Wavelength Response Research Articles

Realization of a filter-free wavelength image sensor and imaging system for visualization of wavelength information

In this study, we fabricated a filter-free wavelength image sensor for the two-dimensional visualization of wavelength information, in addition to light intensity, and realized an imaging system. In this sensor, the electrons generated in silicon in different distributions at each wavelength are divided into two parts by the potential peaks. The wavelength can be identified from the current ratio, and the light intensity can be determined from the total current. In the proposed structure, the pixel current is sequentially output by controlling the rows and columns. The layout was designed based on the proposed structure and was fabricated at the Toyohashi University of Technology LSI factory. An imaging system was constructed using operational amplifiers, A/D converters, and a microcontroller. The ratio and sum of the two signals obtained from the sensor responded linearly to the wavelength and light intensity. The wavelength response was independent of changes in the light intensity. Even when the spot was narrowed down, the response was obtained only at the corresponding pixel and succeeded in the simultaneous imaging of the intensity and wavelength. The above results can be applied to cell analysis, such as multiple fluorescence staining, where multiple fluorescence signals are detected simultaneously.

Enhanced Multiwavelength Response of Flexible Synaptic Transistors for Human Sunburned Skin Simulation and Neuromorphic Computation.

In biological species, optogenetics and bioimaging work together to regulate the function of neurons. Similarly, the light-controlled artificial synaptic system not only enhances computational speed but also simulates complex synaptic functions. However, reported synaptic properties are mainly limited to mimicking simple biological functions and single-wavelength responses. Therefore, the development of flexible synaptic devices with multiwavelength optical signal response and multifunctional simulation remains a challenge. Here, flexible organic light-stimulated synaptic transistors (LSSTs) enabled by alumina oxide (AlOX ), with a simple fabrication process, are reported. By embedding AlOX nanoparticles, the excitons separation efficiency is improved, allowing for multiple wavelength responses. Optimized LSSTs can respond to multiple optical and electrical signals in a highly synaptic manner. Multiwavelength optical synaptic plasticity, electrical synaptic plasticity, sunburned skin simulation, learning efficiency model controlled by photoelectric cooperative stimulation, neural network computing, "deer" picture learning and memory functions are successfully proposed, which promote the development for future artificial intelligent systems. Furthermore, as prepared flexible transistors exhibit mechanical flexibility with bending radius down to 2.5mm and improved photosynaptic plasticity, which facilitating development of neuromorphic computing and multifunction integration systems at the device-level.

Damage identification of railway bridge underwater foundations based on optical images

Diagnosing the health status of the underwater foundation of railway bridges is critical for railway environmental assessment. However, traditional underwater optical image detection technology suffers from image blurring. Thus, this paper develops an underwater foundation damage identification method for railway bridges based on optical images, which exploits the various underwater propagation characteristics of different light source wavelengths. This paper mainly reflects the differential characteristics of underwater basic damage images under different wavelength responses by constructing inter-spectral variance feature functions, local feature functions of images, and global feature functions of images. An optical image enhancement model that integrates different wavelength response features has been proposed to effectively enhance basic underwater optical images. An underwater basic damage area automatic recognition method has been developed that can accurately and efficiently extract the boundary contour of the damaged area. Based on combining field test data, the feasibility and effectiveness of this method were verified through comparative analysis with other methods. The results indicate that the damage identification ability of the method proposed in this paper is improved compared to traditional methods, which can effectively improve the quality of underwater optical images and improve the recognition accuracy of damaged areas. It is a new method of underwater damage identification based on optical image, which can provide more effective technical support for the health diagnosis of underwater railway bridges.

Enhanced performance of photoelectrochemical type ultraviolet photodetector by constructing a KNbO3/ZnO heterojunction

Ultraviolet (UV) radiation has applications in many fields such as environmental monitoring and communication. Hence, high performance ultraviolet photodetectors (UVPDs) are needed to monitor UV radiation. In this work，photoelectrochemical (PEC) type based on KNbO3/ZnO heterojunctions have been fabricated because of their excellent performance and the appropriate band structures of KNbO3 and ZnO. Owing to the internal self-polarization of KNbO3 and the formation of the KNbO3/ZnO heterojunctions, the PEC UVPDs possess higher performances than PEC UVPDs based on pure ZnO and KNbO3. The responsivity of the developed PEC UVPDs based on KNbO3/ZnO reached 0.1 A/W at 360 nm UV light. Meanwhile, the rise and decay time of these PEC UVPDs based on KNbO3/ZnO heterojunctions were 8 ms and 11 ms, respectively. Moreover, the device has an excellent linear wavelength response, roughly from 320 to 400 nm. This work shows that the PEC UVPDs based on KNbO3/ZnO heterojunctions have high sensitivity and responsivity, rapid response, indicating their good potential in UV detection.

Improving the short wavelength response of the multi-crystalline silicon solar cell by using 10% Erbium doped ZnS nanoparticle-based downshifting layer

This study describes how a polymer/ZnS composite layer with antireflection, scattering, and downshifting properties improved the short wavelength response of multi-crystalline silicon (mc-Si) solar cells. The downshifting layer is made up of 10% erbium-doped ZnS nanoparticles that are dispersed in a PMMA matrix at concentrations ranging from 1 mg/ml to 10 mg/ml and applied by spin coating to the front surface of an mc-Si solar cell. The chemical route method is used to create the 10% erbium-doped ZnS NP. XRD, SEM, UV–Vis, and PL techniques are used to identify the synthesized NP. EQE, IQE, reflectance, and J-V measurement serve as distinguishing characteristics for mc-Si solar cells with and without a downshifting layer. The cubic structure of the nanoparticle has been confirmed by the XRD pattern of 10% erbium-doped ZnS NP. The spherical shape of the nanoparticle was visible in the SEM image. The nanoparticle's ability to absorb high-energy photons and release low-energy photons that met the downshifting requirement was confirmed by the UV–Vis and PL spectra. Solar cells with a downshifting layer expose their EQE spectra to enhance their UV response at short wavelengths. Both EQE and J-V measurements are used to calculate the short circuit current density. In the EQE and J-V measurements, it differs from the bare one by 4.76% and 3.57%, respectively. The short circuit current density has confirmed that the 3 mg/ml amount of ZnS:10 %Er NP/PMMA used as a downshifting layer in this study is the right concentration to improve the short wavelength response when applied to the top surface of the mc-Si solar cell.

Carbon dot / thermo-responsive polymer capacitive wavelength-specific photodetector

Organic photodetectors are considered attractive alternatives to inorganic, semiconductor-based devices. We constructed a wavelength-specific capacitive photodetector, comprising carbon dots (C-dots) and poly-(N-isopropylacrylamide) (PNIPAM), a thermo-responsive polymer. Upon illuminating the C-dot/PNIPAM composite, light absorbed by the C-dots is released as heat; the photothermal effect gives rise to a rapid temperature increase con-sequently affecting structural transformation of the PNIPAM host and concomitant change in the recorded capacitance. Importantly, wavelength sensitivity of the photodetector is determined by the excitation wavelength of the embedded C-dots; we show that tuning the wavelength response of the system can be readily achieved through selection of the C-dot species incorporated within the PNIPAM matrix. The C-dot/PNIPAM photodetector ex-hibits reversibility and rapid light response, is synthesized from inexpensive and recyclable building blocks. The technology may be employed for photo-switching, photoinduced ca-pacitors, smart windows, and other photo-electronic applications.

A novel multifunctional double perovskite structure phosphor La2MgTiO6:Mn4+, Eu3+

A series of La2MgTiO6: Mn4+ and La2MgTiO6: Mn4+, Eu3+ phosphors were prepared by high-temperature solid-phase method. The emission spectra of La2MgTiO6: Mn4+, Eu3+ phosphor samples under 328 nm excitation overlap with the absorbing regions of plant phytochromes PR and PFR in the far red and near infrared double wavelength range. The temperature-dependent and wavelength-dependent emission spectra show that the two distinguishable emission peaks of Eu3+ and Mn4+ exhibit clearly different temperature and wavelength responses. Thus, dual-emission optical sensing based on changes in the fluorescence intensity ratio(FIR) of Mn4+ and Eu3+ ions, the relative sensitivity of wavelength detection in the range of 288–348 nm maxima is 8.13% nm−1 at 288 nm, and relative sensitivity of optical temperature sensing in the range of 298–498 K maxima is 2.09% K−1 at 498 K, In this study, La2MgTiO6: Mn4+, Eu3+ phosphor samples with very promising optical properties can provide the possibility of application in multiple fields.

Broadband Response and a Transformation between Dual‐ and Single‐Wavelength Detection in Coupled Doped‐Well Quantum Cascade Detector

AbstractAs a third‐generation infrared detector, the quantum cascade detector (QCD) exhibits an accurately adjustable wavelength, low noise, and ultrafast response characteristics. By introducing an additional doping layer, QCD also shows excellent application prospects in the broadband response. Herein, a coupled doped‐well QCD with an array structure located at very long‐wave infrared (VLWIR, ≈15 µm) is prepared. Based on the energy levels interaction and carrier distribution, the regulatory mechanism of the applied bias on the response characteristics is explored. At zero bias, the detector exhibits dual‐wavelength detection owing to the splitting of the energy levels, then transforms into single‐wavelength detection with the bias increasing. Simultaneously, the QCD device exhibits a broadband response (≈13–16 µm) from 15 to 300 K and an excellent detectivity of 1.52 × 1012 cm Hz1/2 W−1 at 15 K. A high R0A (>106 Ω cm2) and robust detectivity (>109 cm Hz1/2 W−1) are obtained at room temperature. The results of the response characteristics presented in this work provide a strategy for the flexible application of QCD in infrared detection.

Self-powered In2Se3/PtSe2 photodetector with broadband and fast response

The emerging van der Waals semiconductor In2Se3 has recently gained great attention in the field of optoelectronics, due to its high optical absorption and direct bandgap. Different types of In2Se3 photodetectors have been demonstrated with high responsivity, though the detection speed and wavelength range are limited by the device geometry and its absorption edge. In this paper, we have fabricated an α-In2Se3/PtSe2 heterojunction photodetector. Taking advantage of the high mobility and narrow bandgap of PtSe2, a wide wavelength response from 405 to 1550 nm and fast repose speed of ∼ 127 µs has been achieved in the heterojunction photodetector. Also, benefiting from the strong built-in electric field between In2Se3 and PtSe2, the photodetector can work without any external bias with a responsivity highly at 80 mA/W. This fast self-powered In2Se3/PtSe2 photodetector presents an avenue for future low-energy-consumption optoelectronics.

Excitation wavelength-dependent ultrafast THz emission from surface and bulk of three-dimensional topological insulators

Three-dimensional topological insulators possess various interesting properties that are promising for various modern applications, including in the recently emerging fields of ultrafast THz photonics and spintronics. Their gapless spin-momentum-locked topological surface states with the presence of chiral spin structure are relevant for the development of light helicity-sensitive THz emitters and detectors. In this paper, we report femtosecond excitation pulse wavelength and helicity-dependent response of Bi2Te3 and Bi2Se3 for an enhanced broadband THz pulse emission from the surface and bulk states. Specifically, the excitation wavelength has been varied in a large range from near UV to near IR, where it was observed that the photoexcitation at shorter wavelengths enhances the THz emission from both the surface and bulk states but more rapidly from the surface states. These results will be highly relevant for developing chirality-sensitive efficient THz emitters and detectors.

Theoretical investigation of silicon thin film solar cell for improving short and long wavelength response

Theoretical investigation or numerical simulation of performance of a solar cell can provide useful information to maximize power conversion efficiency. In that respect we carried out a set of numerical simulation using AFORS-HET simulation program. Separately the optical absorption of the individual layers of thin film solar cell was analyzed. Current-voltage characteristic curve of a reference cell (Cell-A) was used as the starting device. The power conversion efficiency (PCE) of the reference device was 8.85% with short circuit current density (Jsc) of 15.43 mA/cm2 and fill factor (FF) of 68.3%. After suitable optimization the PCE of this device (Cell-B2) improves to 11.59% (Jsc and FF became 13.0 mA/cm2 and 87% respectively). The results show that the effective optical absorption in the active layer can be improved significantly by optimizing the device structure. The short wavelength response can be improved by reducing the parasitic optical absorption of the doped window layer, while the long wavelength response improves by raising effective absorption length of the active layer. Furthermore, optimum thickness of the active layer, for the highest possible PCE, is found to be dependent upon its defect density.

Axial InN/InGaN Nanorod Array Heterojunction Photodetector with Ultrafast Speed

AbstractDue to wavelength‐selective characteristics, the InGaN‐based photodetectors show bright prospects in visible light communication and fast imaging system. However, the application of InGaN photodetectors with simple structures is limited in the above field owing to the slow response speed. Herein, an ultrafast photodetector based on axial InN/InGaN nanorod array (NRA) heterojunction is prepared through a self‐catalytic high (930 °C) and low (400 °C) temperature two‐step growth method by molecular beam epitaxy (MBE). The performance of photodetectors is analyzed by semiconductor device analyzer, wavelength response system (consists with a xenon lamp and monochromator), and time response system (combined by an optical chopper, a laser, and a digital oscilloscope). The photodetectors present a fast response speed with a rise/full time of 18.2/24.7 µs. Further analyses reveal a responsivity of 9.27 A W−1 and a specific detectivity of 3.54 × 1010 Jones under −1 V bias. As a result, the axial NRA heterojunction photodetectors exhibit great potential for application in ultrafast photoelectric conversion systems and provide valuable guidance for realizing ultrafast photodetectors.

Temperature-insensitive water content estimation in oil-water emulsion using POF sensors

This paper presents the development of an optical fiber sensor for water content estimation in oil–water emulsions. The sensor is based on the surface Plasmon resonance (SPR) principle applied in a polymer optical fiber (POF). For the sensor fabrication, a D-shape is performed on the POF to expose its core region, which has a gold thin film (nanometer thickness) for the SPR signal. The sensor is analyzed as a function of the water content in different emulsions conditions (from 0 to 55.5 M of water content), where a sensitivity of − 0.0288 nm/M was obtained. In addition, the sensor presented a sensitivity of the transmitted optical power variation as a function of the water content. These results lead to the possibility of data fusion between optical power and wavelength responses to obtain a high accuracy in the water content estimation. In this case, a root mean squared error (RMSE) of 1.36 M was obtained on the emulsion characterization. As an important drawback in general sensors applications, the temperature cross-sensitivity was also analyzed, where a sensitivity of 0.118 nm/°C was obtained. Such temperature sensitivity of the sensor leads to errors in emulsion characterization as a function of the temperature. To address this issue, a compensation method based on direct difference between the sensor’s sensitivities is proposed. By applying the proposed compensation method, a reduction of the temperature-induced errors in the water content estimation from 6.46 %±7.81 % to only 0.32 %±1.96 %.

Asymmetric, non-uniform 3-dB directional coupler with 300-nm bandwidth and a small footprint.

Here we demonstrate a new, to the best of our knowledge, type of 3-dB coupler that has an ultra-broadband operational range from 1300 to 1600 nm with low fabrication sensitivity. The overall device size is 800 µm including in/out S-bend waveguides. The coupler is an asymmetric non-uniform directional coupler that consists of two tapered waveguides. One of the coupler arms is shifted by 100 µm in the propagation direction, which results in a more wavelength-insensitive 3-dB response compared to a standard (not shifted) coupler. Moreover, compared to a long adiabatic coupler, we achieved a similar wavelength response at a 16-times-smaller device length. The couplers were fabricated using the silicon nitride platform of Lionix International. We also experimentally demonstrated an optical switch that is made by using two of these couplers in a Mach-Zehnder interferometer configuration. According to experimental results, this optical switch exhibits -10 dB of extinction ratio over the 1500-1600 nm wavelength range. Our results indicate that this new type of coupler holds great promise for various applications, including optical imaging, telecommunications, and reconfigurable photonic processors where compact, fabrication-tolerant, and wavelength-insensitive couplers are essential.

Augmented reality system based on full-color holographic optical elements lens

Holographic optical element (HOE) lens is an imaging element fabricated through recording wavefront by interference. Because of its advantages of small form factor and wavelength, angle selectivity and arbitrary wavefront formation, it has a good application prospect in augmented reality display. To make the system more compact, the HOE lens is adopted as an off-axis optical element. At the same time, according to diffraction principle, its wavelength response is more sensitive than those of traditional refractive and reflective optical elements. Thus the fabrication and design of a full-color HOE lens is a challenge to optimizing the free-space head-up display system. To systematically analyze the HOE imaging system, the conjugate relation between the object and image is derived by scalar diffraction theory. Then the Gaussian conjugate imaging equation is obtained and the off-axis aberration of distortion and astigmatism in the HOE imaging system are analyzed. In addition, A head-up display with field of view (FOV) of 18° and eyebox of 10 mm is simulated and its imaging process is visualized through the geometric optics method of <i> <b>k</b> </i>-vector diagram and ray-tracing. A full-color HOE lens with high diffraction efficiency is fabricated by interference. Its average peak diffraction efficiency is 56.7%, reaching a high level in the world. A prototype of augmented reality system is established by integrating laser pico-projectior with HOE lens. The experimental results of distortion effect and astigmatism effect of the system are obtained, which are consistent with the simulation results. The modulation transfer function (MTF) parameter of the system is measured, and its definition basically meets the requirements of the human eyes for resolution. The aberration of the system is analyzed and the optimization method is proposed. To optimize the monochromatic image quality, an extra cylindrical lens is added to ensure the same optical power of meridian and sagittal plane to eliminate the astigmatism. Besides, a freeform wavefront is designed by the geometric construction method and forms a freeform HOE to deal with the distortion problem. The local recording freeform wavefront can be calculated by the imaging equation. When full-color HOE is applied to the display system, the images of three channels may separate in the space because of their different reconstruction wavelengths and angles. We propose a pre-compensation method of recording process to solve this problem. If these above-mentioned problems can be solved, due to its good image uniformity, sufficient field angle and eyebox area, the head-up display based on HOE lens with extra optical power will have a better application in augmented reality technology.

Ridge waveguide couplers with leaky mode resonator-like wavelength responses.

Integrated photonic resonators based on bound states in the continuum (BICs) on the silicon-on-insulator (SOI) platform have the potential for novel, mass-manufacturable resonant devices. While the nature of BIC-based ridge resonators requires the resonators to be extended in the (axial) propagation direction of the resonant mode, the requirement for excitation from the quasi-continuum extends the resonator structures also in the lateral dimensions, resulting in large device footprints. To overcome this footprint requirement, we investigate the translation of BIC-based ridge resonators into a guided mode system with finite lateral dimensions. We draw analogies between the resulting waveguide system and the BIC-based resonators and numerically demonstrate that, analog to the BIC-based resonators, such a waveguide system can exhibit spectrally narrow-band inversion of its transmissive behavior.

Investigation of the Hue–Wavelength Response of a CMOS RGB-Based Image Sensor

In this study, a non-linear hue-wavelength (H-W) curve was investigated from 400 to 650 nm. To date, no study has reported on H-W relationship measurements, especially down to the 400 nm region. A digital camera mounted with complementary metal oxide semiconductor (CMOS) image sensors was used. The obtained digital images of the sample were based on an RGB-based imaging analysis rather than multispectral imaging or hyperspectral imaging. In this study, we focused on the raw image to reconstruct the H-W curve. In addition, several factors affecting the digital image, such as exposure time or international organization for standardization (ISO), were investigated. In addition, cross check of the H-W response using laser was performed. We expect that our method will be useful as an auxiliary method in the future for obtaining the fluor emission wavelength information.

High Efficiency Formamidinium‐Cesium Perovskite‐Based Radio‐Photovoltaic Cells

Radio‐photovoltaic cell is a micro nuclear battery for devices operating in extreme environments, which converts the decay energy of a radioisotope into electric energy by using a phosphor and a photovoltaic converter. Many phosphors with high light yield and good environmental stability have been developed, but the performance of radio‐photovoltaic cells remains far behind expectations in terms of power density and power conversion efficiency, because of the poor photoelectric conversion efficiency of traditional photovoltaic converters under low‐light conditions. This paper reports an radio‐photovoltaic cell based on an intrinsically stable formamidinium‐cesium perovskite photovoltaic converter exhibiting a wide light wavelength response from 300 to 800 nm, high open‐circuit voltage (VOC), and remarkable efficiency at low‐light intensity. When a He ions accelerator is adopted as a mimicked α radioisotope source with an equivalent activity of 0.83 mCi cm−2, the formamidinium‐cesium perovskite radio‐photovoltaic cell achieves a VOC of 0.498 V, a short‐circuit current (JSC) of 423.94 nA cm−2, and a remarkable power conversion efficiency of 0.886%, which is 6.6 times that of the Si reference radio‐photovoltaic cell, as well as the highest among all radio‐photovoltaic cells reported so far. This work provides a theoretical basis for enhancing the performance of radio‐photovoltaic cells.

Machine Vision System Utilizing Black Silicon CMOS Camera for Through-Silicon Alignment

Current development trends concerning miniaturizing of electronics and photonics systems are aiming at assembly and 3-D co-integration of a broad range of technologies including MEMS, microfluidics, wafer level optics, and silicon photonics. To this end, on-chip integration using silicon-photonics platform offers a wide range of possibilities addressing passive optics functionality, active optoelectronic devices, and compatibility with CMOS fabrication. On the other hand, the hybrid technology enabling volume manufacturing of such system-on-chip components, it is still in an early development stage. Here, a new type of machine vision system enabling precision stacking and bonding processes of III–V components on silicon photonics chips is introduced. In particular, we focus on the ability to see through substrates with high resolution, which is crucial for the alignment of the markers used for assembly of integrated components on silicon wafer. The system is based on the use of a black silicon enhanced CMOS sensor with extended wavelength response beyond transparency cutoff wavelength for silicon substrate. We study the use of the camera system as a microscope with bottom coaxial infrared (IR) illumination scheme and demonstrate the ability of through-silicon vision for one- and two-layered silicon photonic integrated circuit (PIC) samples. The ability of observing objects with dimensions down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2 \mu \text{m}$ </tex-math></inline-formula> is confirmed. This resolution is close to diffraction limit and corresponds to the dimensions of optical waveguide structures on the PICs surface. In addition, we demonstrate the implementation of a 2-D Fourier transform-based autofocusing technique for through-silicon IR microscopy. These building blocks offer a solution for advanced photonic integration processes and other through-silicon vision-related applications, which is instrumental for a large variety of assembly, lithography, and wafer bonding setups.

Fast-Response Amorphous In2Te3 Short-Wave Infrared (SWIR) Photodetector

Despite the wide applications of Short-Wave Infrared (SWIR) imaging, standard imaging materials require lattice matching and complex fabrication techniques which are relatively expensive. In this work, 50 nm thick amorphous indium telluride (In2Te3) films are RF sputtered on a silicon (Si/SiO2) substrate at 300 °C. The films are used to fabricate back-gated field-effect photodetectors (PDs). The In2Te3 detectors exhibit a broad wavelength response from 406 nm to 1600 nm and a photo responsivity of 0.44 AW− under 1310 nm excitation at 5 V bias. Furthermore, an n-type behavior with an electron field-effect mobility of 1.15 cm2V−1s−1 is observed. We also evaluated the frequency response of the PDs under 1310 nm modulated light, a 3dB cut-off frequency of ~0.97 MHz is measured. Further, the device exhibits specific detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.32\times 10^{{9}}$ </tex-math></inline-formula> Jones and very good stability at ambient conditions.