We present an ultrabroadband thin-film infrared absorber made of sawtoothed anisotropic metamaterial. Absorptivity of higher than 95% at normal incidence is supported in a wide range of frequencies, where the full absorption width at half-maximum is about 86%. Such property is retained well at a very wide range of incident angles too. Light of shorter wavelengths are harvested at upper parts of the sawteeth of smaller widths, while light of longer wavelengths are trapped at lower parts of larger tooth widths. This phenomenon is explained by the slowlight modes in anisotropic metamaterial waveguide. Our study can be applied in the field of designing photovoltaic devices and thermal emitters.

We experimentally demonstrate an infrared broadband absorber for TM polarized light based on an array of nanostrip antennas of several different sizes. The broadband property is due to the collective effect of magnetic responses excited by these nano-antennas at distinct wavelengths. By manipulating the differences of the nanostrip widths, the measured spectra clearly validate our design for the purpose of broadening the absorption band. The present broadband absorber works very well in a wide angular range.In the last decade, plasmonic nano-antennas have experienced a drastical boom period due to their enormous capability to compress light into a subwavelength region with an extremely strong amplitude. 1,2 To date, they have found significant application in diverse areas including sensor detection, 3 solar power harvesting, 4 thermal emission, 5 biomedical imaging, 6 ultrafast modulating, 7 etc. Patterned plasmonic antennas play a significant role for the design of thin film light absorbers, which suppress both the transmission and the reflection while maximizing the absorption. The first perfect absorber that composed by metallic split ring resonators and cutting wires was demonstrated by Landy et. al. 8 Then, it was followed by some work to improve the angular and polarization performance. [9][10][11][12] Nevertheless, all of the above absorbers work at a single band frequency which limits the pragmatic applications such as THz multi-frequency spectroscopy detection. 13 By incorporating different patterns of metallic elements, two dual band absorbers were carried out by different groups. 14,15 Recently, it was reported that based on an H-shaped nano-resonator array, a dual band plasmonic metamaterial absorber could also be constructed. 16 But they are still limited to a relative narrow band response. So far, to design a thin film absorber with broadband spectrum is still quite challenge. In our group, we have made some efforts in this aspect, by stacking multiple layers of metallic crosses with different geometrical dimensions to merge several closely positioned resonant peaks in the absorption spectrum. 17 However, this proposal suffers from one crucial drawback, namely that in the fabrication it is difficult to obtain perfect alignment to match the relative position of each pattern in different layers.It is well known that a three layered structure composed by an array of plasmonic nanostrip antennas of a fixed width on top of a ground reflecting mirror and a very thin spacer layer 18 can efficiently absorb electromagnetic wave at a certain frequency. The principle of the light absorbing is that the upper strip and the ground metal layer support a pair of anti-parallel dipoles with quite closed distance in-between, the interference of those two dipoles in far field is destructive due to their π shift phase difference so that the reflection can be totally cancelled.In this letter, also aiming at broadening the absorption band, we borrow the concept of the collective effect of multiple different oscillators 1...

In this paper, we investigate a type of anisotropic, acoustic complementary metamaterial (CMM) and its application in restoring acoustic fields distorted by aberrating layers. The proposed quasi two-dimensional (2D), nonresonant CMM consists of unit cells formed by membranes and side branches with open ends. Simultaneously, anisotropic and negative density is achieved by assigning membranes facing each direction (x and y directions) different thicknesses, while the compressibility is tuned by the side branches. Numerical examples demonstrate that the CMM, when placed adjacent to a strongly aberrating layer, could acoustically cancel out that aberrating layer. This leads to dramatically reduced acoustic field distortion and enhanced sound transmission, therefore virtually removing the layer in a noninvasive manner. In the example where a focused beam is studied, using the CMM, the acoustic intensity at the focus is increased from 28% to 88% of the intensity in the control case (in the absence of the aberrating layer and the CMM). The proposed acoustic CMM has a wide realm of potential applications, such as cloaking, all-angle antireflection layers, ultrasound imaging, detection, and treatment through aberrating layers. In many medical ultrasound or nondestructive evaluation (NDE) applications, ultrasound needs to be transmitted through an aberrating layer [1][2][3][4][5][6][7], where either the transmission is desired to be maximized or the reflection needs to be minimized. One of the most representative examples is transcranial ultrasound beam focusing, which could find usage in both brain imaging and treatment [6,7]. However, transcranial beam focusing is extremely challenging because of the presence of the skull. A common approach to achieve transcranial beam focusing is based on the timereversal or phase-conjugate technique and ultrasound phased arrays [8,9]. Although the focal position can be corrected, one significant shortcoming of this strategy is that it does not compensate for the large acoustic energy loss due to the impedance mismatch between the skull and the background medium (water). Recent development of acoustic metamaterials [10][11][12] could open up the possibility for noninvasive ultrasound transmission through aberrating layers. For example, an acoustic metamaterial could be used to cancel out or cloak the aberrating layer, allowing the acoustic wave to pass through the layer without energy loss (Fig. 1). Conventional cloaking strategies [11,13,14], however, compress the space and hide the object inside an enclosure in which there is no interaction with the outside world; therefore, it is not suited to the problem of interest in this study. Lai et al. demonstrated that cloaking or illusion based on electromagnetic wave (EM) complementary metamaterials (CMM) [15] can open up a virtual hole in a wall without distortion [16,17]. In addition, this type of approach does not require the cloaked object to be inside an enclosure or cloaking shell, and it is valid in free space [18]. Because of the s...

In this letter, a class of honeycomb acoustic metamaterial possessing lightweight and yet sound-proof properties is designed, theoretically proven, and then experimentally verified. It is here reported that the proposed metamaterial having a remarkably small mass per unit area at 1.3 kg/m2 can achieve low frequency (<500 Hz) sound transmission loss (STL) consistently greater than 45 dB. Furthermore, the sandwich panel which incorporates the honeycomb metamaterial as the core material yields a STL that is consistently greater than 50 dB at low frequencies. The proposed metamaterial is promising for constructing structures that are simultaneously strong, lightweight, and sound-proof.