Researchers from the Institute of Optics of CSIC and the Institute of Photonic Sciences (ICFO, Spain) have developed and demonstrated an approach to generating single photons along well-defined directions. The advancement avoids the need for sophisticated 2D or 3D structures to achieve highly directional single-photon emission.

Single-photon sources are fundamental components in quantum optical devices for computing, cryptography, and metrology. These devices use quantum emitters that, after excitation, produce single photons with a probability close to 100% and emission times on the order of a few to tens of nanoseconds. The quality of a single-photon source depends on its ability to extract photons efficiently, to reduce uncertainty in the emission time, to deliver photons at a high repetition rate, and to rule out two-photon events. These criteria rely heavily on the types of emitter and dielectric environments.

The proposed design can be implemented with a variety of materials and stands up well to manufacturing imperfections, according to the ICFO researchers. As a 1D structure, it has a much smaller footprint than 2D photonic crystal structures, making it easier to integrate on a chip. The guided mode is automatically confined to a 1D channel, in contrast to 2D configurations.

In the approach, a quantum emitter is inserted into a 1D waveguide. The periodically patterned, linear waveguide is designed to support a single guided mode of light within the spectral range of the quantum emitter. This mode acquires zero group velocity near the boundary of the first Brillouin zone, which is accompanied by a divergence in the photonic local density of states. This divergence places a dominant weight on the emission through the guided mode. The photons emitted by the quantum emitter are preferentially coupled to this waveguide mode. This results in the production of highly directional single photons and reduces the temporal uncertainty of the emission by two orders of magnitude.

The researchers used the Purcell effect to modify the emission probability of the quantum emitter based on its interaction with its environment to improve extraction efficiency and reduce emission time uncertainty. Placing the emitter in a nanostructured environment helps to increase the indistinguishability of the emitted photons. In principle, the quantum emitter located in the waveguide emits photons along both directions of the waveguide with equal probability. However, different strategies can be adopted to emit photons in only one direction, the researchers said.

For example, it is possible to use circularly polarized emitters, in which the electric field of the photon rotates as the light propagates, by exploiting optical spin-orbit coupling. It is also possible to modify one end of the waveguide to implement a Bragg reflector.

The researchers envision a design of 1D waveguides in which emitters can be activated at different locations, with waveguide interconnects relying on local rearrangements of the scattering particles to maximize coupling to the desired waveguide route. A 3D structure incorporating 1D waveguides would be a possible architecture for optimum light routing. The study — with its design for a single-photon generator with good quantum efficiency — could support this architecture.

According to the researchers, results of the study could be applied to other types of elements, such as periodic corrugations in a rectangular waveguide. The divergence in the local density of states, which is responsible for the enhanced guided emission, could also be used to strongly couple distant quantum emitters sitting at different sites along the waveguide. A scheme like this would have potential application in quantum devices.

In applying their results to more general kinds of scatterers, the researchers said it would also be possible to use two-photon photolithography to pattern scatterers in 3D systems, and/or enable nanoparticle self-assembly. These examples further support the application of quantum optics devices.

The research was published in Nanophotonics (www.doi.org/10.1515/nanoph-2023-0276).