1. An optical circuit comprising:
a thermo-optic device including a waveguide core formed over a semiconductor substrate, said core covered by a cladding; and
an opening in said semiconductor substrate extending under said thermo-optic device, said opening having a first laterally
extending region communicating with a second deeper, vertically extending region, said vertically extending region having
a lateral extent less than the lateral extent of said laterally extending region, wherein said first laterally extending region
is isotropically etched, and said second vertically extending region is anisotropically etched.
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