Collective spontaneous radiation and superradiant d-class lasers

Of all coherent quantum phenomena, collective spontaneous radiation (Dicke supperadiance) has always attracted the attention of IAP researchers. In the recent years, the possibility of such radiation produced by the Bose-Einstein condensate of particles with the internal degrees of freedom was proposed and justified theoretically. The spectrum-correlated properties of such superradiance have been identified for a laser which uses traps serving as low-Q resonators and having dimensions that exceed the radiation wavelength considerably. Specifically, the existence of a superradiant laser generation at the frequency of recombination of dipolar excitons in semiconductor traps with the quantum holes under conditions close to the conditions required for the Bose-Einstein condensation of excitons at a temperature of about 1–10 K has been predicted. A possible design of resonators, where the required partial electromagnetic modes can be produced, has been proposed. Such modes can be generated due to a total internal reflection from the heteroboundaries (surfaces) of the trap, which ensures generation of the normal polariton modes whose polarization (exciton) components prevail over the electromagnetic ones. In this case, the generation is possible not only due to the excitons condensed in the main state of the trap, but also due to the over-condensate excitons, before the threshold of the Bose-Einstein condensation is reached. The obtained result provides a foundation for the development of new methods for the diagnostics and spatial re-distribution of the excitons in the critical region.

Example of a quasi-periodic superradiant generation of a D-class laser: the dynamical spectra of the electromagnetic field (left) and the inversion of the active medium (right) are shown in the “time – frequency” plane (deeper yellow color corresponds to more intense generation)

Another remarkable possibility is a collective recombination of free electrons and holes produced by femtosecond laser pumping in the heterostructures with the quantum holes located in a transverse magnetic field of about 10 T. It was predicted about 20 years ago by A. A. Belyanin, V. V. Kocharovsky, and Vl. V. Kocharovsky, and achieved and studied in detail in the joint Russian-American experiments not long ago. The recombination occurred simultaneously from several Landau levels of electrons and holes. It was accompanied by arbitrarily directed, but coherent pulses of collective spontaneous radiation at each of these interband transitions.

It has been also shown for the first time that the latest achievements of nanotechnology, specifically, growth of submonolayer quantum dot arrays and the IAP-developed methods of the nonlinear dynamical mode selection due to a distributed feedback can be used to make a unique superradiant heterolaser with a millimeter-scale length and an efficiency of over 50%. It is capable of generating sequences of the high-power (multiwatt) supershort (picosecond) pulses of a coherent radiation under continuous optical or injector pumping in the absence of special mode synchronization, and is designed to solve complicated problems of dynamical spectroscopy and data processing. Its radiation has unconventional (for existing lasers) spectro-dynamical and correlation properties, since it is produced in a low-Q multi-mode resonator, where the lifetime of photons is short compared with the lifetime of an optical polarization of the quantum dots (E. R. Kocharovskaya, V. V. Kocharovsky, Vl. V. Kocharovsky). Such lasers, which are called D-class lasers and have not been realized before, can be made of the GaAs/InGaAs heterostructures with several layers of submonolayer quantum dots. Such structures have been designed at IAP in cooperation with the Physical-Technical Institute of RAS and have a record-breaking spectral and spatial density of their states and a sufficiently long time of relaxation of their optical polarization.