A plasmonic modulator based on metal-insulator-metal waveguide with barium titanate core

We design a plasmonic modulator which can be utilized as a compact active device in photonic integrated circuits. The active material, barium titanate (BaTiO3), is sandwiched between metal plates and changes its refractive index under applied voltage. Some degree of switching of ferroelectric domains from the in-plane to out-of-plane orientation provides the change of the refractive index, which can be exploited for effective light modulation. By numerical analysis we prove that the π phase change can be achieved with a 12...15μm length device having propagation losses 0.05...0.2dB/μm. Utilizing plasmonic waves in nanostructures allows efficient manipulation of light on the subwavelength scale [1-3]. In particular, plasmonic modulators and switches are of major interest for ultra-compact photonic integrated circuits [4]. Several promising layouts have been proposed recently, which outperform conventional photonic-waveguide-based designs [5-11]. A metal-insulator-metal (MIM) waveguide provides the most compact layout [12-15]. Efficient performance is achieved because of the high field confinement between the metal layers which also serve as electrodes. For such waveguides, the mode is strongly localized within the core. Although high performance was predicted and experimentally demonstrated, a detailed characterization of the devices encounters several problems, mainly because of the small mode size and high insertion losses. Recently, it has been shown that the efficient coupling from a photonic waveguide to an MIM structure can be realized to launch the signal [16]. Different active materials have been widely studied over the last several years. Transparent conducting oxides provide a large change of refractive indexes and can be utilized for fast signal modulation [5,17-20]. However, they possess high losses, consequently the modal propagation length is fairly small [17,19]. Another approach is to implement gain materials and directly control the absorption coefficient [21-23]. However, such materials can increase the noise level. Barium titanate (BaTiO3) was shown to provide high performance for photonic thin film modulators [24-27]. Here we propose the implementation of BaTiO3 as the * E-mail: [email protected] active core of a MIM waveguide. Under applied voltage, some part of ferroelectric domains can be switched from the in-plane (with an ordinary refractive index no) to outof-plane orientation (with extraordinary index ne) [28, 29]. Thus, the refractive index for a field polarized along one axis can be changed, and control of a propagating signal is achieved. Different voltage provides a different degree of domain switching, and thus the required level of modulation can be realized. A similar option for electrooptic modulation was studied in plasmonic interferometers [30]. A schematic view of the MIM waveguide with BaTiO3 is shown in Fig. 1. We compare three metals: silver, gold, and aluminum. We are interested in modulation on the telecom wavelength only, λ0 = 1.55μm, so metal parameters are fixed: εAg = –129 + 3.3i [31], εAu = – 115 + 11.3i [31], and εAl = –240 + 49i [32]. Silver and gold conventionally show the best plasmonic performance. However, they are not compatible with CMOS fabrication facilities. Aluminum possesses higher losses, but it is CMOS-compatible and can be adopted in manufacturing. BaTiO3 is highly birefringent with the refractive index difference Δn = 0.05. Thus, we consider no = 2.35 and ne = 2.3 [27]. We solve the dispersion equation of a threelayer structure with εxx = no 2 and εzz = ne 2 tensor components of the permittivity of the core (see for example [33]). It corresponds to the device off-state. Under applied voltage, domains are switched and both tensor components become equal εxx = εzz = ne 2 . Fig. 1. Metal-insulator-metal waveguide as plasmonic modulator. A plasmonic modulator based on a metal-insulator-metal waveguide with a barium titanate core Viktoriia E. Babicheva * and Andrei V. Lavrinenko DTU Fotonik, Technical University of Denmark, Oersteds Plads 343, 2800 Kgs. Lyngby, Denmark Received May 13, 2013; accepted June 28, 2013; published June 30, 2013 doi: 10.4302/plp.2013.2.08 PHOTONICS LETTERS OF POLAND, VOL. 5 (2), 57-59 (2013) http://www.photonics.pl/PLP © 2013 Photonics Society of Poland 58 We solved the dispersion equation for different core thicknesses d = 50...500nm. Figures 2-3 show the changes of propagation constant β and absorption coefficient α within the switch. The extinction coefficient of BaTiO3 is negligible, and consequently, the absorption coefficients for onand off-states are almost the same (see Fig. 3). In contrast, the variations of β are highly pronounced: β varies in the range 9.5...11.5μm -1 with corresponding effective index neff = 2.34...2.84. Thus such a mode is suitable for the signal phase control. Fig. 2. Propagation constant for MIM waveguides with different metals. Fig. 3. Absorption coefficient for MIM waveguides with different metals. We calculated the length of the waveguide required to achieve the π phase change between the onand off-states: Lπ = π/(βoff – βon). (1) It has values around 13...15μm in the broad range of d (Fig. 4). The propagation losses are 0.05...0.2dB/μm. Thus, with such a short device length Lπ the modulator has relatively high transmittance: