Femtosecond-scale switching based on excited free-carriers

TL;DR

This paper introduces femtosecond-scale optical switching methods using free carrier excitation in semiconductors, enabling ultrafast control of light with high efficiency and potential applications in ultrashort pulse manipulation.

Contribution

It presents novel switching schemes based on spatially patterned free carriers, utilizing diffusion and controlled pump pulses for femtosecond optical switching.

Findings

Achieved up to 50% switching efficiency with 100 fs pump pulses

Demonstrated potential for ultrafast pulse shaping and spectroscopy

Proposed methods suitable for applications requiring femtosecond features

Abstract

We describe novel optical switching schemes operating at femtosecond time scales by employing free carrier (FC) excitation. Such unprecedented switching times are made possible by spatially patterning the density of the excited FCs. In the first realization, we rely on diffusion, i.e., on the nonlocality of the FC nonlinear response of the semiconductor, to erase the initial FC pattern and, thereby, eliminate the reflectivity of the system. In the second realization, we erase the FC pattern by launching a second pump pulse at a controlled delay. We discuss the advantages and limitations of the proposed approaches and demonstrate their potential applicability for switching ultrashort pulses propagating in silicon waveguides. We show switching efficiencies of up to $50\%$ for $100$ fs pump pulses, which is an unusually high level of efficiency for such a short interaction time, a result of the use of the strong FC nonlinearity. Due to limitations of saturation and pattern effects, these schemes can be employed for switching applications that require femtosecond features but standard repetition rates. Such applications include switching of ultrashort pulses, femtosecond spectroscopy (gating), time-reversal of short pulses for aberration compensation, and many more. This approach is also the starting point for ultrafast amplitude modulations and a new route toward the spatio-temporal shaping of short optical pulses.