effective and stable emissions spanning from visible to near-infrared (NIR) regions. Notably, efficient energy transfer from the host to the dopants enables tunable emissions with good chromaticity, which is rarely reported in the field of lead-free double perovskite. Moreover, density functional theory calculations reveal that the high local electron density around the [LnCl6]3- octahedron in DPSCs plays a key role in the improvement of photoluminescence quantum yields (PLQYs). The optimal PLQYs are up to 84%, which increases around 3 times over that of the undoped sample. Finally, multicolor and NIR LEDs based on Ln3+-doped Cs2NaScCl6 DPSCs were fabricated and had different application functions. Specifically, the single-composite white LED shows adjustable coordinates and correlated color temperatures, while the NIR LED shows good night vision imaging. This work provides new inspiration for the application of efficient multifunctional LEDs based on lead-free double perovskite materials.

Developing Broadband Cr3+-Substituted Phosphor-Converted Near-Infrared Light Sources

Near-infrared (NIR) spectroscopy is a rapidly growing research area due to its noninvasive, inexpensive, and rapid analysis capabilities spanning various applications. The key technology behind the growth of this field involves phosphor-converted NIR light-emitting diodes (pc-NIR LEDs) due to their energy efficiency, long operating lifetimes, and chemical and physical stability. Numerous studies have reported new Cr3+-substituted phosphors that can more effectively convert blue LED emission to NIR radiation with a broad full-width-at-half-maximum (fwhm) that covers a significant portion of the interatomic vibrational spectrum. However, there is still much to learn about these materials’ structure–property relationships. In this Spotlight on Applications, we discuss current research progress in designing broadband Cr3+ phosphors as sources of NIR light, specifically emphasizing our research. We examine results from our work and others that have been used to create an efficient broadband emission in the NIR range. The discussion also emphasizes various techniques to analyze and improve temperature-dependent photoluminescence. Finally, we highlight the potential applications of Cr3+-based pc-NIR LEDs in fields such as night vision, bioimaging, and food freshness and analysis. This Spotlight on Applications aims to provide an overview of the progress of pc-NIR LEDs based on Cr3+ by emphasizing the challenges associated with its advancement while offering a few suggestions for ventures in this field.

Light Extraction of Near-Infrared AlGaAs-Based LED Enhanced by Nanostructured Surface Morphology

To improve the light extraction efficiency of a planar AlGaAs-based light emitting diode (LED) (Device A), three novel LEDs with nanostructured surface morphologies are prepared using a surface texturing approach. We describe the fabrication of a nanostructured light extraction enhancement layer via the annealing of an evaporated Ag layer. Thermal annealing is used to induce the restructuring of a 15–25 nm layer of electron beam evaporated Ag into “nanoparticles”, which are used as an etch mask to transfer a nanopattern into the n-AlGaAs layer. Then, the Ag is removed, and the remaining nanostructured n-AlGaAs surface improves the extraction of light from the device. The novel LEDs have three different nanostructured thicknesses, namely 15 nm (Device B), 20 nm (Device C), and 25 nm (Device D). The morphologies of the Devices B–D are analyzed by an atomic force microscope and a scanning electron microscope. Compared to the planar LED (Device A), the light output powers of the Devices B–D increases by 112.6%, 91.4%, and 63.5%, respectively. The nanopatterns can mitigate the abrupt change of the refractive indexes between the air and the semiconductor to increase the LEDs’ light extraction efficiency. The surface texturing approach can be applied to commercial high-power LEDs.

Efficient Near-Infrared Electroluminescence from Lanthanide-Doped Perovskite Quantum Cutters.

Perovskite nanocrystals (PeNCs) deliver size- and composition-tunable luminescence of high efficiency and color purity in the visible range. However, attaining efficient electroluminescence (EL) in the near-infrared (NIR) region from PeNCs is challenging, limiting their potential applications. Here we demonstrate a highly efficient NIR light-emitting diode (LED) by doping ytterbium ions into a PeNCs host (Yb3+ : PeNCs), extending the EL wavelengths toward 1000 nm, which is achieved through a direct sensitization of Yb3+ ions by the PeNC host. Efficient quantum-cutting processes enable high photoluminescence quantum yields (PLQYs) of up to 126 % from the Yb3+ : PeNCs. Through halide-composition engineering and surface passivation to improve both PLQY and charge-transport balance, we demonstrate an efficient NIR LED with a peak external quantum efficiency of 7.7 % at a central wavelength of 990 nm, representing the most efficient perovskite-based LEDs with emission wavelengths beyond 850 nm.

Efficient Near‐Infrared Electroluminescence from Lanthanide‐Doped Perovskite Quantum Cutters

AbstractPerovskite nanocrystals (PeNCs) deliver size‐ and composition‐tunable luminescence of high efficiency and color purity in the visible range. However, attaining efficient electroluminescence (EL) in the near‐infrared (NIR) region from PeNCs is challenging, limiting their potential applications. Here we demonstrate a highly efficient NIR light‐emitting diode (LED) by doping ytterbium ions into a PeNCs host (Yb3+ : PeNCs), extending the EL wavelengths toward 1000 nm, which is achieved through a direct sensitization of Yb3+ ions by the PeNC host. Efficient quantum‐cutting processes enable high photoluminescence quantum yields (PLQYs) of up to 126 % from the Yb3+ : PeNCs. Through halide‐composition engineering and surface passivation to improve both PLQY and charge‐transport balance, we demonstrate an efficient NIR LED with a peak external quantum efficiency of 7.7 % at a central wavelength of 990 nm, representing the most efficient perovskite‐based LEDs with emission wavelengths beyond 850 nm.