Many new photonic technologies require compact and high-resolution photonic devices, which can manipulate light at sub-wavelength/nanoscale dimensions. At this scale, conventional optical components, such as lenses, are no longer functional and new approaches and components, such as e.g. nanoantennas, should be developed. This has stimulated strong research efforts in the field of nanophotonics, which deals with light properties and manipulation at nanoscale. To date, most of these efforts have been in the scientific domain generating high-impact publications rather than high-impact technologies. One limitation of many nanodevice concepts is the high losses of nanophotonic components based on plasmonic metals as well as incompatibility with traditional CMOS based fabrication plus the costs of the required nanofabrication.
The era of practical nanophotonic technologies is rapidly approaching. Our group was among the pioneers of the new branch of nanophotonics related to nanoantennas based on high-refractive index dielectric and semiconductor materials, which can solve the major problem of losses and is compatible with existing industrial processes (Science, 2016). Our current research is mainly focused on development of the concept of dielectric nanoantennas towards different application directions in flat optics, light-on-chip integration and bioimaging.
In parallel to academic research work, we are continually looking to translate these cutting-edge concepts into real-world applications. For that, we actively seek to engage industrial partners to transfer the technologies and principles developed into practical devices.
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Flat optics with dielectric metasurfaces
Making optical components smaller and thinner is important for their integration inside mobile devices. Nanoantennas offer a way to miniaturize optical components to a size below the wavelength of light. Phased arrays of nanoantennas, also called metasurfaces, can manipulate the wavefront of light at will, thus paving the way to on-demand ultra-thin optics.
Dielectric metasurfaces have demonstrated high efficiency for both transmission and reflection based devices. Dielectric nanoantenna components also offer a wide range of optical magnetic and electric resonances enabling a variety of new magnetic optical phenomena. We focus on the development of dielectric metasurface components based on resonant concepts. In particular, we were one of the first to explore a concept of directional light scattering by dielectric nanoparticles (Nature Communications, 2013) and ultra-thin Huygens’ dielectric metasurfaces based on it (see Fig. 1A-C and Laser & Photonics Reviews, 2015). This approach leads to highly-efficient flat optical devices operating in transmission based on thin (<λ/5) low-aspect ratio nanostructures. Small thickness and low aspect ratio are beneficial for applications requiring large-area, low-cost nanofabrication. Another example of our work is light polarization control with metasurfaces based on the generalized Brewster effect (see Fig. 1D-E and Nature Communications, 2016). In contrast to well-known Brewster’s angle, the generalized Brewster effect relies on both electric and magnetic response of a metasurface and can achieve suppressed reflection for either p or s polarizations at any desired angle. Recent results of our team also demonstrate efficient ultra-high angle (>80 degrees) visible light bending based on silicon (ArXiv, 2017) and titanium dioxide (Nano Letters, 2017) diffractive nanoantenna arrays with asymmetric nanoantenna geometries. In particular this concept was applied to experimentally demonstrate a flat lens with free-space numerical aperture of >0.99, which exceeds all existing flat and bulk optical analogues (as shown in Fig. 1F-H from ArXiv, 2017).
Future work on these topics will continue to develop resonant dielectric metasurfaces and device concepts and their transfer to industry.
Fig.1. (A-C) Visible light bending with silicon-based Huygens’ metasurfaces. (D-E) Generalized Brewster effect in dielectric metasurfaces. (F-H) Near-unity numerical aperture flat lens based on diffractive nanoantenna arrays.
Dielectric nanoantennas for fluorescence enhancement
Fluorescent molecules are widely used in bioimaging and detection systems as a convenient way to observe material properties at the sub-wavelength scale. One major drawbacks of this approach is a weak emission signal from a low quantity of molecules which then limits the detection speed and imposes additional requirements to have highly-sensitive detection systems. Nanoantennas provide a solution to this problem by increasing the fluorescence signal of each molecule by local enhancement of the excitation field and enhancement of the molecule emission signal due to the Purcell effect. Plasmonic nanoantennas were shown to be efficient in field localization and fluorescence enhancement. However, they also provide significant losses transferring a significant portion of absorbed incoming pump energy into heating of the antenna and the surrounding medium. This is particularly undesirable when biological objects are involved.
Dielectric nanoantennas overcome this issue by providing significant fluorescence enhancement with negligible loss. Though the total field enhancement by dielectric nanoparticles is typically lower than that of metals, their high enhancement of the radiative decay rate (with almost zero non-radiative decay) and additional strong emission directivity provide a strong overall enhancement of the detected fluorescence signal. Our group was the first to study field enhancement effects in dielectric dimer nanoantennas (Nano Letters, 2015). In particular we have experimentally demonstrated that both electric and magnetic near-fields can be enhanced by dielectric nanoparticles, as shown in Fig.2, opening ways for emission enhancement not only from electric but also from magnetic dipolar emitters.
Further research on this project is related to design and realization of dielectric nanoantenna configurations, which will bring high fluorescence enhancement and shape the radiation pattern to further improve collection efficiency. The final goal is to develop large-area fluorescence enhancing substrates for real-life biodetection systems.
Fig.2. (i) Schematic of electric and magnetic hotspots in a plasmonic and a dielectric dimer. (ii) Electromagnetic near-field around a silicon dimer measured by NSOM, showing the generation of both electric and magnetic hotspots.
Dielectric nanoantennas for light-on-chip
Silicon photonics is considered to be the future of high-speed computer interconnects at the chip to chip and intra-chip levels. It also has significant application space for the telecommunication and sensing industries. Traditional silicon photonic components are based on waveguides and ring resonators. Silicon nanoantennas fabricated on the SOI platform may help to miniaturize existing devices and bring additional functionalities to the existing silicon photonic platform without disrupting existing fabrication processes. Our group is working on developing a new component library based on the nanoantenna concepts to complement traditional silicon photonics. In particular we have shown that resonant nanoantennas can couple through their electric and magnetic dipole resonances, as shown in Fig. 3, to guide light at millimeter scales with losses comparable to conventional silicon waveguides (Nano Letters, 2017). In contrast to conventional waveguides this guiding has resonant characteristics, which can be used for modulation or sensing purposes. We also explore the use of nanoantennas to design efficient III-V-based on-chip components for light generation and amplification.
The major goal of this project is to extend the existing silicon photonics library with new functionalities and efficient and more compact nanoantenna-based devices.
Fig.3. Resonant light guiding along a chain of silicon nanoantennas: top – schematics; bottom – SEM images of the fabricated structures and NSOM image of the guided mode (all scale bars are 500 nm).