In this paper, we theoretically and experimentally demonstrate broadband metamaterial absorbers that work in the mid-infrared regime. In the absorbers, two or four gold cross resonators with different sizes are multiplexed in a unit cell on SiO(2) spacing layer on top of gold ground plane. Compared with the single cross resonator absorbers with a Q factor of 6.39, the developed absorber with two cross resonators multiplexed reduces the Q factor to 3.78. When four different cross resonators are integrated, the Q factor drops to as low as 1.85, and the bandwidth almost covers the full mid-infrared regime from 3 μm to 5 μm with absorbance higher than 50%.
Metasurfaces, known as ultra‐thin and planar structures, are widely used in optical components with their excellent ability to manipulate the wavefront of the light. The key function of the metasurfaces is the spatial phase modulation, originated from the meta‐atoms. Thus, to find the relation between the phase modulation and the parameters of an individual meta‐atom, including the sizes, shapes, and material's optical properties, is the most important but also time‐consuming part in the metasurface design. Here by developing a backpropagation neural network based machine learning tool, the design process of a high performance achromatic metalens can be greatly simplified and accelerated. A library of the phase modulation data from 15 753 meta‐atoms can be generated in less than 1 s by our backpropagation neural network. In the experiment, it is demonstrated that the designed metalens shows an excellent achromatic focusing and imaging ability in the visible wavelengths from 420 to 640 nm without the polarization dependence.
A purely artificial mechanism for optical nonlinearity is proposed based on a metamaterial route. The mechanism is derived from classical electromagnetic interaction in a meta-molecule consisting of a cut-wire meta-atom nested within a splitring meta-atom. Induced by the localized magnetic field in the split-ring meta-atom, the magnetic force drives an anharmonic oscillation of free electrons in the cut-wire metaatom, generating an intrinsically nonlinear electromagnetic response. An explicit physical process of a second-order nonlinear behavior is adequately described, which is perfectly demonstrated with a series of numerical simulations. Instead of "borrowing" from natural nonlinear materials, this novel mechanism of optical nonlinearity is artificially dominated by the meta-molecule geometry and possesses unprecedented design freedom, offering fascinating possibilities to the research and application of nonlinear optics.Nonlinear optics has become thriving since the first observation of the second harmonic generation (SHG) in 1961 [1], and plays an essential role in many optical devices such as frequency up-converters and mixers, nonlinear spectrometers, and new light sources [2]. Over the past half century, with tons of efforts focused on searching new nonlinear materials [3, 4], researchers endeavor to uncover the physical mechanism behind the optical nonlinearity [5]. As a universal phenomenon, nonlinearity is exhibited by almost all materials interacting with sufficiently strong light [6], and various fundamental mechanisms were proposed, such as distortion of the electron cloud, relative motion of nuclei, reorientation of molecules, electrostriction effect, and thermal effect [7]. Despite the fact that these phenomenological theories extensively advanced the development of new nonlinear materials in the past several decades, they
In this paper, a broadband terahertz (THz) metamaterial absorber using asymmetric split ring resonator (ASR) was designed, fabricated, and characterized. By breaking the symmetry of a split ring resonator, two asymmetric resonances are excited from a dipole resonance, which enhance both the absorption and -factor. With the integration of four different ASRs into one unit cell, a broadband absorber experimentally obtained a 0.82-THz bandwidth with absorptivity of more than 0.9, which is 3.4 times as wide as the 0.24-THz bandwidth of the symmetric dipole peak. The proposed broadband absorber has great application potentials in the THz spectroscopy, imaging, and sensing.
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