Excitonic superradiance to exciton-polariton crossover and the pole approximations

Ultrafast thermal-free photoluminescence of coherently extended single quantum states

The coherent volume of single quantum states of matter is typically smaller than that of photons by several orders of magnitude, and hence, interactions between photons and single quantum states are normally very weak. This limits the speed of radiative decay of matter states in free space. Recent efforts to speed-up radiative processes have been focused on creating a small mode volume of photons using cavity systems, or on realizing spontaneous synchronization among quantum emitters to create a dipole at the macroscopic scale, which accelerates photon emission up to a couple of hundred femtoseconds. Here, we demonstrate the 10-fs class of photoluminescence (PL) of a single quantum state in solid thin films without the use of a photo-cavity system or the spontaneous synchronization effect. Significantly, this speed can beat thermal dephasing of relevant excited states at room temperature, which is typically a couple of tens of femtoseconds. The process occurs due to the giant interaction volume between light waves and the multipole excitonic waves. This result indicates the possibility to realize photoemission processes that complete before the thermal dephasing process activates, which opens up the hidden potential of ubiquitous solids as thermal-free or extremely low-energy-loss photonic materials.

Superradiance-to-Polariton Crossover of Wannier Excitons with Multiple Resonances

We demonstrated the superradiance-to-polariton crossover of the blue excitons in Cu_{2}O by varying the sample thicknesses instead of controlling the cavity quality factor. The crossover behavior was compared with unprecedented calculations based on the nonlocal optical response theory with the inclusion of three exciton resonances. The crossover thickness, found to be 177±2  nm, was smaller than the predicted value for a single resonance by a factor of 5. The fact that there was much larger longitudinal-transverse splitting (40±5  meV) than in the bulk implies a surprisingly fast radiative recombination even without a cavity structure.

Observation of superradiance by nonlocal wave coupling of light and excitons in CuCl thin films

We report the observation of a remarkably strong coupling between light and a multinode-type exciton. The observed radiative decay time reaches the order of 100 fs, which is much faster than the dephasing process of nonradiative scattering. In this high-speed superradiance, the light wave and the excitonic wave in a high-quality thin film form a harmonized wave-wave coupling over a range of multiple wavelengths. This mechanism contradicts the conventional physical description of light-matter interaction based on the long-wavelength approximation.