Between electronics and photonics there exists a frequency gap of approximately two octaves, i.e., the frequency range between 100 GHz and 10 THz, across which there are limited capabilities for signal generation, control, guidance, and processing. Here, we demonstrate that phonon-polaritons in ionic crystals like LiNbO3 or LiTaO3 may be used to bridge this gap. The ability to directly visualize polariton fields through real-space imaging, to generate arbitrary THz waveforms through the use of temporally and/or spatially shaped optical waveforms, and to fabricate integrated functional elements for polariton guidance and control through laser machining yields a THz polaritonics platform that enables advanced signal processing and spectroscopy applications.
Phonon-polariton dispersion is characterized in ferroelectric lithium tantalate and lithium niobate through femtosecond time-resolved impulsive stimulated Raman scattering (ISRS). An improvement in the ISRS setup permits optical heterodyne detection of the signals. In addition to substantially increasing the sensitivity and accuracy of the measurements, the phase sensitivity of heterodyne detection makes it possible to fully characterize the polariton wave after it has propagated outside of the excitation region. The detection of propagating responses with heterodyned ISRS is explored theoretically and experimentally. Discrepancies in earlier results reported for these materials are resolved.
Generation and control of pulsed terahertz-frequency radiation have received extensive attention, with applications in terahertz spectroscopy, imaging and ultrahigh-bandwidth electro-optic signal processing. Terahertz 'polaritonics', in which terahertz lattice waves called phonon-polaritons are generated, manipulated and visualized with femtosecond optical pulses, offers prospects for an integrated solid-state platform for terahertz signal generation and guidance. Here, we extend terahertz polaritonics methods to patterned structures. We demonstrate femtosecond laser fabrication of polaritonic waveguide structures in lithium tantalate and lithium niobate crystals, and illustrate polariton focusing into, and propagation within, the fabricated waveguide structures. We also demonstrate a 90 degrees turn within a structure consisting of two waveguides and a reflecting face, as well as a structure consisting of splitting and recombining elements that can be used as a terahertz Mach-Zehnder interferometer. The structures permit integrated terahertz signal generation, propagation through waveguide-based devices, and readout within a single solid-state platform.
We report the generation of aberration-free cylindrical phonon-polariton wave packets in uniaxial LiTaO3 crystals by nonresonant impulsive stimulated Raman scattering. The unique properties of phonon polaritons with a typical carrier frequency in the THz regime allow direct measurement of the spatiotemporal amplitude and phase distributions. We demonstrate that under these conditions the phase anomaly (Gouy phase) may be visualized directly through spatiotemporal imaging as the cylindrical wave propagates through its focus.
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