Dissertation Defense: Nathan Henry

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Title: Mid-Infrared and Terahertz Frequency Combs from Quantum Cascade Lasers

Abstract: Optical frequency combs (FC) allow for extremely high resolution and broadband spectroscopic measurements that are captured contemporaneously rather than through some scanning action. Spectroscopic access to the infrared and THz is highly coveted as many molecular resonances lie in this region. However, due to a lack of available materials, emission of FC in the IR has been difficult, with many attempts resulting in low power and efficiency. In 2014 [1] the first mid-IR FC was characterized from a free-running QCL, requiring no extra elements. However, due to the inherently short upperstate lifetime of the laser, the FC is atypical in that it is not characterized by pulses but rather frequency modulation (FM). While the QCL FC has advanced significantly, it is not fully understood. As a result, spectroscopic measurements can become unreliable, sensitive to environmental changes, and recovery of absolute frequency can be difficult.

To better understand the FC QCL, a set of rate equations adapted from the optical Bloch equations is developed and found to be fully adequate for describing the origins and dynamics of FM FC. This work addresses two modes of operation (pseudo-random and chirped FM) calculating the dynamics of a QCL modeled after real-world measurements. Using specifications of real world QCLs (THz and IR), the gain is modeled under various operational scenarios and the most efficient state is identified. The period of the FM is postulated to be determined by the relative strengths of the various hole burning mechanisms and stability is shown for multiple regimes.

Further work is presented addressing the stability of QCL FCs. We begin by deriving the linewidth of the FC generating QCL and show that indeed it can be just as narrow as more conventional FCs. Subsequent to this work we use a two-dimensional model to achieve an engineered power-law dispersion, which can mitigate offset frequency drift offering the potential to significantly lower the phase noise. It is the hope of the author that this research will be used to develop a deeper understanding of FC producing QCLs that contribute to many fields of human endeavor such as medical diagnostics, remote sensing, time standardization, etc.

Committee Members

Jacob Khurgin – Department of Electrical and Computer Engineering. Adviser.

Susanna Thon – Department of Electrical and Computer Engineering.

Amy Foster – Department of Electrical and Computer Engineering.

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