Electrically driven sources of photons offer a unique platform for realising applications such as beam steering and free-space optical interconnects. Metal-insulator-metal (MIM) tunnel junctions have been extensively used to electrically generate and manipulate light via inelastic electron tunneling. However, beam steering by dynamically switching the excitation source is still not shown. Here, we numerically demonstrate tunable directional emission of light from electrically-driven nanostrip tunnel junctions. Our device consists of an Ag-SiO2-Ag stack with the top Ag film milled into 16 nanostrips. Two nanostrips at the centre, labelled S1 and S2, act as individual sources with a resonance wavelength of ∼ 695 nm. We show that, by individually exciting S1 or S2, the light emission can be directed to spatially different channels, with an angle of emission depending on the periodicity of the passive elements. On applying a bias to source S2, the calculated far-field radiation pattern showed a highly directional beam with an emission angle of 30° and FWHM of < 12°. When the source is switched to S1, the emission pattern shifts to −30° with a similar FWHM, thereby paving the way towards practical, reconfigurable electrically-driven light sources.
Room temperature-operable mid-infrared (MIR) photodetectors have drawn significant attention due to their potential applications in imaging, security and sensing. The unique and tunable optoelectronic properties of graphene make it an attractive platform for designing tunable and broadband photodetectors. This work demonstrates a tunable mid-infrared photodetector using Graphene Nanoribbons (GNR) operated in the 5 – 12 μm range. We used the tunable plasmonic properties of graphene nanoribbons to model the photodetector. We numerically compute the generation rate using the plasmon-enhanced optical absorption in the GNR. We show a peak extinction of ~35% in the structure with GNR of width 50 nm and Fermi energy 0.3 eV is due to plasmonic resonances. The computed generation rate determines the photoresponse current in the GNR-based FET. The proposed structure shows a ~ 40-fold improvement in the peak photoresponse current in patterned structure over unpatterned structure in the wavelength 5 – 12 μm. Hence the tunable plasmonic resonances and the width dependent bandgap of GNRs enable the realization of room-temperature operable broadband MIR photodetector.
The concept of parity-time (PT) symmetry has recently expanded the toolbox to achieve active tunability in metasurfaces by modulating the imaginary part of the refractive index. In this work, we propose a hybridized static-active platform to dynamically tune the intensity and angular response of light by varying the non- Hermiticity factor in an all-dielectric metasurface. We numerically demonstrate tunable asymmetric transmission with respect to gain or loss side incidence in a vertically stacked Mie-resonant GaInP phased-array metasurface. It should be noted that the proposed system is reciprocal despite asymmetric transmission as the materials considered are in a linear regime. The primary building block consists of four PT-symmetric nanopillars of varying radii to achieve sufficient phase sampling. The overall design parameters are optimized for operation at a wavelength of 655 nm (typical PL emission peak of GaInP). For loss side normal incidence, the transmission is predominantly in the 0th diffraction order (ηl0~ 0:80, ηl1~ 0:18), while for gain side normal incidence, an amplified transmission is in the 1st order (ηg0~ 0:02, ηg1~0:78). The observed asymmetric transmission is due to the near-field coupling between different Mie multipoles, broken in-plane mirror symmetry (meta-atoms with increasing radii along the x-axis), and the broken PT-phase along the propagation direction. An asymmetry factor, ~0:9, is observed at λ = 655 nm. The symmetry in transmission can be restored by reducing the gain-loss contrast. We believe an optimal arrangement of gain-loss resonators combined with tunable pumping (either optically or electrically) could pave the way towards practical reconfigurable metasurfaces.
Optically resonant dielectric nanostructures is a new direction in nanophotonic research which gives a strong promise to compliment or substitute plasmonics in many potential application areas [1]. The main advantages of resonant dielectric nanostructures over conventional plasmonics are low losses, wide range of applicable dielectric materials and strong magnetic resonant response. So far most of research in this field has been conducted with silicon as a material for nanostructures due to its one of the highest value of refractive index at optical frequencies and CMOS compatibility. However, while silicon is an excellent material of choice for operation in the near-IR spectral range its applicability for visible frequencies is limited by increasing losses inside the material. Also, being an indirect bandgap semiconductor it is not a suitable material for making active nanoantenna devices. For these reasons in recent studies research focus starts shifting towards other appropriate materials such as III-V semiconductors, e.g. GaAs or GaP, and wide-bandgap semiconductors such as TiO2. In this presentation we will discuss applicability of different dielectric/semiconductor material platforms for obtaining resonant nanoantennas and metasurfaces operating in the visible frequency range. We will first show that titanium dioxide metasurfaces can be designed to obtain sharp resonances and full phase control at all three RGB wavelengths through Huygens’ metasurface approach, which pave the way towards realization of thin multi-layer metasurfaces with multi-colour operation. Then we will introduce a new III-V material platform based on GaN, which is highly transparent through the whole visible spectrum, and show high-efficiency operation of GaN metasurfaces in the blue and green parts of the visible spectrum. Finally we will discuss active nanoantennas based on GaAs and show the path towards achieving laser emission from resonant semiconductor nanoantenna structures.
References:
1) A. I. Kuznetsov et al., “Optically resonant dielectric nanostructures”, Science 354, aag2472 (2016).
Graphene is one of the emerging active nanophotonics materials with optical properties that can be controlled in real time by an applied bias voltage. Different applications from sensing to active nanophotonics have been discussed in the literature recently and the field is still developing especially with an eye on structured and multi-layer graphene. To design new devices there is a need for precise modeling of multivariate and dynamic optical responses of graphene elements in frequency and time domains. Taking into account the complexity that comes along with multiple unknown parameters, including edge effects in nanostructured graphene elements, graphene impurities, imperfections of characterization optics etc., it is hard to build an adequate multivariate model to reach good quantitative agreement with experiment.
Here, we present an approach that uses optimization methods to retrieve the optical properties of a given graphene sample from experiment. We show that with these techniques good quantitative agreement with experiments can be achieved; additionally we encapsulate our techniques in an online data-fitting tool. The tool includes several options to precisely fit the conductivity function to a given experiment - general spline approximations and physically meaningful random phase approximation models for frequency domain solvers, along with the relaxed Lorentz oscillator models for confident time domain simulations. A pilot version of our free online tool entitled Photonics2D-Fit (to be staged at nanoHUB.org) is presented.
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