Wavelength Response Research Articles

Enhancing Wavelength Tunability of Photonic Crystal Nanolasers by Waveguide-Like Strain Shapers

In this report, we propose an idea of connecting 1D photonic crystal (PhC) nanocavity with novel waveguide-like strain shapers in a polydimethylsiloxane substrate. In theory, the induced film-edge strain by the strain shapers can significantly enhance the wavelength tunability of PhC nanocavity under different applied stress. By our nanofabrication process, we realize this idea in the form of a tunable nanolaser. In experiments, the nanolaser shows the enhanced wavelength tunability both under stretching and compressive stress. By investigating the effects of PhC periods and length of the strain shapers, we obtain an optimized lasing wavelength tunability of −13.9 nm under every percentage strain variation in experiments. When further taking the 2nd -order mode in the nanocavity into consideration, we observe a wide lasing wavelength tuning range over 160 nm, which takes only the applied strain $\Delta \xi _{tot}$ of ±0.05. In addition to the large wavelength response beneficial for tunable nanolasers and optical strain sensors, we also believe that this waveguide-like strain shapers can serve as the optical interconnections at the same time in the flexible photonic circuits.

Inter-Cross De-Modulated Refractive Index and Temperature Sensor by an Etched Multi-Core Fiber of a MZI Structure

We present a relative sensitivity of in-fiber inter-cross demodulation of a Mach-Zehnder interferometer (MZI) based on an etched multi-core fiber (eMCF). The sensor can measure the external refractive index (RI) and temperature with a large fringe visibility of 15 dB. It is tuned using a simple technique of slow chemical etching. When the outer cores of MCF will be exposed to the surrounding, a large difference of relative effective RI is observed, which enhances the sensitivity of the sensor. The sensor's wavelength and intensity responses have displayed that it can function with three different inter-cross-demodulation phenomena. A superior RI sensitivity of 178.20 dB/RIU in the range of 1.334 to 1.370, and temperature sensitivity of 66.73 pm/°C in the range of 30 to 80 °C are obtained, with an adequate linear response. Besides, it can readily resolve the issues of cross-sensitivity. Moreover, it has many advantages including easy fabrication, compact size, multiplex, repeatable, stable, and can easily differentiate RI and temperature, which lack others.

Optical properties of kalihinol derivatives in TPA, ECD and ROA

The kalihinols possess the highest skeletal and functional group complexity of the biologically enigmatic isocyanoterpene (ICT) class. In this paper, one-photon absorption (OPA), two-photon absorption (TPA), electron circular dichroism (ECD) spectroscopy, resonance Raman spectroscopy and Raman optical activity spectroscopy (ROA) are used to explore the optical properties and electronic excitation characteristic of kalihinol D, kalihinol A and kalihinene Y. By using the transition density matrix (TDM) and the charge difference density (CDD) maps, the situation of electron-hole interaction is analyzed. During the analysis of ECD spectra, the strong coupling between the transition electric dipole moment with the transition magnetic dipole moment and the transition electric quadrupole moment with the transition electric dipole moment can be showed obviously because of the visualization method. The molecular characteristics reflect different wavelength response and different polarized light absorption. It also corresponds to different modes of Raman vibration.

Capillary Fiber Bragg Grating Fabricated by Femtosecond Laser for Sensing Applications

We report the demonstration of fiber Bragg grating (FBG) fabricated in silica capillary fiber by use of femtosecond laser. The FBG is inscribed in the silica cladding surrounding the air core by a transversal scanning method through a phase mask. Multimode resonances which result from the overlap of the grating structure and the cladding modes transmitting in the capillary fiber, are observed in the reflection spectra. Sensing capability the FBG is exemplified by infiltrating the hollow core with liquids of different refractive indices (RIs), and shift in resonant wavelengths is observed. The temperature characteristic is investigated with a heating treatment from room temperature to 1000°C. Linear wavelength response is exhibited, with temperature sensitivity of 8 pm/°C. The FBG can be finding applications as optofluidic element for biochemical sensing.

Photogating-controlled ZnO photodetector response for visible to near-infrared light

In recent years, as a direct wide band gap semiconductor, zinc oxide (ZnO) nanomaterial has attracted a lot of attention. However, the widely investigated ZnO materials are strongly limited in fast-response and broadband photodetectors due to their inherent weaknesses, so an effective structure or mechanism of ZnO nanostructure photodetector is greatly needed. In this work, a photogating-controlled photodetector based on a ZnO nanosheet-HfO2-lightly doped Si architecture is demonstrated. Its performance was significantly improved by the photogating-controlled local field at the Si and HfO2 interfaces compared to the findings in other published works on ZnO. Consequently, the photodetector not only effectively balances the responsivity (as high as 5.6 A W−1) and response time (400 µs), but also broadens the wavelength response of the ZnO-based photodetectors from visible to near-infrared light range (~1200 nm). Additionally, the photogating-controlled ZnO photodetector enables high-resolution imaging both in the visible and near-infrared bands. Our photogating-controlled ZnO photodetectors not only exemplify the controllability of the gate electrode in high mobility materials but also provide a basis for the development of fast speed and high responsivity detection of high mobility materials.

Anisotropic localized surface plasmons in borophene.

We present a theoretical study on the plasmonic response of borophene, a monolayer 2D material that is predicted to exhibit metallic response and anisotropic plasmonic behavior in visible wavelengths. We investigate plasmonic properties of borophene thin films as well as borophene nanoribbons and nanopatches where polarization-sensitive absorption values in the order of 50% is obtained with monolayer borophene. It is demonstrated that by adding a metal layer, this absorption can be enhanced to 100%. We also examine giant dichroism in monolayer borophene which can be tuned passively (patterning) and actively (electrostatic gating) and our simulations yield 20% reflected light with significant polarization rotation. These findings reveal the potential of borophene in the manipulation of phase, amplitude and polarization of light at the extreme subwavelength scales.

A Novel Phenoxazine‐based Fluorescent Probe for the Detection of HOCl in Living Cells

Hypochlorous acid (HOCl), one of the reactive oxygen species (ROS), is a key microbicidal agent which is used for natural defense. However, it is also linked to varieties of human diseases owing to the overproduction of HOCl. Much effort has been made to exploit selective fluorescent sensors for the detection of HOCl, but most of them have some disadvantages such as short excitation wavelength, low selectivity, and slow response and so on. These restrict the biological application of the probes. In this work, BR‐O was designed and synthesized on the base of phenoxazine for the detection of HOCl. BR‐O exhibited a violent fluorescence enhancement in the presence of HOCl, showing excellent selectivity and high sensitivity. More importantly, the probe BR‐O was capable of detecting exogenous and endogenous HOCl in living cells.

Simulation of multi-layer antireflection film for crystalline silicon solar cell

Based on the design theory of single-layer, double-layer and multi-layer antireflection films, the thickness and refractive index of each layer of three-layer antireflection films are designed and calculated. The theoretical results of double-layer and three-layer antireflection films was simulated by PC1D, and reflectance, internal and external quantum efficiency and device characteristics of solar cells with double-layer and three-layer antireflection coatings were comparative studied. The results show that the three-layer antireflection film has a wider wavelength response range and higher external quantum efficiency, and the conversion efficiency of the three-layer antireflection film Crystalline silicon solar cells is improved by 0.2% compared with the double-layer antireflection film Crystalline silicon solar cells.

Large Wavelength Response to Pressure Enabled in InGaN/GaN Microcrystal LEDs with 3D Architectures

Optical detection of pressure has the advantage of direct and dynamic indication of the pressure distribution with a high spatial resolution. In this study, microcrystal (µ-crystal) light-emitting ...

In-Situ Tailoring of Vertically Coupled InAs p-i-p Quantum-Dot Infrared Photodetectors: Toward Homogeneous Dot Size Distribution and Minimization of In–Ga Intermixing

The authors report a detailed analysis of an epitaxial growth technique for indium arsenide (InAs) Quantum-dot infrared photodetectors circumvent the detrimental effects arising from the progressively increasing dot-size in vertically coupled heterostructures. Constant overgrowth percentage of the vertically coupled dot-layers has been achieved with the implementation of the growth strategy, which has been validated by cross-sectional transmission electron microscopy (X-TEM) images of the samples. The optical characteristics of these samples have been analyzed through photoluminescence spectroscopy and photoluminescence excitation spectroscopy (PL and PLE) measurements which show longer wavelength response and reduced full width at half-maxima (fwhm) upon implementation of the growth strategy. X-TEM and in-plane and out-of-plane high resolution X-ray diffraction (HR-XRD) measurements suggest morphological improvement upon implementation of the growth strategy, with a reduction in the indium desorption and lowering of defects and dislocation densities. Excellent correlation has been found between the different experimental results and also their theoretical simulations. The fabricated single-pixel photodetectors at low temperature (T = 14 K) show a broad response extending up to the MWIR region (∼4.5 μm) for one of the samples. Also, a strong spectral response in the SWIR region is obtained even at room temperature (T = 300 K). The highest responsivity (Rp) and specific detectivity (D*) values obtained are 166.17 A/W and 8.39 × 1010 cm Hz1/2 W–1 at a bias of 5 V and 300 K temperature.

Effect of front surface light trapping structures on the PERC solar cell

Properties of the front textured surface shape and anti-reflection coating have a great impact on the performance of solar cells. In this paper, the simulation model of the minimum unit cell structure is established and validated, which is based on the framework of Silvaco software and basic parameters of the standard pyramid textures single crystalline silicon PERC solar cell. The effect of the front surface light trapping structures on cell performance is discussed. It is found that the slightly concave pyramid-like textures can improve the response for short wavelengths and the short-circuit current density of the cell is increased by 0.3 mA/cm2, which is improved by 0.80%. In addition, by properly controlling the preparation process of the anti-reflection coating, a gradient-index SiOxNy/Si3N4 double-layer anti-reflection coating (DLARC) can be formed, which can significantly reduce the reflectivity for short wavelengths. And the short-circuit current density of the cell can be increased by 0.32 mA/cm2, which is improved by 0.86%. Finally, the optimized slightly concave pyramid-like textures and the SiOxNy/Si3N4 DLARC can improve the photoelectric conversion efficiency of the PERC solar cell by 0.18% and 0.20%, respectively.

Tandem Mach Zehnder Directional Coupler Design and Simulation on Silicon Platform for Optical Coherence Tomography Applications.

We design and compare the splitting ratio wavelength flatness of directional coupler (DC), Mach-Zehnder directional coupler (MZDC), and tandem MZDC. All coupler responses are analyzed, and tandem MZDC performance is the best in the wavelength insensitivity compared with the other two. An MZDC with any coupling ratio could be utilized to match the maximum flatness in a 40-nm wavelength range. To extend a broad flatness range, the tandem MZDC is proposed and still follows the Mach Zehnder structure taking two MZDCs as couplers connected through a decoupled region. Unlike DC, MZDC with the flat wavelength response has a non-linear output phase. Hence, using two wavelength-insensitive MZDCs as the coupling function in a tandem MZDC could demonstrate a more extensive decoupled phase term to maximize the flat wavelength response. The tandem MZDC theoretically demonstrates the splitting ratio with 100-nm flatness in the wavelength range from 1250 nm to 1350 nm. Finally, a point spread function through the tandem MZDC shows a 24-dB signal-to-noise ratio improvement in optical coherence tomography applications.

A high-accurate and universal method to characterize the relative wavelength response (RWR) in wavelength modulation spectroscopy (WMS)

Considering the importance of the laser wavelength response and the difficulty in its real-scenario measurement in WMS, a high-accuracy and universal method was developed to characterize the relative wavelength response (RWR) by analyzing the laser current response. A coupling term that depends on both the current scan and the modulation characteristic was introduced to describe the coupling effect between the wavelength scan and modulation. The accuracy of the proposed method was verified with different laser working conditions and scan waveforms. All fitting residuals of the RWR result from the proposed method are smaller than 0.1% of the total scan range and the fitting residual of the ramp scanned WMS is twice smaller than the minimum value from literature. The better calibration-free 2f/1f fitting and more accurate CO2 concentration results also suggest the high accuracy and superiority of the proposed method. Finally, based on the precise prediction of RWR with small scan and modulation indices, the spectral parameters, including line strength and self-collisional broadening coefficient, of CO2 transition at 6976.2026 cm-1 were successfully measured using WMS.

Electronic structure and second-order nonlinear optical properties of lemniscular [16]cycloparaphenylene compounds.

Chiral organic compounds are excellent second-order nonlinear optical (NLO) materials due to their inherent non-symmetric electronic structures combined with the advantages of organic compounds. At present, density functional theory (DFT) has become a powerful tool for predicting the properties of novel materials. In this paper, based on chiral lemniscular [16]cycloparaphenylene, three novel compounds are designed by introduction of donor/acceptor units and their combinations. The geometrical/electronic structure, electronic absorption, and the second-order NLO properties of these compounds have been systematically investigated by DFT/TDDFT theory. The simulated UV-Vis/CD spectra of compound 1 are in good agreement with the experimental ones, enabling us to assign their electronic transition characteristics and absolute configuration with high confidence. The investigations show that energy gaps, absorption wavelength and second-order NLO response may be effectively tuned by the introduction of the donor or acceptor units or their combinations. For instance, the second-order NLO value of compound 4 is about 207 times as large as the average second-order polarizability of the organic molecule urea. Thus, the studied compounds are expected to be potential large second-order NLO materials.

Coherence imaging spectroscopy at Wendelstein 7-X for impurity flow measurements.

In the last decade, Coherence Imaging Spectroscopy (CIS) has shown distinctive results in measuring ion flow velocities in the edge of magnetically confined plasma devices. Its 2D spatially resolved measurement capabilities and its high optical throughput are ideal for investigating the impurity behavior in the complex 3D magnetic island topology edge of Wendelstein 7-X (W7-X). However, a highly precise and stable calibration method is required for a reliable diagnostic operation. A new level of precision and stability has been achieved for the two CIS systems installed at W7-X with the use of a new calibration source, a continuous tunable laser commercially available only since 2015. A specific prototype model was successfully adapted to the challenging requirements of W7-X, granting high accuracy (±0.01 pm) and flexibility (spectral range: 450-650 nm) in the wavelength calibration required for measuring low-Z impurity ion flow velocities. These features opened up new investigation possibilities on temperature stability and wavelength response of the CIS components, allowing to fully characterize and validate the W7-X systems. The CIS diagnostic was operational throughout the last W7-X experimental campaign. Measured velocities on the order of ∼20-30 km/s were observed, corroborated by comparisons with measurements with Mach probes.

Spectral Behavior of Pinus elliottii, Subjected to Drought in Southern Brazil

The second most cultivated plant to a forest range in Brazil is the Pinus elliottii. This paper aims to evaluate the spectral behavior of individuals of Pinus elliottii in the electromagnetic radiation (1st) of the visible and (2nd) the infrared, when submitted to the water deficit. The experiment was conducted in the city of Santa Maria located at the Rio Grande do Sul State. The spectral responses of 10 individuals of Pinus elliottii were observed in different climatic seasons at 24 months of age. To evaluate the spectral behavior a spectroradiometer was used, and the behavior in the 450 to 1700 nm range was analyzed. Five individuals were submitted to water deficit treatment and five were used as controls, remaining in adequate water conditions. The spectral pattern for individuals submitted to water deficit was similar, but a trend of wavelength responses in the region of visible green (577-491 nm) and near-infrared (1100-789 nm) was noticeable. The most evident differences occurred in the fall season when differences in spectral amplitudes are noticeable and significantly different from the rest of the year.

Strain shapes the light in a photonic crystal nanocavity

This study proposes and demonstrates a novel nanoclamp structure symmetrically disposed near a one-dimensional (1D) photonic crystal (PhC) nanocavity embedded in a deformable polydimethylsiloxane substrate. These nanoclamps show capabilities of locally shaping (including enhancing and inhibiting) the strain of PhC nanocavity. The produced artificial non-ideal elastomer leads to an enhanced wavelength response of − 12.7 nm for every percentage compressive strain variation from the tunable PhC nanolasers in experiments. This result not only guarantees the excellent tunability of the 1D PhC nanolasers but also promises ultrahigh sensitivity in strain sensing. Moreover, such nanoclamps can further create a reconfigurable conversion between waveguide and nanocavity with a 1–2 order difference in the quality factor when applied to a 1D PhC waveguide. We believe this study provides a possibility for on-demand sculpturing of the optical properties of tunable PhC devices in the nanoscale by inserting additional nano- or micro-structures.

Spin‐On‐Patterning of Sn–Pb Perovskite Photodiodes on IGZO Transistor Arrays for Fast Active‐Matrix Near‐Infrared Imaging

AbstractFlat‐panel imagers have wide applications in industrial and medical inspections. Nonetheless, large area infrared imaging remains a challenge due to the fact that the state‐of‐the‐art infrared sensors are usually based on silicon or germanium technologies, which are limited by the wafer size. Recent advances in low bandgap Sn–Pb perovskite photodiodes (PDs) and indium gallium zinc oxide (IGZO) thin‐film transistors (TFTs) matrix backplane bring new opportunity for developing the large area near‐infrared image sensor. As a proof of concept, a 12 × 12 pixels array with each pixel independently controlled by the gate voltage of a TFT are constructed. Arrays of Sn–Pb based perovskite PDs are spin deposited onto the IGZO TFT drain electrode via self‐assembled patterning process. The low bandgap perovskite PD exhibits a broad spectral response for wavelength from 300 to 1000 nm, featuring a high light to the dark current ratio of ≈104, and a high specific detectivity (D*) of ≈1011 Jones at 850 nm (biased at −0.1 V). The integration takes advantage of the high mobility of IGZO transistors and the high infrared sensitivity of low Sn–Pb perovskite materials, which enables the next generation near‐infrared flat‐panel imager with high frame rate and low operating voltages.

Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin two-dimensional (2D) crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm (3.1 eV) increases by a relative 10 percent, while the long wavelength response at 633 nm (2.0 eV) increases by a relative 36 percent on average and up to 80 percent at localized hot spot regions when photocathodes are deposited onto graphene coated stainless steel. There is a similar effect for photocathodes deposited on hexagonal boron nitride monolayer coatings using nickel substrates. The enhancement does not occur when reflective substrates are replaced with optically transparent sapphire. Optical transmission, X-ray diffraction (XRD) and X-ray fluorescence (XRF) revealed that thickness, crystal orientation, quality and elemental stoichiometry of photocathodes do not appreciably change due to 2D crystal coatings. These results suggest optical interactions are responsible for the QE enhancements when 2D crystal sublayers are present on reflective substrates, and provide a pathway toward a simple method of QE enhancement in semiconductor photocathodes by an atomically thin 2D crystal on substrates.

A Concave Blazed-Grating-Based Smartphone Spectrometer for Multichannel Sensing

A compact imaging smartphone spectrometer utilizing a lightweight aspheric concave blazed-grating is presented for multichannel spectral detection. The lightweight concave diffraction grating is fabricated on a plastic substrate using a two-step thermal-embossing method in a cost-effective and mass-productive way, which enables a large-area, smooth and aspheric concave surface while preserving a blazed-grating structure. The blazed-grating serves as a key single element for aberration-free light focusing and highly-efficient light diffraction. It thus eliminates extra moving optics in the spectrometer and improves the optical characteristics and system integration, which are proven by both simulation and experiments. Spectral linewidth of 1 nm is achieved by combining the concave grating with a smartphone camera. This compact imaging smartphone spectrometer is only about the size of a match-box. Spectra can be obtained directly from the detected images using a developed application (APP) interface. High-throughput plasmonic resonance sensing using microfluidic silver nanoslit arrays under various glycerin environments is demonstrated, and the detected resonance wavelength response is comparable with that detected by a commercial compact spectrometer.