Advances in the synthesis and manufacturing of nanometer-sized structures of various shapes, sizes, and material composition have increased attention to nanotechnology due to fundamental scientific interests, and because of the diversity of potential technological applications in fields such as medicine, optical data storage, light harvesting, and sensors. The challenge, however, remains in accurately determining the properties of such structures, e.g., mechanical, chemical, and optical, which in general, differ significantly from those of bulk materials. That is, “at what point does a nanostructure take on the properties of the bulk?” To help address this question a sparse concentration of nanoparticles are studied. The attraction of these “single particle” studies is that they avoid the problems associated with ensemble measurements, e.g., dispersion in particle size and shape or potential inter-particle interactions. With regards to the optical properties of nanostructures, which are particularly sensitive to their chemical composition, shape, and local environment, far-field optical microscopy is rapidly becoming a convenient tool for characterization and detection at low concentrations.  One of our goals at PROBE is to develop approaches that would enable precise optical experiments of single nanoparticles. In this regard, we have shown theoretically that it is possible to determine both the second-order nonlinear susceptibility elements of a single nanoparticle and its position in 3D space [J. Phys. B 44, 015401 (2011)].To facilitate such a measurement coherent confocal microscopy and polarization diversity (in the form of a vector field) are required. Along with our collaborators, we are currently working on an experimental implementation. In addition, we have also developed the tools to optically trap single plasmonic nanoparticles in solution by exploiting a priori knowledge of their plasmon resonance [Opt. Exp. 15, 12017 (2007)], and are currently working on a robust method for measuring both the refractive index of these trapped particles and their extinction spectra.

While it is interesting to characterize the optical properties of single nanoparticles, there is growing interest in exploiting the potentially high local field enhancements that some plasmonic nanostructures provide. For example, metal bowtie nanoantennas (BNAs) have been shown to concentrate an applied optical field in a dielectric “gap” region as small as 20 nm in diameter. This has resulted in enhancement factors approaching 10³, particularly when placed in an array configuration. We have recently demonstrated this effect through a study of the nonlinear optical response [Nano. Lett. 11, 61 (2011)]. Specifically, we find that near IR, fs-pulsed, input light tightly focused onto a 100 mm x 100 mm area of BNAs, leads to second-harmonic generation emission as well as observation of two-photon photoluminescence and a broad emission spectrum in the visible that we attribute to a type of continuum generation. We show that the nature of the spectrum can be controlled by tuning the input optical polarization as well as the array periodicity. In addition, it turns out that this BNA platform can be very efficient at trapping, manipulating, and sorting micro particles. A demonstration of this work has recently been published in Nano Letters [http://dx.doi.org/10.1021/nl203811q]. Currently, in collaboration with researchers at UIUC and MIT, we are investigating this system, as well as other exotic nanostructures, for potential use in light harvesting applications.

Collaborators: N. Fang (MIT, ME), G. L. Liu (UIUC, ECE), S. Carney (UIUC, ECE), B. Davis (Creare, Inc.)