Coupled exploration of light and matter

n quasi-particles called polaritons, the states of light and matter are strongly coupled. Prof. Ataç İmamoğlu's group has developed a new approach to study the nonlinear optical properties of polaritons in highly correlated electronic states. In doing so, they opened new perspectives to explore the two ingredients of the polariton: new features for photonic devices and a fundamental insight into the exotic states of matter.

The concept of "quasiparticles" is an extremely effective framework for describing complex phenomena that occur in multi-body systems. Polaritons in semiconductor materials are one of the most interesting quasiparticle species in recent years. These are created by projecting light onto a semiconductor, where the photons excite electron polarization waves, called excitons. The creation process is followed by a period during which the dynamics of the system can be described as that of a particle-like entity that is neither light nor matter, but a superposition of two. Once these mixed light matter particles disintegrate – usually at the time scale of picoseconds – photons regain their individual identity. Write in the newspaper NaturePatrick Knüppel and colleagues from Professor Ataç Imamoglu's group of ETH Zurich's Department of Physics now describe experiments in which released photons reveal unique information about the semiconductor they have just left; at the same time, the photons were modified in a way that would not have been possible without interaction with the semiconductor material.

Teach photons new tours

Much of the recent interest in polaritons comes from the perspective that they open up intriguing new capabilities in photonics. More specifically, polaritons allow photons to do something that photons can not do on their own: interact with each other. The rays of light intersect normally. On the other hand, the bound photons in the polaritons can interact through the material part of the latter. Once this interaction can be made sufficiently strong, the properties of photons can be exploited in new ways, for example for the processing of quantum information or in new optical quantum materials. However, getting interactions strong enough for such applications is not a trivial matter.

It starts by creating polaritons. The semiconductor material hosting the electronic system must be placed in an optical cavity, in order to facilitate the strong coupling between the material and the light. The Imamoglu group has been perfected over the years in collaboration with others, especially with the group of Professor Werner Wegscheider, also at the Physics Department of ETH Zurich. A distinct challenge is to make the interaction between the polaritons strong enough to have a significant effect during the short lifetime of the quasiparticles. How to achieve such a polariton-polariton interaction is currently a major open problem on the ground, hindering progress towards practical applications. And here, Knüppel et al. have made a substantial contribution to their latest work.

Brands of strong interaction

ETH physicists have found an unexpected way to improve the interaction between polaritons, including by appropriately preparing the electrons with which the photons are about to interact. Specifically, they started with electrons initially in the so-called fractional quantum Hall regime, where electrons are confined to two dimensions and exposed to a high magnetic field to form highly correlated states entirely driven by electron-electron interactions. . For particular values ​​of the applied magnetic field – which determines the fill factor characterizing the quantum Hall state – they observed that photons shone on the sample and reflected from the sample showed clear signatures of optical coupling with quantum Hall states (see figure).

It is important to note that the dependence of the optical signal on the fill factor of the electronic system has also appeared in the nonlinear part of the signal, a strong indicator that the polaritons have interacted with each other. In the fractional quantum Hall regime, polariton-polariton interactions were up to ten times stronger than in experiments with electrons outside this regime. This improvement of an order of magnitude represents a significant advance over current capabilities and could be sufficient to allow key demonstrations of "polaritonic" (as a strong blockade of polaritons). This no less than in the experiments of Knüppel et al. The increase in interactions is not done at the expense of the life of the polariton, unlike many previous attempts.

The power and challenges of nonlinear optics

Beyond the implications for light handling, these experiments also bring the optical characterization of multi-body states of two-dimensional electronic systems to a new level. They establish how to separate the weak nonlinear contribution of the signal from the dominant linear contribution. This has been made possible thanks to a new kind of experience developed by ETH researchers. A major challenge was to respond to the need to illuminate the sample with a relatively high power light to refine the weak nonlinear signal. In order to ensure that the photons striking the semiconductor do not cause undesirable changes to the electronic system – especially the ionization of the trapped charges – the Imamoglu- team Wegscheider designed a sample structure reducing the sensitivity to light, and conducted experiments with that excitation continues, to minimize exposure to light.

The tools currently being developed to measure the nonlinear optical response of quantum Hall states should allow a new understanding beyond what is possible with linear optical measurements or in traditionally used transport experiments. This is good news for those studying the interaction between photonic excitations and two-dimensional electronic systems – an area in which open scientific problems abound.