DEVICE FOR THz GENERATION AND/OR DETECTION AND METHODS FOR MANUFACTURING THE SAME

Abstract

A terahertz device includes a first waveguide, which is a plasmonic waveguide, having a first core with a nonlinear material, such as a ferroelectric material, and having a cladding with a first cladding portion including, at a first interface with the first core, a first cladding material that is an electrically conductive material. The terahertz device can include an antenna having a first and a second arm (for receiving or for emitting or for both, receiving and emitting electromagnetic waves in the terahertz range); a first and a second electrode arranged close to the first waveguide.

Claims

1. A terahertz device, in particular for detecting or for emitting or for both, detecting and emitting electromagnetic waves in the terahertz range, the device comprising a first waveguide which is a plasmonic waveguide comprising a first core comprising a nonlinear material, in particular a ferroelectric material; and a cladding comprising a first cladding portion comprising, at a first interface with the first core, in particular with the nonlinear material, a first cladding material which is an electrically conductive material; and the device comprising an antenna having a first and a second arm, for receiving or for emitting or for both, receiving and emitting electromagnetic waves in the terahertz range; a first and a second electrode arranged close to the first waveguide.

2. The device according to claim 1, wherein the device is a terahertz emitter for emitting electromagnetic waves in the terahertz range, and wherein the first and second electrodes are provided for picking up an electric field present in the nonlinear material.

3. The device according to claim 1, wherein the device is a terahertz detector for detecting electromagnetic waves in the terahertz range, and wherein, first and a second electrode are provided for producing an electric field in the nonlinear material when a voltage is applied between the first and second electrodes, for modulating an optical property of the nonlinear material, such as for modulating at least a real part of a refractive index of the nonlinear material.

4. The device according to claim 1, wherein the first electrode establishes the first cladding portion, in particular wherein the first cladding material is a metallic material.

5. The device according to claim 1, wherein the cladding comprises a second cladding portion separate from the first cladding portion, comprising, at a second interface with the ferroelectric material, a second cladding material which is an electrically conductive material.

6. The device according to claim 4, wherein the nonlinear material is arranged, in particular laterally arranged, between the first and second cladding portions, in particular wherein the first electrode establishes the first cladding portion and the second electrode establishes the second cladding portion.

7. The device according to claim 1, comprising, in addition, a second waveguide comprising a second core positioned in proximity to the first waveguide, for enabling coupling, in particular evanescent coupling, between the first and second waveguides, in particular wherein the second waveguide is arranged vertically between the substrate and the first waveguide.

8. The device according to claim 7, comprising a substrate comprising one or more substrate layers, wherein both, the first and the second waveguide, are located on and attached to the substrate.

9. The device according to claim 1, comprising an optical structure, in particular a diffractive optical structure, for enhancing a coupling between free-space electromagnetic waves, in particular free-space electromagnetic waves in the infrared, in the visible or in the ultraviolet range, and the first waveguide, in particular wherein the optical structure is a focusing optical structure.

10. The device according to claim 9, wherein the optical structure is provided for focusing the free-space electromagnetic waves into an end of the first waveguide.

11. The device according to claim 9, wherein the optical structure comprises a first part and a second part, for enhancing a coupling of free-space electromagnetic waves to a first end and to a second end, respectively, of the first waveguide.

12. The device according to claim 9, wherein the optical structure is an optical structure exhibiting a variation of optical properties, in particular of a refractive index, on distances below 100 micrometers, in particular on distances between 10 micrometers and 0.01 micrometers.

13. A method for manufacturing a terahertz device according to claim 1, the method comprising providing a first wafer, such as a silicon-on-insulator wafer or a single-crystalline silicon wafer; depositing on the first wafer a nonlinear material, in particular a ferroelectric material; providing a second wafer, such as a silicon-on-insulator wafer or a single-crystalline silicon wafer, optionally with an adhesion layer; bonding the first wafer to the second wafer, so as to obtain a stacked wafer, thereby burying the nonlinear material, in particular with the nonlinear material interfacing the second wafer.

14. The method according to claim 13, further comprising removing a portion of the first wafer from the stacked wafer.

15. The method according to claim 13, comprising structuring the nonlinear material comprised in the stacked wafer, in particular to produce a first core of a first waveguide.

16. The method according to claim 15, comprising producing a cladding comprising a first cladding portion comprising, at a first interface with the structured nonlinear material, a first cladding material which is an electrically conductive material, in particular comprising selectively depositing a metal.

17. The method according to claim 13, wherein the depositing of the nonlinear material comprises epitaxially growing the nonlinear material on the first wafer, in particular on a single crystalline material of the first wafer.

18. The method according to claim 13, comprising, after the bonding, producing a second core of second waveguide, in particular wherein the second core comprises material of a layer of the second wafer and producing the second core comprises structuring said layer.

19. The method according to claim 13, comprising producing on the stacked wafer a first waveguide which is a plasmonic waveguide, and depositing on the structured wafer a material to form an optical structure, in particular a diffractive optical structure, for enhancing a coupling between free-space electromagnetic waves, in particular free-space electromagnetic waves in the infrared, in the visible or in the ultraviolet range, and the first waveguide, in particular wherein the optical structure is a focusing optical structure.

20. A method for manufacturing a terahertz device according to claim 1, the method comprising providing a first wafer made of a nonlinear material, in particular of a ferroelectric material, optionally with an adhesion layer; providing a second wafer, such as a silicon-on-insulator wafer or a single-crystalline silicon wafer, optionally with an adhesion layer; bonding the first wafer to the second wafer, so as to obtain a stacked wafer.

21. The method according to claim 20, comprising structuring the nonlinear material comprised in the stacked wafer, in particular to produce a first core of a first waveguide.

22. The method according to claim 20, comprising producing a cladding comprising a first cladding portion comprising, at a first interface with the structured nonlinear material, a first cladding material which is an electrically conductive material, in particular comprising selectively depositing a metal.

23. The method according to claim 20, comprising, after the bonding, producing a second core of second waveguide, wherein the second core comprises material of a layer of the second wafer and producing the second core comprises structuring said layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Below, the invention is described in more detail by means of examples and the included drawings. In the drawings, same reference numerals refer to same or analogous elements. The figures show schematically:

[0062] FIG. 1 (a) and FIG. 1 (b) are a cross sectional view of examples of nonlinear elements. It illustrates a nonlinear element loaded terahertz wave antenna gap forming a metal-insulator-metal plasmonic slot waveguide based on ferroelectric material.

[0063] FIG. 2 illustrates a perspective view of an example of a terahertz detection and generation device including of a plasmonic waveguide and a terahertz antenna.

[0064] FIG. 3 (a) and FIG. 3 (b) show cross-sectional illustrations of possible coupling schemes between free-space electromagnetic waves and the first waveguide, i.e., the plasmonic waveguide.

[0065] FIG. 4 shows an implementation example of a terahertz detector with an integrated Mach-Zehnder interferometer.

[0066] FIG. 5 schematically illustrates a method for manufacturing a terahertz device.

DETAILED DESCRIPTION OF THE INVENTION

[0067] The described embodiments are meant as examples or for clarifying the invention and shall not limit the invention.

First Embodiments

[0068] FIGS. 1 (a) and 1 (b) show cross-sectional views illustrating examples of a nonlinear elements, more specifically the terahertz antenna arms forming the plasmonic slot waveguide for the propagation of optical signals. The cross-section is taken from the transverse plane of waveguide propagation.

[0069] FIG. 1 (a) illustrates structured nonlinear material 101 with two metal electrodes 102 forming a plasmonic slot, and providing conductive material for the terahertz antenna. A (second) photonic waveguide 103 buried in the substrate 104 provides an access waveguide for optical pump signals. Light propagating in the higher refractive index material defining the photonic waveguide 103, is evanescently coupled to the plasmonic slot waveguide. The optical energy now propagates as surface plasmon polaritons (SPP) at the metal-insulator-metal interface. The electric field of the propagating plasmonic mode is symbolized by an arrow 110. The plasmonic nature of the slot waveguide provides extreme sub-diffraction confinement of the optical field allowing an electrode spacing on the order of 100 nm. This strong confinement of the optical pump signals leads to very strong optical intensities, essential for strong nonlinear effects.

[0070] When two optical pump signals co-propagate along the nonlinear material loaded plasmonic waveguide, strong second order nonlinear interaction between the two optical pump signals result in sum and difference frequency generation. If the two carriers are non-equal, the resulting new signal from the difference frequency generation is equal to the relative frequency offset of the two optical pump signals. This offset can be set to the desired terahertz frequency. The resulting terahertz field leads to charge oscillation in the electrodes resulting in an oscillating current in the conductive electrodes and efficient radiation in the far field by the terahertz antenna. In another instance, the nonlinear third order effect may be exploited to generate a terahertz signal by means of fourwave mixing. These implementations can act as an efficient terahertz wave generation device.

[0071] On the other hand, a terahertz wave incident onto the antenna results in a voltage across the two electrodes 102, and thus the antenna arms. This voltage across the nano-scale antenna gap leads to very strong terahertz electric fields in the nonlinear material 101 with same polarization orientation as an optical probe signal propagating along the plasmonic waveguide 110. The almost perfect overlap between the optical and the terahertz electric fields, and the strong field enhancement provided by the nano-scale slot, result in a very strong and efficient nonlinear interaction between the optical and terahertz signals in the nonlinear material for terahertz wave detection. This implementation provides an efficient way to map the information contained in the terahertz on an optical carrier, providing an efficient terahertz wave detection.

[0072] FIG. 1 (b) illustrates a vertical metal-insulator-metal waveguide. A bottom conductive electrode 106 such as a metal, a semimetal or transparent conductive oxide (TCO) is deposited on a substrate 103. The nonlinear material 101 on top of the first electrode can be structured, e.g., nanorods, or simply a thin fil. In case of a structured nonlinear material 101, an insulating layer 105 can be present. The top electrode 102 is acting as a top cladding. In such a vertical MIM waveguide, coupled/in light would propagate as SPP confined between the top and bottom electrodes. Choosing a dielectric cladding 105 with a smaller refractive index as the nonlinear material 101, gives additionally a horizontal confinement. The electric field of the SPP are polarized in the vertical direction as shown by the arrow 110. Such a structure would provide a very strong sub-wavelength confinement of optical signals, resulting in strong nonlinear interactions in the nonlinear material.

Second Embodiment

[0073] FIG. 2 illustrates a perspective view of an example of a terahertz detection and generation device including a plasmonic waveguide and a terahertz antenna. The plasmonic waveguide is a metal-insulator-metal (MIM) slot waveguide formed by the antenna arms 201 and the nonlinear material as the core 202, present on a substrate 203. The metal can be either a metal or a semimetal. The antenna includes two antenna arms 201 in form of four-clover-leave shape. Many other possible antenna shapes would work similarly, e.g., Bow-Tie and Yagi-Uda antennas. The core material 202 is, e.g., a second order nonlinear ferroelectric material, polymer or crystal. The nonlinear material could also rely on a third order nonlinear effect. Excited surface plasmon polaritons (SPP) propagating along the MIM slot waveguide are strongly confined inside of the plasmonic waveguide, leading to very high optical intensities in the nonlinear material. Two propagating SPP in the plasmonic slot result in generation of sum- and difference-frequency signals. Of interest for terahertz generation is mainly the difference-frequency generation (DFG) of two SPP with optical frequency SPP. The resulting terahertz field is efficiently radiated in the far field by the terahertz antenna. This implementation would act as an efficient terahertz wave generation device.

[0074] A terahertz wave incident on the antenna induces an oscillating current in the conductive antenna arms. Electrical charges accumulate at the antenna gap, resulting in a voltage across the slot. The voltage across the nano-scale slot results in a very strong terahertz electric field in the nonlinear material 202. In addition, the resonant nature of the antenna additionally enhances the terahertz electric field in the nonlinear material. The resulting terahertz electric field in the nonlinear material can interact with a propagating SPP of optical frequencies, and translate the information carried by the incident terahertz wave to the optical signal by means of sum-frequency generation.

Third Embodiments

[0075] FIGS. 3 (a) and 3 (b) show cross-sectional illustrations of possible coupling schemes between free-space electromagnetic waves, in particular infrared, visible or ultraviolet light, and the first waveguide, i.e. the plasmonic waveguide.

[0076] FIG. 3 (a) illustrates one embodiment in which the top silicon layer of a silicon-on-insulator (SOI) wafer 301 is selectively etched to pattern (second) photonic waveguides 302. This photonic waveguide is used to guide optical signals, e.g., coupled from a laser or fiber by means of grating coupler or edge coupling 303. In proximity to the plasmonic waveguide, the optical signal from the photonic waveguide couples to the plasmonic evanescently. In addition, the silicon waveguide 302 is tapered down to enhance the coupling efficiency from the photonic waveguide to the metal-insulator-metal (MIM) interface formed by the terahertz antenna arms 304 and the core 305 including the nonlinear material, i.e., formed by plasmonic waveguide. These SPP strongly confined to the nonlinear material 305 propagate along the MIM slot waveguide, providing, as described for FIGS. 1(a) and 1(b), strong nonlinear interaction between propagating SPP and terahertz field. The propagating SPP, can evanescently couple back to the photonic waveguide at the end of the plasmonic waveguide. These optical signals can be coupled out to a fiber or camera 306 for temporal or spectral analysis.

[0077] FIG. 3 (b) illustrates an efficient coupling scheme between a free-space optical signal 307, and a plasmonic slot waveguide 305, by means of a diffractive optical element 308. In this embodiment, the optical element is in close proximity with the plasmonic slot waveguide and structured in form of a focusing grating made of silicon. A free-space optical signal is diffracted by the grating and focused towards the plasmonic slot. At the edges of the optical element, the optical signal is evanescently coupled to the metal-insulator-metal interfaces. This provides an efficient scheme for coupling free-space electromagnetic waves in (307) and out (309) of the plasmonic waveguide.

Fourth Embodiment

[0078] FIG. 4 shows an implementation example of a terahertz detector with an integrated Mach-Zehnder interferometer. This enables direct translation of the terahertz information to an amplitude modulation of an optical carrier signal (probe signal). In this embodiment, the optical signal guided by a silicon waveguide 401 is split in two path, e.g., by a Y-splitter or multi-mode interferometer (MMI). Both silicon waveguide branches couple evanescently in close proximity with a (first) plasmonic waveguide. The plasmonic waveguide includes the nonlinear material 402 and conductive electrodes, formed by portions of the antenna arms 403. The two inner electrodes interface both waveguide cores, thus functioning as claddings. The two antenna arms form the outer electrodes. An incident terahertz wave produces a voltage across both antenna gaps, which contain the nonlinear material 402. The terahertz electric field in the nonlinear material 402 changes the real part of the refractive index, thus inducing a phase shift of the optical signal (probe signal) proportional to the applied voltage. By applying an opposite bias voltage between the outer and inner electrodes of the two plasmonic slot, an opposite phase shift can be induced. An induced n-phase shift difference between both plasmonic waveguides would result in destructive interference at the output combiner. This way one can modulate the intensity of the optical probe signal.

[0079] Manufacturing Methods

[0080] FIG. 5 schematically illustrates a method for manufacturing a terahertz device. Two wafers are bonded to each other, wherein the first wafer includes a layer of a nonlinear material (indicated in FIG. 5 as Epi-ferroelectric), which can be, e.g., an epitaxially grown layer of a ferromagnetic material. In the obtained wafer (referred to as stacked wafer), the layer of nonlinear material is buried. In a next step, possibly existing layers which are not required in the terahertz device are removed.

[0081] In further steps, the nonlinear material is structured so as to form a waveguide core (of a firstplasmonicwaveguide), and a layer of the second wafer is structure so as to form another waverguide core (of a second waveguide, such as a waveguide for probing and/or pumping). Alternatively to forming the second core from a layer of the second wafer, it is also possible to deposit further material on the stacked wafer and to produce the second core from that further material.

[0082] Electrodes are produced close to the first core, e.g., by selective metal deposition.

[0083] In an alternative method, the first wafer is made of the nonlinear material (possibly with an adhesion layer added). In that case, it is possible to structure the core of the second waveguide from a layer of the second wafer, such as from the layer below the (optional) adhesion layer, such as from the layer indicated Si in FIG. 5.