Epsilon-near-zero (ENZ) materials such as indium tin oxide (ITO), have recently emerged as a new platform to enhance optical nonlinearities. In this talk I will report on our recent theoretical and experimental results on the origin of ultrafast nonlinearities in ITO films that are dominated by intraband and interband transitions. We show that there are two competing factors that jointly contribute to a spectrally-invertible nonlinearity of ITO near its ENZ region i.e. the nonparabolicity of the bands that results in a larger effective mass and the Fermi energy shift, which determines the free carrier density. Our work reveals the relationship between the large nonlinearity and the intrinsic material properties of the ITO films which will enable design and development of photonic materials and nonlinear devices made of transparent conductive oxides.
Nonlocal metasurfaces, which have uniform geometric arrangements but respond to incident angles differently, are uniquely capable of processing images. Phase change materials are promising functional building blocks to make metasurfaces reconfigurable owing to the significant refractive index contrast between amorphous and crystalline states. This presentation shows a novel tunable nonlocal metasurface based on the lossless phase change material Sb2Se3. It demonstrates the integration of two on-demand switchable functions, bright field imaging and second-order spatial differential imaging, on a single device.
Subwavelength nonlinear optical sources with high efficiency have received extensive attention although
strong dynamic tunability of these sources is still elusive. Germanium antimony telluride (GST) as a well-established phase-change chalcogenide is a promising candidate for the reconfiguration of subwavelength
nanostructures. Here, we design an electromagnetically induced transparency (EIT)-based high-quality-factor (high-Q) silicon metasurface that is actively controlled with a quarter-wave asymmetric Fabry-Perot cavity incorporating GST to modulate the relative phase of incident and reflected pump waves. We demonstrate a multi-level third-harmonic generation (THG) switch with a theoretical modulation depth as high as ~ 70 dB for the fundamental C-band crossing through multiple intermediate states of GST. This study shows the high potential of GST-based dynamic nonlinear photonic switches for a wide range of applications ranging from communications to optical computing.
Weak intensity, low emission rate and poor emission directionality are three main challenges of quantum emitters. Here, we report on realizing enhanced photon emission from quantum dots mediated by plasmonic hybrid nanoantennas. Firstly, a plasmonic hybrid structure is proposed, where a silver nanocube is positioned at the center of a metallic concentric-ring structure, to simultaneously enhance the emission directionality and the decay rate of quantum dots embedded in the vertical nanogap while maintaining a high quantum efficiency. Secondly, a crystalline spherical silicon nanoparticle on metal nanoantenna is realized that can largely enhance the spontaneous emission intensity and the emission decay rate. A high quantum efficiency of over 80% is obtained. These hybrid nanoantennas can be combined with various nanoscale optical emitters and easily extended to form large area light-emitting devices for applications such as advanced display and wireless optical communication.
Metal nanoparticles (NPs) have wide applications in various fields due to their unique properties. The accurate and fast characterization of metal NP concentration is highly demanded in the synthesis, metrology, and applications of NPs. The commonly used inductively coupled plasma mass spectrometry (ICP-MS) is a standard method for measuring the mass concentration (MC) of NPs, even though it is time-consuming, expensive, and destructive. While for the number concentration (NC) characterization of NPs, the method is less explored. Here, we present an improved optical extinction-scattering spectroscopic method for the fast, non-destructive characterization of the MC and NC of poly-disperse metal NP colloid simultaneously. By measuring the extinction spectrum and the 90° scattering spectrum of the nanorod (NR) colloid, we can solve an inverse scattering problem to retrieve the two dimensional joint probability density function (2D-JPDF) with respect to the width and the aspect ratio of NR sample accurately, based on which the NC and MC of the colloidal NPs can be calculated. This method is powerful to characterize both the geometric parameters and the concentrations, including the MC and NC, of poly-disperse metal NPs simultaneously. It is very useful for the non-destructive, non-contact, and in-situ comprehensive measurement of colloidal NPs. This method also has the potential to characterize NPs of other shapes or made of other materials.
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