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.
    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.
    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.
    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.
    L. Jiang, C. C. Evans, O. Reshef, and E. Mazur. 2013. “Optimizing anatase-TiO2 deposition for low-loss planar waveguides”. Publisher's VersionAbstract
    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 thin films and achieve higher quality, lower-loss films.
    C. C. Evans, J. D.B. Bradley, E. Armando Marti, and E. Mazur. 2012. “Mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm.” Optics Express, 20, Pp. 3118–3128. Publisher's VersionAbstract
    We observe mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm using the open aperture Z-scan technique. We fit the data with an extended model that includes multiphoton absorption, beam quality, and ellipticity. The extracted two- and three-photon absorption coefficients are below 1 mm/GW and 2 mm3/GW2, respectively. We observe negligible two-photon absorption for 813-nm light polarized along the extraordinary axis. We measure the nonlinear index of refraction and obtain two-photon nonlinear figures of merit greater than 1.1 at 774 nm and greater than 12 at 813 nm. Similarly, we obtain three-photon figures of merit that allow operational intensities up to 0.57 GW/mm2. We conclude that rutile is a promising material for all-optical switching applications around 800 nm.
    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, J. D.B. Bradley, J. Choy, O. Reshef, P. Deotare, M. Loncar, and E. Mazur. 2012. “Submicrometer-width TiO2 waveguides.” In . CLEO: Science and Innovations Waveguides and Passive Components (CM3M). Publisher's VersionAbstract
    We fabricate submicrometer-width TiO2 strip waveguides and measure optical losses at 633, 780, and 1550 nm. Losses of 30, 13, and 4 dB/cm (respectively) demonstrate that TiO2 is suitable for visible-to-infrared on-chip microphotonic devices.
    L. Tong and E. Mazur. 2008. “Nanophotonics and nanofibers.” In Handbook for Fiber Optic Data Communications: A Practical Guide to Optical Networking, edited by Casimer DeCusatis, Pp. 713–728. Academic Press. Publisher's VersionAbstract
    Nanophotonics is a fusion of photonics and nanotechnology, and is defined as nanoscale optical science and technology that includes nanoscale confinement of radiation, nanoscale confinement of matter, and nanoscale photoprocesses for nanofabrication [1.], [2.] and [3.]. While photonics has been widely used for fiber-optic data communication for decades, the application of nanotechnology for optical communication is an emerging technology. The basic motivation for incorporating photonics with nanotechnology is spurred by the requirement of increased integration of photonic devices for a variety of applications such as higher data transmission rates, faster response, lower energy consumption, and denser data storage [2]. For example, to reach an optical data transmission rate as high as 10Tb/s, the size of photonic matrix switching devices should be reduced to 100-nm scale [4].