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Self-holding optical actuator based on a mixed ionic-electronic conductor material Marina Muñoz Castro, Nicolai Walter, Jan K. Prüßing, Wolfram HP Pernice, and Hartmut Bracht ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01708 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019
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Self-holding optical actuator based on a mixed ionic-electronic conductor material Marina Muñoz-Castro,∗,† Nicolai Walter,‡ Jan K. Prüßing,† Wolfram Pernice,‡ and Hartmut Bracht∗,† †Institute of Materials Physics, University of Münster, Wilhelm-Klemm-Str.10, 48149 Münster, Germany. ‡Institute of Physics, University of Münster, Heisenbergstraße 11, 48149 Münster, Germany E-mail:
[email protected];
[email protected] Abstract A new approach for an optical actuator system based on mixed ionic-electronic conductor materials is proposed. The system actuates on light propagating in a waveguide implemented on a photonic integrated circuit by electrochemically changing the composition of the MIEC material, using the characteristic dependence of the optical properties upon stoichiometry. To realize this actuator, a multilayer stack was sputtered and characterized forming a battery-like system where ions reversibly travel from a Liion source to a Lix V2 O5 layer producing the desired change of the optical properties. Modal field FEM simulations were carried out to estimate the influence of the formed actuator on a waveguide fundamental mode implemented on the silicon on insulator platform. The time resolution of the actuator is estimated solving the diffusion profile of Li inside the Lix V2 O5 coating, and its evolution with time. Through simulations and measurements, promising results for a potential actuator system are shown, like small
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device length (< 20 µm), low power consumption (∼ 10 pW per switch), reversibility and long time stability.
Keywords optical actuator, optical properties, Li-ion battery, electrochemistry, silicon waveguide, photonic integrated circuit In the last decades, silicon has come forward as one of the most promising platforms for integrated optical devices. 1–3 While silicon photonics has been mainly present in the data-processing 4,5 and telecommunication industries, 6,7 still new applications such as phased antenna arrays 8,9 and quantum photonic circuits 10 are attracting major attention. All these applications demand a large number of integrated devices and functionalities, similarly to common electronic platforms. However, despite the progress in fabrication technology, the application of silicon photonic integrated circuits (PICs) is hindered by the lack of adequate tools to overcome a number of critical issues, such as functional drifts (aging, environmental conditions) or crosstalk effects (thermal, optical, electrical), all of which need to be addressed in order to set and maintain the circuit to the desired performance. To solve these issues, one key point is the control of light transmission along the circuit in a non-volatile fashion. In a PIC, modulation of light propagation is required for either: (i) Circuit reconfiguration: enabling fast and reliable adjustment of the optical path through a PIC or to fine-tune light transmission in a reversible and stable way; 11–13 (ii) Post-fabrication trimming (PFT): to modify permanently the circuit properties to compensate for inevitable fabrication deviations. 14–16 The full functionality of integrated photonic chips requires a new strategy to tackle the fine-tuning of light propagation in a reversible, stable way, with minimum power consumption. In this context, self-holding optical actuators have been recently proposed for silicon photonic waveguides; these actuators can maintain the switching state without "always on" power dissipation exploiting phase change materials, like GeSbTe compounds, 17
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insulator-metal phase transitions in vanadium dioxide (VO2 ), 18 and memristor-like plasmonic structures. 19
Actuator proposal In this work, we investigate a new mechanism to realize self-holding optical actuators based on the switching properties of mixed ionic-electronic conductors (MIECs). In MIECs, the local stoichiometry of the material can be modified by electrochemical injection of ions and electrons. 20 This mechanism can be exploited to realize an electrically driven self-holding optical actuator for PICs. We envision a thin film battery-like system with the MIEC as upper cladding of the silicon waveguide. In this device, the ion-migration effects and redox processes driven by an applied voltage will induce a controllable, self-holding, and reversible change of the optical properties of the coating material. Applying an external voltage between the electrodes, the composition of the cladding material is changed (Mx B + δM + + δe− Mx+δ B) along with the optical properties. Figure 1 depicts the proposed optical actuator. By modulating the optical properties of the cladding, the light propagating in the waveguide is affected through near-field coupling of the evanescent tail of the waveguide mode. This switching mechanism has been, to the best of our knowledge, neither explored nor exploited in PICs.
V2O5 as actuator material To test the proposed modulation mechanism, we chose V2 O5 as MIEC material, since it is a well-known cathode material for Li ion batteries. 21–26 Its layered structure can uptake/release a great amount of ions, and its optical properties are very sensitive to changes in stoichiometry. 27–29 In particular, the modulation of the refractive index (n) and the extinction coefficient (k) of the optical medium is of interest, since these parameters affect directly the phase and intensity of the light wave. 30 3
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Figure 1: Proposed optical actuator. Voltage applied between the metal electrodes alters the composition of the MIEC material producing a change in its optical properties. This will ultimately affect the light propagating inside the Si waveguide.
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As a starting point, we studied the change of the optical properties of Lix V2 O5 (0
