The Seventeenth Topical Meeting on Laser Applications to Chemical, Security, and Environmental Analysis (LACSEA) was scheduled to be held in Vancouver, Canada, from 22–26 June 2020 as part of the Optical Sensors and Sensing Congress. However, the COVID-19 pandemic rendered a personal meeting impossible, and OSA made the valiant decision to transform the complete meeting into a virtual event. Despite vastly different time zones for speakers and attendees from around the globe and substantially different presentation formats, almost all authors did present their work under the new circumstances, making the meeting a great success. Innovative events, such as virtual coffee breaks, provided a platform for participants to link up and discuss selected topics. The quality of the submitted work remained very high, reflecting the global effort to adapt and maintain scientific and technical activities during the difficult situation. Our thanks go to the entire OSA conference staff for the very professional organization and their rapid adaptation to the online format and for conducting a successful meeting. We believe that lessons learned from the 2020 meeting will—at least partly—be incorporated into future conferences.

The development of lasers, laser systems and detectors is driving new applications in optical sensing in various wide-ranging areas such as industry, security, medicine, material, chemistry and military. In particular for applied experimental work in the field, laser sources need to be compact, rugged, tunable in wavelength, and of sufficient power to provide quality data for temperature, species composition, and velocity determination. Another aspect in laser-based analytical measurements is the speed with which data can be collected, which is limited—except for constraints in detector electronics—by the repetition rate and wavelength flexibility (if required) of the employed laser systems. Considerable progress has been made in recent years in high-repetition-rate pulsed laser systems with pulse energies in the hundreds of millijoule range that make even optical diagnostics methods with inherently low signal intensity, such as Raman spectroscopy, feasible for long range detection. This special issue is aimed at scientists, engineers, and practitioners interested in understanding the basic principles and diagnostic purposes of a variety of laser-based methods for the quantitative detection and monitoring of essential parameters in probed samples of different origin and aggregate state, i.e., gas, liquid or solid. Its intent is to bring together the development areas of the necessary equipment (lasers, detectors, optics), the methods themselves, and new approaches. But it is also diverse in its applications in chemical, biophysical, combustion, and environmental analysis. As such, interactions within the field often involve a wide diversity and a strong interdisciplinary exchange among researchers. The Seventeenth Topical Meeting on Laser Applications to Chemical, Security, and Environmental Analysis was the latest in a long line of similar meetings that have included some of the most advanced research in laser spectroscopic analysis and its applications, while serving as an ideal venue for showcasing exciting new developments in this field.

This LACSEA meeting featured a broad range of distinguished papers that focused on recent advances in analytical laser and optical spectroscopy. A total of 52 outstanding papers were presented at the meeting, including topics such as absorption-based gas phase diagnostics, laser-induced breakdown spectroscopy for remote detection of security related species and explosives, new laser and terahertz sources for remote sensing, and micro-optical laser-based systems. Additionally, the technical program included new fundamental research on laser spectroscopic detection techniques for medical, biochemical, and combustion applications. As a result, the number and breadth of the 17 LACSEA papers contained in this special feature of Applied Optics represent an exceptionally wide range of interesting and new research in this expanding field.

Femto- and picosecond laser electronic excitation tagging (FLEET and PLEET, respectively) was the topic of four contributions. Hill et al. [1] apply FLEET for 1 kHz, pointwise, tracer-free velocity measurements in the hypersonic boundary-layer of an ogive-cylinder model. The potential of the FLEET technique is demonstrated and challenges and strategies of for direct boundary layer velocity measurement are discussed. The FLEET method was also used by Grib et al. [2] They propose a rare-gas-assisted approach taking advantage of a multiphoton-resonance enhancement in argon admixed to a nitrogen flow. This works very well for velocimetric tracking of the nitrogen gas in flow environments by virtue of the long-lived nature of the excited species. It is demonstrated that tuning the laser to a three-photon-resonant transition of argon provides significant improvements compared to earlier work. A related approach using picosecond lasers for electronic excitation tagging is referred to as PLEET. Stereoscopic picosecond laser electronic excitation tagging (S-PLEET) was developed by Colter Russel et al. [3] for time-resolved, tracer-free, three-component flow velocity measurements in flows. A pulse-burst system is used for photodissociation of nitrogen along a line and the subsequent luminescence is captured by two intensified high-speed CMOS cameras for 1-dimensional velocity measurements at a repetition rate of 100 kHz. Analysis shows the capabilities and limitations of the technique. Zhang et al. [4] present a detailed analysis of the PLEET method taking a number of key parameters into account including the lifetime of the nitrogen emissions, the power dependence, the pressure dependence, and local flow heating by the laser pulses. Their results demonstrate that when PLEET is performed with 24-ps pulses high quality data can be obtained at 100 kHz repetition rate. Interestingly, less flow disturbance is observed in comparison to 100-ps PLEET.

Four papers are contributed to atmospheric and environmental sensing development and application. Nee et al. [5] demonstrated aerosol robotic network (AERONET) and light detection and ranging (LIDAR) for remote sensing of volcanic aerosols from eruptions of Nishionshina Island observed in southern Taiwan. Increasing aerosol loadings were observed, beginning on 5 August 2020, based on PM10/PM2.5 and the aerosol optical depth (AOD) of AERONET. LIDAR measurements showed strong aerosol layers at heights of 0–2 km comparable to AERONET AOD. Utilization of the demonstrated remote optical sensing technique allows to accurately monitor daily air quality and identify the pollutant sources. Remote laser sensing is also often used for gas leak detection in the field, particularly methane leak detection motivated by safety, environmental, legal, or economic aspects. Lahyani et al. [6] have developed an all-fiber pulsed laser source emitting light in the vicinity of 2.05 µm for simultaneous remote sensing of ${{\rm{CO}}_2}$ concentration and wind velocity. Two narrow-linewidth master oscillators, for ON-line/OFF-line ${{\rm{CO}}_2}$ differential absorption LIDAR operation, alternately seed a four-stage amplifier chain at a switching rate up to 20 kHz. The developed laser source will allow standalone km-range ground-based ${{\rm{CO}}_2}/{\rm{wind}}$ measurement using coherent detection. The paper by Yadong et al. [7] also concerns optical remote sensing, but with focus on data processing rather than experimental hardware. Laser point cloud registration is a key step in multi-source laser scanning data fusion and application. They propose a hierarchical registration algorithm of laser point clouds, mitigating issues with overlapping regional features and influence of building eaves, which improves registration accuracy. Strahl et al. [8] demonstrate a standoff methane leak detection and localization for leakage rates of 2 and 5 ml/min using tunable laser spectroscopy (TLS) combined with mid-infrared imaging. Sensitive and quantitative visualization of methane gas leaks (pixelwise sensitivity ${\sim}{{1}}\;{\rm{ppm*m}}$) at a frame rate of 125 Hz is achieved. The demonstrated imaging technique could serve as an excellent basis for automated leak identification and localization by means of artificial intelligence systems.

Six papers are contributed to energy- and combustion related research. Hsu et al. [9] present a new high-speed method for imaging temperatures and flame curvature in reacting flows. Using a high-repetition-rate pulsed laser system, single-shot images of temperature distributions in flames are achieved by 2-color laser-induced fluorescence of hydroxyl radicals. The detailed flame dynamics exposed by the method shows that curvature and temperature are correlated. The developed technique is expected to be a valuable asset in fundamental and applied research on turbulent reacting flows. Williams et al. [10] present a method to recover pressure measurements from LIG signals by extracting a pressure-dependent signal-decay-lifetime. This method was applied successfully to an optically accessible engine, yielding an accuracy of better than 10% at all tested conditions above atmospheric pressure. Prenting et al. [11] present a detailed characterization of tracer dyes for liquid-phase thermometry by two-color laser-induced fluorescence. They tested five different dyes dissolved in a variety of solvents that are frequently used in spray flame synthesis of nanoparticles. The dissolved tracers were excited at either 266, 355, and 532 nm for temperatures between 296 and 393 K, and for concentrations ranging between 0.1 and 10 mg/L. The absorption and fluorescence spectra revealed temperature dependences, signal re-absorption levels, the impact of different solvents and varying two-component solvent compositions. Gragston et al. [12] present a comparison of different schemes for fuel-to-air ratio (FAR) measurements using laser-induced breakdown spectroscopy (LIBS). They used laser pulses of 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) duration to induce breakdowns in an atmospheric Hencken flame. It was found that the emission spectra for ns-LIBS and ps-LIBS were very similar in the range of 550–800 nm, with slightly elevated atomic oxygen lines in the ps-scheme. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, but significantly weaker ionic emission from nitrogen. The authors conclude with a number of recommendations for each scheme and highlight that fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS. Coherent anti-Stokes Raman scattering (CARS) has proven to be a widely used technique for temperature measurements in combustion facilities, due to its high spatial resolution, dynamic range and background emission insensitivity. In particular, rotational CARS (RCARS) spectroscopy provides excellent temperature sensitivity for measurements below 2000 K, while allowing simultaneous measurement of multiple specie. However, quantitative measurements employing the RCARS technique require good knowledge of the spectral widths of $S$-branch Raman transitions, especially in the presence of the collisional partner molecules commonly encountered in combusting mixtures. Hölzer et al. [13] performed S-branch Raman linewidth measurements of oxygen using time-domain picosecond RCARS. ${{\rm{O}}_2}$ S-branch Raman linewidths and their broadening in the presence of ${{\rm{N}}_2}$ for the temperature range of 295 K–1950 K are reported. Hsu et al. [14] have designed a compact fiber-coupled hyperspectral imaging sensor operating in the wavelength range from ultraviolet to near-infrared. The HSIS allows remote recording of two-dimensional spectrally-resolved thermal radiation and chemiluminescence from ultra-high-temperature ceramics. The experiments suggest that the sensor could be a valuable tool for hypersonic material characterization in practical arc jet facilities that have limited optical access.

Beyond the development of novel detection approaches, the sensitive detection of trace gases outside of pristine laboratory environments also requires the advancement and miniaturization of the spectroscopic tools used in these approaches. Hoppe et al. [15] discuss and characterize a high-speed GaSb-based external cavity diode laser (ECDL) driven resonantly by a microelectromechanical system (MEMS) actuator. This ECDL spans the 1980–2090 nm wavelength range, allowing access to several molecular absorption bands, including that of ${{\rm{CO}}_2}$, whereas the high tuning speeds (scan rates ${\sim}{\rm{kHz}}$) afforded by the MEMS actuator provide rapid tuning free of mode hopping. This design exhibits the potential to allow rapidly scanned detection of trace gas molecular fingerprints for a variety of applications. Milde et al. [16] demonstrate the miniaturization of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for the mobile monitoring of trace gases. The QEPAS approach, which uses a quartz tuning fork to detect the acoustic resonance induced by absorption of a modulated light source, allows sensitive detection of gases such as ${{\rm{CO}}_2}$ and ${{\rm{CH}}_4}$. By reducing the size of a QEPAS sensor to fit within a standard 14-pin butterfly housing (${18.1}\;{\rm{mm}} \times {{8}.\rm{1mm}}$), the authors present a MicroQEPAS sensor that promises versatile use in mobile, handheld packages. Gomolka et al. [17] present a method for methane detection utilizing photothermal spectroscopy near 1651 nm inside a hollow-core fiber (HCF). The impact of various experimental parameters on the signal amplitude and signal-to-noise ratio is analyzed. With a 1.3-m-long HCF and a fiber amplifier for signal enhancement, the proposed technique is capable of detecting methane at single parts-per-million levels. This makes it interesting for industrial applications such as leak detection of natural gas.

Over the years, the LACSEA meeting has established itself as one the most important meetings in the field of laser-based spectroscopic analytics, in particular for applications associated with combustion diagnostics, chemical analysis, environmental monitoring, and trace species detection related to security and explosives surveillance. The LACSEA 2022 meeting is planned to take place in July, presumably in Vancouver, and we invite you to participate in this stimulating experience and to contribute with your recent results.

Finally, the feature editors thank all the authors and reviewers for their exceptional contributions, which allowed this special feature issue to be completed on time. Further, we want to acknowledge the outstanding help of the OSA staff.