Nonlinear Nanophotonics

Effects of annealing on optical properties of TiO2 planar waveguides, at Photonics West 2011 (San Francisco, CA), Tuesday, January 25, 2011:
Titanium dioxide possesses a high second order nonlinear index, making it a potential material for all-optical switching. In order to exploit these properties, TiO2 thin films with high refractive indices and low losses are required. We deposit TiO2 waveguides via RF reactive sputtering onto oxidized silicon substrates. We find that film characteristics vary greatly with deposition temperature, suggesting that they may also exhibit strong sensitivity to post-deposition annealing. We annealed TiO2 thin films at various temperatures in an oxygen environment using a tube furnace. The annealed... Read more about Effects of annealing on optical properties of TiO2 planar waveguides
Zero-index nanophotonics, nonlinear optics, and quantum entanglement, at Nano-optics: Principles enabling basic research and applications, Centro Ettore Majorana (Erice), Saturday, July 22, 2017
In these interactive lectures we explore how light can be manipulated at the nanoscale. We begin by describing optical propagation in ordinary materials and then show how materials can be engineered to achieve a refractive index of zero. These zero-index materials have remarkable properties and can be integrated in photonic circuits. We also give an introduction to nonlinear optics and discuss how zero-index materials can be used to accomplish phase matching in nonlinear optics and to obtain longer coherence times and lengths among entangled emitters or qubits.

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Read more about Zero-index nanophotonics, nonlinear optics, and quantum entanglement
Optimizing Anatase-TiO2 Deposition for Low-loss Waveguides, at Photonics West (San Francisco, CA), Tuesday, February 5, 2013:
Polycrystalline anatase-TiO2 thin film possesses desirable properties for on-chip photonic devices that can be used for optic computing, communication, and sensing. Low-loss anatase-TiO2 thin films are necessary for fabricating high quality optical devices. We studied anatase-TiO2 by reactively sputtering titanium metal in an oxygen environment and annealing. By correlating key deposition parameters, including oxygen flow rate, deposition pressure, RF power, and temperature to film morphology and planar waveguiding losses, we aim to understand the dominant source of propagation losses in TiO2... Read more about Optimizing Anatase-TiO2 Deposition for Low-loss Waveguides
Simultaneous multiphoton absorption in rutile (TiO2) across the half-bandgap, at Photonics West 2012 (San Francisco, CA), Thursday, January 26, 2012:
Future optical systems require compact, ultra-fast devices capable of switching and logic across a wide range of wavelengths. To realize this goal, ultrafast nonlinearities must be exploited while maintaining manageable linear losses and nonlinear absorption. We present TiO2 as a nonlinear material to meet these needs. TiO2 is highly transparent for wavelengths > 400 nm and possesses both high linear and nonlinear refractive indices. We measurements the nonlinear index and multiphoton absorption in bulk TiO2 (rutile) using the z-scan technique near the half bandgap (800 nm). Using... Read more about Simultaneous multiphoton absorption in rutile (TiO2) across the half-bandgap
J. D.B. Bradley, C. C. Evans, J. Choy, O. Reshef, P. Deotare, F. Parsy, K. Phillips, M. Loncar, and E. Mazur. 2012. “Submicrometer-wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices.” Optics Express, 20, Pp. 23821–23831. Publisher's VersionAbstract
We demonstrate amorphous and polycrystalline anatase TiO2 thin films and submicrometer-wide waveguides with promising optical properties for microphotonic devices. We deposit both amorphous and polycrystalline anatase TiO2 using reactive sputtering and define waveguides using electron-beam lithography and reactive ion etching. For the amorphous TiO2, we obtain propagation losses of 0.12 dB/mm at 633 nm and 0.04 dB/mm at 1550 nm in thin films and 3 dB/mm at 633 nm and 0.4 ± 0.2 dB/mm at 1550 nm in waveguides. Using single-mode amorphous TiO2 waveguides, we characterize microphotonic features including microbends and optical couplers. We show transmission of 780-nm light through microbends having radii down to 2 μm and variable signal splitting in microphotonic couplers with coupling lengths of 10 μm.
C. C. Evans, K. Shtyrkova, J. D.B. Bradley, O. Reshef, E. Ippen, and E. Mazur. 2013. “Spectral broadening in anatase titanium dioxide waveguides at telecommunication and near-visible wavelengths.” Optics Express, 21, Pp. 18582–18591. Publisher's VersionAbstract
We observe spectral broadening of femtosecond pulses in single-mode anatase- titanium dioxide (TiO2) waveguides at telecommunication and near-visible wavelengths (1565 and 794 nm). By fitting our data to nonlinear pulse propagation simulations, we quantify nonlinear optical parameters around 1565 nm. Our fitting yields a nonlinear refractive index of 0.16 × 10−18 m2/W, no two-photon absorption, and stimulated Raman scattering from the 144 cm−1 Raman line of anatase with a gain coefficient of 6.6 × 10−12 m/W. Additionally, we report on asymmetric spectral broadening around 794 nm. The wide wavelength applicability and negligible two-photon absorption of TiO2 make it a promising material for integrated photonics.
C. C. Evans, K. Shtyrkova, O. Reshef, M. Gerhard Moebius, J. D.B. Bradley, S. Griesse-Nascimento, E. Ippen, and E. Mazur. 2015. “Multimode phase-matched third-harmonic generation in sub-micrometer-wide anatase TiO2 waveguides.” Optics Express, 23, Pp. 7832–7841. Publisher's VersionAbstract
Third-harmonic generation (THG) has applications ranging from wavelength conversion to pulse characterization, and has important implications for quantum sources of entangled photons. However, on-chip THG devices are nearly unexplored because bulk techniques are difficult to adapt to integrated photonic circuits. Using sub- micrometer-wide polycrys- talline anatase TiO2 waveguides, we demonstrate third-harmonic generation on a CMOS-compatible platform. We correlate higher conversion effi- ciencies with phase-matching between the fundamental pump mode and higher-order signal modes. Using scattered light, we estimate conversion efficiencies as high as 2.5% using femtosecond pulses, and thus demonstrate that multimode TiO2 waveguides are promising for wideband wavelength conversion and new applications ranging from sensors to triplet-photon sources.
M. Gerhard Moebius, F. Herrera, S. Griesse-Nascimento, O. Reshef, C. C. Evans, G. G. Guerreschi, A. n. Aspuru-Guzik, and E. Mazur. 2016. “Efficient photon triplet generation in integrated nanophotonic waveguides.” Optics Express, 24, Pp. 9932–9954. Publisher's VersionAbstract
Generation of entangled photons in nonlinear media constitutes a basic building block of modern photonic quantum technology. Current optical materials are severely limited in their ability to produce three or more entangled photons in a single event due to weak nonlinearities and challenges achieving phase-matching. We use integrated nanophotonics to enhance nonlinear interactions and develop protocols to design multimode waveguides that enable sustained phase-matching for third-order spontaneous parametric down-conversion (TOSPDC). We predict a generation efficiency of 0.13 triplets/s/mW of pump power in TiO2-based integrated waveguides, an order of magnitude higher than previous theoretical and experimental demonstrations. We experimentally verify our device design methods in TiO2 waveguides using third-harmonic generation (THG), the reverse process of TOSPDC that is subject to the same phase-matching constraints. We finally discuss the effect of finite detector bandwidth and photon losses on the energy- time coherence properties of the expected TOSPDC source.
O. Reshef, K. Shtyrkova, M. Gerhard Moebius, S. Griesse-Nascimento, S. Spector, C. C. Evans, E. Ippen, and E. Mazur. 2015. “Polycrystalline Anatase Titanium Dioxide Micro-ring Resonators with Negative Thermo-optic Coefficient.” J. Opt. Soc. Am. B, 32, Pp. 2288–2293. Publisher's VersionAbstract
We fabricate polycrystalline anatase TiO2 micro-ring resonators with loaded quality factors as high as 25,000 and average losses of 0.58 dB/mm in the telecommunications band. Additionally, we measure a negative thermo-optic coefficient dn/dT of −4.9 ± 0.5 × 10−5 K−1. The presented fabrication uses CMOS- compatible lithographic techniques that take advantage of substrate-independent, non-epitaxial growth. These properties make polycrystalline anatase a promising candidate for the implementation of athermal, vertically-integrated, CMOS- compatible nanophotonic devices for nonlinear applications.

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