The exciton represents an electron-hole pair which can be excited in semiconductors by light. Moldavian physicist S. A. Moscalenco and professor from USA M. A. Lampert predicted in 1958, independently from each other, the existence of biexcitons, i.e. of exciton molecules, representing bound states formed by two electrons and two holes. The biexcitons and multi-exciton complexes have been discovered experimentally later on. For these achievements a group of collaborators of the Russian Academy of Sciences and the collaborator of the ASM Sveatoslav Moskalenko were awarded the State Prize of USSR in 1988. The biexcitons are used nowadays as an efficient source for the generation of entangled photons in quantum information processing.
About 55 years ago, when studying the properties of excitons in semiconductors, the young doctor of sciences Sveatoslav Moscalenco predicted for the first time the phenomenon of Bose-Einstein condensation of the excitons and their superfluidity in semiconductors and described the reversible optico-hydrodynamical processes which can occur in a nonideal exciton gas under the conditions of its Bose-Einstein condensation. The work published in 1962 in the journal „Физика Твердого Тела” (vol. 4, p. 276) triggered the development of a new direction in the solid state physics - the Bose-Einstein condensation of excitons and biexcitons. Over the years, this new direction attracted an increasing number of researchers in different scientific centers of the world, who not only confirmed experimentally the phenomenon predicted by academician Sveatoslav Moscalenco, but also demonstrated its importance for practical applications. Particularly, at the Institute of Applied Physics of the Academy of Sciences of Moldova the phenomenon of Bose-Einstein condensation of excitons and biexcitons was investigated over more than 50 years with the participation of several generations of researchers, more than 1000 papers having been published, including 10 monographs, about 40 PhD theses and 6 doctor habilitat theses having been defended. The results obtained in different scientific centers worldwide in the period of 1962-2000 were reflected in the monograph of S.A. Moskalenko and D.W. Snoke „Bose-Einstein condensation of excitons and biexcitons and coherent nonlinear optics with excitons, Cambridge University Press (2000).
The physics of high density excitons and biexcitons demonstrated a rising development. If the exciton-photon conversion process takes place reversibly and repeatedly, a new type of elementary excitation half-matter/half-light is formed, known as polariton. The phenomenon of Bose-Einstein condensation of such excitations became the basis for the development of a new type of lasers - the polariton laser (
http://www.nature.com/nphoton/journal/v8/n8/fig_tab/nphoton.2014.176_F1.html).
While most of electrons must be in a high energy state (with inversion of the population) for functioning of a usual laser, the polariton laser can work without fulfilling this condition. Among the main advantages of the polariton laser, one can mention an extremely low threshold current density necessary for the diode excitation. Apart from this, the polariton laser emission can be modulated at much higher frequencies as compared to usual lasers (in other words, this type of lasers can be switched on and off much faster).
The discovery of the polariton laser is of particular importance for Moldavian scientists, since the basic theoretical concepts that brought to this scientific and technological success belongs to academician Sveatoslav Moscalenco. We are pleased that the phenomenon predicted by our scientist many years ago not only was confirmed experimentally, but also leads nowadays to the discovery of new lasers, more efficient and cost-effective than the existing ones.
Academician Ion Tighineanu,
First vice-president of the Academy of Sciences of Moldova
