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Polaritons IN the QUAntum Regime (PINQUAR)

Periodic Reporting for period 1 - PINQUAR (Polaritons IN the QUAntum Regime (PINQUAR))

Reporting period: 2015-05-01 to 2017-04-30

Semiconductor microstructures with nonlinear optical response can be used as building-blocks for memories, switches, and other optical devices. For quantum optical technologies, it is important to further enhance and understand the nonlinear response of these microstructures. Thus, the objective of this action was to investigate highly nonlinear and quantum effects in semiconductor microstructures. To this end, the high quality semiconductor cavities developed at the host institution (Center for Nanosciences and Nanotechnologies (C2N), CNRS) where instrumental.
The first part of the action was dedicated to study of highly nonlinear effects in coupled microcavities. In particular, we observed tristability, i.e. the existence of three steady states at a given driving condition, for the first time. Which one of those states the cavities adopted depended on the history of the system, i.e. in which direction we scanned the laser power driving the microcavities. The newly found tristability effect can form the basis of future optical memories. In the second part of the project, the influence of quantum fluctuations on the nonlinear response of single semiconductor cavities was observed for the first time. These experiments demonstrated signatures of a novel kind of phase transition of light, known as a dissipative phase transition. The results of this project bring semiconductor cavities a step closer to the long-sought quantum regime, where microcavities can emit light with non-classical properties suitable for novel quantum technologies.
The researcher worked at the Center for Nanosciences and Nanotechnologies (C2N), CNRS, in close collaboration with Prof. Jacqueline Bloch and Dr. Alberto Amo.

The researcher began the project with a theoretical investigation of nonlinear physics of two coupled resonators. The researcher discovered that, due to the nonlinearity, this system supports multiple steady-states. He predicted that one of these states could enable a novel type of control of the phase that polaritons pick when hopping between coupled cavities. Based on his predictions, the researcher performed numerous experiments with coupled semiconductor microcavities in highly nonlinear regimes. For this purpose, he used the technological infrastructure and samples available at C2N. Indeed, the researcher experimentally observed multiple steady-states at a given driving condition, i.e. multistability. Through interferometry measurements, the predicted nonlinear optical phase control was evidenced. This phase control opens the way to the implementation of artificial gauge fields for light. These results led to a high-impact publication in the journal Nature Communications [S.R.K. Rodriguez et al, Nature Commun. 7, 11887 (2016)]. The results were also disseminated in many international conferences, invited talks, workshops, and seminars, in Europe, USA, Japan, and Mexico.

Towards the end of the second year, efforts shifted towards observing quantum effects with polaritons. This project was performed in close collaboration with the theory group of Prof. Cristiano Ciuti in Paris. For the experiments, the researcher built a new setup at C2N, and he developed a novel protocol to experimentally investigate the influence of quantum fluctuations on the polariton dynamics. The experiments consisted of scanning the driving power up and down at various speeds, and measuring the dynamical optical hysteresis of the microcavity. Due to the influence of quantum fluctuations, the hysteresis area decayed following a double power law as a function of the scanning time. This result was found to be in agreement with recent theoretical predictions from the group of Prof. Cristianio Ciuti. The critical time at which the hysteresis area transitioned from one power law to another was also found to be highly sensitive to the intrinsic polariton-polariton interaction strength. This effect is not captured by the mean-field approximation, and therefore constitutes a remarkable discovery in the field. Furthermore, by investigating cavities with different interaction strengths, signatures of a dissipative phase transition of light were obtained. These results led to a high-impact publication in the journal Physical Review Letters [S.R.K. Rodriguez et al. Phys. Rev. Lett. 118, 247402 (2017)]. These results have also been presented in numerous contributed and invited talks, seminars, and workshops, throughout Europe, USA, Mexico, and Japan.

From a career development point of view, the results of this action enabled the insertion of the researcher into a new scientific community for him. To this end, the transfer of invited talks by Jacqueline Bloch was also helpful.
Thanks to the project PINQUAR, two main results which push the state-of the-art in the field were obtained:

1) A nonlinear optical method for controlling the phase that polaritons pick when hopping between coupled cavities was demonstrated.
2) The influence of quantum fluctuations on the nonlinear dynamics of polaritons was demonstrated, and its connection to a dissipative phase transition was established.

The first result constitutes an important contribution to the emerging field of quantum simulation. The nonlinear control over the hopping phase, when generalized to a lattice of nonlinear cavities, open the door to the creation of synthetic magnetic fields for photons. Since photons (charge neutral particles) normally do not respond to an applied magnetic field, a synthetic magnetic field for photons is highly desired for fundamental studies and applications. Fundamentally, a synthetic magnetic field for photons could enable novel schemes for simulating the physics of charged particles with photons. Application-wise, a synthetic magnetic field for photons could be used to create novel non-reciprocal optical technologies such as optical isolators.

The second result is of large fundamental relevance, as it shows (for the first time) that polaritons can indeed be influenced by quantum effects despite their relatively weak interaction strength. These results will play a pivotal role in future studies of nonlinear quantum dynamics in more complex systems of coupled cavities. For more than a decade, there has been a strong interest (largely fuelled by theoretical works) on non-classical states of light relying on polariton-polariton interactions. The results from this action constitute a step in that direction, by elucidating the conditions under which quantum fluctuations are relevant. Regarding applications, such an understanding will likely have an impact in the design of future optical memories or switches relying on polariton-polariton interactions.
Measurements of dynamic optical hysteresis decaying under the influence of quantum fluctuations