Photon correlations probe the quantized nature of light emission from optoelectronic materials

Even though the quantized interaction between light and matter in the photoelectric effect is considered one of the cornerstones in the development of quantum mechanics, it was long thought impossible to directly observe the quantized nature of light itself. The advent of light-intensity autocorrelation techniques in the 1950s, first developed to study the size of stars, changed little in this perception, until true single-photon correlation spectroscopy from trapped ions was proven to be possible in the late 1970s: Photons from a single quantum emitter tend to arrive in bunches, but each light quantum is spaced apart in time—it is antibunched. Initially considered only as the workhorse of atomic quantum optics, over the past two decades, photon correlation techniques have become a standard in fields as diverse as quantum-information processing, biological imaging, polymer physics, and materials science, in general. The technique is particularly useful for probing materials that are specifically designed to emit or absorb light in optoelectronic devices, such as light-emitting diodes, solar cells, or lasers. The central question in such studies is how large a mesoscopic piece of material can become and still behave as an atom-like source of single photons. What interactions may arise between multiple excitations within a material, and what sort of dark states may give rise to intermittency in the stream of photons? We review the many different classes of optoelectronic materials for which photon-correlation spectroscopy has proven to offer useful insight into excited-state dynamics, ranging from molecular, over semiconductor to metallic nanostructures. The technique is particularly suited to probing mesoscopic aggregates of organic semiconductors since each single molecule acts as a quantum emitter itself.