Laser device based on a photonic crystal with columnar or wall-shaped semiconductor elements, and method for its operation and manufacture

Abstract

Laser device (100) comprising: - a substrate (10) on the surface of which an optical waveguide (11) is arranged, which contains an optical resonator (12, 13) with a resonator length such that at least one resonator mode in the resonator (12, 13) forms a standing wave, and - a gain medium arranged on a surface of the optical waveguide (11) and comprising a plurality of semiconductor elements (21) arranged periodically on the surface of the optical waveguide (11) in a manner projecting from the optical waveguide (11), wherein: - the gain medium is arranged for optical interaction with the at least one resonator mode of the optical resonator (12, 13) and for amplifying light having a wavelength of the at least one resonator mode of the optical resonator (12, 13), - the gain medium comprises a photonic crystal (20) and the semiconductor elements (21) are columnar and / or wall-shaped semiconductor elements (21), and - a diffractive coupling is formed between the photonic crystal (20) and the optical resonator (12, 13), in which standing light waves in the optical resonator (12, 13) are extended to the photonic crystal (20), characterized in that - a periodicity of the arrangement of the semiconductor elements (21) defines a coupling wavelength which is coupled between the semiconductor elements (21) and the waveguide (11), wherein the coupling wavelength and a resonance wavelength defined by the resonator length of the optical resonator (12, 13) are simultaneously selected such that the resonance wavelength in the waveguide (11) matches the coupling wavelength.

Description

[0001] The invention relates to a laser device comprising a photonic crystal with a plurality of columnar and / or wall-shaped semiconductor elements (nanostructured semiconductor elements, in particular nanowires and / or nanowalls), and to methods for their operation and their manufacture. Applications of the invention are in the generation of coherent light, particularly in optoelectronics.

[0002] For a description of the background of the invention, particular reference is made to the following publications: [1] J. Van Campenhout et al. in “Opt. Express” 15, 6744-67 49 (2007); [2] G.-H. Duan et al. in "IEEE J. Sel. Top. Quantum Electron." 20, 6100213 (2014); [3] B. Snyder et al. in "J. Lightw. Technol." 3, 3934-3942 (2013); [4] DE 10 2011 118 273 A1; [5] R. Chen et al. in "Nat. Phot." 5,170-175 (2011); [6] T. Stettner et al. in „Appl. Phys. Lett.“ 108, 011108 (2016); [7] A. C. Scofield et al. in „Nano Lett.“ 11, 5387-5390 (2011); [8] US 2014 / 0286367 A1; [9] H. Kim et al. in „Nano Lett.“ 16, 1833 (2016);

[10] W.-J. Lee et al. in „Appl. Phys. Lett.“ 108, 081108 (2016);

[11] I. Giuntoni et al., „Light coupling between vertical monolithically integrated III-V nanowires and planar silicon waveguides“, Abstracts of „European Materials Research Society Fall Meeting“, 2015;

[12] US 2014 / 0226691 A1;

[13] D. Korn et al. in „Nature Communications“ 7, 10864 (2016); und

[14] B. Jang et al. in „Appl. Phys. Expr.“ 9, 092102 (2016).

[0003] The monolithic integration of laser sources on silicon substrates represents one of the most important steps for new applications of silicon (Si) in photonics and / or electronics, such as the integration of optical components or optical data transmission in conjunction with CMOS technology. Silicon exhibits an indirect band gap, so light emission cannot be achieved directly from silicon. Therefore, combination with other materials that exhibit a direct band gap of suitable energy is required. Silicon substrates, in particular, allow the integration of waveguides for guiding light between optical components.

[0004] Various methods for integrating laser sources on silicon have been proposed, such as bonding III-V semiconductor chips onto the surface of a Si waveguide [1, 2] or hybrid integration by connecting separately prefabricated components [3]. However, these concepts have disadvantages such as limited effectiveness of light coupling into the waveguide, complex structure, or lack of suitability for mass production.

[0005] It is also known to produce light-emitting components from photonic crystals (periodic arrangements of nanostructured semiconductor elements) (see, for example, [7]). The semiconductor elements have characteristic dimensions in the sub-micrometer range. By selectively etching planar semiconductors (“top-down” process) or by epitaxially growing nanostructured semiconductor elements on a substrate (“bottom-up” process), columnar semiconductor elements (nanowires) or wall-shaped semiconductor elements (nanowalls, nanoplates, nanodisks) can be formed on the substrate (see, for example, [4] and publications cited therein). Of particular interest are such semiconductor elements with heterostructures made of compound semiconductors based on arsenides and phosphides of atoms of main group III (main group III arsenide or phosphide).-phosphide semiconductors), which have direct band gaps smaller than the band gap of Si and are thus suitable for the emission of light that can be guided in Si waveguides without large losses.

[0006] Nanostructured semiconductor elements made of compound semiconductors, particularly when manufactured using the bottom-up process, have the advantage of overcoming limitations in planar technology due to lattice mismatch with Si substrates. Base surfaces of the nanostructured semiconductor elements, with typical dimensions of less than a few hundred nanometers, allow the growth of compound semiconductors on Si substrates with better crystal quality than is possible with planar technology.

[0007] Laser sources whose gain medium is formed by a single nanowire or a periodic array of nanowires are described, for example, in [5, 6, 7, 8]. However, these laser sources are characterized by limited performance and limited practical applicability. For example, they are not designed for combination with other optical components.

[0008] The positioning of III-V semiconductor nanowires on the surface of a patterned Si substrate and the coupling of light between the nanowires and a Si waveguide are described in [9, 10]. In these arrangements, the Si on which the nanowires are positioned serves only as a mechanical support. Furthermore, the structures described in [9, 10] do not achieve the emission of coherent laser light. The coupling of light fields between a periodic array of vertical semiconductor nanowires and planar Si waveguides is described in

[11] . Coupling of light fields between a planar Si waveguide and an overlying semiconductor structure can also be achieved if the latter is configured as a photonic crystal by etching holes from a semiconductor block

[12] .

[0009]

[13] describes a laser source comprising a hybrid structure with a silicon waveguide resonator and a capping layer made of a dye-doped organic substance. The capping layer forms a gain medium for the light guided in the waveguide resonator. The hybrid structure according to

[13] has disadvantages due to limited efficiency of light coupling between the gain medium and the resonator, which requires a long resonator length, as well as limited ability to electrically excite (electrically pump) the organic capping layer.

[0010]

[14] describes a laser source comprising a hybrid structure with a silicon waveguide resonator and a gain medium mounted thereon. The latter consists of quantum dots epitaxially embedded in a compound semiconductor heterostructure. This hybrid structure has disadvantages due to the small volume of the quantum dots, which act separately and do not form a photonic crystal, necessitating a long resonator length. Another disadvantage of the laser source arises from its non-monolithic fabrication.

[0011] The object of the invention is to provide an improved laser device based on a photonic crystal with a plurality of nanostructured semiconductor elements, which overcomes the disadvantages of conventional techniques. The laser device should be characterized, in particular, by increased performance, miniaturizability, electrical excitability of the laser emission, and / or improved integrability into Si-based semiconductor devices, in particular Si-based photonics and highly integrated electronics. Furthermore, the object of the invention is to provide improved methods for operating and improved methods for manufacturing such a laser device, which avoid the disadvantages of conventional methods.

[0012] These objects are achieved by the laser device and the operating and manufacturing methods having the features of the independent claims. Advantageous embodiments and applications of the invention are set forth in the dependent claims.

[0013] According to a first general aspect of the invention, the above-mentioned object is achieved by a laser device comprising an optical waveguide and a gain medium. The optical waveguide contains an optical resonator with a resonator length suitable for guiding at least one resonator mode within the resonator. The gain medium is arranged on the surface of the optical waveguide.

[0014] According to the invention, the gain medium comprises a photonic crystal with a plurality of columnar and / or wall-shaped semiconductor elements arranged periodically on the surface of the optical waveguide, projecting from the optical waveguide. The surface of the optical waveguide is an unstructured, in particular planar, surface that directly forms a boundary of the optical waveguide.

[0015] Furthermore, according to the invention, the photonic crystal is configured for direct optical interaction with the at least one resonator mode of the optical resonator through a selection of its geometric properties. The inventors have discovered that the photonic crystal can be formed by selecting the geometric properties, in particular the period (spacing), diameter, and height of the semiconductor elements, such that the coupling between the photonic crystal and the optical waveguide is maximized. Advantageously, the photonic crystal becomes a component of the optical resonator as a gain medium.

[0016] For the laser device to function, it is sufficient if only a section of the waveguide is provided that is configured as an optical resonator. However, another important advantage of the invention is that the waveguide containing the optical resonator can be directly coupled to a waveguide outside the laser device and / or, for example, in the case of a linear resonator, can continue outside the optical resonator.

[0017] In contrast to the techniques described in [9, 10], in which a photonic crystal composed of nanowires only interacts optically with a laterally directed waveguide in an evanescent manner, the present invention couples the photonic crystal directly to a waveguide immediately below it. Advantageously, the limitations resulting from the less efficient optical coupling and the provision of the lattice structure on which the nanowires are grown in [9, 10] are avoided.

[0018] The inventors have further determined that disadvantages of the hybrid structure according to

[13] and

[14] arise from the separation of the amplification in the cover layer on the one hand and the guidance of light in the waveguide on the other. In the hybrid structure according to

[13] , the coupling of light from the waveguide resonator into the cover layer occurs only evanescently, so that the intensity simply drops away from the waveguide in the gain medium and the coupling is determined solely by the geometry of the waveguide resonator and the refractive index of the cover layer. Deviating from this, according to the invention, the coupling of the photonic crystal to the resonator is determined diffractively, in particular by the period of the semiconductor elements in a reference direction along the alignment of the light field in the waveguide, in particular parallel to the optical axis in the waveguide, as well as by their diameter and refractive index.Standing light waves in the optical resonator of the waveguide are extended to the photonic crystal. This type of coupling is advantageously significantly more efficient than evanescent coupling, allowing the laser device to be designed considerably more compactly.

[0019] According to the invention, light is also guided within the photonic crystal under the effect of a jump in the refractive index relative to the surroundings of the semiconductor elements and / or additionally via metal mirrors on the photonic crystal. Since the interaction is simply evanescent and thus weak according to

[13] , the resonator in conventional technology must be relatively long (at least a few mm), resulting in many resonator modes and a very broad wavelength spectrum. This limits the suitability for tuning specific output wavelengths and the miniaturization for optoelectronic applications in

[13] .

[0020] According to the invention, the waveguide containing the optical resonator is arranged on a substrate. The substrate advantageously serves as a mechanical support. Particularly preferably, the waveguide is produced by processing from the substrate.

[0021] Preferably, the optical waveguide is formed protruding above the surface of the substrate. The waveguide has lateral surfaces that are oriented perpendicular or inclined to the surface of the substrate, and an upper surface that is oriented parallel to the surface of the substrate. According to a preferred embodiment of the invention, the photonic crystal is arranged on the upper surface of the optical waveguide, with the semiconductor elements oriented perpendicular to the upper surface and to the surface of the substrate. Particularly preferably, the photonic crystal is arranged exclusively on the upper surface of the optical waveguide, while the lateral surfaces are free of semiconductor elements. In

[13] , an all-side covering of the waveguide with the gain medium is described because the light field exits the waveguide mainly through the lateral surfaces into the cover layer.The inventors have found that due to the significantly more efficient nature of the coupling, lateral coverage of the waveguide with the photonic crystal is not necessary.

[0022] Furthermore, the resonator length of the optical resonator, i.e., the length of the waveguide between the reflectors, defines a resonance wavelength. On the other hand, the periodicity of the arrangement of the semiconductor elements defines a coupling wavelength that is coupled between the semiconductor elements and the waveguide. According to the invention, both parameters are selected simultaneously such that the resonance wavelength in the waveguide matches the coupling wavelength. For this purpose, the semiconductor elements are arranged along the reference direction parallel to the optical axis in the waveguide, preferably at the positions where field maxima of the at least one resonator mode (standing wave in the waveguide) are located. In other words, the period of the semiconductor elements is preferably a multiple of the wavelength in the waveguide.

[0023] Advantageously, the laser device can be provided with various types of resonators. For example, the resonator can be a linear resonator with two reflectors formed on the optical waveguide (first embodiment of the invention) or a closed resonator without reflectors (second embodiment of the invention).

[0024] The linear resonator preferably has the shape of an elongated cuboid formed on the surface of the substrate of the laser device, in particular protruding therefrom. Particularly preferably, the reflectors comprise periodic structures in the waveguide that form Bragg reflectors for the at least one resonator mode. The periodic structures in the waveguide comprise refractive index variations, e.g., through depressions in the upper surface of the waveguide, which are arranged along the optical axis of the waveguide at intervals that satisfy the Bragg condition for the at least one resonator mode.

[0025] According to a first variant of the first embodiment of the invention, it is sufficient if the semiconductor elements are arranged on the upper surface of the linear resonator as a single straight row along the reference direction parallel to the optical axis of the resonator. In this case, the semiconductor elements form a single-row, linear photonic crystal, which is advantageously characterized by simplified production. According to an alternative variant of the first embodiment of the invention, at least two, preferably at least three parallel rows of semiconductor elements are arranged on the upper surface of the linear resonator along the reference direction parallel to the optical axis of the resonator.In this case, the photonic crystal can be configured in such a way that the optical coupling of a central row to the waveguide is stronger than the optical coupling of the outer rows to the waveguide, and the outer rows are adapted for lateral confinement of the light field within the photonic crystal. This advantageously results in increased efficiency of the laser device.

[0026] The closed resonator of the second embodiment of the invention is equipped with a coupling section for coupling light out of the closed resonator. The coupling section preferably includes a linear waveguide extending at a distance along an outer surface of the closed resonator, the distance being selected depending on the wavelength. Alternatively, the coupling section can consist of a multimode interference coupler.

[0027] Advantageously, various shapes of closed resonators are available with which the laser device according to the second embodiment of the invention can be realized. For example, the closed resonator can be a circular ring resonator. In this case, advantages can arise for shaping the at least one resonator mode in the resonator. The photonic crystal comprises a single-row or multi-row, periodic circular arrangement of semiconductor elements on the upper surface of the ring resonator.

[0028] Alternatively, the closed resonator can be a racetrack resonator with a shape composed of straight and curved sections, which can result in advantages from the particularly preferred arrangement of the photonic crystal on at least one of the straight sections. In this variant of the invention, the photonic crystal, as mentioned above with respect to the linear resonator, can comprise one or more (at least two, in particular at least three) straight rows of semiconductor elements extending parallel to the optical axis in the respective straight section of the waveguide.

[0029] According to a particularly preferred embodiment of the invention, the laser device is configured for electrically exciting the photonic crystal. The surface of the substrate preferably carries a first contact electrode adjacent to the photonic crystal, in particular adjacent to the waveguide, which is configured to connect the base ends of the semiconductor elements connected to the substrate to a voltage source device via the substrate.

[0030] A second contact electrode is configured to connect the head ends of the semiconductor elements, opposite the base ends, to the voltage source device. For this purpose, the semiconductor elements of the photonic crystal are preferably arranged in a dielectric embedding layer, which advantageously stabilizes and mechanically protects the semiconductor elements and forms a support for the second contact electrode.

[0031] Particularly preferably, the second contact electrode is formed by an electrical contact layer arranged on the embedding layer and electrically connected to the tip ends of the semiconductor elements. If the electrical contact layer is an opaque layer, e.g., made of a metal, further advantages arise for confining the light field in the photonic crystal.

[0032] Alternatively, the laser device can be configured for optical excitation of the photonic crystal. In this case, no contact electrodes are provided.

[0033] According to a further, particularly preferred embodiment of the invention, the waveguide is made of silicon. The substrate is preferably an SOI substrate with a silicon carrier substrate, a silicon dioxide layer, and a silicon cap layer containing the optical waveguide. The waveguide is particularly preferably etched out of the silicon cap layer. The semiconductor elements are arranged in direct contact with the silicon, which results in several advantages. First, the integration of the laser device according to the invention into a Si-based structure, in particular a Si-based optoelectronic chip, is simplified. Second, when doped, the silicon is sufficiently electrically conductive to provide electrical contact between the semiconductor elements at their base ends facing the substrate.Particularly preferably, the substrate consists of silicon dioxide with a silicon cover layer that forms the substrate surface and supports the semiconductor elements. Advantageously, the silicon dioxide has a particularly large band gap and a low refractive index, so that the confinement of the light guided in the waveguide is supported by the silicon dioxide. In addition to the embodiment based on a silicon waveguide described in detail here, embodiments are also conceivable in which the waveguide consists of a different material, such as InP, GaAs, GaN, or AlN. Depending on the material of the waveguide, light of other wavelengths can be guided without loss, resulting in the respective advantageous materials for the substrate and the semiconductor elements.

[0034] According to a second general aspect of the invention, the above-mentioned object is achieved by a method for operating the laser device according to the first general aspect of the invention. During operation of the laser device, the laser device is coupled to a pumping device, in particular a voltage source device or a pumping light source, the photonic crystal is electrically or optically excited, and laser light is guided in the waveguide and / or emitted from the waveguide. Advantageously, laser light is coupled directly into the waveguide and thus provided for photonic applications based on waveguides. Alternatively, the light can be guided out of the waveguide.

[0035] According to a third general aspect of the invention, the above-mentioned object is achieved by a method for producing a laser device according to the first general aspect of the invention. The semiconductor elements of the photonic crystal are grown on the substrate. Preferably, all semiconductor elements grow simultaneously. Subsequently, contact is made with the photonic crystal via contact electrodes. Preferably, the growth of the semiconductor elements comprises direct epitaxial or gas-liquid-solid-based growth of the semiconductor elements on the substrate and / or mask-based deposition of the semiconductor elements on the substrate. Particularly preferably, the semiconductor elements are embedded in a transparent embedding layer.

[0036] Further details and advantages of the invention are described below with reference to the accompanying drawings. They show: Fig. 1: a schematic plan view of a laser device with a linear resonator according to the first embodiment of the invention; Fig. 2: a schematic plan view of a laser device with a ring resonator according to the second embodiment of the invention Fig. 3 and Fig. 4: schematic perspective views of a laser device with a linear resonator according to the first embodiment of the invention; Fig. 5 and Fig. 6: a schematic side view of a linear resonator and a graph of the Q factor of the linear resonator as a function of the period of the semiconductor elements; Fig. 7 and Fig. 8: a schematic perspective view of a Bragg reflector of the linear resonator according to Fig. 1 and a curve representation of the reflectivity and transmission as a function of the period number of the Bragg reflector; and Fig. 9 and Fig. 10: another schematic side view of a linear resonator and a graph showing the output power of the laser device as a function of the gain factor of the semiconductor elements.

[0037] Embodiments of the invention are described below with exemplary reference to a laser device equipped with a single row of semiconductor elements in the form of nanowires. The use of a single-row photonic crystal has advantages for simplified manufacture of the laser device. However, the practical implementation of the invention is not limited to this embodiment. Rather, the laser device can be realized with a multi-row photonic crystal. Furthermore, the invention is not limited to the use of nanowires, but can be realized in a corresponding manner with a photonic crystal made of nanowires and nanowalls or exclusively of nanowalls. In these variants of the invention, the nanowalls are aligned transversely to the light path in the resonator, in particular transversely to the optical axis in the resonator.

[0038] In general, the semiconductor elements of the photonic crystal in embodiments of the invention comprise, for example, the shape of a column (or: needle, wire, or rod) or the shape of a wall (or: disk). The thickness of the semiconductor elements is preferably at least 1 nm and / or at most 500 nm, and the height of the semiconductor elements is preferably at least 10 nm and / or at most 10 µm. The semiconductor elements are preferably made of a III-V semiconductor, in particular GaAs, GaP, or GaN compound semiconductors, and are optionally doped, e.g., with Si, Be, C, or Te (GaAs) or with Si, Ge, or Mg (GaN).

[0039] Fig. Figure 1 shows the first embodiment of a laser device 100 according to the invention with a linear resonator 12 in a schematic plan view. The laser device 100 comprises a substrate 10, on the surface of which a straight waveguide 11 (see Fig. 3). The surface of the substrate 10 and the waveguide 11 are made integrally of silicon, in particular formed as part of a silicon plate.

[0040] The waveguide 11 has a cuboid shape with lateral surfaces perpendicular or inclined relative to the surface of the substrate 10 and an upper surface parallel to the surface of the substrate 10. The waveguide 11 has a width of, for example, 500 nm and a height of the upper surface relative to the remaining surface of the substrate 10 of, for example, 50 nm. In the longitudinal direction, the waveguide 11 has a length that is selected depending on the specific application of the laser device 100 and can be selected in the range from 5 µm to 5 mm. If the laser device 100 is designed for electrical excitation of the photonic crystal, the substrate 10 and in particular the waveguide 11 comprises doped silicon. The silicon is, for example, doped with boron at a concentration of 10 18 cm-3 doped. If the laser device 100 is designed for optical excitation of the photonic crystal, undoped silicon is preferably used.

[0041] The waveguide 11 has two reflectors 14, 15 that span the linear resonator 12. The linear resonator 12 has a length along the optical axis OA of, for example, 10.2 µm, which corresponds to a resonance wavelength of, for example, 1.3 µm. The length of the resonator is thus almost 1000 times shorter than in

[13] and

[14] , which advantageously improves the integrability of the laser device 100 into an optoelectronic integrated circuit (optoelectronic chip).

[0042] The reflectors 14, 15 have different reflectivities. The first reflector 14, as the resonator end mirror, has a reflectivity of nearly or equal to 100%, e.g., at least 99.9%, while the second reflector 15, as the resonator output mirror, has a lower reflectivity in the range of 95% to 98%. The reflectors 14, 15 are Bragg reflectors with Bragg structures in the material of the waveguide 11 (see Fig. 7). Alternatively, at least one of the reflectors 14, 15 may be a dielectric mirror on an end face of the waveguide 11.

[0043] The photonic crystal 20 comprises a straight row 22 of, for example, 10 semiconductor elements 21, such as cylindrical nanopillars 21, arranged on the upper surface of the waveguide 11 with a period of, for example, 750 nm. This period is optimized for a coupling wavelength of, for example, 1.3 µm, which is matched to the aforementioned resonance wavelength. The nanopillars 21 are formed, for example, by epitaxial growth on the waveguide 11. Advantageously, each nanopillar 21 contains, for example, a heterostructure with at least one quantum well, wire, or dot, which is made, for example, of compound semiconductors with different dopings. The totality of the nanopillars 21 of the photonic crystal 20 forms the gain medium and thus, together with the linear resonator 12, the overall resonator of the laser device 100 according to the invention. Upon electrical or optical excitation of the photonic crystal 20 (see Fig. 3, Fig. 4) an emission and resonant amplification of light takes place and a coupling out into the remaining waveguide 11 or into an adjacent free space through the second reflector 15 (see arrow).

[0044] Alternatively, the laser device 100 according to the invention in the second embodiment can Fig. 2 in a schematic plan view. The ring resonator 13 comprises a circular waveguide 11 with a rectangular cross-sectional area relative to the surface of the substrate 10. On the upper surface of the waveguide 11, which has a diameter of, for example, 4 µm, the photonic crystal 20 is arranged in the form of a circular row of, for example, 10 semiconductor elements 21, such as, for example, cylindrical nanopillars 21. Upon electrical or optical excitation of the photonic crystal 20, an emission and resonant amplification of light occurs, which is guided along the circular optical path OP in the ring resonator 13 (symbolized by a double arrow) and coupled out at a coupling section 30. The coupling section 30 contains a waveguide 31, which is arranged at a distance from the waveguide 11.

[0045] To couple light between the closed ring resonator 13 and the waveguide 31, it is sufficient to select the appropriate lateral distance between the two. Alternatively, through suitable semiconductor processing, the coupling section 30 can be configured with a material bridge between the ring resonator 13 and the waveguide 31.

[0046] The fabrication and characterization of the laser device 100 is described below with reference to the first embodiment (using a linear resonator). The fabrication and characterization of the second embodiment of the laser device 100 (using a ring resonator) is carried out in a corresponding manner with an adaptation of the waveguide shape.

[0047] The Fig. 3 and Fig. 4 show various stages of the manufacture of the laser device 100 according to the invention, e.g., with the following properties. For laser emission at a wavelength of λ = 1.3 µm, the waveguide 11 is made of silicon (optional doping: 10 18 cm -3 ), and the semiconductor elements 21 are nanowires made of group III arsenides and / or phosphides. For example, with a GaAs-(In,Ga)As-GaAs heterostructure, a pn profile is formed in the semiconductor elements 21, preferably as a core-shell structure or alternatively in the longitudinal direction of the semiconductor elements 21, which allows electrical excitation of the photonic crystal and confinement of the charge carriers in the active sections of the semiconductor elements 21.

[0048] In a first step, the substrate 10 with the optical waveguide 11 is provided. The substrate 10 has a layer structure consisting of a silicon carrier substrate (not shown), a silicon dioxide carrier layer 16, and a silicon cap layer 17 on which the waveguide 11 is formed. The silicon carrier substrate represents the mechanical support of the laser device 100, which can advantageously also contain electronic components. The thicknesses of the silicon dioxide carrier layer 16 and the silicon cap layer 17 are, for example, 2 µm and 220 nm. The substrate 10 is manufactured using conventional silicon processing methods, such as ion implantation or wafer bonding. The reflectors 14, 15 are formed on the waveguide 11. Trenches are introduced into the waveguide 11, for example, by etching, which form Bragg structures. The number of trenches (number of periods) determines the reflectivity of the reflectors 14, 15.For example, the first reflector 14 (with a reflectivity of nearly 100%) comprises 100 trenches with a pitch of 230 nm, while the second reflector 15 (with a reflectivity of, for example, 95%) comprises 40 trenches with the same pitch.

[0049] To form the photonic crystal 20, the semiconductor elements 21 are deposited by epitaxial growth on the surface of the waveguide 11. For this purpose, a SiO2 masking layer (not shown) is formed on the upper surface of the waveguide 11. The masking layer contains holes where the waveguide is exposed and the growth of the semiconductor elements 21 takes place.

[0050] Furthermore, a first contact electrode 41 is deposited on the surface of the substrate 10, in particular on the silicon cover layer 17 next to the waveguide 11. The contact electrode 41 comprises, for example, a layer of gold or aluminum for coupling via an electrical line to a pump current source (power supply device) 43 (see Fig. 4).

[0051] The photonic crystal 20 is then Fig. 4 embedded in an embedding layer 23, which is preferably made of an organic polymer, for example benzocyclobutene (BCB) by spin coating.

[0052] The embedding layer 23 is removed until the upper ends (head ends) of the semiconductor elements 21 are exposed.

[0053] Finally, a metal layer, for example, made of gold, is formed as a second contact electrode 42 on the surface of the embedding layer 23 and in electrical contact with the semiconductor elements 21. The second contact electrode 42 fulfills a dual function with respect to electrical contact (connection to the pump current source 43 acting as a pumping device) and as a third reflector, which delimits the photonic crystal 20 in a direction perpendicular to the surface of the substrate 10. The second contact electrode 42 has a thickness of 100 nm, for example.

[0054] The laser device 100 according to the Fig. 3 and Fig. 4 is designed for electrical excitation of a laser emission by injecting a pump current from the pump current source 43 via the first and second contact electrodes 41, 42 into the photonic crystal 20. However, the implementation of the invention is not limited to electrical excitation of the photonic crystal 20, but is optionally possible with optical excitation of the photonic crystal 20. In this case, the first and second contact electrodes 41, 42 are not provided, and the substrate can be made of non-doped silicon. The optical excitation of the photonic crystal 20 is preferably carried out with a pump light source (not shown, e.g., a laser diode) by irradiation through the embedding layer 23.

[0055] Fig. Figure 5 shows a side view of the first embodiment of the laser device 100 according to the invention with the waveguide 11 (without the remaining substrate) and the photonic crystal 20. The linear resonator 12 is formed by the reflectors 14, 15 and the section of the waveguide 11 located between them. The semiconductor elements 21 of the photonic crystal 20 are formed vertically on the surface of the waveguide 11 and embedded in the embedding layer 23. On top of the embedding layer 23, the second contact electrode 42 is in electrical contact with the tip ends of the semiconductor elements 21.

[0056] The two-dimensional numerical simulation of the quality factor (Q-factor) of the linear resonator 12 using commercially available FDTD software (“Finite-Difference Time-Domain Software”) results in Fig. 6 shows the dependence of the Q-factor on the period P of the photonic crystal 20. The Q-factor is a dimensionless quantity that indicates after how many optical periods the energy within the linear resonator 12 drops by the factor exp(-2π).

[0057] The simulation result of the Fig. Figure 6 is obtained with a photonic crystal 20 composed of 10 GaAs / (In,Ga)As nanowires 21 with a pn profile in core-cladding geometry and two ideal reflectors 14, 15 (reflectivity for both 100%) in a Si waveguide with a height of 220 nm and a length of 10.2 µm, with a nanowire height of 3 µm, a nanowire diameter of 260 nm, and a thickness of the (In,Ga)As shell of 20 nm. With a period of P = 0.75 µm, a Q factor above 4000 results.

[0058] After determining the length of the resonator and selecting the period of the photonic crystal 20 based on the maximum Q-factor, the dimensioning of the reflectors 14, 15, in particular the Bragg structures in the waveguide 11, is carried out according to the Fig. 7 and Fig. 8.

[0059] In general, the reflectivities of the resonator end mirror 14 and the resonator output mirror 15 are preferably adjusted by selecting the period number and the modulation depth of the Bragg structure. These parameters of the resonator end mirror 14 are selected, for example, based on tests or simulations using the bidirectional mode propagation (BEP) method, such that a reflectivity of almost 1 is achieved. The period number and the modulation depth of the output mirror 15 are selected accordingly and further dependent on the Q factor of the linear resonator 12 such that a predetermined portion of the light circulating in the linear resonator 12 is output by the resonator end mirror 15.

[0060] Fig. Figure 7 illustrates the Bragg structure of one of the reflectors 14, 15 in a schematic perspective view. The Bragg structure comprises a periodic arrangement of grooves 18 in the waveguide 11 on the substrate 10. The grooves 18 extend perpendicular to the longitudinal extent (optical axis OA) of the waveguide 11. The spectral reflectivity of the Bragg structure is determined by the period A (distance between the grooves), the period number N (number of grooves), and the modulation depth (depth d of the grooves 18).

[0061] For the above example of silicon as the waveguide material and a wavelength of λ = 1.3 µm, a period A = 227 nm is chosen to satisfy the Bragg condition. The depth d of the trenches 18 can extend over part of the height of the waveguide 11 relative to the surface of the substrate 10 or over the entire height of the waveguide 11. In the above example, the depth d of the trenches 18 is 50 nm.

[0062] By varying the period number N, the reflectivity and transmission of the Bragg structure can be adjusted. Fig. The simulation result obtained using the BEP method, illustrated in Figure 8, shows how the reflectivity R increases with increasing period number N, while the transmission T decreases with increasing period number N. For a Bragg structure with more than 50 grooves 18, a reflectivity of almost 1 is achieved.

[0063] For the simulation of the laser device 100 according to Fig. 9, an ideal resonator end mirror 14 (reflectivity close to 1) and a partially transmissive resonator output mirror 15 with a transmission T = 0.35, based on a Bragg structure with 15 periods, are assumed. Light is generated in the semiconductor elements 21 by electrical excitation and, in cooperation with the linear resonator 12, is resonantly fed back and amplified in the semiconductor elements 21, whereby the light becomes coherent. The calculation of the light output power P emerging through the output mirror 15 out depending on the gain g in the (In,Ga)As-based semiconductor elements 21 results in Fig. Function shown in 10.

[0064] To achieve laser emission from the linear resonator 12, the optical gain of the active material (photonic crystal 20) must be greater than the resonator losses. To achieve this, charge carriers are injected into the semiconductor elements 21 by electrical excitation. For gain values ​​above 3600 cm -1 An exponential increase in output power is achieved. To achieve this threshold with the lowest possible pump currents (excitation currents), the resonator losses are kept as low as possible, in particular by designing the output mirror 15 for a lower transmission, preferably less than 5%.

[0065] The features of the invention disclosed in the above description, the drawings and the claims may be important both individually and in combination or sub-combination for the realization of the invention in its various forms.

Claims

[1] Laser device (100) comprising: - a substrate (10) on the surface of which an optical waveguide (11) is arranged, which contains an optical resonator (12, 13) with a resonator length such that at least one resonator mode in the resonator (12, 13) forms a standing wave, and - a gain medium arranged on a surface of the optical waveguide (11) and comprising a plurality of semiconductor elements (21) arranged periodically on the surface of the optical waveguide (11) in a manner projecting from the optical waveguide (11), wherein: - the gain medium is arranged for optical interaction with the at least one resonator mode of the optical resonator (12, 13) and for amplifying light having a wavelength of the at least one resonator mode of the optical resonator (12, 13), - the gain medium comprises a photonic crystal (20) and the semiconductor elements (21) are columnar and / or wall-shaped semiconductor elements (21), and - a diffractive coupling is formed between the photonic crystal (20) and the optical resonator (12, 13), in which standing light waves in the optical resonator (12, 13) are extended to the photonic crystal (20), characterized by , that - a periodicity of the arrangement of the semiconductor elements (21) defines a coupling wavelength which is coupled between the semiconductor elements (21) and the waveguide (11), wherein the coupling wavelength and a resonance wavelength defined by the resonator length of the optical resonator (12, 13) are simultaneously selected such that the resonance wavelength in the waveguide (11) matches the coupling wavelength. [2] Laser device (100) according to claim 1, wherein - the photonic crystal (20) is arranged on a surface of the optical waveguide (11) which runs parallel to the surface of the substrate (10), and the semiconductor elements (21) are aligned perpendicular to the surface of the substrate (10). [3] Laser device (100) according to one of the preceding claims, in which - the semiconductor elements (21) are arranged periodically along the resonator (12, 13) at positions of field maxima of the at least one resonator mode. [4] Laser device (100) according to one of the preceding claims, in which - the semiconductor elements (21) form a single row (22) extending along the resonator (12). [5] Laser device (100) according to one of claims 1 to 3, wherein - the semiconductor elements (21) form at least two rows extending along the resonator (12, 13). [6] Laser device (100) according to one of the preceding claims, in which - the resonator is a linear resonator (12) with two reflectors (14, 15) on the optical waveguide (11). [7] Laser device (100) according to claim 6, wherein - the reflectors (14, 15) comprise periodic structures of the waveguide (11) which form Bragg reflectors for the at least one resonator mode. [8] Laser device (100) according to one of claims 1 to 5, in which - the resonator is a closed resonator (13), and - a coupling section (30) is provided for coupling out light from the closed resonator (13). [9] Laser device (100) according to claim 8, wherein - the closed resonator (13) is a ring resonator with a circular shape or a racetrack resonator with a shape composed of straight and curved sections. [10] Laser device (100) according to one of the preceding claims, wherein - the substrate (10) carries a contact electrode (41) adjacent to the photonic crystal (20). [11] Laser device (100) according to one of the preceding claims, wherein - the semiconductor elements (21) of the photonic crystal (20) are arranged in a dielectric embedding layer (23). [12] Laser device (100) according to claim 11, wherein - the embedding layer (23) carries an electrical contact layer (42), in particular an opaque contact layer (42). [13] Laser device (100) according to one of the preceding claims, wherein - the optical waveguide (11) comprises silicon, and - the substrate (10) comprises an SOI substrate with a silicon carrier substrate, a silicon dioxide layer (16) and a silicon cover layer (17) containing the optical waveguide (11). [14] Method for operating a laser device (100) according to one of the preceding claims, comprising the steps - coupling the laser device (100) to a pumping device, - excitation of the photonic crystal (20) with the pumping device, and - Emission of laser light from the photonic crystal (20). [15] Method according to claim 14, wherein - the pumping device comprises a pumping current source (43) with which the photonic crystal (20) is electrically excited. [16] Method according to claim 14, wherein - the pumping device comprises a pumping light source with which the photonic crystal (20) is optically excited. [17] A method for manufacturing a laser device (100) according to any one of claims 1 to 13, comprising the steps - providing the substrate (10) with the optical waveguide (11), and - growing the semiconductor elements (21) of the photonic crystal (20) on the optical waveguide (11). [18] Method according to claim 17, comprising the step - Contacting the photonic crystal (20) with contact electrodes (41, 42). [19] A method according to claim 17 or 18, wherein - the growth of the semiconductor elements (21) comprises a direct epitaxial or a gas-liquid-solid-based growth of the semiconductor elements (21) on the optical waveguide (11) and / or a mask-based deposition of the semiconductor elements (21) on the optical waveguide (11). [20] Method according to one of claims 17 to 19, in which - the semiconductor elements (21) are embedded in a transparent embedding layer (23).

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