Traditional far-field optical microscopy is limited by the diffraction limit of light. The spatial resolution that can be achieved is a couple hundred nanometers at a minimum. Improved spatial resolution can be achieved using near-field optics; however, there is always a problem of physical intrusion with near-field probes. Even more problematic is the inefficiency in delivering light to probes of sub-wavelength dimensions. There is a need to develop imaging techniques with sub-diffraction limited spatial resolution in order to study numerous chemical systems with important phenomena that occur in the tens of nanometers regime.
TIR Raman was first described by Ikeshoji in 1973. It can provide spatial resolution on the order of the diffraction limit of light in the focal plane, and spatial resolution below the diffraction limit of light in the direction perpendicular to the focal plane (z plane). As with other TIR techniques, the incident angle of light is varied upon a material with a high index of refraction (n1). At angles beyond the critical angle, the incident light is completely reflected and an evanescent wave is created in the adjacent medium (n2), which must have a lower index of refraction than n1. The penetration depth of the evanescent wave varies with the angle of incidence, the wavelength of light, and the indices of refraction of the two media. The penetration depth for Raman scattering is approximately half the exponential decay length of the electric field.
We are developing a high resolution TIR Raman microspectroscopy and imaging system with angle scanning capabilities. A schematic of this instrument is shown in Figure 1. The instrument will be used to study catalytic reactions that take place within functionalized porous silica materials.
Figure 2. Schematic explanation of STED microscopy. Excitation laser beam excites molecules in a diffraction limited spot. A STED laser pulse depletes the excited species with a ring shaped laser profile. The result is a spot of excited species that is smaller than the diffraction limit light. High spatial resolution imaging is possible with this format. Figure modified from Hell, S. et al.
Graduate and Postdoctoral Advisors and Advisees
Graduate advisors and postdoctoral sponsors. Michael Natan (Oxonica Inc., Mountain View, California), Robert Corn (University of California, Irvine, Department of Chemistry), Mary Wirth (Purdue University, West Lafayette, Department of Chemistry), Danny Brower (deceased)
Graduate students (8 total): Neha Arora, Deepak Dibya, Michael Murphy, Suzanne Sander. Working on DOE projects: Jason Lupoi, Kristopher McKee, Matthew Meyer, Chien-Ju Shih (Iowa State University).
Postdoctoral scholar (1 total): Nuha Salem (Clarkson University)