Ultrafast Active Tuning of the Berreman Mode

The Berreman effect, by which thin films of polar dielectric materials exhibit strong, narrow resonances near their longitudinal optic (LO) phonon frequency, results in strong material interactions with infrared radiation and offers tremendous potential for infrared nanophotonics. We report the first implementation of the LO-phonon-plasmon-coupling (LOPC) effect to actively tune the Berreman mode of a semiconductor thin film. Using time-resolved ultraviolet pump, infrared probe reflectance spectroscopy, we excite free carriers in a sub-infrared-wavelength film of GaN and observe substantial shifts of the Berreman mode as a new, simple version of LOPC-based polariton tuning. We demonstrate resonance shifts (Δω) greater than the resonance width (δω), realizing a state-of-the-art tuning figure of merit Δω/δω ~ 1.4, and show that the shift can be modulated on a sub-nanosecond timescale. These results provide substantial promise for future ultrafast, wavelength-tunable infrared photonic devices and novel exper...