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A novel class of programmable integrated photonic circuits has emerged over the past years, strongly driven by approaches to tackle unsolved computing problems in the optical domain. Photonic neuromorphic and quantum computing are examples of optical systems implemented in complex photonic circuits, which are reconfigured before and during operation. However, a key building block to enable efficient reconfigurable optical network architectures is still missing: a non-volatile optical phase shifter. Here we demonstrate such an element—compatible with silicon photonics—based on the monolithic integration of BaTiO3 thin films with silicon waveguides. By manipulating ferroelectric domains in BaTiO3 with electrical control signals, we achieve analogue and non-volatile optical phase tuning with no absorption changes. We demonstrate an eight-level long-term-stable photonic device with non-destructive optical readout and switching energy as low as 4.6 pJ. With our results, an analogue non-volatile photonic element is added to the integrated photonics toolbox, enabling a new generation of power-efficient programmable photonic circuits. ISSN:1749-4885 ISSN:1749-4893

The explosive rise of silicon photonics has led to renewed interest in the electro-optic (EO) or Pockels effect due to its potential uses in many next generation device applications. To find materials with a strong EO response in thin film form, which are essential for low power and small footprint devices, one needs to find a general design rule for strong Pockels materials. To elucidate what makes the Pockels effect strong, we study the effect in LiB3O5 (LBO) and CsB3O5 (CBO) and use these materials as prototypical examples of where conventional wisdom breaks down. We find the Pockels tensor components to be extremely small in both materials, despite the large degree of anharmonicity in the crystals, which has been used as a proxy for the presence of nonlinear electronic effects. We relate the lack of EO response to the large optical phonon frequencies (despite the relatively large Raman susceptibility) in LBO and to the small Raman susceptibility (despite the low phonon frequencies) in CBO, respectively. We shed light on the underlying physical phenomena behind the Raman susceptibility, which we find to be intimately linked to the electron–phonon coupling strength of the near-edge electronic states, and identify a route to discovering new strong EO materials.

Different surface preparation methods for cleaning MgO, a widely used substrate in oxide epitaxy, are summarized and compared. We find that in situ surface preparation methods are preferable to ex situ preparation methods. We show that the complete removal of hydroxide, carbonate, and adventitious carbon from the MgO surface can be achieved via oxygen plasma exposure at 200 °C without high temperature annealing. Using this process, an atomically flat surface with root mean square roughness values of ∼0.1 nm is demonstrated. Surfaces treated thus also exhibit sharp RHEED streaks indicating good crystalline order of the surface. We also show that high temperature annealing of MgO, either by itself or following other ex situ cleaning methods, such as solvent cleaning, is a reasonably effective method for the removal of surface contaminants, enabling one to achieve a surface roughness of ∼0.2 nm. We show that wet etching or other ex situ cleaning methods alone without annealing cannot eliminate all surface contaminants and may even worsen the surface roughness significantly.

SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. The reactivity of the substrate with respect to the film during deposition, particularly with regard to redox reactions, has typically been glossed over. We demonstrate by depositing a variety of metals (Ti, Al, Nb, Pt, Eu, and Sr) and measuring the in situ core level spectra of both the metal and SrTiO3 that, depending on the oxide formation energy and work function of the metal, three distinct types of behavior occur in thin metal films on SrTiO3 (100). In many cases, there will be an interfacial layer of oxygen-deficient SrTiO3 formed at the interface with the overlying film. We discuss how this may affect the interpretation of the well-known two-dimensional electron gas present at the interface between SrTiO3 and various oxides.