Publications

2010
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16. Y. J. Ying, C. L. Sones, A. C. Peacock, F. Johann, E. Soergel, R. W. Eason, M. N. Zervas, S. Mailis. Ultra-smooth lithium niobate photonic micro-structures by surface tension reshaping. Optics Express 18 11508-11513 (2010).
Annealing of micro-structured lithium niobate substrates at temperatures close to, but below the melting point, allows surface tension to reshape preferentially melted surface zones of the crystal. The reshaped surface re-crystallizes upon cooling to form a single crystal again as it is seeded by the bulk which remains solid throughout the process. This procedure yields ultra-smooth single crystal superstructures suitable for the fabrication of photonic micro-components with low scattering loss.
DOI: 10.1364/OE.18.011508 - open access to full text
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15. C. L. Sones, K. S. Kaur, P. Ganguly, D. P. Banks, Y. J. Ying, R. W. Eason, S. Mailis. Laser-Induced-Forward-Transfer: A rapid prototyping tool for fabrication of photonic devices. Applied Physics A, published online 17 June 2010.

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14. A. Dabirian, S. Harada, Yury Kuzminykh, S. Cosmin Sandu, E. Wagner, G. Benvenuti, P. Brodard, S. Rushworth, P. Muralt and P.Hoffmann. Combinatorial chemical beam epitaxy of lithium niobate thin films on sapphire. Accepted for publication in J. Electrochem. Soc. 2010.





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13. A. Dabirian, Y. Kuzminykh, B. Afra, S. Harada, E. Wagner, S. C. Sandu, G. Benvenuti, S. Rushworth, P. Muralt, and P. Hoffmann. Combinatorial discovery and optimization of amorphous HfO2-Nb2O5 mixture with improved transparency. Electrochemical and Solid-State Letters 13, G60-G63 (2010).
Combinatorial high vacuum chemical vapor deposition (HV-CVD) of mixed HfO2–Nb2O5 thin films has been demonstrated to yield amorphous layers at substrate temperatures where individually deposited pure HfO2 and Nb2O5 films are polycrystalline. Spectroscopic ellipsometry of the films shows that adding HfO2 to Nb2O5 improves the transparency of the films while still maintaining a high refractive index. Atomic force microscopy measurements show that the root-mean-square surface roughness of the films is about 1.2 nm.
DOI: http://dx.doi.org/10.1149/1.3407618
2009
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12. Sones, C.L., Ganguly, P., Ying, Y.J., Johann, F., Soergel, E., Eason, R.W. and Mailis, S. Spectral and electro-optic response of UV-written waveguides in LiNbO3 single crystals. Optics Express 17, 23755-23764 (2009).
An experimental study of the spectral and electro-optic response of direct UV-written waveguides in LiNbO3 is reported. The waveguides were written using c.w. laser radiation at 275, 300.3, 302, and 305 nm wavelengths with various writing powers (35-60 mW) and scan speeds (0.1-1.0 mm/sec). Spectral analysis was used to determine the multimode and single mode wavelength regions and, the cut-off point of the fabricated waveguides. Electro-optic characterization of these waveguides reveals that the electro-optic coefficient (r33) decreases for longer writing wavelengths, with a maximum of 31 pm/V for 275 nm and, is reduced to 14 pm/V for waveguides written with 305 nm.
DOI: 10.1364/OE.17.023755 - open access to full text
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11. Y. J. Ying, C. E. Valdivia, C. L. Sones, R. W. Eason and S. Mailis. Latent light-assisted poling of LiNbO3. Optics Express 17, 3490-6 (2009).
The observation of latent light-assisted poling (LAP) in lithium niobate single crystals is reported. More specifically, the nucleation field is reduced and remains reduced for an extended time period (up to several hours) after irradiation with ultrafast (~150 fs) laser light at a wavelength of 400 nm. The maximum nucleation field reduction measured using latent- LAP (62%) was significantly higher in comparison with regular non-time- delayed LAP (41%) under identical irradiation conditions in undoped congruent lithium niobate crystals. No latent-LAP effect was observed in MgO-doped crystals for the experimental conditions used, despite the strong effect observed using regular LAP. The latent-LAP effect is attributed to the formation of a slowly decaying photo-induced space- charge distribution which assists local ferroelectric domain nucleation. The dynamics of latent-LAP are compared with the dynamics of photorefractive grating decay, recorded in lithium niobate crystals of different doping, confirming the space charge hypothesis.
DOI: 10.1364/OE.17.018681    Full Text
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10. F. Johann, Y. J. Ying, T. Jungk, . Hoffmann, C. L. Sones, R. W. Eason, S. Mailis, and E. Soergel. Depth resolution of piezoresponse force microscopy. Appl. Phys. Lett. 94, 172904 (2009).
Given that a ferroelectric domain is generally a three dimensional entity, the determination of its area as well as its depth is mandatory for full characterization. Piezoresponse force microscopy PFM is known for its ability to map the lateral dimensions of ferroelectric domains with high accuracy. However, no depth profile information has been readily available so far. Here, we have used ferroelectric domains of known depth profile to determine the dependence of the PFM response on the depth of the domain, and thus effectively the depth resolution of PFM detection.
DOI: 10.1063/1.3126490
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9. S. Diziain, J.-M. Merolla, M. Spajer, G. Benvenuti, A. Dabirian, Y. Kuzminykh, P. Hoffmann, and M.-P. Bernal, Rev. Sci. Determination of local refractive index variations in thin films by heterodyne interferometric scanning near-field optical microscopy. Rev. Sci. Instrum. 80, 093706 (2009).
We report on a heterodyne interferometric scanning near-field optical microscope developed for characterizing, at the nanometric scale, refractive index variations in thin films. An optical lateral resolution of 80 nm (λ/19) and a precision smaller than 10−4 on the refractive index difference have been achieved. This setup is suitable for a wide set of thin films, ranging from periodic to heterogeneous samples, and turns out to be a very promising tool for determining the optical homogeneity of thin films developed for nanophotonics applications.
DOI: 10.1063/1.3226660
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8. S. Diziain, S. Harada, R. Salut, P. Muralt, and M.-P. Bernal. Strong improvement in the photonic stop-band edge sharpness of a lithium niobate photonic crystal slab. Appl. Phys. Lett. 95, 101103 (2009).
We report on a photonic crystal (PhC) etched into a 380 nm thick lithium niobate (LN) thin film deposited on a MgO substrate by pulsed laser deposition. The transmission properties of this device were assessed by optical near-field measurements and compared to the transmission spectra of the same PhC drilled into bulk LN and calculated by a two dimensional finite-difference time domain method. We show a strong improvement in the transmission properties of the LN PhC by etching it into a thin layer rather than into a 500 m thick wafer. This result appears to be very promising for applications based on LN tunable PhCs.
DOI: 10.1063/1.3223595
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7. P. Ganguly, C. Sones, Y. Ying, H. Steigerwald,  K. Buse,  E. Soergel,  R. Eason, S. Mailis. Determination of Refractive Indices from the Mode Profiles of UV-Written Channel Waveguides in LiNbO3-Crystals for Optimization of Writing Conditions. J. Lightwave Technology 27, 18681-92 (2009).
We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in LiNb $O_{3}$for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of ~ 0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.
DOI: 10.1109/JLT.2009.2015963
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6. G.W. Burr, S. Diziain, and M.-P. Bernal. Theoretical study of lithium niobate slab waveguides for integrated optics applications. Optical Materials 31, 1492-1497 (2009).
We report on the theoretical study of lithium niobate slab and wire waveguides with different kinds of cladding (silicon dioxide, sapphire and air). The mode propagation, the light confinement and radiation losses are simulated using a software based on a beam propagation method. We propose from those results lithium niobate waveguide geometries for optical integrated applications.
DOI: 10.1016/j.optmat.2009.02.013
2008
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5. S. Diziain, J. Amet, F. I. Baida, and M.-P Bernal. Optical far-field and near-field observations of the strong angular dispersion in a lithium niobate photonic crystal superprism designed for double (passive and active) demultiplexer applications. Appl. Phys. Lett., 93(26), 261103-5 (2008).
We report on a lithium niobate photonic crystal (PC) superprism device designed for double demultiplexer applications. In fact, the strong angular beam steering cannot only be reached by passively tuning the wavelength but it can also be actively controlled by the Pockels effect enhanced due to the slow light phenomenon. The performance of the passive device is demonstrated by measuring its transmission properties. Optical far field and near-field experiments, corroborated by two-dimensional finite difference time domain (2D-FDTD) calculations, exhibit an angular dispersion of 1.5/nm. A value as high as 4.3/nm is expected by improving the PC design as supported by 2D-FDTD simulation.
DOI: 10.1063/1.3059558
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4. N. D. Scarisoreanu, G. Dinescu, R. Birjega, M. Dinescu, D. Pantelica, G. Velisa, N. Scintee, and A. C. Galca. SBN thin films growth by RF plasma beam assisted pulsed laser deposition. Appl. Phys. A, 93, 795–800 (2008).
SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr0.6Ba0.4Nb2O6 and ceramic Sr0.5Ba0.5Nb2O6 stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31 in-plane orientation relative to the MgO (100) axis is identified.
DOI: 10.1007/s00339-008-4753-2
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3. V. Ion, A. C. Galca, N. D. Scarisoreanu1, M. Filipescu1, and M. Dinescu. Spectroscopic ellipsometry study of amorphous SrxBa1−xNb2O6 thin films obtained by pulsed laser deposition. Physica status solidi c, 5, pp. 1180-1183 (2008).
Optical properties of amorphous Strontium Barium Niobate (a-SBN) thin films are investigated using spectroscopic ellipsometry. Since the SBN can be applied to optoelectronic devices, the dispersion of refractive index is desirable. The films are obtained by Pulsed Laser Deposition (PLD) onto MgO and Si substrates by targeting a SBN:60 monocrystal. The dielectric function of a-SBN is approximated using a single Tauc-Lorentz oscillator model. Thicknesses of the films and of their rough layer are in agreement with SEM and AFM results. The dispersion of the refractive index is presented in the 1-5 eV range.
DOI:
10.1002/pssc.200777818
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2. Sones, C. L. , Muir, A. C., Ying, Y. J. , Mailis, S., Eason, R. W. , Jungk, T., Hoffmann, . & Soergel, E. Precision nanoscale domain engineering of lithium niobate via UV laser induced inhibition of poling. Appl. Phys. Lett. 92, 072905 (2008).
Continuous wave ultraviolet laser irradiation at λ = 244 nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100 nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution microstructuring approach when followed by chemical etching.
DOI: 10.1063/1.2884185
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1. A. C. Muir , C. L. Sones, S. Mailis, R. W. Eason, T. Jungk, . Hoffmann and E. Soergel. Direct writing of inverted domains in Lithium niobate using a continuous wave ultraviolet laser. Optics Express 16(4) 2336-2350 (2008).
The inversion of ferroelectric domains in lithium niobate by a scanning focused ultra-violet laser beam (λ = 244nm) is demonstrated. The resulting domain patterns are interrogated using piezoresponse force microscopy and by chemical etching in hydrofluoric acid. Direct ultra-violet laser poling was observed in un-doped congruent, iron doped congruent and titanium in-diffused congruent lithium niobate single crystals. A model is proposed to explain the mechanism of domain inversion.
DOI: 10.1364/OE.16.002336

Conference proceedings
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4. A. Dabirian, Y. Kuzminykh, C. Parsons, S. Rushworth, E. Wagner, S. C. Sandu, G. Benvenuti S. Harada, P. Muralt, and P. Hoffmann, Rushworth, P. Muralt, and P. Hoffmann. Efficient optimization of high vacuum chemical vapour deposition of niobium oxide on full wafer scale. IOP Conf. Series: Materials Science and Engineering 8, 012026 (2010).
A systematic study of niobium oxide deposition using niobium tetraethoxy-dimethyl-amino-ethoxide (Nb(OEt)4((dmae)) precursor is presented. The deposition process was conducted in a high-vacuum chemical vapor deposition machine with precursor flux gradient capability. An efficient optimization of the deposition process was achieved and both mass-transport- and chemical-reaction-limited regimes were identified.
DOI: 10.1088/1757-899X/8/1/012026 - open access to full text
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3. G. Benvenuti, C.S. Sandu and E. Wagner. TiO2 laser and electron beam assisted chemical deposition. IOP Conf. Series: Materials Science and Engineering 8, 012006 (2010).
Chemical Beam Deposition is a gas phase deposition technique with chemical precursors, operated under high vacuum conditions (10−6 mbar). One of the advantages over conventional Chemical Vapour Deposition (CVD) technique is that the highly reduced pressure allows the use of highly energetic beam particles (photons with energy over 3–3.5 eV, electrons, or ions) to assist the deposition without any gas phase interactions with the chemical precursor molecules. Densification of TiO2( thin films with anatase crystalline phase by a laser-assisted growth process is discussed.
DOI: 10.1088/1757-899X/8/1/012006 - open access to full text
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2. S. Harada and P. Muralt. Pulsed laser deposition of KNN-based ferroelectric thin films on platinised Si substrates. IOP Conf. Series: Materials Science and Engineering 8, 012004 (2010).
In this paper we report work on a modified potassium sodium niobate composition that has the potential to replace PZT for piezoelectric thin film applications. Pulsed laser depositions were undertaken on platinised Si substrates. The resultant films were found to be well-oriented in the [100] direction and almost phase-pure. TEM analysis of film cross-sections revealed the grain structure to be highly columnar. Values for Ps = 3.8 μC/cm2( and Ec = 20 kV/cm were derived from polarisation hysteresis measurements for a 3.2 μm thick film. Finally, piezoresponse force microscopy was used to map the local piezoelectric activity down to the level of individual grains in the film.
DOI: 10.1088/1757-899X/8/1/012004
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1. A. Dabirian, Y. Kuzminykh, P. Hoffmann, S. C. Sandu, E. Wagner, G. Benvenuti, C. Parsons, and S. Rushworth. Combinatorial chemical vapor deposition of lithium niobate thin films. ECS Transactions 25 (8), 1221 (2009).
Combinatorial lithium niobate deposition on 150 mm naturally oxidized silicon (100) wafers in a high-vacuum chemical vapor deposition reactor using Li(OBut) and Nb(OEt)4(dmae) is presented. The novel precursor supply system allows individual spatial control of precursors impinging rates on the substrate. This results in variations of the film properties in a single experiment at a certain substrate temperature due to the influence of different precursors flow rates and ratios. It efficiently leads to deposition conditions to achieve highly <001> oriented polycrystalline lithium niobate films.
DOI: http://dx.doi.org/10.1149/1.3207727

Patents
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Homogeneous and combinatorial thin film deposition equipment under molecular beam conditions
Inventor(s): G. Benvenuti et al.
Applicant: ABCD Technology Sarl. PCT/IB2008/054129 Oct 2008
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Large area deposition in high vacuum with high thickness uniformity
Publication number: US2007193519 (A1)
Publication date: 2007-08-23
Inventor(s): BENVENUTI GIACOMO [CH]; HALARY-WAGNER ESTELLE [CH]; AMOROSI SIMONE [CH]; HOFFMANN PATRIK [CH]
Applicant(s): ECOLE POLYTECH [CH]