Funding type: COFUND Marie Slodowska Curie Action

This project is under the Marie Skłodowska-Curie Actions (MSCA) program. There are no nationality conditions but the candidates must fulfill the MSCA mobility conditions, which means that she/he must not have stayed more than 1 year in France during the last 3 years immediately before the call deadline (31/05/2019).

Expected start date: 01/10/2020

Contacts:

Pr. Christelle Monat, INL/Ecole centrale de Lyon

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Pr. Arnan Mitchell, RMIT

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Domain and scientific context :

In the past two decades, silicon photonics has emerged as a mature technological platform allowing for multiple optical functions to be integrated onto the same chip [1]. Electro-optic modulators, SiGe photodetectors and low-loss silicon waveguides are now available. However, when it comes to light emission or nonlinear functions, silicon turns out to be intrinsically limited. The heterogeneous integration of III-V materials onto silicon has already provided a way to realize efficient LED or laser devices [2]. Similarly, several material candidates are investigated for their nonlinear properties, with the aim to integrate them onto the mature silicon photonic platform. The nonlinear optical response of materials can enable attractive optical devices such as all-optical switches and even amplifiers that can directly control light signals with other light signals. These all optical nonlinear devices are much faster than their optoelectronic counterparts and perhaps more importantly, they can enable completely new functions such as wavelength conversion, the generation of frequency combs or supercontinuum light. More generally, a wide range of nonlinear devices can be realized for information processing using light control signals [3]. These could advantageously complement the power-hungry and bulky electronic routers that are used in telecommunications. These routers perform data processing and signal routing in the electrical domain, by converting optical signals into the electical domain and after processing back in the optical domain to convey information across the Internet network. However, as the data rate increases so does the energy consumption of the electronic components in routers, calling for the need to develop alternative and disrupting technologies. All-optical devices could play a central role there.

Keywords : nanophotonics, nonlinear optics, integrated optics, lithium niobate

Objectives and scientific challenges:

Despite the high application potential of nonlinear optics for all-optical information processing, no nonlinear material candidate has emerged as a clear choice to complement silicon photonics so far. On the one hand, wide band gap semiconductors have been investigated, but their integration onto silicon photonics is not straightforward. Glass materials have also been explored, but their relatively weak nonlinearity precludes the realization of compact devices. Lithium niobate (LiNBO₃) possesses both a second-order (c⁽²⁾) and third-order (c⁽³⁾) nonlinearity, which proves useful for both electro-optical modulation and also all-optical signal processing devices [1]. However, it has been very difficult to create high performance nanophotonic devices due the low index contrast typically offered by this technology.

Very recently, thin-film lithium niobate on insulator wafers [2] have become commercially available and emerged as a highly promising platform for integrated nonlinear optics [3]. Most importantly, this platform supports tightly confining waveguide geometries, a boost for nonlinearities, while additionally opening opportunities for dispersion engineering, which is key to device efficiency and broadband processes, such as frequency combs and supercontinuum [4,5]. The combination of both c⁽²⁾and c⁽³⁾ responses can, in contrast with silicon where only c⁽³⁾ does exist, provide new ways of electrically tuning all-optical nonlinear functions. Furthermore, the birefringence of lithium niobate and ferroelectric domain inversion capabilities provide opportunities for phase matching and quasi phase matching, which is critical for frequency conversion processes, such as four-wave mixing or high-order harmonic generation.

RMIT has developed a complementary route towards high performance thin-film lithium niobate based devices that exploits strip loading of another thin film, silicon nitride (Si₃N₄) for instance, which is patterned instead of the lithium niobate so as to support low loss guided modes. This approach elegantly alleviates the need for lithium niobate patterning, while still offering relatively tightly confining geometries (Aeff ~2µm²) and the possibility to engineer the dispersion. It might also be useful for the creation of hybrid devices where both the nonlinear response of silicon nitride and lithium niobate can be harnessed to create efficient devices. First reports of nonlinear second-harmonic generation with high efficiency have already been demonstrated on this platform [6], which holds the potential for supporting a much wider variety of nonlinear functions, from tunable frequency combs to highly efficient all-optical signal processing devices and broadband supercontinuum.

The specific objectives of this PhD study will be (1) to exploit strip-loaded lithium niobate on insulator waveguides and resonators to design and realize highly efficient c⁽³⁾ nonlinear devices with anomalous dispersion for frequency conversion as well as supercontinuum and frequency comb generation, (2) to experimentally demonstrate the integration of electro-optical modulators with c⁽³⁾ ring resonators on a single chip, where the modulation frequency of the modulator can be tuned to match the free spectral range of the ring resonator, which will enable the efficient generation of optical frequency combs, (3) explore the alternative path offered by c⁽²⁾ nonlinear optics via quasi-phase matching in lithium niobate for the generation of optical frequency combs and (4) use the c⁽²⁾ response of lithium niobate to realize tunable nonlinear functions

Expected original contributions :

• Contribution to the development of a generic integrated photonic platform made of strip-loaded thin-film lithium niobate on insulator for high performance nonlinear optics on a chip
• Demonstration of all-optical signal processing devices and broadband supercontinuum using low-power pump sources
• Contribution to the field of frequency combs, by investigation of the potential interplay between electro-optic and Kerr combs as well as using c⁽²⁾ for comb generation

Research program and methodology :

The PhD student will be involved in the design, fabrication and test of the lithium niobate on insulator nonlinear devices at telecom wavelengths (l~1,55mm). This will include the patterning of the strip-loaded silicon nitride material deposited on top of the thin-film lithium niobate on insulator wafer for producing the nanophotonic devices by traditional clean-room nanofabrication (nanolithography, etching) at the NANOLYON nanotechnology platform and the MNRF Melbourne (for waveguide structures and periodic poling). The PhD student will also drive the characterization efforts of the resulting optical devices using the characterization setups available at INL (both linear and nonlinear optical test-beds) and RMIT. The student will be working with the Nanophotonics research group at INL hosted by Ecole Centrale de Lyon, the Integrated Photonics and Applications Centre (InPAC) by Prof. Arnan Mitchell at RMIT for integrated photonic devices. The student will benefit from INL's and RMIT’s resources and expertise in silicon photonics, non-linear optics and lithium niobate periodic poling, both in terms of device design and on technology and clean room manufacturing aspects for the production of the first basic demonstrators. S/he will contribute to develop an original nonlinear integrated platform with as long-lasting impact for Datacom and telecom applications.

Tentative timeline for the PhD studies:

Year 1 (in Lyon): Simulation and design of nonlinear optical waveguides in strip loaded lithium niobate on insulator waveguides for c⁽²⁾ and c⁽³⁾ optical nonlinearity as well as anomalous dispersion (RMIT/INL). Investigation of theoretical limitations when using c⁽²⁾ for optical frequency generation and comparison with traditional c⁽³⁾ devices (RMIT/INL). First characterization of nonlinear optical devices provided by RMIT using optical test-beds of INL (frequency conversion, supercontinuum generation).

Year 2 (in Melbourne): Introduction in the fabrication of nonlinear optical lithium niobate on insulator waveguides in the Micro Nano Research Facility (RMIT). Characterisation of the nonlinear optical devices (ring resonators) and use of the measured data to optimize the design (RMIT/INL). Optical frequency comb generation in c⁽²⁾ and c⁽³⁾ nonlinear optical devices and characterisation of typical comb parameters such as comb state, bandwidth, stability, phase noise of comb lines. Preliminary assessment of the usefulness of such combs for communcation experiments.

Year 3 (in Lyon): Realization / characterization of optimized devices and nonlinear demonstrators as per the work of Year 1-2. Use the c⁽²⁾ response of lithium niobate for efficient tuning of the nonlinear devices. Writing of the thesis.

Scientific supervision:

• Description of the supervision committee :

• Integration inside the laboratories (percentage of working time inside these laboratories) : 67% at INL, 33% at RMIT

PhD funding : Co-Fund Marie Sladowska Curie Action (MSCA) ECL/RMIT (ECLAUsion program)

Profile of the candidate :

We seek a talented and ambitious researcher with a good knowledge and a solid background in the fields of solid-state physics, nonlinear and/or integrated optics, and semiconductor devices. S/he should work towards his/her Masters/honours or Engineering degree in a field apposite to one of these areas. An experience in photonics, clean-room fabrication, material deposition or optical modeling and characterization will be strongly appreciated.

Objectives for the valorization of the research work:

The results obtained will be published in peer-reviewed journals with a high impact factor and presented at international conferences in the field (CLEO-US/Europe, SPIE photonics Europe, SPIE Photonic West, NLO...) as well as to national conferences (ACOFT in Australia, Nonlinear optics GDR workshops in France).

Skills that will be developed during the PhD :

The PhD work is rooted in photonics, nanotechnology and nonlinear optics. The student will develop skills related to technological areas that are directly relevant to the telecom/ Datacom industry. The PhD student will gain experience from the device design (simulation and design of optical microcomponents, FDTD-Finite difference time domain, FEMSIM-finite element method, Nonlinear Schrodinger equation), the nanofabrication of these devices in clean room environments (e-beam and optical lithography, dry and wet etching), and their characterization (using the wide range of setups - microreflectivity, Fourier optics, linear and non-linear optical characterization­ available at INL). S/he will also acquire unique know-how on the lithium niobate material which is already used for modulation in telecom applications, but strongly expand the prospects of this material. The highly collaborative and international environment of the project will require the student to develop, in addition to technical and scientific skills, communication, teamwork and project management skills.

Professional opportunities after the PhD:

The conceptual, scientific and technological work carried out by the PhD student on the creation of a new generation of photonic components will result in the emergence of a platform that is completely different from current technologies. The created platform will be very generic for sustaining a wide variety of nonlinear functions (from processing signals to the generation of non-classical light sources), which the PhD student will have direct experience with. Finally, the nanotechnology and computation tools used by the student during the PhD will be relevant for a high number of application fields and will allow him/her to find a job in the photonics or microelectronics industry. The future prospects for the student, at the end of his thesis, include the possibility of pursuing an academic career in a prestigious photonics laboratory or joining an industry in the microelectronics, photonics or telecom sector.

Bibliographic references about the PhD topic :

1. David Thomson et al “Roadmap on silicon photonics” J. Opt. 18, 073003 (2016)
2. Tin Komljenovic, Michael Davenport, Jared Hulme, Alan Y. Liu, Christos T. Santis, Alexander Spott, Sudharsanan Srinivasan, Eric J. Stanton, Chong Zhang, and John E. Bowers, "Heterogeneous Silicon Photonic Integrated Circuits," J. Lightwave Technol. 34, 20-35 (2016)
3. S. M. Hendrickson, A. C. Foster, R. M. Camacho, and B. D. Clader, “Integrated nonlinear photonics: emerging applications and ongoing challenges” J. Opt. Soc. Am. B 31, 3193 (2014)
4. L. Arizmendi, “Photonic applications of lithium niobate crystals”, Physica status solidi (a) 201, 253, (2004)
5. A. Boes, B. Corcoran, L. Chang, J. Bowers, and A. Mitchell, “Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits” Laser & Photonics Reviews 12, 1700256 (2018).
6. C. Wang, M. Zhang, X. Chen, M. Bertrand, A. Shams-Ansari, S. Chandrasekhar, P. Winzer & M. Lončar, “Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages” Nature 562, pages101–104 (2018)
7. M. Zhang et al. « Broadband electro-optic frequency comb generaiton in a lithium niobate microring resonator » Nature 568, 373 (2019)
8. M. Yu, “Coherent two-octave spanning supercontinuum generation in lithium niobate waveguides”, Optics Letters 44, 1222 (2019)
9. A. Boes, L. Chang, M. Knoerzer, T. G. Nguyen, J. D. Peters, J. E. Bowers, and A. Mitchell, “Improved second harmonic performance in periodically poled LNOI wavegudes through engineering of lateral leakage”, Opticals Express 27, 23919 (2019)