Spectral Stability Research Articles

Demonstration of molded glass primary optics for high-efficiency micro-concentrator photovoltaics

Micro-concentrator photovoltaics (micro-CPV) is a branch of the CPV technology where the size of the solar cell is reduced below the millimeter range to permit novel manufacturing methods for cost reductions and maintaining high efficiencies by using high performance optical materials. The reduction in size of all the components demands a very good optical quality of the primary lenses to achieve high concentrations and high optical efficiencies. PMMA or silicon-on-glass are the most used materials for CPV primary optics, but PMMA reliability against UV weathering is questionable and silicone has a large coefficient of thermal expansion (CTE) mismatch with respect to glass, which introduces a large thermal sensitivity in the optical efficiency. In contrast, direct glass molding is a promising alternative due to benefits such as a high Abbe number (lower dispersion losses), great mechanical properties, low CTE, low thermal coefficient of the refractive index (dn/dT), and a proven outdoor reliability with very little degradation. Furthermore, glass can generally be manufactured with high throughput, straightforward, and cost-effective compression or roller molding processes. In this work we present a collaboration with the company Holophane S.A.S. in France that resulted in the manufacturing of several micro-CPV lens arrays of plano-convex lenses with a design tailored to the requirements imposed by the manual compression molding process. A comprehensive experimental characterization has been carried out based on surface metrology and functional measurements with a collimated-light solar simulator, and the results are compared with ray-tracing simulations. The measured optical efficiency at the design concentration of 178X is 79.2 ± 2.5% for the whole array with a best lens efficiency of 82.2%. Furthermore, the best performing array has been used to assemble a demonstrator mono-module, which has been characterized outdoors. The outdoor characterization shows very good thermal and spectral stability of the lens array with an I–V curve fill factor of 84.8 ± 1.9% and an electrical efficiency of 27.2 ± 1.8%. The experience gained with these prototypes will lead to further optimization in the manufacturing process to increase the optical performance, and to a larger scale manufacturing for micro-CPV modules. This works paves the way for the development of high-efficiency robust glass-based micro-CPV modules able to generate maximum energy yield per area in space-constrained applications. • Moldable glass lens arrays for micro-CPV with a 178X geometrical concentration. • Small material volume in micro-CPV allow for more expensive materials with high optical performance. • 5 × 5 lens arrays with optical efficiencies for single lenses up to 82.2%. • A single cell module reached an electrical efficiency of 27.2% measured outdoors. • Stable performance during over 10 °C temperature variations.

Highly coherent, flat, and broadband time-stretched swept source based on extra-cavity spectral shaping assisted by a booster semiconductor optical amplifier.

We demonstrate a flat broadband time-stretched swept source based on extra-cavity spectral shaping. By adjusting the polarization-dependent gain profile and driving current of the booster optical amplifier (BOA), extra-cavity spectral shaping is optimized to generate output with a 1-dB bandwidth of ∼100 nm, 3-dB bandwidth of ∼140 nm and output power of ∼21.4 mW. The short-term and long-term stabilities are characterized. The average cross correlation of 183,485 round trips is 0.9997 with a standard deviation of 2×10-5, indicating high single-shot spectral similarity and high coherence. The noise floor of relative spectral energy jitter is -141.7 dB/Hz, indicating a high short-term spectral energy stability. The proposed highly stable flat broadband time-stretched swept source is applied to an optical coherence tomography (OCT) system. The axial resolution is 10.8 µm. The proposed swept source can serve as excellent light sources in ultra-fast coherent detection systems for high precision sensing and imaging.

High-Performance Ultra-Broadband Absorber for Polarized Long-Wavelength Infrared Light Trapping

Conventional infrared polarization imaging and detection systems are normally complex, bulky, and expensive, since rare existing absorber components possess polarization selectivity, especially in the long-wavelength infrared (LWIR) range. In this study, we propose a novel high-performance metamaterial absorber with ultra-broadband high absorption and polarization selectivity in the entire LWIR atmospheric window. The absorber can realize an absorptance higher than 90% with a 4750 nm bandwidth due to different surface plasmons coupled with optical phonons. It exhibits precise polarization-selective absorption at the target wavelength; specifically, it can selectively trap TM-polarized light while restricting the absorption of TE-polarized light. In addition, the designed metamaterial exhibits remarkable spectral stability when the geometric parameters of the microstructure change, which is of great benefit in its manufacturing process. The proposed ultra-thin absorber provides a promising approach to simplify LWIR imaging and detection systems and further improve their performance.

Spectral stability, spectral flow and circular relative equilibria for the Newtonian n-body problem

For the Newtonian (gravitational) n-body problem in the Euclidean d-dimensional space, d≥2, the simplest possible periodic solutions are provided by circular relative equilibria, (RE) for short, namely solutions in which each body rigidly rotates about the center of mass and the configuration of the whole system is constant in time and central (or, more generally, balanced) configuration. For d≤3, the only possible (RE) are planar, but in dimension four it is possible to get truly four dimensional (RE).A classical problem in celestial mechanics aims at relating the (in-)stability properties of a (RE) to the index properties of the central (or, more generally, balanced) configuration generating it. In this paper, we provide sufficient conditions that imply the spectral instability of planar and non-planar (RE) in R4 generated by a central configuration, thus answering some of the questions raised in [14, Page 63]. As a corollary, we retrieve a classical result of Hu and Sun [11] on the linear instability of planar (RE) whose generating central configuration is non-degenerate and has odd Morse index, and fix a gap in the statement of [6, Theorem 1] about the spectral instability of planar (RE) whose (possibly degenerate) generating central configuration has odd Morse index.The key ingredient is a new formula of independent interest that allows to compute the spectral flow of a path of symmetric matrices having degenerate starting point, and a symplectic decomposition of the phase space of the linearized Hamiltonian system along a given (RE) which is inspired by Meyer and Schmidt's planar decomposition [13] and which allows us to rule out the uninteresting part of the dynamics corresponding to the translational and (partially) to the rotational symmetry of the problem.

Theoretical analysis of beam quality degradation induced by laser spectral characteristics in spectral beam combining system

We establish a model based on the dispersion of the diffractive grating to investigate the influence of the spectral envelope and wavelength stability on the combined beam quality M 2 of the spectral beam combining (SBC) system. We obtain the analytic expressions of combined beam quality corresponding to the second-moment-spectral width of the beamlets, which can be used to forecast the combined beam quality for any integrable spectral envelope shape of beamlets in an SBC system. Additionally, we find that the center wavelength shift of beamlets has a significant impact on the combined beam quality. The second-moment-based width of the center wavelength shift distribution among all beamlets should be less than 0.5 times the spectral width. Here, the spectral width is the weighted root mean square of all beamlets’ spectral width, where the weight is the power of each single channel.

Highly Sensitive Ultrasonic Sensor Based on Polymer Bragg Grating and its Application for 3D Imaging of Seismic Physical Model

A miniaturepolymer Bragg grating (PBG) sensor is fabricated and employed for ultrasonic imaging of seismic physical models (SPMs). The sensing Bragg grating is inscribed into an ultraviolet (UV) glue polymer waveguide with a femtosecond laser. The uniform polymer waveguide is fabricated by sealing the UV glue into a capillary fibre through capillary effect. By using line-by-line inscription technique, the laser beam is scanned transversely to periodically modulate the refractive index of the polymer waveguide, leading to the formation of the PBGs with various grating lengths. The sensor response to ultrasonic waves are investigated experimentally. When compared to single-mode fiber Bragg grating and phase-shifted fiber Bragg grating, the PBG with the same reflectivity presents a higher response amplitude due to its lower Young’s modulus. Besides, the sensor has a good spectral stability when transferred from air to water due to the waterproof coating on the sensor end. Finally, the sensor is used to scan a large-scale 3D SPM and the structural features, such as fault, fluctuation, and depositional termination, can be distinctly reconstructed. The proposed PBG sensor provides a new technique with easy fabrication, high sensitivity, and good stability for high-fidelity ultrasonic imaging of seismicphysical models.

Cysteamine-Gold Coated Carboxylated Fluorescent Nanoparticle Mediated Point-of-Care Dual-Modality Detection of the H5N1 Pathogenic Virus

Globally, point-of-care testing (POCT) is the most preferable on-site technique for disease detection and includes a rapid diagnostic test (RDT) and fluorescent immunochromatographic strip test (FICT). The testing kits are generally insufficient in terms of signal enhancement, which is a major drawback of this approach. Sensitive and timely on-site POCT methods with high signal enhancement are therefore essential for the accurate diagnosis of infectious diseases. Herein, we prepare cysteamine-gold coated carboxylated europium chelated nanoparticle (Cys Au-EuNPs)-mediated POCT for the detection of the H5N1 avian influenza virus (AIV). Commercial nanoparticles were used for comparison. The spectral characteristics, surface morphologies, functional groups, surface charge and stability of the Cys AuNPs, EuNPs, and Cys Au-EuNPs were confirmed by UV-visible spectrophotometry, fluorescence spectrometry, transmission electron microscope with Selected area electron diffraction (TEM-SAED), Fourier-transform infrared spectroscopy (FTIR) and zeta potential analysis. The particle size distribution revealed an average size of ~130 ± 0.66 nm for the Cys Au-EuNPs. The Cys Au-EuNP-mediated RDT (colorimetric analysis) and FICT kit revealed a limit of detection (LOD) of 10 HAU/mL and 2.5 HAU/mL, respectively, for H5N1 under different titer conditions. The obtained LOD is eight-fold that of commercial nanoparticle conjugates. The photo luminance (PL) stability of ~3% the Cys Au-EuNPs conjugates that was obtained under UV light irradiation differs considerably from that of the commercial nanoparticle conjugates. Overall, the developed Cys Au-EuNPs-mediated dual-mode POCT kit can be used as an effective nanocomposite for the development of on-site monitoring systems for infectious disease surveillance.

Universal selection of pulled fronts

We establish selection of critical pulled fronts in invasion processes as predicted by the marginal stability conjecture. Our result shows convergence to a pulled front with a logarithmic shift for open sets of steep initial data, including one-sided compactly supported initial conditions. We rely on robust, conceptual assumptions, namely existence and marginal spectral stability of a front traveling at the linear spreading speed and demonstrate that the assumptions hold for open classes of spatially extended systems. Previous results relied on comparison principles or probabilistic tools with implied nonopen conditions on initial data and structure of the equation. Technically, we describe the invasion process through the interaction of a Gaussian leading edge with the pulled front in the wake. Key ingredients are sharp linear decay estimates to control errors in the nonlinear matching and corrections from initial data.

Investigation of hardware and software techniques to enhance the characteristics of focused ultrasound (FUS) spectra

Objective. Microbubble cavitation generated by focused ultrasound (FUS) can induce safe blood-brain-barrier (BBB) opening allowing therapeutic drug passage. Spectral changes in the hydrophone sensor signal are currently used to distinguish stable cavitation from inertial cavitation that can damage the BBB. Gibbs’ ringing, peak intensity loss and peak width increase are well-known distortions evident when using the discrete Fourier transform (DFT) to transform data containing a few hundred points. We investigate overcoming the fact that FUS time signals (10 ms providing 312 500 points sampled at 32 ns intervals) can generate such sharp spectral peaks that variations in their DFT-related distortions can significantly impact the values of the key metrics used for cavitation characterization. Approach. We introduce low-pass filter hardware to improve how the analogue to digital convertor handles high-frequency noise components and the orders of magnitude differences between FUS harmonic intensities. We investigate the enhanced FUS spectral stability and resolution obtained from a new technique, physical sparsification (PH-SP), customized to the a-priori information that all key FUS components are harmonically related. Results are compared with standard DFT optimizations involving time data windowing and Fourier interpolation. Main results. A new simulation model showed peak intensity, widths and metrics modified by small changes in the transformed signal’s length when removing the noisy starting transient of the FUS hydrophone signal or following minor excitation frequency or sampling rate adjustments. 25%–60% area-under-the-curve changes occurred in phantom studies at different pressure levels. Spectral peak sharpness was best optimized and stabilized with PH-SP. Significance. Special FUS characteristics mean starting transients and minor variations in experimental procedures lead to significant changes in the spectral metrics used to monitor cavitation levels. Customizing PH-SP to these characteristics led to sharper, more stable spectra with the potential to track the impact of microbubble environment changes.

Periodic multi-pulses and spectral stability in Hamiltonian PDEs with symmetry

We consider the existence and spectral stability of periodic multi-pulse solutions in Hamiltonian systems which are translation invariant and reversible, for which the fifth-order Korteweg-de Vries equation is a prototypical example. We use Lin's method to construct multi-pulses on a periodic domain, and in particular demonstrate a pitchfork bifurcation structure for periodic double pulses. We also use Lin's method to reduce the spectral problem for periodic multi-pulses to computing the determinant of a block matrix, which encodes both eigenvalues resulting from interactions between neighboring pulses and eigenvalues associated with the essential spectrum. We then use this matrix to compute the spectrum associated with periodic single and double pulses. Most notably, we prove that brief instability bubbles form when eigenvalues collide on the imaginary axis as the periodic domain size is altered. These analytical results are all in good agreement with numerical computations, and numerical timestepping experiments demonstrate that these instability bubbles correspond to oscillatory instabilities.

On the Stability of the Periodic Waves for the Benney System

We analyze the Benney model for interaction of short and long waves in resonant water wave interactions. Our particular interest is in the periodic traveling waves, which we construct and study in detail. The main results are that, for all natural values of the parameters, the periodic dnoidal waves are spectrally stable with respect to perturbations of the same period. For another natural set of parameters, we construct the snoidal waves, which exhibit instabilities, in the same setup. Our results are the first instability results in this context. On the other hand, the spectral stability established herein improves significantly on the work of Angulo, Corcho, and Hakkaev [Adv. Difference Equ., 16 (2011), pp. 523--550], which established stability of the dnoidal waves, on a subset of parameter space, by relying on the Grillakis--Shatah theory. Our approach, which turns out to give definite answer for the entire domain of parameters, relies on the instability index theory, as developed by Kapitula, Kevrekidis, and Sandstede [Phys. D, 3--4 (2004), pp. 263--282]; Kapitula, Kevrekidis, and Sandstede [Phys. D, 195 (2004), no. 3--4, 263--282], Phys. D, 201 (2005), pp. 199--201]; Lin and Zeng [Instability, Index Theorem, and Exponential Trichotomy for Linear Hamiltonian PDEs, 2021]; and Pelinovsky [Proc. R. Soc. Lond. Ser. A Math. Phys. Eng. Sci., 461 (2005), pp. 783--812]. Interestingly, end even though the linearized operators are explicit, our spectral analysis requires subtle and detailed analysis of matrix Schrödinger operators in the periodic context, which support some interesting features.

Ultra-trace and quantitative SERS detection of polycyclic aromatic hydrocarbons based on Au nanoscale convex polyhedrons with embedded probe molecules

Surface-enhanced Raman scattering (SERS) has great potential for the detection of marine pollutants, but it is still restricted in ultra-trace and quantitative analysis. Here, a strategy for the detection of polycyclic aromatic hydrocarbons (PAHs) was proposed based on Au nanoscale convex polyhedrons (Au NCPs) coated with high-energy facets and embedded with 4-ATP as a probe molecule. Au NCPs acted as SERS substrates and led to limits of detection (LODs) for six common PAHs that reached 0.01 nM. Using internal calibration, the relative standard deviations (RSD) of the spectral stability and reproducibility were as low as 3.36% and 5.11%, respectively. The maximum mean relative errors (AREs) of the predicted and true values were 6.28%. The results indicate that the resulting Au NCPs improved the ultra-trace and quantitative detection of SERS, thus suggesting that the Au NCPs have practical application value in SERS.

Spectral enclosures and stability for non‐self‐adjoint discrete Schrödinger operators on the half‐line

We make a spectral analysis of discrete Schrödinger operators on the half-line, subject to complex Robin-type boundary couplings and complex-valued potentials. First, optimal spectral enclosures are obtained for summable potentials. Second, general smallness conditions on the potentials guaranteeing a spectral stability are established. Third, a general identity which allows to generate optimal discrete Hardy inequalities for the discrete Dirichlet Laplacian on the half-line is proved.

Spatiotemporal dynamics in a twisted, circular waveguide array

AbstractWe consider the existence and spectral stability of nonlinear discrete localized solutions representing light pulses propagating in a twisted multicore optical fiber. By considering an even number, N, of waveguides, we derive asymptotic expressions for solutions in which the bulk of the light intensity is concentrated as soliton‐like pulses confined to a single waveguide. The leading order terms obtained are in very good agreement with results of numerical computations. Furthermore, as in the model without temporal dispersion, when the twist parameter, ϕ, is given by , these standing waves exhibit optical suppression, in which a single waveguide remains unexcited, to leading order. Spectral computations and numerical evolution experiments suggest that these standing wave solutions are stable for values of the coupling parameter less than a critical value, at which point a spectral instability results from the collision of an internal eigenvalue with the eigenvalues at the origin. This critical value has a maximum when .

Test Results From the Prelaunch Characterization Campaign of the Engineering Test Unit of the Ocean Color Instrument of NASA’s Plankton, Aerosol, Cloud and Ocean Ecosystem (PACE) Mission

This paper summarizes the results from the system level test campaign of the Engineering Test Unit (ETU) of the ‘Ocean Color Instrument’ (OCI), the primary payload of NASA’s ‘Plankton, Aerosol, Cloud and ocean Ecosystem’ (PACE) mission. The main goals of the test campaign were to optimize characterization procedures and evaluate system level performance relative to model predictions. Critical performance parameters such as radiometric gain, signal-to-noise ratio, polarization, instantaneous field-of-view, temperature sensitivity, relative spectral response and stability were evaluated for wavelengths from 600 to 2,260 nm and are in line with expectations. We expect the OCI flight unit to meet the PACE mission performance requirements. Building and testing the ETU has been extremely important for the development of the OCI flight unit (e.g. improved SNR by increasing the aperture, optimized thermal design), and we strongly recommend the inclusion of an ETU in the development of future spaceborne sensors that rely on novel technological designs. ETU testing led to the discovery of a hysteresis issue with the SWIR bands, and a correction algorithm was developed. Also, the coregistration of the SWIR bands relative to each other is worse than expected, but this was discovered too late in the schedule to remediate.

Slow localized patterns in singularly perturbed two-component reaction–diffusion equations

Localized patterns in singularly perturbed reaction–diffusion equations typically consist of slow parts, in which the associated solution follows an orbit on a slow manifold in a reduced spatial dynamical system, alternated by fast excursions, in which the solution jumps from one slow manifold to another, or back to the original slow manifold. In this paper we consider the existence and stability of stationary and travelling localized patterns that do not exhibit such jumps, i.e. that are completely embedded in a slow manifold of the singularly perturbed spatial dynamical system. These ‘slow patterns’ have rarely been considered in the literature, for two reasons: (i) in the classical Gray–Scott/Gierer–Meinhardt type models that dominate the literature, the flow on the slow manifold is typically linear and thus cannot exhibit homoclinic pulse or heteroclinic front solutions; (ii) the slow manifolds occurring in the literature are typically ‘vertical’, i.e. given by u ≡ u 0, where u is the fast variable, so that the stability problem is determined by a simple (decoupled) scalar equation. The present research concerns a general system of singularly perturbed reaction–diffusion equations and is motivated by several explicit ecosystem models that do give rise to non-vertical normally hyperbolic slow manifolds on which the flow may exhibit both homoclinic and heteroclinic orbits that correspond to either stationary or travelling localized slow patterns. The associated spectral stability problems are at leading order given by a nonlinear, but scalar, eigenvalue problem with Sturm–Liouville characteristics and we establish that homoclinic pulse patterns are typically unstable, while heteroclinic fronts can either be stable or unstable. However, we also show that homoclinic pulse patterns that are asymptotically close to a heteroclinic cycle may be stable. This result is obtained by explicitly determining the leading order approximations of four critical asymptotically small eigenvalues. By this analysis, that involves several orders of magnitude in the small parameter, we also obtain full control over the nature of the bifurcations—saddle-node, Hopf, global, etc—that determine the existence and stability of the (stationary and/or travelling) heteroclinic fronts and/or homoclinic pulses. Finally, we show that heteroclinic orbits may correspond to stable (slow) interfaces in two-dimensional space, while the homoclinic pulses must be unstable as localized stripes, even when they are stable in one space dimension.

Sub-40 fs optical parametric oscillator beyond the gain bandwidth limit.

We report a compact and passively stable optical parametric oscillator for direct generation of sub-40 fs pulses, five times shorter than the 200 fs pump oscillator. By employing an intracavity all normal dispersion feedback fiber, we achieved low-noise and coherent broadening beyond the parametric gain bandwidth limitation. We demonstrate spectral coverage from 1.1 to 2.0 µm with excellent passive power and spectral stability below 0.1% rms and a footprint smaller than 14 × 14 cm2.

Island-like random scattering layer for light extraction in organic light-emitting diodes with enhanced luminous efficiencies and angular spectral stability

Optical outcoupling efficiency of organic light-emitting diode (OLED) can be improved using nanoscale or sub-micrometer quasi-periodic structures. An embossed nano/micro-structure inserted between the substrate and anode is known to be effective in extracting the waveguide mode. However, the techniques to fabricate these nanoscale or sub-micrometer structures are usually complicated, and the light extraction is wavelength-oriented. Herein, we develop a simple process to enhance the performance of OLED. We texture the glass surface with shrinking Ag droplets inthe form of island structures and then deposit SiNx planarization film to form an island-like random scattering layer (IRSL), which changes the light path of emitting light in a way to reduce light attenuation and extract emission outward. Furthermore, IRSL has a low haze and a high optical transparency, both of which are beneficial to light output. The OLEDs constructed with IRSL as an internal light extraction structure exhibit a significant improvement of current and power efficiencies by 52% and 62%, respectively, compared to the planar OLED device. In addition, compared to the conventional pillar-structured RSL, the IRSL not only facilitates the extraction of light for a broad range of visible light region, but also favors better angular spectral stability and Lambertian emission.

Spectral stability of the Steklov problem

This paper investigates the stability properties of the spectrum of the classical Steklov problem under domain perturbation. We find conditions which guarantee the spectral stability and we show their optimality. We emphasize the fact that our spectral stability results also involve convergence of eigenfunctions in a suitable sense according with the definition of connecting system by Vainikko (1979). The convergence of eigenfunctions can be expressed in terms of the H1 strong convergence. The arguments used in our proofs are based on an appropriate definition of compact convergence of the resolvent operators associated with the Steklov problems on varying domains.In order to show the optimality of our conditions we present alternative assumptions which give rise to a degeneration of the spectrum or to a discontinuity of the spectrum in the sense that the eigenvalues converge to the eigenvalues of a limit problem which does not coincide with the Steklov problem on the limiting domain.

Synthesis and application of the fluorescent furan and imidazole probes for selective in vivo and in vitro cancer cell imaging

Development of imaging probes for identification of tumors in the early stages of growth can significantly reduce the tumor-related health hazards and improve our capacity for treatment of cancer. In this work, three different furan and imidazole fluorescent derivatives abbreviated as Cyclo X, SAC and SNO are introduced for in vivo and in vitro imaging of cancer cells. The fluorescence quantum yield values were 0.226, 0.400 and 0.479 for Cyclo X, SAC and SNO, respectively. The excitation and emission wavelengths of maximum intensity were (360, 452), (350, 428) and (350, 432) nm for Cyclo X, SAC and SNO, respectively. The MTT reduction assay was used to estimate the cytotoxic activity of the proposed derivatives against HT-29 (cancer) and Vero (normal) cell lines. Cyclo X showed no cytotoxic effect, while SAC and SNO showed significantly higher cytotoxicity against the tested cell lines than cisplatin as a well-known anticancer drug. In vitro fluorescence microscopic images obtained using HT-29 cells showed that Cyclo X produced very bright images. The in vivo cancer cell imaging using 4T1 tumor-bearing mice revealed that Cyclo X is selectively accumulated in the tumor without distribution in the mice body organs. The spectral and structural stability, large Stokes shift, non-cytotoxicity and high level of selectivity for in vivo imaging are properties that make Cyclo X a suitable candidate to be used for long-term monitoring of cancer cells.