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.

A microwave ultra-broadband polarization-independent metamaterial absorber is demonstrated. It is composed of a periodic array of metal-dielectric multilayered quadrangular frustum pyramids. These pyramids possess resonant absorption modes at multi-frequencies, of which the overlapping leads to the total absorption of the incident wave over an ultra-wide spectral band. The experimental absorption at normal incidence is above 90% in the frequency range of 7.8−14.7GHz, and the absorption is kept large when the incident angle is smaller than 60 degrees. The experimental results agree well with the numerical simulation.

We demonstrate the bulk self-alignment of dispersed gold nanorods imposed by the intrinsic cylindrical micelle selfassembly in nematic and hexagonal liquid crystalline phases of anisotropic fluids. External magnetic field and shearing allow for alignment and realignment of the liquid crystal matrix with the ensuing long-range orientational order of well-dispersed plasmonic nanorods. This results in a switchable polarization-sensitive plasmon resonance exhibiting stark differences from that of the same nanorods in isotropic fluids. The device-scale bulk nanoparticle alignment may enable optical metamaterial mass production and control of properties arising from combining the switchable nanoscale structure of anisotropic fluids with the surface plasmon resonance properties of the plasmonic nanorods.KEYWORDS Nanorods, liquid crystals, optical metamaterials, self-assembly, plasmonic nanoparticles H aving predesigned structural units different from those in a conventional matter, metamaterials exhibit many unusual properties of interest from both fundamental science and applications standpoints. However, manufacturing such bulk optical metamaterials with three-dimensional (3D) structure 1-4 using lithography techniques presents a significant challenge, especially for the large-scale production. Mass production of bulk optical metamaterials from self-aligning and self-assembling nanoparticles is poised to revolutionize scientific instruments, technologies,andconsumerdevices.5-7 Althoughthemetamaterial self-assembly from nanoparticles remains a significant challenge, recent advances in colloidal science show that it may be realized and the emerging nanoscale alignment and assembly approaches utilize surface monolayers, 8,9 stretched polymer films, 10,11 and functionalized nanoparticles 12,13 but are usually restricted to only short-range ordering, twodimensional rather than three-dimensional assembly, and limited switching.7 Tunable metamaterials may potentially be obtained by nanoparticle self-assembly in liquid crystals (LCs) 14 through the LC-mediated realignment and rearrangement of incorporated nanoparticles in response to applied fields. However, experimental realization of such self-assembling switchable metamaterial composites is lacking. In this work, we demonstrate spontaneous long-range orientational ordering of gold nanorods (GNRs) dispersed in surfactant-based lyotropic LCs and use polarizing optical microscopy, darkfield microscopy, spectroscopy, and freezefracture transmission electron microscopy (FFTEM) to study these composites on the scales ranging from nanometers to millimeters. We find that the anisotropic fluids in both columnar hexagonal and nematic LC phases impose nematic-like long-range orientational ordering of GNRs with no correlation of their centers of mass but with the GNRs aligning along the LC director n (a unit vector describing the average local orientation of cylindrical micelles forming the LC), Figure 1. The unidirectional alignment of nanorods with high order parameter is...

Electromagnetic absorbers have drawn increasing attention in many areas. A series of plasmonic and metamaterial structures can work as efficient narrowband absorbers due to the excitation of plasmonic or photonic resonances, providing a great potential for applications in designing selective thermal emitters, biosensing, etc. In other applications such as solar-energy harvesting and photonic detection, the bandwidth of light absorbers is required to be quite broad. Under such a background, a variety of mechanisms of broadband/multiband absorption have been proposed, such as mixing multiple resonances together, exciting phase resonances, slowing down light by anisotropic metamaterials, employing high loss materials and so on.

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...

Highly efficient planar heterojunction (PHJ) perovskite solar cells (PSCs) with a structure of ITO/ PEDOT:PSS/CH 3 NH 3 PbI 3 /PCBM/Al were fabricated by a low-temperature solution process. As employed silica-coated gold (Au@SiO 2 ) nanorods at the interface between the hole transport layer PEDOT:PSS and the active layer CH 3 NH 3 PbI 3 , the average power conversion efficiency (PCE) showed over 40% enhancement, of which the average PCE was improved from 10.9% for PHJ-PSCs without Au@SiO 2 to 15.6% for PHJ-PSCs with Au@SiO 2 , and the champion one up to 17.6% was achieved. Both experiment and simulation results proved that prominent efficiency enhancement comes from the localized surface plasmon resonance of Au@SiO 2 nanorods which could improve the incident light trapping as well as improve the transport and collection of charge carrier, resulting in the enhancement in device parameters. The results suggest that metal nanorods, e.g., Au@SiO 2 , could be employed to fabricate highefficiency and low-cost PHJ-PSCs.

Three-dimensional (3-D) structures have triggered tremendous interest for thin-film solar cells since they can dramatically reduce the material usage and incident light reflection. However, the high aspect ratio feature of some 3-D structures leads to deterioration of internal electric field and carrier collection capability, which reduces device power conversion efficiency (PCE). Here, we report high performance flexible thin-film amorphous silicon solar cells with a unique and effective light trapping scheme. In this device structure, a polymer nanopillar membrane is attached on top of a device, which benefits broadband and omnidirectional performances, and a 3-D nanostructure with shallow dent arrays underneath serves as a back reflector on flexible titanium (Ti) foil resulting in an increased optical path length by exciting hybrid optical modes. The efficient light management results in 42.7% and 41.7% remarkable improvements of short-circuit current density and overall efficiency, respectively. Meanwhile, an excellent flexibility has been achieved as PCE remains 97.6% of the initial efficiency even after 10 000 bending cycles. This unique device structure can also be duplicated for other flexible photovoltaic devices based on different active materials such as CdTe, Cu(In,Ga)Se2 (CIGS), organohalide lead perovskites, and so forth.

Metal–inorganic semiconductor–metal photodetectors (MSM‐PDs) have received great attention in many areas, such as optical fiber communication, sensing, missile guidance, etc., due to their inherent merits of high speed, high sensitivity, and easy integration. This review focuses on MSM‐PDs with the semiconductor layer made of inorganic materials including traditional semiconductors (such as GaAs and Si), the third‐generation wide bandgap semiconductors (such as GaN, ZnO, and SiC), as well as several emerging semiconductors (such as perovskites and 2D materials). First, the basic structures of MSM‐PDs, including the planar and vertical configurations, are presented. Then, their working principles of MSM‐PDs are discussed. Subsequently, the research progresses on MSM‐PDs consisting of different photosensitive semiconductor materials are described in detail. Additionally, the efforts to optimize MSM‐PDs from the aspects of dark current, response speed, responsivity, spectral adjustment, etc., are also introduced. Finally, the review is concluded with the perspectives of MSM‐PDs from the authors’ vision.