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Topological insulators exhibit great potential in the fields of electronics and semiconductors for their gapless surface states. Intriguingly, most topological insulators are possibly excellent microwave-absorbing materials because of easy adjustment of electrical transport based on conducting surface states in the nanostructure. Herein, topological insulator Bi2Te3 nanosheets are synthesized by a simple solvothermal method. The material demonstrates a unique dielectric behavior based on conducting surface states, resulting in excellent microwave-absorbing performance. Benefiting from the outstanding impedance matching, Bi2Te3 nanosheets exhibit an ultrathin microwave absorption with the qualified frequency bandwidth of 3.0 GHz at only 0.77 mm thickness, which is thinner than other absorbers in reported references. Moreover, a strong reflection loss of -41 dB at 0.8 mm is achieved. The result provides a new approach for developing ultrathin microwave absorption materials at the submillimeter scale.

Since sensors could be used for detecting multiple forms of mechanical deformations, we present a simple method for fabricating highly sensitive strain sensors using a conductive graphene/polymer sponge which is self-assembled by graphene flakes in the porous structure of the sponge. The graphene composite sponges exhibit outstanding electromechanical properties and can be processed into different dimensional sensors easily to detect multiple forms of mechanical deformations including tensile strain, impact, bending, vibrating and twisting. Although the sensitivity of the three-dimensional sensor is only 1.33 N-1, one-dimensional sensor exhibits high sensitivity up to 18.5 N-1. In addition, the one-dimensional sensor can evaluate human life activity, such as finger wag and even the impact caused by a hair. The shape of the graphene composite sponge can easily be arbitrarily customized in order to sense more accurately in practical applications.

For eliminating the limits in the classical models, here we establish a general model for precisely predicting dielectric constant of porous materials. In this model, dielectric constant is independent on pore shapes when the pore size is far smaller than electromagnetic wavelengths; however, it depends on the porosities and correlation between the open pore direction and wave propagation direction. The structure factor β in the effective dielectric formula is 1 and 3 for through-hole and closed-pore materials, respectively. The experimental results are in good agreement with the model, suggesting a favorable tool for predicting dielectric properties of porous materials.

We present a silicon slot waveguide with metallic gratings embedded on the silicon surface in the slot region. The dependence of the optical coupling between two silicon wires on the width of the metal gap and the slot size are studied in detail. The results show that the optical field in the slot region with metallic gratings is significantly enhanced compared with the traditional slot waveguide due to the surface plasmon polaritons coupling on metallic gratings. The extraordinary optical confinement is attributed to the low effective dielectric constant of metallic gratings. The effective dielectric constant decreases with the increasing wavelength, and reaches the minimum when the width of the metal gap is about 0.01 times the wavelength.