The concept of cloaking has never failed to fascinate scientific researchers. In recent years, the idea has ignited a whole new trend of research effort when extended to temporal domain. A typical temporal cloak conceals events from probing light by creating a temporal intensity gap (cloaking time) that will later be closed back by manipulating the speed of light leveraging the chromatic dispersion in dielectric media. Therefore, the observer will only see a continuous probing light but will never realize the temporal events that happens during the temporal gap. Previous experimental demonstrations primarily relied on parametric- or phase-modulator (PM)-based time lens to realize temporal cloaking, which had finite temporal aperture and only concealed events at a fixed time period. Consequently, the cloaking time is so far limited to around 200 ps and practical applications are still unforeseeable.Recently, Zhou et al. [1] has demonstrated a programmable temporal cloak with significantly enhanced cloaking time using a brand-new type of time lens. The time lens first generates a coherent broad-band optical frequency comb and a following electrically-tuned microring resonator (ET-MRR) provides linearly-scanned filtering. Consequently, the output is linearly chirped, which is equivalent to a conventional time lens. Most importantly, the ET-MRR not only achieves large modulation depth as the parametric time lenses, it is also programmable by generating arbitrary electrical driving signals. With the combined strength, ET-MRR opens temporal cloaks at arbitrary time and the cloaking time is also tunable from 0.449 to 3.365 ns. The cloaking capacity is 17 times larger than the previous record, finally bringing temporal cloaking technique to the nanosecond regime.Admittedly the current performance might not directly lead to practical applications, but it will definitely inspires more subsequent research effort. The scalability of the cloaking time is related to the bandwidth of frequency comb generator and the free spectral range of the ET-MRR, which has not been fully explored yet. Moreover, the application of the new ET-MRR-based time lens is certainly not restricted to temporal cloaking. It would be technically fascinating to explore its application in temporal magnification or temporal Fourier transform, which might also bring new breakthroughs. Last but not least, for more useful application, the programmability of temporal cloaking should not only be realized in the time domain, but also in the spatial domain: i.e., concealing events at arbitrary positions along the transmission link, which we expect to see in the near future.

Two-dimensional (2D) layered materials possess sheet-like structures with single-atom or few-atom thicknesses. They exhibit exceptional electronic and optical properties due to quantum confinement in the direction perpendicular to the 2D plane. Besides being the basis of various modulators and photodetectors, 2D materials have realized light sources with femtosecond pulse duration utilizing their ultrahigh optical nonlinearity. Recent developments and explorations of various 2D materials and their heterostructures have prompted intense research on various photonic devices with superior performance and functionalities. These developments can pave the way for realistic applications of 2D materials, and boost the fundamental study of various physical effects and phenomena.This special issue on “2D materials for photonic applications” covers the most recent progresses in photonic applications of 2D materials. In the three reviews and two original research articles included, it introduces light sources, modulators, and fabrication methods of 2D materials. The significant benefits of 2D materials for electronics and photonics are also presented, providing a roadmap for a broad range of application fields.Mustonen et al. [1] summarized the fabrication methods of large-area transparent graphene electrodes for industry, including liquid exfoliation and chemical vapor deposition. Zhong et al. [2] reviewed different graphenebased all-optical modulators, and comprehensively discussed their performances in detail. Yao et al. [3] focused on integrated optical switches enabled by 2D materials and beyond. They summarized state-of-the-art optical switches (e.g., all-optical, thermo-optical, and electro-optical modulators) in terms of their energy consumption and response time, showing several stimulating charts. Mu et al. [4] and Li et al. [5] demonstrated pulse generation based on 2D nanoparticles, which can potentially generate stable and rectangular pulses. These researches are highly promising for practical applications.The five articles selected for this special issue cover only a portion of the recent advances and applications in this rapidly growing domain of 2D materials. In the future, significant developments in 2D materials for photonic applications are expected. We hope that this special issue on “2D materials for photonic applications” will interest readers, and provide useful references that will inspire future research in this exciting field.In addition, we would like to express sincere gratitude to the editors of Frontiers of Optoelectronics for providing a valuable opportunity to organize this special issue. We also thank all authors who have contributed their articles, and the many reviewers who have generously offered their valuable time to provide high-quality reviews of all papers.

Heavily doped colloidal plasmonic nanocrystals have attracted great attention because of their lower and adjustable free carrier densities and tunable localized surface plasmonic resonance bands in the spectral range from near-infra to mid-infra wavelengths. With its plasmon-enhanced optical nonlinearity, this new family of plasmonic materials shows a huge potential for nonlinear optical applications, such as ultrafast switching, nonlinear sensing, and pulse laser generation. Cu3-xP nanocrystals were previously shown to have a strong saturable absorption at the plasmonic resonance, which enabled high-energy Q-switched fiber lasers with 6.1 μs pulse duration. This work demonstrates that both highquality mode-locked and Q-switched pulses at 1560 nm can be generated by evanescently incorporating twodimensional (2D) Cu3-xP nanocrystals onto a D-shaped optical fiber as an effective saturable absorber. The 3 dB bandwidth of the mode-locking optical spectrum is as broad as 7.3 nm, and the corresponding pulse duration can reach 423 fs. The repetition rate of the Q-switching pulses is higher than 80 kHz. Moreover, the largest pulse energy is more than 120 μJ. Note that laser characteristics are highly stable and repeatable based on the results of over 20 devices. This work may trigger further investigations on heavily doped plasmonic 2D nanocrystals as a next-generation, inexpensive, and solution-processed element for fascinating photonics and optoelectronics applications.

In this paper, we have proposed and demonstrated the generation of passively mode-locked pulses and dissipative soliton resonance in an erbium-doped fiber laser based on Fe3O4 nanoparticles as saturable absorbers. We obtained self-starting mode-locked pulses with fundamental repetition frequency of 7.69 MHz and center wavelength of 1561 nm. The output of a pulsed laser has spectral width of 0.69 nm and pulse duration of 14 ns with rectangular pulse profile at the pump power of 190 mW. As far as we know, this is the first time that Fe3O4 nanoparticles have been developed as low-dimensional materials for passive mode-locking with rectangular pulse. Our experiments have confirmed that Fe3O4 has a wide prospect as a nonlinear photonics device for ultrafast fiber laser applications.

Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes; it has low sheet resistance, high optical transmission and is flexible. Whereas the most common transparent electrode material, tin-doped indium-oxide (ITO) is brittle, less transparent and expensive, which limit its compatibility in flexible electronics as well as in low-cost devices. Here we review two large-area fabrication methods for graphene based transparent electrodes for industry: liquid exfoliation and low-pressure chemical vapor deposition (CVD). We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results. State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43:5, slightly over the minimum required for industry of 35, while CVD reaches as high as 419.

All-optical devices, which are utilized to process optical signals without electro-optical conversion, play an essential role in the next generation ultrafast, ultralow power-consumption optical information processing systems. To satisfy the performance requirement, nonlinear optical materials that are associated with fast response, high nonlinearity, broad wavelength operation, low optical loss, low fabrication cost, and integration compatibility with optical components are required. Graphene is a promising candidate, particularly considering its electrically or optically tunable optical properties, ultrafast large nonlinearity, and high integration compatibility with various nanostructures. Thus far, three alloptical modulation systems utilize graphene, namely freespace modulators, fiber-based modulators, and on-chip modulators. This paper aims to provide a broad view of state-of-the-art researches on the graphene-based alloptical modulation systems. The performances of different devices are reviewed and compared to present a comprehensive analysis and perspective of graphene-based alloptical modulation devices.

Applications of optical switches, such as signal routing and data-intensive computing, are critical in optical interconnects and optical computing. Integrated optical switches enabled by two-dimensional (2D) materials and beyond, such as graphene and black phosphorus, have demonstrated many advantages in terms of speed and energy consumption compared to their conventional silicon-based counterparts. Here we review the state-ofthe- art of optical switches enabled by 2D materials and beyond and organize them into several tables. The performance tables and future projections show the frontiers of optical switches fabricated from 2D materials and beyond, providing researchers with an overview of this field and enabling them to identify existing challenges and predict promising research directions.

This review article highlights the exploration of inorganic nanoscintillators for various scientific and technological applications in the fields of radiation detection, bioimaging, and medical theranostics. Various aspects of nanoscintillators pertaining to their fundamental principles, mechanism, structure, applications are briefly discussed. The mechanisms of inorganic nanoscintillators are explained based on the fundamental principles, instrumentation involved, and associated physical and chemical phenomena, etc. Subsequently, the promise of nanoscintillators over the existing single-crystal scintillators and other types of scintillators is presented, enabling their development for multifunctional applications. The processes governing the scintillation mechanisms in nanodomains, such as surface, structure, quantum, and dielectric confinement, are explained to reveal the underlying nanoscale scintillation phenomena. Additionally, suitable examples are provided to explain these processes based on the published data. Furthermore, we attempt to explain the different types of inorganic nanoscintillators in terms of the powder nanoparticles, thin films, nanoceramics, and glasses to ensure that the effect of nanoscience in different nanoscintillator domains can be appreciated. The limitations of nanoscintillators are also highlighted in this review article. The advantages of nanostructured scintillators, including their property-driven applications, are also explained. This review article presents the considerable application potential of nanostructured scintillators with respect to important aspects as well as their physical and application significance in a concise manner.