We experimentally demonstrate a chiral metamaterial exhibiting negative refractive index at terahertz frequencies. The presence of strong chirality in the terahertz metamaterial lifts the degeneracy for the two circularly polarized waves and allows for the achievement of negative refractive index without requiring simultaneously negative permittivity and negative permeability. The realization of terahertz chiral negative index metamaterials offers opportunities for investigation of their novel electromagnetic properties, such as negative refraction and negative reflection, as well as important terahertz device applications.

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Transmission spectra of terahertz pulses through periodic array of subwavelength holes exhibit a characteristic evolution with various hole widths. The peak absolute transmittance approaches a maximum value at a critical hole width, while linewidth broadening and blueshift of peak frequency are observed with increasing hole width. Such characteristic evolution is attributed to the coupling between discrete resonant excitation of surface plasmons and continuum nonresonant transmission through the holes; this agrees well with the numerical analysis based on the Fano model and the measured angle-resolved transmission band structures.

Mass production of ordered and porous three-dimensional (3D) electrodes is a crucial prerequisite for practical energy storage devices. MXenes have drawn considerable attention as pseudocapacitive materials for outstanding electric conductivity and surface redox reactions; however, they face challenges for achieving 3D porous architectures especially at high mass loadings. Herein we propose a reduced-repulsion freeze-casting assembly concept via interlayer interaction engineering for constructing 3D porous Ti3C2T x films, wherein interlayer repulsion is minimized via less electronegative functional groups and charge screening effect based on quantum calculations. 3D Ti3C2T x films deliver a capacitance of 207.9 F g–1 at 10 V s–1, which demonstrates 58.6% capacitance retention with a 1000-fold scan rate increase. The capacitive performance is almost independent of electrode mass loading up to 16.18 mg cm–2, exhibiting ultrahigh areal capacitance of 3731 mF cm–2 and energy density of 336.7 μWh cm–2.