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.
Bound states in the continuum (BIC) to achieve highly efficient frequency conversion using high quality-factor (high-Q) metasurfaces have been demonstrated using symmetry-broken structures with high robustness; however, the breaking-symmetry tactics are typically limited to one of the dimensions of the structures, which restricts the nonlinearity with BIC. In this work, we present a new metasurface structure in the form of an array of unit cells composed of two identical nano-bars with two mirror-symmetric corners cut into each nano-bar to break this limit. By using the high refractive index and large third-order nonlinearity of amorphous silicon (a-Si), we demonstrate ultra-high theoretical Qs up to ~ 2×10^5. Owing to the large nearfield enhancement in the meta-atoms, we observe optical Kerr effect in efficient third harmonic generation from the a-Si BIC metasurfaces via different levels of pump power, which paves the way for variational quasi-BIC for switchable nonlinear generation.
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.
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