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Chinese Physics

Theory of interband coherence and inversionless bistability in semiconductor lasers

Guo-Qin Ge, Leung Pui Tang

Published Nov 1, 2000• 1 citation
Publisher

Abstract

Coherent transitions of charge carriers between the conduction and valence bands of a semiconductor medium are essential for the operation of a semiconductor laser. In this paper, we study how such interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. A laser threshold is obtained, which shows that a strongly coupled high-Q microcavity has a low threshold value. However, when an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. Besides, if the pump-field exceeds a threshold value, it is even possible to achieve a bistability in the inversionless region. Some fundamental macroscopic properties, including polarization, absorption and dispersion for the semicoductor system, are also obtained analytically.

DOI
10.1088/1009-1963/9/11/004

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Coherent transitions of charge carriers between the conduction and valence bands of a semiconductor medium are essential for the operation of a semiconductor laser. In this paper, we study how such interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. A laser threshold is obtained, which shows that a strongly coupled high-Q microcavity has a low threshold value. However, when an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. Besides, if the pump-field exceeds a threshold value, it is even possible to achieve a bistability in the inversionless region. Some fundamental macroscopic properties, including polarization, absorption and dispersion for the semicoductor system, are also obtained analytically.
We study how interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. It shows that a strongly coupled high-Q microcavity has a low lasing threshold value, which corresponds to the threshold intensity being unity. When an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. When the pump-field exceeds a threshold value, it is possible to achieve a bistability in the inversionless region.
From a microscopic set of equations which takes into account spontaneous emission into lasing mode, we derive a macroscopic quantum model of low-threshold semiconductor lasers that includes the parabolic band structure, Pauli blocking of the injection current, and the carrier distribution dependence on the temperature. This model confirms the predictions of lasing without inversion and inversionless intensity squeezing, that were previously made by Yamamoto and co-authors on the basis of a semiclassical two-level approach. In addition, our analysis demonstrates the existence of an optimum temperature value that minimizes the injection current necessary to obtain lasing and intensity squeezing.
Optical frequency bistability and power bistability in semiconductor lasers biased from below to above threshold are presented using the unified description of semiconductor lasers with external light injection. A new method is given to calculate the bistable frequency and power loop widths. When the bias current of a laser is fixed, there is a lowest power for the external light to realize optical frequency bistability. And this lowest power reaches its minimum just at threshold. The optical injection level (defined as the ratio of the injected power to the power emitted by the free-running laser) must be larger than a certain value to obtain the bistability. This level may be larger than unity for a laser biased below threshold. On the other hand, for a given bias current, there is a lowest frequency detuning between the electric field of the laser and the injected field to achieve the power bistability. And this detuning must be negative for any bias current (below or above threshold) of the laser. The results are in good agreement with experiments.>
A fully quantum-mechanical theory of bistable inversionless lasers is developed by generalizing the Scully-Lamb laser theory to this kind of laser systems. By converting the quantum master equation of the system into a Fokker-Planck equation, we obtain the steady-state properties and the fluctuation properties of the laser field for both coherently and incoherently injected atomic systems. It is shown that both the intensity and phase fluctuations of the laser system with coherent atomic injection are reduced corresponding to their counterpart of the incoherent atomic injection. The phase noise of bistable inversionless lasers can be quenched down to the vacuum-noise level for both atomic injections.
Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.
A unified description for semiconductor lasers based from below to above threshold with external light injection is presented using small-signal analysis. The threshold of a semiconductor laser can be reduced by external light injection. The gain change of the laser due to external light injection reaches its maximum just at threshold. Optical bistability in a semiconductor laser is found to be varied continuously from below to above threshold. An analytical expression for the bistable loop bandwidth is presented. There is an optical bias for a laser to maximize the loop bandwidth for a fixed injection power. The results are in good agreement with experiments.>