Over the last year we have been celebrating the 20th Anniversary of Optics Express, which published its first issue in July 1997. The all-electronic, open access model introduced by Optics Express proved to be highly successful. Despite competition from the stream of new open access publications that have since entered the optics and photonics space, Optics Express remains vigorous: we received more than 6,500 submissions in 2017, our most ever, and published more than 3,000 papers! We have an outstanding editorial board, which now includes more than 125 scientists and engineers, and maintain a very fast editorial and production process (2017 median time to publication: only 65 days). Optics Express enjoys a Google Scholar h-5 index of 103, which gives it a #2 ranking among publications in Optics & Photonics.

In a previous editorial I announced a number of special features in celebration of this anniversary year [1]. These include guest editorials from our previous Editors-in-Chief [2–4], Editors’ Picks Collections representative of important topic areas that have been covered by the journal, and a list of our 100 most cited articles. These and other materials may be found at our Anniversary web site.

Finally, we have recruited 14 distinguished authors to write invited review or perspective articles, which have the potential to benefit our research community for years to come. The last four, from lead authors Christoph Hitzenberger, Jian-Wei Pan, Markus Pollnau, and Peter Winzer, are being published along with this editorial. I am excited now to introduce the full list of these articles, which have been released over the past several months and are compiled into a virtual feature issue. Below are short summaries of the articles, grouped into five categories.

1. Lasers, optical amplifiers, and laser optics

Frank Wise of Cornell University and colleagues have prepared a perspective entitled “Several New Directions for Ultrafast Fiber Lasers” [5], accompanied by an extensive bibliography numbering nearly 250 entries. Wise et al. point out that although nonlinearity is a key ingredient for mode-locking, mode-locked lasers have traditionally incorporated “just enough nonlinearity to obtain short pulses, and no more.” However, this philosophy limits attainable peak intensities, particularly for fiber laser systems. The article begins with a discussion of mode-locked fiber lasers that embrace new modes of highly nonlinear operation to enhance pulse energies by orders of magnitude, then moves on to provide a perspective on frontier topics such as nonlinear pulse propagation in multi-mode fibers with possible application to mode-locking in both space and time.

The second paper in this category is entitled “The Harmonic State of Quantum Cascade Lasers: Origin, Control, and Prospective Applications” [6]. Written by Federico Capasso of Harvard University and a group of collaborators, this paper takes a deep dive into a current hot topic involving multimode laser operation. Multimode oscillation of interband semiconductor lasers is well known and has been extensively studied. However, the unipolar, intraband character of quantum cascade lasers (QCLs) brings new physics, such as ultrafast gain recovery dynamics, which may lead to new multimode oscillation phenomena. The so-called “harmonic state” refers to an intriguing multimode regime characterized by oscillation on a series of widely spaced longitudinal modes separated by many free spectral ranges (FSRs), distinct from the more common dense multimode state, which features oscillation on adjacent FSRs. The authors report experimental and theoretical investigations into the physics of this state and discuss potential applications such as short pulse generation and broadband spectroscopy in the mid-infrared.

2. Nanophotonics, metamaterials, and photonic crystals

Metasurfaces consisting of intricate arrangements of subwavelength-thick metallic or dielectric elements offer exciting prospects both for ultrathin optics and for realization of functionalities difficult or impossible to achieve using conventional optical technologies. Since the introduction of metasurfaces less than one decade ago, metasurface research has grown explosively. Din Ping Tsai of National Taiwan University and Academia Sinica, Taiwan, and his coauthors have written a review paper for Optics Express’s 20th Anniversary celebration entitled “Advances in optical metasurfaces: fabrication and applications” [7]. As suggested by the title, the article first surveys a wide range of fabrication methods relevant to metasurfaces and then reviews examples of cutting edge applications involving wavefront shaping, polarization control, and active tuning. Functionalities involving chirality, such as multispectral chiral imaging, appear to be particularly distinctive relative to traditional optics. Throughout the article, the discussion is illustrated by collections of striking images taken from a wide variety of sources.

Two-dimensional (atomically thin, single layer) materials are the subject of enormous research activity in condensed matter physics, electronics, optics, and other fields. The discovery of the first two-dimensional material, graphene, was recognized with a Nobel Prize in 2010. A host of additional two-dimensional materials have since emerged. In an article entitled “Nanophotonics with 2D transition metal dichalcogenides” [8], Andrea Alù of the University of Texas at Austin and the City University of New York and colleagues review the single-layered semiconductors WS₂, WSe₂, MoS₂, and MoSe₂. These single-layered transition metal dichalcogenides (1L-TMDCs) are of particular interest in optics due to their direct bandgaps in the visible range, which brings strong interactions with light and interest in potential applications such as photodetection, light harvesting, light emitters, phototransistors, and emerging areas such as valleytronics. Alù and his colleagues provide a tutorial review on the optical properties of the 1L-TMDCs and their incorporation into plasmonic and dielectric nanocavities, including research showing cavity-enhanced emission of light and photon-matter interaction entering the strong coupling regime.

Also pertaining to nanophotonics is a review article by Yuri Kivshar of Australian National University and a coauthor, entitled “Generalized Kerker effects in nanophotonics and meta-optics” [9]. The Kerker effect refers to a paper by Milton Kerker et al., published in the Journal of the Optical Society of America in 1983, which studied electromagnetic scattering from a magnetic sphere with µ≠1 [10]. The study showed that when ε = µ, the backscattering can be totally eliminated, a surprising finding that can be attributed to destructive interference of electric and magnetic multipole scattering. However, because naturally occurring materials do not exhibit appreciable magnetic effects in the optical regime, this result was not of immediate impact. But with the discovery of artificial magnetism in optical metamaterials, as well as high index dielectric particles, there is a resurgence of interest in manipulation of scattering effects through constructive and destructive interference of electric and magnetic scattering contributions, including both dipolar and higher order contributions. This article reviews such effects in generalized structures, including particles of arbitrary shape and particle clusters, as well as periodic arrangements of such elements (metalattices), and highlights intriguing possibilities such as perfect transmission, reflection, and absorption.

The science and engineering of light at the nanoscale can have significant impact in the area of sustainable energy. In an article entitled “Nanophotonic control of thermal radiation for energy applications” [11], Shanhui Fan and coauthor at Stanford University provide a broad review of opportunities to achieve and exploit thermal radiation properties that are drastically different from those of conventional thermal emitters. Whereas “conventional thermal radiation is incoherent, broadband, un-polarized and near-isotropic in its directionality,” the authors explain that nanophotonic structures can be designed to achieve increased coherence of the radiation and to tailor its spectrum, polarization, and directionality. Exotic ideas such as rapid, dynamic control of thermal radiation and violation of the usual equivalence between absorptivity and emissivity through nonreciprocity are also explored. Finally, several fascinating applications concepts, including daytime radiative cooling, thermal textiles to enhance cooling (summer) or heating (winter) at the human body scale, and thermophotovoltaic and thermophotonics systems that harness and convert waste heat into electricity are introduced, and state-of-the-art research exploiting nanophotonics is reviewed.

3. Imaging systems, microscopy, and displays

William Moerner of Stanford University and his collaborators have prepared a comprehensive review of “Light sheet approaches for improved precision in 3D localization-based superresolution imaging in mammalian cells” [12]. The article introduces two hot topics in modern fluorescence microscopy, single-molecule super-resolution microscopy and light sheet illumination, and then discusses progress towards combining these methods for enhanced three-dimensional superresolution. Optical detection of single molecules and localization of their positions with precision much finer than the diffraction-limited point spread function was recognized with the 2014 Nobel Prize in Chemistry, awarded jointly to Prof. Moerner, Dr. Eric Betzig, and Prof. Stefan Hell. However, the achieved superresolution applies mainly in two dimensions. Light sheet illumination, “where the sample is optically sectioned by a sheet of light illuminating the image plane of the detection optics,” provides a method to capture superresolved 3D images in relatively thick samples such as mammalian cells, while achieving critical reductions in photobleaching and background fluorescence from emitters outside of the image plane.

Next, an article by Dan Mittleman of Brown University is entitled “Twenty Years of Terahertz Imaging” [13]. Mittleman points out that the field of terahertz (THz) imaging began around the same time as the launch of Optics Express and “has for the most part grown up alongside Optics Express.” Furthermore, Optics Express has been a highly attractive venue for papers in this field. As noted by Mittleman, “a quick search of the phrase ‘terahertz imaging’…brings up over 300 articles.” This represents roughly a quarter of all the articles on “terahertz” published in Optics Express. After providing a historical background, the article gives a focused discussion on a selection of recent hot topics, including the push toward faster image acquisition, near-field THz imaging, and applications in nondestructive investigation of works of art and historical artifacts.

Christoph Hitzenberger, one of the pioneers of optical coherence tomography (OCT) technology and current Editor-in-Chief of Biomedical Optics Express, has contributed a paper entitled “Optical coherence tomography in Optics Express” [14]. Hitzenberger notes: “Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express.” This article presents data on OCT publication patterns in Optics Express and other journals and explores reasons why many researchers chose to submit some of their best OCT work to Optics Express. The data suggest that rapid time-to-publication and the ability to publish multimedia content, both of which were unique or nearly unique to Optics Express at a key period in OCT technology development, were especially important motivators.

4. Optical communications and interconnects

Fiber optic communications is an outstanding success story both in its impact on society and in its incredible record of technological innovation. In an ambitious article entitled “Fiber-optic transmission and networking: the previous 20 and the next 20 years” [15], Peter Winzer and his colleagues David Neilson and Andrew Chraplyvy of Nokia Bell Labs recount many of the advances that enabled a factor of 1000 growth in the capacity of wavelength-division multiplexed transmission systems and provide their perspective on an equally exciting future. Within the last 20 years, fiber optics systems design has transitioned from a discipline dominated by device physics and engineering (e.g., better lasers, higher speed optoelectonics, etc.) to a discipline dominated by communications engineering principles (e.g., advanced modulation formats, digital signal processing, error correcting codes). The article presents data on optical technology trends and technology drivers (e.g., aggregate data center footprints, switch capacities vs. year) that both illustrate the past and inform future needs. Finally, the authors discuss space division multiplexing, explored in research labs for about a decade, as the principal new dimension that will sustain further exponential improvements in fiber system capabilities to meet networking needs.

Quantum science and engineering is receiving tremendous attention due to its potential for revolutionary impacts on information technology. One of the earliest recognized opportunities, quantum key distribution (QKD), offers communications security grounded in the laws of physics. Photons play an essential role in QKD due to their natural use for signal transmission. Jian-Wei Pan, Qiang Zhang, and colleagues at University of Science and Technology of China have prepared a paper for the Optics Express 20th Anniversary Celebration entitled “Large scale quantum key distribution: challenges and solutions” [16]. After a brief introduction, the authors first discuss “secure QKD with imperfect devices,” focusing on so-called decoy state and measurement-state independent QKD approaches, which respectively circumvent eavesdropper attacks targeted at nonideal photon transmitters and receivers. They then address the challenge of achieving long-distance QKD despite the extreme fragility of quantum states with regard to loss and report on demonstrations based on satellite and fiber networks that illustrate a “simple prototype for a global quantum communication network.”

In an article entitled “Photonics switching in high performance data centers” [17], Keren Bergman and coauthors at Columbia University review optical technology alternatives and comment on the performance metrics they will have to satisfy to earn wide deployment. The continued growth in server performance and bandwidth demands in data centers, high power consumption, and severe limitations in high speed electrical transmission over distances of even several centimeters provide exciting opportunities for photonics. Pluggable optical transceiver and optical switching bandwidth density are both rapidly increasing. Due to lack of optical buffering technologies, the authors believe that if optical switching is adopted in data centers, it will be used in combination with conventional electronic switches. They also suggest that in order to be competitive, the cost associated with adding optical switching to provide reconfigurable communication links must be considerably lower than the cost of adding additional links, and they provide a perspective on the roles of factors such as switching speed, optical power penalty, electrical power consumption, and port count in establishing the value of optical switching.

5. Integrated optics

Silicon photonics, in which photonic devices are realized in a silicon platform, has received massive attention in recent years. Leveraging high-volume Si CMOS manufacturing platforms promises to provide high process fidelity and low cost for photonics integration with tight coupling to electronics at a systems-on-a-chip level. In an article entitled “Monolithic silicon-photonic platforms in state-of-the-art CMOS SOI processes” [18], Vladimir Stojanović of the University of California, Berkeley, and coauthors at several institutions begin by summarizing and comparing several monolithic and non-monolithic silicon-on-insulator (SOI) technology approaches. They then provide a detailed survey of their own “zero change” approach to silicon photonics using 32 nm and 45 nm process nodes. The development trajectory of silicon photonics is discussed primarily with reference to photonics interconnects, the first potentially high-volume application envisioned for this technology. However, with adoption of silicon photonics by major foundries, the prospect of low-cost, “high connection density between transistors and photonics” may also enable a variety of other emerging applications, such as lidar, or even new applications not yet envisioned.

An important challenge for silicon photonics is how to achieve on-chip optical gain and lasing. In a paper entitled “Optically pumped rare-earth-doped Al₂O₃ distributed-feedback lasers on silicon” [19], Markus Pollnau of the University of Surrey and coauthor Jonathan Bradley of McMaster University have reviewed one of the main approaches. These lasers consist of an amorphous Al₂O₃ layer doped with Er, Yb, Tm, or Ho ions and deposited on top of either a Si or Si₃N₄/Si platform. Although such rare-earth doped lasers are less efficient than hybrid III-V semiconductor lasers on silicon, which constitute the other primary approach, and require extra effort to achieve high gain, factors such as lower propagation loss in the dielectric materials also lead to an important advantage, namely ultranarrow linewidths down to the few KHz range. The article provides a detailed discussion of the structure, fabrication, and performance of such rare-earth-doped waveguide lasers, then concludes with intriguing examples of new capabilities that may become possible by exploiting active and passive designs with high-resolution features on a single platform.

Once again, I would like to express my appreciation to a number of individuals who helped to make our Anniversary celebration possible, including selection of Editors’ Picks and recruitment and processing of invited reviews. Senior Deputy Editor Jim Leger led these selections, assisted by current and former Deputy Editors Miguel Alonso, Chris Dainty, John Dudley, Magnus Karlsson, Guifang Li, Takeshige Omatsu, Vitor Schneider, and Christian Seassal, as well as the Editor-in-Chief of Optical Materials Express, Sasha Boltasseva. I am also very grateful to the authors of the 14 review articles who found time in their busy schedules to contribute to this celebration. Finally, I would like to specifically thank members of the OSA Publications staff for their contributions to many aspects of the Anniversary celebrations: Kelly Cohen, John Long, Bob Sumner, Sika Dunyoh, Julie Rovesti, Daphne Greenwood, Rebecca Robinson, Sharon Jeffress, Jennifer Mayfield, and Marisol Velez.

I hope you have enjoyed our look back at the first 20 years of Optics Express, and I look forward to many more successful years of the journal serving the optics and photonics community.