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).

A gallium nitride-based 2 × 2 optical waveguide coupler (shown in top and cross-section views) has high transparency at 1550 nm. Because their refractive index can be modulated via carrier injection, such waveguides may be incorporated into optical switches.

"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).