The ability to break Kirchhoff’s law is of fundamental importance in thermal radiation. Various nonreciprocal emitters have been proposed to break the balance between absorption and emission. However, the thicknesses of the nonreciprocal materials are usually larger than 1/10 times of the wavelength. Besides, the previous proposed nonreciprocal emitters are complex, thus they can hardly be fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials. In this paper, we investigate the nonreciprocal thermal radiation of the magnetic Weyl semimetal (MWSM) film atop of the metal substrate. It is found that the strong nonreciprocal radiation at the wavelength of 9.15 µm can be achieved when the thickness of the MWSM film is 100 nm. The enhanced nonreciprocity is attributed to the Fabry-Perot resonances. The results indicate that the MWSM film is the promising candidate to engineer the ultrathin and simple nonreciprocal thermal emitters. What is perhaps most intriguing here is that the proposed structure can be more easily fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials.

A high-performance fiber optic interferometric hydrophone based on a push-pull mandrel structure, with high sensitivity and wide-frequency response, is designed and developed. Each arm of the Michelson interferometer is a fiber coil wrapped onto a separate but concentric thin elastic tube as the sensing arm to detect acoustic pressure wave. The fiber optic hydrophone is fabricated and examined experimentally, demonstrating its capacity to detect very low-frequency underwater acoustic pressure variation. The theoretical design is validated experimentally, with results showing that the push-pull fiber optic hydrophone has an average sensitivity of -134.39 dB re rad/ μPa within the frequency range of 1 Hz-2 kHz, which is 5.67 dB higher than that of the analogous one-sensing-arm fiber optic interferometric mandrel hydrophone that we made for comparison. The push-pull fiber optic hydrophone has a minimum detectable pressure (MDP) of 25.37 μPa/√{Hz} at 1 kHz, about 15 dB lower than the deep-sea state zero (DSS0) level. At 10 Hz, the MDP is ~1.71 mPa/ √{Hz}, which is nearly 7 dB lower than the acoustic ambient noise in the ocean at low frequency. The sensitivity and the MDP of the push-pull fiber optic hydrophone have been doubled and improved by about 20% at 1 kHz, respectively, compared to other state-of-the-art flat response fiber optic hydrophones.

In recent years, our research group has been devoting substantial efforts to the research and development of active all-solid-state electronic terahertz (THz) continuous wave imaging systems for nondestructive testing, which is currently benefitting from the increasing amount of transmitting power, high performance/cost ratio and adaptability to engineering. In this paper, an in-house developed broadband linear frequency modulated continuous wave (LFMCW) three-dimensional (3D) THz imaging system is described, and two sets of experimental platforms are set up to assist the planning and completion of the research, including a narrow-band LFMCW 3D imaging radar and a wide-band stepped-frequency modulated continuous wave 3D imaging radar. For 3D imaging systems, to cope with demanding scenarios and to achieve excellent imaging performance, various reconstruction algorithms are explored. The first is a spectral refinement and correction approach based on fast Fourier transform and modern spectral estimation for accurate thickness measurement. The second is the synthetic aperture radar imaging algorithm for surface detection or internal detection of objects with lower refractive index. The third is a 3D reconstruction algorithm based on half space Green’s function and the exploding source model for the interior detection of materials with higher refractive index. The fourth is the frequency interference algorithm combining phase unwrapping to measure uneven and nonplanar surfaces. Exploiting these systems, along with the associated experimental platforms and reconstruction algorithms, we successfully implemented non-destructive testing for objects with various defects and of different materials, such as polymer boards with voids, and foam with inclusions.