Mode locking of far-infrared p-Ge lasers

Active mode locking of the THz p-Ge light-to-heavy-hole-band laser is reported yielding 200 ps FIR pulses with a few Watts peak power. It is achieved via radio-frequency (RF) gain modulation at one end of the laser crystal in Voigt configuration. Applying an additional bias voltage at the RF contacts allows one to optimize the gain and improve mode locking characteristics. This compensates an intrinsic bias onset caused by charging of the laser crystal due to the asymmetry of (warped-band) hole transport in crossed electric and magnetic fields for the orientation B||[112], E||[11¯0] used in our experiments. A transition from single-pulse mode locking to second harmonic mode locking is observed for a crystal with roundtrip frequency equal to the RF frequency, and separation between the two pulses is tuned from zero to half the cavity roundtrip period by changing the external bias to the modulating RF field