Roy, Mithun
(2021)
*Harmonic states in quantum cascade lasers: Frequency-domain analysis and mode-spacing control.*
Masters thesis, Concordia University.

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## Abstract

Quantum cascade lasers (QCLs) are unipolar lasers where lasing transition and carrier transport occur between subbands that result from multiple nanometer-thick quantum wells and barriers formed by the conduction band edges of a semiconductor heterostructure. Since their inception in 1994, QCLs have undergone tremendous improvements with respect to output power, frequency range covered, and maximum operating temperature. As a result, they have become a prominent source of light emitting in the mid- and far-infrared regions of the electromagnetic spectrum.

Multimode behavior of QCLs was a focus of many past works. In a recent comprehensive study, it was found that, if the pumping is increased gradually from threshold, QCLs enter into a harmonic state regime, which is characterized by the lasing of side modes that are separated from each other by multiples of free spectral range (FSR). With a further increase in the pumping, finally, transition into the familiar single-FSR-spaced regime (dense state regime) occurs. Unlike the dense state regime, the harmonic state regime of QCLs has not gone through intense scrutiny, as it is a relatively recent discovery.

In this thesis, a theoretical investigation into the harmonic state regime of QCLs is performed. The work is based on Maxwell’s equation and the density matrix (DM) formalism. The two-level DM equations, although commonly employed, ignore many important details of complex carrier transport through a QCL structure. Therefore, here, a three-level DM formalism is employed, which takes into account phenomena such as resonant tunneling and carrier scattering between the three states.

The thesis is mainly divided into two parts. In the first part, starting from the DM-Maxwell equations, an analytical expression for the instability gain of the side modes is derived. This expression can explain the appearance of harmonic states in QCLs. Using this analytical expression, the effects of group velocity dispersion on the harmonic states are studied. In the second part, multimode behavior of QCLs is analyzed using a more general model than that used in the first part. In particular, openness (non-unity facet reflectivity) of the cavity is considered, which was not taken into account in previous works that used the modal expansion method to study QCLs. Using the theory, it is shown that the coating of a facet can be used to excite harmonic states with different mode spacing. Such a control over the generation of harmonic states could make QCLs invaluable for applications such as microwave and terahertz generation, picosecond pulse generation in the mid-infrared frequency range, and broadband spectroscopy.

Divisions: | Concordia University > Gina Cody School of Engineering and Computer Science > Electrical and Computer Engineering |
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Item Type: | Thesis (Masters) |

Authors: | Roy, Mithun |

Institution: | Concordia University |

Degree Name: | M.A. Sc. |

Program: | Electrical and Computer Engineering |

Date: | 2 August 2021 |

Thesis Supervisor(s): | Kabir, M. Z. |

Keywords: | Quantum cascade lasers, harmonic states, four-wave mixing, open cavity, constant-flux states, facet engineering |

ID Code: | 988722 |

Deposited By: | Mithun Roy |

Deposited On: | 30 Nov 2021 20:53 |

Last Modified: | 20 Aug 2023 00:00 |

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