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physica status solidi (b)

Interband Coherence and Inversionless Bistability in Semiconductor Lasers

Guo Qin Ge, P. T. Leung

Published Sep 1, 2000• 0 citations
Publisher

Abstract

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.

DOI
10.1002/1521-3951(200009)221:1<403::aid-pssb403>3.0.co;2-9

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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.
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.
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.>
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