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 nc ~ 1 µm–2 is obtained at the Bose stimulation threshold N(E = 0) = 1 using thermalized polariton distributions . The low critical density nc arises from the macroscopic thermal de Broglie wavelength λth ~ 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 . 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.
 "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]
 "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]
 "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]