Stand-off Spectroscopy Research Articles

Standoff and Point Detection of Thin Polymer Layers Using Microcantilever Photothermal Spectroscopy

Standoff detection based on optical spectroscopy is an attractive method for identifying materials at a distance with very high molecular selectivity. Standoff spectroscopy can be exploited in demanding practical applications such as sorting plastics for recycling. Here, we demonstrate selective and sensitive standoff detection of polymer films using bi-material cantilever-based photothermal spectroscopy. We demonstrate that the selectivity of the technique is sufficient to discriminate various polymers. We also demonstrate in situ, point detection of thin layers of polymers deposited on bi-material cantilevers using photothermal spectroscopy. Comparison of the standoff spectra with those obtained by point detection, FTIR, and FTIR-ATR show relative broadening of peaks. Exposure of polymers to UV radiation (365 nm) reveal that the spectral peaks do not change with exposure time, but results in peak broadening with an overall increase in the background cantilever response. The sensitivity of the technique can be further improved by optimizing the thermal sensitivity of the bi-material cantilever and by increasing the number of photons impinging on the cantilever.

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N2+ lasing at 428 nm, assigned to the B2Σu+(ν'=0) and X2Σg+(ν''=1) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B2Σu+(ν'=0) and X2Σg+(ν''=1) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N2+ lasing in ambient air.

Modified range equation for exact modeling and design optimization of active laser remote sensing systems

An exact modeling for laser target illumination and electro-optic detection parameters used in most common laser remote sensing systems, such as laser microphones, LiDARs, laser range finders, laser spot tracking, laser designation, stand-off laser spectroscopy (Raman, LIBS, LIF,…), laser imaging of remote targets and all laser transceiver system having a laser source and an electro-optical detection measure in one assembly. The model makes possible to embed most basic parameters in one equation to ease the design of the main system components from engineering point of view, instead of the currently used general models. The resulting equation corrects calculations for short standoff target detection parameters.

Tracking Airborne Molecules from Afar: Three-Dimensional Metal–Organic Framework-Surface-Enhanced Raman Scattering Platform for Stand-Off and Real-Time Atmospheric Monitoring

Stand-off Raman spectroscopy combines the advantages of both Raman spectroscopy and remote detection to retrieve molecular vibrational fingerprints of chemicals at inaccessible sites. However, it is currently restricted to the detection of pure solids and liquids and not widely applicable for dispersed molecules in air. Herein, we realize real-time stand-off SERS spectroscopy for remote and multiplex detection of atmospheric airborne species by integrating a long-range optic system with a 3D analyte-sorbing metal-organic framework (MOF)-integrated SERS platform. Formed via the self-assembly of Ag@MOF core-shell nanoparticles, our 3D plasmonic architecture exhibits micrometer thick SERS hotspot to allow active sorption and rapid detection of aerosols, gas, and volatile organic compounds down to parts-per-billion levels, notably at a distance up to 10 m apart. The platform is highly sensitive to changes in atmospheric content, as demonstrated in the temporal monitoring of gaseous CO2 in several cycles. Importantly, we demonstrate the remote and multiplex quantification of polycyclic aromatic hydrocarbon mixtures in real time under outdoor daylight. By overcoming core challenges in current remote Raman spectroscopy, our strategy creates an opportunity in the long-distance and sensitive monitoring of air/gaseous environment at the molecular level, which is especially important in environmental conservation, disaster prevention, and homeland defense.

Portable standoff spectrometer for hazard identification using integrated quantum cascade laser arrays from 65 to 11 µm

This article presents new spectroscopic results in standoff chemical detection that are enabled by monolithic arrays of Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs), with each array element at a slightly different wavelength than its neighbor. The standoff analysis of analyte/substrate pairs requires a laser source with characteristics offered uniquely by a QCL Array. This is particularly true for time-evolving liquid chemical warfare agent (CWA) analysis. In addition to describing the QCL array source developed for long wave infrared coverage, a description of an integrated prototype standoff detection system is provided. Experimental standoff detection results using the man-portable system for droplet examination from 1.3 meters are presented using the CWAs VX and T-mustard as test cases. Finally, we consider three significant challenges to working with droplets and liquid films in standoff spectroscopy: substrate uptake of the analyte, time-dependent droplet spread of the analyte, and variable substrate contributions to retrieved signals.

Generation of air lasing at extended distances by coaxial dual-color femtosecond laser pulses

We present an approach for generation of air lasing at extended distances by coaxial dual-color femtosecond laser pulses. A strong 800 nm pulse prepares the population inversion in during the filamentation in air, and a weak 400 nm pulse produced coaxially with the 800 nm light by frequency doubling in a BBO crystal seeds the gain medium, producing strong narrow-band lasing emission at ∼391 nm. We demonstrate that this scheme can overcome the difficulty in spatially overlapping two beams at a far distance, and the lasing emissions at a designed position can be manipulated by dispersion components inserted in the propagation path of the coaxial two-color beam. Our results provide a way for remote generation of air laser for standoff spectroscopy and detection.

In situ analysis of magnesium alloy using a standoff and double-pulse laser-induced breakdown spectroscopy system

To monitor the components of molten magnesium alloy during the smelting process in real time and online, we designed a standoff double-pulse laser-induced breakdown spectroscopy (LIBS) analysis system that can perform focusing, collecting and imaging of long-range samples. First, we tested the system on solid standard magnesium alloy samples in the laboratory to establish a basis for the online monitoring of the components of molten magnesium alloy in the future. The experimental results show that the diameters of the focus spots are approximately 1 mm at a range of 3 m, the ablation depth of the double-pulse mode is much deeper than that of the single-pulse mode, the optimum interpulse delay of the double pulse is inconsistent at different ranges, and the spectral intensity decays rapidly as the range increases. In addition, the enhancement effect of the double pulse at 1.89 m is greater than that at 2.97 m, the maximum enhancement is 7.1-fold for the Y(I)550.35-nm line at 1.89 m, and the calibration results at 1.89 m are better than those at 2.97 m. At 1.89 m, the determination coefficients (R2) of the calibration curves are approximately 99% for Y, Pr, and Zr; the relative standard deviations (RSDs) are less than 10% for Y, Pr, and Zr; the root mean square errors (RMSEs) are less than 0.037% for Pr and Zr; the limits of detection (LODs) are less than 1000 ppm for Y, Pr, and Zr; and the LODs of Y, Pr, and Zr at 2.97 m are higher than those at 1.89 m. Additionally, we tested the system on molten magnesium alloy in a magnesium alloy plant. The calibration results of the liquid magnesium alloy are not as favorable as those of the sampling solid magnesium alloys. In particular, the RSDs of the liquid magnesium alloy are approximately 20% for Pr and La. However, with future improvements in the experimental conditions, the developed system is promising for the in situ analysis of molten magnesium alloy.

Application of Stand-off Double-Pulse Laser-Induced Breakdown Spectroscopy in Elemental Analysis of Magnesium Alloy**supported by National Natural Science Foundation of China (No. 61473279), the National High-Tech Research and Development Program of China (863 Program) (No. 2012AA040608) and Equipment Development Programs of the Chinese Academy of Sciences (No. YZ201247)

In this study, a stand-off and collinear double pulse laser-induced breakdown spectroscopy (DP LIBS) system was designed, and the magnesium alloy samples at a distance of 2.5 m away from the LIBS system were measured. The effect of inter-pulse delay on spectra was studied, and the signal enhancement was observed compared to the single pulse LIBS (SP LIBS). The morphology of the ablated crater on the sample indicated a higher efficiency of surface pretreatment in DP LIBS. The calibration curves of Ytterbium (Y) and Zirconium (Zr) were investigated. The square of the correlation coefficient of the calibration curve of element Y reached up to 0.9998.

Active FTIR-based stand-off spectroscopy using a femtosecond optical parametric oscillator.

We presented the first demonstration of stand-off Fourier transform infrared (FTIR) spectroscopy using a broadband mid-infrared optical parametric oscillator, with spectral coverage over 2700-3200 cm⁻¹. For vapor-phase water and nitromethane (NM), stand-off spectra was recorded using a concrete target at from 1-m to 2-m range and showed good agreement with reference spectra, and in NM a normalized detection sensitivity of 15 ppm·m·Hz(-1/2) was obtained. Spectra from 50-μL droplets of liquid thiodiglycol were detected at a stand-off distance of 2 m from aluminum, concrete and painted metal surfaces. Our results imply that OPO-based active FTIR stand-off spectroscopy is a promising new technique for the detection of industrial pollutants and the identification of chemical agents, explosives or other hazardous materials.

Comparative study on fast classification of brick samples by combination of principal component analysis and linear discriminant analysis using stand-off and table-top laser-induced breakdown spectroscopy

Focusing on historical aspect, during archeological excavation or restoration works of buildings or different structures built from bricks it is important to determine, preferably in-situ and in real-time, the locality of bricks origin. Fast classification of bricks on the base of Laser-Induced Breakdown Spectroscopy (LIBS) spectra is possible using multivariate statistical methods. Combination of principal component analysis (PCA) and linear discriminant analysis (LDA) was applied in this case. LIBS was used to classify altogether the 29 brick samples from 7 different localities. Realizing comparative study using two different LIBS setups — stand-off and table-top it is shown that stand-off LIBS has a big potential for archeological in-field measurements.

Generation of an air laser at extended distances by femtosecond laser filamentation with telescope optics

We present the generation of self-induced-white-light-seeded lasing action of nitrogen molecules in air by a Ti:sapphire femtosecond laser (800 nm, 5.5 mJ) and demonstrate that such lasing action is strongly influenced by external focusing conditions. It is found that the self-seeded lasing signal of N(2)(+) at ~391 nm decreases dramatically by orders of magnitude and ultimately disappears when the focal length of an external lens increases from 0.5 m to 1 m. By using a telescope, it is shown that such limitation can be overcome and the 391 nm lasing can be controlled to occur at remotely designated distance, providing a possibility for practical applications in standoff spectroscopy.

Time-resolved stand-off UV-Raman spectroscopy for planetary exploration

The exploration of Mars, Europa and Enceladus has provided evidence that support the existence of present or past potentially habitable environments, which may shelter signatures of extinct or extant life. A search for further evidence for habitability or for life requires the development of sophisticated instruments and techniques that enable detailed investigations of locations, which are of great interest to planetary scientists and astrobiologists. Raman spectroscopy is a powerful and versatile technique; a rover based Raman Laser Spectrometer (RLS) operating at 532nm excitation wavelength has been selected for the 2018 ExoMars mission. In this study, we demonstrate the feasibility of the utilisation of a time-resolved stand-off UV-Raman prototype for the detection and identification of pure organics, organics mixed in a quartz matrix and minerals that have been selected based on their potential relevance to astrobiology and planetary exploration. The samples of organics (β-carotene, l-ascorbic acid, thiamine hydrochloride, l-alanine, l-serine, thymine), carbonates (calcite, dolomite), sulfates (gypsum), silicates (quartz), and natural rock (an altered meta-volcanic rock featuring quartz inclusions) were analyzed at a distance of 6m using a 355nm excitation source and a gated intensified charged-coupled device (ICCD) as the detector. We were able to obtain spectra with clear Raman signals enabling unequivocal identification of all selected samples. We assert for the first time, that such an instrument can effectively identify minerals and a wide range of organics that may serve as geo- and biomarkers thus showing great potential for the exploration of planets and astrobiology.

Self-seeded forward lasing action from a femtosecond Ti:sapphire laser filament in air

428 nm forward lasing action was observed from a femtosecond laser filament in air created by Ti:sapphire laser pulses. The 800 nm femtosecond laser filament not only provides a source for population inversion between two vibrational levels ( and ) of N2+ but also generates a 428 nm seed from filament-induced white light. This simple technique will find more applications in standoff spectroscopy.

Arrayed free space continuous-wave terahertz photomixers

We present free space coherent arrays of continuous-wave terahertz (THz) photomixers and compare the results to on-chip arrays. By altering the relative phases of the exciting laser signals, the relative THz phase between the array elements can be tuned, allowing for beam steering. In addition, the constructive interference of the emission of N elements leads to an increase of the focal intensity by a factor of N2 while reducing the beam width by ∼N(-1), below the diffraction limit of a single source. Such array architectures strongly improve the THz power distribution for stand-off spectroscopy and imaging systems while providing a huge bandwidth at the same time. We demonstrate this by beam profiles generated by a 2×2 and a 4×1 array for a transmission distance of 4.2 m. Spectra between 70 GHz and 1.1 THz have been recorded with these arrays.

Single-mode tapered quantum cascade lasers

We demonstrate tapered quantum cascade lasers monolithically integrated with a distributed Bragg reflector acting as both a wavelength-selective back mirror and a transverse mode filter. Each of the 14 devices operates at a different wavelength between 9.2 and 9.7 μm, where nine devices feature single-mode operation at peak powers between 0.3 and 1.6 W at room temperature. High output power and excellent beam quality with peak brightness values up to 1.6 MW cm−2 sr−1 render these two-terminal devices highly suitable for stand-off spectroscopy applications.

High-power arrays of quantum cascade laser master-oscillator power-amplifiers

We report on multi-wavelength arrays of master-oscillator power-amplifier quantum cascade lasers operating at wavelengths between 9.2 and 9.8 μm. All elements of the high-performance array feature longitudinal (spectral) as well as transverse single-mode emission at peak powers between 2.7 and 10 W at room temperature. The performance of two arrays that are based on different seed-section designs is thoroughly studied and compared. High output power and excellent beam quality render the arrays highly suitable for stand-off spectroscopy applications.

Master-oscillator power-amplifier quantum cascade laser array

We report on the demonstration of an array of master-oscillator power-amplifier quantum cascade lasers (QCLs) operating in single-mode at different wavelengths between 9.2 and 9.8 μm. In each device, the output of a distributed feedback QCL is injected into a tapered QCL section which acts as an amplifier while maintaining a high beam quality due to adiabatic mode spreading. All array elements feature longitudinal as well as transverse single-mode emission at peak powers between 0.8 and 3.9 W at room temperature. The high output power and excellent beam quality render the array highly suitable for stand-off spectroscopy applications.

Plasma-assisted coherent backscattering for standoff spectroscopy

We show that an intense coherent backward signal can be generated through a Raman-type four-wave-mixing process using forward propagating fields only. Phase matching for this process is achieved through a plasma modulation of the refractive index. Applications to standoff spectroscopy are discussed.

Ultra violet decomposition of surface adsorbed explosives investigated with infrared standoff spectroscopy

Here we present a standoff spectroscopic technique where ultraviolet (UV) and infrared radiations are used in tandem to identify explosive residues on a surface 8 m away. Instead of analyzing byproducts, UV illumination induced changes in the adsorbates are investigated. The UV radiation is used to photodecompose the surface adsorbed explosive residues while the surface is probed with a tunable infrared quantum cascade laser. IR spectral changes generated after UV exposure are compared to the IR spectra before UV exposure to yield a signature similar to traditional transmittance plots for each explosive residue tested. Spectral variation the explosive compounds can be clearly identified with a surface concentration of 700 ng/cm 2.

Standoff spectroscopy via remote generation of a backward-propagating laser beam.

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.