A primary limitation of the intensively-researched polaritonic systems compared to their atomic counterparts for the study of strongly correlated phenomena and many-body physics, is their relatively weak two-particle interactions compared to disorder. Here, we show how new opportunities, to enhance such on-site interactions and nonlinearities, arise by tuning the exciton-polaritons dipole moment in electrically-biased semiconductor microcavities incorporating wide quantum wells. The applied field results in a twofold enhancement of excitonexciton interactions as well as more efficiently driving relaxation towards low energy polariton states, thus reducing condensation threshold. 78.67.Pt, 71.36.+c 2018 American Institute of Physics. Achieving the nonlinear quantum regime in photonics where the single-site effective photon interaction energy is larger than the losses, opens a plethora of interesting phenomena such as photon blockade [1], photon crystallisation [2], and opportunities to realize quantum simulators for the study of condesed matter problems such as Mott-insulator to superfluid phase transitions [3] in arrays of optical cavities. So far, the lack of scalable devices with sufficient nonlinearities and low-enough losses has been the main obstacle for the experimental realization of these phenomena. Exciton polaritons are composite quasiparticles resulting from the strong coupling of cavity photons and quantum well (QW) excitons embedded within a microcavity (MC) [4]. Polaritons interact nonlinearly due to their excitonic component and can form macroscopically coherent condensates [5]. They are scalable to form arrays by either etching [6][7][8] or optical patterning [9][10][11] of the microcavity. However, in the presently studied systems, polariton-polariton interaction energy is smaller as compared to their line-broadening. There are two approaches towards overcoming this problem: manufacturing higher quality microcavities, or enhancing the polariton nonlinearities.Here, we take the second approach and demonstrate twofold enhancement of the polariton-polariton interaction using wide QWs in an electrically driven MC. By exploiting the quantum confined Stark effect (QCSE) to form dipolar polaritons we demonstrate tuning of the exciton-exciton interaction. As a direct consequence of this, we obtain an enhancement of the polariton emission in both linear and lasing regimes with a simultaneous reduction of the polariton lasing threshold and shorter polariton condensate formation times due to enhanced exciton scattering. Our results are the first demonstration of the electrical tuning of nonlinearities in exciton-polariton condensates.Exciton-exciton interactions play a key role in the strong nonlinearities present in MC polariton systems. A first at- tempt to control these interactions previously suggested, was to utilise the concept of dipolaritons [12] by incorporating double asymmetric quantum wells in an electrically biased MC. Both direct (DX) and indirect (IX) excitons couple to the same cavity m...

We explore two parabolic quantum well (PQW) samples, with and without Bragg mirrors, in order to optimise the building blocks of a Bosonic Cascade Laser. The photoluminescence spectra of a PQW microcavity sample is compared against that of a conventional microcavity with embedded quantum wells (QWs) to demonstrate that the weak coupling lasing in a PQW sample can be achieved. The relaxation dynamics in a conventional QW microcavity and in the PQW microcavity was studied by a non-resonant pump-pump excitation method. Strong difference in the relaxation characteristics between the two samples was found. The semi-classical Boltzmann equations were adapted to reproduce the evolution of excitonic populations within the PQW as a function of the pump power and the output intensity evolution as a function of the pump-pump pulse delay. Fitting the PQW data confirms the anticipated cascade relaxation, paving the way for such a system to produce terahertz radiation.

We report observation of strong light-matter coupling in an AlGaAs microcavity (MC) with an embedded single parabolic quantum well. The parabolic potential is achieved by varying aluminum concentration along the growth direction providing equally spaced energy levels, as confirmed by Brewster angle reflectivity from a reference sample without MC. It acts as an active region of the structure which potentially allows cascaded emission of terahertz (THz) light. Spectrally and time resolved pump-probe spectroscopy reveals characteristic quantum beats whose frequencies range from 0.9 to 4.5 THz, corresponding to energy separation between relevant excitonic levels. The structure exhibits strong stimulated nonlinear emission with simultaneous transition to weak coupling regime. The present study highlights the potential of such devices for creating cascaded relaxation of bosons, which could be utilized for THz emission.