Active photonic devices based on standard silicon-on-insulator based PICs rely typically on the plasma dispersion effect in silicon to induce a refractive index change as function of an applied voltage. In this case, the silicon waveguide is doped in such a way that it forms a pn-junction. This is, however, accompanied by optical losses due to absorption and fundamental speed restrictions are related to carrier injection and removal.
To overcome these limitations, it is desirable to integrate materials with large off-resonant electro-optical (EO) effect, which allows a change of refractive index by applying a dc or modulated electric field. In this way, no carrier transport is necessary giving rise to ultra-high speed data communication. Here, the silicon waveguide is structured like a capacitor. It consists of two silicon rails in which the active EO material is located in between. Functionalized organic materials are from great interest since they exhibit extraordinary high EO effects. In particular, they can exhibit both, a linear and quadratic EO effect [4]. The advantage of organic EO materials is twofold. First, they exhibit strong EO effects and are highly transparent in the telecommunication wavelength range. Second, they exhibit off-resonant EO effects avoiding unwanted non-parametric processes like two photon absorption and free carrier absorption. The combination of organic materials with silicon-on-insulator based waveguides is known as silicon-organic hybrid (SOH) technology.
Recent research has demonstrated a high-speed SOH phase shifter having bandwidths above 100 GHz [5]. Besides that, EO modulators with energy consumption in the atto-Joule per-bit regime have been realized, and also higher modulation formats have been demonstrated [6]. These results show clearly the potential of SOH devices for a next generation of signal processing.
However, the integration of organic materials into a well-established silicon-on-insulator technology is still challenging because typical semiconductor fabrication processes are used with relatively high process-temperatures destroying the organic materials. To tackle this issue, an integration approach was proposed in which the organic material is deposited in a post-process.
In this work, we review recent results on the hybrid integration of organic EO materials in a silicon-on-insulator technology. We outline some of the identified challenges regarding process compatibility and present preliminary results on the integration of organic materials in a photonic integrated circuit (PIC) technology. Here, we are focusing on EO applications for high speed data transfer employing the linear and quadratic EO effect. As an example, we demonstrate an intensity modulator fabricated in a 0.25 µm SiGe BiCMOS pilot line using 200 mm silicon-on-insulator wafers. This approach gives perspective for monolithically hybrid-integrated photonic devices in an electronic PIC (EPIC)-technology.
References
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