Gallium nitride carries infrared light

The semiconductor materials gallium nitride (GaN) and aluminum gallium nitride (AlGaN) have found fame by becoming the basis of blue- and ultraviolet-emitting lasers and light-emitting diodes; the several-stories-tall full-color display at the Nasdaq Marketsite Tower in New York City, pulsating image-laden backdrops for rock concerts, and DVDs with a fivefold increase in storage capacity are a few of the results. Gallium nitride (GaN)-based semiconductors are also used in ultraviolet photodetectors. But researchers at the University of Kansas (Lawrence, Kansas) and Kansas State University (Manhattan, Kansas) have now found a use for GaN-based materials in a spectral region far from the blue—in fact, in the infrared.

The team is creating GaN-based waveguide devices for long-wavelength (1550-nm) optical communications. Because properties of GaN and AlGaN in the infrared were largely unknown, the researchers had to start from scratch, measuring the refractive index of Al_xGa_1–xN for different molar fractions x of Al ranging from 0.1 to 0.7. The refractive index n showed a monotonic decrease as the proportion of Al was increased; at 1550 nm, n = 0.431x² – 0.735x + 2.335.

Several waveguide configurations were modeled, including straight waveguides and 2 × 2 couplers. Samples of waveguides were fabricated, with a 4-µm-thick Al_xGa_1–xN film (x equal to 0.03) grown on a sapphire substrate and a 3-µm-thick layer of GaN grown on top. Waveguides were then created by lithographic patterning and dry etching. A 2 × 2 waveguide coupler was fabricated (see figure).

"The biggest advantage of using GaN-based material operating in the long-wavelength region can be simply stated as follows: it is transparent in the long-wavelength region—but since it is a semiconductor, its refractive index can be switched by carrier injection," says Ron Hui, one of the University of Kansas researchers. "Therefore, it is possible to build picosecond-level all-optical switches based on optical phasors using this material. In comparison, conventional silicon dioxide-based waveguide technology does not provide tunability."

To test the waveguides, the researchers coupled light in and out with tapered single-mode fibers that had a 6-µm working distance and a 2.5-µm spot size. One fiber carried light from a tunable laser diode to the waveguide; the other carried output light to an optical power meter. Both fibers were mounted to precision five-axis positioning stages. A measurement of a 2 × 2 coupler showed equal power splitting in the two output ports. An attenuation measurement of a 1.395-mm-long waveguide gave a waveguide loss coefficient of 34.4 dB/cm. Scattering at the waveguide boundaries could be reduced by better etching.

Mach-Zehnder modulators and arrayed-waveguide gratings are candidate devices for the GaN-based waveguides; such waveguides would potentially have lower loss than waveguides made from indium phosphide. Because the carrier-induced change in refractive index in GaN-based waveguides is independent of polarization, phase-shifting devices could be made that are polarization-independent (unlike lithium niobate optical devices that rely on a polarization-dependent electro-optic effect). The subnanosecond carrier lifetime in Al_xGa_1–xN may make optical packet switching possible, say the researchers.

REFERENCE

1. R. Hui et al., Appl. Phys. Lett. (March 3, 2003).