# NUSOD Blog

Connecting Theory and Practice in Optoelectronics

## NUSOD 2017 Preview: Strain tuning of absorption spectrum in a topological insulator

2017-06-22

Posted by on Topological insulators have attracted a huge amount of attention in the field of condensed matter physics. This new state of matter is characterized by a bulk band gap and conducting surface states. The surface states have linear dispersion resembling relativistic Dirac fermions, in analogy with graphene. In contrast to graphene the 2D fermions on the surface are non-degenerate, whereas electrons in graphene are spin and valley degenerate. The linear dispersion and 2D nature of the electrons in graphene leads to the universal optical absorbance απ≈2.3% given by the fine structure constant , independent on the material parameters and the photon energy. Due to this relatively large absorption for a single atomic layer, graphene is a promising material for optoelectronic applications e.g. photodetectors, which has been demonstrated [1]. Similarly a topological insulator has an absorbance of απ/2≈1.1% for photon energies below the bulk band gap, due to the surface states at the top and bottom surface. It has been shown that the signal-to-noise ratio of photodetector based on a thin slab of the topological insulator Bi_{2}Se_{3 }can be significantly larger than for a graphene based device[2]. For a slab thickness below 6 nm the surface states on opposing sides interact leading to a band gap, which can be tuned by varying the thickness. However, this way of manipulating the optical properties is not very flexible since Bi_{2}Se_{3} has a layered structure with five atomic layers strongly bound in a quintuple layer, limiting the possible thicknesses to only integer numbers of quintuple layers (QL). If instead strain is used to tune the optical properties, this can be done continuously and dynamically.

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

2017-05-23

Posted by on 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 [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.

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

2017-05-16

Posted by on 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 [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].

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

2017-05-08

Posted by on 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

2017-04-21

Posted by on Integrated 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/mm^{2 }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.