Silicon-plasmonics enables the fabrication of active photonic circuits in CMOS technology with unprecedented operation speed and integration density. Regarding applications in chip-level optical interconnects, fast and efficient plasmonic photodetectors with ultrasmall footprints are of special interest. A particularly promising approach to silicon-plasmonic photodetection is based on internal photoemission (IPE), which exploits intrinsic absorption in plasmonic waveguides at the metal-dielectric interface. However, while IPE plasmonic photodetectors have already been demonstrated, their performance is still far below that of conventional high-speed photodiodes. In this paper, we demonstrate a novel class of IPE devices with performance parameters comparable to those of state-of-the-art photodiodes while maintaining footprints below 1 μm 2 . The structures are based on asymmetric metal-semiconductormetal waveguides with a width of less than 75 nm. We measure record-high sensitivities of up to 0.12 A/W at a wavelength of 1550 nm. The detectors exhibit opto-electronic bandwidths of at least 40 GHz. We demonstrate reception of on-off keying data at rates of 40 Gbit/s.

Structuring optical materials on a nanometer scale can lead to artificial effective media, or metamaterials, with strongly altered optical behavior. Metamaterials can provide a wide range of linear optical properties such as negative refractive index 1,2 , hyperbolic dispersion 3 , or magnetic behavior at optical frequencies 4 . Nonlinear optical properties, however, have only been demonstrated for patterned metallic films 5-10 which suffer from high optical losses 11 . Here we show that second-order nonlinear metamaterials can also be obtained from non-metallic centrosymmetric constituents with inherently low optical absorption.In our proof-of-principle experiments, we have iterated atomic-layer deposition (ALD) of three different constituents, A = Al 2 O 3 , B = TiO 2 and C = HfO 2 . The centrosymmetry of the resulting ABC stack is broken since the ABC and the inverted CBA sequences are not equivalent -a necessary condition for non-zero second-order nonlinearity. To the best of our knowledge, this is the first realization of a bulk nonlinear optical metamaterial.The basic idea of metamaterials is simple, yet powerful: Using ordinary constituents shaped on a sufficiently small spatial scale, effective material properties that go qualitatively beyond those of the ingredients become possible 12,13 . Early examples are stacks of isotropic layers leading to nanolaminates with an effective anisotropic electromagnetic response 14 . Metamaterials may also lead to properties that are typically not present in nature, such as hyperbolic dispersion observed in layered metal-dielectric structures 3 or negative refractive indices which can be exploited to create superlenses 2 .However, while these examples refer to linear optical properties only, nonlinear phenomena are of fundamental technological importance as well. For example, optical nonlinearities are utilized for generating frequencies otherwise hardly accessible 7 , for modulating light at hundreds of GHz 15 , or for creating optical gates 16 . Nonlinear optics additionally enables fundamental components for quantum applications, such as sources of entangled photons by spontaneous down-conversion 17 .Second-order nonlinear optical crystals, such as potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO 3 ) or superlattices grown by molecular-beam epitaxy (MBE) 18 are readily available, but these materials are difficult to incorporate in most photonic platforms. Nonlinear optical metamaterials are therefore interesting not only for answering general physical questions, but also because they may offer a method of incorporating second-order nonlinear materials on photonic platforms.Current research on nonlinear optical metamaterials has shown interesting results already such as high-harmonic generation 9 or giant surface nonlinearities 5 . However, most results available so far report on enhancements of pre-existing nonlinearities which originate at metal-dielectric interfaces 5,[8][9][10]19 . Furthermore, nonlinear metamaterials available so far consist of met...

We show a THz plasmonic modulator with flat frequency response from 40 MHz to 0.325 THz and employ it to demonstrate THz-to-optical conversion of a 30 Gbit/s signal on a 0.294 THz carrier.

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