Rolf Szedlak

Broadband mid infrared spectroscopy enables simultaneous probing of multiple molecules but requires light sources capable of tuning over a broad range. Single mode tunable ring lasers are highly attractive for spectroscopic sensing, as they combine compactness with the ability to smoothly tune over spectral bandwidths exceeding those of distributed feedback lasers. In this work, we demonstrate a single unidirectional ring laser that achieves continuous single mode tuning over 20.43 cm–1 (93 nm) in continuous-wave operation at room temperature. This performance is enabled by the cavity shape that increases the T0 of the material by more than 80 K, thereby extending the accessible thermal tuning range. The resulting operating regime is fully compatible with wavelength modulation spectroscopy, a widely adopted technique that offers high sensitivity and robustness for trace-gas detection and other spectroscopic applications. The substantial increase in the tuning range of a single ring represents a key step towards scalable ring-array architectures, in which multiple rings could be combined to cover the majority of the available gain bandwidth and enable broadband, high-resolution on-chip spectroscopy.

Mid-infrared photonics enables a large number of applications in chemical sensing, medical diagnostics, environmental monitoring, and optical communications, but widespread adoption of this technology is hindered by the lack of compact, CMOS-compatible photodetectors capable of room-temperature operation. This work presents an Al–Si–Al planar heterostructure acting as a plasmonic photodetector that leverages an electrostatically tunable Schottky barrier to detect sub-bandgap mid-infrared photons (5–7 μm) in silicon with a responsivity ∼0.9 mA/W, 2 dark current density, and a broad and uniform spectral response that does not require cooling. Fast internal dynamics with time constants of 3.7 ps and 1.4 ns are observed at 1560 nm, suggesting the potential for high-speed operation. Our monolithic and crystalline Al–Si heterostructure features abrupt interfaces obtained without epitaxy, ensuring CMOS compatibility and scalability. We demonstrate the potential of this device for mid-infrared sensing by detecting and spectrally characterizing water molecule absorption lines. By leveraging plasmonically generated hot-carriers combined with the inherent scalability of silicon-based technology, our work opens new pathways for cost-effective and high-speed infrared photodetectors suitable for next-generation integrated photonic systems.

Highly divergent and asymmetric beam profiles are an inherent limitation of facet-emitting mid-infrared semiconductor lasers and complicate their use in free-space optical systems. In this work, we demonstrate beam collimation of a facet-emitting interband cascade laser using a gold-coated parabolic mirror fabricated by high-resolution two-photon lithography. A three-dimensional adaptive resolution printing strategy with dynamically adjusted voxel size for shorter print times is employed. The additively manufactured mirror is externally coupled to the laser and redirects the emitted beam by 90°, transforming the strongly divergent facet emission into a collimated free-space beam without modifying the laser cavity. Beam profiles are characterized experimentally over a propagation distance of up to 32 cm and show a substantial reduction in divergence compared to the uncollimated emission. Wave-optical simulations performed in Ansys Zemax OpticStudio reproduce the measured beam evolution and provide insight into the influence of finite mirror geometry and modal asymmetry. The results demonstrate the potential of 3D-printed reflective micro-optics as flexible beam-shaping elements for mid-infrared laser sources and highlight a pathway toward integrated, alignment-free optical systems.

Semiconductor photonic devices operating in the midwave infrared (mid-IR, which we roughly define here as wavelengths spanning 3 to 14 µm) uniquely address a wide range of current practical needs. These include chemical sensing, environmental monitoring, industrial process control, medical diagnostics, thermal imaging, LIDAR, free space optical communication, and security monitoring. However, mid-IR device technologies are currently still works in progress that are generally much less mature than their near infrared and visible counterparts. Not only are most of the relevant materials more difficult to grow and process, but attainment of the desired optical device performance is often fundamentally more challenging. This Roadmap will review the leading applications for mid-IR optoelectronics, summarize the status and deficiencies of current device technologies, and then suggest possible roadmaps for improving and maturing the performance, manufacturability, and cost of each device type so the critical needs that are uniquely addressed by mid-IR photonics can be satisfied.

Semiconductor photonic devices operating in the midwave infrared (mid-IR, which we roughly define here as wavelengths spanning 3 to 14 µm) uniquely address a wide range of current practical needs. These include chemical sensing, environmental monitoring, industrial process control, medical diagnostics, thermal imaging, LIDAR, free space optical communication, and security monitoring. However, mid-IR device technologies are currently still works in progress that are generally much less mature than their near infrared and visible counterparts. Not only are most of the relevant materials more difficult to grow and process, but attainment of the desired optical device performance is often fundamentally more challenging. This Roadmap will review the leading applications for mid-IR optoelectronics, summarize the status and deficiencies of current device technologies, and then suggest possible roadmaps for improving and maturing the performance, manufacturability, and cost of each device type so the critical needs that are uniquely addressed by mid-IR photonics can be satisfied.

Semiconductor photonic devices operating in the midwave infrared (mid-IR, which we roughly define here as wavelengths spanning 3 to 14 microns) uniquely address a wide range of current practical needs. These include chemical sensing, environmental monitoring, industrial process control, medical diagnostics, thermal imaging, LIDAR, free space optical communication, and security monitoring. However, mid-IR device technologies are currently still works in progress that are generally much less mature than their near infrared and visible counterparts. Not only are most of the relevant materials more difficult to grow and process, but attainment of the desired optical device performance is often fundamentally more challenging. This Roadmap will review the leading applications for mid-IR optoelectronics, summarize the status and deficiencies of current device technologies, and then suggest possible roadmaps for improving and maturing the performance, manufacturability, and cost of each device type so the critical needs that are uniquely addressed by mid-IR photonics can be satisfied.

This review highlights the development and diverse applications of surface‐emitting ring quantum cascade lasers. These lasers, featuring a ring‐shaped cavity and a second‐order distributed feedback grating, enable controlled surface emission, stable single‐mode operation, and collimated, circularly symmetric beams. These advancements have expanded their use in scientific and industrial applications, with significant progress in extending their operation from the mid‐infrared to the terahertz regime. Innovations such as microring designs, phase‐shifted gratings and buried heterostructures have enhanced their scalability, beam quality, and suitability for continuous‐wave operation at room temperature, unlocking opportunities in spectroscopy, imaging, and communication. On‐chip optical solutions and integrated architectures combining emission and detection within a single chip further pave the way for compact and efficient systems for remote sensing and gas monitoring. Progress in miniaturization, thermal stability, and wavelength consistency strengthens the role of ring‐based lasers in industrial, environmental, and medical diagnostics. These advancements, alongside scalable fabrication techniques and enhanced modulation schemes, position ring quantum cascade lasers as key enablers for the next generation of robust, energy‐efficient sensors tailored to diverse scientific and industrial needs.

Superluminescent diodes are usually hard to fabricate and cannot switch to lasing operation. We demonstrate a practical approach for creating devices that can function as a laser as well as a superluminescent diode. The operation mode is controlled by a distinct contact that enables electrical switching between those modes. For the demonstration of this principle, we fabricated devices based on interband cascade gain media. The devices showed superluminescent power of up to 200 μW. The electrically switchable operation combines the good alignment capabilities of a laser with the broadband, low coherence properties of a superluminescent diode.

Interband cascade lasers (ICLs) are becoming increasingly valuable in mid-infrared applications due to their low power consumption and compatibility with silicon photonic integration, particularly for trace gas sensing. ICLs have demonstrated room-temperature continuous-wave operation in the 3–6 μm range, with excellent performance around 3.3 μm. A key factor limiting ICL performance at longer wavelengths is optical loss, i.e., caused by the intervalence band transitions. These losses increase with hole concentration in the active region, leading to a pronounced current-dependence of the optical losses in ICLs. Conventional methods that infer optical losses from length-dependent variations in parameters such as slope efficiency or threshold current require the assumption of constant optical loss. In this study, we present a direct optical transmission measurement technique to determine waveguide losses. Our experiments confirm strongly increasing waveguide losses with current density, directly impacting the quantum efficiency of ICLs. This approach offers a precise evaluation of optical losses and bears a functional alternative compared to traditional methods, addressing the limitations of assuming constant losses and providing enhanced insight into ICL performance across various wavelengths.

We present a compact vertically emitting ring interband cascade laser (ICL) with low power consumption and the possibility for seamless integration into various CO2 sensing applications. Our devices exhibit desirable performance characteristics in battery-driven handheld devices, including room temperature (20 °C) threshold currents as low as 15 mA, small footprints, and stable single-mode emission, suitable for rapid isotope-resolved CO2 detection. Through epi-down bonding with sub-micron accuracy, we achieved robust integration of substrate-emitting ring ICLs, ensuring reliability and scalability that would be required for mass production. We present comprehensive experimental results validating the efficacy of our approach, including spectral analysis and CO2 sensing capabilities with limits of detection of 24 and 13 ppmv utilizing the 12CO2 P(60) and 13CO2 R(10) transitions in the ν3 fundamental band, respectively. The demonstrated devices hold great promise for a wide range of industrial applications, including environmental monitoring, process control, and atmospheric research, where compact low-power sensors are essential.