Research

Current Projects:

Visible Laser Induced Virus Inactivation

For the better part of half a century, it has been understood that visible light in conjunction with photosensitizer molecules can generate highly reactive oxygen species, such as singlet oxygen. Photosensitizers absorb light and retransmit some of the absorbed energy to excite surrounding oxygen molecules above its ground state, catalyzing the formation of singlet oxygen. These oxygen molecules are highly active chemical species with a lifetime in the range of microseconds in water and other common solvents. They are highly electrophilic, reacting rapidly with unsaturated carbon-carbon bonds and neutral nucleophiles, as well as with anions. As a result, these oxygen species can be used to inactivate viruses and other microorganisms. This photodynamic method is regularly used in the pharmaceutical industry to sanitize biological products. Some common photosensitizers are organic dyes (rose Bengal, methylene blue, and eosin) and phthalocyanines molecules.  Common features of photo-enhancers are that they have multi-ring five and six member ring structures with alternating single and double bonds which form “π” bonds. Blue 405 nm light is used in hospital settings to sanitize patient areas based on lights interaction with porphyrin molecules within bacterial membranes, which produces lethal amounts of singlet oxygen. Several studies over the past few years, now implicate direct sunlight as a means by which viruses are environmentally inactivated. It’s thought that dissolved organic molecules in aqueous settings interact with sunlight to catalyze the formation of singlet oxygen in the environment. While sunlight at ground level does contain some lower energy light within the UV spectra (300 – 400 nm), this UV light does not penetrate well in aquatic environments. Thus, virus inactivation by sunlight is principally due to visible spectra light (400 – 700 nm). Indeed, for some water-borne fecal-oral viruses, solar photoperiods are known to influence environmental persistence. Human norovirus was first known as the winter vomiting disease since its person-to-person transmission peaks during fall and winter months. The high virus presence in sewage and filter-feeding bivalve shellfish is now typically observed in the winter season.

As part of investigating the mechanism of murine norovirus inactivation by femtosecond (fs)  and continuous wave (CW) blue light lasers (400 – 450 nm), we have discovered evidence that intense blue laser light may be capable of inducing singlet oxygen formation in the absence of photosensitizer molecules. First, we find that this light inactivation is oxygen-dependent based on the removal of oxygen from murine norovirus (MNV-1) viruses samples using sodium bisulfate. Second, we have used viral lysate samples that lacked any indicator dye and found the virus was inactivated after non-thermal exposure to the light. This virus stock is a 0.2 µm-filtered supernatant of a virus-infected cell lysate, raising the possibility of cellular proteins could be acting as photo-enhancers. However, purified MNV virions were also shown to be readily inactivated by blue laser light. This work raises one of two possibilities, either 1) the virus acts as its own photosensitizer, or 2) intense laser light is capable of inducing singlet oxygen in the absence of photosensitizer. Currently, we are inclined to discount the first (self-enhancer) possibility based on limited multi-ring or multi-benzene like ring structures within the amino acids that compose the virus capsid.  While viral RNA encased within the virus capsid does have some ring-like structures, attempts to demonstrate damage to RNA in response to blue light treatments based on quantitative real-time RT-PCR and serial dilution long amplicon RT-PCR amplification fail to indicate viral RNA damage. Thus, while we remain open the formal possibility that the MNV capsid may generate singlet oxygen leading to its demise when exposed to blue light, we strongly favor the idea that intense laser light is capable of making singlet oxygen without a photoenhancer to catalyze O₂  formation.

This study seeks to confirm and characterize singlet oxygen enhancer-free production in water and to subsequently investigate the physical-chemical mechanisms of its creation, considering a direct one-photon absorption of visible light or through an indirect two-photon Raman process with the generation of a detectable Stokes-shifted photon. The study also investigates the role of the virus in the production of singlet oxygen molecules. Use of lasers to study the conversion of dissolved oxygen to reactive oxygen species in solution has not been previously undertaken. Continuous wave and pulsed laser sources in the blue region of the spectrum will be used to conduct direct and indirect Raman-based detection of singlet oxygen in water-settings. The research should provide new insights and ideas for detection and identification of singlet oxygen and will perhaps lay the pathway for an active and controllable production of the molecule. Successful completion of this research project would result in novel findings with potential applications for chemical-free sanitization of food, and bioproducts, as well as for disease treatments. Preliminary evidence shows that photosensitizer-free non-thermal generation of singlet oxygen by blue laser light is responsible for a significant reduction of virus activity. The study will expand the initial results and set the best set of conditions for an efficient singlet oxygen generation by using only light.

Funding: NSF HBCU EiR

Nonlinear Optical Characterization of Thin Films and Nanonlaminates Grown by Atomic Layer Deposition

Next-generation of high-speed photonics devices, such as ultrafast integrated modulators and wavelength converters, require materials with large third-order optical nonlinearities. Nonlinear materials are typically cut from bulk crystals or liquids that are not suitable for integration with complementary metal–oxide–semiconductor (CMOS) technology. In addition to all-optical on-a-chip device applications, materials that exhibit high nonlinear absorption and a fast response time are useful in optical limiting applications for the protection of optical sensors and the human eye from high-intensity light such as lasers. Previous materials proposed for optical limiting have been semiconductors, fullerenes, carbon nanotubes, nanostructured materials such as nanoparticles, graphene, nonlinear absorbers doped in xerogels and sol-gel films, glasses, filters, organic/inorganic clusters, as well as 2D atomic crystals and organic dye molecules. For most of these materials, there is a tradeoff between their optical limiting ability and damage thresholds, and response time. The vast majority of these materials are not suitable for covering large-scale areas with consistent reproducibly required for sensitive applications such as infrared countermeasures sensors. Therefore, there is a need for CMOS-compatible materials with sizeable nonlinear optical properties.

A potential solution to the scarcity of CMOS-compatible materials is transition-metal oxides (TMOs).  These materials have been demonstrated to have large third-order optical nonlinearities with a fast response time (~picosecond time scale). Values as large as 8.4×10^-11 cm²/W for the nonlinear refractive index (n₂) for a 33.5 nm thick V₂O₅ film have been measured. In conjunction with having a large and fast nonlinearity, the TMO fabrication process must be compatible with current CMOS technology. There are several techniques, such as atomic layer deposition (ALD), chemical vapor deposition (CVD) and physical vapor deposition (PVD), which are CMOS compatible. However, due to its conformality and control over materials thickness and composition ALD is preferred over other deposition methods. Therefore, this work is exploring the third-order nonlinear properties TMO films where the initial focus in with TiO₂ films. Our latest results show that 4 – 6 order enhancement of the nonlinearity is possible and depend on the initial growth conditions.

Nonlinear Optical Measurements in Indium Flouride Glass and Fiber

To ensure continuum generation in the 2–5 μm spectral band, tellurite, chalcogenide, and fluoride fibers have been successfully adopted as nonlinear media. They have high nonlinearity and relatively low attenuation in the mid-IR region, where silica-based fibers are not transparent. Broadband supercontinuum (SC) generation in the soft-glass fibers, using a variety of pumping sources delivering femtosecond, picosecond, and nanosecond pulses, has also been demonstrated. Unlike other mid-infrared fibers, such as chalcogenides, fluoride fibers can operate at a higher temperature allowing for more power transmission. Zirconium fluoride(ZrF4) and Indium fluoride(InF3) glasses are two commonly used fluoride-based materials that are used in the optical fiber fabrication that is being used for environmental, medical, and military applications. Optical fibers made of InF3 glass are known to have a robust fabrication process, environmental stability, and broad transmission window from the visible wavelengths to 5.5μm. In addition to the transmission, it turns out that the easy-to-manufacture step-index fibers made from InF3 exhibit low dispersion properties for SC generation using a laser in the femtosecond regime, which opens up a way for reliable and high power fiber based pump sources. To our knowledge, such nonlinear optical studies have not been conducted for InF3 glass or fiber.

This project seeks to evaluate the nonlinear optical properties of InF3 bulk glasses and fibers with and without rare earth dopants using both the Z-scan and the induced grating autocorrelation (IGA) optical techniques. Preliminary results show that the closed Z-scan measurements on a 2mm thick disc of InF3 showed a nonlinear index of refraction of 4.3x10^-16 cm²/W at 800 nm, while the result on a 10 m long fiber using IGA produced a nonlinear index of refraction of 3.2x10^-16 cm²/W.