We report the fabrication of buried optical channel waveguides in strontium barium niobate nonlinear ferroelectric crystals by direct ultrafast laser inscription. These waveguides are strongly polarized and can be reversibly switched on and off by changing the temperature of the crystal, a characteristic we attribute to the bistable enhancement of the electro-optic coefficients at the ferro to paraelectric phase transition

Chen, F

Jaque, D

Kar, AK

Martinez Maestro, L

Psalia, ND

Reid, DT

Rodenas, A

Thomson, RR

Jaque, D.

Psaila, N. D.

Thomson, R. R.

Chen, F.

Maestro, L. M.

Rodenas, A.

Reid, D. T.

Kar, A. K.

The University of Manchester - Institutional Repository

Ultrafast laser inscription of bistable and reversible waveguides in strontium barium niobate crystals

University of Salford Institutional Repository

Appl. Phys. Lett. 96, 191104 (2010); https://doi.org/10.1063/1.3429584 96, 191104© 2010 American Institute of Physics.Ultrafast laser inscription of bistable andreversible waveguides in strontium bariumniobate crystalsCite as: Appl. Phys. Lett. 96, 191104 (2010); https://doi.org/10.1063/1.3429584Submitted: 25 December 2009 . Accepted: 18 April 2010 . Published Online: 11 May 2010D. Jaque, N. D. Psaila, R. R. Thomson, F. Chen, L. M. Maestro, A. Ródenas, D. T. Reid, and A. K. KarARTICLES YOU MAY BE INTERESTED INFemtosecond laser writing of waveguides in periodically poled lithium niobate preservingthe nonlinear coefficientApplied Physics Letters 90, 241107 (2007); https://doi.org/10.1063/1.2748328Efficient frequency doubling in femtosecond laser-written waveguides in lithium niobateApplied Physics Letters 89, 081108 (2006); https://doi.org/10.1063/1.2338532Femtosecond laser three-dimensional micro- and nanofabricationApplied Physics Reviews 1, 041303 (2014); https://doi.org/10.1063/1.4904320Ultrafast laser inscription of bistable and reversible waveguidesin strontium barium niobate crystalsD. Jaque,1,a N. D. Psaila,2 R. R. Thomson,2 F. Chen,3 L. M. Maestro,1 A. Ródenas,2D. T. Reid,2 and A. K. Kar21Departamento de Física de Materiales, Universidad Autónoma de Madrid, 28049 Madrid, Spain2School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS,United Kingdom3School of Physics, Shandong University, Jinan 250100, People’s Republic of ChinaReceived 25 December 2009; accepted 18 April 2010; published online 11 May 2010We report the fabrication of buried optical channel waveguides in strontium barium niobatenonlinear ferroelectric crystals by direct ultrafast laser inscription. These waveguides are stronglypolarized and can be reversibly switched on and off by changing the temperature of the crystal, acharacteristic we attribute to the bistable enhancement of the electro-optic coefficients at the ferroto paraelectric phase transition. © 2010 American Institute of Physics. doi:10.1063/1.3429584Ultrafast laser inscription ULI is a versatile, fast, clean,and almost universal technique for the inscription of opticalwaveguides in transparent media, including nonlinear crys-tals and laser gain media.1–3 ULI relies on the microstructuralchanges which can be induced when ultrashort laser pulsesare tightly focused inside a transparent medium. Thesechanges may manifest themselves through a refractive indexmodification which can be used to directly inscribe opticalwaveguides. The nature, magnitude and origin of the refrac-tive index modification depends on the inscription param-eters pulse energy, pulse duration, laser polarization, laserwavelength, scan speed, pulse repetition rate and focusinggeometry as well as on the particular optical response of theirradiated material. The mechanisms at the basis of the re-fractive index modifications have attracted significant atten-tion over recent years, concluding that refractive index modi-fication arise as the result of a complex combination ofdifferent phenomena such as lattice damage, stress-inducedlattice distortions, compositional changes, and photochemi-cal reactions.4–6Interestingly, it has previously been proposed that inferroelectric crystals the refractive index can also be modi-fied through the appearance of space charge fields in combi-nation with the electro-optic effect. Nevertheless, thismechanism has not yet been applied for the fabrication ofoptical buried waveguides. This would be especially relevantin the ferroelectric strontium barium niobate SBN crystalbecause of its prime importance in modern photonics dueto its nonlinear, electro-optic, photorefractive, laser, andbistable properties.7–10 This paper reports on the opticalproperties of ULI buried channel waveguides in SBN. Theresults reveal a bistable and reversible optical behavior whenthe system is thermally driven from the ferroelectric to theparaelectric phase which can be explained in terms of thechanges in the electro-optic coefficients which take placeduring the ferroelectric phase transition PhT.The SBN Sr0.60Ba0.40Nb2O6 sample used in this workis nominally pure and its PhT temperature was measured tobe 75 °C. The SBN sample was multidomain, with the do-main orientation flipping 180° from the +c to −c directionsalong the sample. Waveguides were inscribed by using anYb:fiber laser Fianium Ltd. providing 350 fs pulses at1064 nm, with a pulse repetition rate of 500 kHz. For allexperiments, the laser polarization was adjusted to be per-pendicular to both the incident beam and sample translationdirection. The inscription beam was focused 300 m be-low sample surface by using a 0.4 numerical aperture lens.Structures were inscribed by translating the 6 mm longsample perpendicular to the optical c-axis and at a constantvelocity of 6 mm s−1. The pulse energy was varied between425 and 725 nJ, resulting in all cases in observable refractiveindex modifications. Preliminary investigations indicatedthat the waveguides fabricated using 648 nJ pulses exhibitedthe most interesting guiding properties, and hereafter, wewill refer to these as the optimum waveguides. For the re-mainder of this paper we analyze in detail the properties ofthe optimum waveguides.Figure 1a shows the optical transmission image of theend facet of the waveguide. Two modified regions can bedifferentiated. First, at the focal volume a very elongated andnarrow modified region 3 m in width and 40 m inlength has been created parallel to the pulse propagation di-rection. Second, an ellipsoidal modified region appears cen-tered around the focal volume with a maximum width of25 m and an approximate length of 40 m. The natureof these two regions has been investigated by micro-Ramanand scanning electron microscope experiments not shownhere for the sake of brevity. Measurements suggest that thecentral region corresponds to a slightly damaged regionaElectronic mail: daniel.jaque@uam.es.E Ec axisc axis3 µm25 µm(a) (b) (c)c axisfs fs fsFIG. 1. Color online a Optical transmission image of the laser inducedmodified track generated with a pulse energy of 648 nJ. The spatial extent ofthe damage region and of the heat affected zone are indicated. Near fieldspatial distribution of 980 nm waveguide modes obtained for b ordinaryand c extraordinary polarizations, with the heat affected zone shown, forcomparison, by the dashed ellipse.APPLIED PHYSICS LETTERS 96, 191104 20100003-6951/2010/9619/191104/3/$30.00 © 2010 American Institute of Physics96, 191104-1where the Raman signal slightly decreases, and the ellip-soidal modified region to a strain affected volume character-ized by a local compression slight increase in the Ramanphonon modes energy. We attribute these two regions to thepresence of a direct laser-damaged area localized at the focalvolume, and to the creation of a strained area due to thepresence of a heat-affected zone HAZ Ref. 11 which isexpected due to the high pulse repetition rates used in thiswork.12 The guiding properties of the structures were probedby end-coupling light from a 980 nm fiber-coupled diodeinto one end of the waveguide. Figures 1b and 1c presentthe waveguide’s modes at 980 nm for ordinary o-polarizedand extraordinary e-polarized light, respectively. As can beseen in Figs. 1b and 1c, the o-polarized light is guidedinside the HAZ, whereas the e-polarized light is guided oneither side of the HAZ. By analyzing the intensity profile ofscattered light, the propagation losses of the waveguideswere measured to be 1.8 and 1.0 dB/cm for o- ande-polarized light, respectively.The guiding properties of the o- and e-polarizations werealso observed to differ in their thermal response. Figure 2presents the guided modes for e-top and o-bottom polar-izations at three different temperatures corresponding to theferroelectric phase, the PhT and the paraelectric phase. Inthis figure the dashed line indicates the spatial extension ofthe HAZ. For the e-polarization, guiding was found to beenhanced at PhT more intense and confined mode andstrongly deteriorated in the paraelectric phase, in which onlyvery weak guiding was observed. Surprisingly, for theo-polarization the opposite behavior was found: waveguidingwas completely absent at the PhT, only re-emerging whenthe sample was heated further into its paraelectric phase.These changes in the waveguiding properties have beenfound to be completely reversible. This is illustrated in Fig.3, in which we have included the e- and o-polarized modeintensities, as well as the e-polarization mode size obtainedduring a heating and cooling cycle Figs. 3a–3c, respec-tively. It should also be noted that the heating and coolingprocedures have been found to be nonequivalent, leading toan optical bistable response in the region of the PhT. Thisbehavior can be explained in terms of the characteristic ther-mal hysteresis associated with the first-order ferroelectrictransition of SBN.7The bistable and reversible optical behavior observed forSBN waveguides can be understood in terms of the creationof a space-charge distribution on either side of the strainedHAZ. During irradiation, a highly ionized region is gener-ated in the focal region. After irradiation electrons may bedisplaced along the c-axis due to photovoltaic effect as it isknown to occur in LiNbO3 crystals,13–15 being this chargedisplacement relevant in the HAZ due to the thermal-inducedenhancement of electron mobility. The resultant space-chargedistribution, can be thought of as an electric dipole pointingparallel to the c-axis.15 We suggest that this space-chargefield would modify both the o- and e-polarization indices viathe electro-optic effect, leading to an induced refractive in-dex change given by the following:noesc = − 12noe3 r1333Esc, 1where noe is the oe refractive index, r1333 is the corre-sponding electro-optic coefficient, and Esc is the spacecharge field along the c-axis. Importantly, the complete ordi-nary refractive index map will also have an additional elasto-optic contribution arising from the local lattice strain inducedin the HAZ nostrain. According to this model, the extraor-dinary refractive index change at both sides of HAZ wouldbe proportional to the electro-optic coefficient value. In theparticular case of SBN, these coefficients are known to beenhanced drastically at PhT, suffering then a strong reductionin the paraelectric phase.9 This behavior well explains theobserved improvement in the waveguiding at PhT and thedeterioration of waveguiding in the paraelectric phase seeFigs. 2 and 3. According to this assumption, the spatialmodulation of the ordinary refractive index would be deter-mined by the interplay between the strain-induced refractiveindex increment nostrain and the electro-optic index reduc-tion induced in the HAZ nosc. Experimental data suggestthat at PhT the enhancement in the r13 coefficient reduces theoverall ordinary refractive index increment in the HAZ,therefore, suppressing waveguiding.There are also two additional experimental evidencesthat support this explanation. First, when ULI was performedin absence of thermal accumulation by using 1 kHz lasersno waveguides of any kind were formed. Thus, in the ab-sence of any thermal induced enhancement of the electron’sE E EE E Ec axisc axisc axisc axisc axisc axisFerroelectric (28 ºC) Phase transition (75 ºC) Paraelectric (120 ºC)FIG. 2. Color online Near-field spatial distribution of the 980 nm wave-guide modes obtained for extraordinary top and ordinary bottom polar-izations at three different temperatures corresponding to the ferroelectricphase, to the ferro-to-para PhT and to the paraelectric phase. The extent ofthe heat affected zone is shown schematically by the dashed ellipse.40 80 12045640 80 120 40 80 120WaveguideIntensity(Arb.Units)Modesize(µm)Temperature (ºC)HeatingCooling(a) (b) (c)FIG. 3. Color online Temperature dependence of the a extraordinary andb ordinary waveguide mode intensity. The temperature dependence of theaverage mode size of the extraordinary mode is shown in c. In each case,the red and blue data correspond to heating and cooling procedures. Thedashed line indicates the PhT temperature.191104-2 Jaque et al. Appl. Phys. Lett. 96, 191104 2010mobility, no space charge fields are created. Second, whenthe ULI was further performed with the sample c-axis paral-lel to the propagation direction of the inscription pulses, con-finement of e-polarized light was found to occur at the topand at the bottom of the HAZ instead of at its sides seeFig. 4, this revealing the presence of a dipolelike space-charge field with the dipole axis parallel to the c-axis, i.e.,parallel to the photovoltaic field. Independently of the exactorientation of the space-charge field, we have found that thesame bistable and reversible behavior described for thewaveguides fabricated in the orthogonal geometry see Figs.2 and 3. Thus, the charge displacement and accumulationalong the c-axis provides a satisfactory qualitative explana-tion for the bistable optical behavior of the waveguides illus-trated in Fig. 1. Nevertheless, the exact mechanisms leadingto charge pinning at the sides of the HAZ are still not wellunderstood and further experiments are required such as ULIat different temperatures, the study of the influence of differ-ent impurities that would modify the photorefractive re-sponse of SBN, and the application of external fields.In summary, we have demonstrated the formation ofchannel waveguides in SBN crystals by ULI of single linestructures. The refractive index modifications responsible forthe waveguiding are attributed to the appearance of a heataffect zone and a space-charge induced electric field. Abistable and reversible optical behavior has been found whenthe SBN system is thermally driven through its ferroelectricPhT which can be explained by the strong enhancement ofthe electro-optic during PhT. Due to the inherent three-dimensional 3D capability of the ULI technique, and theferroelectric nonlinear properties of SBN crystals, thewaveguides reported here open an interesting direction forthe fabrication of 3D integrated photonic devices exhibitinguseful bistable and nonlinear properties.The authors gratefully acknowledge support from theScottish Universities Physics Alliance DJ, by the UK En-gineering and Physical Sciences Research Council projectGrant Nos. EP/E016863/1, EP/D047269/1, and EP/G030227/1, by the Spanish Ministerio de Ciencia y Tecnología GrantNo. MAT2007-64686, and by the Universidad Autónoma deMadrid and Comunidad Autónoma de Madrid Grant No.CCG08-UAM/MAT-4434 and by the National NaturalScience Foundation of China Grant No. 10925524. We alsoacknowledge support from Fianium Ltd.1S. Nolte, M. Will, J. Burghoff, and A. Tuennermann, Appl. Phys. A:Mater. Sci. Process. 77, 109 2003.2R. Osellame, G. Della Valle, N. Chiodo, S. Taccheo, P. Laporta, O. Svelto,and G. Cerullo, Appl. Phys. A: Mater. Sci. Process. 93, 17 2008.3B. McMillen, K. P. Chen, H. An, S. Fleming, V. Hartwell, and D. Snoke,Appl. Phys. Lett. 93, 111106 2008.4J. Burghoff, C. Grebing, S. Nolte, and A. Tünnermann, Appl. Phys. Lett.89, 081108 2006.5A. Ródenas, G. A. Torchia, G. Lifante, E. Cantelar, J. Lamela, F. Jaque, L.Roso, and D. Jaque, Appl. Phys. B: Lasers Opt. 95, 85 2009.6F. Vega, J. Armengol, V. Diez-Blanco, J. Siegel, J. Solis, B. Barcones, A.Perez-Rodriguez, and P. Loza-Alvarez, Appl. Phys. Lett. 87, 0211092005.7N. V. Bogodaev, T. R. Volk, L. I. Ivleva, P. A. Lykov, N. M. Polozkov, andV. V. Osiko, Laser Phys. 11, 511 2001.8J. J. Romero, D. Jaque, J. Garcia Sole, and A. A. Kaminskii, Appl. Phys.Lett. 78, 1961 2001.9M. Goulkov, T. Granzow, U. Dörfler, T. Woike M. Imlau, R. Pankrath, andW. Kleemann, Opt. Commun. 218, 173 2003.10M. O. Ramirez, D. Jaque, L. E. Bausa, and J. García Solé, Phys. Rev. Lett.95, 267401 2005.11A. H. Nejadmalayeri and P. R. Herman, Opt. Express 15, 10842 2007.12M. Beresna, T. Gertus, R. Tomasiunas, H. Misawa, and S. Juodkazis,Laser Chem. 2008, 976205 2008.13O. Beyer, I. Breunig, F. Kalkum, and K. Buse, Appl. Phys. Lett. 88,051120 2006.14S. Juodkazis, M. Sudzius, V. Mizeikis, H. Misawa, E. G. Gamaly, Y. Liu,O. A. Louchev, and K. Kitamura, Appl. Phys. Lett. 89, 062903 2006.15Q. W. Song, C.-P. Zhang, and P. J. Talbot, Appl. Opt. 32, 7266 1993.caxisE E EFerroelectric (28 ºC) PhT (75 ºC) Paraelectric (120 ºC)caxiscaxisE E EcaxiscaxiscaxisFIG. 4. Color online Near field spatial distribution of the 980 nm wave-guide modes obtained for extraordinary top and ordinary bottom polar-izations at three different temperatures corresponding to the ferroelectricphase, to the ferro-to-para PhT and to the paraelectric phase. The extensionof the heat affected zone is schematically shown. In this case fematosecondpulses were incident parallel to the c-axis.191104-3 Jaque et al. Appl. Phys. Lett. 96, 191104 2010

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We report the fabrication of buried optical channel waveguides in strontium barium niobate nonlinear ferroelectric crystals by direct ultrafast laser inscription. These waveguides are strongly polarized and can be reversibly switched on and off by changing the temperature of the crystal, a characteristic we attribute to the bistable enhancement of the electro-optic coefficients at the ferro to paraelectric phase transition. © 2010 American Institute of Physics.</p

Jaque, Daniel

Psaila, Nicholas D

Thomson, Robert R

Maestro, L M

Rodenas, Airan

Reid, Derryck Telford

Kar, Ajoy Kumar

Heriot Watt Pure

https://salford-repository.worktribe.com/file/1363176/1/1.3429584.pdf

Ultrafast laser inscription of bistable and reversible waveguides in strontium barium niobate crystals

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Heriot Watt Pure

Heriot Watt Pure

The University of Manchester - Institutional Repository