Nelson's Movie Theater
| The Basics The interference pattern of crossed excitation pulses generates multiple-cycle polariton waves in lithium tantalate. Polaritons propagate at the speed of light (for the material) along the surface of the crystal and away from the excitation region. The polariton frequency may be tuned by changing the interference fringe spacing. |
| Reflections
The polaritons cannot propagate through air and so reflect off the crystal edge. In this movie, single-cycle polariton wavepackets are generated in a 3x8mm lithium niobate crystal by a cylindrically focused "line" of excitation light. |
| Transmission through Air Single-cycle polariton wavepackets are generated as above, but in this case a second lithium niobate crystal is placed adjacent to the first separated by a 100-micron gap. Because electric fields extend well past the surface of the crystal, an incoming polariton induces a wave in the second slab of lithium niobate and effectively transmits itself across the gap. This allows the interesting possibility of performing THz spectroscopy on a sample by placing it between the two crystals, an idea made even more attractive by the control over wave timing and frequency afforded by the versatile technique for polariton generation. In addition, the entire volume is less than 1 cm3 and could be incorporated in a microfluidics device. |
| Pulse-shaping and Simulation Using either a two-dimensional liquid crystal or micromirror array, we are able to introduce phase changes over the entire cross-section of an expanded laser beam. Appropriately specifying these phases provides both spatial and temporal control over the femtosecond laser pulse. Here, more than 40 excitation spots are generated, starting first at the bottom of the image and moving upward. Each spot immediately launches a propagating wave, and they interfere constructively to form this tilted wavefront. The top panel is data, while the bottom is the result of finite difference time domain (FDTD) simulations which accurately model polariton propagation through the crystal. |
| Focusing The same techniques are used as above, but here eight excitation pulses that arrive at eight distinct, vertically arrayed spots with a parabolic temporal sweep. The resulting parabolic THz wavefront is focused within the crystal, and laser pulse shaping allows complete control over both the both location and timing of the focus. Again, there is good agreement between theory and experiment. |
| Waveguides Here the polariton is focused into an integrated THz waveguide fabricated by femtosecond laser machining of a lithium tantalate crystal. Two parallel "trenches" of air were generated, leaving an un-machined region that acts as a 1.8-mm long waveguide. |
| Interference
Polariton field splitting and recombination in a waveguide interferometer in lithium tantalate. |
| A Resonator
Laser machining was used to generate a resonant cavity in lithium tantalate. In this movie, crossed pulses we used to generate an overtone of the fundamental resonator mode. The period was chosen so that an integral number of half-wavelengths would fit in the cavity. | |
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