Sensors based on laser-induced fluorescence (LIF) enable the rapid detection of micron-size airborne pathogens. As with any sensor, the central design issue is the trade between sensitivity and selectivity. In the case of a LIF bio-particle sensor, the objective is to best distinguish a small concentration of “threat” particles against a potentially much larger concentration of harmless “background” particles, without an excessive rate of falsely alarming when threat particles are absent. In this paper, we characterize sensor performance using four inter-related metrics -- sensitivity, probability of detection, false positive rate (FPR) and response time. We develop several sensor design principles and present a new approach to signal processing called the “degree of threat” algorithm. We describe a recent experiment quantifying the performance of a BioLert testbed in distinguishing a biological agent (Bacillus globigii spores) from a mineral dust (kaolin), using a receiver operating characteristic (ROC) curve to show the trade between sensitivity and FPR.

Models of optically-based biological aerosol sensors may help to predict baseline performance and support efficient sensor optimization. Reducing a sensor’s false positive rate while maintaining sensitivity is an important performance goal that must be optimized. To that end, the capacity to theoretically test environmental backgrounds, in an accelerated fashion, would be valuable. Sensor false positives are presumed to occur as a result of complicated transient fluctuations in the environmental aerosol background. Simulating a sensor’s response to such naturally occurring transients, with an appropriate model, is a mechanism for accelerating sensor characterization. These models complement and reduce the need for experimentally challenging interferant tests. Additionally, validated models include the ability to characterize sensor responses to harmful agents or rare materials while simultaneously adjusting many transient parameters. We describe a model of the Lincoln Laboratory Biological Agent Warning Sensor (BAWS), highlighting our general approach to sensor model architecture. The resulting model was utilized to simulate the sensor’s response to a variety of individual background constituents as well as to time varying backgrounds with multiple constituents. The result of the simulation predicts the sensor’s false positive rate to a simulated indoor and outdoor aerosol background, which can be compared to experimental data. Model applications and improvements will be discussed.

Detect-to-warn defense strategies against airborne contamination are based on providing warning to personnel to take temporary protective actions. The effectiveness of such detect-to-warn active strategies is measured by the reduction in contaminant exposure compared to passive exposure. Effectiveness depends on several factors, including the contaminant release and transport properties, the warning sensor performance and the protective actions taken. In this paper we analyze effectiveness for several specific scenarios where certain reasonable protective actions are assumed and sensor performance is varied. One type of scenario analyzed is the protection of outdoor personnel against an upwind instantaneous point release. Meteorological conditions such as wind speed, turbulence level and heat flux, which result in high exposure levels are assumed. Personnel are warned to temporarily use filter masks based on a warning signal from a sensor placed between them and the release point. Another type of scenario is the protection of personnel inside of a building using active ventilation control. The building air handling properties, such as air exchange and recirculation, degree of leakage and filtration and zone volume, are representative of modern office buildings. Different sensor locations and ventilation control strategies are chosen to defend against outside and inside instantaneous point releases. In each scenario, we evaluate the dependence of effectiveness on sensor sensitivity threshold and response time. In addition, we describe desired values of other sensor attributes, such as false positive sensing rate, size, power consumption, maintenance frequency and procurement cost, to support realistic deployment and operations.

We present an analytical model evaluating the suitability of optical absorption based spectroscopic techniques for detection of chemical warfare agents (CWAs) and toxic industrial chemicals (TICs) in ambient air. The sensor performance is modeled by simulating absorption spectra of a sample containing both the target and multitude of interfering species as well as an appropriate stochastic noise and determining the target concentrations from the simulated spectra via a least square fit (LSF) algorithm. The distribution of the LSF target concentrations determines the sensor sensitivity, probability of false positives (PFP) and probability of false negatives (PFN). The model was applied to CO₂ laser based photoacosutic (L-PAS) CWA sensor and predicted single digit ppb sensitivity with very low PFP rates in the presence of significant amount of interferences. This approach will be useful for assessing sensor performance by developers and users alike; it also provides methodology for inter-comparison of different sensing technologies.

We describe a low-cost prototype bio-aerosol fluorescence sensor designed for unattended deployment in medium to large area networks. The sensor uses two compact xenon flash units to excite fluorescence in an aerosol sample volume drawn continuously from the ambient environment. In operation, the xenons are pulsed alternately at 300ms intervals whilst absorption filters restrict their radiation output to UV bands ~260-290nm and ~340-380nm respectively, optimal for exciting the biological fluorophores tryptophan and NADH. Fluorescence from all particles instantaneously present within a sensing volume is measured using two miniature photomultiplier detectors optically filtered to detect radiation in the bands ~320-600nm and ~410-600nm. The second of these bands covers the principal emission from NADH, whilst the difference between the first and second detector channels yields fluorescence in the 320-410nm band, covering much of the tryptophan emission. Whilst each sensor is clearly limited in specificity, the low sensor cost (<$5k) offers potential for the deployment in large networks that would be prohibitively expensive using particle fluorescence sensors based on currently available UV lasers. Preliminary details are also given of a variant of the sensor, currently under development, in which xenon illumination is used to acquire single particle fluorescence data at rates of up to 200 particles per second.

Laser-induced fluorescence (LIF) provides a real-time technique for detecting micron-size airborne pathogens. Early LIF biological particle sensors used harmonic generation of UV in solid-state lasers to excite fluorescence. UV diode lasers have several key advantages over traditional lasers: a greater selection of wavelengths for the efficient and selective excitation of specific fluorescent biological compounds; continuous output so that all sampled particles are interrogated; and the ability to combine several UV diode lasers emitting at different wavelengths into a compact multiple-wavelength source for simultaneously exciting several biofluorophores. The coincident detection of multiple biofluorophores is expected to markedly improve discrimination of airborne pathogens from non-biological background aerosols. In this paper, we describe BioLert 2x16C5+1 - a LIF bio-particle sensor with two diode lasers, detection of sixteen fluorescence emission bands bundled into five user-defined linear combinations, and an elastic scatter detector. BioLert 2x16C5+1 also features fluorescence photon counting for sensitivity sufficient to distinguish between single bacterial spores and similar size inert particles, improved signal processing for optimally distinguishing between airborne pathogens and harmless particles, and a highly integrated air sampling system.

A rationale for evaluating bioaerosol sensor technology for building protection applications is presented. Issues associated with bio-threat sensor systems for buildings include sensor performance metrics, standards and cost. The low-cost AirSentinel bioaerosol sensor is highlighted as an example of an approach that addresses the issue of affordability.

This paper presents the status of an ongoing development of a point detector for biological warfare agent sensing based on ultraviolet laser-induced fluorescence from single particles in air. The detector will measure the fluorescence spectra of single particles in a sheath flow air beam. The spectral detection part of the system consists of a grating and a photomultiplier tube array with 32 channels, which measure fluorescence spectra in the wavelength band from 300 nm to 650 nm. The detector is designed to measure laser induced fluorescence from single laser pulses and has been tested by measuring fluorescence from simulants of biological warfare agents in aqueous solution. The solutions were excited with laser pulses at the wavelengths of 293 nm and 337 nm. The paper also presents preliminary results on the sheath flow particle injector and time-resolved measurements of fluorescence from biological warfare agent simulants in solution.

A lightweight, tactical biological agent detection network offers the potential for a detect-to-warn capability against biological aerosol attacks. Ideally, this capability can be achieved by deploying the sensors upwind from the protected assets. The further the distance upwind, the greater the warning time. The technological challenge to this concept is the biological detection technology. Here, cost, size and power are major factors in selecting acceptable technologies. This is in part due to the increased field densities needed to cover the upwind area and the fact that the sensors, when deployed forward, must operate autonomously for long periods of time with little or no long-term logistical support. The Defense Advanced Research Project Agency’s (DARPA) Solid-state Ultraviolet Optical Source (SUVOS) program offers an enabling technology to achieving a detector compatible with this mission. As an optical source, these devices emit excitation wavelengths known to be useful in the detection of biological aerosols. The wavelength band is absorbed by the biological aerosol and results in visible fluorescence. Detection of a biological aerosol is based on the observed intensity of this fluorescence signal compared to a background reference. Historically this has been accomplished with emission sources that are outside the boundaries for low cost, low power sensors. The SUVOS technology, on the other hand, provides the same basic wavelengths needed for the detection process in a small, low power package. ECBC has initiated an effort to develop a network array based on micro UV detectors that utilize the SUVOS technology. This paper presents an overview of the micro UV detector and some of the findings to date. This includes the overall design philosophy, fluid flow calculations to maximize presentation of aerosol particles to the sources, and the fluorescence measurements.

A compact Ultraviolet Biological Trigger Lidar (UBTL) instrument for detection and discrimination of bio-warfare-agent (BWA) simulant aerosol clouds was developed by us [Prasad, et al, 2004] using a 5mW, 375nm semiconductor UV optical source (SUVOS) laser diode. It underwent successful field tests at Dugway Proving Ground and demonstrated measurement ranges of over 300m for elastic scattering and >100m for fluorescence. The UBTL was modified during mid-2004 to enhance its detection and discrimination performance with increased range of operation and sensitivity. The major optical modifications were: 1. increase in telescope collection aperture to 200 mm diameter: 2. addition of 266nm and 977nm laser transmitters: 3. addition of three detection channels for 266nm and 977nm elastic backscatter and fluorescence centered at 330nm. Also the commercial electronics of the original UBTL were replaced with a multi-channel field programmable gate array (FPGA) chip for laser diode modulation and data acquisition that allowed simultaneous and continuous operation of the UBTL sensor on all of its transmitter and receiver wavelengths. A notebook computer was added for data display and storage. Field tests were performed during July 2004 at the Edgewood Chemical and Biological Center in Maryland to establish the enhanced performance of UBTL subsystems. Results of these tests are presented and discussed.

Bioaerosol weapons pose a threat to both troops and civilians. Remote detection of bioaerosols is important for timely deployment of effective countermeasures against these weapons and for triggering other detection systems. In this paper we describe a new approach for remote bioaerosol detection based on an eye-safe spectrally broadband backscatter LIDAR. This technique illuminates a remote cloud using a spectrally broadband laser centred about 1.5 μm. The spectrally backscattered fraction of the broadband illumination beam is detected. Using an inverse Monte Carlo algorithm, the particle size distribution and refractive index of the cloud particles can be determined. In this way threat clouds containing anomalous man-made distributions of particles could be discriminated from normal background clouds. The laser is a custom designed source based on a special non-collinear optical parametric oscillator configuration. The laser produces Q-switched pulses with a maximum spectral bandwidth covering the 1.4 to 1.8 μm region. In practice the spectral region of 1.52 to 1.75 μm is used as this matches an atmospheric transmission window. A comparison of this broadband backscatter LIDAR technique, with the commonly used UV lidar fluorescence technique will be presented. Progress to date and details of a prototype LIDAR system will be described.

In this paper, we consider the advantages that segmentation can give towards detecting sub-pixel point targets in hyperspectral imagery. We will show that by segmenting the image, we can find a better estimate for the covariance matrix; the resulting target estimation is vastly improved. A method to deal with edge points is discussed.

Unmanned air vehicles (UAVs) today are mostly used for reconnaissance and sometimes weapons delivery. Remote sensing of chemical-biological (CB) agents is another beneficial use of UAVs. While remote sensing of CB agents can be done by LIDAR spectroscopy, this technology is less spatially precise and less sensitive than actual measurements on a collected sample. One family of UAVs of particularly unique benefit for CB sampling and in-flight analysis is the Honeywell family of Organic Air Vehicles (OAVs). This vehicle with its ability to hover and stare has the unique ability among UAVs to collect and analyze chem-bio samples from a specific location over extended periods of time. Such collections are not possible with other micro-air-vehicles (MAVs) that only operate in fly-by mode. This paper describes some of the Honeywell OAV features that are conducive to CB detection.

Major improvement into the sensitivity of broadband Fourier transform infrared (FTIR) spectrometers, used in gas analysis, can be achieved by a photoacoustic detection system, which bases on a recently introduced optical pressure sensor. The sensor is a cantilever-type microphone with interferometric measurement of its free end displacement. By using a preliminary prototype of the photoacoustic gas detector, equipped with the proposed sensor and a black body radiation source, a detection limit in the sub-ppb range was obtained for e.g. methane gas. The limit, obtained in non-resonant operation mode, is very close to the best photoacoustic results achieved with powerfull laser sources and by exploiting the cell resonances. It is also orders of magnitude better than any measurement with a black body radiation source. Furthermore, the ultimate sensitivity leads on to very small detection limits also for several chemical warfare agents (CWA) e.g. sarin, tabun and mustard. The small size of the sensor and its great thermal stability enables the construction of an extremely sensitive portable CWA analyzer in the near future.

We report sensitive and selective detection of Diisopropyl methylphosphonate (DIMP) - a decomposition product of Sarin and a common surrogate for the nerve gases - in presence of several gases expected to be interferences in an urban setting. By employing photoacosutic spectroscopy with broadly tunable CO₂ laser as a radiation source we demonstrate detection sensitivity for DIMP in the presence of these interferences of better than 0.5 ppb in 60 second long measurement time, which satisfies most current homeland and military security requirements and validates the photoacoustic spectroscopy as a powerful technology for nerve gas sensing instrumentation.

The remote detection and identification of liquid chemical contamination is a difficult problem for which no satisfactory solution has yet been found. We have investigated a new technique, pulsed indirect photoacoustic spectroscopy (PIPAS), and made an assessment of its potential for operation at stand-off ranges of order 10m. The method involves optical excitation of the liquid surface with a pulsed laser operating in the 9-11μm region. Pulse lengths are of order 3μs, with energy ~300μJ and repetition rates ~200Hz. Rapid heating of the liquid by the laser pulse produces acoustic emission at the surface, and this is detected by a sensitive directional microphone to increase the signal-to-noise ratio and reduce background clutter. The acoustic pulse strength is related to the liquid's absorption coefficient at the laser wavelength; tuning allows spectroscopic investigation and a means of chemical identification. Maximum coverage rates have been examined, and further experiments have examined the specificity of the technique, allowing a preliminary assessment of false-alarm and missed-signal rates. The practical aspects of applying the technique in a field environment have been assessed.

Recent experimental field trials have demonstrated the ability of both Fourier transform infrared (FTIR) and active light detection and ranging (LIDAR) sensors to detect particulate matter, including simulants for biological materials. Both systems require a reliable, validated, quantitative database of the mid infrared spectra of the targeted threat agents. While several databases are available, none are validated and traceable to primary standards for reference quality reliability. Most of the existing chemical agent databases have been developed using a bubbler or syringe-fed vapor generator, and all are fraught with errors and uncertainties as a result. In addition, no quantitative condensed phase data on the low volatility chemicals and biological agents have been reported. We are filling this data gap through the systematic measurement of gas phase chemical agent materials generated using a unique vapor-liquid equilibrium approach that allows the quantitation of the cross-sections using a mass measurement calibrated to primary, National Institutes of Standards and Technology (NIST) standards. In addition, we have developed quantitative methods for the measurement of condensed phase materials in both transmission and diffuse reflectance modes. The latter data are valuable for the development of complex index of refraction data, which is required for both system modeling and algorithm development of both FTIR and LIDAR based sensor systems. We will describe our measurement approach and progress toward compiling the first known comprehensive and validated database of both vapor and condensed phase chemical warfare agents.

Mid-wave/Long-wave IR (3-14 μm) semiconductor lasers such as QC and Sb can be used for standoff chemical agent sensing in a network architecture that is different from conventional absorption lidars. Compact, potentially inexpensive semiconductor lasers may allow using them in a large number that form a cooperative network in which, the integrated sensing information is much more than the sum of its parts. This paper presents a study of system architecture based on CDMA, similarly to a CDMA optical wireless network, which allows a system of many distributed units to plug-and-play and cooperate with each other for N² information scaling, rather than interfering with each other in non-networked architecture. This paper describes experimental studies with this system architecture, conducted with M/LWIR lasers, near-IR lasers, using wavelength-division-multiplexing (WDM) technique for high spectral fidelity, optical scanner for multi-spectral imaging, and simulated spatially distributed transmitters and receivers for sensor network. Specifically, the use of advanced lasers capable of broad and continuous wavelength tuning and modulation for WMS imaging is described. The experimental results suggest that M/LWIR spectral imaging with WDM multi-spectral transmitters is highly promising for chemical agent detection and visualization.

Fully optimized molecular geometry, parameters of reactivity and vibrational spectra of triacetone triperoxide (TATP) and homologue organic peroxides were calculated using B3LYP/6-31G(d,p) method within the Density Functional Theory formalism. Infrared and Raman Spectroscopy were utilized to obtain vibrational spectra of the energetic compound. The model consists in the relation found between the Raman Shift location of the important symmetric stretch ν(O-O) of the organic peroxides and the reactivity of the organic peroxides. A good correlation between the band location in the series studied and the x-y plane polarizability component and the ionization energy was found. Gas phase IR absorption of TATP in air was used for developing stand-off detection schemes of the important organic peroxide in air. The sublimation properties of TATP were measured using two methods: Grazing Angle Probe-Fiber Coupled FTIR and gravimetric on stainless steel surfaces. Sublimation rates, loading concentration values and absorbance band areas were measured and modeled using the persistent IR vibrational signature of the ν(C-O) mode.

We have used subpicosecond time-resolved photoluminescence (TRPL) downconversion techniques to study the interplay of carrier localization and radiative and nonradiative processes in the active regions of light emitting III-nitride semiconductor ultraviolet optical sources, with the goal of identifying potential approaches that will lead to higher radiative efficiency. Comparison of TRPL in (In)AlGaN multiple quantum well active regions indicate that for addition of only 0.01 In content the PL decay time in an InAlGaN MQW is more than double that in an AlGaN MQW designed to emit at the same wavelength (360 nm), thus indicating the importance of indium for improvement of material quality, most likely through the suppression of point defects. This result is further underscored by TRPL data on 320 nm InAlGaN MQW active regions, which exhibit longer PL lifetimes than expected for growth on GaN templates with dislocation densities in the mid-10⁸cm^-2 range. While the PL lifetimes in these InAlGaN MQWs improve for growth on lower dislocation density HVPE bulk GaN substrates, a similar phenomenon is not observed for deposition on nearly dislocation-free bulk AlN substrates, suggesting that defect generation in the MQWs associated with lattice mismatch or AlN surface preparation may play an important role. The pump intensity dependence of the time zero signal and the TRPL decays in the MQWs implies that internal electric field-induced recombination through the barriers and interface states plays an important role in the radiative efficiency of quantum well active regions for c-axis oriented materials and devices. The effect of these internal electric fields can be mitigated through the use of nonpolar MQWs. The combination of more intense time-integrated PL spectra and shorter PL lifetimes with decreasing well width in GaN/AlGaN MQWs grown on a-plane LEO GaN for low pump intensity suggests that the radiative lifetime becomes shorter due to the accompanying increase in exciton binding energy and oscillator strength at smaller well width in these high quality samples. Finally, it is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create spatial localization that enhances the luminescence efficiency and PL lifetime (300-400 ps) despite high defect density (>10¹⁰cm^-2) by inhibiting movement of carriers to nonradiative sites. Significant enhancement of this phenomenon has been obtained in a DH LED structure grown on a lower defect density (mid-10⁹cm^-2) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10⁷ cm^-2 range, and only a 3.3 times drop in PL intensity when the temperature is raised from 12 K to room temperature, suggesting up to ~ 30% internal quantum efficiency.

Recent progress and outlook in quantum cascade lasers (QCLs) in the mid- to far-infrared wavelength range (3.6-16 μm) are reviewed. Our recent work has focused on the development of high-power continuous-wave (CW) QCLs emitting in wavelengths of 4.3-6.3 μm at room temperature and above. For λ~6 μm, advanced heterostructure geometries, including the use of a thick electroplated gold, epilayer-side heat sink lead to the first remarkable high-power CW QCL performance above room temperature, and a buried-ridge heterostructure are demonstrated to improve significantly laser performance (i.e., 579 mW at 298 K and operation up to 343 K) when combined with narrow laser ridges. Through re-engineering the optimized strain-balanced design, a similar excellent operation is achieved at 4.3-6.3μm. The pulse operations of the shorter wavelength (3.6-4 μm) and the long wavelength (8-16 μm) QCLs at room temperature are also demonstrated. Lastly, these results are put in the perspective of other reported results and possible future directions are discussed.

Type-II Interband cascade lasers combine interband optical transitions with interband tunneling to enable the cascading of type-II quantum well active regions. This combination allows for low threshold current densities and high external slope efficiencies, both of which are important for high temperature, high power operation. Experimental results have already demonstrated some of this potential including high differential external quantum efficiency (>600%), high peak output powers (~6 W/facet at 80 K), high cw power conversion efficiency (>32% at 80 K), and lasing above 315 K under pulsed conditions. However, cw operation at high temperature has not yet been achieved - present generation 3.6-μm-wavelength interband cascade lasers fail to operate under cw conditions at heat sink temperatures above ~214 K. Past performance highlights and recent advances are described, followed by a discussion of issues that continue to limit high temperature, cw performance. The outlook for improving device performance is presented, including a discussion of areas where further research is needed.

Recent progress in wide-bandgap semiconductor optoelectronics resulted in an appearance of deep-UV light-emitting diodes (LEDs), which can be used for fluorescence excitation in a variety of chemical and biological compounds. We used two generations of AlGaN-based UVTOP series deep ultraviolet LEDs developed by Sensor Electronic Technology, Inc. The peak wavelength of these fully packaged devices is 340 nm and 280 nm, line width at half maximum approximately 10 nm, wall-plug efficiency up to 0.9% and output power in the milliwatt range. The second-generation emitters are shown to have an extremely low level of unwanted long-wavelength emission what is important for fluorescence measurements. The UV LEDs were tested for fluorescence excitation in standard fluorophores (organic dyes), autofluorescent biological compounds (riboflavin, NADH, tryptophan, and tyrosine) and medical specimens (fluid secreted by prostate gland). Fluorescence lifetime measurements in the frequency domain were demonstrated using UVTOP-340 and -280 devices. The output of the LEDs was modulated at frequencies up to 200 MHz by high-frequency current drivers and the phase angle of the fluorescence signal was resolved using a radio-frequency lock-in amplifier. Nanosecond-scaled measurements of fluorescence lifetimes, which are the “fingerprints” of chemical and biological compounds, were demonstrated.

A novel, compact and robust UV laser has been developed for laser induced fluorescence spectroscopy of biomolecules in the spectral region from 290 nm to 345 nm. It was based on a frequency-doubled passively Q-switched Nd:YAG laser, emitting at 532 nm, which was pumping a periodically poled KTiOPO₄ optical parametric oscillator with intra-cavity sum-frequency mixing in a BBO crystal. The output was generated in two branches in the UV, 293 nm and 343 nm, with pulse widths of 1.8 ns and pulse repetition rate of 100 Hz. These wavelengths were then used for fluorescence experiments of bioagents.

Optical techniques for the classification and identification of biological particles provide a number of advantages over traditional 'Wet Chemistry’ methods, amongst which are speed of response and the reduction/elimination of consumables. These techniques can be employed in both 'Trigger’ and 'Identifier’ systems. Trigger systems monitor environmental particulates with the aim of detecting 'unusual’ changes in the overall environmental composition and providing an indication of threat. At the present time there is no single optical measurement that can distinguish between benign and hostile events. Therefore, in order to distinguish between these 2 classifications, a number of different measurements must be effected and a decision made on the basis of the 'integrated’ data. Smiths Detection have developed a data gathering platform capable of measuring multiple optical, physical and electrical parameters of individual airborne biological particles. The data from all these measurements are combined in a hazard classification algorithm based on Bayesian Inference techniques. Identifier systems give a greater level of information and confidence than triggers, -- although they require reagents and are therefore much more expensive to operate -- and typically take upwards of 20 minutes to respond. Ideally, in a continuous flow mode, identifier systems would respond in real-time, and identify a range of pathogens specifically and simultaneously. The results of recent development work -- carried out by Smiths Detection and its collaborators -- to develop an optical device that meets most of these requirements, and has the stretch potential to meet all of the requirements in a 3-5 year time frame will be presented. This technology enables continuous stand-alone operation for both civil and military defense applications and significant miniaturisation can be achieved with further development.

Developments in real time optical biological agent detection and sensing are presented which describe start of the art advances in the detection and warning of these pathogens. The following paper describes the basic operating principles of the current BIRAL ASAS (Aerosol Size and Shape) system which measures the optically determined particle properties, on a particle by particle basis, and uses the information to describe the size and shape characteristics of the aerosol. Furthermore, recent development of the existing technology to also encompass fluorescence detection is described, which significantly increases the detection ability of the ASAS aerosol suite. This operational improvement is a major advancement in the field of airborne biological agent detection and allows for near generic detection and warning. Applications of this device include all aspects of bio-aerosol monitoring, including the use as a biological agent detector and generic identifier, use as a general bio-agent monitor and also for use as a hazardous environment monitor. Such a device would be particularly useful in the fields of Armed Forces protection and National Defence either as a point detector or as a "plug and play" biosensor detector in a network.

The use of intrinsic fluorescence to characterise airborne particles is often applied to the detection of biological materials, particularly micro-organisms. However, as a number of particles which are found in the atmosphere also fluoresce (whether natural or artificially generated), simple measures of particle fluorescence alone may not be sufficient to indicate the presence of biological agents in the atmosphere. An instrument has been developed for the real-time measurement of aerosols using UV induced fluorescence emission and elastic scattered light to characterise individual particles in terms of size, shape and fluorescence. Particles are detected as they scatter light from a CW red laser beam, which triggers a pulse of 266nm radiation to induce fluorescence. Elastic scatter from the red laser beam is used to measure particle size and shape parameters, and total fluorescence between ~300 to 500nm is collected. The performance of the instrument has been investigated in laboratory tests and field trials, using a range of biological agent simulants and interferents. An automated classification technique has been applied to assess the ability of the instrument to recognise potential threats against the natural background environment.

A partnership that includes the Naval Research Laboratory (NRL), MIT Lincoln Laboratories and the Edgewood Chemical and Biological Command is engaged in an effort to develop optical techniques for the rapid detection and classification of biological aerosols. This paper will describe two efforts at NRL: development of an improved UV fluorescence front-end trigger and the use of infrared absorption spectroscopy to classify biological aerosol particles. UV Laser-induced fluorescence (UVLIF) has been demonstrated to provide very high sensitivity for differentiating between biological and inorganic aerosol particles. Unfortunately, current UVLIF systems have unacceptably high false alarm rates due to interferences from man made and naturally occurring organic and biological particulates. We have developed a two-wavelength, UVLIF technique that offers a higher level of discrimination than is possible using single wavelength UVLIF. Infrared absorption spectroscopy coupled with multivariate analysis demonstrates a high potential for differentiation among members of biological and chemical sample classes. Two-wavelength UVLIF in combination with the IR interrogation of collected bioaerosols could provide a rapid, reagentless approach to specific classification of biological particles according to an operational level of discrimination - the degree of particle characterization required in order to signal the presence of pathogenic material.

Cavity ring-down spectroscopy (CRDS) can provide high sensitivity, high precision, and absolute calibration in a wide range of environments. We report on a compact cavity ring-down spectrometer that can measure atmospheric toxic industrial compounds such as hydrides and hydrazines. The ring-down spectrometer is fully contained in two 5 ¼" tall, 19" wide rack mount enclosures and utilizes a robust, near-infrared, fiber-coupled tunable diode laser. The instrument has a baseline sensitivity of 8 x 10^-11 cm^-1/Hz^½. We will present the results of this study, which demonstrates the capability to detect toxic gases such as arsine, silane, and hydrazine (simulated using ammonia) in air at parts per billion (ppb) concentrations in less than 1 minute. We will also present results on CRDS instrument performance, including zero drift, precision, absolute accuracy, and linearity over a wide range of environmental operating conditions.

Efforts to develop a single solution for detecting hazardous chemicals and biological organisms for both military and civilian communities often produce conflicting requirements. The detection of biological threats, specifically spores, presents us with the most challenging problem. Raman spectroscopy is an excellent method for unique chemical and biological identification. The applicability of Raman spectroscopy to bacterial identification and analysis has been previously demonstrated. Surface-enhanced Raman scattering (SERS) is a well-known method for improving the signal level in Raman scattering. In order to form a uniform noble metal surface architecture, and therefore reproducible surface enhanced spectra, novel fabrication techniques have been developed. Here we report on our recent efforts using silver-shells around latex spheres as a SERS substrate for bacterial endospores.

A novel sensor for detecting very low concentrations of chemicals in water and other liquids is presented. A cavity ring-down spectrometer has been developed that can measure chemicals in solution in a harsh environment. The high Q Fabry Perot cavity is fabricated in an optical fibre with high reflectivity mirrors on each end. The cavity contains a fused fibre taper, with very low intrinsic loss, for coupling light in the cavity evanescently into a smart surface layer that is bound on to the fibre surface. Small changes in the absorption are detected by changes in the ring-down time of the resonant cavity. The low loss cavity results in ring down times of 1μs for a 2 m cavity, which is equivalent to 100 passes through the smart surface. The ring down time provides a very accurate measure of absorbance because it is independent of source and detector drift and the fibre cavity is unaffected by changes in temperature, vibration or bending. Absorption changes of 5x10^-5 dB can be detected with the current configuration and further improvements can be achieved by optimisation leading to detection of atto-molar chemical concentrations.

The terahertz (1 THz = 10¹² Hz, 3 mm or 33 cm^-1) region of the electromagnetic spectrum is typically defined in the frequency range 100 GHz to 10 THz, corresponding to a wavelength range of 3 mm to 30 microns. Owing to a lack of suitable coherent sources and detectors, this region has only been investigated in earnest in the last ten years for terrestrial imaging and spectroscopy applications. Its role in the medical, pharmaceutical, non-destructive testing and more recently security industries is now being examined. The terahertz frequency range is of particular interest since it is able to probe several molecular interactions including the intermolecular vibrations, large amplitude vibrations and twisting and torsional modes. Molecules have also shown polarization sensitivity to the incident terahertz radiation. The ability of terahertz radiation to investigate conformational change makes it an important part of the electromagnetic spectrum. Terahertz radiation has the potential to provide additional information, which may complement other optically based sensing technologies. The use of terahertz technology in the security and defence industry is discussed, with a specific focus on biological and chemical sensing. The challenges faced in bringing terahertz technology into the market place will be discussed.

There is a renewed interest in the development of chemical and biological agent sensors due to the increased threat of weapons deployment by terrorist organizations and rogue states. Optically based sensors address the needs of military and homeland security forces in that they are reliable, rapidly deployed, and can provide continuous monitoring with little to no operator involvement. Nomadics has developed optically based chemical weapons sensors that utilize reactive fluorescent chromophores initially developed by Professor Tim Swager at MIT. The chromophores provide unprecedented sensitivity and selectivity toward toxic industrial chemicals and certain chemical weapon agents. The selectivity is based upon the reactivity of the G-class nerve agents (phosphorylation of acetylcholinesterase enzyme) that makes them toxic. Because the sensor recognizes the reactivity of strong electrophiles and not molecular weight, chemical affinity or ionizability, our system detects a specific class of reactive agents and will be able to detect newly developed or modified agents that are not currently known. We have recently extended this work to pursue a combined chemical/biological agent sensor system incorporating technologies based upon novel deep ultraviolet (UV) light emitting diodes (LEDs) developed out of the DARPA Semiconductor UV Optical Sources (SUVOS) program.

The parallel analysis of Cy5 fluorophore micro-arrays over polycarbonate and polymethyl metacrilate substrates is reported. Sequential printing of biochemical samples, CCD detection, enhanced analysis by signal processing and assay results recording as digital data on a consolidated substrate is demonstrated. The developed equipment finds its application in low-cost high throughput screening of massive chemical, biochemical or cellular agents.

We are developing a novel method to fluorescently label specific biological aerosols on-the-fly using an in-line electrospray technique. Fluorescently labeled biomarkers such as molecular beacons, aptamer beacons, or those constructed from antibodies, will be used to coat aerosol particles in an air stream. Single biological particles with appropriate receptors will be tagged with biomarkers that fluoresce at a particular wavelength allowing the particle to be identified in near real time using a simple laser induced fluorescence technique. The fluorescent markers are normally quenched in the absence of their target analyte, permitting the use of mixtures of different biomarkers for simultaneously identifying multiple types of biological particles. The technique can also be applied to inorganic particulate with a molecular surface composition that lends itself to epitopic binding. Some of the issues that are currently being investigated include the kinetics of biomarker binding in an aerosol stream, optimal electrospray geometries and the nondestructive charging of biological particles on the fly.

Aptamers, synthetic DNA capture elements (DCEs), can be made chemically or in genetically engineered bacteria. DNA capture elements are artificial DNA sequences, from a random pool of sequences, selected for their specific binding to potential biological warfare or terrorism agents. These sequences were selected by an affinity method using filters to which the target agent was attached and the DNA isolated and amplified by polymerase chain reaction (PCR) in an iterative, increasingly stringent, process. The probes can then be conjugated to Quantum Dots and super paramagnetic nanoparticles. The former provide intense, bleach-resistant fluorescent detection of bioagent and the latter provide a means to collect the bioagents with a magnet. The fluorescence can be detected in a flow cytometer, in a fluorescence plate reader, or with a fluorescence microscope. To date, we have made DCEs to Bacillus anthracis spores, Shiga toxin, Venezuelan Equine Encephalitis (VEE) virus, and Francisella tularensis. DCEs can easily distinguish Bacillus anthracis from its nearest relatives, Bacillus cereus and Bacillus thuringiensis. Development of a high through-put process is currently being investigated.

Terbium-dipicolinic acid can be used to enhance the UV fluorescence detection of selected biological agents and endospore cells. We are using new tunable UV laser near 220nm-280nm to optimize the excitation wavelength and detection sensitivity, and studying the acid solution/buffer mix to optimize the fluorescence yield. Enhancements on the order of a factor of 20 have been observed so far.

We examine how aggregation affects the light-scattering signatures, especially the polarization in the near-backward-scattering direction. We use the discrete dipole approximation (DDA) to study the backscatter of agglomerate particles consisting of oblong monomers. We examine the effects of monomer number and packing structure on the resulting negative polarization branch at small phase angle. We find large a dependence on the orientation of the monomers within the agglomerate and a smaller dependence on the number of monomers, suggesting that the mechanism producing the negative polarization minimum depends strongly on the interactions between the individual monomers. We also examine experimental measurements of substrates composed of biological cells. We find that the light-scattering signatures in the backward direction are not only different for different spore species, but for spores that have been prepared using different methodologies. These signatures are reproducible in different substrates composed of the spores from the same batches.

We describe the construction of a bio-aerosol monitor designed to capture and record intrinsic fluorescence spectra from individual aerosol particles carried in a sample airflow and to simultaneously capture data relating to the spatial distribution of elastically scattered light from each particle. The spectral fluorescence data recorded by this PFAS (Particle Fluorescence and Shape) monitor contains information relating to the particle material content and specifically to possible biological fluorophores. The spatial scattering data from PFAS yields information relating to particle size and shape. The combination of these data can provide a means of aiding the discrimination of bio-aerosols from background or interferent aerosol particles which may have similar fluorescence properties but exhibit shapes and/or sizes not normally associated with biological particles. The radiation used both to excite particle fluorescence and generate the necessary spatially scattered light flux is provided by a novel compact UV fiber laser operating at 266nm wavelength. Particles drawn from the ambient environment traverse the laser beam in single file. Intrinsic particle fluorescence in the range 300-570nm is collected via an ellipsoidal concentrator into a concave grating spectrometer, the spectral data being recorded using a 16-anode linear array photomultiplier detector. Simultaneously, the spatial radiation pattern scattered by the particle over 5°-30° scattering angle and 360° of azimuth is recorded using a custom designed 31-pixel radial hybrid photodiode array. Data from up to ~5,000 particles per second may be acquired for analysis, usually performed by artificial neural network classification.

In this paper we report on the fabrication and characterization of GaN, AlGaN, and AlN layers grown by hydride vapor phase epitaxy (HVPE). The layers were grown on 2-inch and 4-inch sapphire and 2-inch silicon carbide substrates. Thickness of the GaN layers was varied from 2 to 80 microns. Surface roughness, Rms, for the smoothest GaN layers was less than 0.5 nm, as measured by AFM using 10 μm x 10 μm scans. Background Nd-Na concentration for undoped GaN layers was less than 1x10¹⁶ cm^-3. For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 10¹⁶ to 10¹⁹ cm^-3. P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x10¹⁶ to 3x10¹⁸ cm^-3, for various samples. Zn doping also resulted in p-type GaN formation with concnetration N_D-N_A in the 10¹⁷ cm^-3 range. UV transmission, photoluminescence, and crystal structure of AlGaN layers with AlN concentration up to 85 mole.% were studied. Dependence of optical band gap on AlGaN alloy composition was measured for the whole composition range. Thick (up to 75 microns) crack-free AlN layers were grown on SiC substrates. Etch pit density for such thick AlN layers was in the 10⁷ cm^-2 range.

2,4,6-Trinitrotoluene is a high explosive that has been used for military purposes since 1902. Ammunition manufacturing facilities where TNT is made as well as sites across the world used to test military explosives in diverse ways, such as landmines and unexploded ordnance that have been buried in soil; grenades, etc are concerned with the health hazard and environmental problem of TNT. Since TNT is a contaminant that remains in the soil and produces various carcinogenic compounds as a result of photodecomposition and biodegradation, large amounts of the nitroaromatic compounds represent both a threat and a problem. Vibrational spectroscopy is a powerful tool that can be used to characterize TNT in its diverse condensed forms: droplets and crystals of polymorphs. Crystallization of TNT from different solvents: water, methanol, chloroform, acetone, and acetonitrile, was carried out and the vibrational spectra were obtained during crystallization. Crystals produced from evaporation of the mentioned solvent showed a similar crystallization pattern, and their spectroscopic information obtained was found to depend on the physical form of TNT. The nitroaromatic compound exhibits a series of unique characteristic bands that allow its detection and spectroscopic characterization. The spectroscopic signatures of neat TNT samples were determined with Raman Microspectroscopy and used as comparison standards. Strong bands about 1365 and 2956 cm-1 dominate the Raman spectrum of neat TNT. The intensity and even the presence of these bands are found to be remarkably dependent on TNT form and source.

We have been developing a novel infrared fiberoptic system for on-line monitoring of toxic materials in water. The system is based on fiberoptic evanescent wave spectroscopy (FEWS) and it operates in the middle infrared (Mid-IR) spectral range 3µm - 30µm. This spectral range covers the “fingerprint” region where many molecules have characteristic absorption. The system is based on silver halide (AgClBr) fibers which are flexible, non-toxic, non-hygroscopic and highly transparent in the Mid-IR. A short segment of unclad AgClBr serves as a sensing element, which is coupled to a tunable IR source (e.g. FTIR or tunable IR laser) via two long IR fibers. This setup makes it possible to carry out absorption measurements on water, in a remote location (in situ) and in real time. By flattening the short sensor element one can increase the sensitivity. Using this system we have already monitored pollutants in water in concentrations of the order of 1ppm. The system allows a highly sensitive and selective detection of several pollutants, simultaneously. With additional improvement this fiberoptic sensor system will be more sensitive, selective, affordable, robust and easy to operate. Such a system could to detect the presence of toxic chemicals, such as pesticides, in drinking water at levels lower than 1ppm.

Quantum cascade laser (QCL) offer many desirable attributes as mid-infrared laser sources for chemical and remote sensing. Some key advantages are a narrow linewidth, wide bandwidth current modulation characteristics and moderate tunability (15 cm^-1). Combined, these characteristics allow for applications to a wide variety of chemical and remote sensing techniques such as wavelength and frequency modulation based detection techniques, cavity enhanced point sensors as well as techniques such as LIDAR and DIAL. This paper will describe laser development efforts to enhance QCL frequency stabilization and QCL injection locking and to develop robust external cavity QCL designs.