Keldysh field theory for nonequilibrium condensation in a parametrically pumped polariton system

We develop a quantum field theory for parametrically pumped polaritons using Keldysh Green's function techniques. By considering the mean-field and Gaussian fluctuations, we find that the low energy physics of the highly non-equilibrium phase transition to the optical parametric oscillator regime is in many ways similar to equilibrium condensation. In particular, we show that this phase transition can be associated with an effective chemical potential, at which the system's bosonic distribution function diverges, and an effective temperature. As in equilibrium systems, the transition is achieved by tuning this effective chemical potential to the energy of the lowest normal mode. Since the occupations of the modes are available, we determine experimentally observable properties, such as the luminescence and absorption spectra.Comment: 16 pages, 14 figure

Secret objectives: promoting inquiry and tackling preconceptions in teaching laboratories

In its most general form, a `secret objective' is any inconsistency between the experimental reality and the information provided to students prior to starting work on an experiment. Students are challenged to identify the secret objectives and then given freedom to explore and understand the experiment, thus encouraging and facilitating genuine inquiry elements in introductory laboratory courses. Damping of a simple pendulum is used as a concrete example to demonstrate how secret objectives can be included. We also discuss the implications of the secret objectives method and how this can provide a link between the concepts of problem based learning and inquiry style labs

Keldysh field theory for nonequilibrium condensation in a parametrically pumped polariton system

We develop a quantum field theory for parametrically pumped polaritons using Keldysh Green's function techniques with which the occupations of the excitation spectra can be calculated. By considering the mean field and Gaussian fluctuations, we find that the highly nonequilibrium phase transition to the optical parametric oscillator regime is in some ways similar to equilibrium condensation. In particular, we show that this phase transition can be associated with an effective chemical potential, at which the system's bosonic distribution function diverges, and an effective temperature for low energy modes. As in equilibrium systems, the transition is achieved by tuning this effective chemical potential to the energy of the lowest normal mode. Since the nonequilibrium occupations of the modes are available, we determine experimentally observable properties such as the luminescence and absorption spectra

Non-equilibrium phase transition to the polariton OPO regime

Exciton-polaritons are the quasi-particles that form when cavity photons couple strongly to quantum well excitons in semiconductor microcavities. When a pump laser is applied near the point of inflection of the lower polariton dispersion, a phase transition to the polariton optical parametric oscillator regime where two additional, `signal' and `idler', modes with macroscopic occupation appear can occur. The steady state of the non-equilibrium polariton system is maintained by continuous pumping and the Keldysh functional integral approach is used to study the phase transition. Despite its highly non-equilibrium nature, an effective chemical potential is identified and the phase transition occurs when the effective chemical potential crosses the normal modes. The Keldysh formalism also gives access to the occupations of the modes and experimentally observable properties such as the incoherent luminescence and absorption spectra are calculated. One of the key properties of the signal mode is that it occurs near the minimum of the lower polariton dispersion with zero momentum. To calculate the mean field occupation of the three mode optical parametric oscillator regime analytically, the signal momentum has to be chosen explicitly. A simple method to determine the signal momentum by using linear response analysis for any system parameters is proposed and the predictions compared with numerical integration of the complex Gross-Pitaevskii equations describing the system. At weak pump strengths, the signal momentum is found best by the linear response analysis of the three mode description, while at higher pumping, a linear response analysis of the single pump mode gives best agreement with the numerical simulations

Asking the next generation: the implementation of pre-university students' ideas about physics laboratory preparation exercises

It was planned to introduce online pre-laboratory session activities to a first-year undergraduate physics laboratory course to encourage a minimum level of student preparation for experiments outside the laboratory environment. A group of 16 and 17 year old laboratory work-experience students were tasked to define and design a pre-laboratory activity based on experiments that they had been undertaking. This informed the structure, content and aims of the activities introduced to a first year physics undergraduate laboratory course, with the particular focus on practising the data handling. An implementation study showed how students could try to optimise high grades, rather than gain efficiency-enhancing experience if careful controls were not put in place by assessors. However, the work demonstrated that pre-university and first-year physics students can take an active role in developing scaffolding activities that can help to improve the performance of those that follow their footsteps

Geographies of Outer Space : Progress and New Opportunities

Acknowledgements The editors of this forum would like to acknowledge productive contributions to a thematic session at the RGS-IBG Annual International Conference 2016 on ‘Geographies of Outer Space’. We are also grateful to the editorial board of this journal for their support of this endeavour. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.Peer reviewedPostprin

Geographies of outer space: progress and new opportunities

Research into outer space has burgeoned in recent years, through the work of scholars in the social sciences, arts and humanities. Geographers have made a series of useful contributions to this emergent work, however scholarship remains fairly limited in comparison to other disciplinary fields. This Forum explains the scholarly roots of these new geographies of outer space, considering why and how geographies of outer space could make further important contributions. The Forum invites reflections from political, environmental, historical and cultural geographers to show how human geography can present future avenues to continued scholarship into outer space

Properties of the signal mode in the polariton optical parametric oscillator regime

Theoretical analyses of the polariton optical parametric oscillator (OPO) regime often rely on a mean-field approach based on the complex Gross-Pitaevskii equations in a three-mode approximation, where only three momentum states, the signal, pump, and idler, are assumed to be significantly occupied. This approximation, however, lacks a constraint to uniquely determine the signal and idler momenta. In contrast, multimode numerical simulations and experiments show a unique momentum structure for the OPO states. In this work we show that an estimate for the signal momentum chosen by the system can be found from a simple analysis of the pump-only configuration. We use this estimate to investigate how the chosen signal momentum depends on the properties of the drive