We present the first experimental study of the optical properties of HELP InGaAsP (InGaAsP grown by He-plasma-assisted molecular beam epitaxy) relevant to all-optical switching, and the first demonstration of picosecond switching using this material.

We observed an optical response time of 15 ps, a nonlinear index change as large as 0.077, a sharp absorption band edge, and a small absorption tail in HELP InGaAsP. The unique coexistence of ultrafast response, large interband nonlinearity, and small band-tail absorption, never before reported, makes HELP InGaAsP particularly suitable for ultrafast all-optical switching. Additionally, faster response (subpicosecond) was achieved by doping the material with beryllium, and moderate doping (up to ~1018 cm-3) did not significantly alter the absorption edge.

We systematically studied the response time variations with doping concentration, annealing temperature, carrier density, and wavelength. We conclude that, (a) Be doping reduces the response time by compensating for donor-like mid-gap states, thus increasing the electron trap concentration;​ (b) annealing removes defects responsible for fast carrier trapping;​ (c) the response time increases with carrier density due to limited trap states;​ (d) the response time varies with wavelength due to difference in electron and hole trapping cross-sections, which were determined based on experimental results and a phenomenological two-trap-level rate equation model.

We investigated two types of HELP-InGaAsP-based all-optical switching devices, the nonlinear directional coupler (NLDC) and the asymmetric Fabry-Pérot (AFP) switch.

Based on numerical modelling and waveguide loss measurements, we conclude that, while HELP-InGaAsP-based passive NLDCs are in principle viable, practical devices will tend to require high switching energy, and will likely experience low contrast and high insertion loss. We demonstrated that AFP devices will outperform NLDCs in contrast ratio, throughput, bandwidth, switching time, and pol

rization-sensitivity. We designed and fabricated three prototype AFP devices, and achieved polarization-independent switching with a 5 ps switching window, 20 dB contrast ratio, >​=10% throughput, 25-40 nm bandwidth (at >​=10 dB contrast ratio), and 0.5-1.4 pJ/μm² switching energy density.

HELP InGaAsP is a promising material for ultrafast switching and other applications requiring ultrafast nonlinear operations. The HELP-InGaAsP-based AFP device we demonstrated is well suited for demultiplexing high-bit-rate data in practical optical communications systems.