Browsing by Subject "Pockels effect"

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Oxide materials at the two-dimensional limit
(2017-05-05) Kormondy, Kristy Joy; Demkov, Alexander A.; Lai, Keji; De Lozanne, Alejandro L; Tsoi, Maxim; Ekerdt, John G
Emergent phenomena in transition metal oxide films are receiving considerable attention with the development of techniques for the preparation of well-controlled oxide surfaces. On the macroscopic scale, such display novel physics phenomena including superconductivity, magnetism, ferroelectricity, and more. On the nanometer scale, the properties of epitaxial interfaces are further impacted by strain, band alignment, and crystal imperfections that may affect the long-range as well as the short-range order. Furthermore, symmetry lowering at the interface creates entirely new environments that are not accessible in the bulk environment. Thus, thin-film oxide materials are increasingly important in many applications. My work focuses on epitaxial oxides of the perovskite, spinel, and rocksalt structure and covers two main phenomena: (1) the two-dimensional electron gas at epitaxial oxide interfaces, and (2) thin epitaxial electro-optic oxides. Because polar oxides are of prominent interest as a mechanism for the formation of the two-dimensional electron gas, I start with a study of polar semiconductor Co₃O₄. Ellipsometry reveals a direct band gap of 0.75 eV, and magnetic measurements show the signature of antiferromagnetic ordering at 49 K, higher than the typical bulk value. Next, I look closer at the role of defects by studying the highly conducting layer at the crystalline [gamma]-alumina/SrTiO₃ (STO) interface which is attributed to oxygen vacancies. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator. Building upon these results, I show that even at room temperature, out-diffusion of oxygen from SrTiO₃ during epitaxy of highly spin-split semiconductor EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO₃ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas for the development of novel nano-oxide spintronic devices. The large effective Pockels coefficient for high-quality epitaxial BaTiO₃ (BTO) films on Si distinguishes BaTiO₃ as a highly promising material for integrated silicon nanophotonics. However, the linear electro-optic effect in BaTiO₃ thin films determined in previous experiments clearly shows deteriorated properties compared to bulk BTO crystals. First, I study BaTiO₃ films of varied thickness in order to quantify the Pockels coefficient with respect to crystalline orientation. As a next step, I report on the strong dependence of the Pockels effect in BaTiO₃ thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. The 25× enhancement of the Pockels coefficient indicates a promising route to increase the performance of nonlinear oxides in the two-dimensional limit for the development of novel hybrid silicon photonic platform.
The origins of strong Pockels responses
(2019-12) Hamze, Ali Kassem; Demkov, Alexander A.; Chelikowsky, James R; Ekerdt, John G; Niu, Qian; Downer, Michael C
The linear electro-optic (Pockels) effect, which relates the change in the index of refraction of a crystal to an applied electric field, has been subject to increasing study in recent years due to its potential applications in integrated photonics, which include interchip optical interconnects, neuromorphic computing, and photonic chips for quantum computing. The current “gold standard” Pockels-active material is LiNbO₃, which sees wide use as an optical modulator in the telecommunications industry. However, LiNbO₃ has a small Pockels response (~30 pm/V) and does not integrate well with Si. Therefore, finding other Pockels-active materials is of great importance for their potential use in future devices. Most current studies are focused on BaTiO₃, which has an enormous response (~1600 pm/V) in bulk, and which can be epitaxially integrated on silicon (001). However, it of great technological importance to find other strong Pockels materials and to understand the underlying physical principles which drive strong Pockels responses. In this work, we calculate the Pockels response of a wide variety of materials from first principles. We show that SrTiO₃, a centrosymmetric crystal which ordinarily cannot exhibit a Pockels response, can be made to have a strong response through epitaxial strain. The phonon modes driving the large response in SrTiO₃ are very anharmonic. Noticing that other strong Pockels materials are also strongly anharmonic, we investigate whether crystal anharmonicity in non-centrosymmetric crystals is a predictor of strong Pockels responses. We do this by through an in-depth study of LiB₃O₅, which has thermal anharmonicity an order of magnitude larger than that of BaTiO₃ or SrTiO₃. We find that crystal anharmonicity (or rather, soft phonon modes) is a necessary, but not sufficient requirement for strong Pockels responses. Large Raman susceptibilities, which we associate with strong electron-phonon interactions and large deformation potentials, are also required. Finally, we summarize unpublished calculations of the Pockels response of a variety of crystals, many of which have not been considered for the electro-optical applications, and we provide suggestions for future first-principles studies of the Pockels effect