Collective versus single-particle effects in the optical spectra of finite electronic quantum systems

We study optical spectra of finite electronic quantum systems at frequencies smaller than the plasma frequency using a quasiclassical approach. This approach includes collective effects and enables us to analyze how the nature of the (single-particle) electron dynamics influences the optical spectra in finite electronic quantum systems. We present an analytical expression for the low-frequency absorption coefficient of electromagnetic radiation in a finite quantum system with regular ballistic electron dynamics; a two-dimensional electron gas confined to a strip of width a with specular reflection at the boundaries (our approach is not restricted to systems with regular electron dynamics; it applies equally in the case of diffusive or classically chaotic electron motion). By comparing with results of numerical computations using the random-phase approximation we show that our analytical approach provides a qualitative and quantitative understanding of the optical spectrum.