Christopher Matheisen

Pockels effect based fully integrated, strained silicon electro-optic modulator

We demonstrate for the first time a fully integrated electro-optic modulator based on locally strained silicon rib-waveguides. By depositing a Si3N4 strain layer directly on top of the silicon waveguide the silicon crystal is asymmetrically distorted. Thus its inversion symmetry is broken and a linear electro-optic effect is induced. Electro-optic characterization yields a record high value χ(2)(yyz) = 122 pm/V for the second-order susceptibility of the strained silicon waveguide and a strict linear dependence between the applied modulation voltage V(mod) and the resulting effective index change Δn(eff). Spatially resolved micro-Raman and terahertz (THz) difference frequency generation (DFG) experiments provide in-depth insight into the origin of the electro-optic effect by correlating the local strain distribution with the observed second-order optical activity.

Investigation of local strain distribution and linear electro-optic effect in strained silicon waveguides

We present detailed investigations of the local strain distribution and the induced second-order optical nonlinearity within strained silicon waveguides cladded with a Si₃N₄ strain layer. Micro-Raman Spectroscopy mappings and electro-optic characterization of waveguides with varying width w(WG) show that strain gradients in the waveguide core and the effective second-order susceptibility χ(2)(yyz) increase with reduced w(WG). For 300 nm wide waveguides a mean effective χ(2)(yyz) of 190 pm/V is achieved, which is the highest value reported for silicon so far. To gain more insight into the origin of the extraordinary large optical second-order nonlinearity of strained silicon waveguides numerical simulations of edge induced strain gradients in these structures are presented and discussed.

Experimental verification of electro-refractive phase modulation in graphene

Graphene has been considered as a promising material for opto-electronic devices, because of its tunable and wideband optical properties. In this work, we demonstrate electro-refractive phase modulation in graphene at wavelengths from 1530 to 1570 nm. By integrating a gated graphene layer in a silicon-waveguide based Mach-Zehnder interferometer, the key parameters of a phase modulator like change in effective refractive index, insertion loss and absorption change are extracted. These experimentally obtained values are well reproduced by simulations and design guidelines are provided to make graphene devices competitive to contemporary silicon based phase modulators for on-chip applications.

Optical generation of terahertz and second-harmonic light in plasma-activated silicon nanophotonic structures

Plasma-activated silicon structures exhibit symmetry broken surfaces through chemical surface modification leading to a considerable second-order nonlinear optical response. This nonlinear response is demonstrated in second-harmonic and difference frequency generation measurements including the generation of terahertz radiation in silicon photonic nanowires using telecom wavelength excitation pulses.

Electro-optic light modulation and THz generation in locally plasma-activated silicon nanophotonic devices

Silicon is not an electro-optic material by itself but the required second-order optical nonlinearity can be induced by breaking the inversion symmetry of the crystal lattice. Recently, an attractive approach has been demonstrated based on a surface-activation in a CMOS-compatible HBr dry etching process. In this work, we further investigate and quantify the second-order nonlinearity induced by this process. Using THz near-field probing we demonstrate that this simple and versatile process can be applied to locally equip silicon nanophotonic chips with micro-scale areas of electro-optic activity. The realization of a first fully integrated Mach-Zehnder modulator device - based on this process - is applied to quantify the nonlinearity to an effective χ((2)) of 9 ± 1 pm/V. Analysis of the thermal stability of the induced nonlinearity reveals post-processing limitations and paths for further efficiency improvements.

Photoconductive Terahertz Near-Field Detectors for Operation With 1550-nm Pulsed Fiber Lasers

We present a new generation of photoconductive (PC) terahertz (THz) near-field probes based on freestanding Beryllium-doped low-temperature-grown InGaAs/InAlAs cantilevers. The PC probes are compatible to optical gating with a femtosecond laser having a center wavelength of 1550 nm. Therefore, they are well suited for a cost-efficient direct integration with fiber-coupled THz time-domain spectroscopy (TDS) systems. The photoconductor material features electron lifetimes of 500 fs allowing for broadband detection up to 2 THz, which is comparable to existing LT-GaAs-based near-field detectors, which require 800-nm wavelength excitation. We demonstrate the detector operation in a state-of-the-art fiber-based TDS system with fast and coherent data acquisition and show obtained sheet-resistance mappings of conductive thin-films featuring subwavelength resolution.

Novel techniques in terahertz near-field imaging and sensing

Abstract: In this chapter, novel active optoelectronic probe-tips fabricated by micromachining techniques are introduced. These devices are highly attractive for a broad range of applications requiring micron-scale resolution and terahertz (THz) range operation. Using planar waveguide designing modalities they are the first devices combining THz generation, detection, transmission and focussing elements in a single self-contained cantilever microstructure. At the beginning of the chapter a brief overview of current near-field (NF) probing approaches and their applications is given. Passive focussing components such as apertures, metal tips or waveguides are discussed, as well as active (electro-optic and photoconductive) components are able to internally generate and detect THz signals on sub-wavelength scales. Then the new concept is explained and applications in the fields of nonlinear nanophotonic device characterization, failure location in integrated circuits (ICs) and photovoltaic material analysis are demonstrated.

Experimental demonstration of electro-refractive phase modulators based on graphene

We present the experimental demonstration of a compact waveguide-integrated graphene phase modulator marking an essential milestone in outperforming existing silicon based phase modulators. The results are reproduced by simulations.

Terahertz generation in locally plasma-activated silicon nanophotonic waveguides

We present a novel approach for efficient terahertz signal generation at silicon nanophotonic waveguides via electro-optic difference frequency generation using telecom wavelengths excitation pulses. Second-order nonlinearity of silicon is achieved by plasma-activated surface modification.