Narrowband THZ radiation source, emitter therefor and method for producing an emitter

Description

[0001] Some time ago, T. Kampfrath demonstrated a so-called spintronic emitter for THz radiation, in which a THz pulse is generated from a metallic multilayer by irradiation with a femtosecond laser from either the front or back (e.g., Nature Nanotechnology, volume 8, pages 256–260, 2013). The pulse is broadband and non-directional. Since then, various modifications of this basic structure have become known. Among others, CN 110 441 929 A discloses a THz emitter with a substrate and a ferromagnetic layer deposited over the substrate. In this layer, the lattice structure allows the emitter to be rotated around a surface normal, controlling the angle between a lattice orientation and the polarization of the incident laser beam, and thus the intensity of the THz emission.To influence the spectrum of the THz radiation, it is recommended to vary the materials and thickness of the ferromagnetic layer. However, no information is provided that would allow for the targeted optimization of an emitter for a specific wavelength without extensive experimentation, nor is there any apparent reason why the THz radiation, which apparently propagates in the same direction as the laser beam, should be narrower-banded than that of the setup described by Kampfrath. No requirements are specified for the surface properties of the substrate on which the grating is structured.

[0002] One object of the invention is to provide a THz radiation source that is actually capable of providing THz radiation with a bandwidth that is significantly smaller than that of a conventional spintronic emitter with a homogeneous ferromagnetic layer. Further objects are to provide an emitter suitable for such a narrowband radiation source and a method for its fabrication.

[0003] The problems are solved, firstly, by a THz radiation source according to claim 8, in which the THz radiation source comprises an emitter comprising at least one ferromagnetic layer structured as a grating and a laser source for directing short laser pulses onto the emitter in an incident direction, characterized by an aperture that defines a beam path emanating from the emitter in an output direction for THz radiation whose wavelength is smaller than a grating constant of the grating, and wherein the output direction is oriented neither in the incident direction nor in an outgoing direction of laser pulses reflected at the emitter.

[0004] The elements of the grating are excited by the laser pulses with a time and phase shift determined by their positions relative to the direction of incidence, and accordingly generate the THz radiation with a corresponding shift. Each element can be considered a radiation source that emits the THz radiation in a known broadband and omnidirectional manner. For spectral components whose wavelength is greater than the grating constant, i.e., the distance between adjacent elements of the grating, the grating behaves like an extended, featureless source; these spectral components are emitted omnidirectionally by the grating.For spectral components with wavelengths smaller than the grating constant, the time and phase shift with which the grating elements are excited means that the wavefronts emanating from the different elements only constructively superimpose in certain discrete output directions, canceling each other out in between. Therefore, to selectively obtain narrowband THz radiation of a desired wavelength, it suffices to position the aperture in the output direction of the desired wavelength.

[0005] The problems are further solved by an emitter for a THz radiation source according to claim 1.

[0006] At least the first flanks should be level.

[0007] Such an emitter is easy to manufacture by directing a metal-containing material stream onto the surface under such a grazing incidence that every second flank lies in the flow shadow of a first flank, so that metal is deposited only on the first flanks.

[0008] The material stream can contain a gaseous metal-containing compound that deposits metal on the surface by decomposition; in the simplest case, it contains the metal to be deposited as vapor; preferably, it is obtained by cathode sputtering of a metallic target, also known as magnetron sputtering, or by ion beam deposition.

[0009] The grazing incidence simplifies the realization of an emitter, where the first flanks are free of the band in a valley-side portion and covered by the band in a portion farther from the valley. The more grazing the flow, the narrower the bands are formed.

[0010] To create a surface with a wavy cross-section, a substrate made of a semiconductor single crystal is particularly suitable. In such a crystal, the first and second flanks can be formed directly by anisotropic etching. The semiconductor is typically silicon.

[0011] With known anisotropic etching techniques, a

[100] crystal plane is particularly susceptible to etching, while

[111] planes are resistant to etching. Therefore, it is preferred that the mean surface normal corresponds to a

[100] axis of the single crystal and / or that the first flanks are

[111] planes of the single crystal.

[0012] Instead of depositing the metal strips directly onto the etched surface of the semiconductor single crystal, a substrate made of another material, especially plastic, can also be used to create a mold, and the metal strips can then be deposited onto this substrate. This allows the emitter to be manufactured cost-effectively, especially in large quantities.

[0013] If the substrate is flexible, it can be shaped into a cylindrical surface to focus the emitted radiation and to filter the emitted radiation spectrally by appropriately positioning the aperture.

[0014] If the molded first and second flanks of the substrate are, as mentioned above,

[111] planes of the single crystal, then this results in an angle between the flanks of 110-115°.

[0015] To generate THz radiation with high efficiency, it is preferred that one of the incidence and output directions is aligned in the direction of the mean surface normal and / or the other in the direction of a normal of the first flanks.

[0016] To generate narrowband THz radiation with different wavelengths while maintaining a fixed incidence and output direction, the emitter can have zones with different grating constants that can be selectively positioned within the laser source beam. These can be discrete zones whose grating constants change stepwise from one zone to the next; however, a grating in which the spacing between the strips is not constant but changes slowly enough is particularly interesting. This ensures that, on the one hand, the desired diffraction of the THz radiation occurs, and on the other hand, the wavelength of the radiation striking the aperture can be continuously tuned by moving the emitter.

[0017] The invention further relates to a method for manufacturing an emitter according to claim 12.

[0018] Preparatory steps can be taken to prepare the substrate. Forming a lattice of strips of an etch-resistant material on an original surface of a semiconductor crystal, anisotropic etching of the semiconductor crystal to form an etched surface having trenches between the strips, each with a first and a second flank, wherein the surface normals of the first and second flanks are inclined in opposite directions relative to the surface normal of the original surface are carried out.

[0019] The surface exposed to the metal-containing material flow can be the etched surface itself or a surface obtained by molding the etched surface.

[0020] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures. These show: Fig. 1 a silicon crystal with a surface structured for the production of an emitter; Fig. 2 the silicon crystal after anisotropic etching; Fig. 3 the silicon crystal during the deposition of a lattice according to a first embodiment of the invention; Fig. 4 the silicon crystal during molding according to a second embodiment of the invention; Fig. 5 a substrate obtained during molding during the deposition of a lattice; Fig. 6 a THz radiation source with an emitter according to the invention; and Fig. 7 a modification of the THz radiation source according to the invention.

[0021] Fig. 1 Figure 1 shows a schematic perspective view of a section of a single-crystal silicon wafer 1 or another anisotropically etchable crystalline material. Strips 3 of an etch-resistant material, in particular SiN, are formed on a

[100] surface 2 of the wafer 1. The strips 3 are oriented in the

[011] direction. The spacing d of the strips 3 is slightly larger than the wavelength of THz radiation, which is to be generated in a narrowband manner by an emitter to be produced from the wafer 1. In practice, it is on the order of several hundred micrometers; it can, for example, be 300 micrometers.

[0022] Wafer 1 is subjected to anisotropic etching. The technique of anisotropic etching, e.g., with KOH, is widespread in the field of semiconductor device manufacturing and is therefore not described in detail. The

[100] surface 2 is attacked much faster during anisotropic etching than the

[111] surfaces, which gradually penetrate into the interior of wafer 1 along the edges of the strips 3 as the etching front advances into the wafer 1. The etching process practically comes to a standstill when

[111] surfaces, which are essentially resistant to etching and originate from mutually facing edges of two adjacent strips 3, meet at the bottom of a trench formed between the strips 3.

[0023] Fig. 2 Figure 1 shows the wafer 1 after etching. Trenches 4 are formed between the etch-resistant strips 3, the sides 5, 6 of which are each formed by

[111] surfaces. The sides 5, 6 extend at an angle θ = 54.7° to the remnants of the

[100] surface 2 preserved beneath the etch-resistant strips 3.

[0024] The corrosion-resistant strips 3 are removed.

[0025] According to a first embodiment of the invention, the etched wafer 1 is used directly as a substrate 7 for a THz emitter by forming a metal thin-film structure 8 on the wafer 1. A wafer 1 used for this purpose preferably consists of high-purity silicon with low intrinsic conductivity. If desired, an electrically insulating layer, e.g., of SiO₂, can be produced on the surface of the wafer 1. As shown in Fig. 3 A stream 9 of metal vapor or a gaseous metal compound decomposing at the wafer surface is shown being passed at grazing incidence over the surface of the wafer 1. The angle α between the surface 2 and the stream 9 is at most as large as the angle θ, so that of the two flanks 5, 6 of each trench 4, one flank 6 lies completely in the flow shadow and remains uncoated. A portion 11 of flank 5 adjacent to a bottom 10 of the trench 4 also lies in the flow shadow and remains uncoated. In a portion 12 of flank 5 located away from the bottom 10 and in an adjacent portion of the original surface 2 that remains unetched beneath the etch-resistant layer 3, a metal strip 13 is formed. The metal strips 13 together form a lattice whose lattice constant d corresponds to the distance between the etch-resistant strips 3.

[0026] The formation of the stripes 13 of the line lattice proceeds in at least two steps. In a first step, a layer 14 of metal with strong spin-orbit coupling, such as Pt, Ta, or W, is deposited in each stripe 13; in a second step, a ferromagnetic layer 15 is produced. If desired, further pairs of layers of the same composition can be deposited over the layers 14 and 15. According to a preferred embodiment, a single ferromagnetic layer is arranged between layers of two different metals, each with opposite signs of the spin Hall effect.

[0027] According to a second embodiment, the etched wafer 1 serves as a matrix on which a substrate 16 made of plastic is molded, as in Fig. 4 shown. Subsequently, the substrate 16 thus obtained is subjected to a metal deposition as above with reference to Fig. 3 described and subjected to. As in Fig. 5 shown here, unlike in Fig. 3 The ridges 17 between two grooves 4 are sharp-edged, while the base 10, complementary to the unetched remnants of the original wafer surface 2, can have a non-zero width. Therefore, the metal strips 13 are limited to the flanks 5 and are flat, assuming the same angle of incidence α of the current 7. If the angle of incidence is chosen to be larger than shown in the figure, the metal strips 13 can fill the entire flank 5 and, if necessary, a part of the base 10.

[0028] Fig. 6 Figure 1 is a schematic representation of a THz radiation source with an emitter 18 constructed as described above. The radiation source comprises a femtosecond laser 19 whose pulsed beam 20 is directed along an incidence direction 26 onto the surface of the emitter 18. The femtosecond laser 19 illuminates a spot on the surface of the substrate whose diameter is a multiple of the lattice constant d, i.e., each laser pulse 21 strikes a multitude of fringes 13 of the lattice. The influence of the magnetic field of the ferromagnetic layer 15 enables the laser pulse 21 to induce a current pulse in the layer 14, which in turn leads to the broadband emission of electromagnetic radiation 22 in the THz range. The fringes 13 can each be considered (in cross-section) as point sources, each emitting in an undirected manner.Since the excitation by the laser pulse 21 occurs either simultaneously (when the laser beam 20 is incident in the direction of a mean surface normal 23 of the emitter 18 (i.e., perpendicular to the original surface 2) or with a time offset proportional to the distance between the stripes 13 (when the laser beam 20 is not incident perpendicularly), the emission by the stripes 13 occurs with a fixed phase ratio, which allows interference. Therefore, to obtain narrowband THz radiation at a desired wavelength λ, it suffices to place an aperture 24 at an angle ϕ to the surface normal 23, which satisfies the condition for constructive interference between the emissions of the individual stripes. nλ = d sinφ − sin φ i nλ=d(sin ϕ - sin ϕi) is satisfied, where ϕi is the angle of incidence of the laser beam 20 relative to the surface normal 23 (in Fig. 6 is ϕ i =0) and n is an integer and preferably 1 or -1.

[0029] The two diffraction orders n=1 and n=-1 differ in which side of the incident laser beam 20 the output direction of the THz radiation is deflected. The preferred, because more intense, of the two diffraction orders is the one whose output direction 25 deviates less from the surface normal of the strips 13. That is, the aperture 24 is positioned such that, along the output direction 25 passing through it, one looks through it at a smaller angle onto the metal-coated flanks 5 than onto the uncoated flanks 6. Ideally, the output direction 25 can coincide with the direction 28 of the current 9 used for coating; this is also recognizable after completion of the metal thin-film structure by the fact that mutually facing edges 29 (see Fig. 5 ) of two adjacent stripes 13 lie on a straight line extending in direction 28. In this case, the stripes 13 completely fill the part of the surface of the emitter 18 visible along the output direction 25, and the intensity of the radiation emitted in this direction 28 reaches a maximum.

[0030] A maximum of emission can also be achieved if the output direction coincides with a surface normal 27 of the flanks 5, because the emission intensity of a surface radiator is proportional to the sine of the angle between the emission direction and the surface normal and thus reaches a maximum in the direction of the surface normal.

[0031] To achieve the highest possible efficiency in generating THz radiation, the following combinations of incidence and output direction are therefore considered: 1. Incident direction along the mean surface normal 23 of the emitter (the

[100] direction), output direction along the surface normal 27 of the flanks 5; 2. Incident direction along the mean surface normal 23 of the emitter 18, output direction equal to the direction of the current 9; 3. Incident direction along the surface normal 27 of the flanks 5, output direction equal to the direction of the current 9; 4. Incident direction along the surface normal 27 of the flanks 5, output direction along the mean surface normal 23 of the emitter 18; 5. Incident direction equal to the direction of the current 9, output direction along the mean surface normal 23 of the emitter 18; 6. Incident direction equal to the direction of the current 9, output direction along the surface normal of the flanks 5.

[0032] In order to avoid altering the beam path of the laser and THz radiation in the radiation source of the Fig. 6 To enable the coupling of radiation of different wavelengths via aperture 24, emitters 18 with different grating constants d can be used. In the case of the Fig. 6 Three grids with different grid constants d, d', d" are realized on the substrate 16. These grids can be selectively positioned in the laser beam 20 by shifting them perpendicular to the surface normal and transversely to the stripes. Despite the different grid constants, the various grids can be produced in a single etching process because the depth growth of the individual trenches 4 of the different grids essentially ends as soon as the

[100] surface has disappeared at its base. Therefore, the etching process can be continued until even the widest trenches are complete.

[0033] According to one modification, the distances between the stripes can continuously increase or decrease across the width of the emitter 18. Provided that the change is slow enough to allow diffraction to be observed in the section of the emitter 18 illuminated by the laser beam 20, according to the formula above, such an emitter can be used to continuously tune the THz radiation exiting at the aperture 24.

[0034] The same effect can be achieved if the trenches 4 are not spaced equally along their entire length, but are arranged in a fan-like pattern, in which the trenches are divided into sections that each run parallel to an adjacent section of another trench, but where the section between adjacent sections of different trenches varies in the longitudinal direction of the trenches; in this case, the lattice constant in the part of the emitter 18 illuminated by the laser beam 20 changes when it is moved in the longitudinal direction of the trenches 4.

[0035] Both the silicon substrate 7 and the plastic substrate 16 are transparent to THz radiation. The diffraction condition can therefore be met both by THz radiation reflected from the side of the strips 13 facing the laser beam 20 and by THz radiation that propagates through the substrate and is emitted from the side facing away from the laser 16, as shown in Fig. 6 indicated by dashed lines.

[0036] If the substrate is transparent to the laser beam 20, an inverted arrangement of the emitter 18, with the grating on its side facing away from the laser 16, can also be considered.

[0037] Fig. 7 shows a modification of the THz radiation source of Fig. 6 In this variation, this is done by molding according to Fig. 4 The resulting substrate 16 is bent into a cylindrical surface whose axis of curvature runs in the longitudinal direction of the strips 13. As a result, the emitted THz radiation passes through a focal point 30 along the output direction 25, and by placing the aperture 24 around the focal point 30, a particularly narrowband portion of the THz emission can be filtered out. Bezugszeichen

[0038] 1 Wafer 2

[100] -surface 3 Strip 4 Trench 5 Flank 6 Flank 7 Substrate 8 Metal thin-film structure 9 Current 10 Base 11 Part of flank 5 12 Part of flank 5 13 Metal strip 14 Layer 15 Layer 16 Substrate 17 Burr 18 Emitter 19 (Femtosecond) laser 20 Beam 21 Laser pulse 22 THz radiation 23 Surface normal 24 Aperture 25 Output direction 26 Incidence direction 27 Surface normal 28 Current direction 9 29 Edge of a strip 13 30 Focal point

Claims

1. An emitter (18) for a THz radiation source, comprising a substrate (7, 16) and a metal thin-film structure (8) structured as a grating, which comprises a plurality of strips (13) oriented in a longitudinal direction on a surface of the substrate (7, 16) and containing a ferromagnetic layer (15), characterised in that the surface has a wave-like profile in a section perpendicular to the longitudinal direction, with alternating first and second flanks (5, 6) that are inclined in opposite directions with respect to a mean surface normal (23) of the surface, and in that the strips (13) are each located only on the first flanks (5).

2. The emitter of claim 1, characterised in that at least the first flanks (5) are flat.

3. The emitter of claim 1 or 2, characterised respective first and second flanks (5, 6) form a trench (4) in the substrate (7, 16), and the first flanks (5) are free of the strip (13) in a part (11) adjacent to a sole (10) of the trench (4) and are covered by the strip (13) in a part (12) remote from the sole (10).

4. The emitter of one of claims 1 to 3, characterised in that the substrate (7) is a semiconductor monocrystal.

5. The emitter of claim 4, characterised in that the mean surface normal (23) corresponds to a [100] axis of the monocrystal and / or the first flanks (5, 6) are [111] planes of the single crystal.

6. The emitter of one of claims 1 to 3, characterised in that the substrate (16) is a preferably flexible moulding made of plastic.

7. The emitter of claim 6, characterised in that the first and second flanks (5, 6) form an angle of 110-115°.

8. A THz radiation source having an emitter (18) according to one of claims 1 to 6 and a laser source (19) for directing short laser pulses (21) onto the emitter (18) in an incidence direction (26), characterised by a diaphragm (24) which defines a beam path for THz radiation (22) emanating from the emitter (18) in an output direction (25) for THz radiation (22), the wavelength of which is shorter than a grating constant (d) of the grating, and wherein the output direction (25) is oriented neither in the direction of incidence (26) nor in a direction of emergence of laser pulses reflected at the emitter.

9. The THz radiation source of claim 8, characterised in that the direction of incidence (26) and the output direction (25) correspond to two different ones of the following directions: a mean surface normal (23) of the emitter (18), a surface normal (27) of the first flanks (5), a direction (28) in which facing edges (29) of adjacent strips (13) lie on one line.

10. The THz radiation source of one of claims 8-9, characterised in that the emitter (18) is cylindrically curved.

11. The THz radiation source of one of claims 8-9, characterised in that the emitter (18) has zones with different grating constants, which can be placed selectively in the beam (20) of the laser source (19).

12. A method for producing an emitter (18) for a THz radiation source, comprising the steps of: - providing a substrate (7, 16) which has a surface with, in a section, a wave-shaped profile with alternating first and second flanks (5, 6) which are inclined in opposite directions with respect to a mean surface normal (23) of the surface, - directing a metal-containing material stream (9) onto the surface, at a grazing angle of incidence, such that every second flank (6) lies in the flow shadow of a first flank (5), in order to deposit a metal thin-film structure (8) structured as a grating and having strips (13) which are oriented in a longitudinal direction and contain a ferromagnetic layer (15) by depositing metal on the first flanks (5) only.

13. The Method of claim 12, further comprising the preparatory steps of - forming a grating of strips (3) of an etch-resistant material on an initial surface (2) of a semiconductor crystal (1), - anisotropically etching the semiconductor crystal (1) to form an etched surface having trenches (4) between the strips (3), each trench having a first and a second flank (5, 6), wherein the surface normals of the first and second flanks (5, 6) are inclined in opposite directions relative to the surface normal (23) of the original surface (2).

14. A method according to claim 13, wherein the surface exposed to the metal-containing material stream (9) is the etched surface or a surface obtained by moulding the etched surface.