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Using a high-resolution Fabry-Perot interferometer, we have studied the emission of a far-infrared ptype germanium cyclotron laser in a frequency range of strong emission (82 -87 cm '). A nonlinear tuning of the emission frequency with magnetic field, a magnetic field dependent width, and a splitting of the emission line (near 85.8 cm ) were found. We present an analysis showing that the emission behavior can be understood by recognizing that the laser level separation depends not only on the magneticfield strength, but also on the ratio of the electric and magnetic-field strengths. We further relate the observed line splitting to radiation absorption due to transitions between Zeeman-split states of shallow acceptors.In earlier studies of the far-infrared p-type germanium cyclotron laser (FCL) a linear dependence of the emission frequency vL on the magnetic-field strength B has been reported. ' The relation is vL =eB(2m md) ' where md is the dynamic mass of light holes which is slightly larger than the classical cyclotron mass of 0.042m"m, being the free-electron mass. In this paper we report on a high-resolution investigation of the tuning curve and of the linewidth of a FCL. The experimental results, together with a band-structure analysis, give evidence that the dynamic mass depends on both the electric-and the magnetic-field strengths.Our measurements were made using a p-type germanium crystal with a boron doping concentration of 5 X 10' cm . The rectangular bar-shaped crystal had dimensions 37X4X6.5 mm and was cut with the longest dimension parallel to the [110] axis. Two opposite long sides of the sample were covered with aluminum forming Ohmic contacts. The laser crystal was placed in a superconducting solenoid which in turn was immersed in liquid helium. The magnetic field was applied parallel to the long side of the crystal and the electric field was parallel to another [110]axis. The experimental arrangement [ Fig. 1(a)] is similar to that described recently, 5 6i.e. , using a SrTi03 crystal as a high reAectivity mirror.However we used a hybrid metal mesh mirror with nearly constant optical properties in the investigated spectral range as an outcoupling mirror. This mirror was made from a chess-board pattern of molybdenum squares (of 16 pm period) deposited on a sapphire substrate. A 10-pm TeAon film was placed between the metal film and the sample for insulation. The laser radiation was guided out of the cryostat by means of a stainless-steel pipe (20-mm diameter). The emission spectrum of the laser has been analyzed by use of a high-resolution Fabry-Perot interferometer consisting of two parallel 750 lines per inch copper meshes. The device was placed in a vacuum chamber to avoid water vapor absorption and had a finesse of 20 at 90 cm '. The interferometer was operated in 50th order yielding a resolution of 0.1 cm . The radiation transmitted through the interferometer was 88 86-LASER I p -Ge (o) (c) U &-84 C3 C3' CL 82 AOT 80 85 88 87 FREQUENCY (cm ') 4.2 4 Q 4 4 MAGNETIC FIELD (T) FIG. 1. (a) Ske...

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