Realising strong photon-photon interactions with exciton-polaritons
Abstract
The aim of this project is to show how a system consisting of a two-dimensional (2D) semiconductor inside an optical cavity can be used to realize a strong photon-photon interaction. Such a strong interaction paves the way to develop all optical information processing devices, which will offer unprecedented speed compared to present day electronic components. Normally, photons interact very weakly with each other -a fact that is vividly demonstrated by our daily experience of light beams crossing each other unchanged. The reason is that the leading photon-photon scattering process in a vacuum is the exchange of a fermion-antifermion pair. This process is highly off-resonant leading to essentially no photon-photon interaction for visible light. Our aim is to overcome this hurdle by exploiting a resonance effect in a composite system consisting of cavity photons strongly coupled to particle-hole excitations inside a 2D semiconductor. This leads to the formation of exciton-polaritons, which are quasiparticles consisting of a coherent superposition of a photon and a bound electron-hole pair (exciton). Since excitons interact with each other, there is an effective interaction between the photons bound together with the excitons in the polaritons. This direct interaction is however far too weak for realizing photonic information technology components. Our main idea is to introduce an indirect (induced) interaction between the polaritons, by creating polaritons in two distinct spin states, up and down arrows,, using two lasers with different polarizations. One (pump) laser is strong and creates a Bose-Einstein condensate (BEC) of up-arrow-polaritons. The second (probe) laser is weaker and it creates down-arrow-polaritons with a smaller concentration. There will then be an induced interaction between the down-arrow-polaritons mediated by the surrounding BEC of i -polaritons. The key physical effect is that if the transferred energy/momentum between two scattering J, -polaritons is resonant with a sound mode in the BEC, the induced interaction between them will be very strong. This will in tum give rise to a strong interaction between the photons bound in the down-arrow-polaritons. Indeed, such a resonance effect was recently demonstrated by the Pl to exist in a closely related system consisting of impurity atoms in an atomic BEC. The project has several strengths. First, the proposed system of two kinds of exciton-polaritons in a 2D semiconductor inside an optical cavity has already been realized experimentally. Second, the exciton-polariton system is highly flexible. By changing the frequencies and directions of the lasers, one can control the energies and momenta of the down-arrow-polaritons thereby tuning their scattering in and out of resonance. Third, on resonance the scattering probability is essentially 100%, which will be very useful for realizing reliable photonic components. Fourth, the induced interaction between the down-arrow-polaritons depends strongly on their energy. It follows that the scattering can be switched on for a certain laser frequency and off for another frequency, which allows the photonic device to operate on frequencies (0 and l are encoded in the frequency) instead of intensity. This is extremely useful since one avoids possible errors due to unavoidable intensity loss. Finally, given that the Pl explored the same effect in a very similar atomic gas system, he is an excellent position to lead the proposed research project. The project involves collaborations with experts on exciton-polaritons whose competences compliment that of the PI very well. This ensures that the concepts and ideas are transferred most efficiently and accurately from the atomic gas system to the exciton-polariton system.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 04, 2019
- Source ID
- W911NF1910403
Entities
People
- Georg M. Bruun
Organizations
- Aarhus University
- Army Contracting Command
- United States Army