Michael Moebius

Polycrystalline anatase titanium dioxide microring resonators with negative thermo-optic coefficient

We fabricate polycrystalline anatase TiO2 microring 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.

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.

Efficient photon triplet generation in integrated nanophotonic waveguides

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.

Nonlinear Optics in TiO 2 Nanoscale Waveguides

Polycrystalline anatase TiO2 nanoscale waveguides were proposed as a platform for efficient, on-chip generation of triplet photons via third-order spontaneous parametric down-conversion (TOSPDC). TiO2 has a high nonlinear index of refraction (0. 16 × 1018 m2/W at 1565 nm), a wide transparency window (absorption edge at 400 nm) and negligible two-photon absorption above 800 nm, making it an advantageous material for integrated photonics. Efficient generation of three-photon entanglement on an integrated platform are necessary for many experiments in quantum optics, with applications in quantum metrology and communication. We coupled simulations with experiments to determine and confirm optimal waveguide geometries for meeting TOSPDC phase-matching constraints and maximizing the efficiency of triplet-photon generation in TiO2 nanoscale devices.

Femtosecond laser direct writing of monocrystalline hexagonal silver prisms

Bottom-up growth methods and top-down patterning techniques are both used to fabricate metal nanostructures, each with a distinct advantage: One creates crystalline structures and the other offers precise positioning. Here, we present a technique that localizes the growth of metal crystals to the focal volume of a laser beam, combining advantages from both approaches. We report the fabrication of silver nanoprisms—hexagonal nanoscale silver crystals—through irradiation with focused femtosecond laser pulses. The growth of these nanoprisms is due to a nonlinear optical interaction between femtosecond laser pulses and a polyvinylpyrrolidone film doped with silver nitrate. The hexagonal nanoprisms have bases hundreds of nanometers in size and the crystal growth occurs over exposure times of less than 1 ms (8 orders of magnitude faster than traditional chemical techniques). Electron backscatter diffraction analysis shows that the hexagonal nanoprisms are monocrystalline. The fabrication method combines advantages from both wet chemistry and femtosecond laser direct-writing to grow silver crystals in targeted locations. The results presented in this letter offer an approach to directly positioning and growing silver crystals on a substrate, which can be used for plasmonic devices.

Direct-laser metal writing of surface acoustic wave transducers for integrated-optic spatial light modulators in lithium niobate

Recently, the fabrication of high-resolution silver nanostructures using a femtosecond laser-based direct write process in a gelatin matrix was reported. The application of direct metal writing towards feature development has also been explored with direct metal fusion, in which metal is fused onto the surface of the substrate via a femtosecond laser process. In this paper, we present a comparative study of gelatin matrix and metal fusion approaches for directly laser-written fabrication of surface acoustic wave transducers on a lithium niobate substrate for application in integrated optic spatial light modulators.

Extracting loss from asymmetric resonances in micro-ring resonators

Propagation losses in micro-ring resonator waveguides can be determined from the shape of individual resonances in their transmission spectrum. The losses are typically extracted by fitting these resonances to an idealized model that is derived using scattering theory. Reflections caused by waveguide boundaries or stitching errors, however, cause the resonances to become asymmetric, resulting in poor fits and unreliable propagation loss coefficients. We derive a model that takes reflections into account and, by performing full-wave simulations, we show that this model accurately describes the asymmetric resonances that result from purely linear effects, yielding accurate propagation loss coefficients.

Scintillator-based Photon Counting Detector: Is it feasible?

By utilizing finely pitched scintillator arrays where the scintillator has high atomic number and density, fast decay time, and high light output, realizing a scintillator-based Photon Counting Detector (PCD) is conceptually feasible. Fabrication of fine-pitched scintillator arrays however, has been the bottleneck for realizing such detectors. Combining the novel scintillator fabrication technique called laser-induced optical barriers (LIOB) where optical barriers can be placed inside a transparent crystal and act as a reflector without removing the material, with laser ablation, we are now able to overcome the obstacles for developing scintillator-based PCD. In this regard, we are developing an LYSO-based PCD where the LYSO crystal is laser pixelated to sub-mm pixels. The scintillator array will be coupled to an application specific integrated circuit (ASIC) where each ASIC pixel has built-in photodiode, amplifiers and 3–4 energy windows and their associated counters. We have simulated light transport for different scenarios where the crystal is pixelated by a combination of LIOB and laser cut techniques, where the 2 mm thick crystal is first pixelated by LIOB to a depth and then the rest is pixelated by the ablation technique. We also simulated the fraction of collected light in the same scintillator pixel by modeling various surface properties of the pixel cuts as well as optical barrier surface roughness and refractive index (RI). Simulation results show that up to ∼70% of the scintillation light will be contained in the same pixel when only using the LIOB technique with barrier refractive index of 1.0. These results suggest that laser processed arrays can potentially change the paradigm in PCD development as they can replace the traditional array production and thus allow for scintillator-based PCD development in a more robust and cost-effective manner.

Direct laser writing of 3D gratings and diffraction optics

We fabricate 3D gratings and diffraction optics using direct laser writing. Diffraction patterns of gratings agree with Laue theory. We demonstrate zone plates for visible wavelengths. Direct laser writing is promising for integrated diffraction optics.

Femtosecond Laser Micromachining of a-Si:H

Femtosecond laser micromachining has been used to write bulk waveguides and photonic devices in glasses, polymers, and crystalline silicon (Gattass and Mazur, Nat Photonics 2:219–225, 2008). Refractive index changes in these materials tend to be less than a percent, which sets limitations on applications due to low light confinement. Hydrogenated amorphous silicon (a-Si:H) presents a unique, versatile material platform with incredible potential. Variations in the hydrogen content can produce refractive index changes as high as 40–80 % (Fortmann et al. Thin Solid Films 395:142–146, 2001). There is potential for integration on a silicon platform. We use femtosecond laser micromachining to locally reduce the hydrogen content of the material, with the goal of increasing the refractive index. We will use this method to directly write three-dimensional (3D) photonic devices in a-Si:H. This novel, simple method for photonic device fabrication in a-Si:H will facilitate many applications and device integration. Femtosecond laser processing enables 3D fabrication through nonlinear interactions. An ultrafast pulsed laser is focused inside the bulk of a transparent material. Due to tight focusing, material modifications only occur at the focal point of the laser. Complex patterns can by written by translating the sample with respect to the laser focus using x-, y- and z-translation. Intensity dependence of nonlinear processes enables fabrication of features with dimensions below the diffraction limit of light through careful selection of laser exposure. 3D fabrication allows greater versatility in devices and can increase device density, which is critical for modern optics and electronics where space is at a premium. Waveguide fabrication through variations in hydrogen content has been demonstrated in 2D using traditional micro- and nano-fabrication techniques (Fortmann et al. Thin Solid Films 430:278–282, 2003; 501:350–353, 2006). This process requires many steps, including ion implantation to locally control hydrogen content. The waveguides consist of regions with low hydrogen content. Fabrication of 3D photonic devices would require stacking many 2D layers. We are developing femtosecond laser processing to directly write complex 3D patterns in a single-step process, introducing greater versatility and processing speed. We have succeeded in lowering the hydrogen concentration in 2D patterns within a 1-μm film of a-Si:H using a near infrared (1,050 nm) femtosecond laser. Contrast is visible between unaltered and laser processed material under optical microscopy, suggesting index changes due to a reduction in hydrogen content. Contrast increase with laser fluence or a greater number of incident laser pulses. AFM measurements verify that regions with reduced hydrogen content do not exhibit changes in surface topography. We used Raman spectroscopy to verify the reduction in hydrogen content by examining the intensity ratio of Si-H bonding peaks (2,000 and 2,100 cm−1) to Si-Si bonding around 480 cm−1 (Fig. 58.1). A reduction in the Si-H intensity corresponds to reduced hydrogen content (Brodsky et al. Phys Rev B 16:3556–3571, 1977). These initial results provide a proof of concept and valuable knowledge for the fabrication of devices.