Roy specializes in an interdisciplinary applied physics field involving accelerator physics, quantum physics, nanophotonics, electron optics and electron microscopy, and ultrafast lasers. In transmission electron microscopy, he pioneered the field of computer-generated holograms in electron optics using amplitude and phase masks etched into thin silicon-nitride membranes, and authored a series of 6 influential publications along with 2 granted patents. His expertise further encompasses the design of novel nanophotonic structures for electron acceleration, based on traditional and adapted accelerator physics concepts. His efforts on "dielectric laser accelerators" culminated in two Nature publications in the past three years. He also designed a high-resolution, photon-order resolving magnetic spectrometer suited for use in a standard scanning electron microscope, which enabled measuring photon-energy differences in an electron-photon quantum interaction – for the first time in a scanning electron microscope, as reported on in an Editor-selected and Physics-featured Physical Review Letter.