Metamaterial (MM) perfect absorbers are realised over the various spectra from visible to microwave. Recently, different approaches have been explored to integrate tunability into the MM absorbers. Particularly, tuning has...

Metamaterials composed of metallic antennae arrays are used as they possess extraordinary optical transmission (EOT) in the terahertz (THz) region, whereby a giant forward light propagation can be created using constructive interference of tunneling surface plasmonic waves. However, numerous applications of THz meta-devices demand an active manipulation of the THz beam in free space. Although some studies have been carried out to control the EOT for the THz region, few of these are based upon electrical modulation of the EOT phenomenon, and novel strategies are required for actively and dynamically reconfigurable EOT meta-devices. In this work, we experimentally present that the EOT resonance can be coupled to optically reconfigurable chalcogenide metamaterials which offers a reversible all-optical control of the THz light. A modulation efficiency of 88% in transmission at 0.85 THz is experimentally observed using the EOT metamaterials, which is composed of a gold (Au) circular aperture array sitting on a non-volatile chalcogenide phase change material (Ge 2 Sb 2 Te 5 ) film. This comes up with a robust and ultrafast reconfigurable EOT over 20 times of switching, excited by a nanosecond pulsed laser. The measured data have a good agreement with finite-element-method numerical simulation. This work promises THz modulators with significant on/off ratios and fast speeds.

Metasurface analog of electromagnetically induced transparency (EIT) provides a compact platform for generating a narrow-band transmission window with very sharp spectral features. They hold promise for many appealing applications including ultrasensitive detectors, slow-light devices, nonlinear optical devices etc. In particular, reconfigurable EIT metasurfaces are crucial for expanding the capability of light field control, which are promising for terahertz (THz) communications and optical networks. Yet, the investigation on reconfigurable EIT metasurfaces with nonvolatile operation remains scarce. Here, reversible switching of the metasurface-induced transparency in the THz spectrum is experimentally realized. The reconfigurable response (reversible spectral shift) is obtained by integrating a nonvolatile chalcogenide phase change material, Ge 2 Sb 2 Te 5 (GST225) into the meta-atoms. A giant reversible switching of EIT takes place under an excitation of nanosecond laser pulses, showing a reconfigurable group delay of the THz waves. The proposed reconfigurable THz metadevices may provide a new route for the ultrafast laser induced switching and reconfigurable slow-light devices.

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Efficient thermal radiation in the mid-infrared (M-IR) region is of supreme importance for many applications including thermal imaging and sensing, thermal infrared light sources, infrared spectroscopy, emissivity coatings, and camouflage. The capability of controlling light makes metasurface an attractive platform for infrared applications. Recently, different metamaterials have been proposed to achieve high thermal radiation. To date, broadening of the radiation bandwidth of metasurface emitter (meta-emitter) has become a key goal to enable extensive applications. We experimentally demonstrate a broadband M-IR thermal emitter using stacked nanocavity metasurface consisting of two pairs of circular-shaped dielectric (Si3N4) – metal (Au) stacks. A high thermal radiation can be obtained by engineering the geometry of nanocavity metasurface. Such a meta-emitter provides wideband and broad angular absorptance of both p- and s-polarized light, offering a wideband thermal radiation with an average emissivity of more than 80% in the M-IR atmospheric window of 8–14 μm. The experimental illustration together with theoretical framework places a basis for designing broadband thermal emitters, which, as anticipated, will initiate a promising avenue to M-IR source.

Overlapping of reflected ultrasonic echoes is a universal physical phenomenon, when the measured thickness is less than a limit threshold (e.g. 1mm for metallic materials using conventional processing methods). Generally, two typical signal states may be generated in this situation: highly overlapping and distorted overlapping. If the overlapping echoes cannot not be separated precisely, significant measurement errors will inevitably occur. In this research, an improved matching pursuit method for overlapping echo separation has been developed. To approximate the overlapping echoes, a dictionary composed of atoms with truncated Nakagami functions is constructed to replace the commonly used model, which considers scattering conditions. Furthermore, an atomic selection principle combining preselection of the remainder L2 norm and minimum L1 norm of the residual signal is presented. To reduce the correlation of the residual error in iterations and ensure the correct separation of echoes in order, the ultrasonic signal is truncated for matching based on short-time Fourier transform (STFT) analysis. Compared with some competing methods, the proposed method shows superiority in handling highly overlapping echoes and distorted echoes in both simulation and experiments of ultrasonic thickness measurement.

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