Optical Tissue Measurements (Developmental Center-Driven Research Project)

UTSA: DK Sardar, P Hornsby, K Nash, R Yow, TT Jobe; 

AF Research Lab, Brooks City Base: RJ Thomas

Researchers at UTSA, the UT Health Science Center at San Antonio, and the Air Force Research Laboratory at Brooks City Base, San Antonio, have worked on projects of joint interest to CBST since the inception of the Center. We continue to collaborate on projects in the areas of measurement of optical properties of tissues for the data used in optical biopsy projects at UCD, development of tissue compatible windows for in vivo optical monitoring and sensing in animals, development of bioluminescence based biosensors, the development of rare earth nanoparticles as optical tags, and understanding the safety limits of laser based eye measurements.

Relevant to the last research topic, this year we report work on the development of finite-difference beam propagation modeling for lasers in ocular tissues.

This past year we have developed a numerical model based on the finite-difference solution to the non-paraxial wave equation with non-uniform arbitrary grid spacing to study focused laser beam propagation in a cylindrical geometry. The model will incorporate thermal lensing as well as linear and nonlinear absorption effects to model beam propagation within the human eye. The goal of the study is to accurately predict the dynamics of retinal irradiance under conditions of non-linear propagation, with application in the establishment of infrared safe exposure limits.

The beam-propagation method is at present the most widely used tool employed in the study of wave optics, largely owing to its numerical speed and simplicity. This model includes linear and non-linear absorption effects and thermal effects. The two-dimensional paraxial wave equation is equivalent to a (1,0) Padé approximation. Simulation of a Z-scan using this model shows discrepancies when compared to experimental data. These discrepancies are due to the inability of the paraxial equation to capture wide-angle effects. We have thus derived the diagonal system for a non-paraxial wave propagation operator, namely the (1,1) Padé operator.

Future Plan: Development of Rare Earth-based nanobiosensor. 
At UTSA and UTHSCSA, researchers will be involved in development of a novel rare earth (RE)-based biosensor. The RE-based aggregates of nanocrystals will be synthesized from rare earth salts from a urea solution. A method will be developed as to the biological functionalization of the RE nanoparticles for their effective applications as biosensors. The relevant optical and spectroscopic properties of these RE-based nanocrystals will be investigated in detail toward the development of optically efficient RE-based biosensors. One of the most significant biomedical applications of the nanocrystalline rare earth (RE) oxides is likely to be a novel biosensor capable of detecting various disease states. In addition, potential detection of antigens and toxic chemicals is also possible by coupling RE-based nanocrystals with a suitable host protein. The rare earths have many advantages over fluorescent labeling organic dyes and semiconductor-based quantum dots (QD) that have potential of being toxic to cells, and in some cases are chemically unstable. It is well known that the rare earths exhibit both strong absorption and fluorescence, ranging from visible to the near IR region of the optical spectrum, and have long fluorescence lifetimes on the order of milliseconds. Thus, excitation and detection are well separated by wavelength for  convenient observation and analysis Two examples of rare earths oxides that are likely ideal for biosensor applications are Er3+:Y2O3 and Nd3+:Y2O3, which have characteristic emission at 1.5 μm and 0.9 to 1.1 μm, respectively. Furthermore, these nanocrystals can be functionalized more effectively than by techniques employed for quantum dot attachment. The proposed research is relevant and would likely respond to Homeland Security requirements.