Effect of annealing conditions on the luminescence properties and thermometric performance of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphors.

We report on the synthesis, photoluminescence optimization and thermometric properties of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphor powders. The photoluminescence of Sr2.9Al2O5Cl2:0.1Eu2+ phosphors exhibits a blue-shift with an increasing annealing temperature owing to a decrease in the crystal field strength of the host caused by evaporation of Cl from the material. The quenching of the blue band in favour of the red band observed in the luminescence spectra of Sr2.9Al2O5Cl2:0.1Eu2+ with an increased annealing temperature was explained using the mechanism of the Landau-Zener transitions. The quantum yield and the lifetime of the phosphors depend on the annealing temperature. Phosphor samples annealed at 850 °C, 1000 °C, 1200 °C and 1500 °C were found to be potential luminescence thermometers using the luminescence spectral method. For Sr3Al2O5Cl2:Eu2+ annealed at 1000 °C, the temperature-dependent dual-band intensity ratio demonstrated a high-temperature sensitivity of ∼1.47%/°C in the temperature range of 23 °C to 40 °C which is superior to other reported phosphors with a microsecond decay time, suggesting that the material has potential for sensitive thermometry applications at ambient temperatures.

Laser Applications to Chemical, Security, and Environmental Analysis: introduction to the feature issue.

The eighteenth topical meeting on Laser Applications to Chemical, Security, and Environmental Analysis (LACSEA) was held in Vancouver, Canada from 11-15 July 2022, as part of the Optica Optical Sensors and Sensing Congress in a hybrid format allowing on-site and online attendance. The meeting featured a broad range of distinguished papers focusing on recent advances in laser and optical spectroscopy. A total of 52 contributed and invited papers were presented during the meeting, including topics such as photo-acoustic spectroscopy, imaging, non-linear technologies, frequency combs, remote sensing, environmental monitoring, aerosols, combustion diagnostics, hypersonic flow diagnostics, nuclear diagnostics, fs/ps applications, and machine learning and computational sensing.

The Impact of Large Mobile Air Purifiers on Aerosol Concentration in Classrooms and the Reduction of Airborne Transmission of SARS-CoV-2.

In the wake of the COVID-19 pandemic, an increased risk of infection by virus-containing aerosols indoors is assumed. Especially in schools, the duration of stay is long and the number of people in the rooms is large, increasing the risk of infection. This problem particularly affects schools without pre-installed ventilation systems that are equipped with filters and/or operate with fresh air. Here, the aerosol concentration is reduced by natural ventilation. In this context, we are investigating the effect of large mobile air purifiers (AP) with HEPA filters on particle concentration and their suitability for classroom use in a primary school in Germany. The three tested APs differ significantly in their air outlet characteristics. Measurements of the number of particles, the particle size distribution, and the CO2 concentration were carried out in the classroom with students (April/May 2021) and with an aerosol generator without students. In this regard, the use of APs leads to a substantial reduction of aerosol particles in the considered particle size range of 0.178-17.78 µm. At the same time, the three APs are found to have differences in their particle decay rate, noise level, and flow velocity. In addition to the measurements, the effect of various influencing parameters on the potential inhaled particle dose was investigated using a calculation model. The parameters considered include the duration of stay, particle concentration in exhaled air, respiratory flow rate, virus lifetime, ventilation interval, ventilation efficiency, AP volumetric flow, as well as room size. Based on the resulting effect diagrams, significant recommendations can be derived for reducing the risk of infection from virus-laden aerosols. Finally, the measurements were compared to computational fluid dynamics (CFD) modeling, as such tools can aid the optimal placement and configuration of APs and can be used to study the effect of the spread of aerosols from a source in the classroom.

Laser applications to chemical, security, and environmental analysis: introduction to the feature issue.

This Applied Optics feature issue on laser applications to chemical, security, and environmental analysis (LACSEA) highlights papers presented at the LACSEA 2020 Seventeenth Topical Meeting sponsored by The Optical Society (OSA).

Thermographic laser Doppler velocimetry using the phase-shifted luminescence of BAM:Eu2+ phosphor particles for thermometry.

Simultaneous point measurements of gas velocity and temperature were recently demonstrated using thermographic phosphors as tracer particles. There, continuous wave (CW) excitation was used and the spectral shift of the luminescence was detected with a two-colour intensity ratio method to determine the gas temperature. The conventional laser Doppler velocimetry (LDV) technique was employed for velocimetry. In this paper, an alternative approach to the gas temperature measurements is presented, which is instead based on the temperature-dependence of the luminescence lifetime. The phase-shift between the luminescence signal and time-modulated excitation light is evaluated for single BaMgAl10O17:Eu2+ phosphor particles as they cross the probe volume. Luminescence lifetimes evaluated in the time domain and frequency domain indicate that in these experiments, interferences from in-phase signals such as stray excitation laser light are negligible. The dependence of the phase-shift on flow temperature is characterised. In the temperature sensitive range above 700 K, precise gas temperature measurements can be obtained (8.6 K at 840 K) with this approach.

Three-color vibrational CARS thermometry of fuel-rich ethylene/air flames using a potassium gadolinium tungstate Raman-active crystal as a source of narrowband probe radiation.

Three-color broadband vibrational coherent anti-Stokes Raman scattering (CARS) temperature measurements were carried out in laminar fuel-rich sooting ethylene/air flames. Stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman-active potassium gadolinium tungstate [KGd(WO4)2] crystal was employed as a source of narrowband probe radiation. In the three-color CARS experiment, this wavelength-shifted radiation enables N2-based vibrational CARS temperature measurements in sooting flames free of the signal interference with the absorption/emission bands of the flame intermediate radicals C2. Spatial temperature profiles for different fuel-rich atmospheric pressure ethylene/air flames are presented in comparison with the results of two-color broadband vibrational and dual-broadband pure rotational CARS temperature measurements. The comparison shows the suitability of the three-color CARS measurement technique employing the KGd(WO4)2 crystal for accurate, C2 interference-free, temperature measurements in sooting flames.

Soft Tissue Phantoms for Realistic Needle Insertion: A Comparative Study.

Phantoms are common substitutes for soft tissues in biomechanical research and are usually tuned to match tissue properties using standard testing protocols at small strains. However, the response due to complex tool-tissue interactions can differ depending on the phantom and no comprehensive comparative study has been published to date, which could aid researchers to select suitable materials. In this work, gelatin, a common phantom in literature, and a composite hydrogel developed at Imperial College, were matched for mechanical stiffness to porcine brain, and the interactions during needle insertions within them were analyzed. Specifically, we examined insertion forces for brain and the phantoms; we also measured displacements and strains within the phantoms via a laser-based image correlation technique in combination with fluorescent beads. It is shown that the insertion forces for gelatin and brain agree closely, but that the composite hydrogel better mimics the viscous nature of soft tissue. Both materials match different characteristics of brain, but neither of them is a perfect substitute. Thus, when selecting a phantom material, both the soft tissue properties and the complex tool-tissue interactions arising during tissue manipulation should be taken into consideration. These conclusions are presented in tabular form to aid future selection.

Thermographic laser Doppler velocimetry.

We propose a point measurement technique for simultaneous gas temperature and velocity measurement based on thermographic phosphor particles dispersed in the fluid. The flow velocity is determined from the frequency of light scattered by BaMgAl10O17:Eu2+ phosphor particles traversing the fringes like in conventional laser Doppler velocimetry. Flow temperatures are derived using a two-color ratio method applied to the phosphorescence from the same particles. This combined diagnostic technique is demonstrated with a temperature precision of 4%-10% in a heated air jet during steady operation for flow temperatures up to 624 K. The technique provides correlated vector-scalar data at high spatial and temporal resolution.

High-precision flow temperature imaging using ZnO thermographic phosphor tracer particles.

Zinc oxide (ZnO) particles are characterised as a tracer for temperature measurements in turbulent flows, in the context of the thermographic particle image velocimetry technique. Flow measurements are used to compare the temperature precision of ZnO to that obtained using a well-characterised thermographic phosphor, BAM:Eu(2+), under the same conditions. For this two-colour, ratio-based technique the strongly temperature-dependent redshift of the luminescence emission of ZnO offers improved temperature sensitivity, and so at room temperature a threefold increase in the temperature precision is achieved. A dependence of the intensity ratio on the laser fluence is identified, and additional measurements with different laser pulse durations are used to independently show that there is also a dependence on the laser excitation irradiance, irrespective of fluence. A simple method to correct for these effects is demonstrated and sources of error are analysed in detail. Temperature images in a Re = 2000 jet of air heated to 363 K with a precision of 4 K (1.1%) are presented. The sensitivity of ZnO increases across the tested temperature range 300-500 K, so that at 500 K, using a seeding density of 2 x 10(11) particles/m(3), a precision of 3 K (0.6%) is feasible. This new phosphor extends the capabilities of this versatile technique toward the study of flows with small temperature variations.

Needle geometry, target migration and substrate interactions in high resolution.

Recent investigations considering flexible, steer-able needles for minimally invasive surgery have shown the significance of needle shape in determining the needle-tissue interactions leading to the access of targets. Digital Image Correlation has enabled internal deformation and strain caused by needle insertions to be seen in a soft tissue phantom at high resolution for the first time. Here, the impact of tip design on strains and displacements of material around the insertion axis is presented using Digital Image Correlation in a stable, plane-strain configuration. Insight into the shape of needles to minimise tissue trauma and generate interactions that would enable optimal steering conditions is provided. Needle tips with an included bevel angle up to 40° result in asymmetric displacement of the surrounding tissue phantom. Increasing the included tip angle to 60° results in more predictable displacement and strains that may enhance steering forces with little negative impact on the phantom.

Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles.

This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and non-luminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%.

Tissue deformation analysis using a laser based digital image correlation technique.

A laser based technique for planar time-resolved measurements of tissue deformation in transparent biomedical materials with high spatial resolution is developed. The approach is based on monitoring the displacement of micrometer particles previously embedded into a semi-transparent sample as it is deformed by some form of external loading. The particles are illuminated in a plane inside the tissue material by a thin laser light sheet, and the pattern is continuously recorded by a digital camera. Image analysis yields the locally and temporally resolved sample deformation in the measurement plane without the need for any in situ measurement hardware. The applicability of the method for determination of tissue deformation and material strain during the insertion of a needle probe into a soft material sample is demonstrated by means of an in vitro trial on gelatin.

Acetone laser-induced fluorescence behavior for the simultaneous quantification of temperature and residual gas distribution in fired spark-ignition engines.

Although the fluorescence behavior of acetone has already been examined widely, the amount of data is still not sufficient for the quantification of signals over the parameter field relevant for combustion engines. This leads to large uncertainties when new excitation wavelengths are applied or in cases where temperature and pressure and bath gas composition dependences of the fluorescence yield must be extrapolated from models. This work presents calibration results of the fluorescence signal intensities in nitrogen, air, and an exhaust-gas-air mixture in the wide range from 298 to 748 K and from 0.2 bar (0.02 MPa) to 20 bars for the two important excitation wavelengths 308 and 248 nm. Based on this data, measurements of temperature and exhaust gas concentrations in a fired spark ignition engine were performed with high accuracy in single-shot images also.

Flame front detection and characterization using conditioned particle image velocimetry (CPIV).

We investigate the ability of the conditioned particle image velocimetry technique (CPIV) to derive the actual flame front position in turbulent premixed flames. In CPIV, the flame front shape is deduced from the step in the particle number density in PIV images caused by the steep temperature increase in the reaction zone of premixed flames. In a validation experiment the true flame front position is deduced for comparison from simultaneous heat release measurements using planar LIF measurements of OH and CH(2)O. It is found that CPIV yields nearly the same spatial position as the heat release measurements or the steepest slope in the OH distribution. Furthermore, statistical quantities, derived from the extracted flame front shape, like the spatially resolved turbulent flux, the flame surface density and the flame front curvature are compared, showing negligible differences between the applied methods.

Laser-induced fluorescence of ketones at elevated temperatures for pressures up to 20 bars by using a 248 nm excitation laser wavelength: experiments and model improvements.

Laser-induced fluorescence of acetone and 3-pentanone for a 248 nm excitation wavelength was investigated for conditions relevant for internal combustion engines regarding temperature, pressure, and gas composition. An optically accessible calibration chamber with continuous gas flow was operated by using CO2 and air as a bath gas. According to the varying pressure and temperature conditions during the compression stroke of a spark ignition engine, fluorescence experiments were performed under isothermal pressure variations from 1 to 20 bars for different temperatures between 293 and 700 K. The ketone fluorescence behavior predictions, based on a model previously developed by Thurber et al. [Appl. Opt. 37, 4963 (1998)], were found to overestimate the pressure-related fluorescence increase for high temperature and small wavelength excitation at 248 nm. The parameters influencing the model only in the large vibrational energy regime were newly adjusted, which resulted in an improved model with a better agreement with the experiment. The model's validity for excitation at larger wavelengths was not influenced. For the air bath gas an additional collision and vibrational energy sensitive quenching rate was implemented in the model for both tracers, acetone and 3-pentanone.

Combined coherent anti-Stokes Raman spectroscopy and linear Raman spectroscopy for simultaneous temperature and multiple species measurements.

The simultaneous application of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) and vibrational linear Raman spectroscopy (LRS) for the measurement of temperature and species concentrations in combustion systems is demonstrated. In addition to the standard rotational CARS experimental setup, only one detection system (spectrometer and intensified CCD camera) for the collection of the LRS signals was applied. The emission of the broadband dye laser used for CARS was shifted to the deep red to avoid interferences with the LRS signals located in the visible region. First experimental results from a vaporizing propane spray using an engine injection system are shown.

Simultaneous temperature and exhaust-gas recirculation-measurements in a homogeneous charge-compression ignition engine by use of pure rotational coherent anti-Stokes Raman spectroscopy.

Pure rotational coherent anti-Stokes Raman spectroscopy was used for the simultaneous determination of temperature and exhaust-gas recirculation in a homogeneous charge-compression ignition engine. Measurements were performed in a production-line four-cylinder gasoline engine operated with standard gasoline fuel through small optical line-of-sight accesses. The homogenization process of fresh intake air with recirculated exhaust gas was observed during the compression stroke, and the effect of charge temperature on combustion timing is shown. Single-pulse coherent anti-Stokes Raman spectroscopy spectra could not only be taken in the compression stroke but also during the gas-exchange cycle and after combustion. Consequently, the used method has been shown to be suitable for the investigation of two of the key parameters for self-ignition, namely temperature and charge composition.

Application of a beam homogenizer to planar laser diagnostics.

The first application of a microlens array beam homogenizer to planar laser measurement techniques in combustion diagnostics is demonstrated. The beam homogenizing properties of two microlens arrays in combination with a Fourier lens for widespread applications are presented. An uniform line profile with very little temporal fluctuations of the spatial intensity distribution was generated resulting in a significant reduction of measurement noise and enabling an easier and faster signal processing.

Application of an optical pulse stretcher to coherent anti-Stokes Raman spectroscopy.

An external optical cavity pulse stretcher for nanosecond-long laser pulses has been applied to coherent anti-Stokes Raman spectroscopy (CARS). An increased signal-to-noise ratio was achieved for both vibrational and pure rotational CARS, while the power density of the laser beams remained constant. Moreover, it was demonstrated that the use of the pulse stretcher also leads to improved precision of the determined temperatures and concentrations as a result of repeated excitation of the dye laser.