Probing polariton condensation dynamics by multiquantum coherent spectroscopy in organic optical microcavities

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The prospect of 'plastic' lasers, in which each component is composed of polymers or molecules, is exciting because these can be processed simply from solution, reducing significantly their cost of fabrication, and because the flexibility of plastics presents opportunities to conceive flexible lasers. Furthermore, the optical properties of each component can be tuned synthetically; these materials can be designed to emit light very efficiently in the red, green or blue regions of the visible spectrum. However, implementing commercially viable plastic lasers presents many challenges, primarily because excited states of these materials absorb light strongly at wavelengths close to the emission wavelength, and thus high excited-state densities are necessary to induce laser effects. However, a promising alternative approach has been proposed and demonstrated using traditional semiconductors, so-called polariton lasers. These devices exploit the strong interaction of light and electrons, in which new quasi-particles that can be considered to be part photons and part electrons, are generated. These quasi-particles are termed polaritons, and they have the very special property that are so-called bosons. Practically, this means that two or more polaritons may occupy the same quantum-mechanical state. If the boson density is high enough, a large fraction of these occupy the lowest quantum state, at which point quantum effects (coherence; like a large ensemble of cooperative skater pairs all performing in exquisite choreography when the spotlight is upon them) become apparent spontaneously on a macroscopic scale, i.e., the polariton wavefunctions overlap spatially over large lengthscales. The strong photonic character of polaritons permits such coherence to an extent that is challenging for excitons (excited electrons), which are 'dirty' bosons and undergo mutual annihilation at high density. Spontaneous coherence is at the origin lasing in polariton lasers: the optical leakage of out of the spontaneously coherence state is the source of laser light. In this project, we will develop an experimental apparatus that will probe with unprecedented detail the interactions between polaritons that give rise to spontaneous coherence.