The laws of Quantum Mechanics predict that the spontaneous emission process from a collection of identical sources placed in a very small volume is a coherent effect proportional to the number of sources. This phenomenon, known as "superradiance" is generally difficult to observe, regardless of the physical system : atomic clouds, semiconductor quantum dots or superconducting circuits. Identical two-level systems are especially difficult to produce in condensed matter because factors like temperature, disorder and band-structure bring inhomogeneous broadening of the transitions energies, and only a small number of emitters can be considered as identical per energy band. Researchers from the QUAD group showed that a dense electron gas confined in a thin semiconductor layer is the ideal system to observe superradiance. Indeed, the increase of the electron density in such system not only favors the superradiant effects but also the Coulomb interaction between electrons which in turn increases the coherence between the sources. For very high electron densities the ensemble of electronic transitions becomes homogeneous and the spontaneous superradiant emission becomes dominant over all other relaxation phenomena.
The samples that were studied comprise a thin InGaAs layer (quantum well) sandwiched between two layers of AlInAs. The InGaAs layer has a very high electronic concentration (1019 electrons/cm3) obtained by incorporation Si donor during the epitaxial growth. For such high electronic densities, interactions become very intense and phase-lock the electronic oscillations together into a single collective plasmonic mode. To excite the collective mode and observe luminescence, the researchers fabricated transistor-like devices, where the source and drain contacts allow injecting an electrical current directly into the doped InGaAs layer and heat-up the electron gas. The hot electrons then excite superradiant plasmons by a heat transfer.
The phenomenon of superradiance was then demonstrated by a detailed study of the emission linewidth which was found to increase with the electronic concentration and the angle of emission. For some angles the radiative contribution dominated completely the spectral linewidth, which makes this measurement a very reliable estimation of the spontaneous emission rate. Data analysis showed that the spontaneous emission lifetime could be as short as 10 fs, a million times faster than the emission lifetime for a single electron, in perfect agreement with theoretical predictions.
The spontaneous emission thus becomes the fastest process to dissipate the energy which is supplied to the plasmons, much faster than all non-radiative relaxation phenomena characterized by a timescale of about 1 ps. This implies that such superradiant plasmons mainly generate photons than anything else, and could become the active core of new efficient sources of infrared radiation.
Figure :
Color plot of the emission from quantum well plasmons as a function of the energy and the emission angle. The enlargement of the emission spectra at high angles is the evidence of the superradiant phenomena. The inset shows a scanning electron picture of a transistor-like device fabricated for these studies.
Contact :
yanko.todorov@univ-paris-diderot.fr
Reference :
Superradiant Emission from a Collective Excitation in a Semiconductor, T. Laurent, Y. Todorov, A. Vasanelli, A. Delteil, C. Sirtori, I. Sagnes, and G. Beaudoin, Phys. Rev. Lett. 115, 187402 (2015).
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