Optical chaos has a wide range of applications in communications, such as secure communication, high-resolution lidar ranging, optical time domain reflectometer, and high-rate physical random bit generator.

In recent years, external-cavity feedback semiconductor lasers (ECLs) are the most common chaotic laser generation systems due to their characteristics of wide bandwidth, large amplitude, and simple structure, and the dynamic characteristics of chaotic signals have attracted much attention. However, limited by the relaxation oscillation of the laser, the energy of the chaotic signal directly generated by ECL is mainly concentrated at high relaxation oscillation frequency. Thus, the low-frequency component encounters the problem of energy loss.

In practical applications, the signal detection/acquisition device usually responds to a 3-dB low-pass filtering characteristic. Therefore, the available effective bandwidth of the chaotic signal should actually be 3-dB bandwidth. The lack of low-frequency components will limit the energy utilization rate of chaotic signals and restrict the relevant performances of chaotic applications (such as reliability and transmission of chaotic secure communication, randomness and generation rate of physical random bits, measurement accuracy and range of lidar ranging or optical time-domain reflectometer).

In the paper, we propose a broadband chaos generation scheme with simple structure and losing no low-frequency components. Specifically, we experimentally analyze the radio frequency (RF) spectra of the single-mode and the multi-mode output from an optical feedback Fabry-Perot (FP) semiconductor laser after and before filtering. The experimental results show that comparing with the multi-mode chaotic signal, the low-frequency energy of the single-mode chaotic spectrum is enhanced by 25 dB, and the 3-dB bandwidth of the single-mode chaotic signal can reach 6 GHz. Further theoretical analysis demonstrates that the enhancement of low-frequency component in the single-mode chaotic signal is caused by the mode-competing in multi-mode laser. It is concluded that this method can well solve the problem of low-frequency energy loss in conventional optical feedback chaotic systems, and is beneficial to improving the energy utilization rate of chaotic signals, which is of great significance for improving the performance of chaotic secure communication, random bit generation, lidar ranging, optical time domain reflectometer, and other relevant applications.

In order to improve the resistance properties of HfO₂ and increase the consistency and uniformity of conductive filaments formed by oxygen vacancies (VO), the first-principles calculation method based on density functional theory is used to study the micro-properties of Al-doped HfO₂ resistive materials. The results show that the interval Al (Int-Al) is more suitable for being incorporated into HfO₂, and the closer to the relative position of VO the Int-Al, the faster the convergence rate of the resistive material tends to be stable, and the smaller the formation energy. The effects of different Int-Al concentrations on the formation of HfO₂ supercells with VO defects show that when the concentration of doped Int-Al is 4.04%, the fractional charge state density map can form relatively good charge channels. The maximum and critical equipotential surface values are highest, which is conducive to improving the consistency and uniformity of the formation of conductive filaments in HfO₂ resistive materials. The calculation of energy formation shows that the change is slow when the concentration of Int-Al is lower than 4.04%. When the concentration of Int-Al is higher than 4.04%, the abnormal increase occurs, which indicates that the defect system becomes more and more difficult to form with the increase of the concentration of Int-Al.

The introduction of the impurity and the VO defect destroy the original complete crystal structure, which causes the position of the atoms around the impurity to shift, and the valence electron orbit and the energy level of the crystal are changed, and the distribution of the internal charges of the HfO₂ defect system is affected. In order to study the effect of the change of the lattice structure on the formation of the VO conductive filament, the VASP software package is used to calculate the relative ratio of the atoms in the lattice structure of the HfO₂ defect system as the reference and the relative ratio of the HfO₂ defect system after the optimizing the lattice structure. Further study of the change of lattice structure, when the concentration of doped Int-Al is 4.04%, shows that the defect formation energy decreases significantly, which is conducive to the formation of perfect conductive channel. The conductive channel has a certain reference significance for improving the performance of HfO₂ based resistive variable memory materials.

With the development of laser technology, the application scope of nondiffracting beams, such as Bessel beams, Mathieu beams, cosine beams, and parabolic beams, which remain invariant along their propagation, continues to expand. During its propagation, the main lobes of these beams tend to bend towards off-axis position, which is called self-accelerating (or self-bending) property. A Bessel-like beam with self-acceleration can realize the propagation of the main lobe along a curved trajectory while maintaining the non-diffraction. Because of the above property, Bessel-like beams have been utilized in various areas such as guiding particles along arbitrarily curved trajectories, self-accelerating beams in nonlinear medium, plasma guidance, and laser-assisted guiding of electric discharges around objects.

In this paper, we propose a method of bending the trajectory of Bessel-like beams by using a magnetic fluid deformable mirror (MFDM) instead of traditional spatial light modulator (SLM) and Pancharatnam-Berry (PB) phase manipulation. The MFDM provides a method without pixelation, where all parameters can be rapidly modified for fine-tuning. Furthermore, compared with the conventional deformable mirror, the MFDM has the advantages of a continuous and smooth mirror surface, large shape deformation, low manufacture cost, easy extension, and large inter-actuator stroke. Therefore, it is easy for the MFDM to generate the ideal shape of an axicon. Firstly, according to geometric analysis, the asymmetrical mirror profile to produce a self-accelerating Bessel-like optical beam is proposed. The proposed mirror profile can be used to compensate for the difference in optical path length for each annular slice of the axicon. If a collimated Gaussian beam is incident on the mirror combining the axicon and the asymmetrical mirror profiles, which can obtain Bessel-like beams with arbitrarily curved trajectories. Secondly, the resultant of the self-accelerating Bessel-like beams along parabolic trajectories is validated by the simulation in MATLAB. Finally, a prototype of MFDM consisting of the dual-layer arrays of miniature electromagnetic coils, a Maxwell coil and the magnetic fluid filled in a circular container is fabricated for the experiment. The experimental results show that the Bessel-like beams propagate along the parabolic trajectories, with the MFDM used, and the accuracy of the curved trajectories is verified. The proposed method in this paper opens a new experimental way to the study of Bessel-like beam; the theoretical approach can also be generalized mathematically for other non-paraxial beam propagation.

Polarization state of electromagnetic wave has important applications in satellite communication, radar detection, and stereoscopic display imaging. Therefore, the control of polarization state of electromagnetic wave is an important direction in scientific research. The traditional method of manipulating the polarization state is mainly realized based on Faraday effect and birefringent crystal, which has a certain requirement for the material thickness (leading to large volume), and does not have broadband characteristics (leading to narrow band). Recently, metamaterial with subwavelength meta-atoms, has achieved many exotic phenomena and functionalities that cannot be found in nature. As an important branch of metamaterial-based devices, polarization converter has attracted great attention and achieved significant progress. However, most of them cannot realize ultra-broadband, high-efficiency, wide-angle, and simple geometry simultaneously.

In this paper, a linear polarization converter based on a square split ring metasurface is proposed. Due to the anisotropic structure, the amplitudes of the reflected electric field along the two diagonal lines are equal, and their phase difference is 180°. As a result, the polarization direction of the incident wave can be rotated 90°. The simulation results show that the polarization conversion ratio (PCR) is higher than 90% in a frequency range from 7.12 to 18.82 GHz, which means that the relative bandwidth reaches 90%. The significant bandwidth expansion is attributed to the four electromagnetic resonances generated in a square-split-ring unit. We investigate the influence of geometric parameters on PCR in detail. We also examine the performance of the proposed structure under oblique incidence. It has little effect on the co-polarization and cross-polarization reflection coefficients when the incident angle is changed from 0° to 45°. Even if the incident angle reaches 45°, the mean PCR remains above 80%. The PCRs of the four electromagnetic resonant points are all close to 100%. Finally, we fabricate and measure the proposed polarization converter that contains $30\times30$ unit cells. The experimental results are in good agreement with the simulation results, and thus validating the design.

In conclusion, we propose both theoretically and experimentally a linear polarization converter that possesses ultra-broadband, high-efficiency, wide-angle, and simple geometry simultaneously. The proposed scheme can be extended to terahertz and even optical frequencies.