The spectral region between 0.1 and 10 THz is known as the “Terahertz (THz) gap”. Compared with other regions of the electromagnetic spectrum, it is underdeveloped and there is a lack of sources, amplificators and detectors. Le development of new devices in this domain is a major goal of modern opto-electronics. The “ultra-fast THz spectroscopy” group at LPA-ENS has recently developed a THz pulse amplificator based on novel scheme of “gain-switching” in quantum cascade lasers.
Ultra-fast THz spectroscopy is a recent technique and is now used for a number of applications ranging from studies of semiconductor nanostructures to bio-systems, where both the amplitude and phase of the THz wave can reveal interesting information. In these experiments, ultra-fast (sub-ps) THz pulses are generated from near-infrared pulses, by optical rectification or using photoconductive antennas. Nevertheless, the resulting THz pulses display a low power and THz 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, as in all lasers.
The solution developed at LPA-ENS 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 (in our experiment). The device is a gain-switching amplifier.
N. Jukam et al. Nature Photonics 3, 715 (2009)
News and Views par A. Tredicucci et A. Di Carlo Nature Photonics 3, 681 (1 Dec 2009)