Apparatuses and systems for photon detection can include a first optical sensing structure structured to absorb light at a first optical wavelength; and a second optical sensing structure engaged with the first optical sensing structure to allow optical communication between the first and the second optical sensing structures. The second optical sensing structure can be structured to absorb light at a second optical wavelength longer than the first optical wavelength and to emit light at the first optical wavelength which is absorbed by the first optical sensing structure. Apparatuses and systems can include a bandgap grading region.