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Nonlinear optics in KTiOPO4 for spectral management of ultra-short pulses in the near- and mid-IR
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.ORCID iD: 0000-0002-4452-0759
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis explores the possibilities of controlling nonlinear optical interactions in ferroelectric materials for bandwidth tailoring of ultrashort pulses in the pico- and femto-second range. The control is achieved through quasi-phase matching, which is based on alternating the material’s spontaneous polarization into different domains. In the presented work, KTiOPO4 (KTP) is the material of choice as it provides high optical nonlinearity, a wide transparency window in the near- and mid-infrared (IR), as well as a high damage threshold. Furthermore, KTP also enables fabrication of uniform high aspect ratio and fine-pitch domain structures of high quality. These qualities make KTP highly attractive for a vast range of applications and enabled much of the work presented in this thesis.The propagation of ultrashort pulses in domain-structured ferroelectrics was studied numerically with a model based on a single nonlinear envelope equation. This model accounts for the absorption and the dispersion of the material, as well as the second- and the third-order nonlinearities. Supercontinuum generation in the near- and mid-IR was studied in periodically structured KTP for femtosecond pulses at 1.5 μm. The numerical results showed the potential for pulse self-compression with octave-spanning spectral broadening. This process is enabled by cascaded second-order nonlinearities and can be tailored by the phase-matching parameters, which are set by the structure’s periodicity. The proposed design presented in this work resulted in a negative effective Kerr nonlinear coefficient with a magnitude of 1.65x10-14 cm2/W in the positive dispersion regime, which is one order of magnitude higher than the natural Kerr coefficient in KTP. Experimental characterization of single pass propagation of 128 fs-long pulses at 1.52 μm through a periodically structured KTP sample, with a periodicity of 36 μm based on the proposed design, are also presented. The results show a spectral broadening from 1.1 μm to 2.7 μm and a simultaneous compression down to 18.6 fs, thus confirming the numerical findings. Bandwidth tailoring of ultrashort IR pulses in the picosecond range was also studied through devices known as backward-wave optical parametric oscillators (BWOPOs). These devices rely on sub-micrometer domain periods to generate counter-propagating signal and idler waves. In a BWOPO, the forward-generated wave inherits the phase modulation of the pump wave, while the backward-generated wave is inherently narrowband and basically insensitive to pump wavelength tuning. In this work, BWOPOs operated in a cascaded manner, with the forward-generated wave being employed as a pump in a single pass configuration, were studied. The tunability issue of the narrowband backward wave was solved by employing a broadband optical parametric amplifier seeded by the BWOPO forward wave. A tunability of 2.7 THz for a wave at 1.87 μm with a bandwidth of 28 GHz was demonstrated. The coherent phase transfer from the pump to the BWOPO forward wave was investigated in the context of pulse compression. In this experiment, a 220 GHz bandwidth was transferred from 800 nm to 150 ps-long pulses at 1.4 μm, which could be compressed down to 1.3 ps with μJ energy, in a single-grating compressor.

Abstract [sv]

I den här avhandlingen undersöks möjligheterna att kontrollera ickelinjära optiska processer iferroelektriska material. Detta för att kunna anpassa bandbredden hos ultrakorta pulser i piko- ochfemtosekundsområdena. Processerna kontrolleras genom kvasifasmatchning, vilket innebär attmaterialets spontana polarisation alterneras på ett sätt som ger upphov till olika domäner. Arbetetsom presenteras här avser främst användandet av kristallen KTiOPO4 (KTP), då den har en högoptisk ickelinjäritet, en bred transmission i de nära- och mellan-infraröda områdena samt en högskadetröskel. Dessutom möjliggör KTP tillverkning av uniforma samt finstrukturerade domänermed hög kontrast och kvalitet. Dessa egenskaper gör KTP till ett attraktivt material för en stormängd tillämpningar och har möjliggjort mycket av arbetet i denna avhandling.Utbredning av ultrakorta pulser i domänstrukturerade ferroelektriska material studeradesnumeriskt med en modell som baserades på en ickelinjär amplitudekvation. Modellen tar hänsyntill materialets absorption och dispersion så väl som andra- och tredje ordningens ickelinjäriteter.Superkontinuumgenerering i det nära- och mellan-infraröda området undersöktes för periodisktstrukturerat KTP i fallet med femtosekundspulser vid 1.5 μm. Beräkningarna visade att pulsernakan genomgå självkomprimering i samband med en oktavsträckande spektral breddning. Dessaprocesser möjligörs av kaskaderade andra ordningens ickelinjäriteter och kan anpassas medkvasifasmatchningsparametrarna, vilka bestäms av strukturens periodicitet. Designen sompresenteras i detta arbete gav upphov till en negativ effektiv ickelinjär Kerr-koefficient med enstorlek på 1.651014 cm2/W i det positiva dispersionsområdet, vilket är en storleksordning högreän den naturliga Kerr-koefficienten i KTP. Karaktäriseringen av ett enkelpasseringsexperimentmed en 128 fs-puls vid 1.52 μm propagerandes genom en periodiskt strukturerad KTP-kristall,med en periodicitet på 36 μm, presenteras också. Resultaten visar på en spektral breddning från1.1 μm till 2.7 μm samt en komprimering ner till 18.6 fs, vilket således verifierar de numeriskaresultaten.Möjligheten att anpassa bandbredden för ultrakorta infraröda pulser studerades även ipikosekundsområdet med s.k. baklängesvågs-optiska parametriska oscillatorer (BWOPO). Denhär sortens system bygger på domänstrukturer med sub-mikrometerperioder för att genereramotpropagerande signal- och komplementärsignalvågor. I dessa system får den framåtgenereradevågen samma fasmodulation som pumpvågen, medan den bakåtgenererade vågen är intrinsisktsmalbanding och i stort sett okänslig för ändringar i pumpvåglängden. I detta arbete studeraskaskaderade BWOPO-system, där den framåtgenererade vågen används som pump i enenkelpasseringskonfiguration. Problemet med att ändra våglängden på den smalbandigabakåtpropagerande signalen löstes genom att passera den genom en bredbanding optiskparametrisk förstärkare, vilket möjliggjorde ett frekvensskift på 2.7 THz för en våglängd kring1.87 μm med en bandbredd på 28 GHz. Den koherenta fasöverföringen från pumpen till denframåtpropagerande BWOPO-vågen undersöktes även genom pulskomprimering. I dettaexperiment så överfördes 220 GHz bandbredd från 800 nm till en 150 ps lång puls vid 1.4 μm,vilken kunde komprimeras till 1.3 ps i en enkel-gitterkompressor med μJ energi.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 106
Series
TRITA-SCI-FOU ; 2019:15
Keywords [en]
Nonlinear optics; Ferroelectrics
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-251410ISBN: 978-91-7873-183-1 (print)OAI: oai:DiVA.org:kth-251410DiVA, id: diva2:1315525
Public defence
2019-06-14, FA31, Albanova University Center, 10:00 (English)
Opponent
Supervisors
Note

QC20190514

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-14Bibliographically approved
List of papers
1. Cascaded counter-propagating nonlinear interactions inhighly-efficient sub-μm periodically poled crystals
Open this publication in new window or tab >>Cascaded counter-propagating nonlinear interactions inhighly-efficient sub-μm periodically poled crystals
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 8037Article in journal (Refereed) Published
Abstract [en]

Mirrorless optical parametric oscillators (MOPOs) are very attractive parametric devices that rely on the nonlinear interaction of counter-propagating photons to inherently establish distributed feedback, without the use of external mirrors or surface coatings. These devices offer unique spectral and coherence properties that will benefit a large variety of applications ranging from spectroscopy to quantum communications. The major obstacle in exploiting their full potential is ascribed to the difficulty in engineering a nonlinear material in which the generation of counter-propagating waves can be phase matched. Here we present a reliable and consistent technique for fabrication of highly-efficient sub-micrometer periodically poled Rb-doped KTiOPO4. We experimentally demonstrate the first cascaded counter-propagating interactions in which the generated forward signal serves as a pump for a secondary MOPO process, reaching pump depletion larger than 60%. The cascaded process exemplifies the high efficiency of our nonlinear photonic structures. Our domain-engineering technique paves the way to realize counter-propagating schemes and devices that have been deemed unfeasible until now.

Place, publisher, year, edition, pages
Nature Publishing Group, 2017
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-207158 (URN)10.1038/s41598-017-07016-y (DOI)000407559800004 ()28808234 (PubMedID)
Note

QC 20170602

Available from: 2017-05-17 Created: 2017-05-17 Last updated: 2019-05-14Bibliographically approved
2. Supercontinuum generation and soliton self-compression in chi((2))-structured KTiOPO4
Open this publication in new window or tab >>Supercontinuum generation and soliton self-compression in chi((2))-structured KTiOPO4
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2018 (English)In: Optica, ISSN 2334-2536, Vol. 5, no 6, p. 711-717Article in journal (Refereed) Published
Abstract [en]

Ultrafast sources in the mid-IR are indispensable research tools for spectroscopic and medical applications and can also potentially be used to generate attosecond pulses. We investigated a route to directly achieve self-compressed supercontinuum pulses with an octave-exceeding spectrum extending into the mid-IR by employing self-defocusing nonlinearities obtained through cascaded chi((2)) interactions in domain-structured ferroelectrics. A model was developed based on a single-wave nonlinear envelope equation that accounts for cascaded chi((2)) nonlinearities, the native Kerr response from the third-order nonlinear polarization, and the delayed Raman response. Experimental validation of the model was carried out by using an in-house fabricated, periodically poled, Rb-doped KTiOPO4 crystal with a period of 36 mu m. A supercontinuum spectrum spanning from 1.1 to 2.7 mu m was achieved, as well as self-compression down to 18.6 fs, from a 128 fs pump pulse at 1.52 mu m. Using the actual pump pulse and sample parameters, excellent agreement was reached between the model and the experimental results, thus proving the validity of the model. As periodically poled KTiOPO4 can be obtained with large apertures, this approach is energy scalable and hence promising for future development in the field.

Place, publisher, year, edition, pages
Optical Society of America, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-232257 (URN)10.1364/OPTICA.5.000711 (DOI)000435967000008 ()2-s2.0-85048790962 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2016.0104European Regional Development Fund (ERDF), 01.2.2-LMT-K-718
Note

QC 20180720

Available from: 2018-07-20 Created: 2018-07-20 Last updated: 2019-05-14Bibliographically approved
3. Narrowband, tunable, infrared radiation by parametric amplification of a chirped backward-wave OPO signal
Open this publication in new window or tab >>Narrowband, tunable, infrared radiation by parametric amplification of a chirped backward-wave OPO signal
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2019 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 27, no 8, p. 10602-10610, article id 360745Article in journal (Refereed) Published
National Category
Natural Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-249969 (URN)000464614400044 ()2-s2.0-85064454429 (Scopus ID)
Note

QC 20190516

Available from: 2019-04-24 Created: 2019-04-24 Last updated: 2019-05-16Bibliographically approved
4. Coherent phase transfer and pulse compression at 1.4 µm in a backward-wave OPO
Open this publication in new window or tab >>Coherent phase transfer and pulse compression at 1.4 µm in a backward-wave OPO
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-251009 (URN)
Note

QC 20190516

Available from: 2019-05-07 Created: 2019-05-07 Last updated: 2019-05-16Bibliographically approved

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