Deep polarization of the ensemble nitrogen-vacancy centers in diamonds induced by an Airy beam with a long focal depth.

Solid-state spin systems with nitrogen-vacancy (NV) centers in diamonds constitute an increasingly popular platform for quantum sensing. However, most existing platforms designed with ensemble NV centers exhibit a sensitivity that is significantly less than the theoretical maximum. This low sensitivity limits the expansion of the experimental results and application areas. In this study, the sensitivity is improved by increasing the pumping depth of the excitation beam to increase the number of particles involved in spin polarization at a given laser intensity. Compared with the proposed Airy beam with a long focal depth (25.46 λ) and the widely utilized Gauss beam pumping ensemble NV centers, the spin resonance factor fSR can be improved by 10.02%. This sensitivity-optimized approach enhances the functionality of sensors with NV centers.

Arbitrary combinations of helical-conical optical beams in free space.

Helical-conical optical beams (HCOBs) have attracted considerable interest due to their peculiar optical features. Their characteristic helical light intensity distribution has exerted unprecedented advantages in many fields, but multiple combinations of HCOBs have not been reported due to the limitations of algorithms and light field modulation techniques. We propose and experimentally demonstrate arbitrary combinations of multiple HCOBs in free space to construct hybrid HCOB arrays. The similarity between the experimental results and the numerical simulation results is 94.22%. The initial orientation of the HCOBs is flexibly tuned by the rotation factor ß, and the optical pen is used to combine the HCOBs. This approach allows multiple parameters in the array to be precisely tuned, including the type, number, and position of HCOBs, adding more design flexibility. The constructed HCOB arrays have a higher degree of modulation freedom and may find applications in fields where dynamic control is in high demand, including optical tweezers, biological cell sorting, and multiparticle manipulation.

Dynamic pressure surface deformation measurement based on a chromatic confocal sensor: erratum.

This erratum corrects errors in Fig. 4 of the original paper, Appl. Opt.62, 1467 (2023)APOPAI0003-693510.1364/AO.482808.

Effective Signal Extraction Algorithm for Cerebral Blood Oxygen Based on Dual Detectors.

Functional near-infrared spectroscopy (fNIRS) can dynamically respond to the relevant state of brain activity based on the hemodynamic information of brain tissue. The cerebral cortex and gray matter are the main regions reflecting brain activity. As they are far from the scalp surface, the accuracy of brain activity detection will be significantly affected by a series of physiological activities. In this paper, an effective algorithm for extracting brain activity information is designed based on the measurement method of dual detectors so as to obtain real brain activity information. The principle of this algorithm is to take the measurement results of short-distance channels as reference signals to eliminate the physiological interference information in the measurement results of long-distance channels. In this paper, the performance of the proposed method is tested using both simulated and measured signals and compared with the extraction results of EEMD-RLS, RLS and fast-ICA, and their extraction effects are quantified by correlation coefficient (R), root-mean-square error (RMSE), and mean absolute error (MAE). The test results show that even under low SNR conditions, the proposed method can still effectively suppress physiological interference and improve the detection accuracy of brain activity signals.

Free-space creation of a perfect vortex beam with fractional topological charge.

Perfect vortex beams can only propagate stably with integer topological charges. Thus, creating perfect fractional vortex beams capable of stable propagation in free space, as perfect integer vortex beams, is crucial. This study proposed perfect vortex beams carrying fractional topological charge of l + 0.5, which are special solutions of the wave equation, and can maintain stable propagation with physical laws same as integer topological charge. Perfect fractional vortex beams were created in free space, which can break the cognition of traditional fractional perfect vortex beams and promote the development of scientific fields such as optical communication, quantum sensing, and optical imaging.

Generation of vector elliptical perfect optical vortices with mixed modes in free space.

Vector vortex beams are widely used because of their anisotropic vortex polarization state and spiral phase. Constructing mixed mode vector vortex beams in free space still requires complex designs and calculations. We propose a method for generating mixed mode vector Elliptical perfect optical vortex (EPOV) arrays in free space by mode extraction and optical pen. It is demonstrated that the long axis and short axis of EPOVs are not limited by the topological charge (TC). Flexible modulation of parameters in the array is achieved, including number, position, ellipticity, ring size, TC, and polarization mode. This approach is simple and effective, it will provide a powerful optical tool for optical tweezers, particle manipulation, and optical communication.

Orientation of the NV centers are determined using the cylindrical vector beam array.

The determination of nitrogen-vacancy centers plays an important role in quantum information sensing. Efficiently and rapidly determining the orientation of multiple nitrogen-vacancy center s in a low-concentration diamond is challenging due to its size. Here, we solve this scientific problem by using an azimuthally polarized beam array as the incident beam. In this paper, the optical pen is used to modulate the position of beam array to excite distinctive fluorescence characterizing multiple and different orientations of nitrogen-vacancy centers. The important result is that in a low concentration diamond layer, the orientation of multiple NV centers can be judged except when they are too close within the diffraction limit. Hence, this efficient and rapid method has a good application prospect in quantum information sensing.

Magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π focusing system.

This paper explored the optically induced magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π high numerical aperture (NA) focusing system, which is based on the vector diffraction theory and the inverse Faraday effect. The results show that in the case of radial modulation parameter L=0, one longitudinal magnetization chain with adjustable length can be obtained by modulating the truncation parameter ß. When the radial modulation parameter L=1.3, two magnetization chains can be obtained by modulating the truncation parameter ß. By modulating the radial modulation parameter L, two magnetization chains along the optical axis can be generated, each with four dark magnetic traps; meanwhile, the spacing between two magnetization chains can be adjusted. These results may be helpful in high-density all-optical magnetic recording, atom capture, and magnetic resonance microscopy.

Focusing pattern of the Laguerre-Gaussian beam with polarization mixing helical-conical phase modulation.

This paper focuses on the focusing pattern of the Laguerre-Gaussian (LG) beam with polarization mixing helical-conical phase modulation, which is based on the vector diffraction theory. The results show that the topological charge number l can sensitively control the intensity of the intensity peaks. The focal spot will split along the optical axis under different polarization parameters P. When l=1, the spot position and the peak intensity can be modulated by changing the polarization parameter P. The truncation parameter ß makes the focusing spot form an optical trap. By adjusting the eccentricity parameter K, the opening direction of the optical trap can be well controlled. These results may be helpful in optical applications such as optical manipulation, optical focusing, and optical information transmission.

Free-space generation of three-dimensional tunable vector optical cages.

The generation of three-dimensional tunable vector optical cages through full polarization modulation requires complex polarization states. This paper takes the vector Airy optical cage as an example to generate a three-dimensional tunable high-quality optical cage based on the Pancharatnam-Berry phase principle. The proposed method in this paper possesses the capability of arbitrary modulation in various aspects, including the quantity of optical cages and their respective sizes as well as three-dimensional spatial positions. Moreover, the intensity of each optical cage can be modulated independently. This research will improve the capture efficiency of optical tweezers and promote further development in fields of efficient optical trapping, particle manipulation, high-resolution microscopic manipulation, and optical communication.

Dynamic pressure surface deformation measurement based on a chromatic confocal sensor.

To enable the real-time measurement of pressure deformations in sealed cavities, a high-precision method of detecting the deformation of a material surface is proposed. By combining a chromatic confocal displacement sensor with a pressure sensor, we can acquire dynamic online strain measurements that consider the effects of the deformed material and internal environmental conditions. A 90m m×90m m static mechanical cylindrical cavity is simulated using finite element software. The interior of the cylindrical cavity is continuously pressurized at up to 400 kPa with a material deformation of 300 µm. We experimentally obtain the spectral peak wavelengths corresponding to the surface deformation experiment, record the spectral data at 20 kPa intervals, and use the Voigt fitting algorithm to reduce sensor errors. The results show that the experimental results differ from the simulated results by 1.43 µm, with a relative error of 0.083% after sequential pressurization and depressurization using a pressure calibrator, and the uncertainty error of pressure deformation measurement is 1.495 µm. Thus, the proposed method is robust against external disturbances and is suitable for micrometer-level surface deformation monitoring, which has numerous applications in the high-precision inspection industry.

Research on Emotion Recognition Method of Cerebral Blood Oxygen Signal Based on CNN-Transformer Network.

In recent years, research on emotion recognition has become more and more popular, but there are few studies on emotion recognition based on cerebral blood oxygen signals. Since the electroencephalogram (EEG) is easily disturbed by eye movement and the portability is not high, this study uses a more comfortable and convenient functional near-infrared spectroscopy (fNIRS) system to record brain signals from participants while watching three different types of video clips. During the experiment, the changes in cerebral blood oxygen concentration in the 8 channels of the prefrontal cortex of the brain were collected and analyzed. We processed and divided the collected cerebral blood oxygen data, and used multiple classifiers to realize the identification of the three emotional states of joy, neutrality, and sadness. Since the classification accuracy of the convolutional neural network (CNN) in this research is not significantly superior to that of the XGBoost algorithm, this paper proposes a CNN-Transformer network based on the characteristics of time series data to improve the classification accuracy of ternary emotions. The network first uses convolution operations to extract channel features from multi-channel time series, then the features and the output information of the fully connected layer are input to the Transformer netork structure, and its multi-head attention mechanism is used to focus on different channel domain information, which has better spatiality. The experimental results show that the CNN-Transformer network can achieve 86.7% classification accuracy for ternary emotions, which is about 5% higher than the accuracy of CNN, and this provides some help for other research in the field of emotion recognition based on time series data such as fNIRS.

Generation of perfect optical vortex arrays by an optical pen.

Recently, perfect optical vortexes (POVs) have attracted substantial attention, because they have an orbital angular momentum (OAM) and the beam diameter is independent of the topological charges. There are numerous innovative results that have been found by modulating the POV optical field. However, methods for controlling the arbitrary parameters of POV are lacking. In this paper, we use the optical pen to overcome this problem. The optical pen is a high-precision optical field modulation method construction based on the relationship between the optical path difference and phase. Based on this method, we have achieved POV arrays with controllable arbitrary parameters in free space, including the spatial position, numbers, topological charges, beam diameter, and amplitude. This work can be applied not only in the fields of optical tweezers, particle manipulation, and super-resolution microscopic imaging, but also will promote the development of optical communication, quantum information coding, and so on.

Parallel creation of propagable integer- and fractional-order vector vortex beams using mode extraction principle.

Propagable fractional-order vector vortex beams with fractional topological charge provide new possibilities in optics. However, their complicated polarizations give rise to a big challenge in creating multiple vector vortex beam channels. Here, we solve this scientific problem by generating fractional- and integer-order vector vortex beams in parallel using the mode extraction principle along with an optical pen. Based on the principle of mode extraction, integer- and fractional-order vector vortex beams can be extracted in the focal region of an objective lens directly from a single m = 30-order vector vortex beam. Their number, position, amplitude, and phase are further controlled by the parameters of an optical pen. This work not only presents an example of how to manipulate the polarization and phase of a light beam simultaneously using the principle of mode extraction, but also may open new avenues for the applications of integer- and fractional-order vector vortex beams.

Generation of a sub-wavelength sized optical needle with arbitrary longitudinal rotation.

We show that under tight focusing conditions, arbitrarily rotating the longitudinally polarized optical needle in space is possible. Applying the Richards and Wolf vector diffraction methods, the explicit expressions underlying the simultaneous control depth of focus (DoF), intensity suppression of the sidelobes, as well as the orientation of the optical needle can be obtained, and then the strength vectors of the three-dimensional electromagnetic fields can be calculated. Calculations reveal that the sidelobe suppression ratio reaches 5.35% of the principal lobe, the optimal DoF is 5.1λ, and the maximum length is about 18.2λ. In addition, we specify the necessary conditions for rotating the sub-wavelength sized optical needle in the longitudinal field. Such an optical needle with controllable length, purity, and orientation can provide a flexible approach and additional degree of freedom for 3D precise fabrication and some other potential application areas.

Focusing properties of a polarization mixing quadratic space-variant phase-modulated sinh-Gaussian vortex beam.

This research explores the focusing characteristics of a polarization mixing quadratic space-variant phase-modulated sinh-Gaussian vortex beam. Various intriguing evolutionary trends of the focal pattern are demonstrated by the modulation of each parameter. The results show that the adjustable parameter C makes the focusing spot form an optical chain structure. The length of the structure and the number of dark optical traps in the structure can be altered by adjusting C. Simultaneously, the variation of beam order m will cause focal spot deformation. Moreover, the variation of the focal pattern indicates that polarization parameter B has the ability to adjust the position of the spot. In addition, the influence of topological charge l on the component field is also discussed. With the introduction of a polarization mixing quadratic space-variant phase, the focal pattern obtains a series of unique characteristics. These results have potential value for cutting-edge optical applications such as optical shaping, optical transmission, and multiple optical capture.

Circular dichroic metasurface based on a "double L" structure.

Based on the theory of circular polarization dichroism in electromagnetic fields, this paper studies the circular dichroism (CD) characteristics of metasurfaces. Using a stable silicon material, an innovative "double L-shaped" composite structure formed by two L crosses is proposed to improve CD. Under a wide spectrum with wavelengths of 1000-1500 nm, the left circularly polarized (LCP) and right circularly polarized (RCP) lights pass through the structure, and we study the influence of different structural parameters on the CD, in order to obtain the best structural parameters. These realize the cross polarization of left-right circularly polarized light. In addition, at the wavelength of 1302.63 nm, the LCP light illuminates the structure, which realizes the cross polarization of LCP light; that is, the structure realizes the function of a half-wave plate. The RCP light incident structure realizes the function of a filter. It has great application prospects in biological detection, half-wave plates, filters, and other fields.

Control of transverse mode in a He-Ne laser using an astigmatic resonator.

The realization of output of the controllable transverse mode in a laser resonator has always been the key problem in applications of lasers. At present, the theory of optical resonators for passive resonators is relatively mature, but the non-uniformity of gain media greatly affects the output of the laser transverse mode for astigmatic resonators in operation; especially for gas lasers, controllable high-order modes have not been studied. To realize the theory of an astigmatic passive resonator of a gas laser as a good approximation of an active resonator, this paper develops the theory of selecting the laser eigenmode through an astigmatic resonator, and verifies that the two-dimensional tilt of the cavity mirror can break the axial symmetry. Controllable output of the laser mode is realized in real time and conveniently for the first time, to the best of our knowledge. This scheme is not only common to all kinds of lasers, but also has important research significance for the high-order modes required for real-time and rapid regulation of gas lasers under operating conditions.

Focusing pattern of cosh-Gaussian beam with polarization mixing cosine phase modulation.

Based on the vector diffraction theory, this paper focuses on the focusing pattern of cosh-Gaussian beams with polarization mixing cosine phase modulation and discusses the focused optical intensity under different parameters in detail. The results that show by adjusting the polarization parameter C, a tunable focal shift and a continuous shifting of focus on both directions of the optical axis can be achieved and the component that dominates the intensity distribution of the total field can be changed. When the tunable parameter D is positive, the optical chain with tunable length can be structured by increasing D. When D is negative, there will appear one special optical trap focus mode by which the number of optical traps can be changed by tailoring D. When the phase modulation parameter m increases, the focal pattern will wholly enlarge and stretch along the radial direction, changing from two intensity peaks to six optical chains. By increasing the decentered parameter ß, the shape of the light intensity distribution in the focusing region can be remarkably adjusted. Besides, by combining the theories and analyzing the field patterns in the transverse plane, there is an electric field intensity dependence on the azimuthal angle φ. Those novel findings may be helpful in the applications such as optical manipulation, optical focusing, and imaging.

Focusing and propagation properties of Bessel-Gaussian beam with a power-order mixing helical-conical phase wavefront.

Based on the vector diffraction theory, this paper investigates the focusing and propagation characteristics of a Bessel-Gaussian beam with a power-order mixing helical-conical wavefront. It discusses the focused light intensity, optical gradient force distribution, and propagation characteristics under different parameters in detail. The results show that the topological charge number L can finely adjust the opening of the spiral-like curve at the focal plane. The power order n can adjust the energy distribution in the focus area. By increasing the eccentricity parameters K, the position of the maximum intensity spot will move along with the y coordinate axis. When the beam parameter ß increases, the number of intensity peaks on the focal plane increases. As the propagation distance z increases, the pattern of intensity distribution will change from an ellipse to "doughnut-like." Then a circular dark core will appear in the center of the light, and the propagation mode will gradually become an optical trap. Some optical gradient force distributions also are investigated to illuminate the applications of these alterable focal patterns.