We numerically and experimentally demonstrate that metasurfaces can be used to control the light emission from light emitting diodes (LED). This control provides a desired wavefront and functionality of the light emission in addition to enhancing light extraction efficiency. Simply placing the metasurface on top of the LED does not work as conventional metasurface designs require plane wave excitation, which LEDs cannot provide. To overcome this challenge we implement a novel concept using internal and external resonant cavities combined with the LED. Guided by our numerical simulations, we experimentally demonstrate this concept by fabricating Si and TiO2 metasurfaces on top of the resonant cavity LED structures. The integration of these metasurfaces with commercially available GaN and GaP LED devices show full wavefront control, beam deflection and beam collimation. Both the cavity and the metasurface enhance the LED radiation. Moreover, following the proposed principle, any random light emitting sources including fluorescent molecules and quantum dots can be integrated into a similar optical device to achieve focusing, beam deflecting, vortex beam generation and other capabilities.
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).
Optically resonant dielectric nanostructures represent a new and rapidly developing research direction in nanophotonics [1]. They show plenty of useful functionalities and can complement or even substitute resonant plasmonic nanoparticles for many potential application directions. The main advantages over conventional plasmonics are low losses, wide range of applicable dielectric materials and strong magnetic resonant response. In particular, the last feature opens a broad range of opportunities to control light scattering, transmission, reflection and phase characteristics through designed interference between electric and magnetic resonant modes. This has already led to demonstrations of low-loss dielectric Huygens’ metasurfaces operating with very high efficiencies in transmission mode and generalized Brewster effect showing unconventional behaviour of dielectric metasurface in reflection mode [1]. In this presentation, we will review recent magnetic resonant phenomena obtained with high-index dielectric nanoantennas and metasurfaces and show how this might lead to new functionalities, which cannot be achieved neither with conventional metasurface approaches nor with conventional bulk optics. In particular, we demonstrate how the resonance interference effect can be used to control energy distribution between diffraction orders in a nanoantenna array, which leads to light bending at very high angles of >82 degrees with efficiency >50%. This property is used to design and experimentally demonstrate flat lenses having a free-space numerical aperture (NA) of >0.99, which strongly exceeds NA of existing flat lenses and bulk optics analogues. Applications of these new, ultra-high NA, flat dielectric lenses will also be discussed.
References:
1) A. I. Kuznetsov et al., “Optically resonant dielectric nanostructures”, Science 354, aag2472 (2016).
KEYWORDS: Particles, Magnetism, Dielectrics, Dielectric polarization, Electromagnetism, Plasmonics, Scattering, Data storage, Current controlled current source
Metasurfaces are planar arrangements of elements that are designed to present a particular response to an incident electromagnetic field. Due to their ability to control at will the phase, polarization and amplitude of the reflected and/or transmitted waves at a subwavelength scale they have gathered a great deal of attention among the research community.
Although the first metasurface proposals were realized with plasmonic particles, the focus is now turning into all-dielectric approaches, in order to mitigate losses and increase the device efficiencies. Besides the obvious advantage of loss reduction, when high-index, subwavelength particles are considered a whole new family of resonant, magnetic-like modes is accessible. This new set of modes, which cannot be excited in simple metallic particles, brings additional functionalities for these metasurfaces, as will be shown in this talk.
We will focus on the interesting effects that arise as a consequence of the far-field interference between electric and magnetic modes excited in the dielectric particles forming the metasurface and the strong modification of their scattering patterns as a consequence of this interference. In particular, we will show the possibility to realize so called ideal Huygens’ secondary sources to generate a perfectly transmissive metasurface with full phase control. We will also show that these metasurfaces support a generalized version of Brewster’s effect, in which the phenomenon is not restricted to a particular angle or polarization of incidence but can be tuned at will, and the different implications that this concept has.
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