Directive optical leaky wave antennas (OLWAs) with tunable radiation pattern are promising integrated optical modulation and scanning devices. OLWAs fabricated using CMOS-compatible semiconductor planar waveguide technology have the potential of providing high directivity with electrical tunability for modulation and switching capabilities. We experimentally demonstrate directive radiation from a silicon nitride ($Si_3N_4$) waveguide-based OLWA. The OLWA design comprises 50 crystalline Si perturbations buried inside the waveguide, with a period of 1 μm, each with a length of 260 nm and a height of 150 nm, leading to a directive radiation pattern at telecom wavelengths. The measured far-field radiation pattern at the wavelength of 1540 nm is very directive, with the maximum intensity at the angle of 84.4° relative to the waveguide axis and a half-power beam width around 6.2°, which is consistent with our theoretical predictions. The use of semiconductor perturbations facilitates electronic radiation control thanks to the refractive index variation induced by a carrier density change in the perturbations. To assess the electrical modulation capability, we study carrier injection and depletion in Si perturbations, and investigate the Franz-Keldysh effect in germanium as an alternative way. We theoretically show that the silicon wire modulator has a -3 dB modulation bandwidth of 75 GHz with refractive index change of $3\times10^{-4}$ in depletion mode, and 350 MHz bandwidth with refractive index change of $1.5\times10^{-2}$ in injection mode. The Franz-Keldysh effect has the potential to generate very fast modulation in radiation control at telecom wavelengths.
In this paper, we study the radiation from embedded sources in two-dimensional finite-size "photonic-quasicrystal" (PQC) slabs made of dielectric rods arranged according to a 12-fold symmetric aperiodic tiling. The results from our investigation, based on rigorous full-wave simulations, show the possibility of achieving broadside radiation at multiple frequencies, with high-directivity (e.g., 15 dB) and low-sidelobes (e.g., -12 dB). We also show that leaky waves are supported by a PQC slab, and that the beamwidth is directly proportional to the leaky-wave attenuation constant, which provides a…
We provide for the first time the detailed study of the radiation performance of an optical leaky wave antenna (OLWA) integrated into a Fabry-Pérot resonator. We show that the radiation pattern can be expressed as the one generated by the interference of two leaky waves counter-propagating in the resonator leading to a design procedure for achieving optimized broadside radiation, i.e., normal to the waveguide axis. We thus report a realizable implementation of the OLWA made of semiconductor and dielectric regions. The theoretical modeling is supported by full-wave simulation results, which are…
A leaky-wave antenna at optical frequencies is designed and integrated with a ring resonator at 1550 nm wavelength. The leaky wave is generated by using periodic perturbations in the integrated dielectric waveguide that excite the -1 spatial harmonic. The antenna consists of a dielectric waveguides with semiconductor corrugations, and it is compatible with CMOS fabrication technology. We show that integrating the leaky wave antenna in an optical ring resonator that is fed by directional couplers, we can improve the electronic control of the radiation through carrier injection into the semicond…
The integration of a leaky-wave antenna with a ring resonator is presented using analytical guided wave models. The device consists of a ring resonator fed by a directional coupler, where the ring resonator path includes a leaky-wave antenna segment. The resonator integration provides two main advantages: the high-quality factor ensures effective control of radiation intensity by controlling the resonance conditions and the efficient radiation from a leaky-wave antenna even when its length is much smaller than the propagation length of the leaky wave. We devise an analytical model of the guide…
Optical travelling wave antennas offer unique opportunities to control and selectively guide light into a specific direction which renders them as excellent candidates for optical communication and sensing. These applications require state of the art engineering to reach optimized functionalities such as high directivity and radiation efficiency, low side lobe level, broadband and tunable capabilities, and compact design. In this work we report on the numerical optimization of the directivity of optical travelling wave antennas made from low-loss dielectric materials using full-wave numerical …
We present a mixer-duplexer-antenna leaky-wave system based on periodic space-time modulation. This system operates as a full transceiver, where the upconversion and downconversion mixing operations are accomplished via space-time transitions, the duplexing operation is induced by the nonreciprocal nature of the structure, and the radiation operation is provided by the leaky-wave nature of the wave. A rigorous electromagnetic solution is derived for the dispersion relation and field distributions. The system is implemented in the form of a spatio-temporally modulated microstrip leaky-wave stru…
Unbalanced CRLH Leaky-wave Antenna With Broadside Radiation Based On Spin Photonic Topological Insulator Featured Hexagonal Configuration In Armchair Arrangement
The paper includes two contributions. First, it proves that the series and shunt radiation components, corresponding to longitudinal and transversal electric fields, respectively, are always in phase quadrature in axially asymmetric periodic leaky-wave antennas (LWAs), so that these antennas are inherently elliptically polarized. This fact is theoretically proven and experimentally illustrated by two case-study examples, a composite right/left-handed (CRLH) LWA and a series-fed patch (SFP) LWA. Second, it shows (for the case of the SFP LWA) that the axial ratio is controlled and minimized by t…
In this paper, we present the design procedure of modulated metasurface leaky-wave antennas (MMLWAs) to generate coaxial superposition of vortex beams with several orbital angular momentum (OAM) states. Based on the Flat Optics (FO) technique and aperture field estimation (AFE) method, an analytical framework is proposed to facilitate the implementation of MMLWAs generating multiple topological charges in the OAM regimen. Furthermore, using the spectral analysis which has been derived from the proposed model, we have shown that the symmetry of aperture shape can affect the purity of the mode. …
We present a general principle for the rational design of dielectric optical antennas with optimal solar absorption enhancement: leaky mode engineering. This builds upon our previous study that demonstrates the solar absorption of a material with a given volume only dependent on the density and the radiative loss of leaky modes of the material. Here we systematically examine the correlation among the modal properties (density and radiative loss) of leaky modes, physical features, and solar absorption of dielectric antenna structures. Our analysis clearly points out the general guidelineS for t…
Current wireless systems are experiencing unprecedented pressure that could be relieved by shifting carrier frequencies towards the terahertz spectrum. Yet, components are required to enable spectrally efficient communications at such high frequencies. Recently, the photonic crystal platform has been shown to enable signal processing at the chip-scale. Similarly, leaky-wave antennas were used to generate highly directional THz beams. Here, we combine photonic crystals with leaky-wave antennas. Practically, we propose to use an array of cylinders sandwiched between two plates, and we demonstrat…
An important source of innovation in nanophotonics is the idea to scale down known radio wave technologies to the optical regime. One thoroughly investigated example of this approach are metallic nanoantennas which employ plasmonic resonances to couple localized emitters to selected far-field modes. While metals can be treated as perfect conductors in the microwave regime, their response becomes Drude-like at optical frequencies. Thus, plasmonic nanoantennas are inherently lossy. Moreover, their resonant nature requires precise control of the antenna geometry. A promising way to circumvent the…
Amplitude-modulated (AM) leaky-wave antenna (LWA), a concept following amplitude modulation technique from classical communications theory, is a promising structure that enables transforming traveling wave into the radiating wave. In this paper, we provide a different perspective based on the classical holographic theory to gain insight into the physical mechanism of AM LWA and design novel LWAs. In analogy to the classical optical Gabor hologram, we demonstrate that only the amplitude variation of the traveling wave is needed to record both the amplitude and phase information of the object wa…
All-Dielectric Ceramic 3-D Printed Unidirectional Leaky-Wave Antenna at mm-Wave Frequencies
RWTH Aachen University（亚琛工业大学）；University of Duisburg-Essen（杜伊斯堡-埃森大学）；Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR)（弗劳恩霍夫高频物理与雷达技术研究所）
A thermal antenna is an electromagnetic source which emits in its surrounding, a spatially coherent field in the infrared frequency range. Usually, its emission pattern changes with the wavelength so that the heat flux it radiates is weakly directive. Here, we show that a class of hyperbolic materials, possesses a Brewster angle which is weakly dependent on the wavelength, so that they can radiate like a true thermal antenna with a highly directional heat flux. The realization of these sources could open a new avenue in the field of thermal management in far-field regime.
A planar magneto-electric (ME) dipole antenna array is proposed and demonstrated by both full-wave analysis and experiments. The proposed structure leverages the infinite wavelength propagation characteristic of composite right/left-handed (CRLH) transmission lines to form high-gain magnetic radiators combined with radial conventional electric radiators, where the overall structure is excited by a single differential feed. The traveling-wave type nature of the proposed ME-dipole antenna enables the formation of directive arrays with high-gain characteristics and scanning capability. Peak gains…
One of the long-standing problems in antenna engineering is the realization of highly-directive beams using low-profile devices. In this paper we provide a solution to this problem by means of Huygens' metasurfaces (HMSs), based on the equivalence principle. This principle states that a given excitation can be transformed to a desirable aperture field by inducing suitable electric and magnetic surface currents. Building on this concept, we propose and demonstrate cavity-excited HMS antennas, where the single-source cavity excitation is designed to optimize aperture illumination, while the HMS …
With the explosion of wireless networks and automotive radar systems, there is an acute need for new materials and technologies that would not only minimize the size of these devices, but also enhance their performances. The technique of transformation optics-an innovative approach to produce artificial metamaterials that control electromagnetic waves as if space itself was transformed-provides unique opportunities to reach this goal. In this paper we design, fabricate and characterize a new class of metamaterial capable of transforming the source distribution and radiation pattern of an isotr…
Small form-factor, narrowband, and highly directive antennas are of critical importance in a variety of applications spanning wireless communications, remote sensing, Raman spectroscopy, and single photon emission enhancement. Surprisingly, we show that the classical directivity limit can be appreciably surpassed for electrically small multilayer spherical antennas excited by a point electric dipole even if limiting ourselves to purely dielectric materials. Experimentally feasible designs of superdirective antennas are established by using a stochastic optimization algorithm combined with a ri…
We present a highly efficient metagrating-assisted antenna (MGA) architecture with a simple integrated feed. Power from a localized line source is distributed throughout the arbitrarily large antenna aperture with the help of a passive and lossless electromagnetic metagrating (MG). With appropriately designed meta-wire loading, the omnidirectional source field can be efficiently transformed into directive radiation. To aid the design process, a 2-dimensional volume-surface integral equation framework which accurately predicts the radiation pattern of the MGA is developed. Through constrained o…
Spectral singularities appearing in parity-time (PT)-symmetric non-Hermitian optical systems have aroused a growing interest due to their new, exhilarating applications, such as bifurcation effects at exceptional points and the coexistence of coherent perfect absorber and laser (so-called CPAL point). We introduce here how the concept of CPAL action provoked in PT-symmetric metasurfaces can be translated into practical implementation of a low-loss zero- or low-index channel supporting a nearly undamped fast-wave propagation. Such a PT-synthetic metachannel shows the capability to produce a hig…
Spatio-temporally modulated impedance surfaces can be good candidates for generation of radiating waves with arbitrary eigenstates by breaking momentum and energy conservations. Here, we present a theoretical framework based on the holographic technique and generalized Floquet-wave expansion to analyze spatio-temporally modulated impedance surfaces. The holographic technique estimates the required impedance distribution to achieve the desired momentum. Injecting temporal modulation deviates the eigenvalues and changes the radiation frequency. Using the proposed analytical model, the eigenvalue…
Nanoantennas are efficient devices exhibiting large confined electric field enhancements. So far, they have been extensively researched mainly in the receiving mode, which means that the illuminating field is essentially a plane wave. In this paper, we consider the problem of designing an efficient and highly directive transmitting Nanoantenna where the system is energized by a non-plane wave field, a subwavelength laser excitation. Including short-wavelength components allowed us to achieve a 200-nm spot radius, which is a quarter of its incident wavelength (800 nm). Near- and far-field anten…
An ensemble of resonators arranged on a sub-wavelength scale is usually considered as a bulk effective medium, known as a metamaterial, and can offer unusual macroscopic properties. Here, we take a different approach and limit ourselves to the study of only a few number of such elementary components and demonstrate that it still offers uncommon opportunities. Typically, thanks to the multiple scattering and the phase shift that the resonances offer, we observe fields that vary at scales completely independent of the wavelength in freespace. By smartly tuning the resonance frequencies, we can d…
In this paper, theoretical and numerical studies of perfect/nearly-perfect conversion of a plane wave into a surface wave are presented. The problem of determining the electromagnetic properties of an inhomogeneous lossless boundary which would fully transform an incident plane wave into a surface wave propagating along the boundary is considered. An approximate field solution which produces a slowly growing surface wave and satisfies the energy conservation law is discussed and numerically demonstrated. The results of the study are of great importance for the future development of such device…
