Limiting nature of continuum generation in silicon

Abstract

Spectral broadening in silicon is studied numerically as well as experimentally. Temporal dynamics of the free carriers generated during the propagation of optical pulses, through the process of two-photon absorption (TPA), affect the amplitude and phase of the optical pulses, thereby determining the nature and extent of the generated spectral continuum. Experimental results are obtained by propagating pico-second optical pulses in a silicon waveguide for intensities that span two orders of magnitude (1-150 GW/cm 2 ). These results validate the conclusions drawn from numerical simulations that the continuum generation has a self-limiting nature in silicon.

Spectral broadening enhancement in silicon waveguides through pulse shaping

“…Specifically we mention the absorption and dispersion of free carriers produced by two-photon absorption (TPA), which are not present in conventional silica-based devices. The net effect results in a depletion of the achievable spectral broadening for a Gaussian input pulse in comparison with the case when only SPM is acting [8][9][10].Additionally, the phenomenon under study is extremely sensitive to the input pulse characteristics due to the inherent nonlinear nature of the spectral broadening. In fact, up to some extent, pulse shaping techniques have demonstrated to be effective in controlling the nonlinear broadening in photonic crystal fibers [11,12] and other nonlinear materials [13][14][15], using both single-pass [15] and self-learning adaptive configurations [11][12][13][14].…”

“…Specifically we mention the absorption and dispersion of free carriers produced by two-photon absorption (TPA), which are not present in conventional silica-based devices. The net effect results in a depletion of the achievable spectral broadening for a Gaussian input pulse in comparison with the case when only SPM is acting [8][9][10].…”

“…Here the FCA changes induced by pulse shaping do not provide an enhancement of higher frequencies because of the reduction on blueshifted components produced by SPM due to the smooth tail of the asymmetric pulse. At higher (but realistic; see [10]) powers, a notable enhancement appears at both sides of the spectrum since FCA plays a less significant role thanks to the pulse shape. The expected output spectrum is depicted in Fig.…”

“…For low repetition-rate pulses and if the input pulse width T 0 ≪ τ c , the subterm proportional to −N c ∕τ c can be neglected [9], and hence the FCD term can be considered always positive. Accordingly, FCD only produces blueshifted components; by contrast SPM can give rise to both blueshifted and redshifted ones [9,10]. Finally, similarly to [6], we define the threshold power P th 8γ 0 hν 0 A 2 eff ∕T 0 σμβ TPA , beyond which the FCD contribution exceeds that of SPM.…”

Breakthroughs in Nonlinear Silicon Photonics 2011

“…However, the effects of TPA limit the continuum bandwidth that can be achieved when pumped near the silicon bandgap [35], [36]. In 2011, Kuyken et al [37] demonstrated SCG in silicon spanning three-quarters of an octave from 1535 to 2525 nm.…”