Waveguide-enhanced Raman spectroscopy (WERS) efficiently collects Stokes-shifted scattering from target molecules in the evanescent field surrounding nanophotonic waveguides. By using a sorbent material as a top cladding, vapor phase analytes can be detected and identified at ambient densities as low as a few parts-per-billion. Previous demonstrations of vapor-phase WERS have used free-space optical components, such as microscope objectives and bulk Raman filters, to couple and filter light to and from the sorbent-clad waveguide. In this work we demonstrate a complete photonic integrated circuit (PIC) assembly that is packaged and fiber-coupled enabling us to measure WERS from trace vapor concentrations. The PIC comprises low-loss edge couplers from polarization maintaining single-mode optical fibers, sensing trenches with a sorbent top-cladding, and lattice filters for separation of the Stokes signal from the laser. The PICs are fabricated at AIM Photonics using the Silicon Nitride Passive PIC process with the TLX-VIS component library. Then, they are packaged into assemblies with permanent fiber-attach using fiber arrays. The sorbent is deposited in a thin, uniform layer in the sensing trench using one of two deposition techniques: nano-plotting and drip-coating. A laser wavelength of 785 nm enables the use of a compact spectrometer with a thermoelectrically-cooled silicon detector. Spectra are obtained with exposure times of a few seconds and show parts-per-billion detection limits for select vapors. This work successfully demonstrates the use of a compact Raman spectrometer integrated with a fully assembled PIC via optical fibers for the detection of low-density vapor-phase analytes.
Waveguide-enhanced Raman spectroscopy (WERS) using nanophotonic waveguides has been used to demonstrate the detection of vapor-phase chemicals and liquid-phase biomolecules in water. The technique benefits from the fabrication processes and tolerances of CMOS foundries, but successful foundry-based WERS photonic integrated circuits (PICs) have only been demonstrated using excitation wavelengths of 1064 nm and 785 nm. Foundry-based PICS are beginning to operate with low loss at visible wavelengths, and WERS is uniquely poised to take advantage of this capability. Raman scattering cross-sections scale as λ−4, so a visible WERS platform could enable increased sensitivity, decreased exposure times, and/or decreased laser powers. However, increased fluorescence, increased waveguide loss, and decreased feature sizes make WERS in the visible challenging. Here, we demonstrate WERS using 300-mm foundry-based fabrication (AIM Photonics) with 633 nm and 785 nm laser excitation. We also show the successful operation and integration of other required components for a compact WERS system operating in the visible, including edge-couplers and lattice filters.
Under the U.S. Army Forensics Advanced Research Program, the The Portable Chemical Fingerprint Identification System (P-CFIS) is being developed by U.S. Army Combat Capabilities Development Command –Chemical Biological Center (DEVCOM-CBC) enabling trace level (non-visual) detection of solid particulate contamination on surfaces and residual fingerprints. This operationally flexible non-contact, non-destructive methodology is reducing the need to transport potentially contaminated materials and provide a field forward detection capability yielding greater situational awareness of the threat environment. This uniquely developed prototype based on Raman spectroscopy allow the system to scan a one-inch square area of interest, which may be flat or uneven, target any found particles in the field of view, and automatically analyze and report detection events of threat chemicals contained within the spectral database of the system. This presentation will describe the preliminary results from evaluating the performance of the prototype systems, discussing time of analysis, particle size characteristics, analysis of heterogeneous surfaces, and future development of the systems ultimately leading to the development of the next generation of expeditionary systems for military forensic analysis, checkpoint detection, and/or sensitive site exploitation.
Drop-on-demand and inkjet printing technology continues to be a promising method of producing chemical test standards with scalability and flexibility to allow for inexpensive, high-throughput production of samples. This enabling technique provides precise, accurate and highly reproducible test coupons that mimic the hazardous chemicals encountered in various theater scenarios; critical in assessing the performance of existing and future sensors detection capabilities. Under the U.S. Army Forensics Advanced Research Program, the Spectroscopy Branch within the Research and Technology Directorate, DEVCOM CBC, along with internal and external collaborative partners are currently utilizing the Direct Color Systems 1800z flat-bed inkjet printer for deposition of various chemicals on relevant surfaces and GeSiM NP2.1 Nanoplotter for more precise and control droplet deposition to support various optical and non-optical detection objectives. The samples produced under this project are used for the evaluation of trace level energetic materials and illicit drugs of abuse within latent fingerprints, deposition of sorbent polymers onto photonic integrated circuits for vapor detection, point sensors, and more recently exploring enhanced training aids for military working dogs. This work will present results from the characterization of utilized chemical deposition techniques as well as recent experimental results from various assessed detection technologies
Recent work with B. anthracis Delta Sterne spores demonstrated that Raman spectroscopy could be used to discriminate between viable and gamma deactivated spores and provided initial insight into the probable source of discrimination found in the spores. From this previous work, we believe through Raman spectral analyses of viable and deactivated spore samples, significant changes in spectral response can be resolved and ascribed to classes of biomolecules affected by the deactivation processes. We expanded upon this study to include four different Bacillus spores (B. anthracis, B. megaterium, B. thuringiensis, and B. atrophaeus) and probe de-activation techniques to include gamma radiation UV radiation, chemical, and thermal methodologies. We used sequential Raman imaging scanning electron (RISE) microscopy to determine chemical (Raman spectral information) and physical (SEM imaging) variance between viable and deactivated spore samples. Additional use of machine learning algorithms to
Detection of threat materials is an important capability for the military and homeland security to protect soldiers and civilians. Waveguide-enhanced Raman spectroscopy (WERS), a photonic integrated circuit sensing methodology, is being developed for field detection of materials related to chemical warfare agents, explosives, and narcotic threats. Low-fluorescence silicon nitride spiral waveguides with long path lengths are used to obtain high signal levels with nearinfrared excitation (785 nm and 1064 nm). Compact single-mode-fiber-coupled spectrometers with high sensitivity are being utilized for detection of the Raman scattered light. Thermoelectrically cooled charged coupled device (CCD) or InGaAs detectors (-15 °C) provide for low-noise and high-quantum-efficiency spectral measurement. Performance comparable to that obtained with large benchtop spectrometers is observed. The spiral waveguides are coated with functionalized polymer sorbents suitable for concentrating relevant classes of threat materials in the evanescent field of the waveguide. The sorbents are deposited using piezoelectric microdispensers to allow for controlled deposition of thin films without the need for spin-coating. Raman chemical imaging microscopy is used to characterize the uniformity of the sorbent polymers on the waveguides. Library spectral matching can be used in combination with the selectivity of the sorbent materials to provide discrimination of the materials absorbed by the polymer coatings. The ultimate objective is development of a prototype handheld WERS sensor system suitable for defense and security applications in the field. WERS development and spectral measurements will be presented.
Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of vapor-phase analytes using a functionalized chip-scale photonic circuit. Here, we show that WERS signal can be collected from part-per-billion levels of targeted analytes in a backscatter geometry, which, compared to forward-scatter, simplifies component integration and is more tolerant of waveguide loss and modal interference. In addition, we discuss our progress towards a compact Raman sensing system that incorporates a handheld spectrometer and chip-scale optical filters. We demonstrate that a handheld, thermo-electrically cooled spectrometer can be used for backscatter WERS with a comparable signal-to-noise to that of a liquid-nitrogen cooled benchtop spectrometer. Finally, we describe efforts to integrate the dichroic Raman filter on-chip using arrays of unbalanced Mach-Zehnder interferometers. Measurements show filter performance sufficient for integration with WERS: Transmission of >80% of the laser in the cross port and Stokes signal in the through port; and extinction of the laser by >20 dB in the though port and of Stokes signal by >8 dB in the cross port.
The U.S. Army Research Development Engineering Command Chemical Biological Center (RDECOM C&B) continues to develop technologies for the forensic detection of energetic materials and illicit drugs of abuse due to their recent confluence in counter terrorism operations. One specific technology developed here is the use of Raman Chemical imaging to detect these substances located concomitant with residual latent fingerprints. This study demonstrates the ability to identify threat materials non-destructively so that the fingerprint remains intact for further biometric analysis. Utilizing Raman spectroscopy, the Generation I Chemical Fingerprint Identification System (CFIS) semi-autonomously locates and identifies particles of interest found on the friction ridge of a given recovered fingerprint with minimal input from the operator. This work presents results from a collaborative effort between the U.S. Defense Forensic Science Center (DFSC) and RDECOM C&B in which two prototype CFIS systems were assessed with a variety of samples and examines additional practical considerations leading toward the development of the next generation of expeditionary systems for military forensic analysis.
Matrix assisted laser desorption ionization (MALDI) is a powerful technique that improved the mass spectrometry (MS) characterization of biological molecules. However this technique requires the mixing of matrix compound with the analyte of interest. The matrix compound used in MALDI process is not universal and usually depends heavily on the nature of analyte of interest being analyzed. As such there are many matrices that are used and without knowing the nature of your analyte it will be hard to predict which matrix is optimal for the most effective MALDI-MS analysis. Moreover, a high energy laser exposure is needed to initiate the ionization process through a charge transfer process between the matrix and analyte molecules. Recent advancement in the metalorganic framework (MOF) field introduced desirable surfaces that can be modified for various applications. Such MOFs can be synthesized with porous solid, and could have regular or predicted geometry. This project is introducing a novel idea of utilizing a modified MALDI substrate with MOF that can provide charge transfer between immobilized functionalized groups and analyte molecules that mimic the solvation process when a solution matrix is used. Begin the abstract two lines below author names and addresses.
Raman spectroscopy is a powerful tool capable of identifying unknown materials. In the past 20 years, laser sources and detectors have been getting smaller which has led to the development of handheld Raman sensors for use by the military and first responders. One of the advantages of Raman sensors is that it requires no sample preparation, however the incident laser must be able to interrogate the sample which means that the measurement must be taken in the open air or a transparent container. If an unknown material is found in an opaque container, it is typically transferred into a transparent sample jar which represents a handling hazard for the operator. More recently, a technique known as Spatially Offset Raman Spectroscopy (SORS) has shown the ability to measure Raman signals for materials stored in opaque (non-metallic) containers which would eliminate this hazard. Alternatively, advanced algorithm techniques can be used with traditional epi-illumination laser excitation to extract weak Raman signatures from noisy backgrounds or complex mixtures caused when looking at chemicals stored in Raman-active containers. For this study we compare spectral results obtained from two commercially available Raman instruments which use an epi-illumination configuration and SORS against a common set of transparent and opaque containers.
Raman spectroscopy is a powerful tool for obtaining molecular structure information of a sample. While Raman spectroscopy is a common laboratory based analytical tool, miniaturization of opto-electronic components has allowed handheld Raman analyzers to become commercially available. These handheld systems are utilized by Military and First Responder operators tasked with rapidly identifying potentially hazardous chemicals in the field. However, one limitation of many handheld Raman detection systems is strong interference caused by fluorescence of the sample or underlying surface which obscures the characteristic Raman signature of the target analyte. Munitions grade chemical warfare agents (CWAs) are produced and stored in large batches and typically have more impurities from the storage container, degradation, or unreacted precursors. In this work, Raman spectra of munitions grade CWAs were collected using a handheld Raman spectrometer with a 1064 nm excitation laser. While Raman scattering generated by a 1064 nm laser is inherently less efficient than excitation at shorter wavelengths, high quality spectra were easily obtained due to significantly reduced fluorescence of the munitions grade CWAs. The spectra of these less pure, but more operationally relevant, munitions grade CWAs were then compared to spectra of CASARM grade CWAs, as well as Raman spectra collected using the more common 785 nm excitation laser.
Raman spectroscopy is a powerful tool for obtaining molecular structure information of a sample. While Raman spectroscopy is a common laboratory based analytical tool, miniaturization of opto-electronic components has allowed handheld Raman analyzers to become commercially available. These handheld systems are utilized by Military and First Responder operators tasked with rapidly identifying potentially hazardous chemicals in the field. However, one limitation of many handheld Raman detection systems is strong interference caused by fluorescence of the sample or underlying surface which obscures the characteristic Raman signature of the target analyte. Munitions grade chemical warfare agents (CWAs) are produced and stored in large batches and typically have more impurities from the storage container, degradation, or unreacted precursors. In this work, Raman spectra of munitions grade CWAs were collected using a handheld Raman spectrometer with a 1064 nm excitation laser. While Raman scattering generated by a 1064 nm laser is inherently less efficient than excitation at shorter wavelengths, high quality spectra were easily obtained due to significantly reduced fluorescence of the munitions grade CWAs. The spectra of these less pure, but more operationally relevant, munitions grade CWAs were then compared to spectra of CASARM grade CWAs, as well as Raman spectra collected using the more common 785 nm excitation laser.
Raman spectra were taken from whole sheep’s blood with varying levels of acetylcholinesterase (AChE) inhibition using 229 and 532 nm laser excitation wavelengths. AChE levels were inhibited using the organophosphates malathion, paraoxon-ethyl, and octamethyldiphosphoramide and confirmed using the Ellman method. This AChE activity level was investigated with the Raman spectra and analyzed using a partial least squares calibration and cross validation to determine if the AChE activity could be predicted from the Raman spectrum. Correlation scores of 0.78 and 0.26 between the measured and predicted AChE activity were observed using 229 and 532 nm excitation, respectively. A estimate limit of detection was found to be approximately 0.01 ΔA/min.
Raman microspectroscopy and principal component analysis are used to decipher unique biomolecular information by
monitoring the effect of residence time of Bacillus spores suspended in deionized water. Suspensions of viable spores of
Bacillus anthracis Sterne (BA), Bacillus atrophaeus (BG), and Bacillus thuringiensis were prepared and spectrally
monitored from initial deposition (time zero) and intermittently for seven days. Questions addressed include if spectral
variations are significant with bacterial species and residence time under non-germination conditions, is the
discrimination capability affected, and are there markers indicating pre-germination activity. Clear spectral distinction
for the spore suspensions was observed with respect to residence time, however, when the residence time data were
combined, discrimination analyses showed significant overlap between the BA and BG spores. Temporal spectral
analyses at select wavenumbers suggest an increase in pre-germination activity from the freshly suspended to one day
suspensions.
We present the methodology and results of a standard assessment protocol to evaluate disparate SERS substrates that
were developed for the Defense Advanced Research Programs Agency (DARPA) SERS Science and Technology
Fundamentals Program. The results presented are a snapshot of a collaborative effort between the US Army Edgewood
Chemical Biological Center, and the US Army Research Laboratory-Aldelphi Laboratory Center to develop a
quantitative analytical method with spectroscopic figures of merit to unambiguously compare the sensitivity and
reproducibility of various SERS substrates submitted by the program participants. We present the design of a common
assessment protocol and the definition of a SERS enhancement value (SEV) in order to effectively compare SERS active
surfaces.
We are actively investigating the use of Raman spectroscopy for proximal standoff detection
of chemicals and explosive materials on surfaces. These studies include Raman Chemical Imaging of
contaminated fingerprints for forensic attribution and the assessments of commercial handheld or
portable Raman instruments operating with near-infrared (IR) as well as ultraviolet (UV) laser
excitation specifically developed for on-the-move reconnaissance of chemical contamination. As
part of these efforts, we have measured the Raman cross sections of chemical agents, toxic industrial
chemicals, and explosives from the UV to NIR. We have also measured and modeled the effect
interrogation angle has on the Raman return from droplets on man-made surfaces. Realistic droplet
distributions have been modeled and tested against variations in surface scan patterns and laser spot
size for determining the optimum scan characteristics for detection of relevant surface
contamination.
We present an initial bacterial fate study of Gram positive vegetative cells suspended in water and stored at
ambient room temperature via Raman spectroscopy monitoring. Two types of cells were considered for this
study: vegetative cells of Bacillus cereus, Bacillus thuringiensis which contain the polyhydroxybutyric acid
(PHBA) as an energy storage compound and Bacillus subtlilis cells which do not. The cells were cultured
specifically for this project. Immediately following the culturing phase, the bacteria were extracted, cleaned
and at the onset of the study were suspended in de-ionized water and stored at room temperature. Aliquots of
suspensions were deposited onto aluminum slides at different times and allowed to dry for Raman analysis.
Spectra from multiple regions of each dried spot and each deposit time were acquired along with the bright-field
and fluorescence images. Results were examined to investigate the effect of suspension time on the
spectral signatures as well as the fate behavior of the three types of cells investigated. The cells were
monitored daily for over a 14 period during which time the onset of starvation induced sporulation was
observed.
Wide-field Raman chemical imaging (RCI) has been used to detect and identify the presence of trace
explosives in contaminated fingerprints. A background subtraction routine was developed to minimize the
Raman spectral features produced by surfaces on which the fingerprint was examined. The Raman image was
analyzed with a spectral angle mapping routine to detect and identify the explosives. This study shows the
potential capability to identify explosives non-destructively so that the fingerprint remains intact for further
biometric analysis.
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