Frequency conversion is used to generate output radiation from input radiation of different frequencies to cover gaps in spectral regions of interest. For example, Terahertz (THz) radiation is of great interest for communication and imaging applications and shows promise for ultra-wideband wireless communications, atmospheric studies, homeland security, medical imaging, pharmaceuticals (identifying molecular polymorphs), and defense imaging applications, among others. Optical frequency conversion involves fabricating a crystal in which a laser of one frequency is directed through the crystal and a portion of the light is converted to another frequency. The conversion process can be understood by considering it as occurring in the visible light range where a slice of a beam of one color, for example, blue, is converted into a beam of another color, for example, red.
There are a number of nonlinear optical materials that have been successfully used for frequency conversion. Very few however have been successful in the mid-infrared (IR) optical range. This is problematic for atmospheric monitoring as most absorption intensities of common gases are higher in the mid-IR range. Gallium arsenide (GaAs) is a more promising material because it is transparent up to 17 microns, has a larger effective non-linear optical coefficient, and can be easily processed using existing semiconductor fabrication methods. GaAs is not ferroelectric and an inversion of the crystal cannot be achieved by poling.
The unique discovery by Air Force researchers is that a multi-layer GaAs crystal, or crystal wafer, comprising many thin GaAs layers separated by thinner aluminum gallium arsenide (AlGaAs) layers causes a pump laser to undergo many more internal reflections than a similar structure without the AlGaAs layers, thus resulting in increased efficiency of the frequency conversion process.
Advantages over other orientation patterned (OP) GaAs:
- Simplicity of the structure. Does not require preparation of a template by lithography, etching, or MBE (molecular beam epitaxy) growth that OPGaAs requires.
- Uniform planar substrate growth overcomes the difficulty of maintaining patterned GaAs domains throughout epitaxial growth.
- Trouble growing on thin column. Actually easier to grow thinner layers of this material.
- COTS deposition equipment with light-based real-time thickness determination for precise growth of GaAs-AlGaAs pairs.
- In comparison to LiNb03, this new material can be used for conversion of IR and THz sources
- Lower scatter and signal loss in comparison to other quasi phase-matched approaches
- Greater efficiency of conversion over TIR-QPM approach due to dramatically increased number of reflections within the material
- Large tunability range in a compact platform
- Material is easy to produce using common deposition equipment
- Commercially viable technology for mass production of OPGaAs templates
- US patent 8,619,356 available for license
- Prototype material available
- Potential for collaboration with Air Force researchers