Polaritonic materials fabricated and tested with ultrashort-pulse lasers
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Polaritonic materials fabricated and tested with ultrashort-pulse lasers David W. Ward, Eric Statz, Thomas Feurer, and Keith A. Nelson The Massachusetts Institute of Technology, Cambridge, MA 02139, USA ABSTRACT Using femtosecond laser machining, we have fabricated photonic bandgap materials that influence propagation of phonon-polaritons in ferroelectric crystals. Broadband polaritons were generated with impulsive stimulated Raman scattering (ISRS) using an ultrashort laser pulse, and the spatial and temporal evolution of the polaritons were imaged as they propagated through the fabricated structures with polariton real-space imaging. These techniques offer a new approach to optical materials design. INTRODUCTION Polaritonics constitutes an intermediate regime between electronics, with frequencies typically less than 100 GHz, and photonics, with frequencies on the order of 100’s of THz. In the former, radiation is propagated by strong coupling to the motion of free electrons in a transmission line, and in the latter radiation is coupled only weakly to bound electronic motion, if at all, and resembles a freely propagating electromagnetic wave with speeds on the order of light in vacuum. Polaritonics occupies the band of frequencies between roughly 100 GHz and 10 THz and relies on elementary excitations known as phonon-polaritons in which the radiation field is strongly coupled to bound ionic charge motion associated with polar transverse optic (TO) phonon modes (1, 2). This coupling gives rise to a splitting between the TO and longitudinal optic (LO) phonon frequencies, between which electromagnetic propagation is forbidden, and upper and lower polariton branches whose dispersion properties are well known (3, 4). When the crystal is patterned with periodic ‘air’ holes, the dispersive properties are modified such that in addition to the polariton bandgap, a photonic bandgap related to the dielectric periodicity is introduced. Recently the interplay between the intrinsic LO-TO bandgap and a photonic bandgap in an overlapping frequency range that is produced through fabrication of a periodic structure in the crystalline host has been discussed (5, 6). Here we demonstrate the capability for fabrication of polaritonic bandgap structures in the THz range by machining ‘air’ holes into an MgO:LiNbO3 crystal, generating coherent THz phonon-polaritons in the material through impulsive stimulated Raman scattering (ISRS) (7), and monitoring THz wave propagation through the photonic bandgap materials (PBM’s) with polariton imaging (8). EXPERIMENTAL DETAILS A homebuilt multi-pass amplified Ti:Saphire femtosecond amplifier (800 nm, 50 fs FWHM, 1 KHz rep. rate, 700 µJ/pulse) seeded by a KM Labs oscillator (790 nm, 15 fs, 88 MHz rep. rate, 3 nj/pulse) was used in both fabrication and imaging of patterned materials.
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Figure 1: Femtosecond laser machining setup. User specified patterns are milled into MgO:LiNbO3 by moving the sample through the beam focus of the microscope objective. Additional components are for co
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