Origin of Negative Imaginary Part of Effective Permittivity of Passive Materials

The anti-resonant phenomenon of effective electromagnetic parameters of metamaterials has aroused controversy due to negative imaginary permittivity or permeability. It is experimentally found that the negative imaginary permittivity can occur for the natural passive materials near the Fabry Perot resonances. We reveal the nature of negative imaginary permittivity, which is correlated with the magnetoelectric coupling. The anti-resonance of permittivity is a non-inherent feature for passive materials, while it can be inherent for devices or metamaterials. Our finding validates that the negative imaginary part of effective permittivity does not contradict the second law of thermodynamics for metamaterials owing to the magnetoelectric coupling.

Measurement of the echo reduction for underwater acoustic passive materials by using the time reversal technique

A measuring method of the echo reduction of passive materials by using the time reversal（TR） technique is presented. To measure the echo reduction of a sample with this approach, the received signals are firstly focused according to the TR theory. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal.Finally, the echo reduction of the sample is evaluated with these focusing signals. Besides, to calibrate the measured echo reduction via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 mxl.O m x0.005 m axe tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results under the free field at the measured frequency range of0.5-20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dB. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries and low frequency sources.

Inorganic material passivation of defects toward efficient perovskite solar cells

Surface passivation with organic materials is one of the most effective and popular strategies to improve the stability and efficiency of perovskite solar cells(PSCs). However, the secondary bonding formed between organic molecules and perovskite layers is still not strong enough to protect the perovskite absorber from degradation initialized by oxygen and water attacking at defects. Recently, passivation with inorganic materials has gradually been favored by researchers due to the effectiveness of chemical and mechanical passivation. Lead-containing substances, alkali metal halides, transition elements, oxides,hydrophobic substances, etc. have already been applied to the surface and interfacial passivation of PSCs.These inorganic substances mainly manipulate the nucleation and crystallization process of perovskite absorbers by chemically passivating defects along grain boundaries and surface or forming a mechanically protective layer simultaneously to prevent the penetration of moisture and oxygen, thereby improving the stability and efficiency of the PSCs. Herein, we mainly summarize inorganic passivating materials and their individual passivation principles and methods. Finally, this review offers a personal perspective for future research trends in the development of passivation strategies through inorganic materials.

Measurement of insertion loss for underwater acoustic passive materials with the time reversal technique

A method using the time reversal（TR） technique to measure the insertion loss（IL） of passive materials is presented.Firstly the received signals are focused according to the TR theory when there is not a sample between the source and the received array.Then,the sample is placed near the received array and the TR processing is again employed to realize the focus of the received signal.Finally,the IL of the sample is evaluated from these focusing signals.Because the TR processing can focus the energy in spatial domain and time domain,the method can be used to measure acoustic properties of passive materials in a waveguide tank with reflections induced by boundaries or with low source frequencies.Two samples with the same size of 1.1 m×1.0 m×5 mm are tested in the waveguide tank.The method is demonstrated by the comparison of the theoretical and the experimental results in the measured frequency range of 1-20 kHz.