EP4732066A1

EP4732066A1 - Optoelectromechanical device

Info

Publication number: EP4732066A1
Application number: EP24735284.2A
Authority: EP
Inventors: Jouni Ahopelto; Tapani MAKKONEN
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2023-06-22
Filing date: 2024-06-14
Publication date: 2026-04-29
Also published as: FI131375B1; WO2024261394A1; FI20235727A1

Abstract

This disclosure describes an optoelectromechanical device which comprises a substrate with an opening, and an optomechanical structure which extends across said opening. The optomechanical structure comprises a first end section, a middle section and a second end section, and the first end section is fixed to a first edge of the opening, and the second end section is fixed to a second edge of the opening. The middle section comprises an optomechanical cavity. The device also comprises a bulk acoustic wave transducer on the first end section of the optomechanical structure.

Description

OPTOELECTROMECHANICAL DEVICE

FIELD OF THE DISCLOSURE

The present disclosure relates to optoelectromechanical devices, and more particularly to optoelectromechanical devices where an optomechanical structure spans an opening. The present disclosure further concerns the excitation and/or detection of mechanical movement in such an optomechanical structure.

BACKGROUND OF THE DISCLOSURE

Optoelectromechanical devices contain nanometer-sized structures which allow interaction between electrical, mechanical and optical signals. Strong optomechanical interaction can for example be obtained with optomechanical crystals. With suitable patterning and dimensioning, such crystals can form an optomechanical cavity where a photonic field interacts with a resonating phononic field.

An optomechanical crystal may for example be suspended across an opening. The flexibility of the suspension and the dimensioning of the crystal can be selected so that the suspended structure can undergo mechanical oscillation. Furthermore, the optomechanical crystal may be patterned so that light of a certain wavelength is reflected back and forth and trapped in an optoelectromechanical cavity in the crystal when it is illuminated with a probe laser signal.

One possible application for optoelectromechanical devices is the conversion of radiofrequency (RF) signals into optical signals. An RF (0.1 - 12 GHz) electromagnetic signal may be led from the outside to a piezoelectric drive transducer which is connected to a suspended optomechanical crystal. The RF signal can thereby create a mechanical oscillation mode in the optomechanical crystal. The mechanical oscillation mode can in turn shift the resonance spectrum of the photons in the cavity. This shift can be detected as a change in the optical signal passing by the optomechanical crystal or, alternatively, as a change in an electrical readout signal measured with a piezoelectric sense transducer which is connected to the suspended optomechanical crystal. Other applications are also possible. Documents US10578891 and Bochmann et al, Nanomechanical coupling between microwave and optical photons, Nature Physics Vol. 9, November 2013, pages 712 - 716, disclose optical quantum interfaces where microwave-to-optical signal conversion has been implemented. Possible applications for optoelectromechanical devices include 5G and SATCOM communication technology, and photon conversion in quantum computing.

A general challenge in optoelectromechanical devices is that it is difficult to couple an electromagnetic input signal (for example the RF signal mentioned in the preceding example) efficiently to an optomechanical crystal suspended on a wire. The efficiency of optoelectromechanical devices would be increased if a greater proportion of the electrical power in the input signal could be transferred into kinetic energy in the resonating optomechanical crystal.

BRIEF DESCRIPTION OF THE DISCLOSURE

An object of the present disclosure is to provide an apparatus which alleviates the above disadvantage.

The object of the disclosure is achieved by an arrangement which is characterized by what is stated in the independent claim. The preferred embodiments of the disclosure are disclosed in the dependent claims.

The disclosure is based on the idea of forming a piezoelectric transducer at the fixed end of a wire where an optoelectromechanical cavity has been formed. An advantage of this arrangement is that coupling efficiency is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figures 1 a - 1c illustrate an optoelectromechanical device.

Figures 1d - 1f illustrate a bulk acoustic wave transducer.

Figures 2a - 2b illustrated end sections with holes. Figure 3 illustrates an end section with a triangular shape.

Figure 4 illustrates an example embodiment.

Figure 5a illustrates suspended end sections.

Figure 5b illustrates phononic crystals.

Figure 5c illustrates a horn-shaped end section suspended from a phononic crystal.

Figure 6 illustrates a simulation comparison between two suspension arrangements.

DETAILED DESCRIPTION OF THE DISCLOSURE

This disclosure describes an optoelectromechanical device which comprises a substrate with a top surface which defines an xy-plane. The substrate comprises an opening on the top surface of the substrate.

The device comprises an optomechanical structure which extends across said opening in an x-direction. The optomechanical structure comprises a first end section, a middle section and a second end section. The first end section is fixed to a first edge of the opening. The second end section is fixed to a second edge of the opening. The middle section comprises an optomechanical cavity.

The device also comprises a first bulk acoustic wave transducer on the first end section of the optomechanical structure. The first bulk acoustic wave transducer comprises a bottom electrode, a top electrode and a piezoelectric layer between the bottom electrode and the top electrode.

The substrate may be a silicon substrate. Figure 1 a illustrates an optoelectromechanical device 10 with a substrate 17 with an opening 14. The opening has a first edge 141 and a second edge 142. These two edges may be opposite to each other. An optomechanical structure 11 extends across the opening 14. The optomechanical structure has a first end section 111 , a second end section 112 and a middle section 113. The opening 14 has a width 19 in the x-direction, equal to the distance between the first and second edges. The optomechanical structure 11 has the same length.

The opening may be a cavity in the substrate which is open only toward the top surface of the substrate. Figure 1 a illustrates this possibility. A part of the substrate 17 then forms the floor 143 of the cavity. Alternatively, the opening may be a through-hole which is open in two directions, both toward the top surface and toward the bottom surface of the substrate. This option has been illustrated in figure 1 b. These two options apply to all embodiments presented in this disclosure. The device may comprise an optical waveguide (not illustrated) which is adjacent to the cavity. Photons may be coupled to the optomechanical structure from the waveguide by evanescent coupling.

The optoelectromechanical device may for example be configured to convert an electrical signal into an optical signal, and/or to transform a change in an optical signal into a corresponding change in an electrical signal. The coupling between the optical and electrical signal domains occurs through mechanical resonance oscillation in the structure which forms the optomechanical cavity.

The term “optical” refers in this disclosure to electromagnetic radiation in the wavelength range from 100 nm to 2000 nm. The photons that are coupled to the optomechanical structure may have any wavelength within this range, for example 1550 nm. The term “mechanical” refers to actual physical movement. The term “transducer” refers to a device which can convert mechanical movement into an electrical signal or vice versa. The transducers described in this disclosure can be used as drive transducers which set the optomechanical cavity into mechanical oscillation. The transducers are in this case controlled by an electric drive signal which reaches the transducer through electrical connections coupled to the electrodes of the transducer. Alternatively or complementarily, the transducers described in this disclosure can be used as sense transducers which detect the mechanical oscillation of the optomechanical cavity. An electric sense signal is in this case measured from the transducer.

The term “bulk acoustic wave transducer” refers to a piezoelectric layer flanked by an electrode on each side. The electrodes and piezoelectric layer may be stacked upon each other in the z-direction which is perpendicular to the xy-plane. The piezoelectric layer may undergo oscillating bending motion when an alternating electric voltage is applied between the electrodes.

In any embodiment presented in this disclosure, the optomechanical structure may be a nanobeam. The nanobeam can for example be made of silicon, nanocrystalline silicon, single-crystal silicon, or a piezoelectric material such as aluminum nitride. The nanobeam may in some embodiments be made of the same piezoelectric material as the bulk acoustic wave transducer. The width of the middle section 113 of the nanobeam 11 in the y-direction may for example be in the range 200 - 800 nm, or in the range 400 - 600 nm. The length of the nanobeam 11 in the x-direction (equal to the width 19 of the gap 14) may for example be 5 - 100 pm.

Alternatively, in any embodiment presented in this disclosure, the optomechanical structure may be a topological phononic crystal.

In any embodiment presented in this disclosure, the material of the piezoelectric layer may be aluminum nitride, for example columnar aluminum nitride, or zinc oxide, lithium niobate or lead zirconate titanate.

Figure 1c illustrates a top view of the device shown in figure 1 a or 1 b. The illustrated xy- plane is defined by the top surface 171 of the substrate 17. Figure 1 c illustrates a first bulk acoustic wave transducer 121 on the first end section 111 of the optomechanical structure. A second bulk acoustic wave transducer 122 is located on the second end section 112 of the optomechanical structure. The device also includes optional signal transmission lines 15 on the top surface 171 of the substrate. These transmission lines can be used to bring electromagnetic signals to the top and bottom electrodes of the transducers 121 and 122. The cavity 14 has a rectangular shape in the xy-plane figure 1c, but any other shape can also be used.

Figures 1d and 1 e illustrate two alternative designs for the first bulk acoustic wave transducer 121 . The same options apply also to the second bulk acoustic wave transducer 122. Figures 1d and 1 e shows the part of the substrate 17 which the optomechanical structure is attached to. The bottom electrode 1211 and top electrode 1212 of the transducer are formed by a first conductive layer and a second conductive layer, respectively.

In figure 1d the bottom electrode 1211 lies below the first end section 111 of the optomechanical structure 11 . A piezoelectric layer 1213 lies on top of the first end section 111 , and the top electrode 1212 lies on top of the piezoelectric layer 1213 in the transducer 121 . In figure 1 e, on the other hand, the bottom electrode 1211 lies on top of the first end section 111 , the piezoelectric layer 1213 lies on top of the bottom electrode 1211 and the top electrode 1212 lies on top of the piezoelectric layer 1213.

Words such as “top”, “bottom”, “above” and “below” refer in this disclosure only to relative positional relationships in relation to the z-direction which is perpendicular to the xy-plane. They do not refer to the positional relationships in relation to the Earth’s gravitational field. In other words, the device may be oriented in any manner with respect to the gravitational field when the device is in use or when it is manufactured.

An interface 123, which may (but does not necessarily have to) coincide with the first edge 141 of the opening 14 as the figures illustrate, separates the region where the first bulk acoustic wave transducer 121 is located (which overlies the opening 14) from the transmission side 181 on the substrate 17. No transducer is present on the transmission side 181 . The same conductors which form the bottom and top electrodes 1211 and 1212 in the transducer 121 may form transmission lines 15 for electric signals on the surface of the substrate on the transmission side 181 of the interface 123. Furthermore, the layer 119, which forms the optomechanical structure 11 above the opening 14, may extend across the interface 123 onto the transmission side.

As the figures illustrate, the piezoelectric layer 1213 lies on the transducer side of the interface 123, which overlies the opening. On the transmission side 181 of the interface 123, an insulating layer 18 may lie between the two transmission lines 15 so that they are electrically separated from each other.

The two transmission lines 15 may alternatively diverge in different directions in the xy- plane on the transmission side 181 of interface 123, so that they do not overlap in the z- direction. This reduces stray capacitance and energy loss due to radiation of acoustic waves into substrate 17. The piezoelectric layer 1213 can also extend into any part of the transmission side 181 where it is not sandwiched between the lines 15 in the z-direction. This is schematically illustrated in figure 1f, where layer 213 extends across the interface 123, but the conductor which forms the top electrode 1212 does not cover piezoelectric layer 1213 on the transmission side 181 .

The transmission lines 15 and top and bottom electrodes 1211 and 1212 may for example be made of aluminium or molybdenum. The material of the piezoelectric layer 1213 may for example be aluminium nitride, ZnO or LiNbO₃.

The layer from which the optomechanical structure 11 is formed may extend to the transmission side 181 and the top surface of the substrate 17, as figures 1 d - 1 e illustrate.

The device may comprise a first hole in the first end section of the optomechanical structure. The hole may be located at the first edge of the opening. This is illustrated in figure 2a, where reference numbers 211 , 213, 221 , 241 , 27 and 281 correspond to reference numbers 111 , 113, 121 , 141 , 17 and 181 , respectively, in figures 1 a - 1 e. The first end section 211 of the optomechanical structure and the middle section 213 are illustrated with different colours for clarity. The first bulk acoustic wave transducer 221 is stacked on the first end section 211 , as figures 1d - 1 e illustrate.

A first hole 216 extends between the edge 241 and the first end section 211 of the optomechanical structure. In the illustrated case the hole 216 has an elongated shape and is parallel to the edge 241 . Other shapes are also possible. The hole may alternatively be called a gap. It extends all the way through the first end section 211 of the optomechanical structure in the z-direction. The hole may be located exactly at the first edge 241 of the opening, as figure 2a illustrates. The first end section 211 may in this case comprise bridge parts 2111 which extend to the substrate 27 on both sides of the hole 216. The transducer 221 may be built on the remaining parts of the first end section (all parts which are not bridge parts).

Alternatively, the hole 216 could be located a short distance from the first edge 241 of the opening. This has been illustrated in figure 2b, where the hole 216 has an oval shape. Many other hole shapes could be used. The number of holes in the first end section 211 may be larger than one. As in the previous example, the transducer 221 may be built on the part of the first end section which lies between the hole and the middle section 213.

The second end section may comprise a corresponding second hole I holes at the second edge of the opening. The width 291 of the hole 216 in the y-direction may for example be in the range 30% - 95% of the width 292 of the first end section 211 of the optomechanical structure at the first edge 241 . Alternatively, 291 may be 50% - 95% of 292.

In general, the presence of one or more holes 216 on the first end section 211 of the optomechanical structure may reduce energy transfer from the transducer 221 to the substrate 27. Consequently, the holes 216 can increase the coupling efficiency between the transducer 221 and the optomechanical structure.

The width of the first end section of the optomechanical structure in a y-direction, which is perpendicular to the x-direction, may decrease as a function of distance from the first edge of the opening. This has been illustrated in figure 3, where reference numbers 311 , 313, 341 and 37 correspond to reference numbers 111 , 113, 141 and 17, respectively, in figures 1 a - 1 e. It can be seen that the width of the first end section 311 is wider at the fixed end e which is attached to the first edge 341 of the opening than at the free end which is joined to the middle section 313 of the optomechanical structure. In figure 3, the first end section 311 has a triangular shape in the xy-plane. The width of the first end section 311 thereby decreases linearly as a function of distance from the first edge 341 . However, many other shapes are also possible. The left and right sides of the first end section 311 could for example have a curved shape, for example a parabolic shape. The width may in this case for example decrease exponentially as a function of distance from the first edge 341 .

The geometrical options discussed above apply also to the second end section of the optomechanical structure. The second end section of the optomechanical structure may have the same shape as the first end section of the optomechanical structure. Furthermore, the embodiment presented in figure 3 can be combined with any of the embodiments that were discussed with reference to figures 2a - 2b.

The first and second end sections may alternatively have a rectangular shape in the xy- plane, as figures 2a - 2b illustrate.

Figure 4 illustrates an example embodiment where reference numbers 411 , 413, 421 , 441 , 47 and 471 correspond to reference numbers 111 , 113, 121 , 141 , 17 and 171 , respectively, in figures 1 a - 1 c. The device also comprises an open space 433 on the first edge 441 just beneath the first bulk acoustic wave transducer.

The shapes illustrated for the first end section 211 , 311 and 411 in figures 2a - 2b, 3 and 4 are only examples. Many other shapes could be used. The first end section could for example be asymmetric with respect to an axis which is parallel to the x-axis and aligned with the middle section 213 1313. The end section may also comprise one or more holes anywhere on its surface in the xy-plane.

In any embodiment described in this disclosure, the first end section may be fixed to the edge of the opening with a suspension structure. Figure 5a, where reference numbers 511 and 513 correspond to reference numbers 111 and 113 in figures 1 a - 1 c, illustrates a few possible geometries for the suspension structures 52. Each suspension structure 52 may for example be a one-dimensional phononic crystal. Figure 5b illustrates possible geometries for the phononic crystals in the xy-plane. The end section 511 is not itself in contact with the edge of the opening. Instead, each suspension structure 52 extends from the edge of the opening to the end section 511 . The same suspension arrangement may be utilized at each end section of the optomechanical structure.

In other words, the first end section 511 may be fixed to the first edge of the opening with a first suspension structure 52 which comprises a first one-dimensional phononic crystal. The second end section may be fixed to the second edge of the opening with a second suspension structure which comprises a second one-dimensional phononic crystal.

Figure 5c illustrates a device with a horn-shaped end section 511 suspended using 1 D phononic strips, which form suspension structures 52. A horn-shape is similar to a triangle. A straight baseline extends in the y-direction in the horn shape, and two sides extend to the middle section 513 from opposite ends of the baseline. The two sides are curved in the xy-plane. The first end section and/or the second end section may have a horn shape in the xy-plane.

In general, phononic strips prevent leakage of acoustic energy out of the device into the substrate within a designed frequency range. The suspension 52 and the middle section 513 may be terminated using perfectly-matched-layer (PML) regions that are used in modeling to absorb energy entering the PML region.

The options presented above with reference to figures 5a - 5c can be combined with any other embodiment presented in this disclosure.

Figure 6 illustrates a simulation which compares two different suspension structures, a regular straight beam and a 1 D phononic strip. The ratio of energy flux in the optomechanical structure to total power dissipated (here referred to as efficiency) is shown as a function of frequency. One can see that phononic strips enable higher efficiency for most frequencies in comparison with straight beams. One can also see that there is no single narrow frequency range with a clear peak efficiency. Instead, the operating frequency range is wide with several peaks. Efficiency may be increased further by decreasing the area of the transducers. This is because due to viscous losses in the materials, acoustic energy is converted to heat. Viscous losses decrease with decrease in the transducer volume.

In any embodiment described in this disclosure, the mechanical oscillation of the optomechanical structure may for example be a transverse wave where the displacement of the optomechanical structure occurs in the y-direction as the wave propagates in the x- direction. Alternatively, the mechanical oscillation of the optomechanical structure may be a transverse wave where the displacement of the optomechanical structure occurs in the z-direction as the wave propagates in the x-direction. Alternatively, the mechanical oscillation of the optomechanical structure may be a longitudinal wave where the displacement of the optomechanical structure occurs in the x-direction as the wave propagates in the x-direction. In any embodiment described in this disclosure, the mechanical oscillation may form a standing wave in the optomechanical structure. A standing wave can be formed if W = 2L/n, where W is the wavelength, L is a length in the x-direction and n is an integer. The length L may either be the width 19 shown in figure 1a, or it may be the length of the optomechanical cavity (not illustrated but located in the middle section of the optomechanical structure) in the x-direction.

Any device described in this disclosure can be manufactured with microelectronics processes. As an example, fabrication of a device structure shown in Figs. 1 e and 3 is briefly described. As substrate, a silicon-on-insulator (SOI) wafer or an oxidized silicon wafer with a nano/polycrystalline silicon layer can be used. The bottom electrode 1211 , the piezoelectric film 1213 and the top electrode 1212 required for the BAW transducer are deposited, for example by sputtering, on the substrate. The bottom electrode can be molybdenum, the top electron aluminium, and the piezo electric film aluminium nitride. In case the layers are deposited to form the full BAW transducer stack, the layers, including the transducer itself and the leads required for electrical contacting, are patterned by lithography, and etching selectively against the layer underneath. Patterning can also be performed sequentially after deposition of each of the layers. The dimensions are relaxed, and optical lithography can be used in patterning. The optomechanical structure 11/313 is formed in the device layer of the SOI wafer or in the nano/polycrystalline layer of the oxidized silicon wafer after structuring the BAW transducer. The dimensions of the optomechanical structures can be well below 100 nm and typically electron beam lithography is required, leading to mix-and-match lithography in the fabrication of the full device. A high resolution DUV optical stepper provides means for optical lithography of also the optomechanical structures and, thus, for large scale production. The optomechanical structure is dry etched against the silicon dioxide underneath. Finally, the optomechanical structure is released by removing the silicon dioxide below using hydrogen fluoride (HF) vapour. The other structures are manufactured in a similar manner with a slightly altered order of the fabrication steps. The methods described in this paragraph can be used to manufacture any device described in this disclosure.

Claims

1 . An optoelectromechanical device which comprises

- a substrate with a top surface which defines an xy-plane, wherein the substrate comprises an opening on the top surface of the substrate,

- an optomechanical structure which extends across said opening in an x-direction, wherein the optomechanical structure comprises a first end section, a middle section and a second end section, and the first end section is fixed to a first edge of the opening, and the second end section is fixed to a second edge of the opening, and the middle section comprises an optomechanical cavity, characterized in that the device also comprises

- a first bulk acoustic wave transducer on the first end section of the optomechanical structure, wherein the first bulk acoustic wave transducer comprises a bottom electrode, a top electrode and a piezoelectric layer between the bottom electrode and the top electrode.

2. An optoelectromechanical device according to claim 1 , wherein the device comprises a first hole in the first end section of the optomechanical structure.

3. An optoelectromechanical device according to any of claims 1 -2, wherein the width of the first end section of the optomechanical structure in a y-direction, which is perpendicular to the x-direction, decreases as a function of distance from the first edge of the opening.

4. An optoelectromechanical device according to claim 3, wherein the first end section has a triangular shape in the xy-plane.

5. An optoelectromechanical device according to claim 3, wherein the first end section has a horn shape in the xy-plane.

6. An optoelectromechanical device according to any of claims 1 -2, wherein the first end section has a rectangular shape in the xy-plane.

7. An optoelectromechanical device according to any preceding claim, wherein the first end section is fixed to the first edge of the opening with a first suspension structure which comprises a first one-dimensional phononic crystal.

8. An optoelectromechanical device according to any preceding claim, wherein the device comprises a second bulk acoustic wave transducer on the second end section of the optomechanical structure.

9. An optoelectromechanical device according to claim 8, wherein the second end section of the optomechanical structure has the same shape as the first end section of the optomechanical structure.

10. An optoelectromechanical device according to any of claims 8-9, wherein the second end section is fixed to the second edge of the opening with a second suspension structure which comprises a second one-dimensional phononic crystal.

11. An optoelectromechanical device according to any preceding claim, wherein the optomechanical structure is a nanobeam.