There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

Since its discovery, the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional symmetric resonance curves. Recently, the Fano resonance has been found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.

The Fano resonance in plasmonic nanostructures and metamaterials

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation in photonic crystal fiber

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

In order to transfer this gwd noise properties to high output powers two methods we investigated and compared The injection locking of a high power ring slave lasers to the stable master oscillator is well known 45 W of single frequency output has beer demonstrated by us applying SO0 rnW from a NPRO. The resonance of the slave is kept in the vicinity of the master by the Pound-Drever sideband technique.

High Power Low Intensity Noise Single Frequency Light Sources at 1064 nm

A comprehensive material study of different transparent conductive oxides (TCOs) is presented. The layers are deposited by pulsed direct current (DC) magnetron sputtering in an inline sputtering system. Indium tin oxide (ITO) films are studied in detail. The optimum pressure of 0.33 Pa (15Ar:2O2) produces a 300-nm thin film with a specific resistivity \rho of 2.2 \times 10-6 m and a visual transmittance of 81%. Alternatively, ZnO:Al and ZnO:Ga layers with thicknesses of 200 and 250 nm are deposited with a minimum resistivity of 5.5 \times 10-6 and 6.8 \times 10-6 m, respectively. To compare the optical properties in the ultraviolet (UV) range, the optical spectra are modeled and the band gap is determined.

Pulsed DC magnetron sputtering of transparent conductive oxide layers

High power supercontinuum generation based on an ultrafast fiber system

Here, we report on a monolithic continuous wave fiber laser at 1080.5 nm in an Yb-doped large mode area (LMA) fiber realized via a femtosecond inscribed FBG with an output power of 514 W, only limited by the pump power.

514 W monolithic fiber laser with a femtosecond inscribed fiber Bragg grating

It is shown how geometrical optical optimization with ZEMAX can be used to achieve diffraction limited THz systems with the highest possible resolution.

Andreas Tünnermann

Papers

High Power Low Intensity Noise Single Frequency Light Sources at 1064 nm

Pulsed DC magnetron sputtering of transparent conductive oxide layers

High power supercontinuum generation based on an ultrafast fiber system

514 W monolithic fiber laser with a femtosecond inscribed fiber Bragg grating

Optimized THz Systems for Imaging and Spectroscopic Applications