Image sensors with diffractive lenses

An image sensor may include an array of imaging pixels. Each imaging pixel may have a photosensitive area that is covered by a respective diffractive lens to focus light onto the photosensitive area. The diffractive lenses may have a higher index of refraction than the surrounding materials. The diffractive lenses may be formed on an upper or lower surface of a planarization layer or may be embedded within the planarization layer. In some cases, multiple diffractive lenses may be formed over the imaging pixels. Some of the multiple diffractive lenses may have refractive indexes lower than the planarization layer such that the diffractive lenses defocus light. Focusing and defocusing diffractive lenses may be used to tune the response of the imaging pixels to incident light.