Standard Fourier Transform Infrared Spectrometers (FTIR) rely on a Michelson Interferometer scheme which uses a linear delay line to retrieve an interference pattern. Here, we demonstrate a fast FTIR based on a rotational delay line which allows us to achieve kHz acquisition rates. We perform spectrometry measurements using it in combination either with a Mid-IR Quantum Cascade Laser (QCL) frequency comb or a strongly, low-frequency, RF-modulated QCL. Regarding the latter, the modulation enables to broaden the laser emission up to 250cm^-1 (from 6.5µm to 7.5µm) and to reduce its amplitude noise compared to the free-running case. The combination of a strongly modulated QCL with a rotational FTIR opens the possibility to fast and broadband spectroscopy in the Mid-IR region, with possible applications spanning from gas detection to process control
We present a new integrated photonic platform based on active and passive elements integrated in a double-metal, high confinement waveguide layout planarized with a low-loss polymer. An extended top metallization results in low waveguide losses and improved dispersion, thermal and RF properties, as it enables to decouple the design of THz and microwave cavities. Free-running on-chip quantum cascade laser combs spanning 800 GHz, harmonic states over 1.1 THz and RF-injected broadband incoherent states spanning over nearly 1.6 THz are observed. With a strong external RF drive, actively mode-locked pulses as short as 3 ps can be produced, as measured by SWIFTS. We demonstrate as well passive waveguides with low insertion loss, enabling the tuning of the laser cavity boundary conditions and the co-integration of active and passive components. The same platform is employed to demonstrate dispersion compensated ring combs operating at 3 THz.
Non-linear properties of buried heterostructure ring quantum cascade lasers have been investigated. Clear symmetry breaking between two counter-propagating modes has been observed with a transition to a emission that has a solitonic characteristics.
The main obstacle towards widespread industrial adoption of THz quantum cascade lasers is the requirement of cryogenic cooling. Room-temperature operation using the conventional GaAs/AlGaAs material system is inherently limited by the optical phonon energy (ELO=36 meV) being close to the laser transition energy (~10-20 meV). In contrast, the ZnO/ZnMgO material system has a significantly higher ELO of 72 meV, pushing the theoretical high-temperature limit far above room temperature. At the same time, ZnO comes with it's own challenges, such as significantly broadened energy levels and short upper laser state lifetime.
In the present talk, these considerations will be discussed in the light of non equilibrium Green’s function modelling, which is necessary to correctly treat the strong electron-phonon scattering. In addition, design schemes suitable for m-plane (avoiding internal fields) ZnO QCLs will be presented and analysed, providing pathways towards room temperature THz QCLs.
Regardless of all the success of Mid Infrared Quantum Cascade Lasers (QCLs), they still do not operate at room temperature in the THz range. The main temperature degrading mechanism for THz QCLs is not known in time of writing this abstract and it is still a topic of debate by the community [S. Khanal et al, J. Opt. 16 094001, 2014]. This is a challenge to theory and it is crucial to treat all possible scattering channels with the same mathematical footing. A summary of different methods for simulating these structures is found in [C. Jirauschek et al, Appl. Phys. Rev. 1 011307, 2014]. In this work we include and study the effects of electron-electron scattering via the Single Plasmon Pole Approximation (SPPA). In this approximation we capture both the static limit as well as dynamic effects. This gives an energy dependent (non-local in time) interaction beyond the Hartree-Fock approximation. This has been studied in a similar model with promising results [T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, 2009], and with this work we want to adapt the idea into the model described in Ref. [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, 2013]. We start by summarizing the theory underlying the SPPA and we show how it is implemented in the context of our formalism, by showing good agreement with the results for a four well quantum cascade laser [M. Amanti et al, New J. Phys. 11 125022, 2009].
The two main design schemes for Terahertz quantum cascade lasers, based on tunnelling and scattering injection,
respectively, are theoretically compared. We apply our simulation package based on the non-equilibrium Green’s
function technique. Our results provide a good description of the gain degradation with temperature. Thermal
backfilling contributes to decrease of population inversion in both cases. However, the dropping inversion cannot
account for the total reduction of gain.
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