Optical parametric devices in periodically poled LiTaO3 MARTIN LEVENIUS Doctoral Thesis Department of Applied Physics KTH – Royal Institute of Technology Stockholm, Sweden 2013 iii Optical parametric devices in periodically poled LiTaO3 © Martin Levenius, 2013 Quantum Electronics and Quantum Optics Department of Applied Physics KTH – Royal Institute of Technology 106 91 Stockholm Sweden ISBN 978-91-7501-947-5 TRITA-FYS 2013:65 ISSN 0280-316X ISRN KTH/FYS/--13:65—SE Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framläg- ges till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 20 december 2013 kl. 10.00 i sal FA31, Albanova, Roslagstullsbacken 21, KTH, Stockholm. Avhandlingen kommer att försvaras på engelska. Cover picture: Multi-beam parametric generation in the visible supported by 2D quasi-phase matching. Each green beam corresponds to a sum frequency generation process (SFG) driven by the pump beam and a signal beam involved in a simultane- ous process of coupled optical parametric generation (OPG). Each blue beam corre- sponds to a second harmonic generation process driven by the pump beam, which also drives the OPG and the SFG processes. Printed by Universitetsservice US AB, Stockholm 2013 iv Martin Levenius Optical parametric devices in periodically poled LiTaO3 Department of Applied Physics, KTH – Royal Institute of Technology 106 91 Stockholm, Sweden ISBN 978-91-7501-947-5, TRITA-FYS 2013:65, ISSN 0280-316X, ISRN KTH/FYS/--13:65—SE Abstract Optical parametric frequency conversion based on quasi phase matching (QPM) in nonlinear optical crystals is a powerful technique for generating coherent radiation in wavelength ranges spanning from the mid-infrared (mid-IR) to the blue, displaying low thermal load and high efficiency. This thesis shows how QPM in one- (1D) or two-dimensional (2D) lattices can be employed to engineer novel devices for parametric downconversion in the IR, af- fording freedom in designing both spectral and angular properties of the parametric output. Experimental demonstrations of parametric devices are supported by theoreti- cal modelling of the nonlinear conversion processes. In particular, broadband parametric downconversion has been investigated in 1D QPM lattices, through degenerate downconversion close to the point of zero group-velocity dispersion. Ultra-broadband optical parametric generation (OPG) of 185 THz bandwidth (at 10 dB), spanning more than one octave from 1.1 to 3.7 μm, has been achieved in periodically poled 1 mol% MgO-doped near-stoichiometric LiTaO3 (MgSLT) of 25 μm QPM period, pumped at 860 nm. Such broadband gain is of high interest for ultrashort optical pulse amplification, with applications in high harmonic generation, ultrafast spectroscopy and laser ablation. Furthermore, the det- rimental impact of parasitic upconversion, creating dips in the OPG spectrum, has been investigated. By altering the pump pulse duration, energy can be backconverted to create peaks at the involved OPG wavelengths, offering a possible tool to enhance broadband parametric gain spectra. The engineering of the angular properties of a parametric output benefits greatly from 2D QPM, which is investigated in this thesis by the specific example of hexagonally poled MgSLT. It is demonstrated how two OPG processes, supported by a single 2D QPM device, can exhibit angularly and spectrally degenerate signals (idlers). This degeneracy results in a coherent coupling between the two OPG pro- cesses and a spectrally degenerate twin-beam output in the mid-IR (near IR). 2D QPM devices exhibiting such coherently coupled downconversion processes can find applications as compact sources of entangled photon-pairs. This thesis further illus- trates the design freedom of 2D QPM through the demonstration of a device support- ing multiple parametric processes, thus generating multiple beams from the mid-IR to the blue spectral regions. Keywords: nonlinear optics, parametric processes, frequency downconver- sion, quasi-phase matching, LiTaO3, broadband downconversion, quadratic cascading, nonlinear photonic crystals, twin-beam generation v Sammanfattning Denna doktorsavhandling beskriver forskning rörande ickelinjära optiska kristaller som används för att skapa skräddarsydda koherenta infraröda ljuskällor med låg vär- meutveckling, drivna av lasrar vid standardvåglängder. Ljuskällorna baseras på s.k. kvasifasmatchning (QPM), där man genom att periodiskt strukturera den andra ord- ningens ickelinjäritet i kristallen drastiskt kan öka konverteringsgraden till i stort sett vilken våglängd som helst där kristallen är transparent. Avhandlingen visar hur dessa strukturer, i en eller i två dimensioner, kan an- vändas till infraröda ljuskällor, baserade på s.k. optisk parametrisk frekvenskonverte- ring, för att ge frihet att designa det konverterade ljuset både spektralt och vinkel- mässigt. Avhandlingen beskriver genomförda experiment inom parametrisk konverte- ring samt hur dessa experiment förklaras och styrks genom teoretisk modellering. Specifikt beskriver avhandlingen hur endimensionella strukturer har använts till ultrabredbandig parametrisk nedkonvertering som genererade ett 185 Thz brett kontinuum, dvs. som sträcker sig från 1.1 till 3.7 μm våglängd; över mer än en optisk oktav. Detta genomfördes i endimensionellt strukturerade kristaller av stökiometrisk litiumtantalat dopade med 1 mol% magnesiumoxid (MgSLT), drivna av en laser vid 860 nm. Sådant bredbandigt koherent ljus är användbart t.ex. till att förstärka ultra- korta laserpulser, vilka kan användas inom attosekundsfysik, till spektroskopi av ult- rasnabba (kemiska) förlopp eller för industriell bearbetning av material. Vidare pre- senteras en studie i hur ogynnsamma intrinsiska uppkonverteringsprocesser påverkar spektrumet för detta kontinuum. Genom att ändra drivlaserns pulslängd kan deras inverkan förändras, vilket potentiellt erbjuder ett verktyg att förbättra den spektrala profilen hos bredbandiga parametriska ljuskällor. Tvådimensionella (2D) QPM-strukturer erbjuder särskilda möjligheter till att designa de vinkelmässiga aspekterna hos parametriskt genererat ljus. Detta behandlas specifikt i avhandlingen med en studie av hexagonala QPM-strukturer i MgSLT. Stu- dien demonstrerar hur två simultana nedkonverteringsprocesser, upprätthållna av en och samma QPM-struktur, kan sammankopplas koherent. Detta medför att två sepa- rata – men spektralt identiska – strålar av koherent ljus genereras, med tillämpningar inom exempelvis kvantoptik som kompakta ljuskällor av sammanflätade fotoner. Av- handlingen diskuterar vidare hur 2D QPM erbjuder utökade friheter att designa para- metriska ljuskällor, t.ex. genom att upprätthålla flera simultana parametriska konver- teringsprocesser i en enda ickelinjär kristall. Specifikt presenteras en strålkälla där ett flertal upp- och nedkonverteringsprocesser, i samma 2D QPM-struktur, genererar åt- skilliga strålar av koherent ljus med våglängder ända från infrarött till blått. vi vii List of publications This thesis is based on the following peer-reviewed journal articles: I. Levenius, M., V. Pasiskevicius, F. Laurell, and K. Gallo, Ultra-broadband op- tical parametric generation in periodically poled stoichiometric LiTaO3, Op- tics Express 19 (5) 4121-4128 (2011). II. Gallo, K., M. Levenius, F. Laurell, and V. Pasiskevicius, Twin-beam optical parametric generation in χ(2) nonlinear photonic crystals, Applied Physics Letters 98 (16) 161113-3 (2011). III. Levenius, M., M. Conforti, F. Baronio, V. Pasiskevicius, F. Laurell, C. De An- gelis, and K. Gallo, Multistep quadratic cascading in broadband optical par- ametric generation, Optics Letters 37 (10) 1727-1729 (2012). IV. Levenius, M., V. Pasiskevicius, and K. Gallo, Angular degrees of freedom in twin-beam parametric down-conversion, Applied Physics Letters 101 (12) 121114 (2012). viii Description of author’s contributions My contribution to the original articles was the following: I. I performed the experiments under supervision of Valdas Pasiskevicius and Katia Gallo and I performed the data evaluation accompanied by discussions with Katia Gallo and Valdas Pasiskevicius. I wrote the article with assistance from the co-authors. II. I took part in the experiments, conducted by Katia Gallo and Valdas Pasis- kevicius, and assisted Katia Gallo in the data evaluation. I assisted in writing the article. III. I performed the experimental work and the data evaluation, accompanied by discussions with Katia Gallo. I performed the comparison of experimental and numerical results, including discussions with Matteo Conforti and Fabio Baronio (of University of Brescia) and Katia Gallo. I wrote the article with as- sistance from the co-authors, with particular contributions (numerical aspects) from the international collaboration. IV. I performed the experimental work and the data evaluation, accompanied by discussions with Katia Gallo. I built a numerical model verifying the experi- mental results (described in section 5.2.2). I wrote the article with assistance from the co-authors. ix Further publications A. M. Levenius, V. Pasiskevicius, F. Laurell and K. Gallo, "Broadband Optical Parametric Generation in Periodically Poled Stoichiometric LiTaO3", Confer- ence on Lasers and Electro-Optics (CLEO), San José, California, USA, oral presentation CFN2, (2010) B. K. Gallo, M. Levenius, B. Vermersch, V. Pasiskievicius and F. Laurell, “Para- metric frequency downconversion devices in periodically poled Mg-doped stoichiometric lithium tantalate”