Investigator: Madison Hetlage
Atomic and molecular filters are a proven tool in laser based diagnostics. Their use as notch filters has greatly expanded the usefulness of scattering phenomena in both ground testing and remote sensing applications. By tuning a laser to the resonance frequency of a strong transition, intense Mie scattering signals from particles and dust as well as diffuse scattering from windows and walls can be suppressed by several orders of magnitude . The transmitted signal can then be used to measure a variety of parameters. However, current filtering technology limits researchers to the use of the frequencydoubled Nd:YAG signal (532 nm) or more rare and complicated lasers such as Ti:Sapphire and Dye lasers . The visible spectrum presents eye safety issues and lacks the molecular scattering signal strength found in the UV. This work aims to develop a vapor filter functioning at the near UV wavelength of the Nd:YAG third harmonic. The frequency required for this filter, which utilizes an excited state transition in atomic barium vapor, falls between the ozone absorption region and the retinal hazard region, provides a stronger backscattered signal than visible light, and can be easily attained with the robust and commonly used high-power Nd:YAG laser . These benefits have significant implications for remote sensing and ground testing technology, including LIDAR (LIght Detection And Ranging), Thomson and filtered Rayleigh scattering, and flow imaging. This project looks to specifically apply this novel filter to atmospheric LIDAR measurements.
The transition of interest for this absorption occurs between the excited, metastable 3D2 state in barium vapor and the 3F02 state at 28179.367 1/cm, which corresponds to a wavelength of approximately 354.8 nm. The figure to the right introduces a simplified scheme for generating this absorption feature (which is shown by the purple arrow on the right) by optically pumping barium. In this scheme, a low power continuous wave laser is used to induce the spin forbidden transition at 791.3 nm demonstrated by the red arrow. Population in the excited state decays through collisional quenching and spontaneous emission to the 3DJ metastable manifold, where energy pooling is observed due to the forbidden transition back to the ground state . This pumping scheme, along with pumping at 553.7nm (as shown by the green arrow), has been used extensively to study the collisional dynamics and kinetics of the low-lying levels of the barium atom [3-9]. These studies have demonstrated a 60s lifetime of the 3D2 state  and a 75-80\% ground state depletion through this pumping scheme with less than 1mW of pump power [3, 8]. While other atomic species within the tuning range of the Nd:YAG third harmonic were considered, barium was chosen for this work due to its comparatively high vapor pressure and prior demonstration of efficient pumping to its metastable state.