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Light is difficult to store in a small volume for a long time. Semiconductor cavities, which trap light efficiently, are an essential component of many important optical devices and effects, from optical processing to quantum light sources. Cavities are characterized by two main quantities: the modal volume V and the quality factor Q. In many applications, high Qs and small Vs are highly desirable for the high finesse required for laser and filter applications, or for the large Purcell factor required for controlling the spontaneous emission of atoms placed in resonance with the microcavity mode. For a dipole linewidth much smaller than the cavity linewidth, a simple derivation shows that the Purcell factor is equal to 3â(4Ï2)(λân)3QâV, where n is the refractive index of the medium and λ is the resonant wavelength matched with the emission wavelength. This formula holds for a perfect emitter placed at the antinode of the electromagnetic field and with its dipole parallel to the electric field. Thus the QâV ratio is a figure of merit of the cavity alone, which describes the cavity capability to enhance light-matter interaction.
At optical frequencies, due to the lack of good metals, the last decade has seen intense research activity on a new generation of microresonator devices. Total internal reflection is solely exploited in spherical or disk-shaped resonators or in wire rings, and a hybrid confinement that combines photonic bandgaps in one or two dimensions with index guiding is exploited in photonic-crystal (PhC) microcavities such as micropillars, PhC cavities in semiconductor wires, or in two-dimensional photonic-crystal membranes (see Fig. 7.1).
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