Nano-Opto-Biophysics Research Group at San Francisco State University

The nano-opto-biophysics research group at SFSU studies optical properties of nanoscale materials and structures and explores the applications of these properties in bioimaging, single molecule sensing, and technologies for display and information processing.

For a list of publications, please visit the PI's Google Scholar profile here. Current research projects include:

1. Plasmonic nanowire waveguides and their applications

Waveguides capable of achieving high confinement with low loss are a key goal in realizing plasmonic circuits and networks of high efficiency and throughput. These waveguides can find applications in a variety of fields such as high-resolution imaging and sensing, subwavelength lithography, and high-efficiency solar cells. Dielectric waveguides have been studied due to their simplicity in structure. It was shown that propagating modes can exist inside a waveguide of arbitrarily small size when the properties of the dielectric core and the cladding metal are matched (Xu et al., Opt. Commun. 2009). Strong resonant transmission through ZnO nanowire waveguides (see Fig. 1) and patterned ZnO nanowaveguide arrays (see Fig. 2) was observed. Due to the tightly confined illumination volume of the light exiting from the nanowaveguides, these devices can be used to develop a single molecule spectroscopy technique particularly suited for studying the dynamics of cell membrane proteins (see Fig. 3).

Fig. 1. (a) Simulated transmission of 488 nm light through a 40-nm-diameter ZnO waveguide in a 100-nm-thick silver film situated on a fused silica substrate. (b) SEM images of ZnO nanowires embedded inside a 100-nm-thick silver film (Garcia et al., Appl. Phys. Lett. 2012).

Fig. 2. SEM images of ZnO nanowire arrays fabricated using nanofabrication methods (Lamson et al., Opt. Commun. 2018).

Fig. 3. Schematic of nanowaveguide-illuminated fluorescence spectroscopy (NIFS) for studying the dynamics of cell membrane proteins at the single molecule level.

2. Disordered plasmonic systems

Disordered photonic systems have attracted great interest in the past two decades. Here we focus on the study of light scattering and localization in disordered plasmonic systems including disordered plasmonic nanopillar arrays (see Fig. 4) and disordered hybrid nanowaveguide arrays (see Fig. 5) where the inherent attenuation of fields due to the excitation of surface plasmons will interact with spatial disorder induced localization to modify the behavior of light in these systems. Such systems may find potential applications in metasurfaces for flat-optics, see-through head-up displays, and quantum photonic integrated circuits.

Fig. 4. Schematic of disordered plasmonic/photonic nanopillar arrays for applications in transparent display and single molecule sensing.

Fig. 5. Lateral cross section of a disordered array of rectangular dielectric nanowire waveguides, situated above a silver film with a nanometer-sized gap.

3. Metal-dielectric nanostructures for Surface-Enhanced Raman Scattering (SERS), Tip-Enhanced Raman Spectroscopy (TERS), and Tip-Enhanced Fluorescence (TEF) spectroscopy.

Electric field intensity at the tips of silver and gold nano-wedges can be 3-5 orders of magnitude stronger than that of the incident filed (see Fig. 6). These strongly enhanced fields can be used to detect Raman scattering signal of molecules of extremely low concentrations and can find applications in photo-assisted nanochemistry. We also study how the emission property of a dipole emitter may be affected when it is placed near the apex of a metal tip with a radius of curvature around 20 nm. Strong enhancement in fluorescence signal as high as four orders of magnitude can be achieved as a result of the large field enhancement and an increase in quantum yield, in particular with an appropriate choice for the substrate material (see Fig. 7).

Fig. 6. Enhancement of electrical field intensity in a logarithmic scale for a pair of wedges (Ramanauskaite et al., ChemPhysChem, 2016).

Fig. 7. Schematic of Tip-Enhanced Fluorescence (TEF) spectroscopy setup (Isaac and Xu, OSA Continuum, 2018).

4. Nonlinear optical phenomena in plasmonic nanosuspensions

Here we study the creation of spatial optical solitons in a suspension of plasmonic nanoparticles in collaboration with the Chen group at SFSU (http://www.physics.sfsu.edu/~laser/). Guiding of light in plasmonic nanosuspensions (Xu et al., Photonics Research, 2019) and soliton-mediated ordering of gold nano rods (Ren et al., Opt. Lett. 2017) were previously demonstrated.