Neuron-Inspired Steiner Tree Networks for 3D Low-Density Metastructures.

Three-dimensional (3D) micro-and nanostructures have played an important role in topological photonics, microfluidics, acoustic, and mechanical engineering. Incorporating biomimetic geometries into the design of metastructures has created low-density metamaterials with extraordinary physical and photonic properties. However, the use of surface-based biomimetic geometries restricts the freedom to tune the relative density, mechanical strength, and topological phase. The Steiner tree method inspired by the feature of the shortest connection distance in biological neural networks is applied, to create 3D metastructures and, through two-photon nanolithography, neuron-inspired 3D structures with nanoscale features are successfully achieved. Two solutions are presented to the 3D Steiner tree problem: the Steiner tree networks (STNs) and the twisted Steiner tree networks (T-STNs). STNs and T-STNs possess a lower density than surface-based metamaterials and that T-STNs have Young's modulus enhanced by 20% than the STNs. Through the analysis of the space groups and symmetries, a topological nontrivial Dirac-like conical dispersion in the T-STNs is predicted, and the results are based on calculations with true predictive power and readily realizable from microwave to optical frequencies. The neuron-inspired 3D metastructures opens a new space for designing low-density metamaterials and topological photonics with extraordinary properties triggered by a twisting degree-of-freedom.

Nanoprinted high-neuron-density optical linear perceptrons performing near-infrared inference on a CMOS chip.

Optical machine learning has emerged as an important research area that, by leveraging the advantages inherent to optical signals, such as parallelism and high speed, paves the way for a future where optical hardware can process data at the speed of light. In this work, we present such optical devices for data processing in the form of single-layer nanoscale holographic perceptrons trained to perform optical inference tasks. We experimentally show the functionality of these passive optical devices in the example of decryptors trained to perform optical inference of single or whole classes of keys through symmetric and asymmetric decryption. The decryptors, designed for operation in the near-infrared region, are nanoprinted on complementary metal-oxide-semiconductor chips by galvo-dithered two-photon nanolithography with axial nanostepping of 10 nm1,2, achieving a neuron density of >500 million neurons per square centimetre. This power-efficient commixture of machine learning and on-chip integration may have a transformative impact on optical decryption3, sensing4, medical diagnostics5 and computing6,7.

Direct determination of aberration functions in microscopy by an artificial neural network.

Adaptive optics relies on the fast and accurate determination of aberrations but is often hindered by wavefront sensor limitations or lengthy optimization algorithms. Deep learning by artificial neural networks has recently been shown to provide determination of aberration coefficients from various microscope metrics. Here we numerically investigate the direct determination of aberration functions in the pupil plane of a high numerical aperture microscope using an artificial neural network. We show that an aberration function can be determined from fluorescent guide stars and used to improve the Strehl ratio without the need for reconstruction from Zernike polynomial coefficients.

Direct detection of photon spin angular momentum by a chiral graphene mid-infrared photodetector.

We demonstrate the direct detection of photon spin angular momentum in the mid-infrared region by a single surface-plasmon-enhanced graphene photodetector. We utilize chiral surface plasmon nanostructures as photodetector electrodes to generate photocurrents of equal and opposite sign according to incident photon spin. Our detector possesses a current circular dichroism of 60% and a responsivity of 0.80 µA/W at a resonant wavelength of 3.8 µm. The photocurrent dichroism is attributed to the presence of opposite handedness circularly polarized surface plasmon resonances on adjacent electrodes, of which each enhances graphene absorption by a factor of 17. The direct detection of spin angular momentum will be useful in the next wave of multiplexed sensing and communications systems utilizing the optical angular momentum states of light to enhance bandwidth and information collection.

Optomagnetic plasmonic nanocircuits.

The coupling between solid-state quantum emitters and nanoplasmonic waveguides is essential for the realization of integrated circuits for various quantum information processing protocols, communication, and sensing. Such applications benefit from a feasible, scalable and low loss fabrication method as well as efficient coupling to nanoscale waveguides. Here, we demonstrate optomagnetic plasmonic nanocircuitry for guiding, routing and processing the readout of electron spins of nitrogen vacancy centres. This optimized method for the realization of highly efficient and ultracompact plasmonic circuitry is based on enhancing the plasmon propagation length and improving the coupling efficiency. Our results show 5 times enhancement in the plasmon propagation length using (3-mercaptopropyl)trimethoxysilane (MPTMS) and 5.2 times improvement in the coupling efficiency by introducing a grating coupler, and these enable the design of more complicated nanoplasmonic circuitries for quantum information processing. The integration of efficient plasmonic circuitry with the excellent spin properties of nitrogen vacancy centres can potentially be utilized to extend the applications of nanodiamonds and yield a great platform for the realization of on-chip quantum information networks.

Metallic gyroids with broadband circular dichroism.

Circular dichroism is a useful property for filtering or separating beams containing opposite spin angular momentum. Of the many geometries exhibiting circular dichroism, the gyroid has proven to be an excellent template for exploring circular dichroism in three dimensions. However, the bandwidth of the circular dichroism from dielectric gyroids is limited by its narrow circularly polarized stop band. Here we investigate conductive silver gyroid micro-structures using direct laser writing of polymeric templates followed by the electroless deposition of a uniform silver coating. We show that the transformation from dielectric to silver gyroid micro-structure can increase the circular dichroism bandwidth by close to a factor of 3.

Bragg-mirror-like circular dichroism in bio-inspired quadruple-gyroid 4srs nanostructures.

The smooth and tailorable spectral response of Bragg mirrors has driven their pervasive use in optical systems requiring customizable spectral control of beam propagation. However, the simple nature of Bragg mirror reflection prevents their application to the control of important polarization states such as circular polarization. While helical and gyroid-based nanostructures exhibiting circular dichroism have been developed extensively to address this limitation, they are often restricted by the spectral inconsistency of their optical response. Here we present the fabrication and characterization of quadruple-gyroid 4srs nanostructures exhibiting bio-inspired Bragg-mirror-like circular dichroism: a smooth and uniform band of circular dichroism reminiscent of the spectrum of a simple multilayer Bragg-mirror. Furthermore, we demonstrate that the circular dichroism produced by 4srs nanostructures are robust to changes in incident angle and beam collimation, providing a new platform to create and engineer circular dichroism for functional circular polarization manipulation.

Observation of optical activity in dielectric biomimetic 8-srs networks.

We demonstrate the first experimental realization of the 8-srs network by direct laser writing in a dielectric medium. The optical properties of this photonic crystal are both numerically simulated and experimentally measured. The numerical simulations indicate a strong optical activity (OA) for regions of the spectrum close to the unit cell size (3 µm) and a high sensitivity to angular resolution. Experimentally, we observe 40° of OA for a region of the spectrum between 3 and 3.5 µm, confirming both simulation predictions.

Highly efficient plasmonic enhancement of graphene absorption at telecommunication wavelengths.

A hybrid graphene system consisting of graphene and silica layers coated on a metal film with groove rings is proposed to strongly enhance light absorption in the graphene layer. Our results indicate that the excited localized plasmon resonance in groove rings can effectively improve the graphene absorption from 2.3% to 43.1%, even to a maximum value of 87.0% in five-layer graphene at telecommunication wavelengths. In addition, the absorption peak is strongly dependent on the groove depth and ring radius as well as the number of graphene layers, enabling the flexible selectivity of both the operating spectral position and bandwidth. This favorable enhancement and tunability of graphene absorption could provide a path toward high-performance graphene opto-electronic components, such as photodetectors.

Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly.

Capillary force is often regarded as detrimental because it may cause undesired distortion or even destruction to micro/nanostructures during a fabrication process, and thus many efforts have been made to eliminate its negative effects. From a different perspective, capillary force can be artfully used to construct specific complex architectures. Here, we propose a laser printing capillary-assisted self-assembly strategy for fabricating regular periodic structures. Microscale pillars are first produced by localized femtosecond laser polymerization and are subsequently assembled into periodic hierarchical architectures with the assistance of controlled capillary forces in an evaporating liquid. Spatial arrangements, pillar heights, and evaporation processes are readily tuned to achieve designable ordered assemblies with various geometries. Reversibility of the assembly is also revealed by breaking the balance between the intermolecular force and the elastic standing force. We further demonstrate the functionality of the hierarchical structures as a nontrivial tool for the selective trapping and releasing of microparticles, opening up a potential for the development of in situ transportation systems for microobjects.

Adaptive optics enhanced direct laser writing of high refractive index gyroid photonic crystals in chalcogenide glass.

Chiral gyroid photonic crystals are fabricated in the high refractive index chalcogenide glass arsenic trisulfide with an adaptive optics enhanced direct laser writing system. The severe spherical aberration imparted when focusing into the arsenic trisulfide is mitigated with a defocus decoupled aberration compensation technique that reduces the level of aberration that must be compensated by over an order of magnitude. The fabricated gyroids are shown to have excellent uniformity after our adaptive optics method is employed, and the transmission spectra of the gyroids are shown to have good agreement with numerical simulations that are based on a uniform and diffraction limited fabrication resolution.

Simultaneous compensation for aberration and axial elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective.

One of the challenges in laser direct writing with a high numerical-aperture objective is the severe axial focal elongation and the pronounced effect of the refractive-index mismatch aberration. We present the simultaneous compensation for the refractive-index mismatch aberration and the focal elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective. By the use of circularly polarized beam illumination and a spatial light modulator, a complex and dynamic slit pupil aperture can be produced to engineer the focal spot. Such a beam shaping method can result in circularly symmetric fabrication along the lateral directions as well as the dynamic compensation for the refractive-index mismatch aberration even when the laser beam is focused into the material of a refractive index up to 2.35.

Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate.

We present the use of a liquid crystal spatial light modulator to correct for the refractive-index mismatch induced spherical aberration in a high refractive-index lithium niobate crystal when a low repetition rate amplified laser is used for the direct fabrication of three-dimensional micro-structures. By correcting the aberration based on experimentally determined values, we show that the size of written structures decreases dramatically, which allows the fabrication of high quality micro-structures such as three-dimensional photonic crystals. We demonstrate that, through the use of adaptive optics, the fabrication depth and the stopgap strength in the corresponding photonic crystals are increased by a factor of two to three.