For example, a flowcell includes: a nanowell layer having a first set of nanowells and a second set of nanowells to receive a sample; a first linear waveguide associated with the first set of nanowells, and a second linear waveguide associated with the second set of nanowells; and a first grating for the first linear waveguide, and a second grating for the second linear waveguide, the first and second gratings providing differential coupling of first light and second light.