This feature includes a total of 15 papers, 3 of which were invited papers and the remainder contributed papers. The papers at the conference were separated into 4 sessions: (i) Laser Materials, (ii) Optical Fibers, Glasses, and Ceramics, (iii) Semiconductor Based Optical Materials, and (iv) Waveguides and Thin Films.

There are five papers in the Laser Materials category that highlight specific rare-earth-doped crystals (YAG, vanadates, fluorides) and their spectroscopic properties, along with a demonstration of size scaling. Making larger sizes of crystals such as Yb:YAG is important for scaling lasers to higher power, but typically the larger crystals are more inhomogeneous and contain more defects. However, Arzakantsyan et al. [1] used the horizontal direct crystallization method to grow large 90 mm diameter Yb:YAG single crystals with high optical quality. This represents the largest size grown to date. The availability of larger crystals coupled with their superior thermal conductivity compared with glass slabs enables scaling of lasers to higher power. On the other hand, several scientific and industrial applications, such as spectroscopy, photolithography, and micromachining, can benefit from compact and rugged solid-state ultraviolet (UV) sources. Consequently, Bhandari et al. [2] demonstrate a microchip laser at 266 nm with >3 MW peak power using a fluxless grown BBO crystal for fourth-harmonic generation of a linearly polarized Nd:YAG microchip laser, passively Q-switched with [110] cut Cr⁴⁺:YAG in the sub-nanosecond pulse-gap region. Their efforts particularly highlight the need for high purity and quality BBO crystals. Since the pump energy in microchip lasers concentrates in a small volume, thermal effects can cause problems. While thermal lensing and thermal birefringence have been extensively studied, the impact of temperature on the emission cross-sections has seen less activity. Sato and Taira [3] study this effect and develop a general numerical model to describe the impact of temperature on the stimulated emission of Nd in several different crystal hosts (YAG, YVO₄, and GdVO₄). They also present approximations of this model for their samples by a simple polynomial, which can be applicable over a broad temperature range from 15°C to 350°C. In-band (resonant) pumping of eye safer lasers enables better thermal management due to lower quantum defect. Ter-Gabrielyan et al. [4] present the results of a comprehensive comparison of optical, thermal, and spectroscopic properties of resonantly pumped Er:YVO₄ and Er:GdVO₄ single crystals at cryogenic and room temperatures. Both lasers demonstrated operation with a low-quantum defect and slope efficiencies well in excess of 80%, highlighting the practical utility of in-band pumping. Aside from oxide-based crystals, fluoride crystals are also potentially important technological materials for rare-earth doping. For example, Tm:YLF has several applications ranging from surgery to pumping high-energy Ho:YLF slab amplifiers for defense-related applications. The Ho:YLF requires specific pumping at 1890 nm. Strauss et al. [5] demonstrate 1890 nm output from a single 2.5% doped Tm:YLF slab, and a wavelength selective volume Bragg grating (VBG) mirror in order to shift the 1900nm Tm:YLF gain peak. The slab crystal was pumped with a 300 W diode stack, and the output power exceeded 80 W. Their simple architecture can be scaled to higher output powers by adding more crystal slabs and diode-stacks.

In the category of Optical Fibers, Glasses, and Ceramics there are three papers. They highlight novel hollow core gas-filled fiber lasers, the role of refractive index on optical scattering in a ceramic made from anisotropic crystals, and a new photorefractive material for mid-IR applications, respectively. The first paper is a review by Nampoothiri et al. [6] on their work on hollow-core optical fiber gas lasers (HOFGLAS). Their laser combines attractive features of fiber lasers such as compactness and long interaction length of pump and laser radiation with those of gas lasers such as the potential for high output power and narrow line width. This paper summarizes recent developments and describes the demonstration of C₂H₂ and HCN prototype lasers. Because only the evanescent field of the guided pump and laser mode interacts with the host material, undesired nonlinear optical effects like stimulated Brillouin scattering (SBS) and laser damage are expected to occur at much higher energy and power levels compared with solid core fibers. Avenues to extend laser emission further into the IR are discussed. Polycrystalline alumina (PCA) ceramic is a candidate exit aperture for high power lasers owing to its ruggedness and low absorption loss, and a heat sink for high power lasers owing to its excellent thermal conductivity. However, in reality it exhibits very high scattering losses causing it to be translucent. This is attributed to the random orientation of alumina grains, which possess hexagonal symmetry, within the ceramic that leads to optical scattering. However, the actual distribution of the orientation of the alumina grains is not well understood. Thus, Hayakawa et al. [7] applied the polarized Raman mapping technique to determine the orientations of the c axis of each grain in translucent PCA samples. The averaged refractive index difference between neighboring alumina grains was then experimentally estimated from the mapping data, and then the light transmission spectra were simulated using Rayleigh-Gans-Debye (RGD) theory and shown to comparae very favorably with the measured spectra. Development of new photorefractive materials, specifically VBGs, operating in the mid-IR spectral range is essential for new compact mid-IR narrow line laser systems. These are required for many applications, including molecular spectroscopy, noninvasive medical diagnostics, industrial process control, environmental monitoring, atmospheric sensing, free space communication, oil prospecting, and numerous defense related applications such as IR countermeasures, monitoring of munitions disposal, and standoff detection of explosion hazards. Martyshkin et al. [8] demonstrate gamma irradiated LiF color center (LIF:CC) crystals as a material for a VBG operating in the mid-IR spectral range. The absence of active absorptions at wavelengths longer than 1.3 µm results in a VBG that is stable under mid-IR irradiation. Their calculations predict a ~60% diffraction efficiency over the 1 to 6 µm spectral range, which could be realized in a ~1 cm long VBG. To verify the concept, they fabricated periodic structures in LiF:CC crystals using an amplified femtosecond Ti:sapphire laser. Diffraction at 1.56 µm was observed and demonstrates the feasibility of LiF:CC crystals for mid-IR VBG applications.

In the Semiconductor Based Optical Materials category, there are four papers. These highlight the development of periodically oriented GaN (PO-GAN) and orientation-patterned GaAs (OP-GaAs) for optical parametric generation, respectively, and electroluminescence in Er:GaN and an optically pumped vertical external-cavity surface-emitting laser (VECSEL) based on strain compensated GaInAs/GaAs quantum wells, respectively. GaN is well suited for the power electronics arena, and also in enabling higher power applications in the nonlinear optics field, since it has a high laser damage threshold and can be pumped by standard 1 μm pump lasers. The first paper, an invited paper by Hite et al. [9], shows a method for growing periodically alternating polarities of GaN on GaN substrates. The resulting PO-GAN samples demonstrate feasibility of using this method to produce structures for optical parametric generation. To be of practical use, efforts are focused on extending the growth to even thicker material, as well as reducing impurities and absorption in the material. Gallium arsenide has excellent characteristics for parametric frequency conversion and is potentially one of the most attractive mid-IR and long-IR nonlinear optical materials. It has a large second-order nonlinear optical coefficient, a wide transparency range, excellent mechanical properties, and a high thermal conductivity. The crystal is optically isotropic precluding birefringent phase matching; however with suitable quasi-phase-matching (QPM), it can be used for numerous nonlinear optical applications. Grisard et al. [10] describe growth of 2-inch wafers of OP-GaAs with a simple process for initial wafer periodic patterning, resulting in the capability to produce 0.5-mm-thick samples with several centimeters length. Thanks to the optimization of the growth process, absorption losses have been reduced down to 2%/cm or less, and the sample thickness recently increased to the millimeter level. Such material improvements have enabled the demonstration of nanosecond pulsed OPOs with several watts of average output power in the 3 to 5 μm range and optical-to-optical conversion efficiencies of about 50%. They may also soon permit the realization of millijoule level sources and continuous wave devices with large tunability. While conventional EDFAs are optically pumped with lasers either at 980 nm or 1480 nm, electrically pumped IR emitters or optical amplifiers based on Er-doped semiconductors potentially offer compact, rugged components for high-speed photonic integrated circuits. Among various host materials, the wide energy band-gap of III-nitride semiconductor materials reduces thermal quenching and leads to better room temperature performance. In addition, the millisecond lifetime of the 1.54 μm emissions and the excellent crystalline quality of III-nitride epilayers make Er-doped III-nitride semiconductors promising candidates to serve as a new class of IR emitters and optical amplifiers with minimal crosstalk for chip-scale applications. Feng et al. [11] demonstrate that the 1.54 μm emission from GaN:Er p-i-n structures is significantly enhanced by increasing the GaN:Er growth pressure from 10 to 20 torr. They attribute this to possibly fewer Ga vacancy related defects generated under higher GaN:Er growth pressure, which results in a reduced number of competitive recombination defects or impurities. The strong emission at 1.54 μm points to possible applications in optical communications using current injected optical amplifiers based on GaN:Er p-i-n structures. Ranta et al. [12] report on the development of an optically pumped VECSEL emitting near 1120 nm using strain compensated quantum wells. The development is motivated by the need to achieve narrow linewidth emission at ~280 nm via fourth-harmonic generation, which is required to cool Mg⁺ ions. The gain mirror had a top emitting geometry, was grown by molecular beam epitaxy, and comprised GaInAs/GaAs quantum wells strain compensated by GaAsP layers. The strain compensation was instrumental for achieving a dislocation free epitaxial structure. VECSEL operation was demonstrated at a fundamental wavelength close to 1118 nm with a linewidth of less than 300 kHz, and a lithium triborate crystal was used for frequency doubling to ~559 nm with an output power of 1.1W. Their results provide a path forward for true realization of a nonlinear source at 280 nm for Mg⁺ cooling derived from a VESCEL device.

In the Waveguides and Thin Films category, there are three papers. The first is an invited talk that highlights all optical switching in coated microsphere whispering gallery mode (WGM) resonators. The other two papers describe novel magneto-optic (MO) nanocomposite hybrids for magnetic field sensors and sol-gel derived hybrid materials that produce feature sizes <50 nm after two-photon polymerization (2PP) using femtosecond lasers, respectively. All-optical switching at low powers has been theoretically proposed in microspheres coated by a Kerr material and very recently demonstrated experimentally. The main advantage of using the electronic Kerr effect for all-optical switching of WGM resonators is that large refractive index changes can be easily obtained on fast time scales using intensities well below the damage thresholds of the polymers. Murzina et al. [13] have shown experimentally that external pump radiation can result in the spectral tuning of WGM in polymer-coated fused-silica microspheres. Depending on the composition of the polymer, its state, and the geometry of the light-microsphere interaction, different regimes of the switching can be attained: fast electronic Kerr switching, as well slow thermal modification of the WGM spectra that exist even for CW lasers. Magnetic field sensors (i.e., magnetometers) have been an intensive area of research over the past few decades, and optical magnetometers have reached sensitivities that make them interesting candidates for applications such as magneto-encephalography and magneto-cardiography. Lately, the design, synthesis, and study of MO nanocomposite materials with magnetic nanoparticles embedded in a nonmagnetic host matrix have also attracted significant attention, as these mesoscopic materials are expected to exploit the magnetic and optical properties of the nanoparticles alongside the ease of processability of the host. However, magnetic nanoparticles are known to aggregate under external magnetic fields as small as that of Earth’s magnetic field and tend to lose optical transparency over time because of increased scattering and phase separation. Lopez-Santiago et al. [14] describe a novel method of preparing nearly monodisperse cobalt ferrite core polymer shell nanoparticles. These engineered nanoparticles can be further embedded into a polymer host matrix to develop highly transparent polymer based magneto-optic materials. A proof-of-principle all-optical magnetometer has been constructed based on the nanocomposite material. A noise equivalent magnetic field sensitivity of 50nT/√Hz was observed using a 3μT 500Hz control magnetic field. Their results indicate that these highly transparent and magneto-optic responsive materials may be used in magnetic field sensing systems where high sensitivity is required. The two-photon polymerization (2PP) technique is a well-established method for fabrication of high-quality 3D-structures with nanoscale resolution. The most attractive advantage of this method is the potential to realize three-dimensional complex objects without geometrical limitations. So far, a large variety of photonic devices has already been produced with feature sizes below the diffraction limit. Using very short laser pulses with the duration of just a few optical cycles (sub-10 fs in the VIS and NIR spectral range) results in 2PP-thresholds that can be reached at a reduced average power allowing for very small volume creation near the threshold. In combination with a high pulse repetition rate (> 0.2 MHz) the quality of photo polymerization rises with shorter pulses. Emons et al. [15] demonstrate superior 2PP with a new photosensitive, nontoxic inorganic–organic hybrid material, which is synthesized by adding a cross-linker to a zirconium-based hybrid material. They were able to create line structures with a minimum width of 45 nm using 10 fs pulses, considerably smaller than the 82 nm obtained with 50 fs pulses and 150 nm without cross-linker. These results reveal that the addition of cross-linker improves the structural stability and increases the ability of the structure to resist the stress caused during the development process.

The editors believe that the future of AIOM is strong and lies in its diversity and depth to address basic scientific problems within the context of applied practical challenges and limitations. We see great headway being made across the board in optical materials from passive optics to active optics, including rare-earth-doped lasers and nonlinear optics at eye safer wavelengths and also extending into the UV and IR wavelength regions. The editors note that the papers for this special issue represent about a dozen countries. We commend the authors for their outstanding work and encourage researchers to continue to develop exciting theoretical and applied research across the global scientific community. We would like to express our gratitude to all authors and reviewers for their efforts in improving the manuscripts during the review process. We also thank David Hagan, Editor-in-Chief of Optical Materials Express, for his support and encouragement of this feature issue and the OSA journal staff for their excellent support during the review and production.