Evaluation of Transmission Near the Christiansen Wavelength for Dynamic Sand Samples.

Many optical applications, including free-space optical communications, lidar, and astronomical measurements, are impacted by the presence of light-scattering particles also known as obscurants. Scattering from particles consisting of sand, dust, dirt, and other substances can significantly degrade optical signals. For many obscurants, the index of refraction is dependent on the wavelength of light, and there exists a Christiansen wavelength (λc) at which scattering is at a minimum. At λc the index of refraction of the scattering particles (ns) matches that of the surrounding medium, in this case air (with refractive index na). This condition makes the scattering particulates almost invisible to the propagating light, minimizing scattering and increasing transmission at λc. Previously, the authors showed a technique for measuring the index of refraction n(λ) and the extinction coefficient k(λ) using spectroscopic ellipsometry for various sand samples. Spectroscopic measurements on static sand samples demonstrated good agreement with the predicted spectral properties and highlighted the presence of a Christiansen feature near 8 µm. However, in outdoor environments, the scattering particles are never stationary but in a constant state of motion. In this work, spectroscopic measurements on dynamic sand samples (sand that is falling through the optical beam path) show two Christiansen features seen previously in predicted and observed static sand measurements. Additionally, we characterize, for the first time, transmission around a Christiansen feature using a tunable laser and show results consistent with other spectroscopic measurements.

Measurement of the Optical Constants of Sand Samples Using Ellipsometry on Sand-Adhesive Composites.

In order to model the propagation of light through a sand cloud, it is critical to have accurate data for the optical constants of the sand particles that comprise it. The same holds true for modeling propagation through particles of any type suspended in a medium. Few methods exist, however, to measure these quantities with high accuracy. In this paper, a characterization method based on spectroscopic ellipsometry (SE) that can be applied to a particulate material is presented. In this method, a polished disc of an adhesive compound is prepared, and its optical constants are measured. Next, a mixture of the adhesive and a sand sample is prepared and processed into a polished disc, and SE is performed. By treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are extracted. For verification of the proposed method, it is first applied to pure silica powder, demonstrating good agreement between measured optical constants and literature values. It is then applied to Arizona road dust, a standard reference material, as well as real desert sand samples. The resulting optical constant data is input into a rigorous scattering model to predict extinction coefficients for various types of sand. Modeling results are compared to spectroscopic measurements on static sand samples, demonstrating good agreement between predicted and measured spectral properties including the presence of a Christiansen feature near a wavelength of 8 µm.

Surface-Enhanced Raman Spectroscopy for Environmental Monitoring of Aerosols.

Surface-enhanced Raman spectroscopy (SERS) is conducted from single aerosol particles held in a linear electrodynamic quadrupole trap. SERS measurements from two representative types of ambient aerosol particles, semi-liquid and solid aerosols, are demonstrated; aerosol composed of adenine where the metallic nanoparticles (MNPs) are volume distributed throughout the particle and aerosol composed of polystyrene latex (PSL) beads where the MNPs are surface coated. An enhancement factor > 106 is demonstrated from 5 µm aerosols containing trace amounts of adenine (0.1% by mass), with a detection limit of 10-8 M corresponding to 5 × 105 molecules (equivalent to 100 ag in mass or a 50 nm diameter sphere), and a ratio of 100 adenine molecules per Ag NP. SERS signal intensities are linear with particle adenine concentration up to a saturation point. Both the linearity and enhancement factor were confirmed by SERS measurements of adenine as bulk suspensions. The SERS spectra of adenine as bulk suspensions were explored as a function of excitation wavelength ranging from 400 to 800 nm. The two main Raman peaks of adenine at 738 and 1336 cm-1 exhibit SERS maxima for excitation in the 450-500 nm range for commercially available 40 nm spherical Ag nanoparticles (NPs) used in this study, which shifts to longer wavelengths with the addition of NaCl. Shifts in SERS and spontaneous Raman shifts were observed between aqueous and dry adenine, in agreement with the literature, demonstrating the utility of SERS to possibly study water uptake of aerosols. SERS is measured from MNP surface-coated PSL beads with an enhancement factor of 30 for 5 µm PSLs. Theoretical extrapolation demonstrates that the enhancement factor will increase for decreasing particle size with an estimated enhancement factor of 140 for 1 µm PSLs.

Optical measurements from single levitated particles using a linear electrodynamic quadrupole trap.

We have recently made advancements in a linear electrodynamic quadrupole (LEQ) device for capturing and levitating either single or multiple micro-particles that provides significant improvements in capture efficiency, reliability, and optical measurement access. We have used our LEQ to trap particles ranging from 30 to less than 0.5 µm in size and provide a controlled environment to study particle physical/chemical dependencies on temperature, relative humidity, and gas constituents. To demonstrate this approach, we present data and analysis of liquid-droplet evaporation rates for two materials: glycerol and dibutyl sebacate. Droplet size was monitored as a function of time by two independent optical methods: direct imaging and fixed-angle light scattering. This new approach provides a means to rapidly characterize a wide range of aerosol particle properties and a platform for development of new aerosol optical-diagnostic measurements.

Metallic-nanoparticles-enhanced fluorescence from individual micron-sized aerosol particles on-the-fly.

Fluorescence spectra from individual aerosol particles that were either coated or embedded with metallic nanoparticles (MNPs) was acquired on-the-fly using 266 nm and 355 nm excitation. Using aqueous suspensions of MNPs with either polystyrene latex (PSL) spheres or dissolved proteins (tryptophan or ovalbumin), we generated PSL spheres coated with MNPs, or protein clusters embedded with MNPs as aerosols. Both enhanced and quenched fluorescence intensities were observed as a function of MNP concentration. Optimizing MNP material, size and spacing should yield enhanced sensitivity for specific aerosol materials that could be exploited to improve detection limits of single-particle, on-the-fly fluorescence or Raman based spectroscopic sensors.