RF Control and Coherence Spectroscopy of THz Quantum Cascade Laser Frequency Combs
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Author
Date
2021Type
- Doctoral Thesis
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Abstract
Frequency combs generated by mode-locked lasers revolutionized the field of high precision metrology in 1999. They quickly found applications in other research and industrial fields such as astronomy, spectroscopy, atomic clocks or security, as they act as optical rulers with a direct link to the RF domain. Therefore, the concept of the frequency comb is explored in many laser systems. One of these systems is the Quantum Cascade Laser (QCL). QCLs were theoretically predicted in 1971 and rely heavily on quantum well engineering in electrically pumped semiconductor intersubband heterostructures. They were experimentally demonstrated for the first time in 1994 in the mid-IR. They rapidly developed, being able to cover nearly the whole mid-IR range and work nowadays at room temperature. Due to their engineering capability, QCLs also showed in 2002 the emission in the THz region, which are up to today mainly bound to cryogenic temperatures. The high nonlinearities that arise in QCLs allow the formation of frequency combs due to four-wave-mixing (FWM) and spatial-hole-burning (SHB), induced by the Fabry-Pérot cavities.
In this thesis, we will in detail investigate frequency combs emitted by THz QCLs. In chapter 1, we will therefore look into the THz spectral region and the available sources. It will cover briefly the working principle of QCLs followed by a mathematical definition of a frequency comb and a historical review on the developments of them in mode-locked lasers which led to the Nobel Prize. Further, we will show the key developments towards frequency comb emission in THz and mid-IR QCLs.
Chapter 2 will treat the fabrication of THz double-metal ridge waveguide QCLs. We will investigate group velocity dispersion (GVD) effects, induced by the active region (AR), and their influence on the cold cavity modes. We will show how to suppress higher order transverse modes by means of side absorbers and see their effect on the cold cavity mode response. Since the following chapters will rely on RF measurements, we will briefly discuss RF compatible mounting of our devices and characterize the cryostat setup.
In chapter 3, we will go in detail into the mathematical analysis of Fourier-Transform Infrared Spectrometer (FTIR) measurements. We will explain well-known methods for data analysis of standard FTIR data and discuss in detail effects of finite measurements, i.e. the Discrete Fourier Transform (DFT), which lead to spectral leakage. In addition, we will discuss how these spectral leakage effects can be suppressed using window functions. We then extend and adopt these methods for multi-mode and frequency comb laser sources. We will see that accurate data analysis can increase the nominal resolution of 2.25 GHz of our FTIR down to roughly 10 MHz for the frequency comb case. Therefore, we will show that standard FTIR measurements can already give significant insides of the equidistant spacing of optical modes.
Chapter 4 will discuss the broadband emission of homogeneous THz QCLs, which originates from domain formation within the structure. The results are supported by numerical and experimental data. We will also see that such structures emit frequency combs and that we can RF injection lock these states. By increasing the RF modulation, we will observe that the lasing spectrum can be significantly changed.
In chapter 5, we will embed the same active region as in chapter 4 into a lower loss Cu-Cu waveguide. We will see increased performance compared to Au-Au waveguides and that broadband comb formation can be observed up to 80 K. We will then further test the RF injection capabilities of these devices and show broadening of spectra up to 700 GHz at 80 K.
Besides observing frequency combs defined by the fundamental round trip time, we could also observe harmonic combs in the previous mentioned Cu-Cu devices. We will therefore investigate experimentally and theoretically their presence in THz QCLs in chapter 6. We will show that pure and self-starting harmonic combs can be explained by an asymmetric gain due to two optical transitions with different oscillator strengths.
In chapter 7, we will briefly look into a special designed two stack laser which emits two frequency combs spaced by an octave in pulsed operation. The results will reflect the large GVD present in THz QCL devices and we will verify the spectral origin of each comb by means of self-mixing intermode beatnote spectroscopy (SMIBS) and intermode beatnote spectroscopy (IBS) with a Schottky diode mixer.
In chapter 8, we will briefly explore different techniques which allow to measure the coherence or temporal profile of comb emitting laser sources. It will be followed by a detailed overview of Shifted
Wave Interference Fourier Transform (SWIFT), which will be mainly used in chapter 9 and will allow us to access the coherence and temporal profile of our THz QCLs.
In the final chapter 9, we will use a fast antenna coupled, superconducting NbN hot-electron bolometer (HEB) to perform SWIFT measurements on free running and strongly RF modulated homogeneous and heterogeneous THz QCLs. We will analyze fundamental and harmonic frequency combs, and will show clearly for the first time that strong RF injection, resonant and off-resonant, will mainly lead to sub-comb formation in the investigated THz QCLs. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000546201Publication status
publishedExternal links
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Contributors
Examiner: Faist, Jérôme
Examiner: Scalari, Giacomo
Examiner: Burghoff, David
Examiner: Barbieri, Stefano
Publisher
ETH ZurichSubject
THz Quantum Cascade Laser; Frequency comb; QCL; RF injection; THz; FTIROrganisational unit
03759 - Faist, Jérôme / Faist, Jérôme
Funding
724344 - On Chip Terahertz Frequency Combs (EC)
Related publications and datasets
Has part: https://doi.org/10.3929/ethz-b-000406661
Has part: https://doi.org/10.3929/ethz-b-000478146
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