Slow light photonic crystal waveguides (PCW) are a promising tool for optical signal processing as well as integrated optical devices that require strong light-matter interactions, such as modulators or non-linear devices. However, the low group velocity that characterizes these PCWs is typically accompanied by a large group velocity dispersion (GVD), and a low coupling efficiency. Improving both these properties for a large bandwidth forms the design challenge of most optical devices that rely on PCWs.
In this work, we use inverse design methods to, firstly, design slow light PWCs with a large group index-bandwidth product (GBP), and secondly, to design couplers for the PCW, i.e. a mode converter, which couples a ridge waveguide to the PCW, and a grating coupler, which couples free space light directly to the PCW slow light mode. Both couplers are optimized for the PCW’s low group velocity dispersion bandwidth. Unlike pre-existing work, we perform the PCW optimization in full 3D simulations which result in more accurate and fully fabricable devices. The high degrees of freedom associated with inverse design makes it an effective method for these problems and, as such, an essential design tool to optimize future PCW applications.
Particle accelerators are central to applications ranging from high-energy physics to medical treatments. However, the cost and size of conventional accelerators operating in radio-frequencies is prohibitive for widespread proliferation. Operating at optical and near-infrared frequencies, dielectric laser accelerators (DLAs) leverage the high damage threshold of dielectric materials, advances in nanofabrication techniques, and femtosecond pulsed lasers to produce miniaturized laser-driven accelerators. Previous demonstrations of dielectric laser acceleration have utilized free-space lasers directly incident on the accelerating structure. While this is acceptable for proof-of-principle, for DLAs to become a mature technology, it is necessary to integrate the accelerators on-chip to increase scalability and robustness of the system.
Here we demonstrate the first waveguide-integrated dielectric laser accelerator. In this scheme, a grating coupler is used to couple light from femtosecond pulsed laser to a 30 μm wide waveguide, fabricated on a silicon-on-insulator platform. The waveguide is then directly interfaced with an accelerating structure that is patterned with sub-wavelength features to produce near-fields phase-matched to electrons travelling through a vacuum-channel in the device. Both the input grating coupler and accelerator structure have been designed using the inverse design optimization approach.
We have experimentally demonstrated these waveguide-integrated accelerators by showing acceleration of subrelativistic electrons of initial energy 83.5 keV. We observe a maximum energy modulation of 1.19 keV over 30 μm. These results represent a significant step toward scalable and integrable on-chip DLAs for applications in ultrafast, medical, and high-energy technologies.
Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity .
In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3].
Finally, we present the generation of single photons with ultra-low multi-photon probability . Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses.
 K. A. Fischer, et al., New J. Phys. 18, 113053 (2016)
 K. A. Fischer, et al., Nature Physics 13, 649-654 (2017)
 K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017)
 L. Hanschke et al., arxiv: 1801.01672 (2018)