Reflection-Assisted Discrete Achromatic Diffraction

Diffraction redirects light by spatial structuring and is intrinsically chromatic, whereas reflection obeys a wavelength-independent geometric law. Here we show that a deep binary dielectric grating unifies these two principles to realize dispersion-suppressed diffraction across a broad spectral range, without invoking meta-atom group-delay engineering. The period selects a discrete family of diffraction orders that share a common output angle at distinct wavelengths, while the gap drives strong sidewall reflection that renders the modal beat length nearly insensitive to wavelength variation. Together, these two mechanisms redirect multiple discrete wavelengths into the same far-field direction with high channel efficiency and suppressed interorder leakage. Angle-resolved far-field measurements across the visible range confirm this convergence, consistent with full-wave far-field simulations.