Band 4, Heft 2 - Special Issue: Emerging Materials for Nanophotonics / Guest Editors: Jenifer Dionne, Alexandra Boltasseva and Luke Sweatlock

The year 2015 will likely have a unique place in the history books for the optics and photonics community, since it is paired with various events that are exciting for this field. For one it is the 125th birthday of the Optical Society (OSA), and in addition, the United Nations declared 2015 to be the Year Of Light. The first special issue of this year is dedicated to the topic of “Emerging Materials on Nanophotonics”. While the field of nanophotonics has seen tremendous momentum through the support of plasmonics, opto-mechanics, and quantum photonics, it often are both the breakthroughs and continuing developments of materials that bring enabling opportunities for this field. For instance, the area of 2D materials has grown out of its infancy being focused on Graphene into a crossdisciplinary subject area. Here, both scientific and engineering potential are seen in a) novel physical effects, b) higher functionality, and c) smaller form factors all found in one material option. Coincidentally, theUSNational Science Foundation recently held a path findingworkshop on 2D materials Beyond Graphene, and followed through with a dedicated two-year program to fund engineering innovations of the same. Here, the bandgap tunability of trimetal Dichalcogenides (TMD) has found to bear rich bandgap tunability via composition, alloying, and altering design options such as substrate choices or stress, thus providing a large variety of functions. In this context it is interesting to note, that with the many material choices for TMDs, the importance of targeted approaches towards accelerated material-to-marketwas raised in theMaterial Genome Initiative by the US White House. However, with the fundamental challenge of nanophotonics – weak interactions between light and matter – the choice of materials as both device building block and functionality delivery option needs to be synergistically considered. In this regard metal optics is seen as an emerging field that is able to contribute to this design evolution of devices and systems with ever growing constrains. However, materials with new functionalities and *Corresponding Author: Volker Sorger: E-mail: sorger@email.gwu.edu form factors allow utilizing field enhancement techniques in an unprecedented way. This, for instance, enables subwavelength scale photonic and opto-electronic devices with performance improvements such as utilized by the Purcell effect in light emitters, detectors, or electro-optic switching devices. On the other hand, certain novel materials are able to clearly outperform any existing option; for instance transparent-conductive-oxides (TCO) have been found to be able to alter its refractive index by unity. Lastly, with the maturing of silicon photonics as an on-chip optics platform, higher integration options are considered in this special issue; passive devices such as waveguides made out of the electro-optically active Lithium Niobate aid highfunctionality systems on-chip. However, these novel materials and subsequent devices and systems need to be compared and benchmarked in order to be a guide for the next phase of opto-electronic integration and other technologies as carried out by some contributions of this special issue.As the festivities around this Year Of Light continue, this special issue summarizes some of the interesting work around the emerging materials for nanophotonics. Concluding, I would like to thank for the input and help of the fellow Guest Editors, Jenifer Dionne, Alexandra Boltasseva, and Luke Sweatlock along with the Nanophotonics staff, Dennis Couwenberg and Tara Dorrian. Sincerely

Layered materials, such as graphene, transition metal dichacogenides and black phosphorus have attracted lots of attention recently. They are emerging novel materials in electronics and photonics, with tremendous potential in revolutionizing the traditional electronics and photonics industry. Marrying layered material to the nanophotonics is being proved fruitful. With the recent emphasis and development of metasurfaces in nanophotonics, atomically thin materials can find their unique position and strength in this field. In this article, I will focus on one specific two dimensional material: bilayer graphene. Basic physics will be reviewed, such as band-gap opening, electron-phonon interaction, phonon-plasmon interaction and Fano resonances in the optical response. Moreover, I will review the application of bilayer graphene as a sensitive and fast photodetector. An outlook will be given in the final part of the paper.

In this article, we review the various topics on the applications of 2D materials, including both elemental and compound 2D materials, for nanophotonics application from detectors, modulators to plasmonics and light generating devices. With this review, we hope to provide an overview of the past development in this field while offering our perspectives on its future directions.

Silicon photonics has been established as a mature and promising technology for optoelectronic integrated circuits, mostly based on the silicon-on-insulator (SOI) waveguide platform. However, not all optical functionalities can be satisfactorily achieved merely based on silicon, in general, and on the SOI platform, in particular. Long-known shortcomings of silicon-based integrated photonics are optical absorption (in the telecommunication wavelengths) and feasibility of electrically-injected lasers (at least at room temperature). More recently, high two-photon and free-carrier absorptions required at high optical intensities for third-order optical nonlinear effects, inherent lack of second-order optical nonlinearity, low extinction ratio of modulators based on the free-carrier plasma effect, and the loss of the buried oxide layer of the SOI waveguides at mid-infrared wavelengths have been recognized as other shortcomings. Accordingly, several novel waveguide platforms have been developing to address these shortcomings of the SOI platform. Most of these emerging platforms are based on heterogeneous integration of other material systems on silicon substrates, and in some cases silicon is integrated on other substrates. Germanium and its binary alloys with silicon, III–V compound semiconductors, silicon nitride, tantalum pentoxide and other high-index dielectric or glass materials, as well as lithium niobate are some of the materials heterogeneously integrated on silicon substrates. The materials are typically integrated by a variety of epitaxial growth, bonding, ion implantation and slicing, etch back, spin-on-glass or other techniques. These wide range of efforts are reviewed here holistically to stress that there is no pure silicon or even group IV photonics per se . Rather, the future of the field of integrated photonics appears to be one of heterogenization, where a variety of different materials and waveguide platforms will be used for different purposes with the common feature of integrating them on a single substrate, most notably silicon.

Abstract: The ongoing quest for ultra-compact optical devices has reached a bottleneck due to the diffraction limit in conventional photonics. New approaches that provide subwavelength optical elements, and therefore lead to miniaturization of the entire photonic circuit, are urgently required. Plasmonics, which combines nanoscale light confinement and optical-speed processing of signals, has the potential to enable the next generation of hybrid information-processing devices, which are superior to the current photonic dielectric components in terms of speed and compactness. New plasmonic materials (other than metals), or optical materials with metal-like behavior, have recently attracted a lot of attention due to the promise they hold to enable low-loss, tunable, CMOScompatible devices for photonic technologies. In this review, we provide a systematic overview of various compact optical modulator designs that utilize a class of the most promising new materials as the active layer or core— namely, transparent conducting oxides. Such modulators can be made low-loss, compact, and exhibit high tunability while offering low cost and compatibility with existing semiconductor technologies. A detailed analysis of different configurations and their working characteristics, such as their extinction ratio, compactness, bandwidth, and losses, is performed identifying the most promising designs.

Abstract: Molecular plasmonics explores and exploits the molecule–plasmon interactions on metal nanostructures to harness light at the nanoscale for nanophotonic spectroscopy and devices. With the functional molecules and polymers that change their structural, electrical, and/or optical properties in response to external stimuli such as electric fields and light, one can dynamically tune the plasmonic properties for enhanced or new applications, leading to a new research area known as active molecular plasmonics (AMP). Recent progress in molecular design, tailored synthesis, and self-assembly has enabled a variety of scenarios of plasmonic tuning for a broad range of AMP applications. Dimension (i.e., zero-, two-, and threedimensional) of the molecules on metal nanostructures has proved to be an effective indicator for defining the specific scenarios. In this review article, we focus on structuring the field of AMP based on the dimension of molecules and discussing the state of the art of AMP. Our perspective on the upcoming challenges and opportunities in the emerging field of AMP is also included.

Abstract: Advances in opto-electronics are often led by discovery and development of materials featuring unique properties. Recently, the material class of transparent conductive oxides (TCO) has attracted attention for active photonic devices on-chip. In particular, indium tin oxide (ITO) is found to have refractive index changes on the order of unity. This property makes it possible to achieve electrooptic modulation of sub-wavelength device scales, when thin ITO films are interfaced with optical light confinement techniques such as found in plasmonics; optical modes are compressed to nanometer scale to create strong light-matter interactions. Here we review efforts towards utilizing this novel material for high performance and ultra-compact modulation. While high performance metrics are achieved experimentally, there are open questions pertaining to the permittivity modulation mechanism of ITO. Finally, we review a variety of optical and electrical properties of ITO for different processing conditions, and show that ITO-based plasmonic electro-optic modulators have the potential to significantly outperform diffractionlimited devices.

Abstract: Graphene has recently emerged as a viable platform for integrated optoelectronic and hybrid photonic devices because of its unique properties. The optical properties of graphene can be dynamically controlled by electrical voltage and have been used to modulate the plasmons in noble metal nanostructures. Graphene has also been shown to support highly confined intrinsic plasmons, with properties that can be tuned in the wavelength range of 2 μm to 100 μm. Here we review the recent development in graphene-plasmonic devices and identify some of the key challenges for practical applications of such hybrid devices.