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Concept: Photonics


Layered graphitic materials exhibit new intriguing electronic structure and the search for new types of two-dimensional (2D) monolayer is of importance for the fabrication of next generation miniature electronic and optoelectronic devices. By means of density functional theory (DFT) computations, we investigated in detail the structural, electronic, mechanical and optical properties of the single-layer bismuth iodide (BiI3) nanosheet. Monolayer BiI3 is dynamically stable as confirmed by the computed phonon spectrum. The cleavage energy (Ecl) and interlayer coupling strength of bulk BiI3 are comparable to the experimental values of graphite, which indicates that the exfoliation of BiI3 is highly feasible. The obtained stress-strain curve shows that the BiI3 nanosheet is a brittle material with a breaking strain of 13%. The BiI3 monolayer has an indirect band gap of 1.57 eV with spin orbit coupling (SOC), indicating its potential application for solar cells. Furthermore, the band gap of BiI3 monolayer can be modulated by biaxial strain. Most interestingly, interfacing electrically active graphene with monolayer BiI3 nanosheet leads to enhanced light absorption compared to that in pure monolayer BiI3 nanosheet, highlighting its great potential applications in photonics and photovoltaic solar cells.

Concepts: Photon, Optics, Density functional theory, Graphite, Solar cell, Photodiode, Band gap, Photonics


This work demonstrates an attractive low-cost route to obtain large area and high-quality graphene films by using the ultra-smooth copper foils which are typically used as the negative electrodes in lithium-ion batteries. We first compared the electronic transport properties of our new graphene film with the one synthesized by using commonly used standard copper foils in chemical vapor deposition (CVD). We observed a stark improvement in the electrical performance of the transistors realized on our graphene films. To study the optical properties on large area, we transferred CVD based graphene to transparent flexible substrates using hot lamination method and performed large area optical scanning. We demonstrate the promise of our high quality graphene films for large areas with ~400 cm(2) flexible optical modulators. We obtained a profound light modulation over a broad spectrum by using the fabricated large area transparent graphene supercapacitors and we compared the performance of our devices with the one based on graphene from standard copper. We propose that the copper foils used in the lithium-ion batteries could be used to obtain high-quality graphene at much lower-cost, with the improved performance of electrical transport and optical properties in the devices made from them.

Concepts: Optics, Semiconductor, Chemical vapor deposition, Rechargeable battery, Lithium-ion battery, Electronics, Photonics, Optoelectronics


Photonic-integrated devices need to be adequately spaced apart to prevent signal cross-talk. This fundamentally limits their packing density. Here we report the use of nanophotonic cloaking to render neighbouring devices invisible to one another, which allows them to be placed closer together than is otherwise feasible. Specifically, we experimentally demonstrated waveguides that are spaced by a distance of ∼λ0/2 and designed waveguides with centre-to-centre spacing as small as 600 nm (<λ0/2.5). Our experiments show a transmission efficiency >-2 dB and an extinction ratio >15 dB over a bandwidth larger than 60 nm. This performance can be improved with better design algorithms and industry-standard lithography. The nanophotonic cloak relies on multiple guided-mode resonances, which render such devices very robust to fabrication errors. Our devices are broadly complimentary-metal-oxide-semiconductor compatible, have a minimum pitch of 200 nm and can be fabricated with a single lithography step. The nanophotonic cloaks can be generally applied to all passive integrated photonics.

Concepts: Better, Optics, Research, Experiment, Space, Invisibility, Photonics, Cloak


In this work, we propose a novel approach for wafer-scale integration of 2D materials on CMOS photonic chip utilising methods of synthetic chemistry and microfluidics technology. We have successfully demonstrated that this approach can be used for integration of any fluid-dispersed 2D nano-objects on silicon-on-insulator photonics platform. We demonstrate for the first time that the design of an optofluidic waveguide system can be optimised to enable simultaneous in-situ Raman spectroscopy monitoring of 2D dispersed flakes during the device operation. Moreover, for the first time, we have successfully demonstrated the possibility of label-free 2D flake detection via selective enhancement of the Stokes Raman signal at specific wavelengths. We discovered an ultra-high signal sensitivity to the xyz alignment of 2D flakes within the optofluidic waveguide. This in turn enables precise in-situ alignment detection, for the first practicable realisation of 3D photonic microstructure shaping based on 2D-fluid composites and CMOS photonics platform, while also representing a useful technological tool for the control of liquid phase deposition of 2D materials.

Concepts: Spectroscopy, Laser, Integrated circuit, Surface tension, Raman spectroscopy, Photonics, Silicon photonics, Photonic computing


We study experimentally a fine structure of the optical Laue diffraction from two-dimensional periodic photonic lattices. The periodic photonic lattices with the C4v square symmetry, orthogonal C2v symmetry, and hexagonal C6v symmetry are composed of submicron dielectric elements fabricated by the direct laser writing technique. We observe surprisingly strong optical diffraction from a finite number of elements that provides an excellent tool to determine not only the symmetry but also exact number of particles in the finite-length structure and the sample shape. Using different samples with orthogonal C2v symmetry and varying the lattice spacing, we observe experimentally a transition between the regime of multi-order diffraction, being typical for photonic crystals to the regime where only the zero-order diffraction can be observed, being is a clear fingerprint of dielectric metasurfaces characterized by effective parameters.

Concepts: Scientific method, Crystal, Crystallography, Optics, Optical fiber, Metamaterial, Crystal system, Photonics


The imminent limitations of electronic integrated circuits are stimulating intense activity in the area of nanophotonics for the development of on-chip optical components, and solutions incorporating direct-bandgap semiconductors are important in achieving this end. Optical processing of data at the nanometre scale is promising for circumventing these limitations, but requires the development of a toolbox of components including emitters, detectors, modulators, waveguides and switches. In comparison to components fabricated using top-down methods, semiconductor nanowires offer superior surface properties and stronger optical confinement. They are therefore ideal candidates for nanoscale optical network components, as well as model systems for understanding optical confinement. Here, we demonstrate all-optical switching in individual CdS nanowire cavities with subwavelength dimensions through stimulated polariton scattering, as well as a functional NAND gate built from multiple switches. The device design exploits the strong light-matter coupling present in these nanowires, leading to footprints that are a fraction of those of comparable silicon-based dielectric contrast and photonic crystal devices.

Concepts: Optics, Metamaterial, Integrated circuit, Semiconductor, Semiconductor device, Transistor, Photonics, Nanoelectronics


Silicon photonics enables large-scale photonic-electronic integration by leveraging highly developed fabrication processes from the microelectronics industry. However, while a rich portfolio of devices has already been demonstrated on the silicon platform, on-chip light sources still remain a key challenge since the indirect bandgap of the material inhibits efficient photon emission and thus impedes lasing. Here we demonstrate a class of infrared lasers that can be fabricated on the silicon-on-insulator (SOI) integration platform. The lasers are based on the silicon-organic hybrid (SOH) integration concept and combine nanophotonic SOI waveguides with dye-doped organic cladding materials that provide optical gain. We demonstrate pulsed room-temperature lasing with on-chip peak output powers of up to 1.1 W at a wavelength of 1,310 nm. The SOH approach enables efficient mass-production of silicon photonic light sources emitting in the near infrared and offers the possibility of tuning the emission wavelength over a wide range by proper choice of dye materials and resonator geometry.

Concepts: Optics, Light, Electromagnetic radiation, Laser, Electromagnetic spectrum, Photonics, Silicon photonics, Silicon on insulator


In today’s age, companies employ machine learning to extract information from large quantities of data. One of those techniques, reservoir computing (RC), is a decade old and has achieved state-of-the-art performance for processing sequential data. Dedicated hardware realizations of RC could enable speed gains and power savings. Here we propose the first integrated passive silicon photonics reservoir. We demonstrate experimentally and through simulations that, thanks to the RC paradigm, this generic chip can be used to perform arbitrary Boolean logic operations with memory as well as 5-bit header recognition up to 12.5 Gbit s(-1), without power consumption in the reservoir. It can also perform isolated spoken digit recognition. Our realization exploits optical phase for computing. It is scalable to larger networks and much higher bitrates, up to speeds >100 Gbit s(-1). These results pave the way for the application of integrated photonic RC for a wide range of applications.

Concepts: Optics, Machine learning, Computer, Logic, Logic gate, Photonics, Boolean logic, Silicon photonics


Photonic crystals (PhCs) are periodically structured optical media offering the opportunity for spontaneous emission (SpE) to be strongly controlled in spatial terms (directions) or in absolute terms (rates). We discuss the application of this concept for practical light-emitting sources, summarizing the principles and actual merits of various approaches based on two- and three-dimensional PhCs. We take into consideration the numerous constraints on real-world light-emitting structures and materials. The various mechanisms through which modified photonic bands and band gaps can be used are first revisited in view of their use in light sources. We then present an in-depth discussion of planar emitters and enhanced extraction of light thanks to grating diffraction. Applications to conventional III-V semiconductors and to III-nitrides are reviewed. Comparison with random surface roughening reveals some common physical limitations. Some advanced approaches with complex structures or etched active structures are also discussed. Finally, the most promising mechanism to enhance the SpE rate, the Purcell effect, is considered. Its implementation, including through plasmonic effects, is shown to be effective only for very specific sources. We conclude by outlining the mix of physics and material parameters needed to grasp the relevant issues.

Concepts: Crystal, Photon, Optics, Fundamental physics concepts, Band gap, Photonics, Photonic crystal, Spontaneous emission


Label-free biosensing with silicon nanophotonic microring resonator sensors has proven to be an excellent sensing technique for achieving high-throughput and high sensitivity, comparing favorably with other labeled and label-free sensing techniques. However, as in any biosensing platform, silicon nanophotonic microring resonator sensors require a fluidic component which allows the continuous delivery of the sample to the sensor surface. This component is typically based on microchannels in polydimethylsiloxane or other materials, which add cost and complexity to the system. The use of microdroplets in a digital microfluidic system, instead of continuous flows, is one of the recent trends in the field, where microliter- to picoliter-sized droplets are generated, transported, mixed, and split, thereby creating miniaturized reaction chambers which can be controlled individually in time and space. This avoids cross talk between samples or reagents and allows fluid plugs to be manipulated on reconfigurable paths, which cannot be achieved using the more established and more complex technology of microfluidic channels where droplets are controlled in series. It has great potential for high-throughput liquid handling, while avoiding on-chip cross-contamination. We present the integration of two miniaturized technologies: label-free silicon nanophotonic microring resonator sensors and digital microfluidics, providing an alternative to the typical microfluidic system based on microchannels. The performance of this combined system is demonstrated by performing proof-of-principle measurements of glucose, sodium chloride, and ethanol concentrations. These results show that multiplexed real-time detection and analysis, great flexibility, and portability make the combination of these technologies an ideal platform for easy and fast use in any laboratory.

Concepts: Biotechnology, Sodium chloride, Surface tension, Sodium, Microfluidics, Photonics, Cytonix, Digital microfluidics