Ultra-fast THz spectroscopy is a recent technique and is now used for a number of applications and fundamental studies ranging from studies of semiconductor nanostructures to bio-systems. In these experiments, ultra-fast (sub-ps) THz pulses are generated from near-infrared pulses by a non-linear process. Although the amplitude and phase can be measured, the resulting THz pulses display a low power and THz ultrafast techniques would therefore strongly benefit from the development of efficient amplifiers. Even though THz quantum cascade lasers are promising sources, their use as amplifiers is a priori limited. Indeed, in a stationary regime, the laser gain is “clamped” and equal to the losses in the system, limiting their use for amplification.
The solution developed and published recently consists in placing the laser out of equilibrium using an ultra-fast switch (“Auston switch”), integrated into the device. This allows to switch on the quantum cascade laser on a picosecond timescale, considerably faster than the build-up time of laser action, circumventing gain clamping and amplifying THz pulses up to 26 dB. The device is a gain-switched pulse amplifier. These results will be reviewed and the perspectives of these investigations will be presented including the possibility of measuring the phase of the laser emission by replacing the Auston switch by a RF excitation.
Reference : N. Jukam et al. Nature Photonics 3, 715 (2009).