Abstract The coupling between light and collective oscillations of free carriers at metallic surfaces and nanostructures is at the origin of one of the main fields of nanophotonics: plasmonics. The potential applications offered by plasmonics range from biosensing to solar cell technologies and from nonlinear optics at the nanoscale to light harvesting and extraction in nanophotonic devices. Heavily doped semiconductors are particularly appealing for the infrared spectral window due to their compatibility with microelectronic technologies, which paves the way toward their integration in low-cost, mass-fabricated devices. In addition, their plasma frequency can be tuned chemically, optically, or electrically over a broad spectral range. This review covers the optical properties of the heavily doped conventional semiconductors such as Ge, Si, or III–V alloys and how they can be successfully employed in plasmonics. The modeling of their specific optical properties and the technological processes to realize nanoantennas, slits, or metasurfaces are presented. We also provide an overview of the applications of this young field of research, mainly focusing on biosensing and active devices, among the most recent developments in semiconductor plasmonics. Finally, an outlook of further research directions and the potential technological transfer is presented.

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Semiconductor infrared plasmonics

Nanometer-thick active metasurfaces (MSs) based on phase-change materials (PCMs) enable compact photonic components, offering adjustable functionalities for the manipulation of light, such as polarization filtering, lensing, and beam steering. Commonly, they feature multiple operation states by switching the whole PCM fully between two states of drastically different optical properties. Intermediate states of the PCM are also exploited to obtain gradual resonance shifts, which are usually uniform over the whole MS and described by effective medium response. For programmable MSs, however, the ability to selectively address and switch the PCM in individual meta-atoms is required. Here, simultaneous control of size, position, and crystallization depth of the switched phase-change material (PCM) volume within each meta-atom in a proof-of-principle MS consisting of a PCM-covered Al-nanorod antenna array is demonstrated. By modifying optical properties locally, amplitude and light phase can be programmed at the meta-atom scale. As this goes beyond previous effective medium concepts, it will enable small adaptive corrections to external aberrations and fabrication errors or multiple complex functionalities programmable on the same MS.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201901033

Advanced Optical Programming of Individual Meta-Atoms Beyond the Effective Medium Approach.

In this article, we revisit bonding in crystalline GeTe, a simple binary alloy that is also a popular Phase Change Material, and use an ab initio approach that goes beyond the usual one electron description obtained with Density Functional Theory. By considering the electron pair density, we obtain a measure of the number of pairs of electrons that are shared between neighbors. Employing the charge transfer between adjacent atoms as the second quantifier of chemical bonding, we obtain a map which separates ionic, covalent and metallic bonding. Interestingly, GeTe is not located in any of these regions, but instead is located in a region where materials with a peculiar set of properties prevails. The corresponding materials have been coined incipient metals and their bonding ‘metavalent bonding’ (MVB). They often possess a Peierls distortion, which stabilizes the rhombohedral crystal structure by breaking the cubic symmetry. For these materials, the electron population of longer and shorter bonds is close to one-half, and charge transfer between adjacent atoms is quasi-independent of the degree of distortion. The energy gained by the Peierls distortion is much smaller than the energy gained by creating the cubic structure, delocalizing one electron over two bonds. Such Peierls distortions are not observed for aromatic compounds which utilize resonant bonding and have properties which differ significantly from the property portfolio of metavalently bonded materials. This stresses the difference between metavalent bonding and the resonant valence bond view of aromatic compounds and molecules. MVB is also responsible for the anomalies in dielectric properties and the anharmonicity of the solids. The comparison between PbTe, GeTe and GeS is particularly instructive, showing that bonding in these materials shows interesting differences, where metavalent bonds govern the behavior of PbTe and GeTe, while GeS is dominated by the Peierls distortion.

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The interplay between Peierls distortions and metavalent bonding in IV–VI compounds: comparing GeTe with related monochalcogenides

We review the binding and energy level alignment of $\pi$-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with \textit{ab-initio} calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of $\pi$-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and over the entire range from weak to strong coupling.

Binding and Electronic Level Alignment of $\boldsymbol{\pi}$-Conjugated Systems on Metals

We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

Binding and electronic level alignment of π-conjugated systems on metals.

Erbium-doped fiber amplifiers revolutionized long-haul optical communications and laser technology. Erbium ions could provide a basis for efficient optical amplification in photonic integrated circuits but their use remains impractical as a result of insufficient output power. We demonstrate a photonic integrated circuit–based erbium amplifier reaching 145 milliwatts of output power and more than 30 decibels of small-signal gain—on par with commercial fiber amplifiers and surpassing state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We apply ion implantation to ultralow–loss silicon nitride (Si3N4) photonic integrated circuits, which are able to increase the soliton microcomb output power by 100 times, achieving power requirements for low-noise photonic microwave generation and wavelength-division multiplexing optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of various fiber-based devices such as high–pulse-energy femtosecond mode-locked lasers. Description On-chip optical amplification The success of long-haul optical communications and our information society is largely due to the invention of the erbium-doped fiber amplifier. Because the need for faster chips is expected to see a shift from electronics- to photonics-based technologies, erbium ions could form the basis for amplification in optical integrated circuits. Liu et al. used an ultra-low-loss silicon nitride photonic integrated circuit with a waveguide length up to 0.5 meters and erbium ion implantation to fabricate an erbium-doped waveguide amplifier on a compact photonic chip (see the Perspective by Kim). Operating in the continuous-wave regime and providing large optical gain in the telecommunication bands, the results are promising for device applications. —ISO An erbium-doped optical amplifier is fabricated on a silicon-nitride-based optical platform.

A photonic integrated circuit–based erbium-doped amplifier

Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

where ε∞ is the high-frequency permittivity, ωp is the screened plasma frequency corresponding to plasma wavelength λp, and τ is the scattering rate, which characterizes the loss and is related to carrier mobility (μ = τq/m*). We note that for the highest doping densities, τ can acquire a frequency dependence,[14] which we do not consider in this work. The plasma wavelength (Equation (2)) is a function of the effective mass of the free carriers (m*), carrier concentration (N), unit charge (q), free-space speed of light (c), and free-space permittivity (ε0). The plasma wavelength corresponds to the wavelength at which the real part of the permittivity approaches zero, with the material behaving as a (likely lossy) dielectric at shorter wavelengths, and as a metal at longer wavelengths. Materials such as conducting nitrides and oxides[15–18] and traditional group IV[19–29] and III–V[30–33] semiconductors can have doping-tunable plasma wavelengths spanning the infrared spectrum, and have become promising for plasmonics applications. Some of these materials, e.g., titanium nitride, have the added benefit of high-temperature resistance and CMOS compatibility.[15] Prior work using these materials mainly involves uniformly doped substrates,[24] films,[16,17,33,34] or nanocrystals,[35,36] and structuring is usually achieved through selective etching.[20,33] In this study, we demonstrate the potential of dopingpatterned silicon as a CMOS-compatible platform for midand far-infrared optics and plasmonics. We utilized ion-beam doping in conjunction with lithography to locally tune the optical properties of silicon to create embedded diffractive optical elements (e.g., Fresnel zone plates (FZPs)) and plasmonic components (e.g., frequency selective surfaces (FSSs)) that operate in the mid-to-far-IR regime. The resulting devices are monolithic, flat, resilient to thermal and physical damage, and can be easily integrated into other silicon-based platforms. Highly doped semiconductors are an emerging platform for plasmonic devices. Unlike in noble metals, the carrier concentration of semiconductors can vary by many orders of magnitude, resulting in a widely tunable range of plasma wavelengths spanning the mid-infrared and terahertz ranges. In this work, the potential of highly doped, ion-beam-patterned silicon is demonstrated as a fabrication-friendly platform for flat optical devices. Detailed characterization of the optical properties of silicon is performed at various doping levels, and diffractive optical elements and plasmonic frequency-selective surfaces that operate in the mid-to-far-infrared regime are realized. The resulting optical devices are monolithic, flat, resilient to thermal and physical damage, and can be easily integrated into other silicon-based platforms.

Flat Optical and Plasmonic Devices Using Area-Selective Ion-Beam Doping of Silicon

Active optical metasurfaces with dynamic switchable, tunable, and reconfigurable optical functionalities are an emerging field in photonics and optoelectronics. Especially, chalcogenide-based phase-change materials, such as Ge2Sb2Te5 (GST), can be fast and repeatedly switched by external stimuli between crystalline and amorphous states, typically accompanied by a tremendous difference of the electronic and photonic properties. Here, we demonstrate that focused ion beam-induced disorder in highly confined regions can transform phase-change materials in active optical metasurfaces by locally adjusting the phase. A careful control of the amount of disorder can locally tailor the effective refractive index in GST films on the nanometer scale, which is highly promising for multilevel switching applications. In contrast to direct laser writing, focused ion beam irradiation enables the fabrication of subwavelength-scaled, planar, nonvolatile, and reconfigurable optical metasurfaces, with pattern sizes clearly be...

Metasurfaces Enabled by Locally Tailoring Disorder in Phase-Change Materials

We study the molecular structure of one monolayer of picene on a Ag(100) surface. Low energy electron diffraction and scanning tunneling microscopy experiments show that the molecules arrange in a highly ordered manner exhibiting a point-on-line epitaxy with two differently arranged molecules per unit cell. Comparing measured and simulated photoelectron momentum maps allows further conclusions about the composition of the unit cell. The structural basis consists of two parallel molecules; one molecule lies face-on and the other is tilted by ≈45° around its long axis with respect to the surface normal.

Martin Hafermann

Papers

A photonic integrated circuit–based erbium-doped amplifier

Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

Flat Optical and Plasmonic Devices Using Area-Selective Ion-Beam Doping of Silicon

Metasurfaces Enabled by Locally Tailoring Disorder in Phase-Change Materials

Insight into the unit cell: Structure of picene thin films on Ag(100) revealed with complementary methods.