Ultrafast all-optical switching using intersubband transitions in InGaAs/AlAsSb quantum well structures

Abstract

Ultrafast all-optical switching devices will play a crucial role in future optical-communication systems that operate at data transfer rates beyond 1 Tb/s, especially for optical time-division multiplexing (OTDM) and optical code-division multiplexing (OCDM) systems, in which very short light pulses play key roles as signals or codes. From this perspective, the intersubband transitions (ISB-Ts) in semiconductor quantum wells (QWs), which occur within a conduction band, are attractive due to their ultrafast relaxation, large transition dipole moment, and widely tunable transition wavelength. All-optical switching based on ISB-T utilizes optical nonlinearity via the absorption saturation induced by ISB excitation. Ultrashort light pulses having high peak intensities can give rise to considerable temporal changes, which last only from several hundreds femtoseconds to several picoseconds, in absorption coefficients or refractive indices for both interband and intersubband resonant lights. Although there have been proposals to utilize these ultrafast optical nonlinearities for all-optical modulators and switches [1, 2], difficulties in obtaining near-infrared ISB-T have prevented us from realizing such devices. In recent years, progress in the development of semiconductor heterostructures with large conduction-band discontinuities has enabled near-infrared ISB-Ts in the optical-communication wavelength band in systems composed of various materials [3, 4, 5]. InGaAs/AlAsSb quantum wells in particular offer advantages, such as large band offsets of 1.6-1.7 eV, which enable excellent confinement of electrons in high energy subbands, and the capacity to achieve strong absorption through a thick multiple layer that is also compatible with conventional device processing, using the advantage of lattice matching to InP substrates.