Slot modulators with engineered rails for improved performance

A Mach-Zehnder slot modulator coupled to a coplanar transmission line, the slot modulator comprising:

• a substrate;
• the coplanar transmission line positioned on the substrate and including first and second spaced apart elongated conductors with a third elongated conductor positioned midway between the first and second conductors;
• a first pair of spaced apart engineered rails positioned on the substrate between the first elongated conductor and the second elongated conductor and a second pair of spaced apart engineered rails positioned on the substrate between the second elongated conductor and the third elongated conductor, the first and second pairs of spaced apart engineered rails each forming an elongated waveguide slot therebetween, the engineered rails being formed of doped silicon;
• opposed slabs positioned on the substrate and coupling the first and second pairs of engineered rails to the elongated conductors of the coplanar transmission line, the slabs being formed of doped silicon;
• and
• EO polymer cladding material deposited over the first and second pairs of spaced apart engineered rails and the slabs between the elongated conductors and in the waveguide slots.

A method of fabricating a slot modulator coupled to a coplanar transmission line, the method comprising the steps of:

• providing a substrate with the coplanar transmission line thereon, the coplanar transmission line including at least one pair of spaced apart conductors;
• forming a pair of spaced apart elongated engineered rails on the substrate between the pair of spaced apart conductors, the spaced apart engineered rails defining an elongated waveguide slot therebetween, the engineered rails being formed at least partially of highly doped silicon;
• forming opposed slabs on the substrate coupling the engineered rails to the spaced apart conductors of the coplanar transmission line, the slabs being formed at least partially of highly doped silicon;
• and
• depositing an EO polymer cladding layer over the slabs and engineered rails and in the waveguide slot.