Navy

Suspended waveguides for frequency conversion

Nano slot device combines microelectromechanical actuation, semiconductor nanofabrication, and birefringent phase matching to achieve high conversion efficiencies over a large optical bandwidth

Photonics

The power density of three modes involved in DFG in suspended, coupled waveguides.

Integrated nonlinear devices are desired for frequency conversion, particularly to generate optical radiation at wavelengths that are not readily generated by presently available laser devices, for example, in mid-IR wavelengths.

Group III-V semiconductors that crystallize in a zinc blende lattice possess a large second-order nonlinear susceptibility, in excess of 100 pm/V. This quality and their wide use in active optoelectronic devices have made III-V materials of interest for integrated nonlinear devices for frequency conversion.

Second-order nonlinear processes require phase matching between the three waves involved. One technique for phase matching involves the optical anisotropy of nonlinear crystals, a technique usually referred to as birefringent phase matching (BPM). However, bulk zinc blende materials are optically isotropic, which makes them not useful for BPM.

Navy scientists have developed a BPM optical semiconductor device from nanomachined type III-V semiconductor materials configured to produce low loss. The appropriate type III-V materials with a large nonlinear susceptibility include AlGaAs, GaAs, GaP, AlAs, or AlGaAsP. Ideal substrates are GaAs, GaSb, or InP. This integrated non-linear device is useful for frequency conversion, particularly for generating optical pulses at wavelengths that are not readily generated by presently available laser devices

The use of a nano slot has the advantage of large index contrast and uniform material properties within the air gap, in contrast to the composite AlAs/Al2O3 layer typically obtained by oxidation. Compared to typical slot waveguides, where the slot is perpendicular to the substrate, this new Navy design utilizes a single slot parallel to the substrate. This creates suspended air-cladding waveguides with tight optical confinement.

In operation, this optical waveguide device is a frequency mixer, difference frequency generator (DFG), sum frequency generator (SFG), or second harmonic generator (SHG), and can be very useful in the mid-IR range. The device can be incorporated as part of a laser system, or external to a laser source.

The suspended nano slot and nano slab waveguide devices have several advantages over quasi-phase matching (QPM) approaches or other BPM devices with nonlinear wavelength conversion including lower loss and larger transparent range.

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