Active Mode Locking of a P-GE Light-Heavy Hole Band Laser by Electrically Modulating its Gain: Theory and Experiment

Abstract

The p-Ge intervalenceband (IVB) far-infrared laser (50cm−1 < v < 140cm−1), operating in crossed electric and magnetic (E ⊥ B) fields, is based on a population inversion between accumulated light holes and streaming heavy holes in the k-space region below the optical phonon energy ɛop.1 Its broad amplification band (Δv ≈ 100cm−1) allows amplification of far-infrared pulses at a picosecond timescale.2 We have investigated the feasibility of achieving active mode locking in a p-Ge IVB laser by locally modulating the gain in a small part of the p-Ge crystal. By applying a radiofrequency (RF) electric field to small additional contacts along the magnetic field direction, the population inversion in the active part between these contacts is modulated strongly: at peak amplitudes of the RF cycle the light holes are accelerated out of the so-called passive region below Ɛop, while at the RF nodes the light hole population is restored due to the fast pumping by optical phonon scattering from streaming heavy holes. Fig. la shows the expected response of the small-signal gain due to the direct IVB transition during one period of a 500MHz RF electric field, as it has been calculated using a Monte Carlo simulation program.3 From Fig. lb it is clear that a peak-to-peak modulation depth of 30–40 % can be obtained in the modulated part of the p-Ge crystal with only a small RF field amplitude E RF 0 of approximately 20Vcm−1.