The Advanced Solid-State Lasers (ASSL) topical meeting is a premier international conference dedicated to materials and source developments aimed at advancing the state-of-the-art of lasers to meet the demands of an ever-growing number of scientific and commercial applications. Materials have always been a major emphasis of this conference, since the performance of any laser source is ultimately limited by the properties of the material of which it is comprised – whether it be the laser host itself (single crystal or ceramic or glass) or critical components such as the pump source, mirrors, optics, q-switches, mode-lockers, isolators, and nonlinear frequency converters. Consequently new materials often enable significant breakthroughs in solid state laser technology, particularly when composite and highly-engineered material structures are involved. The laser sources themselves, as well as the applications that drive their innovation, are of course the primary focus of the conference, and a broad range of laser designs and architectures vary substantially depending on the temporal regime involved, which can range from continuous-wave (cw) to pulsed, and pulse durations from microseconds to attoseconds. Output powers can range from mW to MW, pulse energies from nJ to MJ, output wavelengths from extreme UV to THz, and bandwidth from single frequency to multiple octaves. ASSL is ultimately a very practical meeting, emphasizing advances in science and technology that improve output power, efficiency, brightness, stability, wavelength coverage, pulse width, cost, environmental impact or other application-specific performance metrics.

The 40 high-level papers accepted for publication in this feature issue are representative of the broad range of technology advances that ASSL strives to facilitate, and we trust readers will find them as stimulating as the conference itself, where these results were first reported. We are thankful to all authors and reviewers for their excellent contributions. We would also like to thank Carmelita Washington from the Optica staff for her outstanding work throughout the launch of this feature issue as well as the review and production processes.

Laser hosts and nonlinear crystals are at the core of solid state laser technology, and consequently new materials and the corresponding lasers continue to be central to the conference. Cubic sesquioxides have attracted great attention due to their high thermal conductivities, and a detailed study of the spectroscopic properties of Er³⁺ in yttria, lutetia, and scandia found that the luminescence lifetime of the ⁴I_11/2 state strongly depends on the cation site, increasing monatonically from Sc₂O₃ to Lu₂O₃ to Y₂O₃ [1]. Brown et al. [2] report on the 3.9-µm emission properties of Ho³⁺ in NaYF₄ and CsCdCl₃ crystals as well as Ga₂Ge₅S₁₃ glass, and reported lifetimes of 0.12, 16.5, and 1.61 ms respectively, which points to the chloride as the preferred host if adequate crystal quality can be achieved. Franz Kaertner’s group investigated the cryogenic performance of Cr:LiCAF, Cr:LiSaF, and Cr:LiSGaF crystals in hopes of scaling their broad-band near-infrared emission to higher powers, and observed modest increases in emission cross-section in each case [3]. The mid-IR emission properties of Er-doped CaF₂, SrF₂, BaF₂, and the mixed crystals (Ca,Sr)F₂ and (Sr,Ba)F₂, were studied by Normani et al. [4], who achieved low-threshold, cw, 2.8-µm operation in each case, with Er:CaF₂ yielding the highest slope efficiency (37.9%). Visible laser performance of Pr-doped ASL was studied by Cassouret et al. using a crystal grown at IKZ: output powers of 206 mW and 90 mW were achieved at 726 nm and 645 nm respectively [5]. The only nonlinear crystal paper in this collection addressed the thermos-mechanical properties of two promising new mid-infrared nonlinear crystals: BaGa₂GeS₆ and BaGa₂GeS₆ [6].

The next group of papers in this special issue focused on “structured materials” ranging from semiconductor diode lasers to laser-inscripted waveguides and gratings to novel monolithic integrated photonics. D. Mitten et al. report on power scaling an 1178-nm VECSEL (for frequency-doubled 589-nm sodium guide star applications) using multiple gain chips in a novel twisted-mode cavity configuration to achieve 39 W (multi-mode) and 21 W (single mode) output [7]. A high-power cw InGaN blue-violet laser diode was designed and fabricated using an asymmetric waveguide structure to produce 4.5 W of 403-nm output at 3A of current at a low threshold current density of 0.97 kA/cm² [8]. Hu et al. demonstrated single-scan fs laser inscription of low-loss mid-infrared waveguides in IG2 chalcogenide glass, achieving ∼1.2 and 2.1 dB/cm propagation losses at 4550 nm in type-II and type-I configurations respectively [9], whereas Perevezentsev et al. used fs lasers and a phase mask inscription technique to produce low-loss chirped volume Bragg gratings in fused silica with a 3×3×12 mm³ volume and ∼190 ps/mm chirp rate around 1030.5 nm, but further work is needed to eliminate severe polarization and phase distortion [10]. A plasmonic whispering gallery mode micro-ring laser was designed and fabricated by depositing gold nanoparticles on a polymer micro-ring written on a silica substrate, and single mode lasing was achieved [11]. Wang et al. designed and simulated a WDM device in the telecom wavelength regime based on all-dielectric silicon topological valley photonic crystal structures [12].

Fiber lasers and amplifiers have been integral to ASSL since their inception, both as stand-alone sources and as pumps for more complex lasers and systems. Yamaizumi, Honda, and Fuji describe a chirped pulse amplification (CPA) system producing ultrashort 1300 nm pulses based on Pr-doped ZBLAN fluoride fiber which yielded 225 fs compressed pulses and 112 mW average power at 40 MHz. [13] Researchers at ISL optimized the performance of a 2.1-µm thulium-holmium co-doped monolithic fiber laser by adapting a fiber Bragg grating to maximize the gain wavelength, reaching a maximum cw output power of 179 W and a 40.2% slope efficiency [14]. The next two papers addressed transverse mode instabilities (TMI), since these represent the primary obstacle to power scaling fiber laser systems with diffraction-limited beam quality. Kholaif et al. [15] developed a novel method to characterize the TMI dynamics even in the presence of power fluctuations using a position sensitive detector, and Li et al. [16] simulated the effect of fiber bending and mode content on the TMI threshold, and optimized these parameters to suppress TMI and SRS in an 8 kW tandem pumped fiber amplifier with an M² of 1.8.

The impact of ultrafast solid state lasers continues to grow, including these four articles on mode-locked lasers and four on optical parametric amplification (OPA) systems. The mode-locked laser submissions are all collaborations between the Max Born Institute in Berlin, the Fujian Institute of Research on the Structure of Matter in Fuzhou, and several other institutions. The first 3 are all diode pumped SESAM-based architectures: Yb-doped YAl₃(BO₃)₄ (Yb:YAB) produced 56 fs pulses and up to 76 mW at ∼67.5 MHz [17]; Yb:Sc₂SiO₅ generated 38 fs pulses at 76 mW average power, but up to 216 mW with 42 fs pulses [18]; and Yb:SrF2 produced 49 fs pulses up to 117 mW output power, which scaled to 313 mW with 70 fs pulses [19]. Finally, Kerr-lens mode-locking of Yb-doped calcium lithium niobium gallium garnet (Yb:CLNGG) delivered soliton pulses down to 31 fs (66 mW output) and up to 203 mW output with 37 fs pulses [20]. Researchers from University of Rochester LLE the demonstrated the first successful optical parametric amplification of broadband spectrally incoherent pulses reaching a 383 mJ signal output energy from a dual stage noncollinear LBO OPA [21]. Petrov et al. reported a 2.3 mJ, 1 kHz, 125 fs OPCPA system based on Yb: CALYO [22], and Bock et al. used spectral pulse shaping and dispersion management to extend the output of their high-energy, mid-IR, 4-stage ZGP-based OPCPA system to 12 µm via DFG in AgGaSe₂, achieving 10 µJ energy and 143 fs (sub-four-cycle) pulses at 1 kHz [23]. Marra et al. reported a cryogenic, high repetition rate chirped pulse amplifier based on Fe:ZnSe to produce 250-fs, 4.59-mJ pulses centered at 4.07 microns [24].

Gain-switched Fe:ZnSe lasers are typically pumped using 2.79-µm Er³⁺ solid state lasers, and D. Martyshkin et al. report such a laser a optimized for this application [25]. Their design features a newly developed electro-optic Q-switch based on a langasite (La₃Ga₅SiO₁₄, or LGS) [26] to enhance their flash-lamp-pumped, Cr:Er:YSGG laser, and a 2.9-m cavity length configuration yielded 190 mJ, 85 ns pulses which are ideal for Fe:ZnSe pumping. The same paper also reports record-level 300-mJ output energy (15 ns, 3 Hz) from a short-cavity master oscillator, and 350 mJ from a long-pulse (90 ns) MOPA configuration.

Three articles on frequency-doubled Raman lasers generating output at yellow, orange, and red wavelengths also appear in this special issue. Hao-Hao Chen et al. report a high beam quality (M² = 1.2) yellow laser based on an AO-q-switched Nd:YVO₄ crystal with undoped end caps, Raman shifting (with beam clean-up) in YVO₄, and SHG in LBO, within a V-shaped cavity to yield 2.85 W at 588 nm (3 ns pulsewidth, 50 kHz repetition rate) [27]. Jian-Cheng Chen et al. report polarization-selectable yellow (579 nm, 0.1 mJ, 80 kW peak power) or orange (589 nm, 0.08 mJ, 50 kW peak power) output from 808-nm diode-pumped Nd:YVO₄ with a Cr⁴⁺:YAG passive q-switch, an Np-cut KGW stimulated Raman scattering crystal, and an LBO SHG crystal in a compact linear cavity [28]. Hui Zhao et al. describe a wavelength-selectable deep-red source based on a diode-pumped, A-O q-switched Nd:YLF with an intracavity KGW Raman crystal and a bismuth borate (BiBO) nonlinear crystal which are angle tuned to achieve SHG/SFG of the fundamental and/or first stokes waves: this yielded emission at 692, 698, 731, and 745 nm with average output powers of 2.4, 2.7, 3.3, and 3.6 W and pulse widths of 3.4, 3.2, 4.3, and 3.7 ns respectively at a repetition rate of 4 kHz [29].

Vortex beams are finding a growing number of applications ranging from improved laser industrial machining processes to enhanced bandwidth in optical communications. Harrison et al. present two articles on power scaling of structured light beams: in the first they demonstrated amplification of higher-order Laguerre-Gaussian modes using a dual-pass 1064-nm MOPA, achieving a gain factor of 17x while preserving beam quality of the input mode [30], while in the second they were able to ameliorate aberration-induced vortex splitting through engineering of the Gouy phase, realizing orbital angular momentum (OAM) beams with 94% vortex purity and an amplification enhancement up to 1200% [31]. Miller et al., reported a new method for generating beams with rapidly tunable angular momentum, using a scanning mirror-based Higher Order Bessel-Beams Integrated in Time (HOBBIT) system to switch between modes in the kHz range at high power and efficiency [32]. Liu et al. demonstrated a compact nanosecond Yb:YAG/V:YAG solid-state laser with cylindrical vector beams modulated by an intracavity S-waveplate mode converter which featured the switchable radially and azimuthally polarized beams with extinction ratios of 92.3% and 89.6% respectively [33].

The last seven papers in this collection are loosely grouped as novel concepts in solid state laser design. Cheng et al. reported room-temperature radiation-balanced lasing in Yb-doped YAG and KYW, achieving record efficiency of 30.5% in 3% Yb³⁺:YAG by frequency-locking the laser cavity to the input light, and developed a detailed quantitative model that agreed well with experiment [34]. CIMAP and their collaborators achieved efficient long-wavelength 2.3-µm emission from Tm:KLu(WO₄)₂ and Tm:CALGO and by upconversion pumping the ³H₄ → ³H₅ transition, and in Tm:YVO₃ they achieved multi-watt dual-wavelength emission at 2-µm and 2.3-µm using a cascaded lasing scheme [35–37]. Danilin et al. observed ∼20-50 nm of spectral broadening in the output of Cr:ZnS and Cr:ZnSe gain media due to spatial hole burning effects, but were able to achieve ∼80-90 pm of line narrowing in a twisted mode cavity by adjusting the orientation of intracavity waveplates [38]. Chang Xu et al. demonstrated a high pulse energy and high beam quality, 100-µs-pulse, 5 Hz, 766.699 nm Ti:sapphire laser with a 0.66 pm linewidth [39]. Finally, researchers from Lawrence Berkeley National Laboratory demonstrated ultra-broadband spectral beam combining of three pulse-shaped, chirped-pulse Yb fiber amplifiers to achieve 42 fs compressed pulses [40].