The mid-infrared spectral regime is technologically important due to its relevance to such applications as thermal imaging, spectroscopic sensing, free-space communications and infrared countermeasures. The field of mid-IR photonics, which spans the generation, manipulation, transmission and detection of mid-IR radiation, has seen significant progress in the past decade, largely spurred by the rapid development of novel materials and device architectures operating at these wavelengths. Examples include the design and fabrication of quantum and interband cascade laser sources [1–4], the growth of unconventional narrow bandgap semiconductors and nanostructures [5,6], the synthesis of new IR glasses and optical ceramics in bulk, planar and fiber form [7–9], the development of novel processing methods for mid-IR photonic materials and device platforms [10–13], as well as the exploration of nonlinear optical interactions in mid-IR transparent materials [14–16]. The papers collected here summarize the forefront of exciting new research in these fields.

The papers in this feature issue can be divided roughly into five main topical categories: 1) laser materials and devices; 2) mid-IR fiber glasses; 3) semiconductors and nanostructures for mid-IR detectors; 4) material platforms for planar integrated components; and 5) nonlinear photon-matter interactions in the mid-IR.

Quantum cascade lasers (QCLs), interband cascade lasers (ICLs), and transition-metal-doped II-VI lasers are three cutting-edge technologies that provide complementary advantages in various overlapping ranges of the mid-IR. In this feature issue, Berry et al. report the demonstration of a Cr-doped ZnSe waveguide laser that produces 1.7 W at 2500 nm wavelength, which represents a six-fold increase compared to previous results for this promising laser class [17]. The authors attribute the higher output power to a reduction of thermal lensing effects in the channel waveguide gain structure, with the maximum output being limited ultimately by thermal quenching of the upper-state lifetime. McCloy et al. study photoluminescence (PL) properties of ZnS prepared by chemical vapor deposition and hot isostatic pressing [18]. Their investigation shows that these materials have highly complicated electronic defect structures, which depend critically on both the sample preparation method and orientation. Zhu et al. [19] demonstrate passive Q-switching of 3 μm Ho³⁺-doped ZBLAN fiber lasers using a Fe²⁺:ZnSe crystal and a graphene deposited fiber mirror as a saturable absorber. Bandyopadhyay et al. [20] and Troccoli et al. [21] review some of the latest advances by QCLs emitting in the 3 - 5 μm and 8 - 12 μm spectral ranges, respectively. Recent achievements highlighted by the Northwestern group include extension of the emission wavelength to as short as 3 μm, a record high QCL wallplug efficiency of 53% at 40 K, a peak output power of 190 W, and a room temperature continuous wave (CW) output power of 5.1 W, with 21% wall plug efficiency. Troccoli et al. compare the performance of longer-wavelength (8-12 μm) QCLs grown by molecular beam epitaxy (MBE) vs. metalorganic chemical vapor deposition (MOCVD). CW output powers exceeding 1 W and double-digit wallplug efficiencies are now attainable at room temperature. It has been noted that the performance of mid-IR interband cascade lasers degrades rapidly with increasing temperature. To identify the critical thermal bottleneck in these lasers, Zhou et al. [22] measure the in-plane and cross-plane thermal conductivities of the cladding layers and active quantum wells of type-II lasers andsuperlattice infrared detectors using the 2-wire 3ω method. The measurement results yield an exceptionally-low isotropic thermal conductivity of 0.49 W/m·K for the AlAsSb digital alloy. Höfling et al. [23] fabricate ICLs with quaternary bulk AlGaAsSb cladding layers, where the bulk claddings provide efficient mode confinement due to their low refractive index, comparable heat conductivity and a reduced current spreading. In an effort to explore an alternative mid-IR laser technology, Almuneau et al. [24] have fabricated and tested a VCSEL structure with lateral oxide confinement and a resonant grating reflector as the top mirror. The electroluminescence sharply peaks at 2.235 μm wavelength, although no lasing action is observed.

Since silica, the dominant optical fiber material for telecommunication bands, becomes opaque beyond 3.5 μm wavelength, alternative fiber-optic materials such as heavy metal oxides and chalcogenides are being explored for mid-IR transmission. Munasinghe et al. [25] report the fabrication and characterization of lead-germanate glass fibers and obtain a high Kerr nonlinearity index n₂ = 56 × 10⁻²⁰ m²/W in an optimized glass composition. Bei et al. [26] discuss methods to reduce optical loss and improve the mechanical strength of fluoroindate glass fibers. Lucas et al. [27] describe telluride chalcogenide glass composition engineering approaches to achieving glass stability, as well as long wave infrared transparency for IR fiber applications. These authors also demonstrate a telluride-based electrophoretic sensor in which the electrically-conductive telluride glass serves as both a capture electrode and an attenuated total reflectance (ATR) optical element for spectroscopic detection of biological species. As an alternative to traditional solid-core optical fibers, hollow glass waveguides (HGWs) capitalizing on metal-dielectric reflective coatings offer potential advantages such as high damage threshold and minimal end facet reflection. Harrington et al. [28] experimentally establish the film growth rates CdS and PbS dielectric thin films that enhance the mid-IR reflection in HGWs. These authors show that optimized deposition conditions allow films with large thicknesses to be achieved for the mid-IR region, as opposed to previous limitations which allowed for optimization in the near-IR only.

Two papers describe the development of advanced materials for mid-IR detectors. Geyer et al. [29] discuss an alternative approach to the multispectral imaging of photons spanning the UV to SWIR bands. UV photons, which normally fall outside the spectral response window of SWIR imagers, are absorbed by a quantum dot (QD) coating and then down-converted to IR photons that can be detected sensitively by the underlying SWIR imager with a high frame rate of 150 kHz. Zhong et al. [30] show in their paper that degenerately-doped InGaBiAs:Si grown by MBE can serve as a new transparent contact material for mid-IR photonics, in contrast to most conventional transparent conductive oxides (such as indium-tin-oxide, ITO) that become opaque due to free carrier absorption. Sheet resistances as low as 7 Ω/􀀀 and high optical transmittances (> 70%) at wavelengths up to 10 μm were obtained simultaneously.

The new field of integrated planar mid-IR photonics is attracting growing research interest. Silicon, the mainstream substrate for microelectronics and silicon photonics, continues to provide an important material platform for mid-IR applications, given its high refractive index and transparency to wavelengths as long as 7 μm. In this issue, Mashanovich et al. [31] present a topical review of passive mid-IR silicon photonic devices, including waveguides, Mach-Zehnder interferometers, multi-mode interference (MMI) splitters, and an angled MMI multiplexer. The heterogeneous integration of other materials with silicon offers versatile photonic functionalities that are essential to an integrated photonic circuit. For example, Roelkens et al. [32] report the integration of GaSb photodetectors, laser sources, and PbS QD photoconductive detectors with silicon waveguides via an adhesive bonding process. Lin et al. [33] evaluate polycrystalline PbTe thin films deposited by thermal evaporation as a potential mid-IR detector material for monolithic integration with chalcogenide glass or pedestal silicon waveguides. Amorphous glasses, in particular chalcogenide glasses (ChG) and heavy metal oxides, represent another promising class of materials for integrated mid-IR photonics, given their broad transparency, high refractive indices, and amorphous structure that is compatible with monolithic integration on different substrates [34,35]. Along this line, Arnold et al. [36] provide an overview of solution processing as an alternative to vacuum-based deposition for ChG film formation. Their method is applied to the hybrid integration of ChG waveguides with QCLs. Zhang et al. [37] model slow light propagation in Ge₂₀Sb₁₅Se₆₅ chalcogenide glass photonic crystal slab waveguides. Bi et al. [38] systematically characterize the structural and optical properties of ZrO₂-TiO₂ thin films deposited by reactive sputtering. Mid-IR transparency of the material at λ = 5.2 μm is demonstrated by optical resonator measurements showing a loaded Q-factor of 11,000.

Six papers in this issue cover the rich physics and material science underlying nonlinear optical interactions in the mid-IR, as well as emerging device technologies that enable these phenomena to be exploited experimentally for practical applications. In particular, mid-IR supercontinuum generation via nonlinear interactions is a topic currently of intensive research interest. Bache et al. [39] describe a method for generating octave-spanning supercontinua and few-cycle pulses, by capitalizing on strongly phase-mismatched cascaded second-harmonic generation (SHG) in mid-IR nonlinear frequency conversion crystals. Yu et al. [40] summarize recent demonstrations of mid-IR supercontinuum generation in ChG materials, and report the experimental demonstration of flat supercontinuum generation in bulk ChG from 2.5 to 7.5 μm. Churin et al. [41] report supercontinuum generation in a liquid-core optical fiber filled with carbon disulfide (CS₂). The paper by Roelkens et al. [32] discuss the nonlinear optical interactions in c-Si and a-Si waveguides in the mid-IR range, including supercontinuum generation and parametric Raman amplification. Jang et al. [42] quantify the mid-IR nonlinear optical properties of KPSe₆ crystals: χ⁽²⁾ = (142.8 ± 10.5) pm/V and χ⁽³⁾ = (4.7 ± 0.6) × 10⁵ pm²/V². The observed strong optical nonlinearities indicate that the material can potentially be utilized for both second- and third-order mid-IR nonlinear optical applications. Mel’nikov et al. [43] demonstrate a pulsed mid-IR source with 10-ns pulse width, few hundred-microjoule energy, and repetition rate adjustable from 10 through 500 kHz based on parametric generation in periodically poled lithium niobate. As an example of applying nonlinear interactions to optical structure fabrication, MacLachlan et al. [44] fabricate and characterize volume phase gratings in gallium lanthanum sulfide ChG using ultrafast laser inscription, and obtain 1st order diffraction efficiencies exceeding 61% at λ = 1.3 μm and up to 24% at 2.64 μm.

In sum, the editors believe that expanding the operation wavelengths of photonic devices from the traditional telecommunications windows to the mid-IR offers immense opportunities for scientific exploration and technological advancement, as well as unique material science and device physics challenges that extend far beyond a simple wavelength scaling. It is our hope that this feature issue offers a timely overview of the dynamic and highly multidisciplinary field of mid-IR photonics, and will spur further research, development, and educational efforts in this area. We especially want to express of our genuine gratitude to all the authors and reviewers for their contributions. We also thank Dr. David Hagan for his support of this feature issue, and the OSA staff for their excellent work throughout the review and production processes.