Spherical Array Research Articles

Fabrication of quartz microlenses using laser ablation

Various quartz microlenses were fabricated in two stages with two lasers. A fiber laser at 1.06 µm was employed for the laser ablation, while a CO2 laser at 10.6 µm was used for direct laser writing (DLW). The experimental data demonstrate the formation of spherical, cylindrical microlenses and microlens array (MLA) with an optimum numerical aperture (NA) of 0.55. Theoretical calculations and simulation were conducted to estimate the temperature at the quartz surface during ablation and simulation of the reshaping. A temperature of 2500 K is achieved in the ablation, and 2015 K was established during the reshaping. The ablation depth is 280 µm, the diameter is 150 µm, and the heat affected zone (HAZ) is approximately 1345 µm after 1 s. Due to the controllable laser micromachining, optoelectronics and biological imaging are viable applications for MLAs.

Point spread function modeling for photoacoustic tomography - I: three-dimensional detection geometries.

Photoacoustic computed tomography (PACT) has been under intensive investigation as a promising noninvasive biomedical imaging modality. Various acoustic detector arrays have been developed to achieve enhanced imaging performance. In this paper, we study the effect of the detection geometry on image quality through point spread function (PSF) modeling based on back-projection image reconstruction. Three commonly-used three-dimensional detection geometries, namely, spherical, cylindrical, and planar detector arrays, are investigated. The effect of detector bandwidth and aperture on PSF in these detection geometries is also studied. This work provides a performance evaluation tool for acoustic detector arrays used in PACT and can be helpful in the design and selection of detector arrays in practical imaging applications.

Investigation of randomly populated cylindrical, spherical, and cubical arrays for application in space, aerial and underwater collaborative beamforming

Investigation of randomly populated cylindrical, spherical, and cubical arrays for application in space, aerial and underwater collaborative beamforming

Prussian blue@Au nanoparticles/SiO2 cavity/ITO electrodes: application as a hydrogen peroxide sensor

A highly sensitive enzyme-free hydrogen peroxide sensor based on PB@Au NPs/SiO2 spherical cavity array electrode is constructed, which is investigated in the reduction of H2O2.

A Compact Spherical Rectifier Antenna Array for Directionally Insensitive Wireless Energy Harvesting Systems

A Compact Spherical Rectifier Antenna Array for Directionally Insensitive Wireless Energy Harvesting Systems

Multi-Scale Volumetric Dynamic Optoacoustic and Laser Ultrasound (OPLUS) Imaging Enabled by Semi-Transparent Optical Guidance.

Major biological discoveries are made by interrogating living organisms with light. However, the limited penetration of un-scattered photons within biological tissues limits the depth range covered by optical methods. Deep-tissue imaging is achieved by combining light and ultrasound. Optoacoustic imaging exploits the optical generation of ultrasound to render high-resolution images at depths unattainable with optical microscopy. Recently, laser ultrasound has been suggested as a means of generating broadband acoustic waves for high-resolution pulse-echo ultrasound imaging. Herein, an approach is proposed to simultaneously interrogate biological tissues with light and ultrasound based on layer-by-layer coating of silica optical fibers with a controlled degree of transparency. The time separation between optoacoustic and ultrasound signals collected with a custom-made spherical array transducer is exploited for simultaneous 3D optoacoustic and laser ultrasound (OPLUS) imaging with a single laser pulse. OPLUS is shown to enable large-scale anatomical characterization of tissues along with functional multi-spectral imaging of chromophores and assessment of cardiac dynamics at ultrafast rates only limited by the pulse repetition frequency of the laser. The suggested approach provides a flexible and scalable means for developing a new generation of systems synergistically combining the powerful capabilities of optoacoustics and ultrasound imaging in biology and medicine.

Evaluation of virtual sensing with spherical array surrounding head for directional noise

Active noise control (ANC) is expected to be applicable to public transportation modalities such as airplanes and trains. In these application systems, passengers may need to avoid wearing devices such as earphones and ear microphones. However, in conventional ANC systems, it is necessary to locate a physical microphone at the ear positions as an error sensor. To overcome this problem, virtual sensing techniques have been studied for estimating sound pressures at ear positions using remote microphones. In this study, we investigated the performance of a virtual sensing method for interpolating the sound pressures at ear positions using a spherical microphone array surrounding the head, based on a spherical harmonic expansion. The experiments were conducted using a rigid sphere instead of a head and dodecahedral spherical microphone array surrounding the sphere. The estimation errors were evaluated for various noise directions at low frequencies (100, 300, 500, and 700 Hz). The experimental results showed that the estimation errors were less than -20 dB for almost all conditions below 500 Hz. By comparison with other virtual sensing methods using a few microphones in the vicinity of the ear positions, we confirmed that our proposed method was effective for any noise direction.

Radiation Pattern Synthesis Using Constraint Convex Optimization Algorithm for Conformal Antenna Array

In this paper, a model based on constraint convex optimization is proposed for the radiation pattern synthesis in cylindrical and spherical microstrip conformal array antennas. The optimization algorithm is modeled to decrease the Euclidean distance between the obtained radiation pattern of the deformed array and the desired radiation pattern of the planar array by calculating the suitable amplitude and phase values for the individual array patches. Four prototypes of 2×2, 3×3, 4×4, and 5×5 microstrip patch array antenna are used as the radiating elements which are deformed from planar arrangement and placed on the faces of prescribed spherical and cylindrical shapes with various radii of curvatures. The results reveal that the performance of optimization algorithm performs better in 5×5 array, followed by 4×4 array, 3×3 array, and 2×2 array. However, 5×5 array provides more degrees of freedom for tuning the radiation pattern in desired form. The 5×5 array computes suitable amplitude and phase values for the radiating elements of the array, which restore the radiation pattern in terms of mainlobe reconstruction, null locations, sidelobe levels, up to the deformation of 20 cm in both spherical and cylindrical configurations of conformal array. The proposed optimization technique when applied to conformal antenna array offers fast computational speed and good convergence accuracy for radiation pattern synthesis, which can be useful for various engineering applications.

Theoretical study on bubble dynamics under hybrid-boundary and multi-bubble conditions using the unified equation

This paper aims to use the unified bubble dynamics equation to investigate bubble behavior in complex scenarios involving hybrid free surface/wall boundaries and interactions between multiple bubbles. The effect of singularity movement on the unified equation’s form is analyzed after deriving the bubble pulsation equation using a moving point source and a dipole, followed by discussions on the effect of migration compressibility-related terms on the bubble dynamics. In addition, the present study accounts for the impact of hybrid boundaries, including crossed and parallel boundaries, by introducing a finite number of mirror bubbles for the former and an infinite number of mirror bubbles for the latter. Spark bubble experiments and numerical simulation are conducted to validate the present theory. The application of the unified equation in multi-bubble interactions is exemplified by computing a spherical bubble array containing more than 100 uniformly distributed cavitation bubbles under different boundary conditions. The bubble cluster-induced pressure peak can reach nearly two times or even higher than that of an individual bubble, highlighting the damage potential caused by cavitation bubble clusters.

Computation of spherical sector harmonics norm with application to blind source separation.

Spherical microphone arrays (SMAs) are widely being used for source localization and separation. However, it is uneconomical to build a full SMA when sources are present in restricted regions of environment. Hence, a spherical sector microphone array is utilized for blind source separation for the first time. In particular, the norm of the spherical sector harmonics basis function is computed for mixing matrix estimation. The estimated steering vectors are clustered using mean-shift algorithm. The number of sources is estimated automatically from the number of clusters. The developed mathematical framework is verified using various simulations and experiments on real data.

Experimental evaluation of bilateral Ambisonics-based binaural room transfer function synthesis with application to personal sound zones

This study experimentally evaluates the bilateral Ambisonics method for synthesizing binaural room transfer functions (BRTFs) and explores its application in generating personal sound zones (PSZs) around listeners’ ears. Bilateral Ambisonics is proposed for improving the spatial sound reproduction accuracy at a limited Ambisonics order, by shifting the origin of Ambisonics representation from the head center to the two ears. While numerical simulations have demonstrated its superiority over traditional Ambisonics, little attention has been given to validating its usability for reproduction in realistic environments with loudspeakers. Experiments are conducted wherein the BRTFs of a dummy head listener are first synthesized by combining the spatial room transfer functions (SRTFs) measured with a spherical Ambisonics microphone array at the ear position and the corresponding head-related transfer functions (HRTFs), and then compared with the actual in situ measured ones. Furthermore, the quality of the synthesized BRTFs is examined in the context of an in-house PSZ system, by objectively evaluating the performance of PSZ filters designed with such BRTFs in terms of acoustic isolation metrics. The application of synthesized BRTFs to PSZ systems circumvents the need for in-situ BRTF measurements, making it highly feasible to deploy such systems while achieving high acoustic isolation.

A hat-shape error microphone array for user-friendly spatial active noise control

Spatial active noise control (ANC) has been developed in the past decades to reduce unwanted acoustic noise over a desired region by generating an anti-noise field with secondary loudspeakers. Unlike conventional multi-channel ANC systems, the integration of spherical harmonic-based sound field analysis allows the ANC system to reduce noise over a continuous region surrounding the user's head rather than discrete points. However, the use of a spherical error microphone array on the boundary of the region for spherical harmonic analysis poses a limitation as it obstructs user access to the controlled area. In this work, we introduce a hat-shaped error microphone array design which can be placed above the listener's head. The proposed error microphone array can achieve spherical harmonic-based residual noise field analysis and is more user-friendly, allowing unrestricted user head movement. We construct the hat-shaped error microphone array and present the results of ANC experiments conducted using the array.

Closed-form multiple source direction-of-arrival estimator under reverberant environments.

Accurate direction-of-arrival (DOA) estimation of multiple sources, simultaneously active in a reverberant environment, remains a challenge, as the multi-path acoustic reflections and overlapped periods dramatically distort the direct-path wave propagation. This article proposes a prominent solution localizing multiple sources in a reverberant environment using closed-form estimates, circumventing any exhaustive search over the two-dimensional directional space. Apart from a low complexity cost, the algorithm has robustness to reverberant, inactive, and overlapped periods and an ease of operation in practice, achieving sufficient accuracy compared to state-of-the-art approaches. Specifically, this algorithm localizes an unknown number of sources through four steps: (i) decomposing the frequency domain signals on a spherical array to the spherical harmonics domain; (ii) extracting the first-order relative harmonic coefficients as the input features; (iii) achieving direct-path dominance detection and localization using closed-form estimation; and (iv) estimating the number of sources and their DOAs based on those pass the direct-path detection. Experimental results, using extensive simulated and real-life recordings, confirm the algorithm with a significantly reduced computational complexity, while preserving competitive localization accuracy as compared to the baseline approaches. Additional tests confirm this low-complexity algorithm even with a potential capacity for online DOA tracking of multiple moving sources.

A three-dimensional gold nanoparticles spherical liquid array for SERS sensitive detection of pesticide residues in apple

High-performance plasmonic substrates have recently attracted much research attention. Herein, a three-dimensional gold nanoparticles (AuNPs) spherical liquid array (SLA) with high “hot spots” and tunable nanometer gap by optimizing the proportion of AuNPs colloids over chloroform was synthesized based on a water–oil interfacial self-assembly strategy. The substrate demonstrated excellent surface-enhanced Raman scattering (SERS) performance using tetrathiafulvalene and rhodamine 6G (R6G) as probe molecules. With a simple extraction and soaking pretreatment process, the SLA exhibited high sensitivity for analysing triazophos on apple peels, with a limit of detection (LOD) of 0.005 µg/mL and recovery ranging from 96 to 110 %. Particularly, the chloroform produced an inherent characteristic peak at 665 cm−1, which was used as the internal standard to correct SERS signal fluctuation, leading to an improvement of the corresponding coefficient R2 from 0.97 to 0.99, thus improving the reproducibility. Therefore the SLA substrate possesses the potential for quantitative analysis of food contaminants.

Wide-baseline view synthesis for light-field display based on plane-depth-fused sweep volume

The advancements in three-dimensional (3D) display technology have led to a wide interest in light-field display. However, the need to simultaneously capture a large number of object views made content generation for light-field displays still a bottleneck. In this paper, we propose a method for light-field content generation based on plane-depth-fused sweep volume (PDFSV), focusing on handling wide-baseline views and exhibiting scene generalization when the camera array remains unchanged. Specifically, the proposed PDFSV exploits the prior depth of the images captured by a 4 × 4 spherical camera array to represent 3D information of scenes. Then two optimized sequential convolutional neural networks (CNN) are employed for implicit depth modeling and final color calculation, respectively. By doing these, the prior depth facilitates the synthesis of regions with complex textures in the target view. We produce a Wide-baseline Multi-view Image Set (WMIS) which has a field of view (FOV) angle reaching 54°and could be publicly available for access. In our experiments, we use only the 4 vertex views as input. Results demonstrate that the proposed approach can synthesize high-quality views at arbitrary positions between sparse views, outperforming existing neural-radiance-fields-based (NeRF-based) methods. Finally, we conduct autostereoscopic display experiments, achieving satisfactory results.

A Novel 3D Beam Domain Channel Model for UAV Massive MIMO Communications

Due to the agile maneuverability, unmanned aerial vehicles (UAVs) have shown great promise for on-demand communications in the next-generation wireless networks. Considering the massive multiple-input multiple-output (MIMO) configuration, this paper proposes a novel three-dimensional (3D) beam domain channel model (BDCM) for UAV communications. Through dividing the large antenna array into several sub-arrays and classifying multipath components as near-field and far-field components, the proposed BDCM takes the spherical wave front (SWF) and array non-stationarity into account. Channel statistical properties including spatial-temporal-frequency correlation function (STF-CF), root-mean-squared (RMS) Doppler spread, beam spread, channel matrix collinearity (CMC), and stationary time interval are derived and simulated for the proposed BDCM. Influences of SFW and non-stationary properties on the statistical properties and system performance are analyzed. Simulation results show that, compared with the equivalent geometry-based stochastic model (GBSM), the proposed BDCM has better temporal correlation, while BDCM and GBSM are equivalent in the system performance evaluation. Furthermore, the performance of the proposed BDCM is evaluated in terms of accuracy, complexity, and pervasiveness. The results show that the proposed BDCM can represent massive MIMO channel properties accurately with low complexity and good compatibility.

Surfaces with modified morphology and wettability arrangement: a potential medium for water harvesting in desertification areas

Abstract The cost of water transport for irrigating drought-endurable plants in desertification areas makes it important to develop other water sources. Research shows that freshwater produced by dewing could satisfy the water requirement during germination of desert plants, therefore, functional materials that promote dew formation provide solutions for reduction of plant irrigation costs in desertification regions. In this review, feasibility of utilizing trace irrigation for ensuring survival of desert plants has been demonstrated. The technologies of preparing water harvesting materials with modified microstructure and wettability arrangement have been introduced, it has been shown that surface embedded with a bump array which has counter wettability compared with its plane section is a common morphology adopted by current works. After comparing climates in desertification regions with those in dewing chambers, it has been found that currently, the expected values of dew productivities of most water harvesting materials in deserted areas will not be as efficient as they performed in experiments due to low relative humidity. However, hydrophobic surface with submicron scale hydrophilic spherical cavity array produced by nanosphere lithography has shown reliable prospects in reducing the energy barrier of heterogeneous nucleation and thus become a more potential medium in dew water production.

Wrap-like transfer printing for three-dimensional curvy electronics.

Three-dimensional (3D) curvy electronics has wide-ranging application in biomedical health care, soft machine, and high-density curved imager. Limited by material properties, complex procedures, and coverage ability of existing fabrication techniques, the development of high-performance 3D curvy electronics remains challenging. Here, we propose an automated wrap-like transfer printing prototype for fabricating 3D curvy electronics. Assisted by a gentle and uniform pressure field, the prefabricated planar circuits on the petal-like stamp are integrated onto the target surface intactly with full coverage. The driving pressure for the wrapping is provided by the strain recovery of a prestrained elastic film triggered by the air pressure control. The wrapping configuration and strain distribution of the stamp are simulated by finite element analysis, and the pattern and thickness of the stamps are optimized. Demonstration of this strategy including spherical meander antenna, spherical light-emitting diode array, and spherical solar cell array illustrates its feasibility in the development of complex 3D curvy electronics.

Phased Array Feed Experiments and Galactic H i Observations

The Phased Array Feed (PAF) is considered as one of the next generation receivers for radio telescopes, which can significantly enlarge the instantaneous Field-of-View of large aperture single dish radio telescopes and enable more flexible observing configurations. Study efforts on PAF development for radio telescopes have been made for more than two decades and have become more and more applicable. We report the development of an ambient-temperature 19 element L-band PAF system and the experimental results including its far field beam pattern and system temperature measurement, which achieve the expectations. Implementing the aperture array beam-forming method, we demonstrate a wide-field Galactic H i observations in the radio camera mode. The results indicate that this system might be applicable for strong Galactic transient detections. This system could be directly equipped to large telescopes like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and FAST array in the future.

Synthesis of sound field from moving complex sources with arbitrary trajectories by linear and spherical loudspeaker arrays.

The spectral division method (SDM) and near-field compensated higher order ambisonics (NFC-HOA) are sound field synthesis techniques based on the spatial Fourier representation of sound fields. Previous studies have derived the driving functions of SDM for sound field synthesis with consideration to uniformly moving point sources and moving point sources with arbitrary trajectories. However, the driving functions of NFC-HOA for synthesizing sound fields from moving sound sources have not been proposed to date. For a more realistic auditory experience, the synthesis of a sound field produced by a moving sound source with a complex radiation property is required. This study focused on deriving the driving functions for synthesizing sound fields produced by moving sound sources with arbitrary trajectories and radiation properties. Sound fields were formulated in the angular spectrum and spherical harmonic domains and applied to SDM and NFC-HOA, respectively. Numerical and measurement experiments were conducted to evaluate the proposed method. The results reveal that the proposed method can synthesize the desired sound fields.