Quantum vacuum radiation in optical media
Westerberg, Niclas Kim Lennart
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In this thesis we study quantum vacuum radiation. This is the radiation that is emitted due to changes of the electromagnetic vacuum in time. Speciﬁcally, we explore the phenomena in optical media at a macroscopic scale by introducing a time-dependent permittivity. We model this by inducing time varying changes to the medium’s resonance frequencies. To start with, we build a perturbative model, from which we learn that the physics of quantum vacuum radiation is well-described in terms of the collective light-matter excitations (polaritons) but that the retarded response of the matter degree of freedom should not be forgotten. In particular, the retarded response of the medium leads to quantum vacuum radiation that can be driven not only by frequencies supported by the spectrum of the modulation, but also local frequencies, such as the beating pattern of two waves. We then apply this model to analyse a ﬁbre optics experiment where photon pair production was measured, and ﬁnd a good agreement between the measured and predicted spectrum. Interestingly, the measured photon pair production coincide with quantum vacuum radiation driven by the beating pattern formed by a travelling polarisation wave and the spatial modulation of the ﬁbre. Following this, we use the perturbative model to study a scenario mimicking a rapidly (∼optical timescales) expanding and contracting spacetime. In this scenario however, the probability to excite quantum vacuum radiation in naturally occurring materials is vanishingly small. Motivated partly by this, we turn to study vacuum radiation in man-made metamaterials, where large changes to the optical properties in time are possible. Speciﬁcally, we study an ε-near-zero metamaterial whose timedependent permittivity has been experimentally measured. In a model that neglects the retarded response of this metamaterial, we ﬁnd that the quantum vacuum radiation becomes strongly peaked around the point where the real permittivity passes through zero. In order to extend the perturbative model to also include large changes to the optical properties in time, we ﬁnish this thesis by mapping macroscopic quantum electrodynamics to a trapped particle in a magnetic ﬁeld. Using the intuition gained from this, we study a variety of non-perturbative settings including bichromatic periodic driving and return to ε-near-zero metamaterials. This conﬁrms some of the previous analysis, as well as provides an intuitive explanation for the physics.