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Bandaoti guangdian

Analysis of full quantum mechanism on possibility for realization of optical gain without population inversion in semiconductor

Guo Changzhi, Chen Shuilian

Published Jan 1, 1997• 0 citations
OpenAlex

Abstract

Based on full quantum mechanical theory,physical mechanism is systematically analyzed for realization of optical gain without population inversion in type-Ⅱ semiconductor quantum well,as well as spectrum of coherent Fano states composed of Γ -valley in the well layer conduction band and X -valley in barrier layer conduction band.The effect of this physical mechanism on Fano quantum interference of optical absorption probabitity is presented,followed by description of the impact of spontaneous emisson resulted from Fano spectrum structure and quantum fluctuation in vacuum field on excited optical gain.The mechanism affords theoretical foundation for the development of semiconductor quantum well laser without population inversion.

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International Conference on Laser and Fiber-Optical Networks Modeling•Dec 22, 2005•1 citation
Based on full quantum mechanical theory,physical mechanism is systematically analyzed for realization of optical gain without population inversion in type-Ⅱ semiconductor quantum well,as well as spectrum of coherent Fano states composed of Γ -valley in the well layer conduction band and X -valley in barrier layer conduction band.The effect of this physical mechanism on Fano quantum interference of optical absorption probabitity is presented,followed by description of the impact of spontaneous emisson resulted from Fano spectrum structure and quantum fluctuation in vacuum field on excited optical gain.The mechanism affords theoretical foundation for the development of semiconductor quantum well laser without population inversion.
We propose a new class of intersubband lasers and amplifiers that achieve net gain without population inversion. The laser scheme is based on a unipolar semiconductor double quantum- well structure where gain occurs at a transition between conduction band subbands. In order to achieve net gain without inversion, we utilize Fano-type interferences. The semiconductor laser scheme that we are considering is analogous to the atomic lambda system that has been extensively analyzed in the context of electromagnetically induced transparency and lasing without inversion. A coherent coupling field however, is not required in the present scheme. The electronic coherence necessary for Fano-type interferences is established by resonant tunneling. For nonlinear optics applications, the asymmetry of the structure allows for (chi) (2) processes and therefore higher conversion efficiency or parametric gain.
In a self-consistent field formalism we investigated the weak-probe optical intersubband response of a three-subband semiconductor quantum well pumped by a strong coherent laser beam. We showed that, when the pumping photon energy is somewhat above the energy separation between the two lowest subbands, a combination of hole-burning effects and coherent pump–probe wave interactions can lead to both optical gain without population inversion and an Autler–Townes-like doublet in the probe optical spectrum. These two absorption peaks, whose positions are blueshifted as the pump intensity is increased, are due to the two-photon absorption process and light-induced intersubband transitions between the two upper states. It is also demonstrated that, in a stepwise two-photon pumping scheme, the optical gain with population inversion can also occur in the quantum-well system when a modestly strong pump field is present.
The quantum interferences arising in three-level atomic systems are investigated in analogous asymmetric semiconductor quantum well structures. These investigations lead to the conditions necessary for induced zero absorption and gain without population inversion in interband transitions. Asymmetric quantum well configurations are proposed and analyzed for induced zero absorption and gain without inversion using a single-particle density-matrix approach. This density-matrix approach is described and shown to give the steady-state solutions for absorption and gain without the approximations required for perturbative or iterative methods. Both excitonic and continuum transitions are examined using typical semiconductor parameters with excitonic transitions contributing the largest fraction of gain without inversion. While interband transitions differ from intraband and atomic transitions, results for the proposed quantum well configurations validate the basic concept of inversionless gain in semiconductors and point the way to ultralow threshold semiconductor microlasers. \textcopyright{} 1996 The American Physical Society.
The absorption and stimulated emission of coherent electromagnetic radiation tuned close to an autoionizing state is investigated with the aim of clarifying some of the aspects behind the proposals of inversionless gain involving such atomic configurations. This is carried out within the framework of individual atomic evolution rather than by the use of density matrix equations, thus providing better insight into the fundamental processes of absorption and emission. In particular, we find that stimulated emission does not overcome absorption in spite of quantum interference of the Fano type being present. It is also shown that gain, in this configuration, although it can take place without population inversion between the involved levels, requires in any case a strong departure from thermal equilibrium conditions. Thereby a strong pumping mechanism, or equivalently a strong excitation power, is needed to reach the lasing threshold. This raises some questions on the relevance of the population inversion indicator of efficient gain implicitly assumed in similar configurations and in many related works.
This thesis is concerned with the amplification of coherent radiation from an active medium under conditions which require no population inversion between the energy levels. Gain without population inversion may be obtained by creating a sufficiently strong correlation between pairs of atomic states, usually with the help of external sources of resonant electromagnetic radiation. Our main objective is to investigate two possible selections for the active medium: a three level atom in a lambda configuration and a Raman-driven four level model. For both we identify the conditions that lead to inversionless laser action, we calculate the small signal gain and investigate the physical mechanisms which are responsible for the emergence of gain. While the ultimate cause of the gain is quantum interference, we propose attractive alternative pictures based on the dressed states of the coupled atom-driving field systems. We address the important practical issue of the effect of atomic motion and related Doppler broadening on the inversionless gain profile and we also analyze the transient response of each laser model with the help of the appropriate Maxwell-Bloch equations. Finally we develop a fully quantum mechanical description of the lambda model and find that the radiated field can display unique quantum features which manifest themselves with sub-Poissonian photon fluctuations.
The basic results of theoretical nonperturbative analysis of spontaneous and stimulated emission from two-band excited intrinsic semiconductor interacting with a single mode of radiation field in a lossless micro-cavity are presented. The self-consistent dynamics of the coupled system ''semiconductor + single mode radiation field", accompanied with spontaneous and stimulated emission was investigated as dependent of the initial level of the excitation of semiconductor in the presence initially of the physical field vacuum. Calculations have been carried out with the help of the commutation version of the evolution method without introducing explicitly the phonon mechanisms. The initial non-equilibrium electron and hole populations in the bands have been supposed to be created by ultra-short laser pulses and described by the Fermi-Dirac quasi-equilibrium distributions with the given chemical potentials. The interaction between the electron-hole subsystem and the mode field has been adopted to be switched on suddenly. The calculations have shown that depending on the initial excitation level of electron-hole subsystem three states of the mode field are realized: the equilibrium Bose-Einstein photon distribution with nonzero chemical potential; the quasi-equilibrium distribution with the average number of photons equal to unit and accompanied with their fluctuations, and finally, the state of photon avalanche. The first regime takes the place when the coefficient of the light amplification is negative. Then, the equilibrium between the electron-hole subsystem and arising photons exists until the average number reaches the unit. For negative and =1 the equilibrium still exists but is accompanied with field fluctuations. Finally, when the steady growth arises the lower but stable values are established