Ellen Schelew

Improved learning in a large-enrollment physics class.

Encouraging active engagement results in enhanced learning. We compared the amounts of learning achieved using two different instructional approaches under controlled conditions. We measured the learning of a specific set of topics and objectives when taught by 3 hours of traditional lecture given by an experienced highly rated instructor and 3 hours of instruction given by a trained but inexperienced instructor using instruction based on research in cognitive psychology and physics education. The comparison was made between two large sections (N = 267 and N = 271) of an introductory undergraduate physics course. We found increased student attendance, higher engagement, and more than twice the learning in the section taught using research-based instruction.

Waveguide integrated superconducting single-photon detectors implemented as near-perfect absorbers of coherent radiation.

At the core of an ideal single-photon detector is an active material that absorbs and converts every incident photon to a discriminable signal. A large active material favours efficient absorption, but often at the expense of conversion efficiency, noise, speed and timing accuracy. In this work, short (8.5 μm long) and narrow (8 × 35 nm(2)) U-shaped NbTiN nanowires atop silicon-on-insulator waveguides are embedded in asymmetric nanobeam cavities that render them as near-perfect absorbers despite their small volume. At 2.05 K, when biased at 0.9 of the critical current, the resulting superconducting single-photon detectors achieve a near-unity on-chip quantum efficiency for ∼1,545 nm photons, an intrinsic dark count rate <0.1 Hz, a reset time of ∼7 ns, and a timing jitter of ∼55 ps full-width at half-maximum. Such ultracompact, high-performance detectors are essential for progress in integrated quantum optics.

Photonic crystal slot-microcavity circuit implemented in silicon-on-insulator: High Q operation in solvent without undercutting

We report the fabrication and characterization of a silicon-based photonic integrated circuit consisting of a photonic crystal slot-cavity, waveguides, and grating couplers, designed as a robust, easy-to-use device for enhancing light-matter interactions at a precise location inside a fluidic medium, while minimizing fabrication complexity. Measured Q values in excess of 7500 for circuits immersed in hexane and operating near 1.5 μm are obtained, in good agreement with simulations. The detection limit for changes in solvent refractive index unit (RIU) for these structures, which have not been optimized, is 2.3×10−5 RIU.

Lithography simulation for the fabrication of silicon photonic devices with deep-ultraviolet lithography

We demonstrate the lithography simulation for the fabrication of silicon photonic devices using deep-ultraviolet lithography. Once the distortions arising from the fabrication process are accounted for, the comparison between predicted and measured results is excellent.

Saturation behaviour of colloidal PbSe quantum dot exciton emission coupled into silicon photonic circuits.

We report coupling of the excitonic photon emission from photoexcited PbSe colloidal quantum dots (QDs) into an optical circuit that was fabricated in a silicon-on-insulator wafer using a CMOS-compatible process. The coupling between excitons and sub-μm sized silicon channel waveguides was mediated by a photonic crystal microcavity. The intensity of the coupled light saturates rapidly with the optical excitation power. The saturation behaviour was quantitatively studied using an isolated photonic crystal cavity with PbSe QDs site-selectively located at the cavity mode antinode position. Saturation occurs when a few μW of continuous wave HeNe pump power excites the QDs with a Gaussian spot size of 2 μm. By comparing the results with a master equation analysis that rigorously accounts for the complex dielectric environment of the QD excitons, the saturation is attributed to ground state depletion due to a non-radiative exciton decay channel with a trap state lifetime ~ 3 μs.

Characterization of Integrated Planar Photonic Crystal Circuits Fabricated by a CMOS Foundry

Integrated planar photonic crystal circuits in silicon on insulator were fabricated with a single-etch-step process by a foundry using complementary metal-oxide-semiconductor processing techniques. The devices studied integrate three key elements: i) input/output grating couplers consisting of 2D uniform arrays of holes, ii) single transverse electric (TE)/single transverse magnetic (TM) mode channel waveguides, and iii) a photonic crystal linear three hole defect (L3) microcavity. Experimentally measured s- and p-polarized transmission, both from grating-to-grating through a uniform silicon slab region, and through the channel waveguide/L3 cavity circuit, were quantitatively compared with finite-difference time-domain simulations. Excellent agreement is achieved assuming circular, vertical side-wall holes, but this requires accurate post-fabrication characterization of actual versus nominal device parameters, including especially the silicon device layer thickness. While s-polarized incident radiation excites TE modes that exhibit typical resonant cavity (filter-like) transmission, p-polarized incident radiation excites TM modes that non-resonantly propagate through the circuit with comparable transmission efficiency. The dependence of the grating coupler tuning range on hole diameter, and the addition of a photoresist covering is determined.

Compact and efficient silicon nanowire to slot waveguide coupler

We propose a Y-branch coupler to efficiently transfer light from a silicon nanowire waveguide to a slot waveguide. The coupler is 500 nm long and functions with greater than 90 % efficiency over a 200 nm bandwidth when operated either in air or solvent. Its versatility makes it a good candidate for photonic circuits with applications including sensing, nonlinear optics and information processing.

Coupling of nanocrystals and photonic crystals for non-linear applications

Modelling results are described that deal with the coupling of excitons in colloidal nanocrystals to silicon-based photonic crystal microcavities, and to the design of structures that couple the microcavities to ridge waveguides and input/output gratings that are realized on 200 mm diameter silicon-on-insulator wafers.

Low pump power spontaneous four-wave mixing source using triple photonic crystal microcavities in silicon

We describe a planar photonic circuit in silicon-on-insulator that efficiently generates pairs of non-degenerate photons in a single mode output channel through spontaneous four-wave mixing when pumped with 10s of μW of continuous-wave radiation at wavelengths near 1.5 μm. The structure is based on a set of three coupled photonic crystal microcavities, each of which is judiciously coupled to single mode input and output channels.

Stimulated Four-Wave Mixing in a Heterostructure Photonic Crystal Triple Microcavity

We report high efficiency nonlinear photon generation achieved through stimulated four-wave mixing in a silicon-on-insulator two-dimensional photonic crystal triple microcavity. The microcavity supports three spatially overlapping resonant modes.