A simple multiple cross-correlations phase delay method (MCC) for unbiased estimation of the phase-shift between two real sinusoidal signals in white noise is proposed. The estimator removes the bias of the correlation methods and enhances the signal-to-noise ratio (SNR) by utilizing multiple cross-correlations of the real signals. The results obtained by MCC in simulations and on real samples are given. A comparison between the phase delay results estimated by MCC, general cross correlation (GCC) method, quadrature delay estimator (QDE), unbiased QDE (UQDE), Sliding Goertzel algorithm (SGA), and discrete-time Fourier transform (DTFT) algorithm is presented. Computer simulations and field test results validate the effectiveness of the proposed method (MCC).
Wave absorption in time-invariant, passive thin films is fundamentally limited by a trade-off between bandwidth and overall thickness. In this work, we investigate the use of temporal switching to reduce signal reflections from a thin grounded slab over broader bandwidths. We extend quasi-normal mode theory to time switching, developing an ab initio formalism that can model a broad class of time-switched structures. Our formalism provides optimal switching strategies to maximize the bandwidth over which minimal reflection is achieved, showing promising prospects for time-switched nanophotonic and metamaterial systems to overcome the limits of time-invariant, passive structures.
Scattering of electromagnetic waves lies at the heart of most experimental techniques over nearly the entire electromagnetic spectrum, ranging from radio waves to optics and X-rays. Hence, deep insight into the basics of scattering theory and understanding the peculiar features of electromagnetic scattering is necessary for the correct interpretation of experimental data and an understanding of the underlying physics. Recently, a broad spectrum of exceptional scattering phenomena attainable in suitably engineered structures has been predicted and demonstrated. Examples include bound states in the continuum, exceptional points in PT-symmetrical non-Hermitian systems, coherent perfect absorption, virtual perfect absorption, nontrivial lasing, non-radiating sources, and others. In this paper, we establish a unified description of such exotic scattering phenomena and show that the origin of all these effects can be traced back to the properties of poles and zeros of the underlying scattering matrix. We provide insights on how managing these special points in the complex frequency plane provides a powerful approach to tailor unusual scattering regimes.
We utilize an effective Hamiltonian formalism, within the Floquet scattering framework, to design a class of driving-induced non-reciprocal components with {\it minimal} complexity. In the high driving-frequency limit, where our scheme is formally applicable, these designs demonstrate a leading order non-reciprocal performance which is inverse proportional to the driving frequency. Surprisingly, the optimal non-reciprocal behavior persists also in the slow driving regime. Our approach highlights the importance of physical loops in the design of these driven non-reciprocal components.
By utilizing Floquet driving protocols and interlacing them with a judicious reservoir emission engineering we achieve extreme non-reciprocal thermal radiation. We show that the latter is rooted in an interplay between a direct radiation process occurring due to temperature bias between two thermal baths and the modulation process which is responsible for pumped radiation heat. Our theoretical results are confirmed via time-domain simulations with RF circuits.
We analytically investigate the heat current I and its thermal fluctuations Δ in a branching network without loops (Cayley tree). The network consists of two types of harmonic masses: vertex masses M placed at the branching points where phononic scattering occurs and masses m at the bonds between branching points where phonon propagation takes place. The network is coupled to thermal reservoirs consisting of one-dimensional harmonic chains of coupled masses m. Due to impedance mismatch phenomena, both I and Δ are non-monotonic functions of the mass ratio μ=M/m. Furthermore, we find that in the low-temperature limit the thermal conductance approaches zero faster than linearly due to the small transmittance of the long-wavelength modes.
Abstract We report the dynamical behaviors of a magnetic vortex driven by an out‐of‐plane spin‐polarized current in a point‐contact geometry using a micromagnetic simulation method. Polarity, chirality, and polarity plus chirality‐switching diagrams are presented corresponding to different current states. Besides vortex core gyrotropic motion, polarity switching, chirality switching, and polarity plus chirality switching, we also observe non‐switching and partial switching behaviors in some cases. Investigations of these switching behaviors find that polarity is switched by nucleation and annihilation of the vortex–antivortex pair for the polarity‐switching only regime, and chirality reversal is realized by moving, nucleation, and annihilation of vortices in the disk for lower current density, but by propagating of spin waves for higher current density. Moreover, the reversal of chirality is always accompanied by polarity switching, and the accompanied polarity switching for the higher current density case is realized by the expansion and compression of the vortex core, not the traditional vortex–antivortex pair‐mediated mechanism.
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