Introduction
Physical phenomena in the terahertz (THz) spectral range are intensively
studied, which is associated with the fundamental importance of new terahertz
physics, as well as with a considerable number of feasible applications. In
particular, many researches aim at constructing new sources and
detectors of THz radiation.
In this context, of great interest are the researches of
semiconductor heterostructures with quantum wells (QWs), in which
collective oscillations of a two-dimensional electron gas (2DEG),
i.e. plasmons, can be excited. The frequencies of plasmon
oscillations belong to the THz spectral range. In a homogeneous
2DEG, plasmons are stable, i.e. their oscillations attenuate in time
owing to the electron scattering by various crystal defects and by
means of the known mechanism of Landau damping. Various methods were
proposed in order to achieve the instability and obtain
oscillation-growth effects intended to be used for the amplification
and the generation of THz radiation. Among those, a possibility of
the instability excitation owing to the electron drift in an
electric field \cite{rev-1, rev-2, Wilkins}, various variants of
two-beam instability \cite{Wilkins, Gribnikov}, and others were
examined. However, for such instabilities to be realized in a
spatially uniform plasma, very high drift velocities, which are
difficult to be obtained experimentally, are required. The situation
essentially changes in spatially non-uniform or finite systems
\cite{f-length-1}. For example, as was shown in works
\cite{Dyakonov}, if special types of contacts are applied to a
quantum well with a finite length, the electron system becomes
unstable already at moderate drift velocities of electrons. This
instability has been studied experimentally \cite{Knap-2004,Knap-2005,Knap-2008}.
Another class of objects, which are active in the THz spectral range, includes
quantum dots \cite{Demel_1990, old-QDs-1, old-QDs-2, old-QDs-3}, molecules and
some molecular compounds \cite{Yu_2004, Maistrenko_1999, Balu_1999}, shallow
impurity centers \cite{Burghoon_1994, Kalkman_1996, Allen_2005}, and so forth.
For brevity, let us call such “zero-dimensional” objects as nanoparticles (NPs).
Hybrid systems consisting of nanoparticles and heterostructures with
free electrons constitute a new type of heterodimensional
objects, which should demonstrate essentially new properties and
effects. In particular, if the oscillation frequencies of
zero-dimensional NPs and two-dimensional plasmons are in the THz
spectral range, hybrid systems may reveal new properties at
extremely high frequencies. Therefore, the study of the interaction
between such NPs and plasmons under equilibrium and nonequilibrium
conditions seems well-timed.