Theoretically predicted "dipole lasing" as spontaneous excitation of coherent oscillations of dipole momentum of metal nano-particles placed inside or near the surface of the medium with optical amplification. It has close analogy with ordinary lasing, but the polarization of nano-particles stands for the optical cavity mode. Oscillations of polarization cause coherent radiation from nano-particles acting as "nano-antennas". Optical cavity is not necessary, so that the minimum size of the dipole laser can be on the nano-scale. Dipole laser frequency corresponds to the localized plasmon resonance of nano-particles. The manifestation of the dipole lasing is in the divergence of the nano-particle polarisability, that is a second-order phase transition. Threshold conditions, enhancement of the spontaneous emission and optical bistability is dipole lasers are found.
Ultrafast intraband carrier dynamics strongly influence many important characteristics in bulk and quantum well lasers and amplifiers through Spectral-Hole Burning (SHB) leading to nonlinear gain effects. In Quantum Dot (QD) devices, where the inter-level relaxation times can be even longer than the intraband relaxation times in conventional devices, SHB effects should also be substantial. A number of promising applications of QD amplifiers in high-speed optical processing (Cross-Gain Modulation, for instance) are based on features of the carrier dynamics in QD structures. In the present paper, based on a density matrix approach, we develop a theory of SHB-based nonlinear gain in QD lasers and amplifiers, which can affect such important characteristics as the modulation bandwidth in QD lasers and the saturation power and pulse energy in QD amplifiers. We give an expression for the nonlinear gain in QD devices, and show how it depends, particularly, on the capture/escape and relaxation/excitation rates.
We analyse the sensitivity of quantum dot semiconductor lasers to optical feedback. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and strongly damped relaxation oscillations.
We have developed a theory of the nonlinear refractive index in Quantum Dot (QD) Semiconductor Optical Amplifiers (SOAs) due to Spectral Hole Burning (SHB). Estimates show that this SHB nonlinear refractive index can be of order of 4x10-16 m2/W that is by four orders higher than the nonlinear refractive index in silica, and offers the possibility of an efficient ultrafast Cross-Phase-Modulation (XPM) in QD SOAs. The opportunity of XPM without patterning effects via this refractive index nonlinearity is discussed. The Pattern-Effect-Free (PEF) XPM is possible in QD SOAs at high pumps, when maximal (constant) gain is achieved in SOAs, and the linear and nonlinear refractive indices also become independent of the total carrier density in the QD structure. In whole, use of the ultrafast refractive index nonlinearity in the regime of maximum gain in QD SOAs can lead to the development of a new generation of nonlinear interferometers for ultrafast optical switching.
We have developed a theory of high-speed operation of quantum dot (QD) semiconductor optical amplifiers (SOAs), and showed that pattern-effect-free amplification of pulse trains, cross gain modulation (XGM) and cross phase modulation (XPM) can take place in QD SOA in the regime with maximum gain. Formulas, which relate the maximum bit-rate for the pattern-effect-free operation and the average SOA output power to the SOA pump current density, were derived. XGM without pattern effect can be realized in the regime with maximum gain due to spectral hole burning effects. Possibility of ultrafast frequency conversion and demultiplexing of data pulse streams through this nonlinearity is illustrated. Expression for the nonlinear refractive index ηnl due to spectral hole burning in QD structure was obtained. The value of ηnl in QD SOAs can be by 4-5 orders larger than ηnl in silica; and efficient ultrafast XPM without pattern effects can be carried out in QD SOA through this nonlinearity. In whole, usage of the regime with maximum gain in QD SOAs can lead to development of new generation of high-speed devices for ultrafast optical processing and communications.
We analyse the sensitivity of quantum dot semiconductor lasers to optical. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and of strongly damped relaxation oscillations.
The effects of carrier cooling on the saturation characteristics of quantum well absorbers are theoretically investigated. A comprehensive many-body model is employed, and both excitonic effects and bandgap renormalization are shown to play a significant role. The detailed interplay between band-filling and many-body effects at low carrier temperatures is shown to lead to saturation characteristics that are highly spectrally dependent, and that cannot in general be identified by a linear saturation characteristic. In addition, a spectral regime of enhanced fast saturation is identified at or slightly below the exciton peak energy, which may provide a mechanism for previously observed mode-locking behavior.
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