Light-induced metal-like surface of silicon photonic waveguides

Abstract

The surface of a material may exhibit physical phenomena that do not occur in the bulk of the material itself. For this reason, the behaviour of nanoscale devices is expected to be conditioned, or even dominated, by the nature of their surface. Here, we show that in silicon photonic nanowaveguides, massive surface carrier generation is induced by light travelling in the waveguide, because of natural surface-state absorption at the core/cladding interface. At the typical light intensity used in linear applications, this effect makes the surface of the waveguide behave as a metal-like frame. A twofold impact is observed on the waveguide performance: the surface electric conductivity dominates over that of bulk silicon and an additional optical absorption mechanism arises, that we named surface free-carrier absorption. These results, applying to generic semiconductor photonic technologies, unveil the real picture of optical nanowaveguides that needs to be considered in the design of any integrated optoelectronic device.

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“…(3). The sublinear dependence of the conductance variation versus the optical power (γ < 1) results from a power dependent recombination time of the photogenerated carriers [6], [8]. At low power, the slope changes in agreement with other observations on photoconductiity [7].…”

“…To this aim, on the transmitter side, the channel is intensity modulated with a frequency f q below the reading frequency of the CLIPP [2]. To not degrade the performance of the transmitted channel, a low modulation index µ (< 8%) shall be used [8]. This can be achieved with a Mach-Zehnder (MZ) modulator driven with an amplitude much smaller than its V π , as shown in Fig.…”

“…This change is induced by free carrier generation, which in turn is due to photon absorption occurring in the semiconductor waveguide core. The change in the waveguide resistance versus the optical intensity exhibits different slopes at different power levels [7], depending on whether the dominant effect generating free carriers is either Surface State Absorption (SSA) or Two Photon Absorption (TPA) [8]. The general relation has a nonlinear dependence on the optical power and obtaining this relation is the first step to measure the optical power in the waveguide.…”

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“…The conversion efficiency has a slightly lower value than expected at pump power larger than 200 mW. Since the effects of three‐photon absorption are negligible at this power level (see ), the effects of surface states may play a role here and need to be suppressed for higher power operations . Figure b shows the measured conversion efficiency as a function of the coupled pump power where the conversion efficiency follows a quadratic relation with the pump power.…”

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“…To this aim, on the transmitter side, the channel is intensity modulated with a frequency f q below the reading frequency of the CLIPP [17]. To not degrade the performance of the transmitted channel, a low modulation index µ (< 8%) can be used [18]. This can be achieved with a MZ modulator driven with an amplitude much smaller than its V π .…”