Abstract - Non-collinear pump-probe spectroscopy with phase-stable multi-THz pulses reveals the ultrafast scattering dynamics of intersubband polaritons in a dispersive microcavity. For resonant excitation of the lower polariton at a finite in-plane momentum, $k_{||}$, a narrowband maximum emerges in the probe reflectivity at the polaritonic ground state at $k_{||}=0$. A quantum mechanical model reproduces the experimental key results quantitatively and confirms the underlying microscopic process as stimulated polariton-polariton scattering. These results are an important milestone on the way to bosonic lasing in custom tailored polaritonic systems in the multi-THz and THz spectral regions.

The ultrafast scattering dynamics of intersubband polaritons in dispersive cavities embedding GaAs/AlGaAs quantum wells are studied directly within their band structure using a noncollinear pump-probe geometry with phase-stable midinfrared pulses. Selective excitation of the lower polariton at a frequency of ∼25  THz and at a finite in-plane momentum k_{‖} leads to the emergence of a narrowband maximum in the probe reflectivity at k_{‖}=0. A quantum mechanical model identifies the underlying microscopic process as stimulated coherent polariton-polariton scattering. These results mark an important milestone toward quantum control and bosonic lasing in custom-tailored polaritonic systems in the mid and far infrared.

Non-collinear multi-THz pump-probe spectroscopy reveals the intersubband polariton dynamics in a dispersive microcavity. Selective pumping of the lower polariton at finite in-plane momentum leads to coherent polariton-polariton scattering towards the ground state.

We demonstrate that, in cavity-embedded doped quantum wells, strong light-matter interaction can create a bound excitonic state. Such a cavity-stabilised state is spectroscopically observed as a discrete resonance below the ionisation threshold.

Applications relying on mid-infrared radiation ( λ ~ 3-30 μm) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs like mid-infrared cameras and quantum cascade lasers. On the other side, standalone and broadband devices allowing control of the beam amplitude and/or phase at ultra-fast rates (GHz or more) are still missing. Here we show a free-space amplitude modulator for mid-infrared radiation ( λ ~ 10 μm) that can operate at room temperature up to at least 1.5 GHz (−3dB cutoff at ~750 MHz). The device relies on a semiconductor heterostructure enclosed in a judiciously designed metal–metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300 K. By applying an appropriate bias, the device transitions towards the weak-coupling regime. The large change in reflectance is exploited to modulate the intensity of a mid-infrared continuous-wave laser up to 1.5 GHz.

We have developed a free-space amplitude modulator for mid-IR radiation (λ = 10μm) that can operate up to at least 1.5 GHz (-3dB cut-off at _ 750 MHz) and at room-temperature. The device relies on a semiconductor heterostructure enclosed in a judiciously designed metal-metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300K. By applying an appropriate bias, the device transitions to the weak coupling regime. The large change in reflectivity due to the disappearance of the polaritonic states is exploited to modulate the intensity of a mid-IR continuous-wave laser up to speeds of more than 1.5 GHz.

Applications relying on mid-infrared radiation (Mid-IR, λ∼ 3-30 μm) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs. Mid-IR cameras have propelled the field of thermal imaging. And the invention of the quantum cascade laser (QCL) has been a milestone, making compact, semiconductor-based mid-IR lasers available to a vast range of applications. All the recent breakthrough advances stemmed from the development of a transformative technology. In addition to the generation and detection of light, a key functionality for most photonics systems is the electrical control of the amplitude and/or phase of an optical beam at ultra-fast rates (GHz or more). However, standalone, broadband, integrated modulators are missing from the toolbox of present mid-IR photonics integrated circuits and systems developers. We have developed a free-space amplitude modulator for mid-IR radiation (λ∼ 10 μm) that can operate up to at least 1.5 GHz (-3dB cut-off at ∼ 750 MHz) and at room-temperature. The device relies on a semiconductor hetero-structure enclosed in a judiciously designed metal-metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300K. By applying an appropriate bias, the device transitions to the weak coupling regime. The large change in reflectivity due to the disappearance of the polaritonic states is exploited to modulate the intensity of a mid-IR continuous-wave laser up to speeds of more than 1.5 GHz.

We demonstrate a free-space amplitude modulator for mid-IR radiation ( λ around 10 µm) that can operate up at least 400 MHz (-3dB cut-off at ∼100/150 MHz) and at room-temperature. The device is based on a semiconductor hetero-structure enclosed in a judiciously designed optical resonator based on a metallic meta-surface. At zero bias, it operates in the strong light-matter coupling regime up to 300K. By applying an appropriate bias, the device transitions to the weak coupling regime: the important change in reflectivity due to the disappearance of the polaritonic states is exploited to modulate the intensity of a mid-IR laser source up to at least 400 MHz .

In contrast to interband excitons in undoped quantum wells, doped quantum wells do not display sharp resonances due to excitonic bound states. In these systems the effective Coulomb interaction between electrons and holes typically only leads to a depolarization shift of the single-electron intersubband transitions. Non-perturbative light-matter interaction in solid-state devices has been investigated as a pathway to tune optoelectronic properties of materials. A recent theoretical work [Cortese et al., Optica 6, 354 (2019)] predicted that, when the doped quantum wells are embedded in a photonic cavity, emission-reabsorption processes of cavity photons can generate an effective attractive interaction which binds electrons and holes together, leading to the creation of an intraband bound exciton. Spectroscopically, this bound state manifests itself as a novel discrete resonance which appears below the ionisation threshold only when the coupling between light and matter is increased above a critical value. Here we report the first experimental observation of such a bound state using doped GaAs/AlGaAs quantum wells embedded in metal-metal resonators whose confinement is high enough to permit operation in strong coupling. Our result provides the first evidence of bound states of charged particles kept together not by Coulomb interaction, but by the exchange of transverse photons. Light-matter coupling can thus be used as a novel tool in quantum material engineering, tuning electronic properties of semiconductor heterostructures beyond those permitted by mere crystal structures, with direct applications to mid-infrared optoelectronics.

We experimentally demonstrate the existence of bound excitonic resonances in doped quantum wells whose electron and hole are bind by the exchange of virtual cavity photons.

This paper deals with an experimental investigation on the shear capacity of BFRP (basalt fibre-reinforced polymer) reinforced concrete (RC) members without shear reinforcement. The difference of BFRP compared to steel reinforcement is its corrosion resistance, electromagnetic neutrality and low thermal conductivity. On the other hand, the modulus of elasticity of BFRP is significant lower than that of steel rebars. This leads to a different behaviour regarding the bond between concrete and BFRP reinforcement and load capacity of RC members. For the application of BFRP RC structures, the load-bearing behaviour has to be evaluated. To study the shear capacity of BFRP RC members, concrete beams under short-term static and centric load were experimentally determined. The results were compared to geometrically same configured specimens with steel reinforcement while several parameters were monitored during the tests. The obtained experimental results were used to evaluate existing shear models of current design guidelines for FRP RC.

We report on the systematic study of two main scattering mechanisms on intersubband transitions, namely ionized impurity scattering and interface roughness scattering. The former mechanism has been investigated as a function of the dopants position within a multiple GaAs/AlGaAs quantum well structure and compared to the transition of an undoped sample. The study on the latter scattering mechanism has been conducted using the growth interruption technique. We report an improvement of the intersubband (ISB) transition linewidth up to 11% by interrupting growth at GaAs-on-AlGaAs interfaces. As a result, the lifetime of intersubband polaritons could be improved up to 9%. This leads to a reduction of 17% of the theoretical threshold intensity for polaritonic coherent emission. This work brings a useful contribution towards the realization of polariton-based devices.

Probabilistic analysis of concrete members is mainly needed for safety evaluations of existing structures. The well-known Monte Carlo simulation is very time-demanding and is therefore not suitable for use in practice, especially for nonlinear finite element analysis of concrete structures. This paper presents a new optimal Latin hypercube sampling method for design of a computer experiment taking into account the sensitivity factors regarding the behaviour of the considered system. The method avoids extensive numerical calculations. The basic idea of this sampling strategy is to use a weighting factor for each dimension of the probability space in order to consider the different influences of the random variables in the optimisation process of Latin hypercube samples. The method is evaluated using examples of reinforced concrete structures, including an example regarding the spatial variability of concrete properties. The results show that with only 10 simulations the statistical values of the load-bearing capacities of concrete structures can be determined with high accuracy, if the new weighted Latin hypercube sampling is used.

We report quantum well mid-infrared photodetectors operating in the strong light-matter coupling regime. It is an ideal system to elucidate the elusive problem of the injection of electrons (single-particles) into polaritonic modes (bosonic excitations). We have obtained experimental information on the transfer function between a polaritonic system and an electronic reservoir. This approach opens promising avenues in view of adding previously unavailable functionalities to quantum well detectors (mid-IR and THz) and improving their performance.

Recent experimental works on concrete fatigue show that there is a significant increase of load-induced temperature in ultra-high performance concrete, especially for high loading rates. The temperature field is not uniformly distributed in concrete which causes internal stresses and leads to a reduction of concrete fatigue strength. The main issue to be examined is the mechanism of heat-generation in concrete due to cyclic loading. In this paper, the development of load-induced temperature is modelled by a heat transfer equation. The heat generation rate is theoretically determined and experimentally validated. The influence of the load frequency and the maximum size of aggregate of concrete on the temperature development and on the fatigue behaviour of concrete is also taken into account. The obtained results show that the load-induced temperature is generated by conversion of plastic work, which mainly occurs at the interfacial transit zone between cement paste and concrete aggregates.

The development of the construction industry moves towards resource-saving and filigree constructions. Ultra-high performance concrete (UHPC) was developed to reach a higher efficiency of the material usage. As a result, the structures will be leaner and lighter. However, due to reduction of self-weight through the optimized use of materials, structures are more susceptible to induced vibrations. This is particularly important to structures under cyclic loading, such as long-span bridges and wind turbines, which are exposed to very high load cycles. A thorough understanding of the fatigue behaviour of UHPC is thus required. The focus of the project "load-induced increase of temperature in UHPC" is to study the mechanism of heat-generation in concrete members due to cyclic loading and its influence on the fatigue behaviour of concrete. The hypothesis is that the induced temperature fields and the resulting concrete stresses have a non-negligible influence on the damage process and the fatigue strength of UHPC. The investigation focuses on the influence of the frequency and regime of load on the temperature development in concrete during the fatigue process. A systematic experimental study within three different testing frequencies and maximum compressive stress levels is carried out. The surface and inner temperature as well as the longitudinal and transverse strain of the specimens are measured continuously. In this paper, the main results of measurement are presented and discussions on the development of load-induced temperature of concrete are also given.

We developed a technique that enables replacement of a metallic waveguide cladding with a low-index (n≈1.4) material - CaF2 or BaF2. It is transparent from the mid-IR up to the visible range: elevated confinement is preserved while introducing an optical entryway through the substrate. Replacing the metallic backplane also allows double-side patterning of the active region. Using this approach, we demonstrate strong light-matter coupling between an intersubband transition (λ∼10 μm) and a dispersive resonator at 300 K and at 78 K. Finally, we evaluate this approach's potential as a platform for waveguiding in the mid-IR spectral range, with numerical simulations that reveal losses in the 1-10 cm-1 range.

Cracks in reinforced concrete members are almost unavoidable due to the low tensile strength of concrete, but are usually uncritical and limited in width by design at the serviceability limit state, so that durability and visual appearance are not impaired. However, crack spacing and crack width also influence the deformation behaviour as well as the shear strength, so that precise knowledge of the mechanical behaviour remains essential. In this article, models for the description of crack spacings are presented and discussed. This is followed by a validation based on experimental investigations carried out by the author and taken from the literature. Finally, the transferability of these models to textile reinforced concrete members is validated and modifications of the models are suggested.

We report experimental evidence of longitudinal optical (LO) phonon-intersubband polariton scattering processes under resonant injection of light. The scattering process is resonant with both the initial (upper polariton) and final (lower polariton) states and is induced by the interaction of confined electrons with longitudinal optical phonons. The system is optically pumped with a mid-IR laser tuned between 1094 cm−1 and 1134 cm−1 (λ = 9.14 μm and λ = 8.82 μm). The demonstration is provided for both GaAs/AlGaAs and InGaAs/AlInAs doped quantum well systems whose intersubband plasmon lies at a wavelength of ≈10 μm. In addition to elucidating the microscopic mechanism of the polariton-phonon scattering, it is found to differ substantially from the standard single particle electron-LO phonon scattering mechanism, and this work constitutes an important step towards the hopefully forthcoming demonstration of an intersubband polariton laser.

We show that intersubband (ISB) polaritons are robust to inhomogeneous broadening. In a series of multiple quantum well samples with mid-infrared ISB absorption transitions with broadenings varying by a factor of 5 (from 4 meV to 20 meV), we observed polariton linewidths lying in the 4 - 7 meV range only. We have verified the dominantly inhomogeneous origin of the broadening of the ISB transition, and that the linewidth reduction of the modes persists up to room-temperature. The immunity to inhomogeneous broadening is a direct consequence of the coupling of the large number of ISB oscillators to a single photonic mode.