New horizons in nanophotonics

Many conventional optoelectronic devices consist of thin, stacked films of metals and semiconductors. In this presentation, I will demonstrate how one can improve the performance of such devices by nano-structuring the constituent layers at length scales below the wavelength of light.

The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical transmission, absorption, reflection, and refraction properties of device layers. This is accomplished by encoding the optical response of nanoscale resonant building blocks into the effective properties of the films and surfaces. To illustrate these points, I will show how nanopatterned metal and semiconductor layers may be used to enhance the performance of solar cells, photodetectors, and enable new imaging technologies. I will also demonstrate how the use of active nanoscale building blocks can facilitate the creation of active metafilm devices.

The concept of Huygens metasurfaces has recently emerged as a powerful platform for complete manipulation of light properties, including phase, amplitude, polarisation, and even colour. Their operation is based on the interference of the electric and magnetic dipolar responses of the constituent metasurface elements, called meta-atoms, such that they can only scatter in forward direction, while back-scattering is inhibited. Dielectric Huygens metasurfaces stand out as a prominent example, due to their negligible optical losses and easy fabrication. Such dielectric metasurfaces are composed of small high-refractive-index nano-particles, which exhibit Mie-type resonances of both electric and magnetic origin and comparable strength. By designing the geometry of the individual meta-atoms it is possible to exactly match the spectral position of these resonances, thus enabling unitary transmission through the Huygens metasurface, while simultaneously being able to control the phase of transmitted light in the full range of 0-2π.

This talk will review the fundamental designs and principles of operation of such dielectric metasurfaces, as well as will overview the plethora of their functionalities, including frequency selectivity, wavefront shaping, and polarization control. In particular, we demonstrate experimentally beam shaping in complex holographic shapes with near unity transmission efficiency. We further utilise our Huygens metasurfaces for generation of beams carrying orbital angular momentum, including vortex and vectors beams with azimuthal/radial polarisations operating over a broad spectral range. Finally, we will present some of their recent applications in nonlinear light sources, biosensing, and quantum optics.

Absorption loss continues to be one of the primary impediments to the application of plasmonic metamaterials and metasurfaces at optical frequencies. Dielectric metamaterials offer one potential solution to this issue by eliminating ohmic loss, allowing the realization of highly transparent materials. As with their plasmonic counterparts, manipulation of the unit cell structure of all-dielectric metasurfaces also offers a means to engineer a wide variety of optical functionalities. 

In this talk, I will discuss our recent experimental efforts to demonstrate silicon-based metasurfaces within the telecommunications band. I will talk about how simple unit cell geometries allow these metasurfaces to be scaled to large areas using self-assembly based patterning techniques. Importantly, defects in such materials are found to have little effect on the performance of the surfaces. On the other hand, I will discuss how more complicated unit cells can be used to realize wavefront control as well as high quality factor resonances. The high-Q resonances can be used for sensing and the large local field enhancement within the silicon unit cells results in a third harmonic conversion enhancement factor of 10⁵ with respect to an unstructured silicon slab. Such surfaces could potentially be applied for all-optical switches in the future.

Plasmonic metasurfaces enhance light-matter interaction by focusing light into extremely subwavelength dimensions. These carefully designed structures have been used in extremely thin optical component which can mold the wavefront, with exciting applications in optical lenses, beam steering, and biosensing applications. Adding dynamic tunability to these devices opens up the possibility for new application in single pixel detection and 3D imaging as well as optical modulators and switches. However the existing approaches for designing active optical devices in infrared, are either slow or have small refractive index change. Integrating plasmonic metasurfaces with single-layer graphene (SLG) opens exciting opportunities for developing active plasmonic devices because the amplitude and phase of the transmitted and reflected light can be rapidly modulated by injecting charge carriers into graphene using field-effect gating. I will describe our recent experimental results demonstrating strong phase modulation of mid-infrared light. The phase shifting due to electric gating of the SLG was measured using a Michelson interferometer, and further utilized to demonstrate an electrically controlled (i.e. no moving parts) interferometry capable of measuring distances with sub-micron accuracy. Because of the potentially nanosecond-scale measurement time, active metasurfaces represent a promising platform for ultra-fast standoff detection. Finally, we demonstrate that, by the judicious choice of a strongly anisotropic metasurface, the graphene-controlled phase shift of light can be rendered polarization-dependent, thereby modulating the polarization state (e.g., the ellipticity) of the reflected light. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.

We use single crystalline gold flakes on atomically flat silicon substrates to generate ideally suitable metals for plasmon propagation. By electrochemical means, the thickness is tunable from a few tens to over 100 nm. Using sub-20 fs laser pulses around 800 nm, we excite surface plasmons, whose dynamics can be observed using time-resolved two-photon excitation electron emission (PEEM).

Plotting the dispersion of surface plasmons in a thin gold slab on silicon, one finds that excitation at 800 nm can lead to extreme wavelength reduction due to the dispersion slop of over five. Using focused ion beam for cutting rings with appropriate periodicity into the samples, we can excite concentric surface plasmons that create a nanofocus of only 60 nm width for 800 nm excitation.

Using Archimedean spirals with broken n-fold radial symmetry, it is possible to excite surface plasmons with angular orbital momentum on the gold flakes. This leads in case of 4-fold symmetry to cloverleaf-type nanofoci on the order of 100 nm, which rotate during four optical cycles by 360 degrees.

Using two-pulse experiments with a subwavelength-stabilized Michelson interferometer, it is possible to observe the dynamics of the surface patterns with a (sub-)femtosecond resolution, thus giving insight into the dynamics of the nanofocus formation as well as on the plasmonic spin-orbit coupling.

We present recent optical wave-mixing measurements showing the excitation of surface plasmons in planar graphene. A large enhancement of non-linear signal in regions of high density of states suggests a strong coupling to propagating plasmons in graphene for a variety of in-plane wave-vectors. Estimates regarding the non-linearity of graphene and the efficiency of the DFG process are inferred from our experiment. These results demonstrate a promising route to efficient generation of surface plasmons in graphene using free-space radiation.

Recent development of deep UV light sources opens a new world of nanophotonics, as exemplified by deep UV photo-lithography, photo-catalysis, sterilization, and molecular-sensing, analysis and imaging. If deep UV optics is combined with Raman scattering microscopy, the distribution of nucleotides and proteins in a cell is imaged and analyzed without labelling. However, the use of deep UV light for bio-imaging is limited because it can destroy or denature target bio-molecules. Recently, we proposed a method for suppressing the photo-degradation of molecules using lanthanide ions in solution as energy quenchers. This approach directly removes excited energy at the fundamental origin of cellular photo-degradation. For sub-wavelength imaging, we need a plasmonic tip that effectively works in deep UV to enhance Raman scattering at molecules. We have found that aluminum is one of the best metals that exhibit plasmonic field enhancement in deep-UV, while not in visible range because the imaginary part of the dielectric constant for aluminum is very large in visible. We also found that Indium is another good candidate in deep UV and is useful in practice for vapour deposition due to the relatively low melting point, which is important for producing multi-grain tips for highly reproducible enhancement. Experimental results of this topics will be shown for bio-photonic nano-imaging.

Recently, a new field of resonant dielectric nanostructures has emerged demonstrating a huge promise to substitute plasmonic elements with low-loss high-refractive index dielectric materials. A unique advantage of dielectric nanostructures over nanometallic or plasmonic structures is the low dissipative losses which provide new and competitive alternatives for nanoantennas and metamaterials. Another unique peculiarity of high-refractive index dielectric nanoparticles is their ability to scatter light unidirectionally, i.e. mainly in a preferred direction. This property is a consequence of far-field Kerker-type interference of electric and magnetic dipoles that can be excited coherently inside such nanoparticles. These high-refractive index dielectric nanostructures can substitute plasmonics in some applications. Resonant properties of dielectric particles with high refractive index together with weak dissipation permit to form efficient dielectric antennas and 2D metasurfaces. These 2D artificial interfaces can be designed to possess specialized electromagnetic properties which do not occur in nature. For example recently it was shown that such metasurfaces permit to realize generalized Brewster effect for any polarization and arbitrary angle of incidence. Structured dielectric surfaces permit to provide locally phase control on subwavelength scale which yields many promising applications. The last but not least we should mention an anapole mode which can be viewed as a composition of electric and toroidal dipole moments, resulting in destructive interference of the radiation fields due to similarity of their far-field scattering patterns. Such anapole excitations exist in most of high-index dielectric nanostructures with relatively large size parameter.