At the Laser Research Laboratory of the University of Texas at Santonio (UTSA), we have been actively involved in the investigation of optical and spectroscopic properties of a variety of laser materials, including trivalent rare earth (RE) and transition metal ions doped in single crystals, glasses, and polycrystalline ceramics. Our main objectives are to characterize these materials, and discover and understand the fundamental physical mechanisms such as energy transfer, fluorescence quenching, nonradiative relaxation, electron-phonon interaction, etc. The experimental measurements include absorption, excitation, emission, and fluorescence lifetimes of electronic transitions at various physical conditions. We have also pursued the spectroscopic and laser studies on solid-state tunable dye lasers. Several solid-state dye laser plastic (HEMA) materials have been developed in our laboratory. We have been able to observe lasing in these materials. Although the thermal effect in these plastic hosts is major concern, currently, we are hopeful that better host and curing agent for the dyes will be found and also better operating design will be possible in order to remedy the thermal stress.

Currently, we are also investigating the optical properties of biological tissues as well as studying the laser-tissue interaction, an exciting area of biophysics. Since the Beer's law only applies to the special situation where the absorption dominates the scattering, this law cannot be used in the case of turbid media such as tissue, where scattering is a dominant factor with the exceptions being limited to certain wavelengths. Therefore, optical absorption measurement using spectrophotometer alone is not sufficient to accurately determine the absorption coefficient which is an important parameter for laser dosimetry that is especially important for medical applications of lasers. Therefore, we employ optical integrating spheres to measure diffuse transmission and reflection.  The measured values of reflectance and transmittance are applied to the Inverse Adding and Doubling (IAD), a computational method, that iteratively solves the Chandrasekhar’s radiative transport equation to determine the absorption and scattering coefficients. The measured values of reflectance and transmittance are finally compared with those obtained from the Monte Carlo simulation technique; this comparison helps us determine how accurate are the optical properties of biological materials.

We also have a joint collaboration between University of Texas Health Science Center at San Antonio and our Laboratory to develop a model for tissue-based biosensors in experimental animals, specifically mice. Tissue-based biosensors can be used for a variety of purposes in experimental physiology and for the detection of substances in the environment, including toxins of importance in bioterrorism. Tissue-based biosensors are formed from genetically modified adrenocortical cells transplanted in immunodeficient (scid) mice. Our goal is to develop a tissue-based biosensor for bioactive insulin in mice. We aim to show the feasibility of using biophotonics to enhance the utility of such tissue-based biosensors. We plan to develop tissue-based biosensors that can be used in freely moving conscious animals, thus avoiding potential artifacts caused by the need to immobilize animals by anesthetizing them in order to take measurements. Moreover this would allow continuous noninvasive monitoring of a physiological parameter (e.g. insulin in blood) or of the exposure of the animal to toxins in the environment. Although biosensors based on BRET (bioluminescence resonance energy transfer) have been described previously, to the best of our knowledge, biosensors that use BRET have not yet been adapted to use in the living animal. Thus the proposed research will be the first demonstration of the feasibility of this technology. The successful completion of this first phase of the work will allow expansion of the concept into a variety of biomedical research areas as well as bioterrorism applications (toxin detection).

One of our current projects involves design, fabrication, and testing of a variety of electro-optic materials (tagants, nanometer-size particles) that have unique absorption, fluorescence and reflectivity properties, coupled with selective adhesion and/or self-powered properties.

Based on our experience, we are now embarking on a critically important area of research: "Noninvasive luminescent rare earth-based nano-biosensors for biomedical applications."  Our long-range goal is to develop RE-doped metal oxide nanoparticles for biosensors. It is expected that the RE-doped nanoparticles will serve as a superior luminescent tags or reporters over the conventional, fluorescent organic dye molecules.