Antenna structure, light emitting device and method for designing an antenna structure
An antenna structure for directing light is disclosed. The antenna structure includes a reflector having a reflective surface and a ring-shaped dielectric grating arranged at the reflective surface and extending concentrically along a center axis perpendicular to the reflective surface and forming an omnidirectional reflector surrounding a low-index center portion of the ring-shaped dielectric grating. The antenna structure is configured to outcouple light emitted inside the low-index center portion through an upper end of the dielectric grating along the center axis with a Gaussian beam profile projection efficiency η of at least 65%. A light emitting device and a method for designing an antenna structure are also disclosed.
1. An antenna structure for directing light, the antenna structure comprising:
a reflector having a reflective surface; and
a ring-shaped dielectric grating arranged at the reflective surface and extending concentrically along a center axis perpendicular to the reflective surface and forming an omnidirectional reflector surrounding a low-index center portion of the ring-shaped dielectric grating,
wherein the antenna structure is configured to outcouple light emitted inside the low-index center portion through an upper end of the dielectric grating along the center axis with a Gaussian beam profile projection efficiency η of at least 65%.
2. The antenna structure according to claim 1 , wherein the Gaussian beam profile projection efficiency η represents a measure for an overlap of the emitted beam profile with a Gaussian beam profile.
3. The antenna structure according to claim 1 , wherein the antenna structure is configured such that the Gaussian beam profile projection efficiency η is at least 70%.
4. The antenna structure according to claim 1 , wherein the low-index center portion has an extension D in at least one direction perpendicular to the center axis.
5. The antenna structure according to claim 4 , wherein the low-index center portion has a round cross-sectional shape in a plane perpendicular to the center axis and a diameter corresponding to the extension D.
6. The antenna structure according to claim 4 , wherein the extension D is not more than 1 mm.
7. The antenna structure according to claim 4 , wherein the extension D is at least 100 nm.
8. The antenna structure according to claim 4 , wherein the ring-shaped dielectric grating has a height along the center axis of not more than D.
9. The antenna structure according to claim 1 , wherein the ring-shaped dielectric grating has a height along the center axis of not more than 10 mm.
10. The antenna structure according to claim 1 , wherein the ring-shaped dielectric grating has a height along the center axis of at least 50 nm.
11. The antenna structure according to claim 1 , wherein the ring-shaped dielectric grating comprises an alternating arrangement of first layers having a first refractive index and second layers having a second refractive index, wherein the second refractive index is higher than the first refractive index and wherein the refractive index of the low-index center portion is lower than the first refractive index.
12. The antenna structure according to claim 11 , wherein one of the second layers forms an innermost layer confining the low-index center portion.
13. The antenna structure according to claim 11 , wherein the second refractive index has a value of more than 1.75 and/or wherein the first refractive index has a value in a range from 1.1 to 1.75.
14. The antenna structure according to claim 11 , wherein the first layers consist of or comprise at least one of the following materials: MgF 2 , SiO 2 , PMMA, Diamond, Cubic Zirconia, GaAs, InGaP, InGaAs, and AlGaAs.
15. The antenna structure according to claim 11 , wherein the second layers consist of or comprise at least one of the following materials: TiO 2 , GaAs, Diamond, and InGaAs.
16. The antenna structure according to claim 1 , wherein the low-index center portion is empty or at least partly filled with a filling material having a lower refractive index than the first refractive index, and wherein the low-index center portion is at least partly filled with air or nitrogen.
17. The antenna structure according to claim 1 , wherein the low-index center portion has a refractive index in a range from 1 to 1.1.
18. The antenna structure according to claim 1 , wherein the low-index center portion is configured to establish a distribution of the electric field intensity resembling a standing wave along the center axis from light emitted inside the low-index center portion, and wherein the standing wave exhibits at least two maxima of the electric field intensity along the center axis.
19. The antenna structure according to claim 17 , wherein the ring-shaped dielectric grating has a height, such that the upper end of the dielectric grating is positioned in the rising or falling slope of one of the maxima of the distribution of the electric field intensity in a range, in which the electric filed intensity has a value between 0.3 and 0.8 of the maximum field intensity.
20. The antenna structure according to claim 1 , wherein the antenna structure is configured to receive one or more light emitters in the low-index center portion.
21. The antenna structure according to claim 1 , further comprising a top layer element arranged at the upper end of the ring-shaped dielectric grating.
22. The antenna structure according to claim 21 , wherein the top layer element has a half-sphere shape.
23. A light emitting device comprising:
the antenna structure according to claim 1 ; and
one or more light emitters arranged in the low-index center portion of the antenna structure, wherein the light emitted inside the low-index center portion is emitted by the one or more light emitters.
24. The light emitting device according to claim 23 , wherein the one or more light emitters comprise at least one single photon emitter.
25. A method for designing an antenna structure for a light emitter having a predetermined emission wavelength, the method comprising the steps:
determining a thickness of first layers and a thickness of second layers of a ring-shaped dielectric grating to extend concentrically along a center axis perpendicular to a reflective surface and to form an omnidirectional reflector surrounding a low-index center portion, wherein the thickness of the first layers and the thickness of the second layers are determined such that their optical thickness corresponds to a quarter wavelength of the predetermined emission wavelength;
optimizing the thicknesses of the first and second layers, a height of the dielectric grating, a position for the light emitter in the low-index center portion of the ring-shaped dielectric grating along the center axis and a diameter of the low-index center portion using a numerical optimization such as to maximize the Gaussian beam profile projection efficiency n of the outcoupled emission light emitted by the emitter.