Nonlinear Optics: feature issue introduction

: This joint issue of Optics Express and Optical Materials Express features 18 state-of-the art articles that witness actual developments in nonlinear optics, including those by authors who participated in the international conference Nonlinear Optics held in Waikoloa, Hawaii from July 15 to 19, 2019. As an introduction, the editors provide a summary of these articles that cover all aspects of nonlinear optics, from basic nonlinear eﬀects and novel frequency windows to innovative nonlinear materials and devices, thereby paving the way for new nonlinear optical concepts and forthcoming applications. (NLO)


Introduction
Vol. 10, No. 3 / 1 March 2020 / Optical Materials Express unpublished work, and we follow the regular review process for the journals with its clear criteria of scientific value and impact being independent of the conference review.
We hope readers will enjoy this issue with the resulting 19 top-level articles that highlight the state of the art in the field. We are thankful to all of the authors and reviewers for their contribution and commitment to the field. We also would like to thank Carmelita Washington from the OSA staff for their outstanding support for making this feature issue in all its steps a success.
The topical issue follows the categories that are typically falling into three categories: 1. Fundamental Studies and New Concepts including topics from nonlinear light propagation and quantum nonlinear optics to ultrafast phenomena, 2. Nonlinear Materials including fields as cold atoms, dielectric, semiconductor, metallic and polymer material as well as nanostructured and integrated systems, and 3. Applications, covering such diverse path breaking fields as data storage, quantum information processing, artificial intelligence, or biomedical nonlinear optical devices.
Following a trend in the last NLO conferences as well as in the optics, photonics and laser community, a number of papers address complex basic effects in applied information processing devices. Such, solitons [1,2] or supercontinuum generation [3,4] have been investigated in fiber laser and amplifier systems as well as at telecommunication wavelengths. Also, quantum encryption and communication takes advantage from higher order nonlinear effects [5].
A related central theme of NLO is ultrafast nonlinear optical phenomena that are also heading for applications as ultrafast nonlinear refraction measurements [6,7], few cycle pulses in integrated nonlinear photonic circuits [8], or ultrashort pulses in epsilon-near-zero metamaterials at the telecom wavelength [9].
The advent of nanoscaled and structured materials paves the way to tomorrows' applications of nonlinear optics, as thin film semiconductor nonlinear platforms [10], integrated parametric oscillators included into two-dimensional materials [11], or nonlinear optical circuits based on complex light two-photon absorption for inspection in microelectronics [12]. In turn, nonlinear optical effects itself as e.g. z-scan can be developed into tools that allow characterizing optically relevant parameters of solutions and dyes [13].
With increasing application relevance, questions of bandwidth, phase-sensitivity, and fluctuations gain more importance. Thus, control of bistable signals [14] in resonators, bandwidth in optical networks [15], phase modulation in interconnection and network transmission [16], fluctuations in fiber lasers [17], or self-phase modulation in two-dimensional materials [18] are all based on nonlinear optical light matter interaction, especially third order wave mixing effects.
With the upcoming 60th birthday of the laser in 2020, the field of Nonlinear Optics is entering its 7th decade of innovations in optics and photonics. Nonlinear optics is not only as vibrant and stimulating as ever before, but is becoming more and more groundbreaking, being a trendsetter that fosters applications of disruptive technologies as quantum technologies, machine learning, or brain science within optics and photonics.
Thus, we can be curiously looking forward to the next OSA Nonlinear Optics conference that surely will continue this exiting trend, to be held in 2021 again in Waikoloa, Hawaii, USA.