Periodic Reporting for period 1 - TOPEX (Extreme confinement of topologically protected slow light)
Reporting period: 2022-08-01 to 2024-07-31
The importance of quantum technologies in our future society is nowadays becoming clear and goes far beyond the highly praised quantum computers. One platform for quantum technologies is integrated photonics, where the interaction between light and matter can be tailored using advanced top-down nanofabrication like that employed for commercial electronics. Integrated photonic cavities and waveguides, where the electromagnetic field intensity is enhanced through either spatial or spectral confinement -or both- are particularly attractive. However, the former trade light-matter interaction strength for optical bandwidth, and the latter trade that strength for propagation losses. Therefore, strong light-matter interaction over long distances and over a large bandwidth is complicated to achieve. TOPEX primarily aimed at developing a novel platform for quantum photonics based on topological bowtie photonic-crystal waveguides, for which such propagation losses may be suppressed due to their topological nature and for which the field intensity is only limited by the smaller feature size -at the few nanometer scale- that can be reliably manufactured.
More specifically, the main objective of TOPEX was to experimentally explore the existence of topologically protected light transport in the slow-light regime of a waveguide made from a conventional semiconductor material, i.e. silicon, where light-matter interaction becomes notable and technologically meaningful. In addition to that, the project aimed at studying the role of bowtie-shaped features at such topological interfaces to exponentially enhance the field intensities locally and thus boost the interaction with point-like emitters.
More specifically, the main objective of TOPEX was to experimentally explore the existence of topologically protected light transport in the slow-light regime of a waveguide made from a conventional semiconductor material, i.e. silicon, where light-matter interaction becomes notable and technologically meaningful. In addition to that, the project aimed at studying the role of bowtie-shaped features at such topological interfaces to exponentially enhance the field intensities locally and thus boost the interaction with point-like emitters.
The main activities and associated achievements of TOPEX are the following:
- Inverse design of two important photonic components, a free-space grating coupler and a 50:50 power splitter, for efficient quantum-photonic circuitry. These were carried out using either the finite-difference-time-domain method and shape optimization in Lumerical or the finite-element-method and density-based topology optimization in COMSOL Multiphysics. The devices were fabricated and characterized, finding quantitative agreement with the simulated optical response.
- Design of a topological bowtie waveguide and study of its waveguide quantum electrodynamics properties, namely the achievable Purcell factor for a point-like emitter. Such waveguide can achieve a 20-fold light-matter interaction strength enhancement over conventionally employed line-defect photonic-crystal waveguides over a broad bandwidth in the telecom C-band.
- Fabrication and experimental characterization of the propagation optical losses in silicon topological waveguides of the valley-Hall type and comparison to conventional line-defect photonic-crystal waveguides. The fabrication was carried out using a high-resolution silicon nanofabrication process leading to state-of-the-art propagation losses, yet the measurements have shown that there is no sign of topological protection in the topological waveguides.
- Far-field optical scattering measurements of light propagating in the slow-light region of the topological mode of a valley-Hall waveguide. These measurements project the near field in the waveguide and evidence the presence of randomly localized (spectrally and spatially) cavity modes instead of propagating fields, which is known as Anderson localization. This demonstrates, contrary to more-or-less explicit indications in the literature, that backscattering is very strong in the topological mode.
- Fabrication and experimental characterization of the propagation optical losses in silicon bowtie photonic-crystal waveguides and comparison to conventional line-defect photonic-crystal waveguides. A decreasing bowtie width is seen to increase the propagation losses, which puts bounds to the waveguide lengths for which the expected light-matter enhancement can be leveraged.
- Development of a method for self-assembling suspended photonic structures with void features well beyond the capabilities of top-down nanofabrication, down to the atomic scale. Such a method has been used precisely for the fabrication of air bowtie photonic cavities and waveguides that confine telecom photons to gaps with an aspect ratio above 100, leading to light confinement more than 100 times below the diffraction limit.
- Inverse design of two important photonic components, a free-space grating coupler and a 50:50 power splitter, for efficient quantum-photonic circuitry. These were carried out using either the finite-difference-time-domain method and shape optimization in Lumerical or the finite-element-method and density-based topology optimization in COMSOL Multiphysics. The devices were fabricated and characterized, finding quantitative agreement with the simulated optical response.
- Design of a topological bowtie waveguide and study of its waveguide quantum electrodynamics properties, namely the achievable Purcell factor for a point-like emitter. Such waveguide can achieve a 20-fold light-matter interaction strength enhancement over conventionally employed line-defect photonic-crystal waveguides over a broad bandwidth in the telecom C-band.
- Fabrication and experimental characterization of the propagation optical losses in silicon topological waveguides of the valley-Hall type and comparison to conventional line-defect photonic-crystal waveguides. The fabrication was carried out using a high-resolution silicon nanofabrication process leading to state-of-the-art propagation losses, yet the measurements have shown that there is no sign of topological protection in the topological waveguides.
- Far-field optical scattering measurements of light propagating in the slow-light region of the topological mode of a valley-Hall waveguide. These measurements project the near field in the waveguide and evidence the presence of randomly localized (spectrally and spatially) cavity modes instead of propagating fields, which is known as Anderson localization. This demonstrates, contrary to more-or-less explicit indications in the literature, that backscattering is very strong in the topological mode.
- Fabrication and experimental characterization of the propagation optical losses in silicon bowtie photonic-crystal waveguides and comparison to conventional line-defect photonic-crystal waveguides. A decreasing bowtie width is seen to increase the propagation losses, which puts bounds to the waveguide lengths for which the expected light-matter enhancement can be leveraged.
- Development of a method for self-assembling suspended photonic structures with void features well beyond the capabilities of top-down nanofabrication, down to the atomic scale. Such a method has been used precisely for the fabrication of air bowtie photonic cavities and waveguides that confine telecom photons to gaps with an aspect ratio above 100, leading to light confinement more than 100 times below the diffraction limit.
TOPEX has explored the role of and the crossroads between two recently emerged notions in photonics, topological transport and sub-diffraction spatial confinement of light, for photonic integrated waveguides. In both such fields, the outcomes of the project have pushed the boundaries of the state of the art. First, recent developments in topological photonics had fostered the vision of backscattering-protected waveguides made from topological interface modes, but, surprisingly, measurements of their propagation losses were so far missing. Therefore, the measurement of propagation losses concomitant with those found in conventional waveguide modes, combined with the observation of strong backscattering, are a landmark for topological photonics and, more generally, for integrated photonics. Second, recent developments in nanophotonics had experimentally confirmed that light confinement in dielectric structures is not bound to the diffraction limit but are only limited by the smallest feature that can be made. However, state-of-the-art nanofabrication sets such length-scale to few tenths of nanometers. The development of a simple self-assembly procedure for creating features close to the atomic scale while still preserving the scalability of top-down nanofabrication is therefore a crutial step towards very strong light-matter interaction in bowtie photonic structures. In addition, the method may find applications far beyond nanophotonics, such as in solid-state nanopore sequencing, ultra-high-quality shadow masks for superconducting quantum electronic devices, to name a few.