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THz photonics

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The group has recently demonstrated a novel technique that allows the phase-locking of THz QC lasers to a harmonic of the repetition rate of a fs-fiber laser .3 The heart of the stabilization technique is based on an ultrafast electro-optic modulator based on a ZnTe birefringent crystal, which down-converts the emission frequency of the QCL from the THz directly to the MHz range. In this frequency range standard RF electronics can be used to provide a feedback for the active control of the QCL drive current, and, ultimately, of its emission frequency. With this technique, which was recently patented, we demonstrated a frequency stability < 1Hz, for a 2.7 THz laser. This performance opens the doors to coherent imaging/spectroscopy using THz QC lasers with very high sensitivity and acquisition speed.

has allowed reconstructing the electromagnetic wave emitted by the QCL. We have exploited this possibility to demonstrate that THz QCLs can be operated in a regime of active-mode locking4 by modulating their drive current (see figure 2). The demonstration of this regime has been the last achievement of a long lasting research effort on the microwave modulation of these devices that we have started several years ago. In particular we have shown that THz QCLs are particularly suitable for high speed modulation, due to intrinsically low propagation losses, as well as good optical confinement of the microwave propagating mode. Thanks to these properties modulation frequencies up to 35GHz have been reached. These results represent a major breakthrough for THz technology, paving the way to the generation and detection of ultrashort THz pulses and THz frequency combs.

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