Tailoring the properties of optical metamaterials

In this contribution we review our latest achievements of combined experimental and theoretical studies to tailor the properties of optical metamaterials(MMs) at will. We give three examples of metamaterial designs that have been realized by means of electron-beam lithography and whose spectroscopic characteristics have been comprehensively investigated. In every case, our experiments are complemented by rigorous numerical simulations. Particular emphasis is put on the significance of such tailored effective properties of optical MMs.

Optical metasurfaces:fundamentals and applications

Optical metasurfaces are currently an important research area all around the world because of their wide application opportunities in imaging,wavefront engineering,nonlinear optics,quantum information processing,just to name a few.The feature issue“Optical Metasurfaces:Fundamentals and Applications”in Photonics Research allows for archival publication of the most recent works in optical metasurface and provides for broad dissemination in the photonics community.

Spatially engineered nonlinearity in resonant metasurfaces

Spatial engineering of the nonlinear susceptibility χ^((2)) in resonant metasurfaces offers a new degree of freedom in the design of the far-field response of second-harmonic generation(SHG). We demonstrate this by applying electric field poling to lithium niobate(LN) thin films, which inverts the spontaneous polarization and thus the sign of χ^((2)). Metasurfaces fabricated in periodically poled LN films reveal the distinct influence of theχ^((2))-patterning on the spatial distribution of the second harmonic. This work is a first step toward far-field engineering of SHG in metasurfaces with electric field poling.

Dielectric metasurfaces for distance measurements and three-dimensional imaging

Ultrathin metasurfaces have shown the capability to influence all aspects of light propagation.This has made them promising options for replacing conventional bulky imaging optics while adding advantageous optical properties or functionalities.We demonstrate that such metasurfaces can also be applied for single-lens three-dimensional(3-D)imaging based on a specifically engineered point-spread function(PSF).Using Huygens’metasurfaces with high transmission,we design and realize a phase mask that implements a rotating PSF for 3-D imaging.We experimentally characterize the properties of the realized double-helix PSF,finding that it can uniquely encode object distances within a wide range.Furthermore,we experimentally demonstrate wide-field depth retrieval within a 3-D scene,showing the suitability of metasurfaces to realize optics for 3-D imaging,using just a single camera and lens system.

Material-specific high-resolution table-top extreme ultraviolet microscopy

Microscopy with extreme ultraviolet(EUV)radiation holds promise for high-resolution imaging with excellent material contrast,due to the short wavelength and numerous element-specific absorption edges available in this spectral range.At the same time,EUV radiation has significantly larger penetration depths than electrons.It thus enables a nano-scale view into complex three-dimensional structures that are important for material science,semiconductor metrology,and next-generation nano-devices.Here,we present high-resolution and material-specific microscopy at 13.5 nm wavelength.We combine a highly stable,high photon-flux,table-top EUV source with an interferometrically stabilized ptychography setup.By utilizing structured EUV illumination,we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm.Moreover,we propose mixed-state orthogonal probe relaxation ptychography,enabling robust phase-contrast imaging over wide fields of view and long acquisition times.In this way,the complex transmission of an integrated circuit is precisely reconstructed,allowing for the classification of the material composition of mesoscopic semiconductor systems.

Near-field launching and mapping unidirectional surface plasmon polaritons using an automated dual-tip scanning near-field optical microscope

The spatial distribution of electromagnetic fields emitted from the aperture tip of a scanning near-field optical microscope(SNOM), which is called the emission pattern, depends on the geometry of the apex and the material composition of the tip’s coating. In previous works, experimental measurements of the emission pattern from the aperture tip were performed mostly in the far field. Moreover, the corresponding theoretical models were also developed based on these far-field measurements. Here, we have used the automated dual-tip SNOM to systematically characterize the emission from the aperture tip in the near field. In this regard, we have considered three different pairs of excitation and detection tips with distinct geometries. The emission patterns of the excitation tips were mapped using detection tips. Unidirectional surface plasmon polaritons(SPPs) at the surface of a gold platelet were launched by an excitation tip and measured in the near field by the detection tip. The experimental results were numerically reproduced by means of the Bethe–Bouwkamp model. This work puts into evidence the applicability of the automated dual-tip SNOM as the only available characterization technique to measure the emission from aperture tips in the near field. The reported asymmetric SPP radiation patterns can find applications in photonic integrated circuits or in biological and chemical sensing.

Nonlinear quantum spectroscopy with parity-time-symmetric integrated circuits

We propose a novel quantum nonlinear interferometer design that incorporates a passive parity-time(PT)-symmetric coupler sandwiched between two nonlinear sections where signal-idler photon pairs are generated.The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation,whose absorption can be inferred by measuring only the signal intensity at a shorter wavelength where efficient detectors are readily available.Remarkably,we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values,which is distinct from the previously studied PT symmetry breaking.We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.