Giant excitons manipulate quantum light

A recent publication in Nature Communications by Valentin Walther and Thomas Pohl explores giant excitons in a semiconductor.

2018.04.19 | Grete Flarup

FIG.1: Some special semiconductor materials can host exotic excitons with a vastly extended Rydberg-state wave function.

Strongly interacting fluids of light are highly coveted but equally difficult to attain in semiconductor structures. A recent publication in Nature Communications by Valentin Walther and Thomas Pohl points a way to such states using Rydberg excitons inside a semiconductor microcavity.

After a long winter, many would perhaps agree that light is an intriguing phenomenon. Electromagnetism, as dating back to the mid 19th century, explains the physics of light in vacuum and nearly as well in air. It is relatively simple because it does not dependent on how much light there is. In other words, photons, the elementary quanta of light, propagate independently from each other.

A new situation arises when a beam of light falls onto a semiconductor. In its ground state, such a material contains electrons in low-energy non-conducting states. Incident light can kick some of these electrons and promote them to states of higher energy. Each of these excited electrons leaves behind a positively charged hole to which it remains attracted and may form a so-called exciton. Excitons can be considered as solid-state analogs of hydrogen atoms, and just like their atomic counterparts experience mutual interactions. This opens up a way to affect the behavior of photons. When a photon moves through a semiconductor, it can be absorbed to create an exciton, which re-emits light when it decays. The emitted photon in turn can create another exciton, and so on. Now imagine that two photons in a semiconductor both get converted into excitons, which, when they interact, change their state and are finally converted back into photons. The resulting change of the two-photon quantum state can be seen as the cause of an effective photon-photon interaction. In most cases, however, this effect is very weak and effective only at close distances because excitons are extremely small and virtually have to bump into each other to feel their presence. Typically, very many excitons are, thus, needed to make such photon interaction notable.

A few years ago, spectroscopic measurements on a special type of semiconductor revealed the existence of a particularly exciting type of exciton with gigantic dimensions. In these so-called Rydberg excitons, the electron and hole orbit each other at an astonishing radius of up to one micrometer; their size is about a billion times larger than that of the  atoms in the semiconductor. Being that big, the observed excitons are very sensitive to polarizating each other, which lets them interact very strongly even at large distances well beyond their actual size.

 

FIG.2: Giant Rydberg excitons of an atomically thin semiconductor can generate effective interactions between photons (sketched in green) in a planar cavity.

 

In their work, Valentin Walther and Thomas Pohl from IFA, together with colleagues from the Max Planck Institute for the Physics of Complex Systems, calculate how light propagates in a semiconductor with such gigantic excitons. Their results show that Rydberg excitons could indeed be used to generate effective photonic interactions that exceed those in conventional semiconductors by several orders of magnitude. This exaggerated optical response provides an exciting outlook for nonlinear optics and is expected to make light behave as a strongly correlated fluid. Besides offering a platform for such exotic phenomena, the strength and long range of the optical nonlinearities suggest viable ways to low-energy optical switching, and even to manipulating light at the single-photon quantum level.

Until such applications can become reality, much remains to be learned about the properties of Rydberg excitons and their control in real semiconductors. The latest results in this new emerging field will soon be on display at Aarhus University, during the “2nd International Workshop on Rydberg Excitons in Semiconductors” on May 3-4.

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