PZT based acoustic resonator for the refractive index modulation Irfan Ansari Photonics Research Group INTEC dept Ghent University-IMEC Email: irfan.ansari@ugent.be Jeroen Beeckman Liquid Crystal & Photonics Group ELIS dept Ghent University Dries Van Thourhout Photonics Research Group INTEC dept Ghent University-IMEC Abstract—We propose a PZT based acoustic resonator to modulate the refractive index of a Silicon waveguide. We use an inter-digital transducer (IDT) defined on the PZT film to create a standing acoustic wave resonance in the suspended device. We calculate the effective index change from the acousto-optic overlap integral and illustrate its dependency on the IDT pitch. Index Terms—Acousto-optic, PZT, Silicon photonics, Ferro- electric thin film, Microwave photonics. I. I NTRODUCTION Integrated acousto-optic devices are emerging as a potential candidate to achieve efficient microwave to optical conversion [1]. In such devices, the actuated acoustic waves dynami- cally perturb the refractive index of the material through the photoelastic effect. This perturbation has been achieved with both travelling waves and standing waves. However in case of travelling waves, only partial acoustic energy is utilised for the index modulation, and most of the energy propagates away. On other hand, standing waves in a resonance based device have been demonstrated to achieve higher modulation efficiency [1]. Realization of such acousto-optic devices has been demon- strated mostly on piezoelectric active photonic platforms such as LiNbO 3 and GaAs. In case of the silicon photonics plat- form, integration of a piezoeletric thin film is needed, for example AlN [2]. Recently, a sol-gel process using an optically transparent buffer layer (LaO 2 CO 3 ) to grow a PZT thin film was reported [3]. This opens the possibility to directly integrate a PZT thin film on Si PICs. In this work, we analyse a PZT based acoustic resonator directly integrated on a Si waveguide, as shown in figure 1. We numerically investigate the interaction between the IDT actuated acoustic field and the optical field in the waveguide and calculate the strain induced index change through the acousto-optic overlap integral. II. PHOTOELASTIC EFFECT The strain field associated with the acoustic waves perturbs the effective refractive index (n eff ) in the Si waveguide through the photoelastic effect. This index change can be described as follows [1]: Δn eff = n 3 eff 2  D E * pS E dr  D E * E dr (1) Fig. 1. a 2D schematic of the suspended IDT/PZT/Si waveguide stack. The thickness of the IDT, PZT-layer and Si waveguide is 70 nm, 200 nm and 220 nm respectively. The width of the Si waveguide is 450 nm. The number of finger-pairs in the IDT is 4. Here D is the 2D cross-section of the waveguide, E is the electric field of the waveguide mode, p is the photoelastic tensor of the waveguide medium and S is the strain field induced by the acoustic wave. Equation 1 can be approximated as: Δn eff = n 3 eff 2  D  (p 11 s xx + p 12 s yy )E 2 x +(p 21 s xx + p 22 s yy )E 2 y  dr  D E * E dr (2) In our case, since the light propagates along the Si [110] direc- tion, we apply a rotational transformation on the photoelastic tensor of Si taken from [4]. Thus we obtain the following coefficients that we used for calculating the overlap integral: p 11 = -0.090, p 12 = 0.013, p 21 = p 12 and p 22 = p 11 . III. ACOUSTO- OPTIC SIMULATION The schematic of the proposed device is shown in figure 1. We define an IDT with 4 finger-pairs on the PZT film to actuate acoustic waves. We set the lateral distance between edge-IDT, IDT-waveguide, and waveguide-edge as 3 μm,2 μm and 3 μm respectively. We set the PZT thickness as 200 nm, and the waveguide width and thickness as 450 nm and 220 nm respectively. In the first step of the simulation, we calculate the electric field from the IDT through an electrostatic simulation. Then, we align the PZT domain polarization along these electric field lines. This is done to account for the process whereby the PZT is poled with the IDT itself as discussed in [5]. In the second step, we apply an RF input signal (amplitude 1V) to the