Field-resolved detection of ultrabroadband infrared pulses is implemented with GaSe crystals. Via phase matching, we extend the detectable frequency range toward the near infrared (λ=2.5μm) and directly record transform-limited pulses as short as 28fs with a 3dB bandwidth of 41THz. The continuous tunability of the center frequency over a wide interval ranging from 31THz to the far-infrared is demonstrated using thick sensors.

The ultrafast transition of an optical phonon resonance to a coupled phonon-plasmon system is studied. After 10-fs photoexcitation of i-InP, the buildup of coherent beats of the emerging hybrid modes is directly monitored via ultrabroadband THz spectroscopy. The anticrossing is mapped out as a function of time and density. A quantum kinetic theory of microscopic carrier-carrier and carrier-LO-phonon interactions explains the delayed formation of the collective modes. The buildup time is quantitatively reproduced to scale with the oscillation cycle of the upper branch of the coupled resonance.

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Electro-optic detection with unprecedented bandwidth is implemented with GaSe sensors utilizing phase matching. We directly record transform-limited 28-fs pulses containing spectral components beyond 120 THz (h= 2.5 pm). Using thicker crystals the continuous tunability of the detector response is demonstrated between 14 THz and 31 THz..