Optically pumped sensors or references with die-to-package cavities

A method of forming a stacked optoelectronic packaged device (packaged device), comprising:

• providing a package comprising a package substrate providing side walls and a bottom wall;
• positioning a bottom die having a bottom surface with a comb structure thereon comprising a material having a thermal resistance above that of a substrate material for said bottom die comprising a plurality of spaced apart teeth separated by gaps onto said bottom wall of said package, wherein said bottom die has a top surface including at least one electrical trace and a light source die for emitting light thereon;
• positioning a first cavity die onto said top surface of said bottom die or to legs of said package which extend above said bottom wall;
• positioning an optics die on said first cavity die, a sealing die on said optics die, and a second cavity die on said sealing die;
• positioning a photodetector (PD) die in a location to be coupled to receive said light originating from said light source die;
• creating a vacuum within said package, and
• sealing a lid on a top of said side walls for sealing said package under said vacuum.