Supercontinuum generation is one of the most spectacular and visually perceptible nonlinear effects produced by the propagation of intense laser pulses in a transparent nonlinear medium. Since its discovery in the early years of nonlinear optics [1,2], supercontinuum generation has continuously attracted a great deal of scientific and technological interest. This is because supercontinuum light represents a unique and versatile source of ultra-broadband radiation with high power spectral density and high degree of coherence equivalent to a white-light laser, finding numerous applications in diverse fields of modern optical sciences, ranging from frequency comb metrology and the generation of few optical cycle pulses to optical sensing and imaging. The steady technological advances in the development of novel laser sources, in the design and fabrication of optical fibers and waveguides, as well as the availability of highly nonlinear materials, continue to foster new studies on supercontinuum generation in various nonlinear media, and in yet unexplored parts of the optical spectrum.

This feature issue brings together the latest developments in supercontinuum generation in confined and bulk geometries, which, although sharing some common features, are often considered as independent research areas due to differences in the underlying physics [3,4]. The research presented herein covers a wide range of topics related to various aspects of supercontinuum generation in bulk nonlinear media, photonic crystal fibers and waveguides, new results of design and fabrication of these structures with engineered dispersion, development of new diagnostic tools, as well as applications of spectral broadening and supercontinuum generation for pulse compression, imaging, and sensing. Below, we briefly highlight the main findings of the featured articles. Yang et al. report on the generation of broadband supercontinuum spanning 4.7 octaves, from 350 to 9000 nm, by multiple-filamentation in a laser-modified bulk ${\mathrm{PbF}}_2$ crystal [5]. Šuminas et al. demonstrate multi-octave supercontinuum generation in polycrystalline ZnS and ZnSe with mid-infrared laser pulses, which is facilitated by efficient generation of multiple harmonics via random quasi-phase matching [6]. Chekalin et al. investigate the general features of supercontinuum generation by self-compressed light bullets in the regime of anomalous dispersion in various transparent dielectrics: fused silica, ${\mathrm{CaF}}_2$, ${\mathrm{BaF}}_2$, and LiF [7]. Ramachandran et al. investigate the effect of multiple scattering on supercontinuum generation by intense laser pulses in water containing randomly suspended polystyrene microspheres [8]. Kudryashov et al. study supercontinuum generation with multiple filaments in pure water and gold nanoparticle colloidal solutions of variable extinction coefficients [9]. Jukna et al. demonstrate that focusing the incident beam either onto the input face or inside a bulk nonlinear medium results in very different evolutions of the supercontinuum spectrum depending on the input pulse energy [10]. Shipilo et al. study third harmonic generation and spectral broadening in air produced by fusion of the initially separated filaments into a superfilament [11]. Diouf et al. report on the simulation of an ultra-broadband coherent mid-infrared supercontinuum extending from 1.25 to 20 μm and pulse compression down to 15 fs in a novel ${\mathrm{AsSe}}_2\text{-}{\mathrm{As}}_2{\mathrm{S}}_5$ hybrid multimaterial photonic crystal fiber designed to have a zero-dispersion wavelength at 3.3 μm with an overall highly engineered group velocity dispersion shifted to the mid-infrared wavelength region [12]. Soboń et al. investigate the shot-to-shot stability of an octave-spanning supercontinuum generated with ultra-short pulses at 1:9–2:0 μm in a commercial highly nonlinear fiber cut to different lengths and demonstrate that the shot-to-shot stability is significantly improved using shorter fiber lengths while maintaining the broad spectral coverage [13]. Vengelis et al. investigate supercontinuum generation in a polarization-maintaining highly nonlinear photonic crystal fiber using subnanosecond pulses from Nd:YAG microlaser and show that a part of the supercontinuum light is generated in the cladding modes that exhibit different dispersion [14]. Eslami et al. show that the low damage threshold and power limitation associated with the use of soft glass fiber could be overcome by using large core multimode fibers and report the generation of octave-spanning supercontinuum from 1200 nm to over 2500 nm with 600 mW average power in a short length of multimode fluoride fiber with 100 μm core diameter [15]. Jain et al. reveal the unexploited potential of germania-doped fibers for mid-infrared supercontinuum generation, demonstrating a supercontinuum source with a 20 dB bandwidth from $\sim 1$ to $\sim 3\mathrm{\mu m}$ and with output power exceeding 6 W based on a ${\mathrm{GeO}}_2$-doped silica fiber [16]. Sinobad et al. present a simple post-process technique to fine-tune the dispersion profile of a hybrid nonlinear SiGe/Si waveguide through adding a top chalcogenide cladding for changing the properties of chip-based generated supercontinuum [17]. Klimczak et al. report on the design, fabrication, and experimental characterization of highly nonlinear, tellurite glass photonic crystal fibers with engineered normal dispersion characteristics for coherent supercontinuum generation in the infrared spectral range [18]. Lemiere et al. experimentally develop arsenic- and antimony-free chalcogenide optical fibers with or without a suspended core and standard step-index fibers with varying core diameter and with low residual losses, used for supercontinuum generation with a spectrum spanning the 2–14 μm range [19]. Christensen et al. numerically study supercontinuum generation with femtosecond pulses in a silicon nitride waveguide and in a standard silica microstructured fiber, and demonstrate that directional supercontinuum generation is observable in both fibers and integrated photonics waveguides with two zero-dispersion wavelengths [20]. Genier et al. present a detailed numerical study of the impact of pump laser amplitude noise on the coherence of the supercontinuum generated in all-normal dispersion photonic crystal fibers with femtosecond high peak power pulses [21]. Saghaei and Van numerically investigate near- and mid-infrared supercontinuum generation in dispersion-engineered silicon-on-insulator waveguides employing a novel side-slotted core structure, which allows achieving optimum dispersion profiles for supercontinuum generation in the anomalous and normal dispersion regimes [22]. Brown et al. propose a numerical diagnostic tool that is useful for interpreting the role of self-phase modulation, third harmonic generation, the effects of plasma and ionization, and the interference of radiations produced by these effects, in supercontinuum generation and conical emission [23]. Jensen et al. develop an assumption-free model for the excess noise of a supercontinuum source and investigate the effect of pulse-to-pulse fluctuations of supercontinuum sources on the noise in spectral domain optical coherence tomography [24]. Dasa et al. report on the development of a cost-effective supercontinuum laser-based multispectral photoacoustic sensing system for in vitro glucose monitoring, which was used to identify the absorption characteristics of two major analytes (glucose and cholesterol) over a wavelength region of 1540–1840 nm [25]. Farinella et al. report on the experiments of spectral broadening in fused silica wafers and compression of very intense pulses performed with the HERCULES laser and at the LASERIX facility [26]. Beetar et al. demonstrate near 20-fold compression of commercial Yb:KGW laser amplifier pulses in a relatively simple scheme utilizing a xenon-filled hollow-core fiber and chirped mirror compressor [27]. Putnam et al. develop a few-cycle, carrier-envelope phase-stable laser source based on supercontinuum generation in a highly nonlinear optical fiber and driven by an amplified Er:fiber-based laser system [28].

This feature issue is complemented by two excellent reviews. Poudel and Kaminski give a brief review on the generation and properties of supercontinuum radiation for imaging applications, which include supercontinuum-enabled microscopies, such as wide-field and laser scanning techniques, spectral imaging, lifetime imaging, two-photon absorption, super-resolution, optical coherence tomography, and coherent anti-Stokes Raman scattering microscopy [29]. In his review paper, Zheltikov discusses the concepts, physical scenarios and current laser technologies for production of multioctave supercontinuum and subcycle pulse generation in the optical range and beyond [30].

We expect that this feature will provide a useful reference on the latest developments of supercontinuum generation in fiber, waveguide, and bulk geometries, and will stimulate further studies in this exciting research field. Finally, we express our gratitude to Editor-in-Chief Grover Swartzlander, journal staff, authors, and dedicated reviewers, without whose efforts and collaboration this feature issue would not have been possible.