We analyze the spectrum and stability of transverse electromagnetic excitations in a dc-biased semiconductor superlattice in a microstrip line geometry. We show the existence of a critical dc field strength that corresponds to equality of the Bloch frequency and the electron plasma frequency at the bottom of the miniband. At this field strength the frequency-wave-number dispersion curves change their character. For a subcritical dc field, the spectrum of electromagnetic excitations consists of propagating transverse electromagnetic wave and quasistatic Bloch oscillations. In the case of overcritical dc field, transverse electromagnetic wave and Bloch oscillations are hybridized. There is a low-frequency branch that is unstable, whereas the high-frequency one is always stable. Instability of the low-frequency mode makes possible the creation of a tunable Bloch-oscillator-based tetrahertz (THz) amplifier and generator using microstrip line and microcavity devices. We also find that the low-frequency branch displays a negative refraction region which is associated with the backward wave excitation in a superlattice; this gives the potential possibility of the subwavelength focusing of electromagnetic radiation in THz frequency domain by means of a superlattice flat slab.
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