Partially etched reflection-modification layer

An optical element to modify reflection of an optical signal with the operational wavelength λ0 propagating incident thereto, the optical element comprising:

• a transmissive layer comprising first and second optical media;
• a third optical medium configured to be substantially transparent to the operational wavelength λo of the optical signal and being characterized by a bulk refractive index at the operational wavelength n3(λ0);
• a solid substrate configured to be substantially transparent to the operational wavelength λo of the optical signal and being characterized by a bulk refractive index at the operational wavelength nsub(λ0),
• wherein:
• the transmissive layer has a substantially uniform thickness D, a first surface of the transmissive layer faces the solid substrate on which the transmissive layer is supported, a second surface of the transmissive layer is positioned against the third optical medium, whereby the transmissive layer is interposed between the substrate and the third optical medium;
• the first optical medium comprises a solid material;
• the first optical medium and the second optical medium are configured to be substantially transparent to the operational wavelength λo of the optical signal and are characterized by respective bulk refractive indices at the operational wavelength n1(λ0) and n2(λ0), where n1(λ0)≠n2(λ0);
• the first and second optical media are arranged within the transmissive layer as a contiguous multitude of discrete volumes, and each discrete volume (i) extends longitudinally from the first surface to the second surface, and (ii) comprises either the first optical medium or the second optical medium, but not both;
• (i) each discrete volume of the first optical medium is less than a first distance d1 in transverse extent in one or both transverse dimensions and is separated from at least one other discrete volume of the first optical medium by a transverse distance less than a second distance d2 through an intervening discrete volume of the second optical medium, and a relationship of the first and second distances d1 and d2 is defined as a sum of the first and second distances (d1+d2) being less than a ratio of the operational wavelength λo, to which the first, second, and third optical media and the solid substrate are configured to be substantially transparent, relative to a sum of the bulk refractive indices at the operational wavelength for the solid substrate and the third optical medium (nsub(λ0)+n3(λ0)), whereby d1+d2<λ0/nsub(λ0)+n3(λ0), or (ii) each discrete volume of the second optical medium is less than a second distance d2 in transverse extent in one or both transverse dimensions and is separated from at least one other discrete volume of the second optical medium by a transverse distance less than a first distance d1 through an intervening discrete volume of the first optical medium, and a relationship of the first and second distances d1 and d2 is defined as a sum of the first and second distances (d1+d2) being less than a ratio of the operational wavelength λo, to which the first, second, and third optical media and the solid substrate are configured to be substantially transparent, relative to a sum of the bulk refractive indices at the operational wavelength for the solid substrate and the third optical medium (nsub(λ0)+n3(λ0)), whereby d1+d2<λo/nsub(λ0)+n3(λ0);
• the discrete volumes of the multitude are sized and distributed on the transmissive layer so that, for the optical signal within the operational wavelength λo and incident on the transmissive layer, the optical element exhibits (i) a first field amplitude reflectivity r1 from the first surface, (ii) a phase delay Δφ for single-pass propagation through the transmissive layer, and (iii) a second field amplitude reflectivity r2 from the second surface, each of which is substantially constant and non-zero, when averaged with a sampling area about equal in transverse extent to the operational wavelength λo in both transverse dimensions, as a function of two-dimensional transverse position along the transmissive layer;
• and
• the substantially constant values of r1, Δφ, and r2 result in a net power reflectivity of the transmissive layer that differs from a power reflectivity exhibited by an interface between the substrate and the third optical medium without the transmissive layer therebetween.