Jonathan D. B. Bradley

Submicrometer-wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices.

We demonstrate amorphous and polycrystalline anatase TiO(2) thin films and submicrometer-wide waveguides with promising optical properties for microphotonic devices. We deposit both amorphous and polycrystalline anatase TiO(2) using reactive sputtering and define waveguides using electron-beam lithography and reactive ion etching. For the amorphous TiO(2), we obtain propagation losses of 0.12 ± 0.02 dB/mm at 633 nm and 0.04 ± 0.01 dB/mm at 1550 nm in thin films and 2.6 ± 0.5 dB/mm at 633 nm and 0.4 ± 0.2 dB/mm at 1550 nm in waveguides. Using single-mode amorphous TiO(2) 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.

Integrated TiO2 resonators for visible photonics.

We demonstrate waveguide-coupled titanium dioxide (TiO(2) racetrack resonators with loaded quality factors of 2.2×10(4) for the visible wavelengths. The structures were fabricated in sputtered TiO(2) thin films on oxidized silicon substrates using standard top-down nanofabrication techniques, and passively probed in transmission measurements using a tunable red laser.

Mixed two- and three-photon absorption in bulk rutile (TiO 2 ) around 800 nm

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.

Spectral broadening in anatase titanium dioxide waveguides at telecommunication and near-visible wavelengths

We observe spectral broadening of femtosecond pulses in single-mode anatase-titanium dioxide (TiO(2)) 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) m(2)/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 TiO(2) make it a promising material for integrated photonics.

Multimode phase-matched third-harmonic generation in sub-micrometer-wide anatase TiO 2 waveguides

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 polycrystalline anatase TiO₂ waveguides, we demonstrate third-harmonic generation on a CMOS-compatible platform. We correlate higher conversion efficiencies 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.

Low-loss TiO 2 planar waveguides for nanophotonic applications

We deposit TiO2 planar waveguides on oxidized silicon substrates by reactive sputtering. The films exhibit Raman spectra consistent with an amorphous or anatase phase and have losses as low as 0.4 dB/cm at 826 nm.

Ultrafast All-Optical Switching in TiO2

Titanium dioxide (TiO2) is a promising yet unexplored material for ultrafast, on-chip nonlinear optical devices. Here, we explore TiO2′s capacity for nonlinear applications and then fabricate linear on-chip devices using this material. We measure TiO2′s Kerr nonlinearity to be 30 times that found in silica glass by using the Z-scan technique with a bulk sample. During the same experiment, the low two-photon absorption observed can enable all-optical applications around 800 nm. To realize devices, we require waveguides made from thin films of TiO2. We deposit our thin films on oxidized silicon wafers using reactive sputtering of titanium metal in an oxygen environment. This method produces thin films with a high refractive index (2.4) and low planar waveguiding losses (<0.4 dB/cm). Using these films, we define structures with electron-beam lithography. Next, we form waveguides using reactive-ion etching to achieve feature sizes in TiO2 down to 100 nm. We show both visible and 780-nm light propagation in 300-nm wide waveguides. Lastly, we test simple linear devices such as bends, directional couplers, and Sagnac interferometers. From our observations of the nonlinear optical properties in bulk samples and our demonstration of basic on-chip devices, we conclude that TiO2 is a viable material for all-optical applications.

Submicrometer-width TiO 2 waveguides

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.

Spectral broadening of femtosecond pulses in polycrystalline anatase titanium dioxide waveguides

We observe the first nonlinear spectral broadening of femtosecond pulses in single-mode anatase TiO2 waveguides at 793 and 1565 nm. The broad applicability and low two-photon absorption of TiO2 makes it a promising material for integrated photonics.

Maximizing Intensity in TiO2 Waveguides for Nonlinear Optics

Titanium dioxide (TiO2) represents an attractive candidate for nonlinear optical devices due to its large refractive index and large Kerr nonlinearity. These properties can strongly enhance confinement and nonlinear interactions. TiO2 also possesses high transparency, exhibiting no linear absorption within the entire visible spectrum and no two-photon absorption at wavelengths above 800 nm. Considering these qualities, TiO2 is capable of outperforming most other widely transparent materials, such as fused silica, silicon nitride, and diamond. Using electron beam lithography and a liftoff procedure followed by reactive ion etching, we structure both amorphous TiO2 as well as polycrystalline anatase thin films to create photonic devices that exploit this material’s properties in order to study nonlinear optics (Bradley et al. Opt Express 20:23821–23831, 2012; Evans et al. Opt Express, submitted) Nonlinear optics benefit from prolonged interactions, necessitating large intensities along extended waveguide lengths. For this reason, waveguide losses need to be minimized. We study the effects of mask materials and annealing procedures on waveguide propagation losses. We also study a variety of taper structures and optimize the insertion losses of these waveguides. For ultrafast pulses, dispersion becomes an important parameter since strong dispersion can elongate a pulse and lower the large peak intensities needed for nonlinear optics. Within nano-scale structures, this parameter can be tailored without difficulty by changing the waveguide geometry. We present a finite-element analysis that demonstrates the geometries necessary to obtain negligible or anomalous dispersion and thus maintain large pulse intensities. These techniques can readily be applied to other novel photonic material platforms.