Diffractive optical element
A diffractive optical element may include sub-wavelength period stack-and-gap structured layers providing transmissive phase delay at a wavelength. The sub-wavelength period stack-and-gap structured layers may include a set of thin anti-reflection layers that are index matched to an environment or a substrate over a range of fill factors of the sub-wavelength period.
1. A diffractive optical element, comprising:
sub-wavelength period stack-and-gap structured layers providing transmissive phase delay at a wavelength,
wherein the stack-and-gap structured layers include a set of thin anti-reflection layers that are index matched to an environment or a substrate over a range of fill factors of the sub-wavelength period of the stack-and-gap structure layers, and
wherein the range of fill factors varies within a period that includes the sub-wavelength period.
2. The diffractive optical element of claim 1 , wherein the set of thin anti-reflection layers are index matched to the environment and to the substrate over the range of fill factors of the sub-wavelength period.
3. The diffractive optical element of claim 1 , wherein the set of thin anti-reflection layers include thin film layers.
4. The diffractive optical element of claim 1 , wherein the stack-and-gap structured layers are castellated in a single dimension planar to a top surface of the substrate.
5. The diffractive optical element of claim 1 , wherein the stack-and-gap structured layers are castellated in two dimensions planar to a top surface of the substrate.
6. The diffractive optical element of claim 1 , wherein the stack-and-gap structured layers include a spacer layer; and
wherein the transmissive phase delay corresponds to a thickness of the spacer layer.
7. The diffractive optical element of claim 6 , wherein the spacer layer is a thin film layer.
8. The diffractive optical element of claim 1 , wherein a transmission efficiency of the diffractive optical element is greater than 85% over the range of fill factors.
9. The diffractive optical element of claim 1 , wherein a transmission efficiency of the diffractive optical element is greater than 95% over the range of fill factors.
10. The diffractive optical element of claim 1 , wherein the stack-and-gap structured layers include at least one silicon layer and at least one silicon dioxide layer.
11. The diffractive optical element of claim 1 , wherein the diffractive optical element is polarization independent at the wavelength.
12. The diffractive optical element of claim 1 , wherein a maximum transmissive phase delay between areas of the stack-and-gap structured layers is greater than or equal to 2π.
13. The diffractive optical element of claim 1 , wherein a maximum transmissive phase delay between areas of the stack-and-gap structured layers is greater than or equal to 4π.
14. The diffractive optical element of claim 1 , wherein the stack-and-gap structured layers is disposed on a first surface of the substrate; and
wherein other sub-wavelength period stack-and-gap structured layers providing another transmissive phase delay at the wavelength is disposed on a second surface of the substrate, and
wherein the other sub-wavelength period stack-and-gap structured layers include another set of thin anti-reflection layers that are index matched to the environment or the substrate over the range of fill factors of the sub-wavelength period.
15. The diffractive optical element of claim 1 , wherein a fill material at least partially covers the stack-and-gap structured layers.
16. A diffractive optical element, comprising:
a stack of layers including a set of anti-reflection layers,
wherein the stack of layers is transmissive for a wavelength,
wherein a width of the stack of layers is divided into a set of periods,
wherein a width of each period, of the set of periods, is shorter than the wavelength,
wherein a period, of the set of periods, has a fill factor defining a width of a gap in the period,
wherein fill factors of different periods are different,
wherein a depth of the gap extends through the stack of layers and through the set of anti-reflection layers,
wherein each period, of the set of periods, provides phase delay, at the wavelength, associated with a corresponding fill factor, and
wherein, over a range of different fill factors, the set of anti-reflection layers is index matched to an environment in the gap or to a substrate.
17. The diffractive optical element of claim 16 , wherein the set of anti-reflection layers form at least two anti-reflection structures.
18. The diffractive optical element of claim 16 , wherein the set of anti-reflection layers form at least three anti-reflection structures.
19. A diffractive optical element, comprising:
stack-and-gap structured layers including at least one layer sandwiched between a first set of anti-reflection layers and a second set of anti-reflection layers,
wherein the stack-and-gap structured layers provide transmissive phase delay at a wavelength,
wherein the stack-and-gap structured layers have a period shorter than the wavelength,
wherein a material composition, a thickness, and a quantity of layers of the first set of anti-reflection layers and a material composition, a thickness, and a quantity of layers of the second set of anti-reflection layers are selected such that, at the wavelength, a greater than 85% transmission efficiency is achieved over a range of fill factors of the period, and
wherein the range of fill factors varies within a wavelength period that includes the period.
20. The diffractive optical element of claim 19 , wherein the diffractive optical element is disposed on a first side of a substrate and another diffractive optical element is disposed on a second side of the substrate.