Room-temperature magnetoresistance in an all-antiferromagnetic tunnel junction.

Antiferromagnetic spintronics1-16 is a rapidly growing field in condensed-matter physics and information technology with potential applications for high-density and ultrafast information devices. However, the practical application of these devices has been largely limited by small electrical outputs at room temperature. Here we describe a room-temperature exchange-bias effect between a collinear antiferromagnet, MnPt, and a non-collinear antiferromagnet, Mn3Pt, which together are similar to a ferromagnet-antiferromagnet exchange-bias system. We use this exotic effect to build all-antiferromagnetic tunnel junctions with large nonvolatile room-temperature magnetoresistance values that reach a maximum of about 100%. Atomistic spin dynamics simulations reveal that uncompensated localized spins at the interface of MnPt produce the exchange bias. First-principles calculations indicate that the remarkable tunnelling magnetoresistance originates from the spin polarization of Mn3Pt in the momentum space. All-antiferromagnetic tunnel junction devices, with nearly vanishing stray fields and strongly enhanced spin dynamics up to the terahertz level, could be important for next-generation highly integrated and ultrafast memory devices7,9,16.

Optical design and pupil swim analysis of a compact, large EPD and immersive VR head mounted display.

Virtual reality head-mounted displays (VR-HMDs) are crucial to Metaverse which appears to be one of the most popular terms to have been adopted over the internet recently. It provides basic infrastructure and entrance to cater for the next evolution of social interaction, and it has already been widely used in many fields. The VR-HMDs with traditional aspherical or Fresnel optics are not suitable for long-term usage because of the image quality, system size, and weight. In this study, we designed and developed a large exit pupil diameter (EPD), compact, and lightweight VR-HMD with catadioptric optics. The mathematical formula for designing the catadioptric VR optics is derived. The reason why this kind of immersive VR optics could achieve a compact size and large EPD simultaneously is answered. Various catadioptric forms are systematically proposed and compared. The design can achieve a diagonal field of view (FOV) of 96° at -1 diopter, with an EPD of 10 mm at 11 mm eye relief (ERF). The overall length (OAL) of the system was less than 20 mm. A prototype of a compact catadioptric VR-HMD system was successfully developed.

Freeform beam splitting system design for generating an array of identical sub-beams.

Laser beam splitting by freeform optics is promising but less studied. Instead of directly forming a target spot array, we propose to first convert the input beam into a closely connected Gaussian sub-beam array. All the Gaussian sub-beams have the same optical field distributions which thus can produce identical discrete spots on the target plane. Such a design concept is very beneficial to ensure the consistency for laser processing. Importantly, the introduction of an intermediate Gaussian sub-beam array can reduce diffraction effects when the size of each Gaussian sub-beam is sufficiently larger than that of the corresponding sub-area within the input beam. The desired transformation can be achieved by two typical systems. The first system consists of two plano-freeform lenses. The second system is composed of a plano-freeform lens and a lens with an entrance freeform surface and an exit surface of freeform lens array. The two freeform beam splitting systems can be determined based on appropriate ray mappings among the input, intermediate and target irradiance distributions and a subsequent double-surface construction. Geometrical and physical simulations verify the effectivenesses of the two beam splitting systems.

Designing double freeform surfaces for large ray bending irradiance tailoring of extended LED sources.

Many illumination applications require redistributing the irradiance distributions of LED sources with large ray bending. The problem becomes even more challenging for a compact design where the LED size is no longer ignorable. We tackle this problem by simultaneously designing two freeform optical surfaces. An iterative wavefront tailoring (IWT) method is adapted for obtaining the entrance and exit base freeform surfaces with a predefined ray bending regulation under stereographic coordinates (u, v). The simulated annealing (SA) algorithm is employed for deforming the two base freeform surfaces using the 'uv' polynomials with the purpose of minimizing the relative root-mean-square deviation (RRMSD) between the simulated irradiance distribution and the prescribed one. The optimizations are implemented in an automated workflow which links the optimization engine, 3D modeling software and ray tracing software. The effectiveness of the proposed method is illustrated by designing several double-freeform-surface lenses (central heights: 10 mm) with different ray bending regulated base surfaces and 10-th order uv polynomial departures for generating 500 × 200 mm2 uniform irradiance distributions at a distance of 100 mm from 2 × 2 mm2 and 3 × 3 mm2 sources, respectively.

Compact integrator design for short-distance sharp and unconventional geometric irradiance tailoring.

A compact microlens array (MLA) integral homogenizer composed of a projection MLA, a condenser MLA, and a subimage array mask based on Kohler illumination is presented herein. By adopting the optimal design of an aspheric projection sublens, a short-distance integrator for unconventional geometric irradiance tailoring can be acquired. Compared with the traditional integrator, the integral lens is removed in the proposed integrator. An incident beam with a larger divergence angle is permitted while a sharp illumination performance is maintained. In addition, the illumination distribution with a predefined geometrical profile can be obtained by introducing a subimage array mask without replacing the MLA elements. Through the introduced mask, the illumination attenuation area is cut, the distortion is caused by the large NA, and the peak effect caused by the integral lens of the traditional integrator can be eliminated. The subimage array mask is generated by combining ray tracing and the radial basis function image warping method. By introducing the array mask into the system, an 80% edge relative illumination and an 85% overall illumination uniformity are realized for a compact large-NA integrator with ${\rm NA} = {0.3}$ and thickness of 4.5 mm. An 88% edge relative illumination and a 92% overall illumination uniformity are realized for a small-NA integrator with ${\rm NA} = {0.15}$.

Transferring freeform lens design into phase retrieval through intermediate irradiance transport.

Freeform lens design for the prescribed irradiance without thin element and paraxial approximations is very complicated. We propose to transport a point source irradiance before the required freeform lens into an intermediate irradiance estimate next to the lens. In this way, the complicated freeform lens design problem could be transferred into a simpler problem of retrieving the phase from the intermediate and target irradiances. The phase retrieval solution can help construct an approximate lens, which is subsequently used to update the intermediate irradiance. Several iterations could bring a good performance, as demonstrated by two challenging designs.

Iterative wavefront tailoring to simplify freeform optical design for prescribed irradiance.

The direct formulation of a freeform optical surface for producing a prescribed irradiance from a point source is very complicated. Instead of directly determining the freeform optical surfaces, we derive a general equation of a parameterized outgoing wavefront, regardless of the structure of the optical element. A separate process is employed to construct the freeform optics following the solution of the wavefront equation. We iteratively revise the wavefront and accordingly update the freeform optics to improve performance. The new method combines two features: (i) the formula derivation process is simplified, and (ii) it could flexibly generate a variety of freeform optical structures with high accuracy.

Simplified freeform optics design for complicated laser beam shaping.

Control of the optical fields of laser beams, i.e., laser beam shaping, is of great importance to many laser applications. Freeform optics offers plenty of advantages for complex beam shaping requirements, including precise beam control, energy efficiency, compact structure, and relatively low cost. We present a modified ray-mapping method to simplify the freeform optics design for complicated optical field control and achieve a challenging task of producing two prescribed beam profiles on two successive target planes. This method begins by calculating an approximate output ray sequence that connects the two prescribed beam profiles and a corresponding input ray sequence. After setting an initial profile of the first freeform optical surface on the input ray sequence, we can obtain the second freeform optical surface based on the optical path length constancy between the given input wavefront and the computed output wavefront. Then, we can acquire all the normal vectors of the first freeform optical surface using Snell's law and approximately reconstruct the first freeform optical surface by solving a relationship between the surface points and normal vectors using a fast least squares method. The surface construction process is repeated until the stop criterion is satisfied. We design three freeform lenses, and Monte Carlo simulations demonstrate their abilities of simultaneously producing two challenging beam profiles from a divergent Gaussian beam.

Freeform illumination optics construction following an optimal transport map.

We present a modified optimal transport (OT) ray-mapping approach for designing freeform illumination optics. After mapping the source intensity into a virtual irradiance distribution under stereographic projection, we employ an advanced OT map computation method with the ability to tackle nonstandard boundary conditions. Following the computed map, we construct the freeform optical surface directly from normal vectors by requiring that the chord between two adjacent points is perpendicular to the average of the two normal vectors at these two points and enforcing this relationship with a least squares method. Examples of designing freeform lenses for LED sources show that we can produce various uniform illumination patterns with high optical efficiencies.

Deconvolution method in designing freeform lens array for structured light illumination.

We have developed a deconvolution freeform lens array design approach to generate high-contrast structured light illumination patterns. This method constructs the freeform lens array according to the point response obtained by deconvoluting the prescribed illumination pattern with the blur response of the extended light source. This design method is more effective in designing a freeform lens array to achieve accurate structured light patterns. For a sinusoidal fringe pattern, the contrast ratio can be as high as 97%, compared to 62% achieved by the conventional ray mapping method.

Tailoring freeform illumination optics in a double-pole coordinate system.

We have developed a new method to design freeform illumination optics by introducing a double-pole coordinate system in ray mapping. This method establishes a much more accurate ray mapping by moving the two poles of the spherical coordinate system to the southernmost point of the sphere and overlapping them together. It can reduce surface error and improve illumination uniformity significantly. The residual surface error (RSE) of the freeform lens designed in the double-pole coordinate system is one magnitude smaller than that of the lens designed in the (θ,φ) coordinate system and is only 1/3 of that of the freeform surface designed in the (u,v) coordinate system.

Composite method for precise freeform optical beam shaping.

We present a composite freeform surface construction method for creating a high-accuracy irradiance distribution from a given incident beam under the influence of diffraction. The main idea is that we first determine a fully continuous freeform surface estimate by solving a standard Monge-Ampère equation and then refine it using an iterative Fourier-transform algorithm associated with over-compensation. Although this method can only be implemented in the paraxial approximation, it can significantly simplify the design and is applicable to many examples that fulfill this restriction. The resulting optical surface, unwrapped from the final phase, is an unusual discontinuous freeform surface that can produce very promising performances in terms of surface roughness and irradiance accuracy.

Creating unconventional geometric beams with large depth of field using double freeform-surface optics.

We consider here creation of an unconventional flattop beam with a large depth of field by employing double freeform optical surfaces. The output beam is designed with continuous variations from the flattop to almost zero near the edges to resist the influence of diffraction on its propagation. We solve this challenging problem by naturally incorporating an optimal transport map computation scheme for unconventional boundary conditions with a simultaneous point-by-point double surface construction procedure. We demonstrate experimentally the generation of a long-range propagated triangular beam through a plano-freeform lens pair fabricated by a diamond-tuning machine.

Focal-plane irradiance tailoring using the concept of Woofer-Tweeter deformable mirrors.

Deformable mirror (DM) is a common-used active freeform optical element. We introduce the concept of Woofer-Tweeter DM system for controlling focal-plane irradiance profiles. We firstly determine a freeform reflective surface for transforming a given incident laser beam into the desired focal-plane irradiance distribution by numerically solving a standard Monge-Ampère equation. Then, we use a low-bandwidth Woofer DM to approximate the required freeform reflective surface and a high-bandwidth Tweeter DM to compensate the residual error. Simulation results show that, compared with single DMs, the Woofer-Tweeter DM system brings the best focal-plane irradiance performances.

Beam shaping system design using double freeform optical surfaces.

A numerical double-freeform-optical-surface design method is proposed for beam shaping applications. In this method, both the irradiance distribution and the wavefront of the output beam are taken into account. After numerically obtaining the input-output ray mapping based on Energy conservation using the variable separation method, the two freeform optical surfaces can be constructed simultaneously and point by point corresponding to the ray mapping based on Snell's law and the constancy of the optical path length. The method is only applicable for separable irradiance distributions. However, such a restriction is fulfilled by many practical laser beam shaping examples. Moreover, the restriction can simplify the computation considerably. Therefore, the method may be quite useful in practice, although it is not applicable to more general cases. As an example, the method was applied to design a two-plano-freeform-lens system for transforming a collimated 20 mm Gaussian laser beam (beam waist: 5mm) into a uniform 10 × 40 mm(2) rectangular one without changing the wavefront. Simulation results show that we can obtain a dual lens beam shaping system with the relative root mean square deviation of the irradiance ranging from 0.0652 to 0.326 and the power ratio concentrated on the desired region ranging from 97.5% to 88.3% as the output beam transfers from 0mm to 1000mm.

Designing double freeform optical surfaces for controlling both irradiance and wavefront.

We propose an improved double freeform-optical-surface design method for shaping a prescribed irradiance distribution whilst forming a desired wavefront from a given incident beam. This method generalizes our previous work [Opt. Exp. 21, 14728-14735 (2013)] to tackle non-separable beam irradiances. We firstly compute a proper ray mapping using an adaptive mesh method in the framework of the L2 Monge-Kantorovich mass transfer problem. Then, we construct the two freeform optical surfaces according to this mapping using a modified simultaneous point-by-point procedure which is aimed to minimize the surface errors. For the first surface, the modified procedure works by firstly approximating a value to the next point by only using the slope of the current point and then improving it by utilizing both slopes of the two points based on Snell's law. Its corresponding point on the second surface can be computed using the constant optical path length condition. A design example of producing a challenging irradiance distribution and a non-ideal wavefront demonstrates the effectiveness of the method.

Orientation dependent wavefront correction system under grazing incidence.

Making use of the stretching effect of grazing incident laser beam, a novel method of wavefront correction was promoted. Without adding any extra beam expanding components, aberrations of wavefront could achieve satisfying correction by two grazing reflections along orthogonal directions on the deformable mirrors. The stretching effect expanded the beam size along grazing direction and the orientation dependent varying aberrations were well compensated as more actuators took effect in the correction process. Analysis showed that the fitting coefficient of all the first 30 order Zernike polynomials could be controlled within 5% by this method.

Research on the particular temperature-induced surface shape of a National Ignition Facility deformable mirror.

We investigate the changes in the shape of a deformable mirror used at the National Ignition Facility caused by differences in temperature between the working environment and the mounting temperature of the mirror. In general, the temperature-induced profile change of the mirror is dominated by a few low-order aberrations, which mainly result in defocus. However, after these low-order distortions are corrected, there remain special, higher-order, surface distortions caused by the particular arrangement, construction, and mounting of the mirror actuators. This work analyzes these special aberrations, and their dependence on the particular actuator design, using the finite element method. Experiments are carried out to verify the computational results, and finally, design considerations to help minimize the temperature-induced high-order aberrations are suggested.

Topological Hall Effect in Thin Films of an Antiferromagnetic Weyl Semimetal Integrated on Si.

Since the large room-temperature anomalous Hall effect was discovered in noncollinear antiferromagnets, Mn3Sn has received immense research interest as it exhibits abundant exotic physical properties including Weyl points and enormous potential for antiferromagnetic spintronic device applications. In this work, we report the emergence of the topological Hall effect in Mn3Sn films grown on Si that is the workhorse for the modern highly integrated information technology. Importantly, through a series of systematic comparative experiments, the intriguing topological Hall effect phenomenon related to the appearance of the noncoplanar chiral spin structure is found to be induced by the Mn3Sn/SiO2 interface. Furthermore, it was found that the current injection to a Pt/Mn3Sn bilayer Hall bar device can effectively manipulate the chiral spin structure of Mn3Sn, which demonstrates the feasibility of Si-based noncollinear antiferromagnetic spintronics.

Chemical Potential Switching of the Anomalous Hall Effect in an Ultrathin Noncollinear Antiferromagnetic Metal.

The discovery of the anomalous Hall effect in noncollinear antiferromagnetic metals represents one of the most important breakthroughs for the emergent antiferromagnetic spintronics. The tuning of chemical potential has been an important theoretical approach to varying the anomalous Hall conductivity, but the direct experimental demonstration has been challenging owing to the large carrier density of metals. In this work, an ultrathin noncollinear antiferromagnetic Mn3 Ge film is fabricated and its carrier density is modulated by ionic liquid gating. Via a small voltage of ≈3 V, its carrier density is altered by ≈90% and, accordingly, the anomalous Hall effect is completely switched off. This work thus creates an attractive new way to steering the anomalous Hall effect in noncollinear antiferromagnets.