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arXiv:physics/0105091

Optical absorption in a Bose gas

Sungkit Yip

Published May 28, 2001• 0 citations
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Abstract

We here develop a theory on optical absorption in a dilute Bose gas at low temperatures. This theory is motivated by the Bogoliubov theory of elementary excitations for this system, and takes into account explicitly the modification of the nature and dispersion of elementary excitations due to Bose-Einstein condensation. Our results show important differences from existing theories.

DOI
10.48550/arxiv.physics/0105091

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We here develop a theory on optical absorption in a dilute Bose gas at low temperatures. This theory is motivated by the Bogoliubov theory of elementary excitations for this system, and takes into account explicitly the modification of the nature and dispersion of elementary excitations due to Bose-Einstein condensation. Our results show important differences from existing theories.
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The paper deals with the properties of Bose-gases in inhomogeneous external fields at very low temperatures, when a Bose–Einstein condensation occurs. This particularly refers to trapped alkali gases, which are of great experimental and theoretical interest. The system is considered here in the frame of the developed approach close to the Bogoliubov one with necessary modifications. In particular the generalization of Bogoliubov transformations made it possible to find the spectrum of quasiparticles (low-energy thermal excitations) with account of purely quantum parts of ψ-operators in first order.
Optical excitations in a Bose gas are demonstrated to be sensitive to many-body effects. At low temperature the momentum relaxation is provided by momentum exchange collisions, rather than by elastic collisions. A collective excitation mode forms, which in a nondegenerate gas is manifest in a collision shift and dramatic narrowing of spectral lines. In the Bose-Einstein condensate state, each spectral line splits into two components. The doubling of the optical excitations results from the physics analogous to that of the second sound. We present a theory of the line doubling, and calculate optical absorption spectrum for a uniform density system.
In the case of structureless bosons, cooled down to low temperatures, the absorption of electromagnetic waves by their Bose-Einstein condensate is usually forbidden due to the momentum and energy conservation laws: the phase velocity of the collective modes of the condensate called bogolons is sufficiently lower than the speed of light. Thus, only the light scattering processes persist. However, the situation might be different in the case of composite bosons or the bosons with an internal structure. Here, we develop a microscopic theory of electromagnetic power absorption by a Bose-Einstein condensates of cold atoms in various dimensions, utilizing the Bogoliubov model of a weakly-interacting Bose gas. Thus, we address the transitions between a collective coherent state of bosons and the discrete energy levels corresponding to excited internal degrees of freedom of non-condensed individual bosons. It is shown, that such transitions are mediated by one and two-bogolon excitations above the condensate, which demonstrate different efficiency at different frequencies and strongly depend on the condensate density, which influence varies depending on the dimensionality of the system.