There are numerous applications of nonlinear optical frequency conversion in the infrared, ranging from generation of coherent radiation for spectroscopy and military applications, to wavelength conversion in communication systems. Semiconductors, such as Al xGa1-xAs and GaP have excellent properties for nonlinear frequency conversion, in particular large nonlinear coefficients and transparency throughout the mid-infrared. However, due to the absence of birefringence, quasi-phasematching (QPM) has to be used for the phasematching, requiring a modulation of the sign of the nonlinear optical coefficient along the optical path in the material. In this work, we have developed an all-epitaxial process to fabricate orientation-patterned AlxGa1-xAs and orientation-patterned GaP structures, used for both bulk-like and waveguide devices. Various nonlinear optical interactions have been demonstrated which show that orientation-patterned AlxGa1-xAs is a promising candidate for infrared applications.
Our orientation-patterned GaAs template is fabricated in three steps. First, we use the polar-on-nonpolar growth of GaAs/Ge/GaAs heterostructure to control the lattice inversion. The orientation pattern is then defined by a combination of photolithography and a series of selective chemical etching steps. Template and waveguide growth is completed using MBE regrowth. Critical regrowth issues are elimination of antiphase defects within each single domain of the template while still maintaining the induced antiphase domains at the pattern boundaries. Appropriate growth conditions were developed which met these challenges and produced vertical propagation of domain boundaries under all MBE conditions tested. Low-corrugation template has been achieved by optimizing the growth conditions of GaAs on Ge. QPM periods demonstrated are short enough to phasematch any interaction in the transparency range of AlxGa1-xAs.
Using this technique, we fabricated low-loss Al xGa1-xAs QPM waveguide devices and demonstrated second harmonic generation with a pump laser at 1.55 m. A waveguide loss, ~4.5 dB/cm at 1.55 m, was measured, which is close to that of the unpatterned waveguides. Record-high conversion efficiency, 43 %W-1, was demonstrated, which is the highest value reported to date for AlGaAs nonlinear waveguides. These achievements provide solid basis for the fabrication of highly efficient nonlinear optical devices based on the GaAs material system.
In addition to the orientation-patterned GaAs growth, we also investigated the growth of single-phase GaP on Si, aiming at transferring growth technologies of GaAs on Ge to GaP on Si. By controlling proper growth conditions, we successfully grew two distinct single-phase GaP on Si and fabricated the first orientation-patterned GaP template on Si. Further progress will lead to GaP-based nonlinear devices for high power operation with a broader wavelength region.