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Optical metasurfaces have emerged as a new class of optical elements superior to the conventional ones owing to their unique ability to tailor the optical wavefront on a subwavelength scale. High efficiency, compactness and lightweight make metasurfaces a strong candidate for on-chip integrated optical devices, a core constituent of next-generation augmented reality displays, light ranging and detection (LiDAR) and optical computing. Here, utilizing the concept of Huygens metasurfaces modulated by liquid crystals (LCs), we demonstrate an all-solid-state tunable transmissive spatial light modulator (SLM) prototype. In contrast to conventional LC SLM, our device features reduced interpixel crosstalk, enabling small pixel size of ~1 um. By programming individually addressable electrodes, we demonstrate reconfigurable beam steering with high Side Mode Suppression Ratio (SMSR) of ~6 dB and transmission efficiencies &gt;20 %. Combining the device with a two-photon polymerization assisted 3D printed doublet microlens, we expand the viewing angle up to 78º. The demonstrated characteristics make the proposed technology very promising for a variety of applications, such as tunable imaging, dynamic holography and LiDAR.

MATERIALS FOR HUMANITY (MH 21)

Large field-of-view all-solid-state transmissive metasurface spatial light modulator.

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

Optical metasurfaces have emerged as a new class of optical elements superior to the conventional ones owing to their unique ability to tailor the optical wavefront on a subwavelength scale. High efficiency, compactness and lightweight make metasurfaces a strong candidate for on-chip integrated optical devices, a core constituent of next-generation augmented reality displays, light ranging and detection (LiDAR) and optical computing. Here, utilizing the concept of Huygens metasurfaces modulated by liquid crystals (LCs), we demonstrate an all-solid-state tunable transmissive spatial light modulator (SLM) prototype. In contrast to conventional LC SLM, our device features reduced interpixel crosstalk, enabling small pixel size of ~1 um. By programming individually addressable electrodes, we demonstrate reconfigurable beam steering with high Side Mode Suppression Ratio (SMSR) of ~6 dB and transmission efficiencies >20 %. Combining the device with a two-photon polymerization assisted 3D printed doublet microlens, we expand the viewing angle up to 78º. The demonstrated characteristics make the proposed technology very promising for a variety of applications, such as tunable imaging, dynamic holography and LiDAR.

technical paper

Inverse design of photonic devices [1] (especially nanophotonics [2]) is challenging due to the large set of geometries andmaterial choices accessible to nanofabrication; Deep Learning is currently being explored [2] for these problems. Previous reportshave exclusively used traditional feed-forward networks with some papers using convolutional layers. With complicated datasets,increasing the number of layers is not effective as vanishing gradient makes it harder to train; resulting saturated or degradedpredictability. Residual learning (ResNet) [4] addresses this issue by identity shortcuts that propagate the gradient by skipping oneor more layers (Fig-1D-i). In DenseNet(Fig-1D-ii,iii), identity mappings are further exploited [5] by directly connecting all layers;that not only alleviates the vanishing-gradient but also strengthens feature propagation by reusing them. Our study explores thesedense network architectures for adequate global predictability to design 2-D extended unit-cell dielectric metagratings [3]. Eachextended unit cell consists of two non-overlapping elliptical nanoresonators whose positions, axes lengths and orientations are freeparameters (Fig-1A) and can lead to an output 3D tensor representing wavelengths, diffraction orders, and polarizations(Fig-1C).Fig-1Bshows the sample network designed for this learning; where the deconvolution layer with sigmoid activation works as thefinal layer. Geometrical parameters, bound by corresponding lower and upper limits, are uniformly sampled. After eliminatingstructures with overlapping ellipses and lattice overshoot, total 54980 feasible structures approximate the global mapping to belearned. Their diffraction spectra were calculated usingS4RCWA solver. Throughout the study, these data were considered fortraining and testing. As shown in Fig-1E, when trained with the complete dataset, all three network architectures were observed tobe giving better accuracies as compared to feed-forward network. By increasing the number of hidden layers, global predictabilitywas also improved.

Densely Connected Neural Networks for Nanophotonic StructureDiscovery

Optical fibers are the main photonic technology platform that serve as the crucial high-speed connections between network nodes and servers across large distances; however, current electro-optic signal conversions present an energy intensive, bandwidth and data rate limited bottleneck. To realized next-generation communication transfer speeds (beyond 5G), a change in hardware architecture is required: the implementation of an all-optical network where signal generation, processing and routing is brought into the optical domain. Recently, planar phase change chalcogenide metamaterials have demonstrated all-optical switching, capable of intensity and phase modulation with built-in memory functionality (non-volatility) at a fraction of the operation wavelength in size[1-5]; thereby presenting an energy efficient, reconfigurable, and small footprint platform for integration onto optical fibers to functionalize this traditionally passive light guiding component. Here we present metadevices made from reconfigurable phase change chalcogenide (specifically Ge2Sb2Te5 composition) metamaterials on both optical fiber-tip and side-polished configurations. Integrated on a commercial single mode fiber tip, the cuboid GST metasurface exhibits large transmission contrasts between amorphous and crystalline phases and both normal and anomalous dispersion profiles ranging across the telecom bands for applications in signal broadening compensation. The metasurface periodicity enables spectral position control of dielectric Mie resonances, allowing for tunable operation across the telecom band. We also demonstrate that side-polished fiber integrated metadevices exhibit two types of resonances: one arising from thin film Fabry-Perot modes and another originating from metamaterial grating modes. By varying the period and thickness of the nanograting, the two resonant modes can be spectrally shifted to address applications in multi-band filtering and wavelength division multiplexing.

Fiber integrated chalcogenide metasurface enabled phase and intensity modulation with built-in memory functionality

Multispectral Liquid Crystal Spatial Light Modulator based on Fabry-Perot Cavity Resonances

Conductance state of a single atom, which exclusively affects the charge transferring rate, plays a key role in determining the electronic properties of the single atom/molecule devices [1, 2]. Therefore, it is significantly important to precisely characterize and deliberately switch the conductance states of single atoms/molecules. In this study, we systematically investigated the dependence of the oxygen adatom (Oad) conductance switching on its charge states by noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM) at 78 K. We firstly clarify the Oad charge and conductance states on rutile TiO2(110) using simultaneous nc-AFM/STM measurement (Figure 1). It suggests that the Oad- has much higher conductance than Oad2-, and the Oad conductance switching between high and low levels can be controlled by deliberately manipulating the Oad charge states through ramping the bias voltage. The charge and conductance states switching of the Oad can be attributed to the tunneling electrons transferring from the tip into the Oad or from the Oad to the substrate polaron [3,4,5]. Our study provides a good reference for the systematic investigation of the atomic conductance, and has a potential overriding effect on revolutionizing the investigation of the single atom/molecule devices.

Charge state dependence of oxygen adatom conductance switching

The combination of inversion symmetry breaking and spin-orbit coupling gives rise to symmetry-dependent spin physics, such as spin-orbit torque (SOT). The lack of vertical inversion symmetry in multilayer structures gives rise to a conventional SOT (or in-plane SOT), which can be used to switch the perpendicular magnetization with the assistance of an external magnetic field. To realize the magnetic field-free switching of the perpendicular magnetization, one solution is to generate an unconventional SOT (or out-of-plane SOT) by using the lack of lateral inversion/mirror symmetry. In the presentation, I will present our recent progress in engineering the out-of-plane SOT through the crystal symmetry at the interface of CuPt/CoPt bilayer.

Perpendicular magnetization switching through crystal symmetry-engineered spin-orbit torque

The spin-orbit torque (SOT) has been used to efficiently manipulate the magnetization in the ferromagnet (FM)/heavy metal (HM) heterostructures for non-volatile storage applications. Engineering of the interface in FM/HM has attracted great attention due to its crucial role in the spin transfer and generation. However, the underlying mechanism for the interfacial contribution to SOT generation is still a subject of debate. Interfacial transparency has been widely used to illustrate the interfacial contribution to the SOT efficiency, for example, the Pt/Co bilayer has a larger interfacial transparency and thus a larger SOT efficiency than Pt/Py bilayer despite the same spin Hall effect of Pt. However, this explanation has been questioned since the Pt/Co interface also has a large spin memory loss, which can lead to a substantial drop of the SOT efficiency [1]. The orbital hybridization is another phenomenon at the FM/HM interface, which is particularly prominent for Pt/Co. Theoretical studies have predicted a strong effect of hybridization on the SOT generated at the Pt/Co interface [2,3], which has not been proved experimentally.
In this work, we perform the STFMR on the epitaxially grown Pt/Ni81Fe19 (as known as permalloy, Py) and Pt/Co/Py heterostructures with (111) crystalline orientation to investigate both the strength and components of their SOT. We demonstrate that inserting an ultra-thin Co layer between epitaxial Pt and Py substantially enhances both the in-plane (IP) and out-of-plane (OP) damping-like SOTs, both of which can be attributed to the strong interfacial hybridization between Co and Pt.

Role of interfacial orbital hybridization in spin-orbit torque generation in Pt-based heterostructures

Ultrashort-laser-induced terahertz (THz) emission is a contact-less probe for charge current generation, which has been recently highlighted in electronics by exploring ultrafast charge dynamics. Here we show it is also applicable in magnetics by exploring spin dynamics. In particular, the generation and transport of an ultrashort spin current, preferably reaching a frequency up to THz, is the core of modern spintronics. First, we demonstrate a picosecond-long spin current generation in a Weyl semimetal Td-WTe2 thin film. The anisotropic dependence of the circular photogalvanic effect in the THz emission indicates that the spin current is generated along the crystalline low-symmetric a-axis, which can be further modulated by the helicity of the laser excitation. Moreover, the generated spins are polarized along the surface normal, which potentially provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. Since the prior studies have been limited to the static measurements with in-plane spin orientation, our work provides an insight into dynamic feature of the anisotropic spin current and shows a potential for the THz-spintronic devices with Weyl semimetals. Second, we further show the transport of an ultrashort spin current using THz emission. In a magnetic multiplayer Bi2Se3/NiO/NiFe, we show that a spin current generated in the ferromagnetic NiFe layer is transferred to the Bi2Se3 layer through the antiferromagnetic insulator NiO layer. The magnetic and insulating features of NiO enables magnons, i.e., the quanta of spin waves, to carry spin currents without moving charges, which therefore can avoid substantial energy dissipation caused by Joule heating and subsequently provides an efficient means to manipulate the magnetization. Our works, which are relevant to the high-speed and energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.

Exploring generation and transport of ps-long spin currents with terahertz probes

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and
unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ~0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~10^6 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

We describe a simple experimental method to detect electron paramagnetic resonance (EPR) in polycrystalline 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample, the standard g-marker for EPR spectroscopy, without using a cavity resonator or a prefabricated waveguide. It is shown that microwave (MW) current injected into a layer of silver paint coated on an insulating DPPH sample can excite the paramagnetic resonance in DPPH. As the applied dc magnetic field H is swept, the high-frequency resistance of the Ag-paint layer, measured at room temperature with a single port impedance analyzer in the MW frequency range f = 1 to 2.5 GHz, exhibits a sharp peak at a critical value of the dc field (H = Hr) while the reactance exhibits a dispersion-like behavior around the same field value for a given f. Hr increases linearly with f. We interpret the observed features in the impedance to EPR in DPPH driven by the Oersted magnetic field arising from the MW current in the Ag-paint layer. We also confirm the occurrence of EPR in DPPH independently using a co-planar waveguide based broadband technique in the frequency range 2 – 4 GHz. This technique has the potential to investigate other EPR active inorganic and organic compounds.

Electrically Detected Paramagnetic Resonance in Ag-paint Coated Polycrystalline DPPH

Domain wall (DW) devices are gaining additional attention for their suitability in emerging neuromorphic computing (NC) 1. Underlying building blocks of NC are neurons and synapses, where neurons process the information and synapses control the information passing from pre-neurons to post-neurons. Such synaptic functions can be achieved by realizing multiple conductance (resistance) states. In DW devices, this virtue can be achieved by controllably stopping the DW at multiple specific positions. However, DW motion is stochastic, and therefore, there is a need to artificially form centers by amending magnetic/geometric properties that can controllably pin the DWs 2.
In the present study, we have proposed and studied the concept of nanoscale interfacial Dzyaloshinskii-Moriya interaction (iDMI) region to pin the DW. When an electrical current is passed through the ferromagnetic (FM) nanowire, the DW moves from the left end of the wire and gets pinned at the pinning site for a large range of iDMI constant, the width of the pinning site, and current density (J) values. The DWs can be depinned from the pinning site by applying a higher J. We found that local iDMI forms a barrier in energy locally and hence pins the DW. Besides, the magnetostatic interaction between DW and the pinning site plays an important role in pinning the DWs. In addition to DW pinning, we observed that the DW oscillates at GHz frequencies at the pinning site 3. Later, we introduced five pinning sites along the FM wire and observed that the DW can be pinned (and depinned) successively at every pinning site. Therefore, multiple resistance states and hence synaptic operations can be achieved. The results of this study will be discussed in full detail during the conference. 


References 
1. A. Sengupta and K. Roy, Applied Physics Express, 11 (2018) 030101. 
2. D. Kumar et al., IEEE Transactions on Magnetics, 55 (2018) 1-8. 
3. D. Kumar et al., Journal of Magnetism and Magnetic Materials, (2020) 166807.

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