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Nanophotonic devices in thin film lithium niobate
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. (Nonlinear Quantum Photonics)ORCID iD: 0000-0002-3889-6223
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photonic devices play a fundamental role in today’s society and are a central building blocks for numerous applications, ranging from modern internet to sensing and manufacturing, and photonics is foreseen to be the backbone of future quantum internet and quantum communication systems thanks to the long coherence time of light. At variance with electronic integrated circuits, where silicon has been the material of election for many decades, in the case of photonic integrated circuits (PICs) there are numerous options available for the material substrate. One of the most promising platforms for future PICs is thin film lithium niobate (TFLN). 

Lithium niobate (LN) is a ferroelectric crystal characterized by a wide transparency window and excellent electro- and nonlinear optical properties. Additionally the thin film format allows the implementation of submicrometric devices, characterized by a footprint similar to the one realized on silicon and silicon nitride but with improved capabilities in terms of coherent electrical control of light and more efficient on-chip photon-photon interactions. This thesis demonstrates a few novel nanophotonic devices and contributes to the quickly developing TFLN technology platform, encompassing also hybrid integration processes. In term of monolithic TFLN nanowaveguide components, nanostructuring capabilities for ultrasmall footprint photonic devices have been developed and utilized to implement high quality factor resonators based on waveguide integrated phase shifted Bragg gratings (PSBG). The response of these devices, operating at telecom wavelength and characterized by a transmission bandwidth as narrow as 8.8 pm (corresponding to a quality factor Q in excess of 1.7×105) and by a footprint smaller than 500 μm2, was analyzed with the help of a model based on coupled mode theory (CMT) showing excellent agreement with the experimental data. This model provides insights on fabricated device losses and useful guidelines for the design of optimized PSBG. While spanning the full parameter space for device fabrication and optimization, deviations from such a model were also experimentally observed. Upon further investigation, these effects were explained as the results of a Fano resonance occurring in the PSBG structure involving the interaction of TE00 and TM01 modes in the supporting waveguide. The effect results in a narrowband and asymmetric response which can be tuned upon changing the waveguide design, as confirmed by experiments and simulations. 

The excellent sensitivity to refractive index changes of PSBG devices was leveraged to develop a comprehensive study of the electro- and the thermo- optical properties of X-cut TFLN. The study highlights the advantages and the limits of both approaches to device trimming and reconfigurability. The thesis includes also experimental contributions to dispersion-engineered TFLN waveguides, whose properties were characterized for fundamental TE and TM modes as function of the waveguide geometry in the telecom band by means of dual comb spectroscopy. 

This thesis addresses also hybrid photonic devices for on-chip light detection and emission, specifically demonstrating the integration of superconducting nanowire single-photon detectors (SNSPDs) and erbium emitters in TFLN. SNSPDs based on niobium-titanium-nitride (NbTiN) were integrated onto single mode TFLN nanowaveguides and their spectral sensitivity was leveraged to implement on-chip wavelength meters working in the telecom C- and L- bands, achieving photon counting and spectral sensitivity on a single waveguide- integrated device. Furthermore, a process for the effective incorporation of Er ions in TFLN was successfully developed in collaboration with researches at the university of Manchester. Structural and optical characterization of the Er:TFLN samples indicates essentially no disruption to the intrinsic properties of the LN crystal. The photoluminescence from the implanted films, emitting in the C-band, was studied as a function of temperature. 

The results hold promise for the implementation of a complete platform for on-chip quantum photonic circuits in the 1550 nm band, capable of operating at cryogenic temperature comprising on-demand single photon sources, electro-optic photon routing and manipulation as well as detection in TFLN circuits.

Abstract [sv]

Fotoniska komponenter spelar en väsentlig roll i dagens samhälle och utgör centrala byggstenar i många tillämpningar, allt från dagens internet till sensorer och tillverkning, och fotonik förväntas bli ryggraden i framtidens kvantinternet och kvantkommunikationssystem tack vare ljusets långa koherenstid. Till skillnad från elektroniska integrerade kretsar, där kisel har varit det föredragna materialet i många decennier, finns det många tillgängliga alternativ för materialsubstrat för fotoniska integrerade kretsar (PIC). En av de mest lovande plattformarna för framtida PIC:er är tunnfilmslitiumniobat ("thin film lithium niobate", TFLN). 

Litiumniobat (LN) är en ferroelektrisk kristall som kännetecknas av ett brett transparensfönster och utmärkta elektro- och ickelinjära optiska egenskaper. Dessutom tillåter tunnfilmsformatet att man kan lägga till dessa funktioners möjlighet att implementera sub-mikrometriska komponenter som kännetecknas av ett fotavtryck som liknar det som realiseras på kisel och kiselnitrid men med förbättrade egenskaper när det gäller koherent elektrisk styrning av ljus och effektivare foton-foton-interaktioner på chipet. Denna avhandling beskriver några nya nanofotoniska komponenter och bidrar tilltunnfilmslitiumniobat-teknologiplattformen som är under snabb utveckling. Avhandlingen inkluderar också hybrida integrationsprocesser. För monolitiska TFLN nanovågledarkomponenter har nanostruktureringskapabilitet för fotoniska enheter med ultrasmå fotavtryck utvecklats och dessa används för att implementera resonatorer med hög kvalitetsfaktor (Q) baserade på vågledarintegrerade fasförskjutna Bragg-gitter ("phase shifted Bragg gratings", PSBG). Den optiska responsen hos dessa komponenter, som arbetar vid telekomvåglängd och kännetecknas av en överförings-bandbredd så smal som 8.8 pm (motsvarande ett Q överstigande 1.7 x 105) och av ett fotavtryck mindre än 500 um2, analyserades med hjälp av en modell baserad på modkopplingsteori ("coupled mode theory", CMT) som visar utmärkt överensstämmelse med experimentell data. Modellen ger insikter om förluster hos tillverkade komponeneter och användbara riktlinjer för design av optimerad PSBG. Med hänsyn till alla parametrar för komponenttillverkning och optimering observerades även experimentella avvikelser från en sådan modell. Vid ytterligare undersökning förklarades dessa effekter som resultaten av en Fano-resonans vilket inträffar i PSBG-strukturen som involverar en interaktion mellan moderna TE00 och TM01 i vågledaren. Effekten resulterar i en smalbandig och asymmetrisk transmissionstopp som kan justeras genom modifiering av vågledardesignen, vilket har bekräftats av experiment och simuleringar. Den utmärkta känsligheten för förändringar av brytningsindex hos PSBG-enheter har utnyttjats för att utveckla en omfattande studie av elektro- och termo-optiska egenskaper hos X-kluven TFLN. Studien belyser fördelarna ochbegränsningarna för båda metoderna för komponenttrimning och omkonfigurerbarhet. Avhandlingen inkluderar även experimentella bidrag till dispersionskonstruerade TFLN-vågledare, vars egenskaper karakteriserades för fundamentala TE och TM moder som funktion av vågledargeometrin i telekombandet med hjälp av dubbelkamsspektroskopi ("dual comb spectroscopy", DCS).

Detta arbete tar även upp hybridfotoniska komponenter för ljusdetektering och emission på chip, vilket demonstrerar integrationen av supraledande nanotråds-enkelfotondetektorer ("superconducting nanowire single photon detectors", SNSPD) och erbiumsändare i TFLN.SNSPD:er baserade på NbTiN integrerades i enkelmods TFLN-nanovågledare och deras spektrala känslighet utnyttjades för att implementera en våglängdsmätare som arbetar i telekom C- och L-banden för att uppnå fotonräkningoch spektral känslighet på en enda vågledarintegrerad komponent. Vidare utvecklades enprocess för effektiv inkorporering av Er-joner i TFLN framgångsrikt i samarbete med forskare från University of Manchester. Strukturelloch optisk karakterisering av Er:TFLN-proverna indikerar ingen störning av LN-kristallens inre egenskaper. Fotoluminescensenfrån de implanterade filmerna, som emitterar i C-bandet, studerades som en funktion av temperatur.

Resultaten är lovande för framtida kvantfotoniska kretsar på chip i 1550 nm-bandet, som kan fungera vid kryogeniska temperaturer med på-begäran-enkelfotonkällor, elektrooptisk fotondirigering och manipulation samt detektering i TFLN-kretsar.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2024. , p. 93
Series
TRITA-SCI-FOU 2024:19
Keywords [en]
Integrated optics, Electro-optics, Thin film lithium niobate, Phase shifted Bragg grating, Waveguide-integrated single photon detectors, Erbium implantation
Keywords [sv]
Integrerad optik, elektrooptik, tunnfilmslitiumniobat, fasfo ̈rskjutet Bragg-gitter, V ̊agledare-integrerade singelfotondetektorer, Erbiumimplantation.
National Category
Atom and Molecular Physics and Optics
Research subject
Physics, Optics and Photonics
Identifiers
URN: urn:nbn:se:kth:diva-344838ISBN: 978-91-8040-885-1 (print)OAI: oai:DiVA.org:kth-344838DiVA, id: diva2:1847802
Public defence
2024-04-16, Room Pärlan, Hus 1, Albano Campus, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

QC 2024-04-03

Available from: 2024-04-03 Created: 2024-03-29 Last updated: 2024-04-03Bibliographically approved
List of papers
1. Tunable Ultranarrowband Grating Filters in Thin-Film Lithium Niobate
Open this publication in new window or tab >>Tunable Ultranarrowband Grating Filters in Thin-Film Lithium Niobate
2021 (English)In: ACS Photonics, E-ISSN 2330-4022, Vol. 8, no 10, p. 2923-2930Article in journal (Refereed) Published
Abstract [en]

Several applications in modern photonics require compact on-chip optical filters with a tailored spectral response. However, achieving subnanometric bandwidths and high extinction ratios is particularly challenging, especially in low-footprint device formats. Phase-shifted Bragg gratings implemented by the sidewall modulation of photonic nanowire waveguides are a good solution for on-chip narrowband operation with reasonable requirements in fabrication and scalability. In this work we report on their implementation and optimization in thin film lithium niobate, a photonic platform that affords reconfigurability by exploiting electrooptic effects. The phase-shifted Bragg grating filters have a footprint smaller than 1 mu m x 1 mm and operate at telecom wavelengths, featuring extinction ratios up to 25 dB. We demonstrate transmission bandwidths as narrow as 14.4 pm (Q = 1.1 x 10(5)) and 8.8 pm (Q = 1.76 x 10(5)) in critically coupled structures and multiwavelength Fabry-Perot configurations, respectively, in full agreement with theoretical predictions. Moreover, by taking advantage of the strong electro-optic effect in lithium niobate, in combination with the tight light confinement of nanophotonic wires and the ultranarrow spectral resonances of optimized grating structures, we demonstrate an electric tunability in peak wavelength and transmission of 25.1 pm/V and 2.1 dB/V, respectively, and a 10.5 dB contrast at CMOS voltages. The results pave the way for reconfigurable narrowband photonic filters with a small footprint and low consumption, to be exploited toward on-chip quantum and nonlinear optics, as well as optical sensing and microwave photonics.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2021
Keywords
Bragg grating, microcavity, nanophotonics, reconfigurable photonics, lithium niobate, electro-optics
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-304785 (URN)10.1021/acsphotonics.1c00383 (DOI)000710954200014 ()2-s2.0-85116242842 (Scopus ID)
Note

QC 20211117

Available from: 2021-11-17 Created: 2021-11-17 Last updated: 2024-03-29Bibliographically approved
2. Electro- and Thermo-Optics Response of X-Cut Thin Film LiNbO3Waveguides
Open this publication in new window or tab >>Electro- and Thermo-Optics Response of X-Cut Thin Film LiNbO3Waveguides
2023 (English)In: IEEE Journal of Quantum Electronics, ISSN 0018-9197, E-ISSN 1558-1713, Vol. 59, no 3, article id 0600108Article in journal (Refereed) Published
Abstract [en]

Lithium niobate has been for decades the material of election for integrated nonlinear and electro-optics. Its recent availability in thin films affording subwavelength confinement of light and nanostructuring capabilities has led to ground-breaking results in numerous applications, ranging from ultrafast signal processing to efficient nonlinear optics, where electro-optic (EO) and thermo-optic (TO) functionalities can be further leveraged for enhanced tunability and reconfigurability. This work provides a consistent comparison between these two approaches in the most widely used configuration in LiNbO3 nanophotonics at telecom wavelengths. Using state of the art Bragg grating technology for high precision index measurements, we evaluate the guided-wave EO and TO tunability to be 3\× 10-5 V-1 and 3.6× 10 -3W-1 , respectively, and study further operation and design tradeoffs, cross-talk effects and long-term stability. The results provide useful insights to identify the most appropriate strategies for implementing reconfigurable integrated photonic circuits effectively leveraging the unique features of LiNbO3.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2023
Keywords
electrooptics, integrated bragg filter, lithium niobate on insulator, Photonic integrated circuits, thermooptics
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-330040 (URN)10.1109/JQE.2023.3234986 (DOI)000994562600003 ()2-s2.0-85147226480 (Scopus ID)
Note

QC 20230706

Available from: 2023-06-27 Created: 2023-06-27 Last updated: 2024-03-29Bibliographically approved
3. Tailoring guided-wave Fano resonances in LiNbO3 nanophotonic wires
Open this publication in new window or tab >>Tailoring guided-wave Fano resonances in LiNbO3 nanophotonic wires
2023 (English)In: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023, Institute of Electrical and Electronics Engineers (IEEE) , 2023Conference paper, Published paper (Refereed)
Abstract [en]

Fano resonances in nanophotonic structures are attracting significant attention for the engineering possibilities they may offer in applications such as lasers, sensing and optical signal processing [1]. Most widely explored device architectures in the quest for fully integrated implementation scenarios involve the side-coupling of different waveguide-cavity systems providing the narrowband and broad resonances (discrete and continuum states, respectively) whose interference gives rise to the signature asymmetric Fano lineshapes. Here we present a brand-new approach to achieve Fano resonances in ultracompact 1D waveguide formats through a polarization diversity scheme, exploiting the longitudinal field components of guided modes in high confinement photonic wires in combination with integrated Bragg-resonant structures achieved by sidewall modulation of the same.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2023
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-339727 (URN)10.1109/CLEO/EUROPE-EQEC57999.2023.10231810 (DOI)2-s2.0-85175726828 (Scopus ID)
Conference
2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023, Munich, Germany, Jun 26 2023 - Jun 30 2023
Note

Part of ISBN 9798350345995

QC 20231116

Available from: 2023-11-16 Created: 2023-11-16 Last updated: 2024-03-29Bibliographically approved
4. Dual-comb dispersion measurement in LiNbO3-on-insulator waveguides at telecom wavelengths
Open this publication in new window or tab >>Dual-comb dispersion measurement in LiNbO3-on-insulator waveguides at telecom wavelengths
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-344837 (URN)
Available from: 2024-03-29 Created: 2024-03-29 Last updated: 2024-04-02
5. Wavelength-Sensitive Superconducting Single-Photon Detectors on Thin Film Lithium Niobate Waveguides
Open this publication in new window or tab >>Wavelength-Sensitive Superconducting Single-Photon Detectors on Thin Film Lithium Niobate Waveguides
Show others...
2023 (English)In: Nano Letters, ISSN 1530-6984, E-ISSN 1530-6992, Vol. 23, no 21, p. 9748-9752Article in journal (Refereed) Published
Abstract [en]

Lithium niobate, because of its nonlinear and electro-optical properties, is one of the materials of choice for photonic applications. The development of nanostructuring capabilities of thin film lithium niobate (TFLN) permits fabrication of small footprint, low-loss optical circuits. With the recent implementation of on-chip single-photon detectors, this architecture is among the most promising for realizing on-chip quantum optics experiments. In this Letter, we report on the implementation of superconducting nanowire single-photon detectors (SNSPDs) based on NbTiN on 300 nm thick TFLN ridge nano-waveguides. We demonstrate a waveguide-integrated wavelength meter based on the photon energy dependence of the superconducting detectors. The device operates at the telecom C- and L-bands and has a footprint smaller than 300 × 180 μm2 and critical currents between ∼12 and ∼14 μA, which ensures operation with minimum heat dissipation. Our results hold promise for future densely packed on-chip wavelength-multiplexed quantum communication systems.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023
Keywords
on-chip single-photon detector, on-chip wavelength meter, superconducting nanowire single-photon detector, thin film lithium niobate
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-340106 (URN)10.1021/acs.nanolett.3c02324 (DOI)001101957200001 ()37871304 (PubMedID)2-s2.0-85176509696 (Scopus ID)
Note

QC 20231128

Available from: 2023-11-28 Created: 2023-11-28 Last updated: 2024-03-29Bibliographically approved
6. Erbium implantation in thin film Lithium Niobate
Open this publication in new window or tab >>Erbium implantation in thin film Lithium Niobate
Show others...
2023 (English)In: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023, Institute of Electrical and Electronics Engineers (IEEE) , 2023Conference paper, Published paper (Refereed)
Abstract [en]

Lithium niobate on insulator (LNOI), thanks to its electro-optic properties and second order nonlinearity, is one of the most promising photonic materials for on-chip implementation of a complex photonic integrated circuit (PIC) [1]. Integration of rare earth ion emitters (RIE), characterized by high coherent transitions in both optical and microwave domains, into LNOI is a very attractive perspective to fully exploit the potential of this material in quantum optics applications and for on chip light generation and amplification. By choosing Erbium ions these functionalities can be implemented at telecom wavelengths (~1550 nm). Erbium integration in LNOI can be achieved using the smart cut technique [2]. However, this approach implies heating the material up to ~1100 ºC, approaching the Curie temperature of lithium niobate (~1200 ºC). Ion implantation also permits the incorporation of RIE into the lithium niobate (LN) crystal structure, operating at lower temperature with high spatial precision of the doped region in a complex PIC.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2023
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-339699 (URN)10.1109/CLEO/EUROPE-EQEC57999.2023.10232542 (DOI)2-s2.0-85175715141 (Scopus ID)
Conference
2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023, Munich, Germany, Jun 26 2023 - Jun 30 2023
Note

Part of ISBN 979-835034599-5

QC 20231116

Available from: 2023-11-16 Created: 2023-11-16 Last updated: 2024-03-29Bibliographically approved

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