Efficient refractive index modulation in an open access silicon photonic platform

Abstract

Optical modulators are one of the critical components of integrated photon-ics. Modulators enable control of light phase through an external excitation. This excitation can take different forms, including mechanical or electrical excitation. The maximum phaseshift, the bandwidth, power consumption as well as area footprint of the modulator are the most important key per-formance parameters of optical modulators. Bandwidth and phaseshifting efficiency are common to be of top priority in the telecom industry. This thesis focuses on the realization of highly efficient optical modulators in a silicon photonic open access platform. The modulators discussed within this work aim at achieving specific key performance indicators for application in an optical phased antenna array. An optical phased antenna array allows for full solid state laser beam steering. Its applications cover a diverse set of solutions within mobility, smart wearables, high precision measurement and meteorology industries. Unlike modulators for the telecom industry, an optical phased antenna ar-ray does not require a high operation bandwidth. As such an array involves a large number of optical modulators. Power consumption is a critical param-eter. This is implied by ensuring a low power density to prevent unwanted thermal effects. In this thesis, design and proposal as well as the experimental realization of novel concepts of integrated optical modulators is pursued. Fabrication of proposed designs is primarily done using an open access standard technol-ogy. The term open access refers to a standard industrial process, that is available to use publicly, under specific conditions, and usually for paying a considerable participation fee. However, such technologies largely ensure re-producibility and mass-producible industrially compatible designs. The work focuses on two approaches. The first is the optomechanical modulation, and the second is the electro-optic modulation. The optomechanical modulator poses a larger fabrication challenge, due to requiring free-standing nanophotonic waveguides to function. This, in turn, requires developing a releasing process, which is heavily discussed in this work. The electro-optic modulators in this work are realized by direct fab-rication in an open access technology without further processing. In the framework of this thesis, a fundamentally novel electro-optic modulator was proposed. This is the bi-junction electro-optic modulator. This modulator implements an implant profile, that is almost identical to the implant profile commonly found in a bipolar junction transistor, however, implemented in an optical waveguide. The phaseshifting performance of these modulators significantly exceeds the reported phaseshifting performance of the commonly reported lateral and interleaved modulators. Also, in some cases, the bi-junction modulators achieve lower or slightly higher optical losses. The phaseshift/optical loss figure of merits are calculated from experimental measurements. The NPN polarity of the bi-junction modulator achieves more than 200 % increase over the stander PN-lateral modulator and 8.3 % increase over the interleaved modulator. Interestingly, in this run, the losses were limited by employing a specific electrical contacts layout that added 1 dB of optical losses to the bi-junction modulators. Removal of such lossy elements can improve the performance gains to 50 % and 300 % more than interleaved and lateral modulators respectively. The bi-junction electro-optic modulator achieves these performance in-dicators under an operation bandwidth of almost 2 MHz and an electrical power consumption of less than 1 nW. These modulators are thought to be-come disruptive modulation devices for optical phased antenna arrays, which are currently a very hot topic in silicon photonics.