An experimental traveling-wave photomixing device generates narrow-band electromagnetic radiation at frequencies up to a few terahertz. The device is, potentially, a prototype of terahertz local oscillators for heterodyne instrumentation for submillimeter-wavelength spectrometry and related scientific applications.

Devices that exploit traveling-wave photomixing to generate radio-frequency signals have been developed previously, but not for the terahertz frequency range. Conventional photomixers (that are not based on traveling waves) with terahertz outputs have also been developed previously, but have been limited as follows: In a conventional photomixer, the output power is proportional to the square of the photocurrent, and the bandwidth is limited by the lifetimes of photoexcited charge carriers and by the electrode capacitance. Therefore, such a photomixer must be designed to have (1) a narrow electrode gap for high photocurrent and (2) a small active area for small capacitance in order to obtain adequate bandwidth. Unfortunately, the smallness of the area of such a device limits its power-handling capability and thus its terahertz output power.

Two Laser Beams With Different Frequencies are aimed at the same device area at different angles to generate traveling difference-frequency charge-density waves accompanied by terahertz electromagnetic waves. Proper phase matching through adjustment of q results in coherent superposition of the terahertz waves, which are then radiated by the antenna.

The present device is designed to overcome the limitations of conventional photomixers. It exploits a traveling-wave principle to distribute the generation of the terahertz signal over a relatively large area, so that a relatively large amount of power can be handled without exceeding the damage-threshold laser power density. Another essential element of the design is that the illuminated traveling-wave area is occupied by a transmission-line structure, which is not subject to the electrode-capacitance bandwidth limitation.

The device (see figure) consists of a dc-biased coplanar strip line terminated by an antenna fabricated on a low-temperature-grown GaAs film. The active area is illuminated by two laser beams that differ somewhat in frequency and are tilted at an angle with respect to each other in order to generate optical interference fringes that move along the strip line. The heterodyne mixing process generates charge-density waves that oscillate at the difference frequency and that are accompanied by terahertz traveling electromagnetic waves. If the velocity of the optical fringes and the group velocity of the terahertz waves are equalized, then the terahertz waves become coherently superposed and are effectively emitted by the antenna. For a given difference frequency, the angle between the two laser beams is adjusted to obtain the phase and velocity match needed for coherent superposition.

This work was done by Rolf Wyss and Shuji Matsuura of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Electronic Components and Systems category.


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Traveling-Wave Photomixer With Angle-Tuned Phase Matching

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This article first appeared in the January, 2001 issue of Photonics Tech Briefs Magazine.

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