Why a magnetized quantum wire can act as an optical amplifier: a short survey

Abstract

This paper 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: [Formula: see text] and [Formula: see text], 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 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.