Showing papers by "Min Gu published in 2019"

Artificial neural networks enabled by nanophotonics.

Abstract: The growing demands of brain science and artificial intelligence create an urgent need for the development of artificial neural networks (ANNs) that can mimic the structural, functional and biological features of human neural networks. Nanophotonics, which is the study of the behaviour of light and the light-matter interaction at the nanometre scale, has unveiled new phenomena and led to new applications beyond the diffraction limit of light. These emerging nanophotonic devices have enabled scientists to develop paradigm shifts of research into ANNs. In the present review, we summarise the recent progress in nanophotonics for emulating the structural, functional and biological features of ANNs, directly or indirectly.

Three-dimensional femtosecond laser nanolithography of crystals

Abstract: So far, nanostructuring of hard optical crystals has been exclusively limited to their surface, as stress-induced crack formation and propagation render high-precision volume processes ineffective1,2. Here, we show that the rate of nanopore chemical etching in the popular laser crystals yttrium aluminium garnet and sapphire can be enhanced by more than five orders of magnitude (from <0.6 nm h−1 to ~100 µm h−1) by the use of direct laser writing, before etching. The process makes it possible to produce arbitrary three-dimensional nanostructures with 100 nm feature sizes inside centimetre-scale laser crystals without brittle fracture. To showcase the potential of the technique we fabricate subwavelength diffraction gratings and nanostructured optical waveguides in yttrium aluminium garnet and millimetre-long nanopores in sapphire. The approach offers a pathway for transferring concepts from nanophotonics to the fields of solid-state lasers and crystal optics. Direct laser writing is shown to dramatically enhance the chemical etch rate of laser crystals yttrium aluminium garnet and sapphire, allowing nanostructuring.

Three-dimensional femtosecond laser nanolithography of crystals

Abstract: Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.

Extraordinary optical fields in nanostructures: from sub-diffraction-limited optics to sensing and energy conversion

Abstract: Along with the rapid development of micro/nanofabrication technology, the past few decades have seen the flourishing emergence of subwavelength-structured materials and interfaces for optical field engineering at the nanoscale. Three remarkable properties associated with these subwavelength-structured materials are the squeezed optical fields beyond the diffraction limit, gradient optical fields in the subwavelength scale, and enhanced optical fields that are orders of magnitude greater than the incident field. These engineered optical fields have inspired fundamental and practical advances in both engineering optics and modern chemistry. The first property is the basis of sub-diffraction-limited imaging, lithography, and dense data storage. The second property has led to the emergence of a couple of thin and planar functional optical devices with a reduced footprint. The third one causes enhanced radiation (e.g., fluorescence), scattering (e.g., Raman scattering), and absorption (e.g., infrared absorption and circular dichroism), offering a unique platform for single-molecule-level biochemical sensing, and high-efficiency chemical reaction and energy conversion. In this review, we summarize recent advances in subwavelength-structured materials that bear extraordinary squeezed, gradient, and enhanced optical fields, with a particular emphasis on their optical and chemical applications. Finally, challenges and outlooks in this promising field are discussed.

Manipulating Terahertz Plasmonic Vortex Based on Geometric and Dynamic Phase

Abstract: Electromagnetic waves carrying orbital angular momentum (OAM), namely, vortex beams, have a plethora of applications ranging from rotating microparticles to high-capacity data transmissions, and it is a continuing trend in manipulating OAM with higher degrees of freedom. Here, an approach to control terahertz (THz) near-field plasmonic vortex based on geometric and dynamic phase is proposed and experimentally demonstrated. By locally tailoring the orientation angle (geometric phase) and radial position (dynamic phase) of aperture arrays embedded in an ultrathin gold film, the excited surface waves can be flexibly engineered to form both spin-independent and spin-dependent THz plasmonic vortex field distributions, resulting in multi-degree of freedom for controlling OAM of THz surface plasmon polaritons (SPPs). Arbitrary OAM values of THz plasmonic vortex and coherent superposition between two OAM states are investigated based on near-field scanning terahertz microscopy (NSTM) system. The proposed approach provides unprecedented freedom to modulate THz near-field plasmonic vortex, which will have potential applications in THz communications and quantum information processing.

Large-scale waterproof and stretchable textile-integrated laser- printed graphene energy storages.

Abstract: Textile integrable large-scale on-chip energy storages and solar energy storages take a significant role in the realization of next-generation primary wearable devices for sensing, wireless communication, and health tracking. In general, these energy storages require major features like mechanical robustness, environmental friendliness, high-temperature tolerance, inexplosive nature, and long-term storage duration. Here we report on large-scale laser-printed graphene supercapacitors of dimension 100 cm2 fabricated in 3 minutes on textiles with excellent water stability, an areal capacitance, 49 mF cm−2, energy density, 6.73 mWh/cm−2, power density, 2.5 mW/cm−2, and stretchability up to 200%. Further, a demonstration is given for the textile integrated solar energy storage with stable performance for up to 20 days to reach half of the maximum output potential. These cost-effective self-reliant on-chip charging units can become an integral part for the future electronic and optoelectronic textiles.

Enhancing monolayer photoluminescence on optical micro/nanofibers for low-threshold lasing.

Abstract: Although monolayer transition metal dichalcogenides (TMDs) have direct bandgaps, the low room-temperature photoluminescence quantum yields (QYs), especially under high pump intensity, limit their practical applications. Here, we use a simple photoactivation method to enhance the room-temperature QYs of monolayer MoS2 grown on to silica micro/nanofibers by more than two orders of magnitude in a wide pump dynamic range. The high-density oxygen dangling bonds released from the tapered micro/nanofiber surface are the key to this strong enhancement of QYs. As the pump intensity increases from 10-1 to 104 W cm-2, our photoactivated monolayer MoS2 exhibits QYs from ~30 to 1% while maintaining high environmental stability, allowing direct lasing with greatly reduced thresholds down to 5 W cm-2. Our strategy can be extended to other TMDs and offers a solution to the most challenging problem toward the realization of efficient and stable light emitters at room temperature based on these atomically thin materials.

Topological Insulator Materials for Advanced Optoelectronic Devices

Abstract: Topological insulators are quantum materials that have an insulating bulk state and a topologically protected metallic surface state with spin and momentum helical locking and a Dirac-like band structure. Unique and fascinating electronic properties, such as the quantum spin Hall effect, quantum anomalous Hall effect, and topological magnetoelectric effect, as well as magnetic monopole images and Majorana fermions, have been observed in the topological insulator materials. With these unique properties, topological insulator materials have great potential applications in spintronics and quantum information processing, as well as magnetoelectric devices with higher efficiency and lower energy consumption. On the other hand, topological insulator materials also exhibit a number of excellent optical properties, including Kerr and Faraday rotation, ultrahigh bulk refractive index, near-infrared frequency transparency, unusual electromagnetic scattering, and ultra-broadband surface plasmon resonances. Specifically, Dirac plasmon excitations have been observed in Bi2Se3 micro-ribbon arrays at THz frequencies. Ultraviolet and visible frequency plasmonics have been observed in nanoslit and nanocone arrays of Bi1.5Sb0.5Te1.8Se1.2 crystals. High transparency has been observed in Bi2Se3 nanoplates. An ultrahigh refractive index has been observed in bulk Bi1.5Sb0.5Te1.8Se1.2 crystals as well as in Sb2Te3 thin films. These excellent optical properties mean that topological insulator materials are suitable for various optoelectronic devices, including plasmonic solar cells, ultrathin holograms, plasmonic and Fresnel lens, broadband photodetectors, and nanoscale waveguides. In this chapter, we focus on the excellent electronic and optical properties of topological insulator materials and their wide applications in advanced optoelectronic devices.

Super-resolution optical microscope: principle, instrumentation, and application

Abstract: Over the past two decades, several fluorescence- and non-fluorescence-based optical microscopes have been developed to break the diffraction limited barrier. In this review, the basic principles implemented in microscopy for super-resolution are described. Furthermore, achievements and instrumentation for super-resolution are presented. In addition to imaging, other applications that use super-resolution optical microscopes are discussed.

Hole Blocking Layer-Free Perovskite Solar Cells with High Efficiencies and Stabilities by Integrating Subwavelength-Sized Plasmonic Alloy Nanoparticles

Abstract: Perovskite solar cells hold great promise as prospective alternatives of renewable power sources. Recently hole blocking layer-free perovskite solar cells, getting rid of complex and high-temperatu...

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

Abstract: 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.

Terahertz spatial sampling with subwavelength accuracy.

Abstract: A simple terahertz (THz) spatial sampling method offers kilohertz (kHz) level sampling rates and greatly preserves the energy of a THz pulse, which enables THz imaging detection with a high signal-to-noise ratio, micron-grade accuracy, and subwavelength resolution.

Optomagnetic plasmonic nanocircuits

Abstract: 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.

Laser Printing of a Nano-Imager to Perform Full Optical Machine Learning

Abstract: Applications of artificial intelligence techniques, specifically machine learning and more recently deep learning [1], are transforming several fields ranging from clinical medicine to optical computing. Integrating full-optical neuromorphic architectures in opto-electronic devices (Fig. 1a) will lead to the near-term availability of clinically and industrially relevant applications such as real-time features detection and classification, image processing and optical implementation of computational intensive tasks such as matrix multiplication with low-power consumption, high-accuracy and ultra-fast processing speed [2].

Stable copper nanowire-graphene oxide thin films for nonlinear photonics

Abstract: Solution-based copper nanowire-graphene oxides are promising building blocks in future optoelectronics ranging from transparent electrodes to electrochromic displays to stretchable electronics. In this paper, a detailed study of the optical properties extending to non-linear properties is demonstrated. An enhancement of 43% is observed for the nonlinear optical properties of these thin films in comparison to a graphene oxide thin film. Moreover, its application as transparent conductive electrodes with 93% of the optical transmittance and a sheet resistance of 10 Ω/sq is manifested.

Optical data storage in a graphene oxide thin film integrated with upconversion nanoparticles (Conference Presentation)

Abstract: The huge volume of digital information generated across the world represents an insuperable challenge for the currently-available data storage devices and compels for the development of novel techniques and storage media. Nanomaterials, which have unique mechanical, electronic and optical properties owing to the strong confinement of electrons, photons and phonons at the nanoscale, are enabling the development of disruptive methods for optical data storage with ultra-high capacity, ultra-long lifetime and ultra-low energy consumption. In this context, upconversion nanoparticles, which feature the interesting property of photon upconversion and emit in a range from ultraviolet to near-infrared, have attracted considerable attention for optical data storage applications through the modulation of their upconversion fluorescence emission. However, it has been difficult to find an effective quencher for upconversion nanoparticles to entirely quench their anti-Stokes type of emission. Graphene oxide (GO) and reduced graphene oxide (r-GO) have proved useful as effective quenchers due to their strong broadband absorption. Herein, we demonstrate optical data storage in a GO and upconversion nanoparticles thin film. Core-shell nanoparticles were prepared via co-precipitation method and measurements of upconversion fluorescence emission intensity and fluorescence lifetime have been performed. Subsequently, the upconversion nanoparticles have been conjugated to GO and deposited through vacuum filtration to form a thin film. The nanocomposite was then irradiated using laser at different powers to produce the reduction of GO to r-GO. The encoded optical data bits were readout through the variation of fluorescence intensity from the upconversion nanoparticles accompanied by the reduction of the GO to r-GO.

Deep-Learning-Aided Three-Dimensional Direct Laser Writing of the Complete Connectome of Mushroom Body from an Insect Brain

Abstract: Creation of biomimetic neuronal structures that emulate the topology of biological neural networks (BNNs) has been an active area of research in engineered neural networks over the last decade, these biomimetic neuronal structures simulate the microenvironment that the neurons could adhere, proliferate and differentiate as well as the drug release through temporal and spatial control [1]. Owing to the capability of three-dimensional processing and the fabrication resolution down to nanometers, three-dimensional (3D) direct laser writing (DLW) based on multi-photon absorption [2,3], has been widely studied and utilized to produce 3D biomimetic structures, such as biomimetic human ovarian malignancy structures and single biomimetic neuron structures [4,5]. However, large scale biomimetic neuronal structures couldn't be facilely achieved using 3D DLW due to the structural complexity of BNNs. This challenge can be articulated into two aspects: i. Unachievable fabrication time. BNNs are 3D networks structures spinning in a very large scale, take human brain as an example, the total neuron circuit length is estimated to be 100 billion μm, resulting in a fabrication time of 1 billion seconds (roughly 31.7 years); ii. Structural data of neuronal circuit at synaptic resolution are rarely available from neuroscience.

High-performance ultrafine-structured graphene supercapacitors fabricated by femtosecond laser nanolithography

Abstract: The microsized structure of laser-scribed supercapacitor electrodes is a barrier for achieving high performances. Here we report on femtosecond laser lithography for ultrafine-structured supercapacitor fabrication. A performance breakthrough can be achieved.

Optical data storage material and method

Abstract: An optical data storage material comprising graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, nanoparticles configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission, and a support material that (i) embeds the GO and the nanoparticles and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas.

High Numerical Aperture Aberration Compensation Enabled by an Artificial Neural Network

Abstract: The determination of phase aberrations is essential to ensure diffraction limited performance in optical imaging and three-dimensional laser fabrication [1]. It is especially crucial as the photonics community transitions from microscopy to nanoscopy and from microfabrication to nanofabrication. However, the determination of phase aberrations is often a highly parallel and nonlinear problem that necessitates specialist equipment, intensive computation, or prolonged iteration.

Matériau de stockage de données optiques et procédé

Abstract: L'invention concerne un materiau de stockage de donnees optiques comprenant de l'oxyde de graphene (GO) concu pour etre reduit photochimiquement sur des zones selectionnees pour le stockage de donnees optiques, des nanoparticules concues pour reduire photochimiquement le GO sur les zones selectionnees par emission de conversion-elevation optique, et un materiau de support qui (i) incorpore le GO et les nanoparticules et (ii) comprend un conducteur thermique en contact thermique avec le GO afin d'attenuer la reduction photo-thermique des zones selectionnees.

Three-Dimensional Laser Nanostructuring of Optical Crystals: Towards Nanophotonic-Engineered Solid-State-Media

Abstract: Nanophotonics, where light control is achieved by means of subwavelength-structured media, are so far mostly fabricated by surface lithographic techniques. Inevitably, this dictates that all nanophotonic elements have planar geometry, and lay at (or close to) the surface of dissimilar material substrates. Planarity is an eminent design restriction against the three-dimensional nature of light propagation, and the use of diverse materials imposes limitations to device robustness against environmental factors, such as temperature changes, vibrations and stress. Regardless of these limitations, two "killer" applications of planar nanophotonics, photonic integrated circuits (PIC) (based on suborn waveguides and 2D photonic bandgap crystals) and optical metasurfaces (i.e. wavefront shaping devices based on subwavelength-structured wavelength-thick surfaces) are already revolutionizing the communications and sensing industries. Such devices are typically based on silicon (or silicon nitride) on lower index silicon dioxide, thus providing good compatibility with the semiconductor CMOS industry. Applying surface lithography or other methods to directly nanostructure dielectric optical materials in the three dimensions has however been out of reach, so far. As a result, the general fields of solid-state lasers and crystal optics still have a very weak connection to the field of nanophotonics. In fact, the use of crystals for either laser light generation or non-linear frequency conversion is fundamentally equal today as it was in 1960: a homogenous crystal is put inside an optical setup.

Multi-photon Imaging with Rare-earth Doped Nanocrystals Beyond the Diffraction Limit

Abstract: Multi-photon imaging beyond the diffraction limit has been demonstrated with rare-earth doped nanocrystals. A resolution of 100 nm has been predicted with the excitation and depletion beam at a wavelength of 980 nm.

Highly efficient modulation of upconversion luminescence at a millisecond timescale

Abstract: The highly efficient modulation of the luminescence from upconversion nanoparticles combined with graphene oxide in a thin film was achieved at a millisecond timescale through the photochemical reduction of graphene oxide under UV irradiation. The experimental design comprised the integration of the upconversion nanoparticles with graphene oxide to form a reproducible and scalable thin film. This design enabled convenient testing of the sample with a home‐built optical system setup comprising an UV CW laser at 375 nm for the photochemical reduction of graphene oxide and a near-infrared CW laser at 980 nm for the excitation of the upconversion nanoparticles. The recovery of the graphene‐like structure through the photochemical reduction of graphene oxide was accompanied by a variation in the absorption coefficient of the thin film, which enabled super‐quenching of the luminescence from the upconversion nanoparticles under near‐infrared excitation with values of up to ~90%. Further, the instantaneous reduction in the intensity upon UV irradiation offered decreased modulation time of upconversion luminescence down to milliseconds at microwatt‐level power. Optical patterning was successfully produced in the thin film: representations of a leaf, the Sydney Opera House and a kangaroo were fabricated in the thin film and recovered by raster scanning the sample. The resulting patterns had high spatial resolution for upconversion luminescence modulation down to the diffraction limit for the considered wavelengths. These findings pave the way toward prompt use of this novel thin film for display technologies, photoswitching in optoelectronic devices, and optical data storage applications.

Dancing of electromagnetic spectra driven by metasurfaces.

Abstract: In 2011, Capasso proposed the concept of manipulating electromagnetic (EM) waves with phase modulation by an ultrathin flat metasurface composed of nanoantenna arrays [1]. Later, researchers developed different kinds of new metasurfaces, such as hyperbolic metasurfaces, tunable metasurfaces and non-linear metasurfaces [2]. The development of metasurfaces has greatly enhanced the capability to manipulate EM waves with multiple functionalities on a flat surface. Non-linearmetasurfaces combine the advantages of metasurface and non-linear EM phenomena, offering new degrees of freedom for the improvement of the performance ofmetasurfaces. The non-linear response of metasurfaces can be used to realize functions such as frequency mixing, harmony generation and ultrafast switching [3], which are essential for classical and quantum communication. However, the strength of the non-linear EM phenomena of the metasurfaces is a naturally given constant that cannot be changed after the metasurfaces have been fabricated. Further, the efficiency of non-linear EM phenomena in metasurfaces is usually very weak due to the limitation of the non-linear materials of the metasurfaces. The invention of digital coding metasurfaces (DCM) offers a programable way to control the EM waves with one physical configuration [4,5]. The unit cells of the DCM have a digital response (‘0’ or ‘1’) controlled by a biased diode that can be switched in the time domain. Suchmetasurfaces offer a new dimension to control the non-linear response with an external voltage bias. Recently, using a DCMwith varying reflectivity controlled by the biased diode, scientists in China and the USA have demonstrated harmony generation with high conversion efficiency, which is not limited by the optical properties of materials [6]. The teamofTie JunCui,QiangCheng and Shi Jin (Jie Zhao, Xi Yang, Jun Yan Dai, Xiang Li, Ning Hua Qi, Jun Chen Ke, Guo Dong Bai, and Shuo Liu), at Southeast University in China, and AndreaAlù, at theCityCollegeofNewYork, designed a reflective time-domain digital coding metasurface to control the spectra of the illuminated signal (Fig. 1). The

Direct plasmonic photodetection of optical angular momentum

Abstract: Optical angular momentum provides a useful dimension for data multiplexing and quantum entanglement. However, the demultiplexing of optical angular momentum is often performed by complex bulk optics and multiple detectors. Here we demonstrate that chiral plasmonic electrodes can be used to selectively detect optical angular momentum in a compact and integrated format. We demonstrate direct detection of photon spin and investigate methods for the detection of orbital angular momentum.