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Effective sound energy transmission and beam manipulation through stiff and dense materials such as metal remain daunting tasks. It is in part attributable to the vast impedance mismatch between those materials and ambient media. Adding openings may facilitate to better bridge energy over, yet ineffective in many applications and may also damage the structural integrity. Here, we present an auxiliary ultrasound focusing meta-lens for stiff and dense materials. It offers significantly enhanced ultrasound transmission and focusing through a stiff metal plate yet without enforcing any through holes or openings. The simple, one-sided only meta-structures are designed and optimized by an inverse strategy based on the genetic algorithm. We numerically and experimentally demonstrate the much enhanced ultrasound transmission when the meta-lens is added to a flat brass plate, along with the capability to offer simultaneous ultrasound focusing. This design methodology can be easily extended to deal with more complex shaped target in a straightforward manner, offering a practical solution to the efficient tunneling of ultrasound energy through stiff and dense materials. With simple grating structures, the meta-lens can be easily fabricated, showing great application prospects in medical imaging and disease treatment. [ABSTRACT FROM AUTHOR]

Zero index metamaterials have shown the ability to achieve total transmission or reflection by embedding particular defects with various effective parameters. Here, we present that tunable sound transmission can be realized by configuring a subwavelength-sized dopant inside zero index acoustic metamaterials. Despite its small spatial signature, the dopant is able to strongly interact with the acoustic waves inside the whole zero index metamaterials. It is due to the essence of the zero effective index that can homogenize the pressure field within the metamaterials. Sound transmission can thus be fully switched on and off by adjusting the dopant's surface impedance. A simple rectangular cavity with varied lengths is proposed to provide the required impedance boundary. Our model of correlating the dopant design with sound transmission performance is validated theoretically and numerically. We further demonstrate the utilization of the proposed design to effectively modulate the sound focusing effect. Such a dopant-modulated sound transmission scheme, with its simplicity and capability, has potential applications in fields like noise control and ultrasonography. [ABSTRACT FROM AUTHOR]

In this work, we propose a simple scheme to realize an acoustic coherent perfect absorber (CPA) and laser modes by embedding a non-Hermitian dopant in a zero index metamaterial. When the dopant is filled with a loss medium at a specific level, the sample can absorb the incident waves completely. On the other hand, when the dopant is filled with a gain medium, the sample can act as a laser oscillator to boost the incident waves. The theoretical derivation based on the scattering matrix and the numerical simulation based on the finite element method are performed and both show good agreement with each other. We also discover that the CPA and laser modes are very sensitive and can be controlled by adjusting the structure parameters or the relative phase of the incident waves. Moreover, the case that asymmetric incidences have different beam widths is considered. We envision that our work may have potential applications in designing acoustic devices, such as absorbers, transducers, and receivers. [ABSTRACT FROM AUTHOR]

Efficiently extracting hole from a 3-dimensional bulk heterojunction is critical to polymer photovoltaics. One possible approach is to incorporate efficient hole transport pathway within each individual nanoscale donor-phase component. We present a study employing electrochemically prepared metallic carbon nanotube (CNT)-polythiophene (PT) core-shell nanowire networks trying to realize such a desired structure. Phenyl-C60-butyric acid methyl ester was infiltrated into the openings of the core-shell nano-networks as electron acceptor. The thin PT shell can ensure high exciton dissociation rate and efficient free hole transport to the CNT core which serves as an efficient network for extracting hole out of the heterojunction. [ABSTRACT FROM AUTHOR]

The determination of the low-temperature plasma propagation of SF6/N2 in gas discharge will accelerate the application of SF6/N2 in advanced electrical equipment. This study investigates the positive streamer propagation characteristics of SF6/N2 by establishing a plasma discharge model and discusses the effects of SF6 ratio on the electron density, electric field, chemical reaction rates, ion concentrations, and streamer thickness and velocity. As the streamer propagates, the shapes of the electron density and electric field become "narrower and taller." Moreover, the ionization and attachment reaction rates increase, and the streamer thickness and velocity decrease with increasing SF6 mixing ratio and propagation time. As the SF6 ratio increases, the electric field distortion degree and particle mobility near the high-voltage electrode decrease due to the increase in the ionization and attachment rates. Consequently, the streamer velocity remains nearly unchanged, but the streamer thickness obviously decreases in the corona discharge process. In the streamer propagation process, the decrease in the streamer thickness and velocity with increasing SF6 ratio is attributed to the reduction in the electric field distortion and particle mobility, which stems from the increase in the DC breakdown voltage with the SF6 mixing ratio. The study results clarify the influence mechanisms of the SF6 mixing ratio on the particles' spatiotemporal evolution during streamer propagation. [ABSTRACT FROM AUTHOR]

Non-Hermitian scattering systems respecting parity-time symmetry exhibit unidirectional invisibility or reflectionlessness at an exceptional point of the scattering matrix. In this study, we investigate the scattering properties of a one-dimensional acoustic parity-time-symmetric multilayered medium. Parity-time symmetry is defined by a spatial small-amplitude square-wave modulation of the medium parameters in the complex plane. Such a multilayered medium has been demonstrated to reach an exceptional point and support unidirectional invisibility for balanced real and imaginary part modulations of the refractive index. Through the analysis of interference within the medium, we show that the complex modulation of acoustic impedance instead of the refractive index governs the exceptional point of the scattering matrix and determines the occurrence of the unidirectional invisibility. This unidirectional invisibility is further investigated for more general cases of complex parameter distribution including unequal modulation amplitudes and rectangular wave modulation. Our study provides an intuitive physical picture of the occurrence of unidirectional invisibility induced by an exceptional point in acoustic wave systems. Moreover, this study could facilitate the design and practical realization of acoustic parity-time-symmetric structures. [ABSTRACT FROM AUTHOR]

A correction to the article "Coaxial carbon nanotube-polythiophene core-shell nanowire for efficient hole transport in heterojunction photovoltaic device," that was published in "Applied Physics Letters" online on February 27, 2012 is presented.

The pursuit of tunable asymmetric sound transmission has been a long-term topic since it could contribute to providing more flexibilities in many areas of acoustic engineering. The interference effect can be a feasible approach in which two waves with the same frequency superposed to form the resultant wave with manipulated amplitude according to the relative phase difference between them. However, strictly speaking, restricted by the spatial variance of phase, the manipulated domain created by the specific phase difference is always limited to a spot with dimensions much smaller than the wavelength. Here, we proposed a design to break this barrier that can realize the tunable asymmetric transmission via the combination of zero-index metamaterials and the binary metasurface. The zero-index metamaterial can provide the effective extremely large speed to shrink the infinite domain into a spot acoustically and the binary metasurface can be used to tune the specific phase difference. Numerical simulations and experimental measurements have good agreement and show that the acoustic waves impinged from the side of metasurface will be manipulated to have controllable transmission, while the acoustic waves impinged from the side of zero-index metamaterials will keep a high transmission. We think the proposed design is full of physical significance, which may find potential applications in many fields, like noise cancelation, acoustic imaging, and ultrasound therapy. [ABSTRACT FROM AUTHOR]

Sleep quantification and automatic scoring of sleep stages via electroencephalogram (EEG) signals has been a challenge for years. It is crucial to investigate the correlation of brain waves by sleep EEG analysis due to the association between rhythmic oscillations of neuronal activity and neocortical synchronization. Multiscale joint permutation entropy (MJPE) had been proven to be capable of measuring the correlation between time series from the view of multiple time scales and thus can be a promising approach to address the challenge. Instead of simulation, we tested the MJPE method on a widely used open dataset of sleep EEG time series from healthy subjects and found that the correlation index obtained by MJPE had the capability of quantifying the brain wave correlations under different sleep stages, reflecting the typical sleep patterns and indicating the weakened correlation with aging. A higher level of correlation was found as the sleep stage advanced. The findings based on the MJPE results were in accordance with previous studies and existing knowledge in sleep medicine. In the second part of the study, we applied MJPE on sleep EEGs from subjects under pathological conditions (sleep apnea and sleep at high altitude). Likewise, the correlation index also properly revealed their sleep architectures, with consistent trends of the correlation through the nights. The effectiveness and practicability of the MJPE method had been verified on sleep EEGs. Therefore, the MJPE method should be encouraged to be widely used for analyzing large-scale sleep EEGs under various pathological conditions to provide insight into the mechanisms of the sleep process and neuron synchronization. [ABSTRACT FROM AUTHOR]