We investigate second-harmonic generation in III-V semiconductor wire waveguides aligned with a crystallographic axis. In this direction, because of the single nonzero tensor element coming from the zinc-blende symmetry of III-V semiconductors, only frequency conversion by mixing with the longitudinal components of the optical fields is allowed. We experimentally study the impact of the propagation direction on the conversion efficiency and confirm the role played by the longitudinal components through the excitation of an antisymmetric second-harmonic higher-order mode.
The large index contrast and the subwalength tranverse dimensions of nanowires induce strong longitudinal electric field components. We show that these components play an important role for second harmonic generation in III-V wire waveguides. To illustrate this behavior, an efficiency map of nonlinear conversion is determined based on full-vectorial calculations. It reveals that many different waveguide dimensions and directions are suitable for efficient conversion of a fundamental quasi-TE pump mode around the 1550 nm telecommunication wavelength to a higher-order second harmonic mode.
Non-degenerate two-photon absorption (TPA) is investigated in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be neglected. It is shown that a signal wave can strongly be absorbed by cross-TPA by interaction with a low energy pump pulse (1.2 pJ), close to the half-bandgap and experiencing low nonlinear absorption. The experiments are very well reproduced by numerical simulations of two-coupled generalized nonlinear Schrödinger equations (GNLSE), validating the usual approximation made to compute the cross nonlinear coefficients in indirect-gap semiconductors. We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon wire waveguides and contrary to optical fibers, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on cross-TPA, as well as on the study of supercontinuum and frequency comb generation in integrated semiconductoron-insulator platforms.
We theoretically and experimentally investigate type II second harmonic generation in III-V-on-insulator wire waveguides. We show that the propagation direction plays a crucial role and that longitudinal field components can be leveraged for robust and efficient conversion. We predict that the maximum theoretical conversion is larger than that of type I second harmonic generation for similar waveguide dimensions and reach an experimental conversion efficiency of 12%/W, limited by the propagation loss.
Novel integrated photonics platforms are having a large impact on nonlinear optics. The inherent strong confinement and highly nonlinear materials allow for ultra-efficient frequency conversion. Recent reports on second harmonic (SH) generation for example posted record conversion efficiencies in millimetre long waveguides [1,2]. These promising results predict an exciting future for frequency conversion in subwavelengths structures. Yet, the vectorial nature of waves propagating in high index contrast nanowaveguides is often overlooked. Here we demonstrate second harmonic generation enabled by the longitudinal component of both the pump and the second harmonic wave. We use a 680 nm wide, 320 nm thick Indium Gallium Phosphide nanowaveguide [3]. As in other III-V materials, only the "#$ (&) component is nonzero. Previous demonstrations of second harmonic generation used waveguides that are rotated 45° in order to split the main transverse component along two axes [1,4]. Conversely, we use a 1.5 mm long waveguide whose propagation direction is aligned along the z crystallographic axis.is the spatial distribution of the electric field in the transverse plane. Full-vectorial simulations predict phase matching and a nonzero overlap between a fundamental quasi-transverse magnetic (TM) pump mode and a higher order SH TM mode around 1575 nm. Importantly, such conversion would not be permitted without strong longitudinal field components. The effective index of both the pump and SH as well as the different field components are shown in Fig. 1. As can be seen, most field components have nonnegligible amplitudes and hence contribute to the effective nonlinearity. We launch a 3 mW telecom band pump in our waveguide through a lensed fiber and collect the second harmonic by use of a high NA (0.9) objective. Our results are shown in Figure 1. We find a maximum conversion of 0.2 %/W/cm 2 , around 2 orders of magnitude less that the theoretical prediction, indicating that the second harmonic mode likely suffers from strong propagation losses. Our experimental image of the SH mode confirms the excitation of an antisymmetric TM higher order mode. In conclusion we demonstrated second harmonic generation through mixing of transverse and longitudinal field components. Not only does it demonstrate the vector nature of the propagating waves, it also allows to excite higher order modes with different symmetries. Furthermore, full-vectorial simulations show that similar wave-mixing is the most efficient conversion scheme for waveguides fabricated in thick (>200nm) InGaP layers. Fig. 1 (a) Simulation of the effective indices of a pump mode (black) and a SH mode (green). The spatial distribution of the different electric field components is shown as inset. (b-c) Measured and computed spatial distribution of the intensity of the SH at the output of the waveguide. (d-e) Measured and computed spatial distribution of the intensity of a 775 nm TM fundamental wave for comparison. The field of view for theoretical modes is 1.5 µm x 1 µm. (f) Secon...
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