Novel quasi-phase-matched semiconductor nonlinear crystals for generation of the IR radiation
The development of broadly tunable optical sources covering the mid-IR (3 - 16 µm and beyond) spectral region is an important research area for a wide range of applications relating to gas sensing, atmospheric transmission, biomedicine and potentially for the detection of concealed explosive materials. For maximum applicability, the laser sources that generate these wavelengths should be compact (potentially portable) and low-cost. The recent progress in the development of electric-field poling (or periodic-poling) techniques for patterning the domain structure of ferroelectric materials has led to the implementation of different quasi-phase-matched (QPM) structures for efficient nonlinear optical interactions. The novel technology for QPM of semiconductors, so-called orientation-patterning (OP) or sublattice reversal epitaxy, allows the fabrication of high quality domain inverted semiconductor structures (for example, periodically-poled GaAs-based crystals), which demonstrate significantly better conversion efficiency.
GaAs material, which is more attractive in comparison with AlGaAs due to large nonlinear coefficients, is commonly used as bulk OP-GaAs devices for frequency conversion. However, the fabrication of bulk devices requires long growth time (more than 24 hours) and is very expensive. The fabrication of OP-GaAs in the waveguide design offers strong optical wave confinement and increases the optical power density inside the crystal, and this correspondingly reduces the pump power threshold for non-linear phenomena and provides better compatibility for diode lasers with such a crystal. Therefore, the waveguide design of orientation-patterned semiconductor structure is better suited to, and can be successfully realised for up- and down-frequency conversion with “low-power” laser sources.
Our group has designed low-loss orientation-patterned GaAs waveguides and demonstrated the possibility of obtaining second harmonic generation in these OP-GaAs waveguides. Frequency doubled light at 1621 nm in periodically poled GaAs waveguide was demonstrated. For this work, an optical parametric oscillator system used as the pump source and based on the periodically poled 5 mol% MgO-doped Congruent Lithium Niobate crystal, generating light in the wavelength range between 1430 nm and 4157 nm, was developed.
Extremely promising results for fabrication of high-performance frequency conversion devices shown by orientation-patterned GaAs waveguides and their further optimisation can lead to realisation of significantly higher power nonlinear devices and extending the generated wavelength up to 16 μm by using optical parametric oscillation technology
This work was funded by EPSRC (2006-2009).