Peer ReviewedPostprint (published version
While practical realizations of optical invisibility have been achieved so far by various ingenious methods, they generally rely on complex materials which prevent the wide implementation of such schemes. Here, we propose an alternative indivisibility procedure to design objects (i.e. self-cloaked structures) that have optical properties identical to the surrounding environment and are, thereby, intrinsically invisible to an external observer as such (without the necessity of an external cloak). The proposed method is based on the uncoupling of the scattered waves from the incident radiation by judiciously manipulating the scattering potential of a given object. We show that such a procedure is able to yield optical invisibility for any arbitrarily shaped object within any specified frequency bandwidth by simply employing isotropic non-magnetic dielectric materials, without the usage of loss or gain material. The validity of the design principle has been verified by direct experimental observations of the spatial electric field profiles and scattering patterns at the microwave regime. Our alternative self-cloaking strategy may have profound implications especially in noninvasive probing and cloaked sensor applications, where the wave penetrability into the sensor region is essential together with its invisibility to minimize the field distortion.
We introduce a new class of systems holding Parity Time (PT)-symmetry locally whereas being globally Psymmetric. The potential is globally symmetric, U=U(|r|), and fulfills PT-symmetry with respect to periodically distributed points r 0 : U(|r 0 +r|)=U*(|r 0 -r|) being r 0 ∫ 0. We show that such systems hold novel properties arising from the merging of the two different symmetries, leading to a strong field localization and enhancement at the double-symmetry center, r=0, when the coupling of outward to inward propagating waves is favored. We explore such general potentials in 1D and 2D, which could have actual realizations in different fields, in particular in optics, combining gain/loss and index modulations in nanophotonic structures. As a direct application, we show how to render a broad aperture VCSEL into a bright and narrow beam source.PACS numbers: 78.20. Bh, 42.25.Bs PT-symmetric systems, introduced as a curiosity in quantum mechanics [1,2], are recently being explored in the field of optics, acoustics, plasmonics or Bose-Einstein condensates [3][4][5][6][7][8]. A necessary condition for a system to be PT-symmetric is that the complex potential fulfills U(r)=U* (-r). Such complex systems with real spectra may support novel unexpected properties [9][10][11].Most PT-symmetric systems can be regarded as belonging to two limiting situations of complex periodic potentials. On one extreme, there are purely real-valued potentials holding real periodic modulations in space; which potential, in the simplest harmonic modulation case, reads: U(r) = n Re cos(qx), being q the spatial period of the modulation and n Re its amplitude. On the other extreme, we find purely imaginary potentials only exhibiting gain-loss modulations, which in the simplest harmonic case may be expressed as: U(r) = n Im cos(qx). Both limits lead to a symmetric coupling of resonant modes, i.e. the two counter-propagating modes with wavevector |k|, exp( ) ikx and exp( ), ikx are coupled symmetrically at resonance, for 2 q k . The most peculiar situation arises when both the real and imaginary parts of the potential are simultaneously modulated, with a / 2 phase shift: U(r) = n Re cos(qx) + n Im sin(qx). When both modulations are balanced, n Re = n Im , the complex potential can be simply expressed as: U(r) = n exp(±iqx), which evidences that the coupling becomes strongly unidirectional. E.g. for such a complex modulation the left-propagating mode exp(, is efficiently coupled to the right propagating mode, exp( ), ikx but not vice versa. The point n Re = n Im is precisely the so-called phase transition, separating two extreme situations. Mathematically, the coupling between left/right propagating modes is conveniently described via linear coupling matrices, M = {{0,n Re +n Im },{n Re -n Im ,0}} which at the PT-phase transition point degenerate to M ={{0,2n},{0,0}}. Generally, in the presence of several modes (or mode continuum) the situation becomes more engaged, however the phase transition separating the two extreme limits of ...
We propose and analyze a beam-shaping mechanism that in broad-area semiconductor amplifiers occurs due to spatial pump modulation on a micrometer scale. The study, performed under realistic parameters and conditions, predicts a spatial (angular) filtering of the radiation, which leads to a substantial improvement of the spatial quality of the beam during amplification. Quantitative analysis of spatial filtering performance is presented based on numerical integration of the paraxial propagation model and on analytical estimations.
Designing invisible objects without the usage of extreme materials is a long-sought goal for photonic applications. Invisibility techniques demonstrated so far typically require high anisotropy, gain and losses, while also not being flexible. Here we propose an invisibility approach to suppress the scattering of waves from/to given directions and for particular frequency ranges, i.e. invisibility on demand. We derive a Born approximationbased generalized Hilbert transform for a specific invisibility arrangement relating the two quadratures of the complex permittivity of an object. The theoretical proposal is confirmed by numerical calculations, indicating that near-perfect invisibility can be attained for arbitrary objects with low-index contrast. We further demonstrate the cases where the idea can be extended to high-index objects or restricted to within practical limits by avoiding gain areas. The proposed concept opens a new route for the practical implementation of complex-shaped objects with arbitrarily suppressed scatterings determined on demand.Full invisibility, or cloaking, was proposed using transformation optics or, equivalently, conformal mapping 1,2 . The idea is elegant and fascinating; however, it can hardly cross the limits of science fiction, since the complexity of the required metamaterials severely limit practical realizations. Therefore, actual cloaking schemes generally scarify the perfect wavefront reconstruction or operate under a narrow bandwidth, as for instance in carpet cloaking 3-7 , plasmonic cloaking 8,9 , or mantle cloaking with thin patterned metasurfaces 10 , metallic scatterer 11 or dielectric coating 12 based cloaking, among others 13,14 .A completely different approach to the concept of invisibility, referred as "unidirectional invisibility", relays on systems described by non-Hermitian Hamiltonians 15-17 . The concept is based on the property of an object to be invisible when probed by a wave from one side only. Such effect is accomplished by specific complex-modulated potentials (in optical terms: specific refraction index and gain/loss distributions), that allow suppressing the scattering of radiation from an object.Unidirectional invisibility was first proposed for parity-time (PT) symmetric periodical systems (defined by symmetric index modulations accompanied by anti-symmetric gain/loss distributions), close to so-called PT-symmetry breaking point. Initially proposed for narrow frequency bands (due to the resonances of the periodic structure), and for particular incidence directions 18-20 , the idea was extended to broad band radiation (both in frequency and in propagation direction) also by considering non-PT-symmetric potentials 21-25 .More recently, "unidirectional invisibility" has been related to the more general class of non-Hermitian potentials fulfilling the spatial Kramers-Kronig (KK) relations 26 . In the same way as the causality in time imposes KK relations in frequency, analogously, the KK theory may be directly extended to attain unidirectional invisibility...
We propose a simple realistic two-dimensional complex parity-time-symmetric photonic structure that is described by a non-Hermitian potential but possesses real-valued eigenvalues. The concept is developed from basic physical considerations to provide asymmetric coupling between harmonic wave components of the electromagnetic field. The structure results in a nonreciprocal chirality and asymmetric transmission between in-and out-coupling channels into the structure. The analytical results are supported by a numerical study of the Bloch-like mode formations and calculations of a realistic planar semiconductor structure.
Postprint (published version
We propose a new approach based on a local Hilbert transform to design non-Hermitian potentials generating arbitrary vector fields of directionality, ⃗( ⃗), with desired shapes and topologies. We derive a local Hilbert transform to systematically build such potentials, by modifying background potentials (being either regular or random, extended or localized). In particular, we explore particular directionality fields, for instance in the form of a focus to create sinks for probe fields (which could help to increase absorption at the sink), or to generate vortices in the probe fields. Physically, the proposed directionality fields provide a flexible new mechanism for dynamically shaping and precise control over probe fields leading to novel effects in wave dynamics.PACS numbers: 78.20. Bh, 42.25.Bs Systems described by non-Hermitian potentials, first introduced in quantum mechanics and linear electrodynamics [1,2], have recently found realizations in optics [3][4][5][6][7][8], by using coherent gain and losses, thus opening the new discipline of non-Hermitian optics. One of the most fascinating properties of such non-Hermitian systems is that the parity (in one dimension), or generally the space symmetry (in two or higher dimensions) can be broken at around so named exceptional points. As a consequence, different counterintuitive physical effects arise, such as unidirectional invisibility [6-8], unidirectional transmission [9], unidirectional lasing [10,11] antibandgaps [12], perfect unidirectional absorption [13,14], nonreciprocal Bloch oscillations [15], and generally the unidirectional transfer of energy in linear [3][4][5] and nonlinear [16][17][18] systems. Most of the new intriguing features were initially proposed in a particular kind of such non-Hermitian systems, namely in those holding PT-symmetry [1].
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