Terahertz Beam Shaping Is Demonstrated

Richard Gaughan

The region of the spectrum between long-wave infrared and microwaves promises significant applications in structural imaging, telecommunications and molecular spectroscopy. Detectors and sources of terahertz radiation do not yet approach the capabilities of devices in higher- and lower-frequency regimes, but a group at Massachusetts Institute of Technology in Cambridge has demonstrated the ability to generate, direct and amplify terahertz waves in a nonlinear optical crystal.

Pulses from a Ti:sapphire laser create propagating terahertz waves within a nonlinear crystal that can be amplified and directed. These false-color images, created from experimental data, illustrate how the temporal shaping of the incident pulses causes a generated wave to come to a focus. Courtesy of Keith A. Nelson.

In the group's experimental apparatus, 50-fs pulses from a Ti:sapphire laser were split to generate an array of pulses from each laser shot. Pulses incident on a 2-mm-thick LiTaO₃ crystal generated a phonon-photon "polariton." This coupled terahertz-frequency lattice vibration/ electromagnetic field propagated through the crystal at approximately one-sixth the speed of light.

MIT researchers Keith A. Nelson, Thomas Feurer and Joshua C. Vaughan predicted that they should be able to direct the incident beams to create arbitrary terahertz waves by manipulating the array of pulses in time and space. To verify this, they used the same source to generate the terahertz waves and to image the vibrations as they propagated through the LiTaO₃. The latter modify the index of refraction of the crystal, so radiation transmitted through the crystal to a CCD camera provided a snapshot of the propagation pattern.

Analogous to the use of multiple transceivers in phased-array radar, different patterns of incident pulses generated linear, angled and focused wavefronts. Additionally, by matching the displacement of incident pulses to the group velocity of the polaritons, the researchers amplified the propagating wave well beyond the amplitude possible with a single pulse.

The ability to send terahertz waves to arbitrary locations with arbitrary time-dependent profiles enables the parallel transmission of many signals or the transmission of complex signals to different addresses without a separate physical structure such as an electronic wire or photonic waveguide for each address, Nelson said. Also, because the waves oscillate slowly enough to perturb both the electrons and nuclei of atoms and molecules, the capacity to amplify and control polariton propagation could lead to coherent control over material structure.

The group has extended the degree of control over terahertz propagation beyond that reported in its recent paper, and it is beginning to apply the generated fields to systems with highly nonlinear responses. Nelson hopes the work will lead to fundamental advances in the understanding of condensed matter as well as to practical applications in areas such as ultrahigh-bandwidth signal processing.