Nanoscale modal confinement is known to radically enhance the effect of intrinsic Kerr and Raman nonlinearities within nanophotonic silicon waveguides. By contrast, stimulated Brillouin-scattering nonlinearities, which involve coherent coupling between guided photon and phonon modes, are stifled in conventional nanophotonics, preventing the realization of a host of Brillouin-based signal-processing technologies in silicon. Here we demonstrate stimulated Brillouin scattering in silicon waveguides, for the first time, through a new class of hybrid photonic–phononic waveguides. Tailorable travelling-wave forward-stimulated Brillouin scattering is realized—with over 1,000 times larger nonlinearity than reported in previous systems—yielding strong Brillouin coupling to phonons from 1 to 18 GHz. Experiments show that radiation pressures, produced by subwavelength modal confinement, yield enhancement of Brillouin nonlinearity beyond those of material nonlinearity alone. In addition, such enhanced and wideband coherent phonon emission paves the way towards the hybridization of silicon photonics, microelectromechanical systems and CMOS signal-processing technologies on chip.

We demonstrate and distinguish experimentally the existence of a special type of Fano resonances at k≈0 in a macroscopic two-dimensional photonic crystal slab. We fabricate a square lattice array of holes in a silicon nitride layer and perform an angular resolved spectral analysis of the various Fano resonances. We elucidate their radiation behavior using temporal coupled-mode theory and symmetry considerations. The unique simplicity of this system whereby an ultralong lifetime delocalized electromagnetic field can exist above the surface and consequently easily interact with added matter, provides exciting new opportunities for the study of light and matter interaction.

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.

The ability to display graphics and texts on a transparent screen can enable many useful applications. Here we create a transparent display by projecting monochromatic images onto a transparent medium embedded with nanoparticles that selectively scatter light at the projected wavelength. We describe the optimal design of such nanoparticles, and experimentally demonstrate this concept with a blue-color transparent display made of silver nanoparticles in a polymer matrix. This approach has attractive features including simplicity, wide viewing angle, scalability to large sizes and low cost.

COMMUNICATIONIn this article, we introduce a metasurface-based broadband fl at-lens array functioning in the terahertz regime. Being functional devices consisted of a number of regularly arranged lenslets, lens arrays not only possess superior focusing and imaging functionalities but also have many intriguing characteristics beyond what a single lens would approach. Lens arrays have been widely used in photography, communications, and photomemory. For instance, ideal wavefront measurement method can be achieved by combining lens array with Hartmann-Shack sensor at visible light frequencies. [ 31 ] Besides, they are widely used in digital cameras to increase the fi lling factor of the detector array. In the terahertz regime, the lens array is one of the key components in terahertz cameras and terahertz communication systems. Generally, by employing a terahertz detector behind each lenslet, the lens array enables multipixel receiving. Moreover, lens arrays with tunable focal length have potential applications in 2D and 3D switchable displays and tunable photonic devices. However, conventional fabrication approaches including direct laser writing in photoresist [ 32 ] and silicon machining technique [ 33 ] are complex and very costly due to requiring interconnections, storage systems, and photovoltaic photography. There is a strong demand for a low-cost, broadband, and fl exible terahertz lens array for various applications of terahertz technology. Here, by adopting the C-shape split-ring resonators (CSRRs) with phase discontinuities, we proposed a metasurface-based terahertz fl at-lens array. We experimentally examined and explored the unique characteristics of the fl atlens array by using near-fi eld scanning terahertz microscopy (NSTM). We show that the proposed fl at-lens array is fl exible, robust, and broadband. Compared to the lens arrays developed with conventional approaches, the metasurface-based lens array is much thinner and lighter, more fl exible, and has broader numerical aperture (NA) variation range. The proposed metasurface-based fl at-lens array paves a novel way to planar terahertz device designs and may have important applications in terahertz imaging and communications.To realize the focusing functionality using metasurfaces, it is crucial to achieve a group of unit elements that enable abrupt phase shift covering a 2π range and a nearly constant transmission amplitude simultaneously. Ultrathin fl at lenses based on metasurfaces have already been demonstrated using 2D V-shaped or rotated bar antennas. [20][21][22][23] Here, the CSRR which reveals strong response to the terahertz radiation is designed as the basic unit structures, [ 34 ] as shown in Figure 1 a. The symmetry lines of the CSRRs are oriented ±45° with respect to the x -axis. Due to structure symmetry, when the incidence polarization is along the x -axis, it partially converts into the y -polarized component in the resonance frequencies range. This feature essentially lies on the superposition of different resonance Recently, growin...