Reciprocity is a fundamental principle of wave physics related to time-reversal symmetry. Nonreciprocal wave behaviors have been pursued for decades because of their great scientific significance and tremendous potential applications. However, nonreciprocity devices have been based on manipulation of non-topological charge (TC) in most studies to date. Here, we introduce the rotational Doppler effect (RDE) into the acoustic system to achieve nonreciprocal control of the TC beam. We use the metasurface to generate a vortex beam with a defined TC. By rotating the metasurface with specific angular velocity, the wave vector of the transmitted wave obtains positive and negative transition flexibly due to the RDE. As a result, isolated and propagating states of the vortex beam can be realized by controlling the rotation direction, representing nonreciprocal propagation. Our work also provides an alternative method for the application of TC beams and the realization of nonreciprocity.

Objective . The imaging photoplethysmography (IPPG), which is applied to heart rate measurement, is susceptible to interference caused by illumination variations during photography. To improve the accuracy, robustness, and real-time performance of IPPG, a new method combined with modified ensemble empirical mode decomposition (EEMD) and fast independent component analysis (FastICA) was proposed. Approach . On the modified EEMD, to reduce the error caused by the residual white noise of the algorithm, it was characterized by recurrently updating zero-mean white noise for ensemble averaging based on the standard deviation of the input signal. On the modified FastICA, its constructed nonlinear function was superseded by Huber’s approximation function to improve the robustness and running speed. Main results . Comparison experiments were conducted between the MAHNOB-HCI database and own collected data. In the hybrid natural light and computer screen light scenario, the mean absolute error (MAE) of heart rate amounted to 0.93 beats per minute and the correlation coefficient r reached 0.85. In the experiments of MAHNOB-HCI database, the MAE amounted to 6.03 beats per minute and the correlation coefficient r reached 0.75. Furthermore, the modified method decreased the running time by approximately ten times compared to the original algorithms. Significance . Various experimental results demonstrated that the proposed method significantly improves the accuracy, timeliness, and interference resistance.

Focusing is extensively researched in bioimaging, medicine, and quantum computation. However, single focal point and short focal length restrict imaging and optical manipulation at long distances. In this study, we propose what we believe to be a novel method, namely the coherent superposition of double-ring circular Airy Gaussian beams (DR-CAiGBs) to achieve multiple autofocusing over long distances. Without complex structures, theoretical simulations and experimental verifications demonstrate that the DR-CAiGBs can generate multiple autofocusing points along the optical axis, whose positions and quantities can be flexibly adjusted. Furthermore, we demonstrated the two-dimensional (2D) particle manipulation of the DR-CAiGBs by trapping multiple particles at different autofocusing points over 100 µm. Our research and findings establish new avenues for practical applications in biological cell analysis, particle transportation, and lithography.

By using a high-contrast grating (HCG, high transmittance > 90 % ) to control the phase shift of incident light, we theoretically designed a novel-structured HCG-integrated superconducting nanowire single-photon detector (HCG-SNSPD) with a high-efficiency and large light-receiving area. Without enlarging the typical single-pixel SNSPD nanowire area ( 10 μm × 10 μm ), the effective detection area is expanded to 115 μm, while the absorption efficiency of the nanowire reaches 84.9% at a wavelength of 1550 nm. The effective detection area of HCG-SNSPD is increased by 11.5 times compared to that of conventional single-pixel SNSPDs. Moreover, the absorption efficiencies of HCG-SNSPD exceed 70% at wavelengths ranging from 1460 nm to 1650 nm, indicating high-efficiency broadband detection. This study promotes new possibilities for the application of SNSPDs.

We propose a novel strategy for optical trapping and sorting of micro-nano particles that does not involve the flow of the immersion medium. The method employs the size- and velocity-dependent response of suspended polystyrene particles to the optical sieve of a dynamic fringelike pattern generated by a spatial light modulator. We not only experimentally demonstrate the trapping and transport of polystyrene particles with 2 μm diameter, but also study the sorting of a polydisperse suspension of polystyrene beads with diameters of 0.8 and 2 μm. The results provide new insights into particle manipulation and separation, and reveal that there is still scope for sorting smaller particles based on dynamic optical sieve.

Dynamic light scattering is a standard technique of nano-particle sizing in colloid systems. However, it is difficult to measure the particle size accurately in the flowing dispersion, which is inevitable in many applications, such as on-line measurement. In this paper, we present that the wave vector of the scattered light, which is averaged to a constant in traditional technique, varies with time when the particles undergoing not only the random motion but also the directional movement. This variation results in an additional term of sinc function to the traditional intensity autocorrelation function of the scattered light, thus affect the final particle size determination, especially for large velocity of the directional movement. The experimental results agree well with our derivation. We believe this will facilitate the extension of the technique of the dynamic light scattering to a wider range of application, especially of the on-line measurement in the flowing dispersion.