Slow optical solitons via intersubband transitions in a semiconductor quantum well

We show the formation of bright and dark slow optical solitons based on intersubband transitions in a semiconductor quantum well (SQW). Using the coupled Schrödinger-Maxwell approach, we provide both analytical and numerical results. Such a nonlinear optical process may be used for the control technology of optical delay lines and optical buffers in the SQW solid-state system. With appropriate parameters, we also show the generation of a large crossphase modulation (XPM). Since the intersubband energy level can be easily tuned by an external bias voltage, the present investigation may open possibilities for electrically controlled phase modulators in solid systems. Copyright c © EPLA, 2008 Solitons describe a class of fascinating shapingpreserving wave propagation phenomena in nonlinear media. Over the past few years, the subject of extensive theoretical and experimental investigations on solitons in optical fibers [1,2], cold-atom media [3–7], Bose-Einstein condensates (BEC) [8,9], and other nonlinear media [10], have received a great deal of attention mainly due to the fact that these special types of wave packets are formed as a result of the interplay between nonlinearity and dispersion properties of a medium under excitations, and can lead to undistorted propagation over extended distance. In the optical domain, most optical solitons are produced with intense electromagnetic fields, and far–offresonance excitation schemes are generally employed in order to avoid unmanageable optical-field attenuation and distortion [1]. As a result, optical solitons produced in this way generally travel with a propagation speed very close to the speed of light in vacuum. As well known, the wave propagation velocity in a highly resonant medium can be significantly reduced via the electromagnetically induced transparency (EIT) technique [11] or (a)E-mail: [email protected] Raman-assisted interference effects. Recently, ultraslow optical solitons including two-color solitons with very low group velocities based on the EIT technique or on Raman-assisted interference effects, have been studied in an atomic medium [3–7]. There is a great interest in extending these studies to semiconductors, not only for the understanding of the nature of quantum coherence in semiconductors, but also for the possible implementation of optical devices such as XPM phase shifters [12], switches [13], etc. It is well known, in the conduction band of a semiconductor quantum structure, that the confined electron gas exhibits atomiclike properties. For example, it has been shown that they can lead to gain without inversion [14–16], coherently controlled photocurrent generation [17], electron intersubband transmissions [18], and EIT [19,20], slow light [21], interferences [22], optical bistability [23], etc. Devices based on intersubband transitions in SQW structures have many inherent advantages such as large electric dipole moments due to the small effective electron mass, high nonlinear optical coefficients, and a great flexibility in device design by choosing the materials and structure dimensions. Furthermore, the transition energies, dipoles