NUSOD Blog

Connecting Theory and Practice in Optoelectronics

NUSOD 2017 Preview: Dynamic simulation of quantum cascade laser structures with optical nonlinearities

QuantuQCL_combm 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 [1]. Besides leading to multimode instabilities [1], 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 [4] 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 [5]. In the figure, simulation results for a QCL-based terahertz frequency comb source [3] 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.

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NUSOD 2017 Preview: Theory and simulations of self-pulsing in photonic crystal Fano lasers

fano_laserPulsed 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 [1].

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 [2].

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NUSOD 2017 Preview: Novel method for tuning and locking of coupled microring resonator filters

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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

NUSOD 2017 Preview: Simulation of Nonlinear Polariton Dynamics in Microcavity wires for Polaritonic Integrated Circuits

Microcavity_wireIntegrated optical technologies for quantum computing based on linear optics have been under active development in the recent decade. However, the nonlinear optical interactions ‘on a chip’ offer new unexplored functionalities and may play a key role. Recently a new class of nonlinear system has emerged in which light and matter play equally important roles. The basic building blocks of these systems are quantum states of matter coupled to enhanced optical fields found in microstructures. One example is the microcavity exciton-polariton: a mixed light-matter quasiparticle, resulting from the strong coupling of quantum well (QW) excitons to cavity photons.

Microcavity exciton-polaritons have numerous advantages over bare photons and excitons. For instance, due to the excitonic component, they exhibit weaker diffraction and tighter localisation, and the strong interparticle interactions result in lower operational powers ~fJ/mm2 and faster switching speeds ~a few ps. Polariton waves can be confined in structures with sub-micron size, which opens up possibilities for fabrication of polaritonic integrated circuits based on structured semiconductor microcavities on a chip. Laterally etched microcavity wires [1] (Fig. 1a) enhance further the polaritonic nonlinearities and thus are a particularly promising integration platform due to broad transparency window, mature fabrication technology and the possibility of monolithic integration with semiconductor diode lasers and VCSELs. In this respect, new theories and numerical methods are needed to model the nonlinear polariton dynamics in non-planar microcavity wires.

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NUSOD 2017 Preview: Maximum GaN-laser power limited by Auger recombination

AugerT80Auger recombination inside the light-emitting InGaN quantum wells (QWs) was recently identified as major cause of output power limitations in GaN-based blue light-emitting diodes (LEDs) which are the core of many modern light sources. In this electron-hole recombination process, the released energy is transferred to another carrier (electron or hole) without light emission. The Auger recombination rate rises strongly with the QW carrier density and therefore intensifies with stronger current injection into the LED.

In contrast to LEDs, GaN-based blue laser diodes are expected to suffer less from Auger recombination,  based on the popular opinion that the QW carrier density does not rise with increasing current injection above lasing threshold. Shuji Nakamura, who received the 2014 Nobel Prize in physics for his pioneering work on GaN-LEDs, stated in his Nobel lecture that “Auger recombina­tion, with the resulting efficiency droop, does not appreciably occur in blue laser diodes”.  We dispute this claim based on our numerical analysis of  high-power InGaN/GaN laser measurements.

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