^newsbreaks
Terahertz waves generated and detected
in ambient air
Multicore glass
optical fiber to
aid auto safety

Researchers at Rensselaer Poly-

technic Institute ( Troy, NY) have demonstrated the use of ambient air as a sensing medium for the broadband detection of terahertz waves. _Parabolic

λ/2 waveplate

^Lens Si filter ^THz _pulses

High data rates and immunity from electromagnetic interference are bringing fiberoptic communications into the automobile, via the so-called MOST (Media Oriented Systems Transport) bus; the result will be intravehi-cle communications networks to handle everything from on-board computing to DVD signals (see www.laserfocusworld.com/ articles/250384). Polymer optical fibers paired with light-emitting diodes (LEDs) are usually chosen, but these fibers are limited to temperatures below 85°C, bending radii above 25 mm, and medium data rates.

Combined with terahertz-wave gen-mirror Plasma eration in air with femtosecond laser _{beams, the all-air and all-optical Lens} BBO _{Lens Filter} sensing approach is expected to en- _{Detector Delay} able remote terahertz-wave sensing and spectroscopy in high-humidity ^Beamsplitter atmospheric environments.

Laser pulse
120 fs, 800 J
800 nm, 1 kHz

Multicore glass optical fibers developed by Schott (Mainz, Germany) are designed to push past these restrictions, allowing the use of fiber for active safety applications in the engine compartment, such as tying together cameras or radar systems that are installed in the bumper for precrash analysis of traffic. The fibers have hot-fused end surfaces, single-fiber diameters of 53 µm, and withstand 125°C, bend to a 5-mm radius, and transmit at 1 Gbit/s, allowing real-time transmission without data compression. The multicore fibers are paired with near-IR ver-tical-cavity surface-emitting lasers, which can be modulated at much higher rates than LEDs, but whose wavelengths are not transmitted well by polymer fibers. Contact Kate Pepler at ka-tie.pepler@us.schott.com.

The terahertz wave was experimentally generated by mixing the fundamental pump beam and its second harmonic at the air-plasma point (right). A parabolic mirror (right) collimated the terahertz beam. A high-resistivity silicon filter blocked the residual pump and second-harmonic beams. A second parabolic mirror (left) focused the collimated terahertz beam. The terahertz wave was detected by the reciprocal process of its generation. A second-harmonic signal was produced by mixing the fundamental probe beam (polarization controlled by a λ/2 waveplate) and the terahertz field, both focused at the same point, with estimated focusing spot sizes of 800 and 20 µm for the terahertz wave and optical beam, respectively. By measuring the time-resolved second-harmonic signal, the amplitude and phase of the terahertz field were coherently detected using homodyne detection. Contact X.-C. Zhang at zhangxc@rpi.edu.

Petawatt-class laser to aim for
four-wave mixing in a vacuum

Researchers from Umea University (Umea, Sweden) and Rutherford Appleton Laboratory (Oxfordshire, England) have advanced a proposal for exploring the laws of quantum electrodynamics (QED) by performing a four-wave mixing experiment in a vacuum. They propose colliding three laser pulses to stimulate emission of a fourth with a new propagation direction and wavelength, due to elastic photon-photon scattering.

According to QED, such scattering can occur in a vacuum because of the interactions of virtual electron-positron pairs. But doing so at detectable levels would require an exceptionally powerful laser source with a fast pulse-repetition rate. The researchers intend to use the high-repetition-rate petawatt-class Astra-Gemini laser at Rutherford Appleton Laboratory in 2007 to generate two independently configurable 0.5-PW, 800-nm pulses with 15 J of energy and focused intensities in excess of 10²² W/cm², at a pulse-repetition rate of one shot per minute. In contrast with previous 2-D beam-mixing proposals, this team will attempt a 3-D process by frequency doubling and splitting one of the laser beams into two. The researchers expect to create 0.07 new photons per pulse at a wavelength of about 267 nm. Contact Mattias Marklund at mattias.marklund@physics.umu.se.