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.
Highlights
Developing the ability to control the optical properties of matter dynamically has been a topic of intense research for many decades
We propose two alternative approaches for femtosecond-scale free carrier (FC)-based switching Unlike standard optical switching, which predominantly employs a single intense pump pulse that is spatially uniform, we consider a configuration based on a highly spatially non-uniform pump pulse
This second pump erases the Bragg gratings (BGs) generated by the first pump by making the FC distribution uniform in z; we refer to this approach as a “write-erase” technique
Summary
Developing the ability to control the optical properties of matter dynamically has been a topic of intense research for many decades. In one realization (see Fig. 1(a)), we rely on an intrinsic property of the FCs, namely, diffusion, to induce self-erasure of the BG; in the second realization (see Fig. 1(b)), we rely on a slightly more complex switching scheme consisting of two spatially staggered pump pulses By using these approaches, we demonstrate theoretically switching times that are as short as a few tens of femtoseconds, i.e., up to 2 orders of magnitude shorter than the fastest standard switching times in semiconductors.
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