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Register a new userPolariton condensation phase diagram in wide bandgap planar microcavities: GaN versus ZnO
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GaN and ZnO microcavities have been grown on patterned silicon substrate. Thanks to a common platform these microcavities share similar photonic properties with large quality factors and low photonic disorder which gives the possibility to determine the optimal spot diameter and to realize a complete comparative phase diagram study. Both systems have been investigated under the same experimental condition. Experimental results are well reproduced by simulation using Boltzmann equations. Lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity as expected due to a larger Rabi splitting. However the threshold is strongly impacted by LO phonons through phonon-assisted polariton relaxation. We observe and discuss this effect as a function of temperature and detuning. Finally the polariton lasing threshold at room temperature is quite similar in both microcavities. This study highlights polariton relaxation mechanism and their importance for threshold optimization.
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A pressure-induced phase transition, associated with an increase of the coordination number of In and Ta, is detected beyond 13 GPa in InTaO4 by combining synchrotron x-ray diffraction and Raman measurements in a diamond anvil cell with ab-initio calculations. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure which shares the same space group with the low-pressure phase (P2/c). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell volume collapse and an electronic bandgap collapse observed by experiments and calculations. Additionally, a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calculations also provide information on IR-active phonons and bond distances. These findings provide insights into the effects of pressure on the physical properties of InTaO4.
We present a theoretical model that allows us to describe the polariton dynamics in a semiconductor microcavity at large densities, for the case of non-resonant excitation. Exciton-polariton scattering from a thermalized exciton reservoir is identified as the main mechanism for relaxation into the lower polariton states. A maximum in the polariton distribution that shifts towards lower energies with increasing pump-power or temperature is shown, in agreement with recent experiments. Above a critical pump-power, macroscopic occupancies (5 times 10^4) can be achieved in the lowest energy polariton state. Our model predicts the possibility of Bose-Einstein Condensation of polaritons, driven by exciton-polariton interaction, at densities well below the saturation density for CdTe microcavities.
Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2-315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K.
Planar microcavities filled with active materials containing excitonic resonances host radiative exciton-polariton (polariton) modes with in-plane wave vectors within the light cone. They also host at least one mode guided in the cavity plane by total internal reflection and which is not radiatively coupled to the vacuum modes except through defects or sample edges. We show that polariton lasing mediated by polariton stimulated scattering can occur concomitantly in both types of modes in a microcavity. By adjusting the detuning between the exciton and the radiative photon mode one can favor polariton lasing either in the radiative or in the guided modes. Our results suggest that the competition between these two types of polariton lasing modes may have played a role in many previous observations of polariton lasing and polariton Bose Einstein condensation.
ZnO/GaN alloys exhibit exceptional photocatalyst applications owing to the flexibly tunable band gaps that cover a wide range of the solar spectrum, and thus have attracted extensive attentions over the past few years. In this study, first-principles calculations were employed to investigate structural stabilities and electronic properties of (1-100) and (11-20) ZnO/GaN heterostructured nanofilms. The effects of nanofilm thickness and GaN ratio were explored. It was found that all studied heterostructured nanofilms were less stable than the corresponding pure ZnO film but more stable than pure GaN one, exhibiting a much thicker film with better stability. Electronic band structures displayed that both two types of (1-100) and (11-20) heterostructured nanofilms were semiconductors with band gaps strongly depending on the GaN ratios as well as the thicknesses. Of particular interesting is that the band gaps decreased firstly, and then increased with the increasing GaN ratio. Furthermore, electronic contribution to the valence band maximum and the conduction band minimum, and optical absorption were discussed. Our results of ZnO/GaN heterostructured nanofilms with spatial separation of electrons and holes, and flexibly tunable band gaps hold great promise for applications in visible-photovoltaic field.
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