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 AlxGa1–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 AlxGa1–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.
Click here to enlarge image

"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 AlxGa1–xN may make optical packet switching possible, say the researchers.

REFERENCE

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

Most Popular Articles

50 YEARS OF GAS LASERS


Durable survivors evolve new forms

Webcasts

Laser Measurements Critical to Successful Additive Manufacturing Processes

Maximizing the stability of the variables going into any manufacturing process is what ensures ts consistency and high quality. Specifically, when a laser is...

Ray Optics Simulations with COMSOL Multiphysics

The Ray Optics Module can be used to simulate electromagnetic wave propagation when the wavelength is much smaller than the smallest geometric entity in the ...

Multichannel Spectroscopy: Technology and Applications

This webcast, sponsored by Hamamatsu, highlights some of the photonic technology used in spectroscopy, and the resulting applications.

Handheld Spectrometers

Spectroscopy is a powerful and versatile tool that traditionally often required a large and bulky instrument. The combination of compact optics and modern pa...
White Papers

Wavelength stabilized multi-kW diode laser systems

Wavelength stabilization of high-power diode laser systems is an important means to increase the ...

Narrow-line fiber-coupled modules for DPAL pumping

A new series of fiber coupled diode laser modules optimized for DPAL pumping is presented, featur...

Accurate LED Source Modeling Using TracePro

Modern optical modeling programs allow product design engineers to create, analyze, and optimize ...
Technical Digests

ADHESIVES, SEALANTS, AND COATINGS: Solutions for optical technologies

A vast array of optical systems of various types and degrees of complexity require the use of adh...

WAVELENGTH-SWEPT LASERS: Dispersion-tuned fiber laser sweeps over a 140 nm range for OCT

By eliminating the use of mechanical tunable filters and instead tuning by intensity-modulation i...

Keeping pace with developments in photonic materials research

For demanding or custom spectroscopy solutions, care must be taken in selecting and integrating a...

HIGH-POWER FIBER LASERS: Working in the kilowatt regime

High-power materials-processing fiber lasers are available in an increasing variety of forms, as ...

Click here to have your products listed in the Laser Focus World Buyers Guide.

RELATED PRODUCTS

Social Activity
  •  
  •  
  •  
  •  
  •  
Copyright © 2007-2014. PennWell Corporation, Tulsa, OK. All Rights Reserved.PRIVACY POLICY | TERMS AND CONDITIONS