Angular Gratings Research Articles

Sensing azimuthally symmetric objects by a single-pixel detector via COAM matrix

A coherence-orbital angular momentum (COAM) matrix describes the second-order spatial correlations among the pairs of the OAM states in a light beam at two radial positions. It provides a detailed information regarding the azimuthal features of the light beam itself or of the objects interacting with it. In this Letter, we reveal a close relationship existing between the COAM matrix's elements of a partially coherent light beam and the angular gratings that it passes through. It is demonstrated both in theory and in experiment that all the parameters of the angular gratings can be identified by measuring the COAM matrix elements. While the off-diagonal elements possess information about the angular shifts and the rotation speed of the angular grating, the diagonal elements account for the energy distribution among the OAM modes. The experimental results agree reasonably well with the theoretical analysis. One practically significant capability is that our technique offers the determination of angular shifts of azimuthally symmetric objects and/or their rotational speed, by means of a single-pixel detector.

Double slot micro ring resonators with inner wall angular gratings as ultra-sensitive biochemical sensors.

We simulate and demonstrate experimentally an inner-wall grating double slot micro ring resonator (IG-DSMRR) with a center slot ring radius of only 6.72 µm based on the silicon-on-insulator platform. This novel photonic-integrated sensor for optical label-free biochemical analysis boosts the measured refractive index (RI) sensitivity in glucose solutions to 563 nm/RIU with the limit of detection value being 3.7 × 10-6 RIU (refractive index units). The concentration sensitivity for sodium chloride solutions can reach 981 pm/%, with a minimum concentration detection limit of 0.02%. Using the combination of DSMRR and IG, the detection range is enlarged significantly to 72.62 nm, three times the free spectral range of conventional slot micro ring resonators. The measured Q-factor is 1.6 × 104, and the straight strip and double slot waveguide transmission losses are 0.9 dB/cm and 20.2 dB/cm, respectively. This IG-DSMRR combines the advantages of a micro ring resonator, slot waveguide, and angular grating and is highly desirable for biochemical sensing in liquids and gases offering an ultra-high sensitivity and ultra-large measurement range. This is the first report of a fabricated and measured double-slot micro ring resonator with an inner sidewall grating structure.

Photonic Integrated Circuit for Rapidly Tunable Orbital Angular Momentum Generation Using Sb2Se3 Ultra‐Low‐Loss Phase Change Material

AbstractThe generation of rapidly tunable optical vortex (OV) beams is one of the most demanding research areas of the present era as they possess orbital angular momentum (OAM) with additional degrees of freedom that can be exploited to enhance signal‐carrying capacity by using mode division multiplexing and information encoding in optical communication. Particularly, rapidly tunable OAM devices at a fixed wavelength in the telecom band stir extensive interest among researchers for both classical and quantum applications. This article demonstrates the realistic design of a Si‐integrated photonic device for rapidly tunable OAM wave generation at a 1550‐nm wavelength by using an ultra‐low‐loss phase change material (PCM) embedded with a Si‐ring resonator with angular gratings. Different OAM modes are achieved by tuning the effective refractive index using rapid electrical switching of Sb2Se3 film from amorphous to crystalline states and vice versa. The generation of OAM waves relies on a traveling wave modulation of the refractive index of the micro‐ring, which breaks the degeneracy of oppositely oriented whispering gallery modes. The proposed device is capable of producing rapidly tunable OV beams, carrying different OAM modes by using electrically controllable switching of ultra‐low‐loss PCM Sb2Se3.

Microcavity-based generation of full Poincaré beams with arbitrary skyrmion numbers

A full Poincar\'e (FP) beam possesses all possible optical spin states in its cross-section, which constitutes an optical analogue of a skyrmion. Until now, FP beams have been exclusively generated using bulk optics. Here, we propose a generation scheme of an FP beam based on an optical microring cavity. We position two different angular gratings along with chiral lines on a microring cavity and generate an FP beam as a superposition of two light beams with controlled spin and orbital angular momenta. We numerically show that FP beams with tailored skyrmion numbers can be generated from this device, opening a route for developing compact light sources with unique optical spin fields.

Direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters

Spatial modes have received substantial attention over the last decades and are used in optical communication applications. In fiber-optic communications, the employed linearly polarized modes and phase vortex modes carrying orbital angular momentum can be synthesized by fiber vector eigenmodes. To improve the transmission capacity and miniaturize the communication system, straightforward fiber vector eigenmode multiplexing and generation of fiber-eigenmode-like polarization vortices (vector vortex modes) using photonic integrated devices are of substantial interest. Here, we propose and demonstrate direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters. By exploiting vector vortex modes (radially and azimuthally polarized beams) generated from silicon microring resonators etched with angular gratings, we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km large-core fiber, showing low-level mode crosstalk and favorable link performance. These demonstrations may open up added capacity scaling opportunities by directly accessing multiple vector eigenmodes in the fiber and provide compact solutions to replace bulky diffractive optical elements for generating various optical vector beams.

Integrated optical vortex beam receivers.

A simple and ultra-compact integrated optical vortex beam receiver device is presented. The device is based on the coupling between the optical vortex modes and whispering gallery modes in a micro-ring resonator via embedded angular gratings, which provides the selective reception of optical vortex modes with definitive total angular momentum (summation of spin and orbital angular momentum) through the phase matching condition in the coupling process. Experimental characterization confirms the correct detection of the total angular momentum carried by the vortex beams incident on the device. In addition, photonic spin-controlled unidirectional excitation of whispering-gallery modes in the ring receiver is also observed, and utilized to differentiate between left- and right-circular polarizations and therefore unambiguously identify the orbital angular momentum of incident light. Such characteristics provide an effective mode-selective receiver for the eigen-modes in orbital angular momentum fiber transmission where the circularly polarized OAM modes can be used as data communications channels in multiplexed communications or as photonic states in quantum information applications.

Importance of Contrast Sensitivity to High and Low Spatial Frequency and Artificial Neural Networks for Visual Impairment in Multiple Sclerosis

This research compared contrast sensitivity (CS) to high and low spatial frequency of angular frequency stimuli and sine-wave gratings, respectively, to assess adults diagnosed with multiple sclerosis (MS), MS with optic neuritis (ON) (MSON) and control. We also investigated whether the involved cognitive impairments affect CS, and whether the technique of artificial neural network (ANN), coupled with the CS to only two spatial frequencies, correctly predict MS. The present cross-sectional study measured CS of adults MS, MSON and control, 22-50 years-old, that used a psychophysical procedure designed with the forced-choice method. The t test for independent samples showed that the CS was better in the control, followed by MS and MSON. The MSON group had lower CS values at all tested angular frequency stimuli and low sine-wave gratings as well (p<0.0001). The frequency 24 cycles/360° and 4.0 cpd were the ones that best differentiated the groups surveyed. The neuropsychological test stroop was the only one to yield a difference between groups (p<0.0001). There was no correlation between the psychophysical measures and the protocols of optical coherence tomography. These data suggest that visual impairments found in MS and MSON are related to CS. The ANN 44221 architecture was successful in separating people with MS from controls, by using the spatial frequencies of 24 cycles/360o and 4.0 cpd that showed good correlation performance (0.71) in the test phase. Research employing this perspective will proceed with the main purpose of verifying the extent to which the use of CS for sine-wave gratings and angular frequency stimuli are relevant in the early assessment of MS.

Mode control for microring resonators with inner-wall gratings

Microring resonators with angular gratings distributed along the inner wall are proposed to achieve single-mode microlasers. Based on a numerical investigation using the finite element method, we find that the inner-wall gratings create a periodic perturbation and couple the whispering-gallery modes into low-angular-momentum components, resulting in an enhancement of radiation loss. By properly arranging the inner-wall grating protrusion number as a nontrivial divisor of 2ν, we can have the mode with azimuthal mode number ν as the only high Q mode for single-mode operation. The impacts of grating depth, microring width, and grating duty cycle are investigated by 2D simulation for the optimization of mode coupling and radiation loss. Finally, the mode selection mechanism is verified based on a 3D simulation.

Integrated label-free optical biochemical sensor with a large measurement range based on an angular grating-microring resonator.

We propose and design a photonic-integrated optical biochemical sensor, which comprises a microring resonator and angular gratings in a silicon-on-insulator waveguide. With the combination of the angular gratings, the measurement range of the angular grating-microring resonator-based sensor significantly increases without the restriction of a free spectral range. Optimization of the several key structural parameters is investigated to achieve favorable transmission properties. A high-quality factor of more than 1.03×105 can meet the requirements of high sensitivity and low detection limit. The simulation results on the biochemical bulk sensing show that a concentration sensitivity of more than 95.27 pm/% and detection limit of less than 0.329% can be obtained. A large measurement range of 50.2 nm is achieved by the combination of the angular gratings. The investigation on the combination of microring resonator and angular grating is a valuable exploration of the liquid and gas biomedical sensing for the ultra-large measurement range.

Influence of engineered surface on cell directionality and motility

Control of cell migration is important in numerous key biological processes, and is implicated in pathological conditions such as cancer metastasis and inflammatory diseases. Many previous studies indicated that cell migration could be guided by micropatterns fabricated on cell culture surfaces. In this study, we designed a polydimethylsiloxane cell culture substrate with gratings punctuated by corners and ends, and studied its effects on the behavior of MC3T3-E1 osteoblast cells. MC3T3-E1 cells elongated and aligned with the gratings, and the migration paths of the cells appeared to be guided by the grating pattern. Interestingly, more than 88% of the cells cultured on these patterns were observed to reverse their migration directions at least once during the 16 h examination period. Most of the reversal events occurred at the corners and the ends of the pattern, suggesting these localized topographical features induce an abrupt loss in directional persistence. Moreover, the cell speed was observed to increase temporarily right after each directional reversal. Focal adhesion complexes were more well-established in cells on the angular gratings than on flat surfaces, but the formation of filipodia appeared to be imbalanced at the corners and the ends, possibly leading to the loss of directional persistence. This study describes the first engineered cell culture surface that consistently induces changes in the directional persistence of adherent cells. This will provide an experimental model for the study of this phenomenon and a valuable platform to control the cell motility and directionality, which can be used for cell screening and selection.

Theoretical model for angular grating-based integrated optical vortex beam emitters

We develop a theoretical model for the recently reported integrated optical vortex beam emitters that incorporate angular gratings in microring resonators. Using azimuthally polarized dipole oscillators to represent emissions scattered from the grating elements that are located along the inner wall of the ring waveguide, we obtain expressions for far-field components under the paraxial approximation. The results show that the emission is of the form of cylindrical vector Bessel beams with exactly defined optical orbital angular momentum, and can have azimuthal, radial, and longitudinal field components after propagation. The calculation results for field distributions in both near and far zone agree well with the experimental results.

Integrated Compact Optical Vortex Beam Emitters

Emerging applications based on optical beams carrying orbital angular momentum (OAM) will probably require photonic integrated devices and circuits for miniaturization, improved performance, and enhanced functionality. We demonstrate silicon-integrated optical vortex emitters, using angular gratings to extract light confined in whispering gallery modes with high OAM into free-space beams with well-controlled amounts of OAM. The smallest device has a radius of 3.9 micrometers. Experimental characterization confirms the theoretical prediction that the emitted beams carry exactly defined and adjustable OAM. Fabrication of integrated arrays and demonstration of simultaneous emission of multiple identical optical vortices provide the potential for large-scale integration of optical vortex emitters on complementary metal-oxide-semiconductor compatible silicon chips for wide-ranging applications.

Comparing contrast sensitivities to angular frequency stimuli and sinewave gratings in aged

Comparing contrast sensitivities to angular frequency stimuli and sinewave gratings in aged

Contrast sensitivity to angular frequency gratings is not higher than to Cartesian gratings.

When contrast sensitivity functions to Cartesian and angular gratings were compared in previous studies the peak sensitivity to angular stimuli was reported to be 0.21 log units higher. In experiments carried out to repeat this result, we used the same two-alternative forced-choice paradigm, but improved experimental control and precision by increasing contrast resolution from 8 to 12 bits, increasing the screen refresh rate from 30 Hz interlaced to 85 Hz non-interlaced, linearizing the voltage-luminance relation, modulating luminance in frequencies that minimize pixel aliasing, and improving control of the subject's exposure to the stimuli. The contrast sensitivity functions to Cartesian and angular gratings were similar in form and peak sensitivity (2.4 cycles per visual degree (c/deg) and 32 c/360 degrees, respectively) to those reported in a previous study (3 c/deg and 32 c/360 degrees, respectively), but peak sensitivity to angular stimuli was 0.13 log units lower than that to Cartesian stimuli. When the experiment was repeated, this time simulating the experimental control level used in the previous study, no difference between the peak sensitivity to Cartesian and angular stimuli was found. This result agrees with most current models that assume Cartesian filtering at the first visual processing stage. The discrepancy in the results is explained in part by differences in the degree of experimental control.

Angular Multiplexing Gratings Characterization in Dichromated Gelatin and Thoughts on Diffractive Optical Memory

We were mainly concerned here with multiple exposure gratings that allow interactions and coupling between gratings. We started the study of interactions and coupling with five gratings recorded in dichromated gelatin (DCG). Having determined experimentally the characteristic parameters of these five gratings, we used a method developed by Oxford University to synthesize the modulation profile. Entering these parameters of each grating into our computing programs (based on the alternative theory of diffraction in modulated media) and by varying the amplitude modulation, the latter was deduced by adjusting the numerical curve to the experimental one. We then extended the study to nine multiplexed gratings. This study allowed us to know the range of the amplitude modulation such that a certain number of angular multiplexing can be done in DCG. Several undesirable effects which we had experimentally observed appeared then during the reconstruction of any one of the multiplexed images, such as cross-talk effect. We noted the good concordance between experimental and theoretical results. We finally illustrated cross-talk effect in a diffractive memory where, if the amplitude modulation was first calculated, the number of multiplexing for an angular range could then be determined and the angular separation achieved to avoid cross-talk.

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings