Joseph gave a poster presentation titled "Effects of Strong Interaction between Exciton-Polaritons" in the International Conference on Physics of Light-Matter Coupling in Nanostructures (PLMCN18) held in Würzburg, Germany.

Demonstration of single-photon nonlinearities, a long-standing goal in quantum information science, has been achieved in various cavity quantum electrodynamics (QED) systems where a single oscillating dipole is strongly coupled to a cavity mode. In contrast, exciton-polaritons can be described by the Tavis-Cummings model that predicts a collective and linear response to the number of excitation photons, i.e., no density-dependent energy shift.

Recently, we have demonstrated that the strong polariton-polariton interaction breaks this linearity (shifts the energy) even at very low particle densities below 1 µm^{–2}, where the critical density for condensation n_{c} ~ 1 µm^{–2} is obtained at the Bose stimulation threshold N(E = 0) = 1 using thermalized polariton distributions [2]. The low critical density n_{c} arises from the macroscopic thermal de Broglie wavelength λ_{th} ~ 1 µm at 10 K due to their light effective mass on the order of 10^{–4} times the electron mass.

With the strong repulsive polariton interaction, the single-photon nonlinearity is anticipated, similar to Rydberg polariton blockade experiments [3]. The measured blue shift ΔE = 0.5 meV at n = 1 µm^{–2} is comparable to the linewidth 0.5 meV at resonance, which predicts the second-order correlation function g^{(2)}(τ = 0) = 0.5 from a simple model. The strong polariton interaction will enable the development of extremely nonlinear optoelectronic devices in a solid state.

In the first part, the linewidth of the exciton-polaritons in our long lifetime sample is discussed. The relatively steep cavity gradient is attributed to the observed broad linewidth. In the second part, the first direct measurement of polariton-polariton interaction strength is reported. Two key requirements to this measurement and mechanisms that might lead to stronger-than-expected blue shift (which are all rejected) are discussed. In the last part, we discuss how the single-photon nonlinear regime can be achieved with exciton-polaritons.

The poster can be found in the link.

[1] "Direct measurement of polariton–polariton interaction strength," Y. Sun, Y. Yoon, M. Steger, G. Liu, L. N. Pfeiffer, K. West, D. W. Snoke, and K. A. Nelson,*Nat. Phys.* (2017), doi:10.1038/nphys4148 [url]

[2] "Bose-Einstein condensation of long-lifetime polaritons in thermal equilibrium," Y. Sun, P. Wen, Y. Yoon, G. Liu, M. Steger, L. N. Pfeiffer, K. West, D. W. Snoke, and K. A. Nelson, *Phys. Rev. Lett.* 118, 016602 (2017) [url]

[3] "Quantum nonlinear optics with single photons enabled by strongly interacting atoms," T. Peyronel, O. Firstenberg, Q.-Y. Liang, S. Hofferberth, A. V. Gorshkov, T. Pohl, M. D. Lukin and V. Vuletić, *Nature* **488**, 57 (2012) [url]