Sava Denev

Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

Linear and nonlinear optical properties of BiFeO3

Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02, d_33/d_22 = -11.4 +/- 0.20 and |d_22| = 298.4 +/- 6.1 pm/V at a fundamental wavelength of 800 nm.

Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO3

A prominent band centered at ∼1000–1300cm−1 and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO3 thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, TN∼375°C. This band is composed of three peaks, assigned to 2A4, 2E8, and 2E9 Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E8 mode, in particular, nearly disappears at TN on heating, indicating a strong spin-two-phonon coupling in BiFeO3.

Spin-charge-lattice coupling through resonant multimagnon excitations in multiferroic BiFeO3

Spin-charge-lattice coupling mediated by multimagnon processes is demonstrated in multiferroic BiFeO3. Experimental evidence of two- and three-magnon excitations as well as multimagnon coupling at electronic energy scales and high temperatures are reported. Temperature dependent Raman experiments show up to five resonant enhancements of the two-magnon excitation below the Neel temperature. These are shown to be collective interactions between on-site Fe d-d electronic resonance, phonons, and multimagnons.

Probing mixed tetragonal/rhombohedral-like monoclinic phases in strained bismuth ferrite films by optical second harmonic generation

Epitaxial strain can induce the formation of morphotropic phase boundary in lead free ferroelectrics like bismuth ferrite, thereby enabling the coexistence of tetragonal and rhombohedral phases in the same film. The relative ratio of these phases is governed by the film thickness and theoretical studies suggest that there exists a monoclinic distortion of both the tetragonal as well as the rhombohedral unit cells due to imposed epitaxial strain. In this work we show that optical second harmonic generation can distinguish the tetragonal-like phase from the rhombohedral-like phase and enable detection of monoclinic distortion in only a pure tetragonal-like phase.

Large nonlinear optical coefficients in pseudo-tetragonal BiFeO3 thin films

Biaxial strain induces a phase transition from a pseudo-rhombohedral (R) to pseudo-tetragonal (T) phase in BiFeO3 (BFO) thin films. Using optical second harmonic generation, we measure the nonlinear optical dij coefficients at a fundamental wavelength of 1550 nm for R and T-BFO thin films. A large increase of the dij magnitudes is observed for T-BFO in comparison to R-BFO. The dij magnitudes for T-BFO were measured to be: |d33|=18.1±2.4, |d31|=60.8±8.1, and |d15|=47.0±4.2, and for R-BFO: |d33|=15.1±2.1, |d31|=8.5±1.2, |d15|=0.9±0.1, and |d22|=18.7±2.6 (pm/V). The strain-enhanced nonlinear optical properties of T-BFO thin films make them potentially useful for optical applications.

Linear and nonlinear optical properties of multifunctional PbVO3 thin films

Lead vanadate (PbVO3) is a multifunctional material which is both polar and magnetic. Its optical properties, important for linear and nonlinear optical spectroscopy of the material, are presented. Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of lead vanadate thin films at 295K is reported. Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d15∕d31=0.20±0.02, d33∕d31=316.0±4.4, and ∣d33∣=10.40±0.35pm∕V at a fundamental wavelength of 800nm.

Polarization rotation transitions in anisotropically strained SrTiO3 thin films

Commensurately strained epitaxial SrTiO3 thin films on a GdScO3 substrate with anisotropic in-plane tensile strains of 1.46% and 1.59% were grown. By using optical second harmonic generation and polarization measurements, a ferroelectric transition from a tetragonal 4∕mmm phase to a ferroelectric mm2 phase at ∼400K and an antiferrodistortive transition to a multiferroic phase at 150–175K are observed, which are in agreement with thermodynamic calculations. In addition, a series of polarization rotation transitions between ⟨100⟩p to ⟨l00⟩p (0

Two dimensional dynamic focusing of laser light by ferroelectric domain based electro-optic lenses

The authors demonstrate the proof of concept of two dimensional focusing of laser light. This has been achieved by using a combination of two cylindrical electro-optic ferroelectric domain lens stacks in an orthogonal geometry. The devices were fabricated on z-cut lithium tantalate (LiTaO3) wafers and tested with helium-neon laser at 633nm. Continuously tunable optical power ranging from −129m−1to129m−1 is obtained in both directions by varying the applied voltage.

Laser-improved protein crystallization screening.

Screening of proteins for crystallization under laser irradiation was investigated using six proteins: ribonuclease B, glucose dehydrogenase, lysozyme, sorbitol dehydrogenase, fructose dehydrogenase and myoglobin. Shining 532 nm green circularly polarized laser light with a picosecond pulse and 6 mW power for 30 s on newly set-up protein drops showed a marked improvement in the number of screen conditions amenable for crystal growth compared with control drops under identical conditions but without laser exposure. For glucose dehydrogenase and sorbitol dehydrogenase, larger and better quality crystals were formed and the resolution of X-ray diffraction was improved. The speed of crystallization increased in the case of ribonuclease B, lysozyme and sorbitol dehydrogenase. During laser irradiation, the amount of precipitation in the screened drops increased, indicating a transient decrease in protein solubility. At the optimized laser settings, there was no deleterious effect of the laser on crystal growth or on the protein. In the cases of ribonuclease B and lysozyme the crystal packing did not change owing to the laser exposure.