OPTICAL DIODE

Abstract

An optical diode (1) comprising an optical wave guide for guiding light, preferably of a light mode, with a vacuum wavelength λ.sub.0, wherein the optical wave guide has a wave guide core (2, 3, 14) with a first index of refraction (n.sub.1), and the wave guide core (2, 3, 14) is surrounded by at least one second optical medium which has at least one second index of refraction (n2), wherein n.sub.1>n.sub.2 applies, wherein the wave guide core (2, 3, 14) has at least in sections a smallest lateral dimension (7) which is a smallest dimension of a cross section (6) perpendicular to a propagation direction (5) of the light in the wave guide core (2, 3, 14), wherein the smallest lateral dimension (7) is greater than or equal to λ.sub.0/(5*n.sub.1) and less than or equal to 20*λ.sub.0/n.sub.1, wherein the optical diode (1) additionally comprises at least one absorber element (10, 11, 15, 16) which is arranged in a near field, wherein the near field consists of the electromagnetic field of the light of the vacuum wavelength λ.sub.0 in the wave guide core (2, 3, 14) and outside of the wave guide core (2, 3, 14) up to a standard interval (12) of 5*λ.sub.0, wherein the standard interval (12) is measured starting from one surface (8) of the wave guide core (2, 3, 14) forming an optical interface and in a direction perpendicular to the surface (8). The invention provides that the at least one absorber element (10, 11, 15, 16) for the light of the vacuum wavelength λ.sub.0 has a strongly different absorption for left circular polarization (σ.sup.−) and the right circular polarization (σ.sup.+).

Claims

1. An optical diode comprising an optical waveguide for conducting light, preferably a light mode, having a vacuum wavelength λ.sub.0, where the optical waveguide has a waveguide core having a first refractive index n.sub.1 and the waveguide core is surrounded by at least one second optical medium, which has at least one second refractive index n.sub.2, where n.sub.1>n.sub.2, where the waveguide core has, at least in segments, a smallest lateral dimension, which is a smallest dimension of a cross section perpendicular to a direction of propagation of the light in the waveguide core, where the smallest lateral dimension is greater than or equal to λ.sub.0/(5.Math.n.sub.1) and is less than or equal to 20.Math.λ.sub.0/n.sub.1, where the optical diode additionally comprises at least one absorber element, which is disposed in a near field, where the near field consists of the electromagnetic field of the light of the vacuum wavelength λ.sub.0 in the waveguide core and outside of the waveguide core up to a standard distance of 5.Math.λ.sub.0, where the standard distance is measured from a surface of the waveguide core that forms an optical interface and in a direction perpendicular to the surface, wherein the at least one absorber element exhibits a differently strong absorption for the light having the vacuum wavelength λ.sub.0 when it has left-circular polarization (σ.sup.−) on the one hand and when it has right-circular polarization (σ.sup.+), on the other.

2. The optical diode as in claim 1, wherein a plurality of absorber elements is provided.

3. The optical diode as in claim 1, wherein the waveguide core is disposed on a substrate.

4. The optical diode as in claim 3, wherein the waveguide core is, at least in segments, recessed in substrate.

5. The optical diode as in claim 1, wherein at least one, preferably singly charged, quantum dot is provided as the at least one absorber element.

6. The optical diode as in claim 5, wherein the at least one quantum dot is disposed outside of the waveguide core.

7. The optical diode as in claim 6, wherein a plurality of quantum dots is provided, where the quantum dots are disposed parallel to the direction of propagation.

8. The optical diode as in claim 5, wherein the plurality of quantum dots comprise quantum dots that have differently strong absorption for light of different wavelength in the case of left-circular polarization (σ.sup.−) on the one hand and in the case of right-circular polarization (σ.sup.+) on the other.

9. The optical diode as in claim 8, comprising a resulting interval of wavelengths, in which the differently strong absorption, has a width that is greater than 1 nm, preferably greater than or equal to 10 nm, especially preferably greater than or equal to 30 nm.

10. The Optical diode as in claim 2, characterized in that foreign atoms are provided in the waveguide core as absorber elements.

11. The optical diode as in claim 10, wherein the waveguide core consists of a semiconductor material and the foreign atoms are doping atoms for the semiconductor material.

12. The optical diode as in claim 11, wherein the optical waveguide consists of silicon.

13. The optical diode as in claim 12, wherein the foreign atoms are boron atoms.

14. The optical diode as in claim 1, comprising means for generation of at least one magnetic field at the position of the at least one absorber element are provided in order to enhance the difference between the strength of absorption of left-circularly polarized (σ.sup.−) light of vacuum wavelength λ.sub.0 by the at least one absorber element and the strength of absorption of right-circularly polarized (σ.sup.+) light of the vacuum wavelength λ.sub.0 by the at least one absorber element.

15. The optical diode as in claim 14, wherein the at least one magnetic field is made so that different parts of the plurality of absorber elements are exposed to magnetic fields of different strengths.

16. The optical diode as in claim 14, wherein at least one electric conductor, through which current can be passed to generate the at least one magnetic field, is provided.

17. The optical diode as in claim 16, wherein the at least one electric conductor is disposed at least in segments on and/or in the substrate.

18. The optical diode as in claim 14, wherein at least one permanent magnet is provided in order to generate the at least one magnetic field.

19. The optical diode as in claim 18, wherein the at least one permanent magnet is disposed at least in segments on and/or in the substrate.

20. The optical diode as in claim 14, wherein the at least one magnetic field is a so-called imaginary magnetic field, where light having a vacuum wavelength that is being guided in the optical waveguide is provided for generation of the imaginary magnetic field, where λ′≠λ.sub.0 and the at least one absorber element is disposed in another near field, which other near field consists of the electromagnetic field of the light of vacuum wavelength λ′ in the waveguide core and outside the waveguide core up to a standard distance of 5.Math.λ′.

21. The optical diode as in claim 14, wherein the at least one magnetic field is variable at the position of at least one absorber element and preferably at least temporarily amounts to at least 1 T, preferably at least 3 T, especially preferably at least 5 T.

22. The optical diode as in claim 1, comprising means for generation of at least one electric field at the position of the at least one absorber element are provided in order to change the value of the wavelength λ at which the at least one absorber element absorbs left-circularly polarized (σ.sup.−) light and right right-circularly polarized (σ.sup.+) light with different strength.

23. The optical diode according to claim 22, wherein the at least one electric field is variable at the position of at least one absorber element.

24. The optical diode as in claim 1, wherein at least one plasmonic nanostructure, the greatest dimension of which is smaller than the vacuum wavelength λ.sub.0 of the light guided in the optical waveguide, is provided as absorber element.

25. The optical diode as in claim 24, wherein the at least one plasmonic nanostructure is made of metal, preferably gold.

26. The optical diode as in claim 24, wherein the at least one plasmonic nanostructure has, at least in segments, the shape of a spiral.

27. The optical diode as in claim 1, wherein the optical diode is laid out so that the optical diode is essentially transparent for light having vacuum wavelength λ.sub.0 in a preset direction of propagation and in a direction opposite the direction of propagation absorbs at least 50%, especially at least 75%, preferably at least 90%, especially preferably at least 99% of the optical power of the light having vacuum wavelength λ.sub.0.

28. An integrated optical circuit comprising an optical diode as in claim 1.

Description

BRIEF DESCRIPTION OF FIGURES

[0059] The invention will now be explained in more detail by means of embodiment examples. The figures are exemplary and are intended to present the ideas of the invention, but not in any way to limit it or even to present it in a final way. For clarity the drawings are not true to scale,

[0060] Here:

[0061] FIG. 1a is an illustration of the SOCL principle in the propagation of light in a direction of propagation in a waveguide disposed on a substrate.

[0062] FIG. 1b is a representation analogous to FIG. 1a, but with opposite direction of propagation of the light.

[0063] FIG. 2a is a schematic sketch of an optical diode according to the invention, where light propagates in a direction of propagation.

[0064] FIG. 2b is a representation analogous to FIG. 2a, but with the opposite direction of propagation of the light.

[0065] FIG. 3 is a schematic axonometric view of an optical diode according to the invention with quantum dots as absorber elements.

[0066] FIG. 4 is a schematic axonometric view of an optical diode according to the invention with foreign atoms as absorber elements.

[0067] FIG. 5 is a sectional view of an optical diode according to FIG. 4, where an electric conductor track is additionally provided.

[0068] FIG. 6 is a schematic axonometric view of an optical diode according to the invention with plasmonic nanostructures as absorber elements.

[0069] FIG. 7 is a detailed representation of a plasmonic nanostructure from FIG. 6.

WAYS TO IMPLEMENT THE INVENTION

[0070] FIG. 1a shows an illustration of the SOCL principle in the propagation of light in a waveguide core 14 disposed on a substrate 9 in a direction of propagation 5. The light is quasi-linearly polarized with a primary polarization component 17, which in this example lies in the plane of the drawing and is perpendicular to the direction of propagation 5. The waveguide core 14 has a refractive index n.sub.1. The waveguide core 14 is surrounded on one side by substrate 9 and on the other by air 13, and both the substrate 9 and air 13 have refractive indices that are smaller than n.sub.1. The air being guided penetrates both into the air 13 and into the substrate 9 and is present there in each case as an evanescent wave. The intensity of the relevant evanescent wave diminishes exponentially with the distance to a surface 8 of the waveguide core 14, where the surface 8 forms an optical interface between waveguide core 14 and substrate 9/air 13.

[0071] Because of the small lateral measurements of the waveguide core 14 the SOCL effect arises, which is expressed in that local circular polarization of the light occurs in the near field. This circular polarization is dependent on the direction of propagation 5 of the light. In FIG. 1a the direction of propagation 5 runs from left to right. The evanescent wave propagating in the air 13 therefore has a nearly complete left-circularly polarization σ.sup.−, for example, near the surface. On the opposite of the waveguide core 14 the evanescent wave propagating in substrate 9 has exactly the opposite polarization near surface 8, in this example therefore a nearly complete right-circularly polarization σ.sup.+.

[0072] If the direction of propagation 5 is reversed, the local circular polarization caused by the SOCL effect also is reversed. This is illustrated in FIG. 1b, where a direction of propagation 5 opposite to that in FIG. 1a is shown. Correspondingly, the evanescent wave propagating in the air 13 near the surface 8 has a nearly completely right-circular polarization σ.sup.+ and the evanescent wave propagating on the opposite side of the waveguide core 14 in substrate 9 near the surface 8 has a nearly completely left-circular polarization σ.sup.−.

[0073] FIG. 2a now shows an optical diode according to the invention, whose structure differs from that shown in FIGS. 1a and 1b only in that near the surface on the side of the waveguide core 14 turned toward the air 13 there are absorber elements 15, which essentially exclusively absorb right-circularly polarized light. Based on the direction of propagation 5 selected in FIG. 2a, which corresponds with the direction of propagation 5 in FIG. 1a, locally a nearly complete left-circular polarization σ′ of the light is present in the region of the near field in which the absorber elements 15 are disposed. Consequently no absorption takes place, and after passing the absorber elements 15 the light has essentially the same optical power as before passing absorber elements 15. This is illustrated by the two arrows pointing in the direction of propagation 5, which were essentially of the same size.

[0074] The optical diode in FIG. 2b has exactly the same structure as in FIG. 2a, but in this case the direction of propagation 5 runs the opposite way. Consequently, a reversed polarization is now present at the positions of the absorber elements 15, i.e., in the region of the near field in which the absorber elements 15 are positioned, the light has a nearly complete right-circular polarization σ.sup.+. Therefore, an absorption occurs, where each absorber element 15 reduces the overall optical power of the light guided in the waveguide, or in the optical diode 1, by a certain fraction. Overall, therefore, the light guided in the diode 1 has a lower optical power after passing the absorber elements 15 than before passing the absorber elements 15. This is indicated in FIG. 2b by the difference in sizes of the arrows pointing in direction of propagation 5.

[0075] FIG. 3 shows a concrete embodiment having a GaAs waveguide core 2, which is recessed in substrate 9, which can consist, for example, of SiO.sub.2. Perpendicular to its lengthwise direction 4, the waveguide core 2 has a rectangular cross section 6, which extends along a vertical direction 20 and a transverse direction 21 of the waveguide core 2. The vertical direction 20 in the example shown is perpendicular to a substrate surface. Along the vertical direction 20 the cross section 6 has its smallest dimension, which defines the smallest lateral dimension 7 of the waveguide core 2. The main polarization component 17 of the light guided in the waveguide core 2 runs parallel to the transverse direction 21. In the case of light whose vacuum wavelength λ.sub.0 lies in the infrared region in which it is intended to pass through the optical diode 1, the dimension of the cross section 6 in the transverse direction 21 can be, for example, about 100 nm.

[0076] Quantum dots 10, which have different absorption for left- and right-circularly polarized light of a specific wavelength λ, are provided as absorber elements. The quantum dots 10 are disposed so that they are in the near field of the light guided in wave [guide] core 2, which has a vacuum wavelength λ.sub.0. This is achieved because a standard distance 12 between the surface 8 and the individual quantum dots 10 is less than 5.Math.λ.sub.0.

[0077] In substrate 9, the quantum dots 10 are disposed at half the height of the waveguide core 2 in the vertical direction in order to enable optimum coupling of the quantum dots 10 to the near field. In each case according to whether the light in waveguide core 2 propagates in the indicated direction of propagation 5 or in the opposite direction, opposite circular polarization of the light at the site of the quantum dots 10 occurs. Correspondingly, the quantum dots 10 absorb the light in dependence on its direction of propagation 5 to a different degree/. The strength of absorption can be about 15% for a quantum dot 10, for example, in the case of a waveguide core 2 with a width of about 100 nm, a standard distance 12 between the quantum dot 10 and the surface 8 of about 50 nm and for light having a vacuum wavelength λ.sub.0 of 920 nm.

[0078] By varying the number of absorber elements or quantum dots 10 the strength of absorption of the optical diode in the blocking direction can be varied. Furthermore, through the choice of different quantum dots 10 the bandwidth of the optical diode can be varied in that the different quantum dots 10, for different wavelengths λ, have differently strong absorption for left- and right-circularly polarized light.

[0079] FIG. 4 shows another embodiment of an optical diode 1 according to the invention. In this case foreign atoms in the waveguide core 2 are used as absorber elements. In the example shown this is an Si waveguide core 3, and doping atoms 11, for example boron atoms, are used as foreign atoms. They are disposed in the region of surface 10 on one side of the waveguide core 3. Correspondingly, it is of less importance to recess the waveguide core 3 in substrate 9, which can be, for example, an SiO.sub.2 substrate. Therefore, the waveguide core 3 is disposed only on the substrate 9.

[0080] The main polarization component 17 of the light guided in waveguide core 3 runs parallel to the vertical direction 20. Correspondingly, the doping atoms 11 are disposed near a surface 8 of the waveguide core 3, //which is stretched from the lengthwise axis 4 and the width direction 21.

[0081] Independent thereof, the smallest lateral dimension 7 of the waveguide core 3 is in turn determined by the dimension of the cross section 6 along the vertical direction 20.

[0082] By the doping atoms 11 being disposed in waveguide core 3, they are necessarily in the near field. In each case according to whether the light in waveguide 3 propagates in the indicated direction of propagation or in the opposite direction, an opposite circular polarization of the light at the site of the doping atoms 11 occurs. Correspondingly, the doping atoms absorb the light with different strength depending on its direction of propagation.

[0083] By varying the number of absorber elements or doping atoms 11 the degree of absorption of the optical diode 1 in the blocking direction can be varied.

[0084] For example, an absorption of 0.0003% can be assumed for a single boron atom for light having a vacuum wavelength λ.sub.0 of 920 nm at an absorption bandwidth of 1 nm. Thus around 1.5×10.sup.6 boron atoms are necessary for an absorption of 99% in the blocking direction in this example. If the boron atoms are distributed on a portion of the surface 8 having an edge length of 100 nm and an average boron atom spacing of 5 nm, one needs in this case a waveguide core 3 that is 1.5 mm long. However, if one, more or realistically, assumes a three-dimensional distribution of the boron atoms, the said length decreases further by, for example, a factor of 10, if the boron atoms are distributed in 10 atomic layers on top of each other in waveguide core 3 close to the surface 8. Therefore, in the latter case one would provide doping atoms only on a length of the waveguide core 3 of 150 μm.

[0085] In order to be able to dynamically adjust the optical diode 1 for light having different vacuum wavelengths λ.sub.0 and/or to be able to vary the bandwidth of optical diode 1, the quantum dots 10 and/or the foreign atoms can be exposed to magnetic fields and/or electric fields. FIG. 5 illustrates this by means of the embodiment example from FIG. 4. Here an electric conductor 18 is disposed in substrate 9 under the waveguide core 3. By passing current through the conductor 18, a magnetic field can be generated at the location of the doping atoms. It can be dynamically adjusted by varying the current.

[0086] Because of the small distance 19 between the conductor 18 and the doping atoms 11 high magnetic fields can already be generated at the site of the doping atoms with moderate currents. In the embodiment example shown the distance 19 can be, for example, about 1 μm. However, too tight a spacing 19 can have a negative effect on the conduction of the light in the waveguide, since the conductor track 18 in general has a much higher refractive index than the waveguide core 3. The distance 19 therefore should preferably be at least greater than or equal to 200 nm, especially preferably greater than or equal to 300 nm.

[0087] Of course, in a completely analogous way permanent magnets can also be disposed in or on the substrate 9 alternatively or in addition to conductor track 18, even though constant magnetic fields can be generated with them.

[0088] Finally, FIG. 6 shows another embodiment of the diode 1 according to the invention, which is made completely analogously to the embodiment of FIG. 3, due to which one is basically referred to the description for FIG. 3. However, in contrast to FIG. 3, plasmonic nanostructures 16 are provided as absorber elements instead of the quantum dots 10. These plasmonic nanostructures 16 are made of metal, preferably gold, and are dimensioned so that their largest dimension is clearly smaller than the vacuum wavelength λ.sub.0 of the light to be guided in waveguide core 3. For example, a diameter of 30 nm and a height or thickness of 5 nm of the plasmonic nanostructure 16 are conceivable for visible light.

[0089] FIG. 7 shows a magnified view of a plasmonic nanostructure 16. Here one can readily see that the plasmonic nanostructure is made segmentally in a spiral shape. Specifically, the plasmonic nanostructure 16 that is shown has the shape of two intermeshing spirals. The helicity of these spirals is positive in the representation in FIG. 7, i. e., the spirals in this figure have a positive direction of rotation. This geometric design correspondingly enhances the absorption of the said plasmonic nanostructure 16 for left-circularly polarized light.

REFERENCE NUMBER LIST

[0090] 1 Optical diode [0091] 2 GaAs waveguide core [0092] 3 Si waveguide core [0093] 4 Lengthwise axis of the waveguide [0094] 5 Direction of propagation [0095] 6 Waveguide cross section [0096] 7 Smallest lateral dimension of the waveguide core [0097] 8 Waveguide surface [0098] 9 Substrate [0099] 10 Quantum dot [0100] 11 Doping atom [0101] 12 Standard distance between the surface and a quantum dot [0102] 13 Air [0103] 14 Waveguide core [0104] 15 σ′ absorber [0105] 16 Plasmonic nanostructure [0106] 17 Main polarization component of light guided in waveguide core [0107] 18 Electric conductor [0108] 19 Distance between the electric conductor and a doping atom [0109] 20 Vertical direction of the waveguide core [0110] 21 Width direction of the waveguide core [0111] σ.sup.− Left-circular polarization [0112] σ.sup.+ Right-circular polarization