The complexity of biological agents can make it difficult to identify the important factors impacting
scattering characteristics among variables such as size, shape, internal structure and biochemical composition, particle
aggregation, and sample additives. This difficulty is exacerbated by the environmentally interactive nature of
biological organisms. In particular, bacterial spores equilibrate with environmental humidity by absorption/desorption
of water which can affect both the complex refractive index and the size/shape distributions of particles - two factors
upon which scattering characteristics depend critically. Therefore accurate analysis of experimental data for
determination of refractive index must take account of particle water content. First, spectral transmission
measurements to determine visible refractive index done on suspensions of bacterial spores must account for water (or
other solvent) uptake. Second, realistic calculations of aerosol scattering cross sections should consider effects of
atmospheric humidity on particle water content, size and shape. In this work we demonstrate a method for determining
refractive index of bacterial spores bacillus atropheus (BG), bacillus thuringiensis (BT) and bacillus anthracis Sterne
(BAs) which accounts for these effects. Visible index is found from transmission measurements on aqueous and
DMSO suspensions of particles, using an anomalous diffraction approximation. A simplified version of the anomalous
diffraction theory is used to eliminate the need for knowledge of particle size. Results using this approach indicate the
technique can be useful in determining the visible refractive index of particles when size and shape distributions are
not well known but fall within the region of validity of anomalous dispersion theory.
Most system analyses of CW high-power lasers propagating in the atmosphere assume a simple additive linear relation of the impact of thermal blooming and optical turbulence in the atmosphere to the propagated laser beam spreading. In other words, both effects are treated as if they would follow Gaussian statistics in an RMS sense.
While the statistics of optical propagation in a turbulent atmosphere can be modeled as Gaussian to first order, thermal blooming is a deterministic nonlinear optical phenomenon. To the best of our knowledge, there is no reason for adding linearly the beam spreading due to these two optical effects. In fact, assuming no interplay in the presence of a strong nonlinear optical interaction is
counter-intuitive. As a result, we have performed extensive numerical Monte-Carlo optical wave-propagation simulations, >50,000 realizations, in the presence of thermal-blooming and
atmospheric turbulence to varying degrees. During the propagation, the amplitude and the phase of a high power laser
field are coupled by the interplay of diffraction, refractive turbulence and thermal blooming. In some cases, we have
observed in our numerical experiments a strong coupling between turbulence and nonlinear thermal blooming.
Aerosol backscatter and extinction cross-sections are required to model and evaluate the performance of
both active and passive detection systems. A method has been developed that begins with laboratory
measurements of thin films and suspensions of biological material to obtain the complex index refraction of
the biological material from the UV to the LWIR. Using that result with particle size distribution and shape
information as inputs to T-matrix or discrete dipole approximation (DDA) calculations yields the extinction
cross-section and backscatter cross section as a function of wavelength. These are important inputs to the
lidar equation.
In a continuing effort to provide validated optical cross-sections, measurements have been made
on a number of high purity biological species in the laboratory as well as measurements of material
released at recent field tests. The resulting observed differences between laboratory and field
measurements aid in distinguishing between intrinsic and extrinsic effects, which can affect the
characteristic signatures of important biological aerosols. A variety of biological and test aerosols are
examined, including Bacillus atrophaeus (BG), and Erwina, ovalbumin, silica and polystyrene.
Optical cross-sections of biological warfare simulants, killed agents, and live agents are needed to assess the
standoff detection performance of active lidar and passive FTIR systems. To aid in this investigation, Johns Hopkins
University Applied Physics Laboratory (JHU/APL) has developed a technique to determine the index of refraction of
biological materials in the visible region using a combination of transmission measurements and anomalous diffraction
theory (ADT). The spectral measurements using a dual beam grating spectrometer provide a basis for calculating the
optical cross section of suspended particles. ADT is then used to convert the cross section result into index of refraction.
A summary of this procedure is described along with the results for silica microspheres and Bacillus globijii (BG). A
comparison of these results to published data is also presented.
Calculation of scattering properties of biological materials has classically been addressed using numerical calculations
based on T-matrix theory. These calculations use bulk optical properties, particle size distribution, and a limited selection
of shape descriptors to calculate the resulting aerosol properties. However, the most applicable shape available in T-matrix
codes, the spheroid, is not the best descriptor of most biological materials. Based on imagery of the spores of
Bacillus atrophaeus and Bacillus anthracis, capsule and egg shapes are mathematically described and programmed into
the Amsterdam Discrete Dipole Approximation (ADDA). Spectrally dependent cross sections and depolarization ratios
are calculated and a comparison made to spheroidal shapes of equivalent sizes.
High optical quality polycrystalline yttrium aluminum garnet (YAG) is now available. The optical
properties of pure polycrystalline YAG and 1% Nd doped polycrystalline YAG are reported from the
midwave infrared to the visible. The absorption and scatter properties are represented in terms of standard
models.
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