Advanced solid-state lasers (ASSL) is an international conference devoted to recent advances in both materials and sources aspects of solid-state lasers. The materials section covers advances in optics, materials science, condensed matter physics, and chemistry relevant to the development, characterization, and applications of new materials and components for lasers and photonics. These include crystals, glasses, and ceramics, as well as functionalized composite materials, from fibers and waveguides to engineered structures with pre-assigned optical properties. Materials used for the fabrication of basic laser components also belong to a core part of this topic. Coherent and high-brightness radiation sources include lasers as well as pump and nonlinear devices. Emphasis is on advances in science and technology, for improved power, efficiency, brightness, stability, wavelength coverage, pulse width, cost, environmental impact, or other application-specific attributes.

We hope that readers will enjoy this joint feature issue of 17 top-level articles which cover recent advances in both materials and sources aspects of solid-state lasers. We thank all the authors and reviewers for their important contributions. We are especially grateful to Carmelita Washington, Keith Jackson, and John Long from the OSA staff for their outstanding work throughout the launch of this feature issue as well as the review and production processes.

Development of novel materials processing schemes, spectral filters, and resonator architectures paves the way for the design of next generation solid-state lasers. K. Hasse and C. Kraenkel reported on fast, direct laser inscription of waveguide laser structures with a 1 MHz-repetition rate fs-laser. They inscribed and characterized more than 100 tracks in Yb:CALGO crystals and waveguide lasing was achieved with slope efficiencies of up to 57% under pumping with an optically pumped semiconductor laser [1]. V. Llamas et al. employed femtosecond direct-laser-writing at kHz repetition rates to produce depressed-index buried and surface channel waveguides (type III) in a bulk 3.5 at.% Tm³⁺:CALGO crystal and characterized the waveguides using confocal microscopy and Raman spectroscopy. In-band pumping at 1679 nm generated a maximum continuous-wave output power of 0.81 W at 1866-1947 nm with a slope efficiency of 71.2% with respect to absorbed pump power [2]. In another study, Y. Baravets et al. used a new type of metallic resonant leaky-mode diffraction grating to construct a fiber laser that is broadly tunable in the wavelength range from 1058 nm to 1640 nm. Experimentally measured diffraction efficiency of more than 90% was obtained into the first reflected order over the 1417-1700 nm spectral window [3]. Q. Berthome et al. further used a volume Bragg grating and a YAG etalon to construct an actively Q-switched, single longitudinal mode Tm:YAP laser which generated 230µJ, 50 ns pulses at 1 kHz and which was tunable from 1940 to 1960 nm [4].

Attaining improved performance of coherent sources in terms of energy, power, and brightness scaling is among the major targets of research in solid-state lasers. Mackonis and Rodin reported a compact optical parametric chirped-pulse amplifier (OPCPA) that is scalable to TW peak power. By using 1.2 ps pump pulses delivered from a Yb:YAG laser, they achieved amplified pulse energy of 2.1 mJ in a three-stage OPCPA [5]. L. Rishoj et al. employed the process of soliton self-mode conversion in a multi-mode fiber to demonstrate an energetic, dual-wavelength ultrashort pulsed fiber source which delivers pulses at the wavelengths of 1205 and 1273 nm with pulse energies of 30 nJ [6]. In another study, Y. Chen et al. employed a graded-index passive fiber to construct a high-power continuous-wave Raman fiber amplifier and achieved 2.087 kW at 1130 nm with an optical conversion efficiency of 90% [7]. W. Wu et al. further reported on a sub-nanosecond burst-mode MOPA Nd:YAG laser at 1.06µm. Output energies of 29.8 mJ and 81 mJ were obtained during 1 kHz and 10 Hz operation, respectively, resulting in peak power of as high as 90 MW during 10 Hz operation [8].

Nonlinear frequency conversion techniques play a crucial role in attaining new spectral ranges with solid-state lasers. F. Cassouret et al. reported on efficient third harmonic generation at 355 nm by using the single crystal Ca₅(BO₃)₃F (CBF). The output of a high peak power, passively Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG microlaser was mixed with its second harmonic in the CBF crystal, resulting in the generation of 479-µJ, 568-ps pulses at 355 nm with a peak power of 0.843 MW [9]. In another study, G. Scurria et al. reported on the first experimental demonstration of multi-watt level mid-infrared super continuum generation in an InF₃ fiber. They used a single, actively Q-switched mode-locked Tm³⁺-doped fiber oscillator with an average output power of 15 W and generated a supercontinuum with a maximum all bands output power of 7W. The spectrum extended up to 4.7 µm [10]. Furthermore, J. T. Meyer et al. employed a passively mode-locked vertical external cavity surface emitting laser (VECSEL) to produce green output at 528 nm based on intra-cavity second harmonic generation in a lithium triborate crystal. They obtained 760-fs pulses at 528 nm, at a pulse repetition rate of 465 MHz and with an average output power of 230 mW [11]. Finally, M. Li et al. reported on the first demonstration of second Stokes lasing in a diamond Raman ring cavity laser resonantly pumped by a continuous-wave tunable Ti:sapphire laser. A maximum output power of 364 mW was generated at 1101.3 nm with a slope efficiency of 33.4% [12].

Search for new laser gain media and characterization of optical materials play a crucial role in the development of new solid-state lasers and resonator components. A. Volokitina et al. reported on the crystal growth, spectroscopy, and first laser operation of a novel double molybdate compound – Tm:KY(MoO₄)₂. Diode-pumped, continuous-wave laser operation of Tm:KY(MoO₄)₂ configured in the form of crystalline films and plates could be obtained, yielding a maximum output power of 0.88 W near 1.9µm with a slope efficiency of 65.8% [13]. M. Nemec et al. reported on the measurement of the spectroscopic and laser parameters of the Er-doped Gd₃Ga₃Al₂O₁₂ crystal (Er:GGAG) in the temperature range of 80-340 K. The best power performance was obtained at 80 K, where they generated a maximum output power of 2.8W with a slope efficiency of 54% [14]. In another study, O Wada et al. employed terahertz(THz) time-domain spectroscopy and optical reflection measurements to study the dielectric properties of high refractive index oxyfluorosilicate (OFS) glasses and developed a unified dielectric model for the sub-THz frequency region [15]. K. Hamamoto et al. further reported on the first measurement of the piezo-optic coefficients of ceramic YAG based on the four-point bending method and analyzed the resulting depolarization effects [16]. Finally, X. Ma et al. performed simulations to investigate the effect of heat load on the power performance of chirally-coupled-core fibers [17].