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Connecting Theory and Practice in Optoelectronics
Quantum cascade lasers (QCLs) utilize quantized electron states as laser levels, which can be custom-tailored to the specific application by adequately designing the multi-quantum-well active region. With dephasing times between the upper and the lower laser level of about a picosecond, coherent light-matter interaction, along with other effects such as dispersion and spatial hole burning, governs the laser dynamics . Besides leading to multimode instabilities , the ultrafast dynamics in QCLs is increasingly exploited to implement innovative functionalities, such as the generation of frequency combs [2,3] and picosecond optical pulses  in the mid-infrared and terahertz regime.
For a targeted design of such structures and a deeper understanding of the complex QCL dynamics, a detailed theoretical model is required. We have developed a multi-domain simulation approach, which couples a Maxwell-Bloch type description of the light-matter interaction with advanced ensemble Monte Carlo (EMC) carrier transport simulations to eliminate empirical electron lifetimes . In the figure, simulation results for a QCL-based terahertz frequency comb source  are presented. The laser dynamics gives rise to a two-lobed power spectrum consisting of equidistant discrete comb lines [Fig. (a)]. Furthermore, a periodic temporal shifting of the power between the two spectral lobes is observed [Fig. (b)]. The two-lobed comb spectrum and the associated temporal switching dynamics have also been observed in experiment [3,5], validating our simulation model.
Pulsed lasers are utilised in a wide variety of applications, especially optical communication systems, and in particular lasers that support passive pulse generation (self-pulsing). The phenomenon of self-pulsing was exclusively found in macroscopic lasers until recently, where self-pulsing in a microscopic photonic crystal Fano laser was reported .
The Fano laser (fig. 1) consists of a line-defect waveguide in a 2D photonic crystal membrane coupled to a nearby point-defect, with active material embedded directly in the membrane. This coupling yields a strong, narrowband suppression of transmission, due to the interference of the continuous waveguide modes with the discrete mode of the nanocavity, effectively forming the right-most laser mirror at the symmetry line, with the left formed by termination of the waveguide .
The problem of circuit control can be particularly challenging in high-index-contrast photonic platforms, due to the large sensitivity of the optical parameters of the devices even to tiny geometrical variations occurring during fabrication, mainly inducing random phase errors. This is particularly true for high-order coupled microring resonator filters. The use of these devices cannot be addressed without proper tuning and locking techniques that can be significantly more complex than the approaches required for the control of a single ring, due to the larger number of degrees of freedom, the existence of non-negligible coupling between rings and the risk of trapping in sub-optimal local solutions. Read more of this post
In the last couple of decades, photonic quantum computing has become a leading contender as a platform for quantum information processing . Recently, CMOS-fabrication technology has been used for quantum optics applications using compact silicon-on-insulator (SOI) photonic circuits. Integrated photonic components allow us to have more complex, stable and scalable quantum photonics devices.
Single photon detectors (SPD) are one of the fundamental building blocks for quantum information processing and therefore highly efficient and fast SPDs with potential for integration are crucial. Read more of this post
Two long sought-after goals for the semiconductor community have been (i) to develop long-wavelength semiconductor lasers on GaAs substrates, to enable exploitation of vertical-cavity architectures as well as monolithic integration with GaAs-based high-speed microelectronics, and (ii) to realise uncooled operation of semiconductor lasers, whereby the external cooling equipment typically required to maintain operational stability in long-wavelength devices can be removed in order to significantly reduce energy consumption without degrading the device performance.