With balanced spatial resolution, imaging depth, and functional sensitivity, photoacoustic tomography (PAT) hold great promise for human brain imaging. However, the strong acoustic attenuation and aberration of the human skull (∼8 mm thick) are longstanding technical challenges for PAT of the human brain. In this work, we numerically investigated the impacts of the stratified human skull on photoacoustic wave propagation (i.e., the forward model) and PAT image formation (i.e., the inverse model). We simulated two representative transcranial PAT implementations: photoacoustic computed tomography (PACT) and photoacoustic macroscopy (PAMac). In the forward model, we simulated the detailed photoacoustic wave propagation from a point or line source through a digital human skull. The wave attenuation, refraction, mode conversation, and reverberation were thoroughly investigated. In the inverse model, we reconstructed the transcranial PACT and PAMac images of a point or line target enclosed by the human skull. Our results demonstrate that transcranial PAMac suffers mainly from wave reverberation within the skull, leading to prolonged signal duration and reduced axial resolution. Transcranial PACT is more susceptible to the skull’s acoustic distortion, mode conversion, and reverberation, which collectively lead to strong image artifacts and deteriorated spatial resolutions. We also found that PACT with a ring-shaped transducer array shows more tolerance of the skull’s adverse impacts and can provide more accurate image reconstruction. Our results suggest that incorporating the skull’s geometry and acoustic properties can improve transcranial PAT image reconstruction. We expect that our results have provided a more comprehensive understanding of the acoustic impact of the human skull on transcranial PAT.

Recently, the study of analog optical computing raised renewed interest due to its natural advantages of parallel, high speed and low energy consumption over conventional digital counterpart, particularly in applications of big data and high-throughput image processing. The emergence of metamaterials or metasurfaces in the last decades offered unprecedented opportunities to arbitrarily manipulate the light waves within subwavelength scale. Metamaterials and metasurfaces with freely controlled optical properties have accelerated the progress of wave-based analog computing and are emerging as a practical, easy-integration platform for optical analog computing. In this review, the recent progress of metamaterial-based spatial analog optical computing is briefly reviewed. We first survey the implementation of classical mathematical operations followed by two fundamental approaches (metasurface approach and Green’s function approach). Then, we discuss recent developments based on different physical mechanisms and the classical optical simulating of quantum algorithms are investigated, which may lead to a new way for high-efficiency signal processing by exploiting quantum behaviors. The challenges and future opportunities in the booming research field are discussed.

A angular radial staggered vane (ARSV) slow wave structure (SWS) is proposed for high power vacuum devices. Cooperating with an angular radial sheet electron beam (ARSEB), the SWS is designed for a W-band backward wave oscillator (BWO). The high frequency characteristics of the proposed SWS are studied by using simulation. The effects of the angle have been found out. The steady transmission of an angular sheet electron beam has been realized using a radial uniform magnetic field of ~ 1 T. The beam wave interactions are studied using particle-in-cell (PIC) simulation, a peak output power of 21 kW has been achieved at 100 GHz, with an electron efficiency of about 5%. The study of frequency versus beam voltage shows that the proposed BWO can be tunable in the frequency range of 79.8 GHz - 104.7 GHz and the output power is higher than 13 kW. The fabrication and cold test results are presented at the end.

Because of helical phase wavefront distribution, vortex electromagnetic waves are considered to carry more information and additional degrees of freedom than traditional spherical waves. Therefore, a vortex wave carrying orbital angular momentum (OAM) can improve inversion and imaging accuracy. In this work, we revisit the reconstruction of three-dimensional objects in layered composite structures extended with OAM. In forward modeling, the concentric uniform circle array is used to generate electromagnetic vortex beams. To analyze the difference of vortex beams, the electric field radiation pattern and phase pattern distribution of OAM waves with different modes are calculated. Then, the scattered field of layered media illuminated by different OAM beams is determined by the dyadic Green's function and the stabilized biconjugate gradient technique with a fast Fourier transform algorithm. In the inversion, the variational Born iterative method is used to reconstruct targets in layered composite structures, and multiple OAM modes are used to improve the reconstruction results. The numerical results prove that the permittivity of the target can be better reconstructed by using the multiple OAM modes rather than the traditional spherical wave. With the increase of OAM mode number, the reconstructed target parameters are closer to the true value. We expect that our results will provide a better understanding of the OAM and pave the way for the improvement of inversion and optical imaging technology using vortex waves.

This study proposes a novel method for generating virtual orbital angular momentum (OAM) using a synthetic uniform circular array (SUCA). In the proposed method, the base array is rotated to various spatial locations, with the feeding phases modified accordingly, and the generated fields at various spatial locations are superimposed. The proposed method exchanges the cost of the time domain for more spatial degrees of freedom, which can overcome the limit of space and configure more virtual array elements to generate vortex electromagnetic waves carrying virtual multimode OAMs. To validate the effectiveness of the proposed method, a radially placed synthetic uniform circular array is designed with four actual elements of a circularly polarized horn antenna and sixteen virtual elements. The SUCA is then simulated and measured, and it successfully generates fifteen virtual OAMs in the frequency range of 5-6.5 GHz. Furthermore, 2-D object detection experiments of three metal targets with different scales at different positions based on the SUCA were conducted. The experimental results show that the three targets can be distinguished clearly, indicating that our work has the potential to generate more virtual OAMs and improve object detection resolution in practical applications.