Brian J. Roxworthy

Mode-division multiplexing (MDM) enables a large increase in the information-carrying capacity of an optical network. Recently, chip-scale MDM devices that can switch different mode orders to different output waveguides have been demonstrated. However, an important milestone showing dynamically tunable mode-order conversion in a single compact device has so far not been reported. In this work, we demonstrate via simulation and measurement a new, to the best of our knowledge, approach for reconfigurable mode conversion using optical micro-electro-mechanical systems (MEMS) to locally modify the effective index in an asymmetric coupler. Modeling shows that dynamic tuning to increase or decrease the mode order is possible. Measurements on fabricated devices are consistent with simulations of reconfigurable mode conversion based on tunable phase matching. Our experimental results demonstrate reconfigurable TE₀-TE₂ to TE₀-TE₁ conversion and validate this new tunable phase-matching approach for mode-division multiplexing.

The advent of interconnected devices, sensors, and systems requires increasingly large data transmission rates. Optical communications has met these bandwidth needs due to the large information-carrying capacity of light, although new modulation and multiplexing approaches are required to keep up with increasing demand. To this end, mode-division multiplexing (MDM), in which light propagates in on-chip waveguides with distinct and orthogonal spatial mode shapes, may offer a promising solution. To date, however, most chip-scale MDM devices have relied on (de)multiplexing on-chip of fixed optical modes as opposed to tunable mode conversion. In this Letter we demonstrate a new approach to mode conversion and mode-division multiplexing (MDM) that takes advantage of MEMS-tunable optical phase matching. We show via simulations that the approach enables mode conversion with high purity. Measurements on fabricated devices demonstrate the feasibility of this new MEMS index perturbation and phase matching approach for on-demand and dynamic mode conversion for MDM data/communications links. [2020-0107]

The optical properties of several hydrogen-bond acidic sorbent materials are evaluated in situ to assess their suitability for waveguide-enhanced Raman spectroscopy (WERS) of vapor-phase organophosphonates. A number of characteristics critical to WERS are evaluated for each sorbent: infrared absorption, Raman spectral background, and the limit of detection for a test hydrogen-bond-basic analyte (dimethyl methylphosphonate, DMMP). We describe the chemical properties of the sorbents that differentiate their optical properties for sensing. Then, we introduce a sorbent figure-of-merit that quantifies these differences and provides a framework to assess the quality of newly-developed sorbent materials.

We report a grating coupler capable to efficiently project a constant-intensity free-space beam due to exceptional points in a photonic band diagram. Our device enables a new route to project arbitrarily wide free-space top-hat beams.

Cavity optomechanics enables large-scale index tuning (Δn_eff). However, most demonstrations exhibited small Δn_eff or have not considered optical loss at large Δn_eff. We demonstrate Δn_eff-tuning using gradient electric forces and analyze optomechanically-induced loss and mitigation.

Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment. Here, we develop an extreme mode converter, which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 µm waist, which corresponds to an increase in the modal area by a factor of >10⁵. The beam quality is thoroughly characterized, and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated. To achieve low phase error over a beam area that is >100× larger than that of a typical grating coupler, our approach separates the two-dimensional mode expansion into two sequential separately optimized stages, which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space. Developed at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication, or other wavelengths. The technique can be expanded to more arbitrary phase and intensity control of both large-diameter, free-space optical beams and wide photonic slab modes.

Plasmomechanical systems – formed by introducing a mechanically compliant gap between metallic nanostructures – produce large optomechanical interactions that can be localized to deep subwavelength volumes. This unique ability opens a new path to study optomechanics in nanometer-scale regimes inaccessible by other methods. We show that the localized optomechanical interactions produced by plasmomechanics can be used to spatially map the displacement modes of a vibrating nanomechanical system with a resolution exceeding the diffraction limit. Furthermore, we use white light illumination for motion transduction instead of a monochromatic laser, and develop a semi-analytical model matching the changes in optomechanical coupling constant and motion signal strength observed in a broadband transduction experiment. Our results clearly demonstrate the key benefit of localized and broadband performance provided by plasmomechanical systems, which may enable future nano-scale sensing and wafer-scale metrology applications.

Integration of nanophotonics with micromechanical, biological, and chemical systems can miniaturize the reference samples for SI traceable calibration. However, an efficient conversion between photonic modes and free-space beams remains a main hurdle due to a gigantic mode mismatch (10 ⁶ times in modal area). Here, we report an extreme mode converter realized on a photonic chip, which enables projecting a 160-micrometer wide Gaussian beam in free space by a sequential mode expansion from a waveguide to a slab mode and then to free space using an apodised grating. The design can be easily adapted for visible, telecom, and UV wavelengths.

Plasmomechanical systems are an emerging class of devices that hold great promise for manipulating light-matter interactions with high speed and subdiffraction spatial resolution. However, realizing their potential requires developing active plasmomechanical systems that can localize their functionality to the level of an individual subwavelength plasmonic resonator. Here, we present an active, electrically tunable plasmomechanical system that uses a localized-gap plasmonic resonator to mediate optical, thermal, and mechanical interactions within a subwavelength footprint. Our device enables facile electromechanical modulation of localized plasmons, selective subdiffraction transduction of nanomechanical motion, and functions as a plasmomechanical oscillator that can be injection-locked to, and thus amplify, weak external stimuli. These functionalities benefit applications in nanomechanical sensing, spatial light modulators, and reconfigurable metasurfaces.