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Defense & Security

New deep-ultraviolet sources for detecting biological threats

Novel optical systems show excellent properties for laser-induced fluorescence detection of dangerous aerosols, hydrosols, chemicals, and explosives.
2 February 2010, SPIE Newsroom. DOI: 10.1117/2.1201001.002570

Biological weapons have been used for warfare, criminal activities, and terrorism for centuries.1 Recently, bio-aerosol terrorist attacks have been carried out against civilians in the US, and recurrence appears inevitable.

Historically, biothreat-detection functions have been broken down into four components: trigger, rapid confirmer/identifier, sample collector, and final confirmer/identifier. Greenwood and colleagues recently presented2 a more in-depth description of the individual functional components of the detection strategy. The biothreat trigger must operate continuously, or nearly so, and respond rapidly to any anomalous aerosol, hydrosol, or surface contamination. Because of the horrific potential of biological weapons to destroy human health, human life, food supplies, and the general habitat, and the virtual certainty that these weapons will be used in the future, the need for a fast, reliable, specific, and inexpensive threat trigger has been recognized for decades. It could be an important tool for detect-to-protect and detect-to-treat countermeasure scenarios. These triggers have been employing laser (but recently LED) light for light-induced native or autofluorescence (LIF) for decades, but without a generally accepted solution. No single device has all of the desirable traits. We recently reviewed3 LIF bio-aerosol threat triggers.

It is widely recognized that compact, efficient, long-lived, low-power, affordable deep-UV (DUV) sources would dramatically push LIF detection forward. (Here DUV covers wavelengths of 200–250nm.) In comparison to violet, UV-A, and UV-B (see Figure 1), DUV wavelengths offer LIF detection that is more sensitive because of strong fluorescence cross sections from fluorescent amino acids in proteins, and more discriminating because it targets intrinsic biological properties rather than growth-media residues. It is also less susceptible to threat-washing techniques, residual threat moisture, and ambient humidity. Finally, it is capable of detecting purified biotoxins and allows fusion of Raman-fluorescence detection with fluorescence-free Raman detection.

Figure 1. Advantages of deep-UV (DUV) light for fluorescence detection of biological threat agents.

If LIF detection is to progress, new DUV sources are needed. We have been developing two new categories of DUV sources and sensors with an eye to biothreat, chemical, and explosives detection.4,5 Our sources include two miniature, narrow-linewidth gas lasers and p-n junction-free, electron (e)-beam-pumped semiconductor sources. We are also developing DUV detection systems, such as standoff sensors for trace contamination on surfaces by chemical, biological, and explosives hazards, reagent-less bacterial and viral-flow cytometers, water-quality sensors, DUV Raman and photoluminescence spectrometers, and other standard commercial products.

In addition, we produce 224nm helium-silver (HeAg) and 248nm neon-copper (NeCu) lasers that have the size, weight, and power consumption of a typical HeNe laser, but with DUV emission. Emission linewidths are less than 0.1 wave numbers, making them suitable for resonance Raman spectroscopy as well as LIF. Excitation at these wavelengths enables both fluorescence-free Raman spectroscopy and simultaneous measurement of Raman and LIF emission. Recently, we described6 the use of the wavelength range associated with these lasers for LIF/Raman sensing.

The e-beam-pumped semiconductor source is a new optical excitation device in the DUV. These devices can be built to emit DUV radiation peaked from ~220nm into violet wavelengths. Figure 2 shows the optomechanical layout for the approximately 1cm3 volume of these e-beam-pumped DUV sources (also known as light-emitting triodes). Figure 3 shows their recent performance characteristics compared to commercial products that emit at longer wavelengths.

Figure 2. Schematic of an electron (e)-beam-pumped DUV semiconductor source.

Figure 3. Optical output for various DUV e-beam-pumped sources compared to to commercial p-n junction UV-B LEDs. Results normalized to input electrical power.

We recently demonstrated two new prototype DUV LIF sensors. The first employs intracavity detection with optical buildup of recirculating optical power more than 100 times over optimally outcoupled laser radiation from the same DUV laser7 (see Figure 4). Using this design, we have achieved large fluorescence yields for highly sensitive LIF detection. Real-time, on-the-fly single-particle Raman signatures from low-volatility chemical threats should be possible. The second laboratory prototype is a sensitive DUV LIF trigger for detecting biological agents in flowing water.

Figure 4. Intracavity optical buildup and detection concept. High-reflectance end mirrors and a Brewster intracavity window generate high recirculating optical power for flow-through detection of fluorescence or Raman scattering from biological or chemical threats.

In summary, we have developed new DUV sources to help protect human health, human life, food supplies, and the general habitat. We are currently developing a variety of Raman and native fluorescence-based proximity and standoff surface- and water-quality sensors that employ these sources. These sensors can detect and classify single bacterial spores, cells, and trace levels of cellular debris and dissolved chemicals. In addition, we are developing a native fluorescence microscope that can image individual, live bacterial spores on natural opaque backgrounds. Other ongoing work is focusing on DUV semiconductor sources for a wide range of biological applications.

Richard DeFreez
Met One Instruments Inc.
Grants Pass, OR

Richard DeFreez is a senior scientist. He received his BSc in physics in 1980 and a PhD in applied physics in 1985. His current research interests include detection of bio-warfare and bio-terrorism agents, laser detection and sizing of particulate matter, laser remote sensing of gaseous chemical species, and biological applications of lasers.

William Hug, Ray Reid
Photon Systems
Covina, CA

William Hug is chairman and chief executive officer. He received his BS from the University of Notre Dame, and his MS and PhD degrees from Northwestern University. Previously, he was a founder and president of Omnichrome Corporation until its acquisition by Melles Griot Corp.

Ray Reid is president and chief operating officer. He received his BS and MS from Colorado State University. He helped found the company, and previously helped found Omnichrome Corp.

Rohit Bhartia
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA

Rohit Bhartia is a staff scientist and a member of the Instrumentation and Advanced Spectroscopy Group in the Planetary Science Section. He received his BS from the University of Wisconsin and his MS from the University of Southern California, where he currently is a PhD candidate.