Wenjun Qiu

Transparent displays enabled by resonant nanoparticle scattering

We create a transparent display by projecting monochromatic images onto a polymer film embedded with nanoparticles that selectively scatter light at the projected wavelength. This approach features simplicity, wide viewing angle, scalability, and low cost.

Stimulated brillouin scattering in nanoscale silicon step-index waveguides: A general framework of selection rules and calculating SBS gain

We develop a general framework of evaluating the Stimulated Brillouin Scattering (SBS) gain coefficient in optical waveguides via the overlap integral between optical and elastic eigen-modes. This full-vectorial formulation of SBS coupling rigorously accounts for the effects of both radiation pressure and electrostriction within micro- and nano-scale waveguides. We show that both contributions play a critical role in SBS coupling as modal confinement approaches the sub-wavelength scale. Through analysis of each contribution to the optical force, we show that spatial symmetry of the optical force dictates the selection rules of the excitable elastic modes. By applying this method to a rectangular silicon waveguide, we demonstrate how the optical force distribution and elastic modal profiles jointly determine the magnitude and scaling of SBS gains in both forward and backward SBS processes. We further apply this method to the study of intra- and inter-modal SBS processes, and demonstrate that the coupling between distinct optical modes are necessary to excite elastic modes with all possible symmetries. For example, we show that strong inter-polarization coupling can be achieved between the fundamental TE- and TM-like modes of a suspended silicon waveguide.

Fundamental Limits to Extinction by Metallic Nanoparticles

O. D. Miller, C. W. Hsu, 3 M. T. H. Reid, W. Qiu, B. G. DeLacy, J. D. Joannopoulos, M. Soljačić, and S. G. Johnson Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 Department of Physics, Harvard University, Cambridge, MA 02138 U.S. Army Edgewood Chemical Biological Center, Research and Technology Directorate, Aberdeen Proving Ground, MD 21010

Broadband circulators based on directional coupling of one-way waveguides

Resonator-based optical circulators are fundamentally bandwidth-limited by their quality factors. We propose a new type of circulator based on directional coupling between one-way photonic chiral edge states and conventional two-way waveguides. The operational bandwidth of such circulators is tied to the bandwidth of the directional waveguide coupler and has the potential for simultaneous broadband operation and small device footprint.

Optimization of broadband optical response of multilayer nanospheres

We propose an optimization-based theoretical approach to tailor the optical response of silver/silica multilayer nanospheres over the visible spectrum. We show that the structure that provides the largest cross-section per volume/mass, averaged over a wide frequency range, is the silver coated silica sphere. We also show how properly chosen mixture of several species of different nanospheres can have an even larger minimal cross-section per volume/mass over the entire visible spectrum.

Layer-by-layer self-assembly of plexcitonic nanoparticles

Colloidal suspensions of multilayer nanoparticles composed of a silver core, a polyelectrolyte spacer layer (inner shell), and a J-aggregate cyanine dye outer shell have been prepared for the first time. Absorption properties of the colloid were measured in the visible region. This multilayer architecture served as a framework for examining the coupling of the localized surface plasmon resonance exhibited by the silver core with the molecular exciton exhibited by the J-aggregate outer shell. The polyelectrolyte spacer layer promotes the formation of an excitonic J-aggregate while serving as a means of controlling the plasmon-exciton (i.e. plexciton) coupling strength through changing the distance between the core and the shell. An analytical expression based on Mie Theory and the Transfer Matrix Method was obtained for describing the optical response of these multilayered nanostructures. Computational and experimental results indicate that the absorption wavelength of the J-aggregate form of the dye is dependent on both the distance of the dye layer from the silver core and the degree of dye aggregation.

Transparent Displays Enabled by Resonant Nanoparticle Scattering

We create a transparent display by projecting monochromatic images onto a polymer film embedded with nanoparticles that selectively scatter light at the projected wavelength. This approach features simplicity, wide viewing angle, scalability, and low cost.

Operation bandwidth of forward stimulated Brillouin scattering in silicon Brillouin active membrane waveguides

We studied the operation bandwidth of forward SBS with the optical waveguide width in Brillouin-active-membrane waveguides. Narrow waveguide width yields Brillouin features over wide frequency range, but waveguides with broad width induce less nonlinear absorption.

Coherent interference of nonlinearities in nanoscale silicon waveguides: The interplay between Kerr, free-carrier dispersion, and Brillouin nonlinear responses

The nonlinear of a nanoscale Brillouin-active silicon waveguide is examined through heterodyne four-wave mixing experiments. The interference between Brillouin scattering, Kerr, and dispersive free-carrier nonlinearities are analytically described to explain the characteristic line-shapes observed.

Tailorable stimulated Brillouin scattering in silicon nanophotonics

We examine the physics of traveling-wave photon-phonon coupling within nanoscale silicon waveguides and explore a host of new Brillouin-based signal processing technologies enabled by tailorable stimulated Brillouin processes in silicon photonics. Theoretical analysis of Brillouin coupling at sub-wavelength scales is presented, revealing that strong light-boundary interactions produce large radiation pressures mediated Brillouin nonlinearities. Experimental results demonstrating stimulated Brillouin scattering in silicon waveguides for the first time are also presented, revealing 1000 times larger forward stimulated Brillouin gain coeffcients than any prior system.