Illustration of local deformation in an isolated carbon nanotube due to the pressure applied through the apex of a nano-tip. By sensing this local deformation by means of Raman shift in TERS, the sample can be imaged with extremely high spatial resolution.(Picture: P. Verma et al., pp. 548-561, in this issue)

Basic concepts of slow and fast light as well as recent advancements based on nonlinear wave-mixing processes are described. As a nonlinear medium, the authors focus on a liquid crystal light valve showing that it allows obtaining a large control of the group delay. A theoretical model accompanies the observations and accounts for them in the general framework of two-wave mixing in the light valve. At the end, a high-sensitivity interferometer is presented as an example of slow light applications.

Basic light-matter interactions and optical properties of chip-based single photon sources are reviewed, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction. The limitations, challenges, and exciting prospects of developing on-chip quantum light sources using integrated photonic crystal structures are discussed.

Hydrogen is potentially one of the most attractive and environmentally friendly fuels for energy applications. Its generation from water splitting represents a holy grail in energy science and technology, as water is the most abundant hydrogen source on the Earth. Among different methods, hydrogen generation from photoelectrochemical (PEC) water splitting using semiconductors as photoelectrodes is one of the most scalable and cost-effective approaches. Compared to bulk materials, nanostructured semiconductors offer potential advantages in PEC application due to their large surface area and size-dependent properties.

Non-diffracting beams do not spread as they propagate. This property is useful in many areas. Here, the theory, generation, properties, and applications of various non-diffracting beams, including the Bessel beam, Mathieu beam, and Airy beam is reviewed. Applications include imaging, micromanipulation, nonlinear optics, and optical transfection.

The spatial resolution in optical imaging is restricted by the so-called diffraction limit, which prevents it to be better than about half of the wavelength of the probing light. Tip enhanced Raman spectroscopy (TERS), which is based on the surface plasmon polaritons induced plasmonic enhancement and confinement of light near a metallic nanostructure, can however, overcome this barrier and produce optical images far beyond the diffraction limit.

Techniques for active modulation and control of plasmonic signals in future highly-integrated nanophotonic devices have advanced rapidly in recent years, with recent innovations extending performance into the terahertz frequency and femtojoule-per-bit switching energy domains. As thoughts turn towards the development of practical device structures, key technologies are compared in this review and prospects are assessed for the future development of the field.

Interference lithography (IL) is emerging as one of the most powerful yet relatively inexpensive methodologies for creating large-area patterns with micron- to sub-micron periodicities. $N$-dimensional periodic structures ($N\leqslant 3$) can be obtained by interfering (${N + 1}$) non-coplanar beams in a photoresist. IL done with shorter wavelength lasers and/or liquid immersion lithography can create features with sub-50\,nm dimensions. Such periodic structures are beginning to find wide use in photonic crystal science, optical telecommunications, data storage, and the integrated circuit~industry.