A Novel Robust Zero-Watermarking Algorithm for Audio Based on Sparse Representation

Behind the prevalence of multimedia technology,digital copyright disputes are becoming increasingly serious.The digital watermarking prevention technique against the copyright infringement needs to be improved urgently.Among the proposed technologies,zero-watermarking has been favored recently.In order to improve the robustness of the zero-watermarking,a novel robust audio zerowatermarking method based on sparse representation is proposed.The proposed scheme is mainly based on the K-singular value decomposition(K-SVD)algorithm to construct an optimal over complete dictionary from the background audio signal.After that,the orthogonal matching pursuit(OMP)algorithm is used to calculate the sparse coefficient of the segmented test audio and generate the corresponding sparse coefficient matrix.Then,the mean value of absolute sparse coefficients in the sparse matrix of segmented speech is calculated and selected,and then comparing the mean absolute coefficient of segmented speech with the average value of the selected coefficients to realize the embedding of zero-watermarking.Experimental results show that the proposed audio zerowatermarking algorithm based on sparse representation performs effectively in resisting various common attacks.Compared with the baseline works,the proposed method has better robustness.

Terahertz wide-angle metalens with nearly ideal object-image relation

Metalenses are essential components in terahertz imaging systems.However,without careful design,they show limited field of view and their practical applications are hindered.Here,a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of±25°.Simulation and experiment results show that the focusing efficiency,spot size,and modulation transfer function of this lens are not sensitive to the incident angle.More importantly,this wide-angle metalens follows the ideal Gaussian formula for the object-image relation,which ensures a wider field of view and better contrast in the imaging experiment.

Arbitrary terahertz chirality construction and flexible manipulation enabled by anisotropic liquid crystal coupled chiral metasurfaces

Chiral metasurfaces integrated with active materials can dynamically control the chirality of electromagnetic waves,making them highly significant in physics,chemistry,and biology.Herein,we theoretically proposed a general and feasible design scheme to develop a chiral metadevice based on a bilayer anisotropic metasurface and a monolayer liquid crystal(LC),which can construct and flexibly manipulate arbitrary terahertz(THz)chirality.When the twist angle between the anisotropic axes of two metasurfacesθis not 0°,the spatial mirror symmetry of the chiral metadevice is broken,resulting in a strong THz chiral response.In addition,the introduction of anisotropic LCs not only enhances the chiral response of the metadevice but also induces the flipping modulation and frequency tunability of the chirality.More importantly,by optimizing theθ,we can flexibly design the arbitrary chiral response and the operating frequency of chirality,thereby promoting the emergence of various chiral manipulation devices.The experimental results show that the maximum circular dichroism can reach-33 d B at 0.94 THz and flip to 28 d B at 0.69 THz by rotating the LC optical axis from the x to y axis,with the maximum operating frequency tunable range of~120 GHz.We expect this design strategy can create new possibilities for the advancement of active THz chiral devices and their applications,including chiral spectroscopy,molecular recognition,biosensing,and fingerprint detection.

Excellent Terahertz shielding performance of ultrathin flexible Cu/graphene nanolayered composites with high stability

Electromagnetic interference(EMI)shielding at Terahertz(THz)frequency range attracts increasing attention due to the rapid development of THz science and technologies.EMI shielding materials with small thickness,high shielding effectiveness(SE),good flexibility and stability are highly desirable.Herein,an ultrathin flexible copper/graphene(Cu/Gr)nanolayered composite are prepared,which can reach the average EMI SE of 60.95 dB at 0.1–1.0 THz with a thickness of only 160 nm,indicating that more than 99.9999%of the THz wave power can be shielded.Furthermore,the Cu/Gr nanolayered composite also exhibits excellent oxidation resistance,with a 93.09%maintenance rate for EMI SE value after heating at 120℃for 3 h in air,far higher than that of the bare Cu film(62.15%).Besides,the Cu/Gr nanolayered composite exhibits good mechanical flexibility and flexural fatigue resistance.The EMI SE value of the Cu/Gr nanolayered composite shows a maintenance rate of 98.87%even after 1500 times bending cycles,obviously higher than that of multilayer Cu film(93.07%).These results demonstrate that the ultrathin flexible Cu/Gr nanolayered composites with excellent shielding performance and good stability have a broad application prospect in THz shielding for wearable devices and next generation mobile communication equipment.