Telecommunications systems are limited in speed by the electronics used to perform switching and wavelength conversion functions. While this has been achieved in large system based on bulk crystals, this approach can be expensive and fault prone. Instead, monlithically integrating both active and passive microphotonic components into the same device would increase component density and reduce cost in the same fashion as has been done for microelectronics. The strong third-order nonlinear optical effects in compound semiconductors provide a solution by enabling practical all-optical switching and frequency conversion devices. We have been investigating advanced nanostructured semiconductor-based waveguides, such as GaAs/AlGaAs superlattices. The ability to tune the bandgap and nonlinear properties of these engineered materials by quantum well intermixing (QWI) postgrowth techniques make them an attractive platform for photonic integrated circuits. Using ultrafast laser systems at the University of Toronto, we have shown that superlattices have unique nonlinear optical properties which may be useful for a number of applications.

Values for the two-photon absorption coefficient, α2, and nonlinear refractive index coefficients, n2, are measured for a waveguide core made of 14:14 monolayer GaAs/AlAs superlattice at photon energies below the half-bandgap. Two-photon absorption coefficients show significant anisotropy with the TE mode having α2 values up to four-times greater than the TM mode. For nonlinear refraction, we determined values of the nonlinear index for the TE mode that are as much as two-times larger than the TM mode. Measurements of cross-phase modulation between the TE and TM modes gave ratios of cross-phase modulation to self-phase modulation that were highly anisotropic. The optical Kerr effect was measured by observing self-phase-modulation in GaAs/AlGaAs superlattice-core waveguides modified by ion-implantation quantum well intermixing. The Kerr effect was suppressed by up to 71% in the TE polarization after intermixing. A reduced polarization dependence of the self-phase-modulation was observed after intermixing.

Optical sensors for gas and chemical detection systems are limited to laser sources which emit photons with energies at the bandgap of the active material. Second-order nonlinear effects provides a solution to this problem. While systems based on bulk crystals with weak second-order properties are available, their large size and cost limit their use to research laboratories. Compound semiconductors have strong second-order properties, but lack natural birefringence such that efficient phase matching can be achieved. One solution to this problem is to utilize domain-disordered quasi-phase matching (QPM). In this technique, the second-order nonlinearity is periodically supressed in a GaAs/AlGaAs superlattice-core waveguide by quantum-well intermixing (QWI).

We have already demonstrated efficient second-harmonic generation (SHG) using QPM waveguides with pulsed and continuous-wave lasers in both Type-I and Type-II polarization configurations. Using data from these experiments, difference frequency generation (DFG) has also been demonstrated. Using an all continuous-wave setup, conversion from optical C-band wavelengths to L- and U-Band wavelengths was achieved. This represented a conversion bandwidth of at least 100 nm.

Parametric processes based on second-order optical nonlinearities in III-V semiconductors are receiving attentions for the development of novel photonic devices such as integrated self-pump optical parametric oscillators (OPOs). The efficiency of parametric processes chiefly rely on the phase-matching technique to be employed. Due to the lack of natural birefringence in compound semiconductors, phase-matching can be challenging in these materials. Our group has proposed and successfully illustrated the exact phase-matching of second-harmonic generation (SHG) using Bragg reflection waveguides (BRW) in AlxGa1-xAs material system. BRW phase-matching is an exact phase-matching technique which employs the modal dispersion properties of the interacting harmonics propagating as either a Bragg mode or as a total internal reflection mode. The technique benefits from large nonlinear conversion efficiencies thanks to the phase-matching of the fundamentals of both Bragg and TIR modes. In our group, we are interested to improve the conversion efficiency of the nonlinear processes by investigating advanced transverse Bragg reflectors as well as extending the technique to other second order nonlinear processes including sum- and difference-frequency generation.

1. Second-harmonic generation Second-harmonic (SH) generation in matching layer enhanced AlGaAs Bragg Reflection waveguides was demonstrated using a 1.8-ps pulsed pump around 1550 nm with an average power of 3.3 mW, peak SH powers of 28 and 60 &mu W were measured in type-I and type-II interactions, respectively, for a sample with a length of 2.2 mm. The associated normalized conversion efficiency were estimated to be 5.3x10^{3} % and 1.14x10^{4} %W^{-1}cm^{-2} with SH process bandwidths of 1.7- and 1.8-nm full-width at half maximum.

2. Frequency conversion Efficient type-II sum-frequency (SF) generation in AlGaAs Bragg reflection waveguides was demonstrated. Continuous-wave signal and pump in 1550 nm wavelength window were used for upconversion of photons to the 775 nm region. For a pump and signal with powers of 0.69 and 0.35 mW, SF power of 35 nW was measured. The normalized conversion efficiency was estimated to be 298 %W^{-1}cm^{-2} in a device with a length of 2.2 mm. The bandwidth of the process was found to exceed 60 nm. Type-II difference-frequency (DF) generation around 1550 nm was demonstrated in AlGaAs Bragg reflection waveguides using a pump around 778 nm and a signal within the C-band range. The DF power level was -62.5 dB lower than that of the signal for an external pump power 62.9 mW in a sample with a length of 1.5 mm length. It was ob served that a detuning of the pump wavelength by 0.4 nm resulted in a span of 40 nm between signal and DF wavelengths. Further detuning of the pump from degeneracy was expected to offer broader separation between signal and DF.

Dispersion engineering of semiconductor waveguides is indispensable for many useful passive/active photonic devices including semiconductor amplifiers, modulators as well as nonlinear devices based on second and third order optical nonlinearities. In our research group, we are particularly interested in investigating first and second order dispersion properties of phase-matched AlxGa1-xAs Bragg reflection waveguides (BRWs) in the context of harmonics generation. We have theoretically shown that quarter-wave BRWs benefit from versatile dispersion properties where, for example, Bragg mode with zero group velocity dispersion (GVD) is attainable. In a second harmonic generation with ultra-short pulses, a design with zero-GVD allows the up-converted signal to propagate over long distances before severe pulse distortion. Also the dispersion sign change around the zero-GVD point enables the generation of bright and dark temporal solitons in a self-focusing material. For quarter-wave BRWs, analytical expressions for the first and second order dispersion terms are derived using a perturbation on the modal dispersion equation. These analytical formulas significantly simplify the design and optimization of the waveguides. Investigating dispersion properties of BRWs with non-quarter wave stacks is also part of the group interest. One such design is the Bragg waveguide with matching layers where extra designable layers, referred to as the matching layer, are place between the core and the periodic claddings. We have shown that the matching layer technique offers additional degree of freedom in tailoring the dispersion properties while maintaining the phase-matching condition. It also enhances the nonlinear conversion efficiency by a few orders of magnitude in comparison to an optimized quarter-wave structure.

Dispersion control is also significant in quantum optics in generating photon-pairs with frequency correlation properties using spontaneous parametric down conversion. In an integrated source of photon-pairs where all interacting harmonics are far from material resonances, waveguide dispersion can be large enough to set the mode dispersion to the desired value. One such waveguide parameter which allows significant variation of dispersion is the ridge size which can simply implemented using lithographical technique. Sources of biphotons with ultra-short and ultra-broad temporal correlations can be then be realized in an integrated platform to favour some emerging disciplines such as quantum optical coherence tomography and generation of photonpairs with high dimensional spectral entanglement.

Bragg reflection waveguides are used to achieve phase-matching for spontaneous parametric down-conversion in monolithic AlGaAs waveguides. Through the dispersion control afforded by this technique, bandwidth tunability between 1 nm and 450 nm could be achieved using the same vertical wafer structure. The tuning was achieved by patterning waveguides with different ridge widths and also by utilizing both type-I and type-II phase-matching conditions. This technology offers a promising route for realization of electrically pumped, monolithic photon-pair sources on a chip with versatile characteristics.