The further development of lasers and laser systems drives applications in the field of optical sensing and analysis in various fields in industry, military, security, medical and chemistry. For analytical purposes in the field laser sources need to be compact, rugged, tunable in wavelength and of sufficient power to provide data of sufficient quality for temperature, species composition and velocity. Another aspect in laser-based analytical measurements is the speed with which data can be collected which, is limited–besides 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 diagnostics 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) as well as 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 Eleventh Topical Meeting on laser applications to chemical, security, and environmental analysis (LACSEA) 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 and served as an ideal venue for showcasing exciting new developments in this field.

The Fifteenth biennial LACSEA meeting was held in Heidelberg, Germany, 25–28 July, 2016, as part of the OSA Imaging and Applied Optics Congress and featured a broad range of distinguished papers that focused on recent advances in analytical laser and optical spectroscopy. Approximately 80 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 16 LACSEA papers contained in this special feature of Applied Optics represent an exceptionally wide range of interesting and new research in this expanding field. The papers cover a wide range of topics.

Surya Cheemalapati et al. present a method that estimates the local thickness of weathered oil slicks formed on artificial seawater using light transmission and Beer-Lambert’s law. This study has an important practical value and successfully addresses unique challenges related to measurements involving complex, viscous, paste-like heterogeneous substances such as weathered crude oil. Anna Michel et al. report open-path spectroscopic methane detection using a broadband monolithic distributed feedback-quantum cascade laser (DFB-QCL) array. Direct absorption spectroscopy was utilized with intrapulse tuning. Ambient methane levels were detectable and an instrument precision of 70 ppb with 100 second averaging was achieved. The sensor system was designed to work “off the grid” and utilizes batteries that are rechargeable with solar panels and wind turbines. Rongchao Pan et al. performed a simultaneous NIR absorption-based multi-parameter measurement of film thickness, temperature, and concentration of a solute coupled with the Bayesian methodology to infer probability densities for the obtained data. As a practical demonstration, the temporal variation of film thickness, urea concentration, and liquid temperature were recorded during evaporation of a liquid film deposited on a transparent heated quartz plate. Viktor Werwein et al. measured high-accuracy line strengths in the 0002–0000 main isotopologue band of nitrous oxide using Fourier transform infrared spectroscopy with pressure, temperature and optical path length being metrologically traceable to the SI. The obtained results agree with previous studies within the given uncertainties.

Javis A. Nwaboh et al. report on an interband cascade laser (ICL)- absorption spectrometer for absolute, calibration free, atmospheric CO amount fraction measurements, addressing direct traceability of the results. The system combines first principles direct tunable diode laser absorption spectroscopy (dTDLAS) with a metrological validation. Using a multi path cell with 76 m path length the instrument is able to detect close to $0.5\text{nmol}/\mathrm{mol}$ with an integration time of 14 s. The sensors absolute response is in excellent agreement with the gravimetric values of a set of primary gas standards used to test the sensor accuracy. Alireza Moosakhani et al. investigated the effect of hydrocarbon molecular decomposition onpalladium assisted laser induced plasma ablation using a Q-switched Nd:YAG laser focused on palladium target in an optically accessible stainless steel chamber filled to atmospheric pressure with various hydrocarbon atmospheres (C1-C4). Plasma spectra generated from the laser-induced breakdown are analyzed to study the effect of hydrocarbons on the Pd ablated mass, gas reaction products as well as the corresponding plasma parameters such as electron density and plasma temperature during the molecular decomposition processes. Daniel R. Richardson et al. performed a detailed comparison of CARS thermometry when either a chirped-probe-pulse (CPP) or a hybrid femtosecond/picosecond (fs/ps) CARS instrument was used currently the two standard instruments employed in high repetition rate combustion diagnostics technique. It is observed that initial calibration fits to the spectrally broad CPP results require more fitting parameters and a more robust optimization algorithm and therefore significantly increase computational cost and complexity compared to the fitting of hybrid fs/ps CARS data. Andrew D. Cutler et al. demonstrated a novel procedure for determining concentration levels and gas phase temperature from the fitting of coherent anti-Stokes Raman spectra (CARS) of ethylene in combustion. Their WIDECARS technique has been developed for spatially and temporally resolved, simultaneous measurement of temperature and mole fraction of most major species in ethylene-air flames. The method is used to measure mole fraction ethylene in a supersonic combustor burning premixed ethylene and air with single-laser-shot precision (one standard deviation) of $\pm 0.0025$ and bias uncertainty of approximately $\pm 0.001$ when the temperature is close to 1000 K.

Jan Menser et al. investigated the temporal luminescence behavior of silicon atoms during and after laser-heating of gas-borne silicon nanoparticles. A prompt signal at 251.61 and 288.15 nm showed a lifetime of 16 ns that matches the documented excited-state lifetime for the respective transitions. A secondary delayed signal contribution was observed commencing approximately 100–300 ns after the laser pulse and is attributed to electron impact excitation or recombination after electron impact ionization of the silicon evaporated. The observations support a nanoparticle evaporation model that can be used to recover nanoparticle sizes from time-resolved LII data. Weixiong Zhao et al. developed a blue light-emitting diode based incoherent broad-band cavity-enhanced absorption spectroscopy instrument for the measurement of the aerosol extinction coefficient. With an effective absorption path length of 2.8 km, an optimum detection limit of $0.05{\mathrm{Mm}}^{-1}$ ($5\times {10}^{-10}{\mathrm{cm}}^{-1}$) was achieved with an averaging time of 84 s. Field performance of the aerosol extinction spectrometer was demonstrated at the Hefei Radiation Observatory site. Waed Ahmad et al. studied fundamental vibrational signatures of guaiacol and syringol. Nitroderivatives of these two compounds have been also studied experimentally for nitroguaiacol and theoretically for nitrosyringol using the ATR-FTIR synchrotron spectroscopy. All the active fundamental vibrational bands have been assigned and compared to available gas phase measurements providing a vibrational database of the main precursors for the analysis of SOA produced by atmospheric oxidation of methoxyphenols. The vibrational study confirms that nitroguaiacol and nitrosyringol are the main oxidation products of methoxyphenols by OH and are key intermediate in the SOA production. Benjamin Halls et al. demonstrated tracer-free mixture-fraction measurements in a free jet using femtosecond-laser electronicexcitation tagging (FLEET). Measurements were conducted across a turbulent jet both in a pure-nitrogen jet exiting into an air–nitrogen mixture and in a jet containing an air–nitrogen mixture exiting into pure nitrogen. The spatial resolution of the measurement was $\sim 180\mathrm{\mu m}$ and the measurement uncertainty ranged from 5–15%, depending on the mixture fraction and location within the beam, under constant-temperature-and-pressure conditions.

Nader Shokoufi et al. used a combined flow injection-photo thermal lens microscopy system (FI-PTLM) for highly sensitive determination of captopril. Captopril has no absorption in visible range but due to its thiol group could interact with Gold nanoparticles. Gold nanoparticles have absorption in visible range and the PTLM with visible light source enable us sensitive measurement of this compound. The developed method was successfully applied for the determination of captopril in human Serum and pharmaceutical sample. Alexey Porfirev et al. studied theoretically and experimentally the propagation of vortex laser beams in a random aerosol medium. The simulation showed that the stability of vortex beams to fluctuations of an optical medium falls with rising order of optical vortices. Moreover, a coherence length (radius) of the random medium is of great importance. Experimental studies in the random aerosol medium showed that at considerable distances, vortex beams start to demonstrate greater stability that may be explained by their capacity to be regenerated after they passed obstacles. Christian Meissner et al. used Stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman-active KGd(WO4)2 crystal as a light source in a three-color broadband vibrational coherent anti-Stokes Raman scattering (CARS) experiment. $N_2$-based vibrational CARS temperature measurements were performed in sooting flames free of the signal interference with the absorption/emission bands of the flame intermediate radicals $C_2$. A comparison with pure rotational CARS showed the suitability of this method. Yejun Wang and Waruna Kulatilaka simulate and investigate beamsteering effects arising from pressure and temperature gradients in gas-phase media using an optical ray tracing approach. Specific cases involving two-beam and three-beam crossing configurations are simulated by using the ZEMAX OpticStudioTM software package. The results show that thermal gradients in the range of 300–3000 K have minimal effects, while pressure gradients in the range of 1–50 atm result in pronounced beam steering and resulting signal fluctuations in the geometries investigated.

Over the years the LACSEA meeting has become one of the premier must-attend meetings in the field of laser-based spectroscopic analytics, especially for applications associated with combustion diagnostics, chemical analysis, environmental monitoring, and trace species detection related to security and explosives surveillance. Consequently, the next LACSEA meeting is already being planned for June 2018 in Orlando, Florida, and we invite you to participate in this stimulating experience and to contribute with your recent results.

Finally, the feature editors thank 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.