  This article reviews the fundamental issues associated with the
magnetoplasmon excitations investigated in a semiconducting quantum wire
characterized by a harmonic confining potential and subjected to an applied
(perpendicular) magnetic field. We embark on the charge-density excitations in
a two-subband model within the framework of Bohm-Pines's random-phase
approximation. The problem involves two length scales: ${\it
l}_0=\sqrt{\hbar/m^*\omega_0}$ and ${\it l}_c=\sqrt{\hbar/m^*\omega_c}$, which
characterize the strengths of the confinement and the magnetic field ($B$).
Essentially, we focus on the device aspects of the intersubband collective
(magnetoroton) excitation, which observes a negative group velocity between
maxon and roton. Consequently, it leads to tachyon-like (superluminal) behavior
without one's having to introduce the negative energies. Existence of the
negative group velocity is a clear manifestation of a medium with population
inversion brought about due to a metastable state caused by the magnetic field
that satisfies the condition $B> B_{th}$; $B_{th}$ being the threshold value
below which the magnetoroton does not exist. The interest in negative group
velocity is based on anomalous dispersion in a medium with inverted population,
so that gain instead of absorption occurs at the frequencies of interest. A
medium with an inverted population has the remarkable ability of amplifying a
small optical signal of definite wavelength, i.e., it can serve as an {\em
active} laser medium. An extensive scrutiny of the gain coefficient suggests an
interesting and important application: the electronic device designed on the
basis of such magnetoroton modes can act as an optical amplifier. Examining the
magnetic-field dependence of the life-time of magnetorotons leads us to infer
that relatively smaller magnetic fields are optimal.
