Artificial resonator waveguide

Abstract

A photonic circuit and an optical resonator are described. The optical resonator element comprises at least one photonic layer formed of a material having selected electro-optic properties, an electrode arrangement comprising at least a first top electrode arrangement, and a second bottom electrode arrangement positioned at first and second opposite facets of the at least one photonic layer. At least one of the first top electrode arrangement and the second bottom electrode arrangement comprises an electrode arrangement defining a ring or straight artificial resonator structure. In response to electric potential applied to one or more electrodes of the arrangement of electrodes, a refractive index varies in a respective region of the at least one photonic layer affected by the one or more electrodes, selectively defining a ring-shaped or straight waveguide within the at least one photonic layer.

Description

[0001] ARTIFICIAL RESONATOR WAVEGUIDE

[0002] TECHNOLOGICAL FIELD

[0003] The present disclosure relates to photonic resonators, and specifically relates to controllable artificial and virtual photonic resonators.

[0004] BACKGROUND

[0005] The demand for highly integrated photonic circuits continues to rise, posing greater challenges in developing high-fidelity high-density photonic chips. These challenges are further amplified in the context of photonic circuits for quantum applications, which operate at the single photon level.

[0006] Optical resonators, including straight and ring resonators, are typically formed of certain optical paths, where signals having specific parameters (typically wavelength) resonate. An optical ring resonator is typically formed of a ring-shaped waveguide, typically associated with one or more input / output waveguides. When light is coupled into the ring resonator through an access waveguide, the coupled light may propagate through the ring, undergoing a plurality of round trips. Due to constructive and destructive interference, if the optical path length around the ring resonator is equal to an integer multiple of the wavelength of the light, the input signal meets resonance conditions. As a result, the ring resonator provides a sharp transmission and amplification of peak at specific resonance wavelengths. Typical straight, or linear, resonators are formed of two reflecting surfaces (e.g., mirrors) facing each other with an optical medium such as a waveguide section between them. The linear resonators allow light to bounce back and forth between the mirrors, forming a standing wave pattern. The distance between the mirrors and refractive index of the optical medium determines the resonant frequencies of the cavity, allowing only certain wavelengths of light to constructively interfere and form resonant modes. Such resonators are fundamental in laser design, enhancing the coherence and stability of the laser output. Fidelity and quality of the optical resonators are often determined by parameters such as the Q-factor and finesse. The rising demand for integrated photonic circuits poses similar, and at times stricter challenges in developing improved optical resonators.

[0007] To address this challenge, a great effort is being made to develop new techniques and materials suitable for use in photonic circuits and respective optical elements. Various such studies are focused on electro-optical properties of different materials, generally described by the Pockels effect, or Kerr effect, defining variation in the refractive index of the material in response to an external electric field. For example, see:

[0008] Ali K. Hamze, et al, Design rules for strong electro-optic materials, NPJ Computational Materials 6, 130 (2020).

[0009] Beskin et al, Growth and Structure of Strong Pockels Material Strontium Barium Niobate on SrTiOs and Si by Molecular Beam Epitaxy (2021). Adv. Photonics Res., 2: 2100111. https: / / doi.org / 10.1002 / adpr.202100111.

[0010] Valentin et al, Lead-Free Perovskite Thin Films with Tailored Pockels-Kerr Effects for Photonics (2023). ACS Applied Materials & Interfaces 15 (31), 38039-38048, DOI: 10.1021 / acsami.3c06499.

[0011] Mian Zhang et al, Integrated lithium niobate electro-optic modulators: when performance meets scalability (2021). Optica 8, 652-667.

[0012] Agham B. Posadas et al, RF-sputtered Z-cut electro-optic barium titanate modulator on silicon photonic platform (2023). J. Appl. Phys. 134 (7): 073101.

[0013] K. Nashimoto et al., High-speed PLZT optical switches for burst and packet switching (2005). 2nd International Conference on Broadband Networks, Boston, MA, USA, 2005, pp. 1118-1123 Vol. 2, doi: 10.1109 / ICBN.2005.1589732.

[0014] G. Chen, H. -L. Lin, J. D. Ng and A. J. Danner, Integrated Electro-Optic Modulator in Z-Cut Lithium Niobate Thin Film With Vertical Structure (2021). IEEE Photonics Technology Tetters, vol. 33, no. 23, pp. 1285-1288, doi: 10.1109 / LPT.2021.3114993.

[0015] A. Karvounis, F. Timpu, V. V. Vogler-Neuling, R. Savo, R. Grange, Barium Titanate Nanostructures and Thin Films for Photonics. Adv. Optical Mater. 2020, 8, 2001249.

[0016] Chengli Wang et al, Lithium tantalate electro-optical photonic integrated circuits for high volume manufacturing, arXiv:2306.16492. Quack, N., Takabayashi, A.Y., Sattari, H. et al. Integrated silicon photonic MEMS. Microsy st Nanoeng 9, 27 (2023). https: / / doi.org / 10.1038 / s41378-023-00498-z.

[0017] Suraj, Shankar Kumar Selvaraja, Highly Oriented PZT Platform for Polarization- Independent Photonic Integrated Circuit and Enhanced Efficiency Electro-Optic Modulation, arXiv:2305.19126.

[0018] Li, Z., Wang, R.N., Lihachev, G. et al. High density lithium niobate photonic integrated circuits. Nat Commun 14, 4856 (2023). https: / / doi.org / 10.1038 / s41467-023- 40502-8.

[0019] Xianwen Liu, Alexander W. Bruch, and Hong. X. Tang, "Aluminum nitride photonic integrated circuits: from piezo-optomechanics to nonlinear optics," Adv. Opt. Photon. 15, 236-317 (2023).

[0020] Ma, X., Cai, Z., Zhuang, C. et al. Integrated microcavity electric field sensors using Pound-Drever-Hall detection. Nat Commun 15, 1386 (2024). https: / / doi.org / 10.1038 / s41467-024-45699-w.

[0021] Zhang, K., Sun, W., Chen, Y. et al. A power-efficient integrated lithium niobate electro-optic comb generator. Commun Phys 6, 17 (2023). https: / / doi.org / 10.1038 / s42005-023-01137-9.

[0022] Churaev, M., Wang, R.N., Riedhauser, A. et al. A heterogeneously integrated lithium niobate-on- silicon nitride photonic platform. Nat Commun 14, 3499 (2023). https: / / doi.org / 10.1038 / s41467-023-39047-7.

[0023] Li, M., Ling, J., He, Y. et al. Lithium niobate photonic-crystal electro-optic modulator. Nat Commun 11, 4123 (2020). https: / / doi.org / 10.1038 / s41467-020-17950-7.

[0024] Various publications relate to circular resonators and EO tunability, including the inventors’ work, utilizing mode confinement by air on the outside, as in conventional state-of-the-art:

[0025] M. Rosenblit, P. Horak, E. Fleminger, Y. Japha and R. Folman, Design of microcavity resonators for single-atom detection. J. Nanophoton. 1, 011670 (2007), Special issue.

[0026] M. Rosenblit, Y. Japha, P. Horak and R. Folman, Simultaneous optical trapping and detection of atoms by microdisk resonators. Phys. Rev. A 73, 063805 (2006).

[0027] M. Rosenblit, P. Horak, S. Helsby and R. Folman, Single-atom detection using whispering gallery modes of microdisk resonators, Phys. Rev. A 70, 053808 (2004). Various materials and techniques are described in the literature, enabling electro- optical effects, and allowing integration in photonic and electro-optical circuits. These techniques utilize waveguides and electro-optical elements formed of Barium titanate (BTO), lithium niobate, Lead zirconate titanate (PLZT), and other materials having properties that give rise to significant Pockels coefficient values.

[0028] GENERAL DESCRIPTION

[0029] Optical processing and photonic circuits can provide high-speed computing in classic and quantum regimes, as well as forming the base for a wide range of light sources, sensors, and communication systems. Some typical elements in such circuits include optical resonators, including linear and ring resonators. Realization of various optical resonators currently faces numerous obstacles. For example, a significant obstacle in utilizing resonators and photonic circuits relates to intensity loss due to light scattering. While this may be solved in certain classical photonic circuits using increased power, such loss provides limitation to quantum applications, and reduces the Q-factor and finesse of the resonators. Generally, even if a material is free from internal material absorption, a waveguide circuit generally has several loss sources. This includes for example bending losses associated with circuit geometry, losses from internal geometrical defects, and losses induced by the scattering of light due to edge roughness associated with waveguide fabrication techniques. In this connection, edge roughness relates to both side-walls roughness and surface roughness. While top and bottom surfaces can be treated to reduce roughness (e.g., with chemical mechanical polishing (CMP)), side-wall roughness remains a hard problem for standard techniques.

[0030] Additional obstacles, typically associated with conventional photonic circuits, relate to control of the phase of the propagating light. Typically, conventional waveguides are made of materials having limited to no electro-optical (EO) effect, e.g., are made of Silicon Nitride (SiN), and phase shifting elements are typically achieved by heating waveguide regions. This technique has several drawbacks, such as chip heat load, long rise / fall times due to thermal capacity and conductivity of the materials and their surroundings, and significant crosstalk. The latter drawback is associated with thermal conductivity of the waveguides and their surroundings, as heat is not a well-localized property. Alternatively, efforts are being made for using EO materials combined with conventional waveguides, by placing the EO materials outside the waveguide, so that they interact with the evanescent field of the light mode. The fabrication of such systems requires complex efforts and provides only a small effect on most of the light mode. Another issue of conventional waveguides and photonic circuits relates to the stability of the circuit against uncontrollable phase noise due to thermal and vibration noise. As indicated above, conventional waveguides are typically made of non-EO materials, resulting in limited ability to correct phase fluctuations with high bandwidth in real time. An additional problem in conventional circuits is high fidelity, which is required for high- end applications, such as Al accelerators or quantum information processing. This fidelity is typically hindered by fabrication imprecision (and the fact that circuit parameters are fixed by fabrication and cannot be dynamically altered).

[0031] An additional example of current drawbacks, and possibly one of the biggest problems, is the fixed nature of the currently existing photonic circuits, which are based on fixed fabricated elements. As waveguides and other photonic elements are based on fixed fabricated elements, this limits variability (i.e., there is no programmability, such as in an FPGA), limits correction of any variation due to manufacturing issues, and limits the ability to quickly react to thermal and vibration noise, as mentioned above.

[0032] A new generation of electro optical materials with a sufficiently high Pockels coefficient enables, for the first time, to confine an optical mode solely due to the index of refraction difference created by the application of an electric field on the material. Namely, the applied electric field may define a waveguide without having air around the waveguide (or without physical intervention, e.g. ion implantation, to affect the refractive index), thus not requiring any etching of or around the waveguide. Such etching may include physical and / or chemical etching or other processes that permanently change structure of the material, including e.g., ion bombardment. For example, circuits made of the popular EO material, Lithium niobate (LiNbO3), typically require etching to generate an arrangement of waveguides.

[0033] In this connection, Various materials including BaTiOs (BTO) and Lithium niobate (LiNbOs or LN) are of the first examples of this new generation of materials, and the roadmap for production of new materials having a stronger Pockels effect is described in Ali K. Hamze, el al listed above. Accordingly, various photonic elements and resonators according to the present disclosure may be utilized with BTO or Lithium niobate and can be improved continuously in accordance with the development roadmap listed above. While BTO still has significant internal absorption and is suitable for photonic circuits with classical light (e.g. for Al accelerators), the new material roadmap will provide materials having lower absorption coefficients, becoming similar to the low absorption of the quartz crystal, thus being suitable for quantum optics applications. In some configurations, BTO may have a small absorption, whereby a small frequency gap between absorption modes may enable narrow line width lasers to operate with low absorption. Alternatively, increasing BTO band gap, e.g., by stretching the material when thin layers of it are sandwiched between layers with other lattice constants may also eliminate or at least significantly reduce absorption of the BTO layer. The techniques of the present disclosure may thus be utilized for BTO, lithium niobate, or any other material having a high Pockels coefficient. For example, the present disclosure may utilize material having Pockels coefficient of 100 pm / V and higher, or of 200 pm / V and higher, or of 300 pm / V and higher, or of 400 pm / V and higher, or of 500 pm / V and higher, or of 600 pm / V and higher, or of 700 pm / V and higher, or of 800 pm / V and higher, or of 900 pm / V and higher or of 1000 pm / V and higher.

[0034] Generating artificial optical resonator elements in accordance with the description of the present disclosure, may provide various advantages over conventional optical resonators that are fabricated by etching. For example, variation of the electric voltage applied between electrodes, as described in more detail below, varies the refractive index in the photonic layer. As a result, the resonance wavelength of the optical resonator may be fine-tuned to the desired wavelength by varying the electric field applied by the respective electrodes. Similarly, as resonators typically suffer from drifts in their resonance wavelength due to temperature and acoustic fluctuations, the present technique enables implementation of fast control (large bandwidth) over the index of refraction to enable efficient corrections (locking) of the resonance wavelength.

[0035] In addition, and most importantly, the use of an electric field for varying the refractive index of regions of a photonic layer to create an artificial waveguide, reduces scattering due to edge (sidewall) roughness. This has the potential to significantly reduce the complexity and cost of resonator fabrication, e.g., eliminating or at least reducing the need for special treatment to reduce roughness, and at the same time further increase the quality factor Q, which is the figure of merit for these resonators. More specifically, the edge roughness in resonators according to embodiments of the present disclosure may be reduced for two reasons: 1. there is more technological know-how on how to make low-roughness edges for metallic layers, especially when they can be very thin; 2. the effective roughness drops exponentially with the ratio of the distance between the source of an electric field (in this case the metallic electrode) to the point of interest within the photonic layer, and the wavelength used. In practice this means that if the distance between the metallic electrode and the center of the photonic layer is 500 nm, then roughness with wavelength below 500 nm is made exponentially small. Hence, the roughness of the metallic electrodes is hardly transferred to the artificial walls of the photonic waveguide created by the electric field.

[0036] Additionally, the use of an electrode arrangement enables control over the light coupling into or out of the resonator. As described in more detail below, the photonic circuit may utilize one or more coupling control electrodes located in the vicinity of one or more ring resonator electrodes. The one or more coupling control electrodes may be configured to generate a controllable coupling gate. Hence, by applying voltage to the coupling control (or gate) electrodes (assuming the resonator electrodes are under electric voltage), the electric field generated by the gate electrodes varies the refractive index, enabling coupling of light between the resonator and a waveguide of the circuit. If no voltage is applied to the gate electrode, light crossing is prevented, and the resonator becomes isolated. For perfect resonators, having high Q and finesse values would allow light to circulate within the resonator, without coupling. Scattering losses are generally minimized by the technique of the present disclosure, eliminating rough edges, however absorption due to material properties may reduce the lifetime of the signal within the resonator. The ability to isolate the ring resonator may enable the use of ring resonators as memory units or delay lines. Having no coupling to a nearby waveguide allows the light to circulate within the resonator unperturbed and enables the resonator to achieve high Q and finesse values. Accordingly, enabling to electronically turn the coupling on and off at highly accurate times enables the use of ring resonators as memory units or delay lines.

[0037] Finally, as indicated above, the present disclosure also provides a linear (straight) artificial resonator configuration. Such linear resonator may be made of a straight waveguide portion with Bragg mirrors at each side forming a Fabry-Perot resonator, whereby the Bragg mirrors are made by modulating the electric field along the axis of the waveguide (or by some other method like implanting ions to modulate the index of refraction). In this case, the problem of bending losses disappears, and the resonator may be made of any dimension selected in accordance with desired resonance wavelength using available materials such as Lithium Niobate. The advantage of the new technique of the present disclosure is that: 1. The edge roughness is again a limiting factor and as explained the new technique reduces the edge roughness, 2. If the mirrors are made by modulating the electric field, the reflectivity of the mirrors can be selectively changed at any time by changing the voltage, thereby controlling when the light leaves the cavity. 3. Changing the waveguide index of refraction can change the effective length of the cavity thereby having the ability for high bandwidth changes to the resonance wavelength (e.g., to fight acoustic and thermal noise).

[0038] Thus, according to a broad aspect, the present disclosure provides a photonic circuit comprising at least one resonator element, the at least one resonator element comprising at least one photonic layer formed of a material having selected electro-optic properties, an electrode arrangement comprising at least a first top electrode arrangement and a second bottom electrode arrangement positioned at first and second opposite facets of the at least one photonic layer; wherein at least one of the first top electrode arrangement and the second bottom electrode arrangement comprises an electrode arrangement defining a ring structure, and wherein, in response to electric potential applied to one or more electrodes of the arrangement of electrodes, a refractive index varies in a respective region of the at least one photonic layer affected by the one or more electrodes, selectively defining a ring-shaped waveguide within the at least one photonic layer.

[0039] According to some embodiments, the at least one resonator element is an artificial resonator. More specifically, the at least one resonator element is defined by a region of the at least one photonic layer affected by the at least a first top electrode arrangement and a second bottom electrode arrangement. The at least one resonator element is manifested by variation of refractive index of a region of the at least one photonic layer in response to electric field applied by the electrode arrangements.

[0040] According to some embodiments, at least one of the first top electrode arrangement and the second bottom electrode arrangement may comprise a circular electrode defining a ring resonator waveguide. According to some embodiments, the photonic circuit may further comprise a first and a second clad layers located at first and second surfaces of the at least one photonic layer, the first clad layer is located between the at least one photonic layer and the first top electrode arrangement, and the second clad layer is located between the at least one photonic layer and the second bottom electrode arrangement.

[0041] According to some embodiments, at least one of the first top electrode arrangement and second bottom electrode arrangement may comprise a plurality of independently controlled point-like electrodes positioned on at least one facet of the at least one photonic layer, and wherein applying electric voltage to a set of point-like electrodes along a selected region defines a photonic circuit within which light propagates within a respective region of the at least one photonic layer.

[0042] According to some embodiments, the selected electro-optic properties comprise a Pockels coefficient defining refractive index variation in response to a DC electric field.

[0043] According to some embodiments, the at least one photonic layer comprises a material having at least one Pockels coefficient of 100 pm / V or higher. According to some embodiments, the at least one photonic layer comprises a material having at least one Pockels coefficient of 200 pm / V and higher, or of 300 pm / V and higher, or of 400 pm / V and higher, or of 500 pm / V and higher, or of 600 pm / V and higher, or of 700 pm / V and higher, or of 800 pm / V and higher, or of 900 pm / V and higher or of 1000 pm / V and higher.

[0044] According to some embodiments, the at least one photonic layer is formed of Lithium Niobate or BTO (BaTiCh).

[0045] According to some embodiments, the photonic circuit may further comprise a control unit comprising at least one processor and memory circuitry, the control unit being electrically connected to electrodes of the first top electrode arrangement and electrodes of the second bottom electrode arrangement and configured to selectively apply electric voltage to the electrodes for operating the at least one resonator element.

[0046] According to some embodiments, the control unit is further configured for selectively applying electric voltage to the electrodes for determining one or more resonance wavelengths of the at least one resonator element.

[0047] According to some embodiments, the electrode arrangement comprises electrodes formed of transparent electrically conducting material. According to some embodiments, the electrode arrangement comprises metallic electrodes, wherein the metallic electrodes are sufficiently thin to eliminate absorbing of light by the metallic electrodes.

[0048] According to some embodiments, the electrode arrangement comprises crystalline electrodes characterized by narrow absorption peaks. For example, the crystalline electrodes may be formed on graphene or other crystalline materials.

[0049] According to some embodiments, the photonic circuit may further comprise at least one top gate electrode located in vicinity of the first top electrode arrangement defining the at least one resonator element, such that applying electric voltage to the at least one top gate electrode provides for a varying refractive index in a region of the photonic layer, thus operating as a gate for coupling optical signals to the at least one resonator element.

[0050] According to some embodiments, the at least one resonator element is operable as a filter.

[0051] According to some embodiments, the at least one resonator element is operable as a switch.

[0052] According to some embodiments, the at least one resonator element is operable as a reflector or delay line.

[0053] According to some embodiments, the at least one resonator element is operable as a reference standard to frequency lock on.

[0054] According to some embodiments, the at least one resonator element is operable as a sensor or detector for single or minute amount of particles.

[0055] According to some embodiments, the at least one resonator element is operable as a quantum device, e.g., frequency comb, or entangler of photons and / or atoms.

[0056] According to some embodiments, the photonic circuit may comprise at least first and second ring resonator elements coupled between them, wherein each of the first and second resonator elements is defined by at least a first top electrode arrangement having a circular portion.

[0057] According to some embodiments, the photonic circuit may further comprise at least one additional electrode arrangement configured to define at least one linear waveguide configured to couple optical signals into or out of the at least one resonator element. According to some embodiments, variation in electric potential applied to one or more electrodes of the arrangement of electrodes enables controlled variation of resonance wavelength of the at least resonator element.

[0058] According to one other broad aspect, the present disclosure provides a photonic circuit comprising at least one resonator element, the at least one resonator element comprising at least one photonic layer formed of a material having selected electro-optic properties, an electrode arrangement comprising at least a first top electrode arrangement and a second bottom electrode arrangement positioned at first and second opposite facets of the at least one photonic layer; wherein at least one of the first top electrode arrangement and the second bottom electrode arrangement comprises an electrode arrangement defining at least first and second discontinuity regions in electrodes along an elongated region; and wherein, in response to electric potential applied to one or more electrodes of the arrangement of electrodes, a refractive index varies in a respective region of the at least one photonic layer affected by the one or more electrodes, selectively defining at least first and second refractive index interfaces associated with said first and second discontinuity regions, defining said resonator within the at least one photonic layer.

[0059] According to some embodiments, the at least one resonator element is an artificial resonator. More specifically, the at least one resonator element is defined by a region of the at least one photonic layer affected by the at least a first top electrode arrangement and a second bottom electrode arrangement. The at least one resonator element is manifested by variation of refractive index of a region of the at least one photonic layer in response to electric field applied by the electrode arrangements.

[0060] According to some embodiments, the artificial resonator is defined by straight waveguide electrode portion and two or more electrodes defining said at least first and second discontinuity regions and configured to vary refractive index at two ends of the straight waveguide electrode portion, thereby defining a straight waveguide portion with Bragg mirrors at each side forming a Fabry-Perot resonator.

[0061] According to some embodiments, the photonic circuit may further comprise a first and a second clad layers located at first and second surfaces of the at least one photonic layer, the first clad layer is located between the at least one photonic layer and the first top electrode arrangement, and the second clad layer is located between the at least one photonic layer and the second bottom electrode arrangement. According to some embodiments, at least one of the first top electrode arrangement and second bottom electrode arrangement comprises a plurality of independently controlled point-like electrodes positioned on at least one facet of the at least one photonic layer, and wherein applying electric voltage to a set of point-like electrodes along a selected region defines a photonic circuit within which light propagates within a respective region of the at least one photonic layer.

[0062] According to some embodiments, the selected electro-optic properties comprise a Pockels coefficient defining refractive index variation in response to a DC electric field.

[0063] According to some embodiments, the at least one photonic layer comprises a material having at least one Pockels coefficient of 100 pm / V or higher. According to some embodiments, the at least one photonic layer comprises a material having at least one Pockels coefficient of 200 pm / V and higher, or of 300 pm / V and higher, or of 400 pm / V and higher, or of 500 pm / V and higher, or of 600 pm / V and higher, or of 700 pm / V and higher, or of 800 pm / V and higher, or of 900 pm / V and higher or of 1000 pm / V and higher.

[0064] According to some embodiments, the at least one photonic layer is formed of Lithium Niobate or BTO (BaTiCh).

[0065] According to some embodiments, the at least one photonic circuit may further comprise a control unit comprising at least one processor and memory circuitry, the control unit being electrically connected to electrodes of the first top electrode arrangement and electrodes of the second bottom electrode arrangement and configured to selectively apply electric voltage to the electrodes for operating the at least one resonator element.

[0066] According to some embodiments, the at control unit is further configured for selectively applying electric voltage to the electrodes for determining one or more resonance wavelengths of the at least one resonator element.

[0067] According to some embodiments, the at least one electrode arrangement comprises electrodes formed of transparent electrically conducting material.

[0068] According to some embodiments, the at least one electrode arrangement comprises metallic electrodes, wherein the metallic electrodes are sufficiently thin to eliminate absorbing of light by the metallic electrodes.

[0069] According to some embodiments, the at least one electrode arrangement comprises crystalline electrodes characterized by narrow absorption peaks. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0071] Fig. 1 illustrates schematically a photonic circuit system including at least one ring resonator element according to some embodiments of the present disclosure;

[0072] Fig. 2 illustrates schematically an additional photonic circuit system including at least one ring resonator element and utilizing clad layers according to some embodiments of the present disclosure;

[0073] Fig. 3 shows the Pockels coefficients of several materials having electro optic response;

[0074] Figs. 4A and 4B show Comsol simulation of optical modes supported by a BTO waveguide (Fig. 4A) and Lithium Niobate waveguide (Fig. 4B) formed according to some embodiments of the present disclosure;

[0075] Fig. 5 illustrates a side view of a section of a photonic circuit system according to some embodiments of the present disclosure, and exemplifying waveguide formation in the circuit;

[0076] Fig. 6 exemplifies a photonic circuit including a ring resonator according to some embodiments of the present disclosure;

[0077] Figs. 7A and 7B exemplify ring resonators coupled to a waveguide section (Fig. 7A) and a ring resonator coupled via a controlled coupling (gated resonator) (Fig. 7B) according to some embodiments of the present disclosure;

[0078] Fig. 8 exemplifies an arrangement of two coupled ring resonators providing a selectively (filtering) reflecting element according to some embodiments of the present disclosure;

[0079] Figs. 9A and 9B exemplify a ring resonator coupled to waveguides in two sides providing a filtering transfer element from one waveguide to the other according to some embodiments of the present disclosure, Fig. 9A exemplifies parallel waveguides and Fig. 9B exemplifies intersecting waveguides; Fig. 10 exemplifies an additional arrangement of two coupled ring resonators according to some embodiments of the present disclosure;

[0080] Fig. 11 exemplifies a pixel-like electrode configuration providing a configurable photonic circuit system according to some embodiments of the present disclosure;

[0081] Fig. 12 shows simulated finesse (number of turns) for a resonator formed using the technique of the present disclosure with a vertical electric field of 107V / m in a 1 pm thick layer of LiNbO3 (6n = 0.0016) and BaTiO3 (6n = 0.06), with their present-day existing absorption coefficients;

[0082] Fig. 13 shows analytically calculated and numerically simulated data of quality factor and lifetime for BTO ring resonators having different radii using electric field of 107V / m, assuming the BTO Pockels coefficient;

[0083] Fig. 14 shows analytically calculated and numerically simulated quality factor and lifetime obtainable for ring resonators having different radii and different refractive index changes.

[0084] Figs. 15A and 15B illustrate a linear resonator configuration according to some embodiments of the present disclosure, Fig. 15A exemplifies a side view of the linear resonator showing electrode arrangement placed on top and bottom of the photonic layer, and Fig. 15B illustrates top view of the linear resonator exemplifying resonator cavity and reflective ends.

[0085] DETAILED DESCRIPTION OF EMBODIMENTS

[0086] As indicated above, the present disclosure provides a photonic circuit system, suitable for numerous applications such as photonic sensors, clocks, information processing and communication, as well as quantum photonics, and one or more photonic resonator elements suitable for use in a photonic circuit. Such photonic resonator elements may include one or more ring resonators formed of circular electrode arrangement defining a ring-shaped resonator, as well as linear resonator elements formed of an electrode arrangement defining a linear waveguide section including back and front reflecting regions (e.g., mirror). The photonic circuit system of the present disclosure utilizes at least one optical resonator based on at least one photonic layer, also referred to as active substrate layer (ASL), and an electrode arrangement positioned and operable for applying an electric field onto selected regions of the photonic layer. The at least one photonic layer is formed of a material having selected electro-optic properties, and more specifically from a material having a property of a varying refractive index in response to an electric field applied thereon.

[0087] Reference is made to Fig. 1 schematically illustrating a photonic circuit system 100 including at least one ring resonator element according to some embodiments of the present disclosure. The system 100 includes at least one photonic layer 50, and an electrode arrangement 110 positioned on facets of the photonic layer 50. Fig. 1 illustrates top electrode arrangement 110, which, in this non-limiting example, includes electrodes 112, 114, 116, and 118, and bottom electrode 120. Generally, bottom electrode 120 may be used as a common electrode, electrically conductive substrate, or it may be formed as an additional electrode arrangement having a plurality of electrode portions mirroring arrangement of the electrode arrangement 110 placed on a top facet of the photonic layer 50. When used as a common bottom electrode or electrically conductive substrate, bottom electrode 120 may be held at ground potential. It should be understood, and as is described in more detail below, that arrangement of the electrodes may vary in accordance with the desired layout of the photonic circuit. Further, as described below, the arrangement of the electrodes may support multiple possible photonic circuit layouts in accordance with operation of the electrodes.

[0088] In Fig. 1 top electrodes 112, 114, and 116 are illustrated as elongated electrodes, generally defining waveguides 52a, 52b, and 54. Additionally, top electrode 118 is a circular electrode. Accordingly, when an electric voltage is held between first top electrode 118 and bottom electrode 120, the electric field between the top and bottom electrodes defines a circular waveguide within the at least one photonic layer 50. Such a circular waveguide acts as an optical ring resonator.

[0089] The electrodes of the electrode arrangement 110 may be operably connected to a control unit 500 configured to selectively apply electric voltage to selected electrodes at selected times and with selected voltage values, in accordance with a desired waveguide arrangement and operation of the photonic circuit 100. Control unit 500 may include one or more processor and memory circuitries (PMCs) and an input / output interface. Additionally, control unit 500 may include an electronic circuit configured to apply selected electrical voltage to selected electrodes of the electrode arrangement 110, and specifically to top electrode 118. Generally, the one or more PMCs of the control unit may be operable in accordance with selected input and / or pre- stored data, to operate the electronic circuit for applying selected electric voltage to selected electrodes of electrode arrangement 110, thereby forming and operating a selected photonic circuit within the at least one photonic layer 50.

[0090] When operated, the control unit 500 may utilize an electronic circuit thereof to generate selected electric voltage difference between selected electrodes, applying a selected electric field on regions of the photonic layer 50. For example, a voltage difference between electrodes 118 and 120 generates an electric field in the regions between the electrodes forming a ring resonator waveguide. Changing the magnitude of the voltage changes the resonance conditions of the resonator. Further, applying an electric voltage difference between electrodes 112, 114, and / or 116 and electrode 120, generates waveguides 52a, 52b, and 54 in the region between the electrodes. Waveguide 52b can be used for coupling optical signals in and / or out of the ring resonator, or as a coupling gate for coupling signal from waveguide 54 into the ring resonator. Further, the waveguides may also operate to apply phase shift to light passing through them. For example, waveguide region 54 may be used to promote coupling of light between the two waveguides 52a and 52b, by increasing the refractive index in a region between the waveguides, thus increasing coupling between the waveguides 52a and 52b.

[0091] Generally, a waveguide portion and / or a ring resonator waveguide may be formed by two or more electrodes together forming a spatial structure of the waveguide / resonator and held in a generally similar or identical electric voltage. Such two or more electrodes may be separated from each other, while forming a common waveguide. The separation between the electrodes is selected to keep the electrodes electrically insulated between them, with a distance that is generally smaller than a distance between the electrodes and the mode supported by the respective waveguide, to maintain a continuous waveguide, or smaller than a wavelength of optical radiation used within the waveguide.

[0092] Although the waveguides 52a, 52b, 54 and / or the ring resonator formed by electrode 118 are formed within the photonic layer 50, i.e., in regions where the refractive index varies based on the electric field, modes of light passing through the waveguides may extend beyond the photonic layer 50. To prevent absorption of light by the electrodes of the electrode arrangement 110, the electrodes may be formed of materials selected to suppress absorption. For example, the electrodes may be formed of crystalline material with absorption peaks distant from the wavelength of light propagating in the waveguides . In some other embodiments, the electrodes may be formed of transparent electrically conducting material such as Indium Tin Oxide (ITO), or formed of thin metal layers, e.g., having thickness in the range of tens of nanometers, making the electrodes effectively transparent. Generally, it should be understood that the electrodes of electrode arrangements 110 and 120 are configured to maintain electric voltage, while need not transfer currents, other than to accumulate the required voltage. Accordingly, the electrodes may be electrically conducting (e.g., metal electrodes), or have limited electrical conductivity such as provided in ITO electrodes or other non-metal electrodes. Further, graphite and / or graphene electrodes may also be used.

[0093] In some further embodiments, e.g., as illustrated in Fig. 2, the photonic circuit system may utilize clad layers 60 located on two facets of the photonic layer 50, between the photonic layer 50 and the electrodes 110 and 120. The clad layers 60 distance the electrodes from the optical modes of the waveguides within the photonic layer 50. This configuration may allow the use of various types and materials for the electrodes. This is partly due to the fact that the use of clad layers 60 generates a distance between the electrodes of electrode arrangement 110 and the photonic layer 50, resulting in reduced interaction / ab sorption between the optical modes of the waveguide and the electrodes. Additionally, the use of clad layers 60 makes the electric field at the photonic layer act as a far field, thus further eliminating sharp field variations due to comers or edges of the electrodes. The use of clad layers 60 also provides for distancing the electrodes from light modes supported by the waveguides, allowing various combinations of two or more electrodes to be operated together, forming a common waveguide. This enables the electrode arrangement to include point-like, or pixel electrodes (e.g., arranged as pixels in a CCD as exemplified in Fig. 11 below), which may be operated together to generate various spatial formations of photonic circuits, as described in more detail below.

[0094] As indicated above, the at least one photonic layer 50 is formed of a material having selected electro-optic properties. More specifically, the photonic layer 50 may be formed of a material having a selected, and sufficiently high, Pockels coefficient, being second order electric susceptibility / 2’. Pockels coefficients indicate the level of Pockels effects, or DC Kerr effect in the material, where an electric field applied on the material causes variations in the refractive index of the material.

[0095] Generally, the Pockels effect is an electro-optic effect in which the refractive index of an optical medium can be changed by applying an electric field on the medium. A Pockels effect is generally a linear electro-optic effect, where the change in refractive index (An) is directly proportional to the strength of the electric field (E) applied on the medium. This effect typically occurs in crystals that lack inversion symmetry, such as the materials listed above. Although Pockels coefficients measured for most known materials are quite small, various materials such as Barium titanate (BaTiO3 or BTO) exhibit a relatively large Pockels effect, characterized by a stress-free Pockels coefficient of r42~1300±100 pm / V. Ongoing research is being carried out for developing additional materials having an increased Pockels effect. Generally, the technique of the present disclosure may utilize materials having significant Pockels coefficients. For example, the material of the photonic layer may be characterized by Pockels coefficient of 100 pm / V or higher. In some examples, the material may have Pockels coefficient of 200 pm / V or higher, or 300 pm / V or higher, or 400 pm / V or higher, or 500 pm / V or higher, or 600 pm / V or higher, or 700 pm / V or higher, or 800 pm / V or higher, or 900 pm / V or higher or 1000 pm / V or higher.

[0096] The Pockels effect stems from the behavior of the electric polarization, which, in a non-linear medium, can be expressed as a power series of the electric field E. The higher order terms are the source of the variation of the dielectric constant (also associated with the refractive index) as a function of the external electric field applied onto the material. In particular, the quadratic term, ^2j,kEjEk induces the Pockels effect. The Pockels coefficients depend on the direction. They are generally different for the three principal axes of the lattice structure of the medium. Fig. 3 shows Pockels coefficients for a few selected materials. As shown, certain materials such as Gallium Arsenide (GaAs) and Silicon (Si) show minimal response to an electric field. This is while other materials such as Lithium niobate (LiNbO3), Lead zirconate titanate (PZT), and various organic compounds show increased electro optic properties. The currently known “champion” in this field is Barium titanate (BTO), having a stressfree Pockels coefficient of -1300 pm / V. It should be noted that the technique of the present disclosure is not limited to the use of a specific material and relates in general to the use of Pockels effect, and material having a significant Pockels coefficient. Such materials including, e.g., BTO and Lithium niobate, may be used for the at least one photonic layer of the photonic circuit system described herein.

[0097] Typically, to maintain light trapping within a ring resonator having no physical (etched) waveguide, the refractive index difference between the resonator waveguide and the surrounding environment should be sufficiently high to overcome output coupling (tunneling) associated with bending of the waveguide. Stress-free Pockels coefficients of Lithium niobate or of BTO can support ring resonators having a relatively large radius, to reduce bending loss.

[0098] Figs. 4A and 4B exemplify dimensions of an optical mode calculated by COMSOL (at 1000 nm wavelength), and formed in a straight waveguide generated in a BTO photonic layer (Fig. 4A) and Lithium Niobate photonic layer (Fig. 4B) under electric field generated by electrodes 110 and 120. Fig. 4A exemplifies a BTO photonic layer having a thickness of 1000 nm, and Fig. 4B exemplifies a Lithium Niobate photonic layer of equal thickness. The photonic layer is formed of a continuous layer of BTO or LN placed on clad layers formed of SiO2 followed by air environment. For example, BTO has a refractive index of n=2.4517 at a wavelength of 550 nm, and SiO2 has a refractive index of n=1.46 at a wavelength of 550 nm, or 1.4585 at a wavelength of 1550 nm. Lithium Niobate has a refractive index of n=2.3149 at wavelength of 550 nm and n=2.2128 at 1550 nm. In these simulated examples, electrodes 110 and 120 are held in a voltage difference of 1 V between them, generating an electric field of 106V / m in a direction between the electrodes. Due to the electric field, the refractive index in the BTO region between the electrodes changes by An=0.005 following a Pockels coefficient of r42=1300±100 pm / V for a wavelength of 1550 nm. The refractive index of the BTO layer on the sides of the region associated with the electrodes 110 and 120 remains unchanged and is thus lower than the refractive index between the electrodes. The LN photonic layer has lower Pockels coefficient, resulting in refractive index change of An=0.00017, and as visible in Fig. 4B the light mode is significantly larger. Data on the optical and electro- optical properties is well known, and can be found for example in A. Karvounis et al, Barium Titanate Nanostructures and Thin Films for Photonics. Adv. Optical Mater. 2020, 8, 2001249. As exemplified in Figs. 4A and 4B, to support an optical mode, the waveguide may be formed with a width of 10000 nm (or 10 microns).

[0099] These results illustrate the ability to maintain waveguides and stable modes within Lithium Niobate and BTO using applied voltage within limits of a 106V / m electric field, which prevents spontaneous discharge (the typical limit stands at 107V / m). Furthermore, using a photonic layer having 1 pm thickness, and electrodes wielding 10 pm wide waveguides, can provide flexible photonic circuits for various applications.

[0100] Reference is now made to Fig. 5 exemplifying a cross-section view of a portion of a photonic circuit system 100 according to some embodiments of the present disclosure. As shown, the photonic circuit system 100 includes at least one photonic layer 50, formed of a material having selected electro-optic properties. The photonic layer 50 may be sandwiched by clad layers 60. Additionally, the photonic circuit system 100 includes top electrode arrangement 110 and bottom electrodes 120. In this example, top electrode arrangement includes electrodes 112, and ring electrode 118, and bottom electrodes 120 includes two separate electrode regions, one being aligned with electrode 112 and a portion of ring electrode 118, and the other being aligned with another portion of ring electrode 118. Generally, the bottom electrodes 120 may be grounded, while an electric voltage may be applied to selected top electrodes 110.

[0101] The clad layer 60 is formed of a selected material having a refractive index lower than that of the photonic layer 50. When applying electric voltage between selected electrodes, e.g., electrodes 112 and 120, the electric field between the electrodes varies the refractive index of the photonic layer 50, and generates one or more waveguides, e.g., waveguide 52 and ring resonator 58. More specifically, given that the photonic layer 50 is aligned such that the electric field affects the proper axis of the photonic layer 50, the refractive index in the region between the electrodes increases and reaches the value m which is larger than , the refractive index in the region outside the electrodes, and ns the refractive index of the clad layer 60, i.e., m>n2>n3. This variation in the refractive index in the region between the electrodes yields a waveguide 52 and ring waveguide 58 in the region. Generally, in the example of Fig. 5, light may be coupled from waveguide 52 into ring waveguide 58 in accordance with the distance between them and the length of the coupling region. In some embodiments, the photonic circuit may utilize a gate waveguide, typically formed by an additional one or more electrodes. The gate waveguide may enable controlled coupling of optical signals in and out of ring waveguide 58.

[0102] Generally, due to the anisotropy of the Pockels effect, the variation of the refractive index of the photonic layer 50 depends on its orientation with respect to the electrodes. More specifically, in some materials, while the refractive index of the ordinary axis n0goes up with the electric field, the refractive index along the extraordinary axis negoes down with the electric field. Accordingly, it should be understood that the photonic circuit system described herein may be operated by orienting the field along either of these axes. For an orientation along the extraordinary axis, the waveguide is formed outside the electrode region. Furthermore, some organic and polymeric materials exhibit the Pockels effect. Such materials may be advantageously used to provide a photonic layer according to some embodiments of the present disclosure. For example, manufacturing of the at least one photonic layer 50 using organic or polymeric materials may be easier over crystalline materials.

[0103] Generally, as described above, the present disclosure provides a photonic circuit system including one or more artificial ring resonator elements. Alternatively, the present disclosure provides an artificial ring resonator element for use with a photonic circuit. According to the present disclosure, the ring resonator waveguide, and optionally additional one or more waveguides of the photonic circuit, are formed based on an electric field applied on at least one photonic layer using an electrode arrangement. The use of an electric field for affecting the refractive index of the photonic layer enables simplification of a photonic circuit and provides various photonic elements affecting optical signals transmission through the photonic circuit.

[0104] For example, Fig. 6 exemplifies a photonic circuit section including an electrode arrangement 110 and 120. The electrode arrangement includes top electrodes 110 and bottom electrodes 120. The bottom electrodes 120 may be formed as an arrangement of electrodes or as a single common electrode held at a selected (e.g., ground) potential. Top electrode arrangement 110 includes a ring electrode 118, coupling gate electrode 116 and waveguide electrode 112. When operated with selected electric voltage, the electrodes generate waveguide sections in the photonic layer regions under the electrodes.

[0105] Generally, light propagating through the waveguide generated by electrode 112 may be coupled into the ring resonator (generated by electrode 118), in accordance with operation of electrode 116 for varying the refractive index under it, and thus generating a coupling gate. This enables controlled coupling of light into and out of the ring resonator.

[0106] Figs. 7A and 7B exemplify two configurations of a photonic circuit section 250 including at least a ring resonator waveguide 254 coupled to an input / output waveguide 52. In the example of Fig. 7A the ring resonator 254 is coupled directly to input / output waveguide 52, and in the example of Fig. 7B a coupling gate 252 is used to control coupling between the waveguide 52 and ring resonator 254. While the configuration in Fig. 7A can also act as a passive delay line, the configuration in Fig. 7B can act as an active delay line. A more advanced passive delay line can be made by a spiral-type waveguide achieving a very long path. Such spiral-type waveguide may be formed using corresponding spiral- shaped electrode arrangement applying respective electric field onto the photonic layer.

[0107] Fig. 8 exemplifies a photonic circuit section 250 including two ring resonators 254a and 254b coupled to an input / output waveguide section 52. This configuration may be used to provide a selective reflecting element, where signals are filtered and the element only reflects signal portions selected in accordance with resonance frequencies (wavelengths) of the ring resonators 254a and 254b. Figs. 9A and 9B exemplify a photonic circuit section 250 configured for selective transfer of signal portions between waveguides 52 and 54. The transfer element include a ring resonator 254 placed between and coupled to two waveguides 52 and 54, such that an optical input signal in one waveguide can be transmitted through another waveguide. Selected (filtered) portions of the input signal are coupled to the resonator 254, and from there to the other waveguide. Fig. 9A exemplifies transfer between parallel waveguides and Fig. 9B exemplifies transfer between intersecting waveguides. Fig. 10 exemplifies a photonic circuit section including a first waveguide 52, coupled to a first ring resonator 254a. The first ring resonator 254a is coupled to a second ring resonator 254b, which is in turn coupled to a second waveguide 54.

[0108] The various configurations of ring resonator circuits, including one or more ring resonators and corresponding to one or more waveguides as exemplified herein, may provide various photonic circuit components, such as transmission filter, reflector filter, switches, memory elements, delay lines, narrow laser linewidth formation, and all the way to micro frequency combs.

[0109] As shown in Figs. 6-10, one or more waveguide electrodes 118 may be operated to define a respective resonator waveguide within the photonic layer. The resonator may be defined by a circular electrode 118, which, when operated to hold selected electric voltage, can define a corresponding circular waveguide within the photonic layer. Although most circular resonators (so-called micro-ring, or micro-disk, or toroid resonator) do not make use of a coupling control gate, as exemplified in the above listed references by the inventor relating to circular resonators, a coupling electrode 116 may be placed between the waveguide electrode 112 and the resonator electrode 118, forming a coupling gate 252. Voltage applied to the coupling electrode 116 can vary the refractive index with a space between the waveguide and the resonator defined by the circular electrode 118, thereby varying coupling of light into and out of the resonator. Furthermore, the coupling may be controlled by narrowing waveguide 112, so that the evanescent field is bigger, making the coupling through the gap larger. For example, waveguide 112 may be defined using an arrangement of several parallel electrodes, or a plurality of point-like electrodes (as exemplified in Fig. 11). In such configurations, width of the waveguide 52 may be controlled by selection of the number of electrodes 112 defining the waveguide. Additionally, selection of electric voltage applied on the resonator electrode 118 may be used to determine and / or fine-tune resonance frequency of the resonator and its operation characteristics. In some configurations, an optical resonator 254, such as that formed by resonator electrode 118, may be operable in a photonic circuit as a delay line, filter, reflector, switch, or other circuit elements. In some configurations, the photonic circuit system may include one or more electrodes, or sets of electrodes, configured and positioned to define, when activated with selected electric voltage, a bent or curved waveguide portion to define a delay line in a photonic circuit (e.g., in the shape of a spiral).

[0110] It should be noted that ring resonators generally require enhanced light trapping conditions as compared to straight waveguides. This is due to bending of the resonator waveguide, which may lead to bending losses. Accordingly, the higher the refractive index variation between the waveguide and the surroundings, the resonator radius may be smaller. This is while for a relatively low refractive index difference, based on what can be achieved within electrical voltage limits, and using the Pockels coefficients of available materials, a ring resonator may require a relatively large radius to maintain light trapping.

[0111] An artificial resonator can also be made of a straight waveguide portion with Bragg mirrors at each side forming a Fabry-Perot resonator, whereby the Bragg mirrors are made by modulating the electric field along the axis of the waveguide. In such a resonator there is no bending loss as noted above. This is exemplified in Figs. 15A and 15B below.

[0112] Generally, in some embodiments, the photonic circuit system of the present disclosure may utilize selected photonic structures for various applications. For example, one or more optical resonators may be used as delay lines, frequency filters, narrow linewidth lasers, micro-frequency combs, quantum memory, and various other photonic applications. Additionally, a typical waveguide structure may be operable with selected electric voltage, varying the refractive index therein, thereby affecting optical path and phase accumulated by the optical signals transmitted in the photonic circuit. High bandwidth changes in the voltage and consequently index of refraction of a resonator, enable to effectively compensate for acoustic and thermal noise prevalent in chip-scale devices.

[0113] The electrode arrangement 110 used in the different photonic circuit configurations described herein above may include various electrodes. During operation, selected electrodes may be activated using selected electric voltage to define respective waveguides, coupling between waveguides, phase affecting elements, and other optical elements required for operation of a selected photonic circuit. Additionally, elements of the photonic circuit may be changed dynamically by varying the electric voltage on the respective electrodes. This may be used for dynamically varying coupling between waveguides, phase variation along selected waveguides, or through selected phase elements, operation of selected resonators, reduce and / or compensate for noise or other fluctuations, etc. Accordingly, the photonic circuit system of the present disclosure provides flexibility in circuit design and operation and provides grounds for dynamically varying photonic circuits.

[0114] The photonic circuit system 100 according to some embodiments of the present disclosure may be configured to provide increased flexibility in supported photonic circuits. Fig. 11 exemplifies a photonic circuit system 100 according to some embodiments of the present disclosure. The photonic circuit system 100 exemplified in Fig. 11 includes an electrode arrangement 110 located on one facet of at least one photonic layer 50, where an additional one or more electrodes are located on the other facet of the at least one photonic layer 50 and are not specifically shown in Fig. 11. The electrode arrangement 110 is formed of a 2-dimentional array of a plurality of electrodes 112. The electrodes 112 are preferably of a dimension not exceeding the wavelength of light used in the photonic circuit 100 (in order not to cause reflections from changes in the index of refraction). This dimension of the electrodes 112 enables operation of a group of two or more electrodes to form together a common waveguide, or common region having an effective refractive index within the photonic layer 50. More specifically, operation of two or more electrodes to be in a selected electric voltage V, provides an electric field applied on the photonic layer 50 by the two or more electrodes 112. To provide an effectively smooth electric field within the photonic layer, a spacing between the electrodes 112 is preferably selected to be much smaller than wavelength of light used, and preferably on the order of tens of nanometers or below. Accordingly, the different electrodes 112 of electrode arrangement 110 are preferably configured with lateral (x, or y axes) dimension being below the wavelength of light used in the photonic circuit. This configuration of the electrodes enables increased (FPGA-like) flexibility in the photonic circuit structure that can be formed by the electrode arrangement 110, enabling operation of one or more selected groups of electrodes to form together a straight or curved (e.g., ring or spiral) artificial waveguide arrangement, with selected or varying widths and indices of refraction, and / or wide variety of optical elements, within the photonic layer 50. For example, the electrodes 112 may have a width and / or length being within a range between 10 and 100 nanometers, in the case of a photonic circuit that is configured for using in an infrared illumination of a wavelength of 1550 nm.

[0115] As indicated above, bending losses are one of the sources of loss in photonic circuits. Specifically, bending losses may limit the ability to maintain an optical signal within a ring resonator, and limit the possible radii of such ring resonators.

[0116] The inventors have conducted analytical calculations and numerical analysis of such ring resonators with available materials as well as with materials expected to soon be available with modest improvements of Pockels coefficient and transparency over existing materials.

[0117] Fig. 12 and 13 show analytical calculated and numerically analyzed data on Cofactor and finesse of ring resonators formed according to some embodiments of the present disclosure, where the photonic layer is formed of BTO or LiNbOs.

[0118] Fig. 12 shows analytically calculated finesse, indicative of number of turns for a disk induced by circular electrodes according to some embodiments of the present disclosure. The electrodes operate on a photonic layer of LiNbOs or BTO providing an electric field of 107V / m on a 1 pm thick photonic layer. Refractive index changes are 6n = 0.0016 for LiNbOs (dashed curve), and 6n = 0.06 for BTO. In reality, the finesse is limited by the absorption of 1 dB / cm and reaches a maximum of F ~ 27 for BTO at a ring radius of about 0.3 mm. For LiNbOs the absorption was taken as 0.28 dB / m and the finesse is limited to about F ~ 40 at a ring radius of about 57 mm. The term ‘about’ relates to standard error in numerical calculations, including variations of up to 20%.

[0119] Additionally, Fig. 13 shows analytical calculations and numerical analysis of the Quality factor of micro disk modes and the corresponding mode lifetime as a function of disk radius R and refractive index change 6n = 0.06, for BTO. Solid curves and markers relate to calculation of bending loss generally ignoring absorption loss, and horizontal dashed lines represent limits set by absorption of BTO (BaTiCh), SiN and SiCh respectively. The absorption limit is set at 1 dB / cm for BTO, 0.13 dB / cm for SiN, and 0.1 dB / km for SiO2.

[0120] The analytical calculation is based on quantum tunneling (QT) and the numerical simulation on ID Bessel functions using COMSOL. This data indicates a stable resonator mode possible for R > 40 pm, using the 6n of the BTO photonic layer. For these calculations the disk thickness was taken to be 1 pm and the electric field was taken as 107V / m. The dotted line represents the Q values corresponding to a finesse of 100 providing 100 turns of a signal within the resonator. From this plot it is clear that even with the existing Pockels coefficient of BTO, if more transparency is achieved, then small resonators, a few hundred micro-meter in size, can achieve Q-factors as high as Q=1010.

[0121] Accordingly, while BTO and LiNbOs may provide limited functionality, the technique of the present disclosure can be effectively implemented using the existing known materials. As indicated above, new materials having greater Pockel coefficients and / or lower absorption are sought in a world-wide effort and expected to be identified in the very near future in accordance with the new existing material roadmaps.

[0122] Fig. 14 exemplifies expected quality factor for ring resonators formed using the technique of the present disclosure for different ring radius between 10 and 100 pm, and for different refractive index change 6n, as expected to be soon available. As shown, for greater refractive index change, the ring resonator provides greater Q factor for smaller ring radius. For example, an index change 6n of 0.3 (just a factor 6 larger than that of BTO) already enables, for low absorption, a ring of radius 40 pm to achieve Q=1012.

[0123] In summary, the technique of the present disclosure enables a ring resonator reaching a finesse of 30 using BTO and finesse of 40 using LiNbOs photonic layers. Various material improvements, such as reducing BTO absorption to Quartz-crystal level (e.g., 1.7 dB / km), will provide a finesse of 106at radius of R=400 pm (with existing BTO Pockels coefficient). The use of materials having refractive index change of 0.3 will provide a finesse of 106for ring resonators having a radius of R=40 pm.

[0124] Furthermore, as indicated above an optical (artificial) resonator can also be configured of a straight waveguide portion with back and front reflectors (e.g., Bragg mirrors at each side). This configuration may form a Fabry-Perot resonator, whereby, according to the present disclosure, the front and back reflectors (Bragg mirrors) are made by modulating the electric field along the axis of the waveguide at two selected regions along the waveguide portion. In this case, the problem of bending losses disappears, and the optical resonator may be formed at desired dimension using available materials such as Lithium Niobate. The linear optical resonator according to some embodiments of the present disclosure provide various advantages over the conventional photonic linear resonators including for example: 1. The edge roughness is again a limiting factor and as explained above, the use of electric field applied on a photonic layer eliminates, or at least significantly reduce edge roughness, 2. The use of electric field for generating and / or modulating the mirrors, enables selectively varying reflectivity of the mirror by varying voltage applied to the respective electrodes, thereby controlling when the light leaves the cavity. 3. Changing the waveguide index of refraction within the waveguide section defining cavity of the resonator by varying electric field applied by the respective electrodes can change the effective length of the cavity thereby having the ability for high bandwidth changes to the resonance wavelength (e.g., to fight acoustic and thermal noise). Reference is thus made to Figs. 15A and 15B exemplifying a straight (e.g., Fabry- Parot) resonator according to some embodiments of the present disclosure. Fig. 15A illustrates layered structure of the straight resonator exemplifying the electrode arrangement 110 and 120, Fig. 15B illustrates a top view exemplifying the effective waveguide region including regions of selected refractive index variations resulting in the resonator structure.

[0125] In the example of Fig. 15A, a region of a photonic layer is used to define a waveguide using electrode arrangement 110 (top) and 120 (bottom). Electrodes 116 extend beyond the region of the page defining the waveguide, electrodes 114 define front and back partly reflecting regions or interfaces of the resonator, and electrode 112 defines the resonator cavity. Fig. 15B illustrates a top view of a similar structure, where waveguide 52 is defined by electrodes 116, resonator interfaces 256 and 258 are defined by electrodes 114 and resonator cavity 250 is defined by electrode 112. Straight / linear resonators may provide various features within a photonic circuit, while eliminating issues associated with bending loss. Such straight resonator 350 can be formed with any available material such as e.g., Lithium Niobate, having sufficient Pockels coefficient and limited absorption.

[0126] Generally, electrodes 114 may be formed of a number of electrodes generating a structured interface such as a dichroic or Bragg reflector. This may be used to control reflection / transmission properties of the interfaces 256 and 258 providing control over Cofactor of the resonator 350, resonance wavelength, etc.

[0127] Generally, an arrangement of pixel-like electrodes may be operable together to form a ring resonator waveguide or a straight waveguide resonator within a photonic circuit. This enables improved flexibility in circuit configurations and allows operation of a photonic circuit in accordance with a digital map of the circuit layout.

[0128] Accordingly, the present disclosure provides a photonic circuit system utilizing at least one photonic layer and at least one electrode arrangement positioned to apply an electric field on selected regions of the photonic layer. The use of an electric field applied by selected electrodes of the electrode arrangement defines a selected arrangement of one or more waveguides, where strength of the electric field is used to determine refractive index variation in the respective regions of the photonic layer. In some embodiments, the selected arrangement of one or more waveguides is formed without etching or structural manipulation of the photonic layer.

[0129] As indicated above, the photonic circuit and photonic resonators according to various embodiments of the present disclosure may provide for various applications including for example, elements of photonic circuits, delay lines, filters, etc. Additionally, such resonators and specifically ring resonators may provide a reference standard to frequency lock other devices such as lasers, operate as a sensor or detector for single or minute amounts of particles, or operate as a quantum device, e.g., frequency comb, or entangler of photons and / or atoms.

[0130] In this connection, atomic clocks, which currently provide the best frequency standard, utilize short and long-term stability control modules. The long-term stability is provided by the atoms while the short-term stability is provided by a high-finesse optical cavity. Currently such cavities are very large and cumbersome. The use of a high-finesse photonic resonator with high stability made in a chip-scale device in accordance with the above-described technique, may replace the present-day large cavity in frequency standards providing enhanced short-term stability control.

[0131] Further, as described in the provided references of Rosenblit et al., a ring resonator may form a detector (sensor) for minute amounts of materials down to a single atom. Hence, creating a high-quality chip- scale ring resonator will enable a high-performance chip-scale sensor for minute amounts of materials (e.g., contaminants). Additionally, frequency combs are currently an essential part of any optical- frequency atomic clock. However, currently they are large and cumbersome. Generally, a frequency comb may be formed by non-linear phenomenon inside a waveguide. However, for this to happen large light intensities are required. A high finesse resonator creates exactly these high intensities as it accumulates light which enters the cavity (resonator), so that such a photonic resonator according to some embodiments of the present disclosure may form a miniature frequency comb. Such nonlinear effects happen with high light intensities even in materials with weak nonlinear properties, such as those currently demonstrated with SiO or SiN.

[0132] It is to be noted that the various features described in the various embodiments can be combined according to all possible technical combinations.

[0133] It is to be understood that the invention is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Hence, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception upon which this disclosure is based can readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the presently disclosed subject matter.

[0134] Those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore described without departing from its scope, defined in and by the appended claims.

Claims

CLAIMS:

1. A photonic circuit comprising at least one resonator element, the at least one resonator element comprising at least one photonic layer formed of a material having selected electro-optic properties, an electrode arrangement comprising at least a first top electrode arrangement and a second bottom electrode arrangement positioned at first and second opposite facets of the at least one photonic layer; wherein at least one of the first top electrode arrangement and the second bottom electrode arrangement comprises an electrode arrangement defining a ring structure, and wherein, in response to electric potential applied to one or more electrodes of the arrangement of electrodes, a refractive index varies in a respective region of the at least one photonic layer affected by the one or more electrodes, selectively defining a ring-shaped waveguide within the at least one photonic layer.

2. The photonic circuit of claim 1, wherein at least one of the first top electrode arrangement and the second bottom electrode arrangement comprises a circular electrode defining a ring resonator waveguide.

3. The photonic circuit of claim 1 or 2, further comprising first and second clad layers located at first and second surfaces of the at least one photonic layer, the first clad layer is located between the at least one photonic layer and the first top electrode arrangement, and the second clad layer is located between the at least one photonic layer and the second bottom electrode arrangement.

4. The photonic circuit of any one of claims 1 to 3, wherein at least one of the first top electrode arrangement and second bottom electrode arrangement comprises a plurality of independently controlled point-like electrodes positioned on at least one facet of the at least one photonic layer, and wherein applying electric voltage to a set of point-like electrodes along a selected region defines a photonic circuit in which light propagates within a respective region of the at least one photonic layer.

5. The photonic circuit of any one of claims 1 to 4, wherein the selected electro-optic properties comprise a Pockels coefficient defining refractive index variation in response to a DC electric field.

6. The photonic circuit of claim 5, wherein said at least one photonic layer comprises a material having at least one Pockels coefficient of 100 pm / V or higher.

7. The photonic circuit of any one of claims 1 to 6, wherein the at least one photonic layer is formed of Lithium Niobate or BTO (BaTiCh).

8. The photonic circuit of any one of claims 1 to 7, further comprising a control unit comprising at least one processor and memory circuitry, the control unit being electrically connected to electrodes of the first top electrode arrangement and electrodes of the second bottom electrode arrangement and configured to selectively apply electric voltage to the electrodes for operating the at least one resonator element.

9. The photonic circuit of claim 8, wherein the control unit is further configured for selectively applying electric voltage to the electrodes for determining one or more resonance wavelengths of the at least one resonator element.

10. The photonic circuit of any one of claims 1 to 9, wherein the electrode arrangement comprises electrodes formed of transparent electrically conducting material.

11. The photonic circuit of any one of claims 1 to 10, wherein the electrode arrangement comprises metallic electrodes, wherein the metallic electrodes are sufficiently thin to eliminate absorbing of light by the metallic electrodes.

12. The photonic circuit of any one of claims 1 to 11, wherein the electrode arrangement comprises crystalline electrodes characterized by narrow absorption peaks.

13. The photonic circuit of any one of claims 1 to 12, further comprising at least one top gate electrode located in vicinity of the first top electrode arrangement defining the at least one resonator element, such that applying electric voltage to the at least one top gate electrode provides for a varying refractive index in a region of the photonic layer, thus operating as a gate for coupling optical signals to the at least one resonator element.

14. The photonic circuit of any one of claims 1 to 13, wherein the at least one resonator element is operable as a filter.

15. The photonic circuit of any one of claims 1 to 14, wherein the at least one resonator element is operable as a switch.

16. The photonic circuit of any one of claims 1 to 15, wherein the at least one resonator element is operable as a reflector or delay line.

17. The photonic circuit of any one of claims 1 to 16, wherein the at least one resonator element is operable as a reference standard to frequency lock on.

18. The photonic circuit of any one of claims 1 to 17, wherein the at least one resonator element is operable as a sensor or detector for single or minute amount of particles.

19. The photonic circuit of any one of claims 1 to 18, wherein the at least one resonator element is operable as a quantum device, e.g., frequency comb, or entangler of photons and / or atoms.

20. The photonic circuit of any one of claims 1 to 19, comprising at least first and second ring resonator elements coupled between them, wherein each of the first and second resonator elements is defined by at least a first top electrode arrangement having a circular portion.

21. The photonic circuit of any one of claims 1 to 20, further comprising at least one additional electrode arrangement configured to define at least one linear waveguide configured to couple optical signals into or out of the at least one resonator element.

22. The photonic circuit of any one of claims 1 to 21, wherein variation in electric potential applied to one or more electrodes of the arrangement of electrodes enables controlled variation of resonance wavelength of the at least resonator element.

23. A photonic circuit comprising at least one resonator element, the at least one resonator element comprising at least one photonic layer formed of a material having selected electro-optic properties, an electrode arrangement comprising at least a first top electrode arrangement and a second bottom electrode arrangement positioned at first and second opposite facets of the at least one photonic layer; wherein at least one of the first top electrode arrangement and the second bottom electrode arrangement comprises an electrode arrangement defining at least first and second discontinuity regions in electrodes along an elongated region; and wherein, in response to electric potential applied to one or more electrodes of the arrangement of electrodes, a refractive index in a respective region of the at least one photonic layer is affected by the one or more electrodes, selectively defining at least first and second refractive index interfaces associated with said first and second discontinuity regions, defining said resonator within the at least one photonic layer.

24. The photonic circuit of claim 23, wherein said resonator is defined by straight waveguide electrode portion and two or more electrodes defining said at least first and second discontinuity regions and configured to vary refractive index at two ends of the straight waveguide electrode portion, thereby defining a straight waveguide portion with Bragg mirrors at each side forming a Fabry-Perot resonator.

25. The photonic circuit of claim 23 or 24, further comprising first and second clad layers located at first and second surfaces of the at least one photonic layer, the first clad layer is located between the at least one photonic layer and the first top electrode arrangement, and the second clad layer is located between the at least one photonic layer and the second bottom electrode arrangement.

26. The photonic circuit of any one of claims 23 to 25, wherein at least one of the first top electrode arrangement and second bottom electrode arrangement comprises a plurality of independently controlled point-like electrodes positioned on at least one facet of the at least one photonic layer, and wherein applying electric voltage to a set of point-like electrodes along a selected region defines a photonic circuit within which light propagates within a respective region of the at least one photonic layer.

27. The photonic circuit of any one of claims 23 to 26, wherein the selected electro-optic properties comprise a Pockels coefficient defining refractive index variation in response to a DC electric field.

28. The photonic circuit of any one of claims 23 to 27, wherein the at least one photonic layer is formed of Lithium Niobate or BTO (BaTiCh).

29. The photonic circuit of any one of claims 23 to 28, further comprising a control unit comprising at least one processor and memory circuitry, the control unit being electrically connected to electrodes of the first top electrode arrangement and electrodes of the second bottom electrode arrangement and configured to selectively apply electric voltage to the electrodes for operating the at least one resonator element.

30. The photonic circuit of claim 29, wherein the control unit is further configured for selectively applying electric voltage to the electrodes for determining one or more resonance wavelengths of the at least one resonator element.

31. The photonic circuit of any one of claims 23 to 30, wherein the electrode arrangement comprises electrodes formed of non-absorbing electrically conducting material.

32. The photonic circuit of any one of claims 23 to 31, wherein the electrode arrangement comprises metallic electrodes, wherein the metallic electrodes are sufficiently thin to eliminate absorbing of light by the metallic electrodes.

33. The photonic circuit of any one of claims 23 to 31, wherein the electrode arrangement comprises crystalline electrodes characterized by narrow absorption peaks.

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