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14 results on '"Christmann, G."'

1. Sculpting oscillators with light within a nonlinear quantum fluid

Author

Tosi, G., Christmann, G., Berloff, N. G., Tsotsis, P., Gao, T., Hatzopoulos, Z., Savvidis, P. G., and Baumberg, J. J.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Quantum Gases, Quantum Physics
Abstract

Seeing macroscopic quantum states directly remains an elusive goal. Particles with boson symmetry can condense into such quantum fluids producing rich physical phenomena as well as proven potential for interferometric devices [1-10]. However direct imaging of such quantum states is only fleetingly possible in high-vacuum ultracold atomic condensates, and not in superconductors. Recent condensation of solid state polariton quasiparticles, built from mixing semiconductor excitons with microcavity photons, offers monolithic devices capable of supporting room temperature quantum states [11-14] that exhibit superfluid behaviour [15,16]. Here we use microcavities on a semiconductor chip supporting two-dimensional polariton condensates to directly visualise the formation of a spontaneously oscillating quantum fluid. This system is created on the fly by injecting polaritons at two or more spatially-separated pump spots. Although oscillating at tuneable THz-scale frequencies, a simple optical microscope can be used to directly image their stable archetypal quantum oscillator wavefunctions in real space. The self-repulsion of polaritons provides a solid state quasiparticle that is so nonlinear as to modify its own potential. Interference in time and space reveals the condensate wavepackets arise from non-equilibrium solitons. Control of such polariton condensate wavepackets demonstrates great potential for integrated semiconductor-based condensate devices., Comment: accepted in Nature Physics

Published
2011
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2. Bragg Polaritons: Strong Coupling and Amplification in an Unfolded Microcavity

Author

Askitopoulos, A., Mouchliadis, L., Iorsh, I., Christmann, G., Baumberg, J. J., Kaliteevski, M. A., Hatzopoulos, Z., and Savvidis, P. G.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Periodic incorporation of quantum wells inside a one--dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic bandgap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification., Comment: 4 pages, 4 figures

Published
2010
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3. Spontaneous Polarisation Build up in a Room Temperature Polariton Laser

Author

Baumberg, J. J., Kavokin, A. V., Christopoulos, S., Grundy, A. J. D., Butte, R., Christmann, G., Solnyshkov, D. D., Malpuech, G., von Hogersthal, G. Baldassarri Hoger, Feltin, E., Carlin, J. -F., and Grandjean, N.

Subjects
Condensed Matter - Materials Science
Abstract

We observe the build up of strong (~50%) spontaneous vector polarisation in emission from a GaN-based polariton laser excited by short optical pulses at room temperature. The Stokes vector of emitted light changes its orientation randomly from one excitation pulse to another, so that the time-integrated polarisation remains zero. This behaviour is completely different to any previous laser. We interpret this observation in terms of the spontaneous symmetry breaking in a Bose-Einstein condensate of exciton-polaritons.

Published
2008
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4. Over the counter. Configuration and refiguration of ticket-sales conversation through institutional architectures for interaction

Author

Christmann, Gabriela B, Knoblauch, Hubert, Löw, Martina, Christmann, G B ( Gabriela B ), Knoblauch, H ( Hubert ), Löw, M ( Martina ), Hausendorf, Heiko, Christmann, Gabriela B, Knoblauch, Hubert, Löw, Martina, Christmann, G B ( Gabriela B ), Knoblauch, H ( Hubert ), Löw, M ( Martina ), and Hausendorf, Heiko

Abstract

How is space communicatively achieved in face-to-face interaction and how can we account for the means and forms of such an achievement of space among those present? Answering these questions within an ethnomethodological framework, we propose the assumption of a general interactional problem of situational anchoring; that is, of a participant’s task to ascertain and to communicate the actually relevant aspects of their spatial surroundings. In solving this problem, space becomes interactively achieved as “interactional space”. Interactional space draws heavily on resources prior to the ongoing interaction. Among these are architectures for interaction, which provide a rich and powerful set of usability cues for participants. Especially within institutionalized communication, participants systematically utilize architectural manifestations in terms of buildings, settings, furnishings, and technologies. This abstract view is elaborated on with evidence from data of over-the-counter ticket-sales conversation, which is currently undergoing a process of significant refiguration affecting architectural, bodily, and verbal resources of situational anchoring.

Published
2022

6. Influence of multi-exciton correlations on nonlinear polariton dynamics in semiconductor microcavities

Author

Massachusetts Institute of Technology. Department of Chemistry, Wen, P., Nelson, Keith Adam, Christmann, G., Baumberg, J. J., Massachusetts Institute of Technology. Department of Chemistry, Wen, P., Nelson, Keith Adam, Christmann, G., and Baumberg, J. J.

Abstract

Using two-dimensional spectroscopy, we resolve multi-polariton coherences in quantum wells embedded inside a semiconductor microcavity and elucidate how multi-exciton correlations mediate polariton nonlinear dynamics. We find that polariton correlation strengths depend on spectral overlap with the biexciton resonance and that up to at least four polaritons can be correlated, a higher-order correlation than observed to date among excitons in bare quantum wells. The high-order correlations can be attributed to coupling through the cavity mode, although the role of high-order Coulomb correlations cannot be excluded.

Published
2013

7. Geometrically locked vortex lattices in semiconductor quantum fluids

Author

Tosi, G, Christmann, G, Berloff, NG, Tsotsis, P, Gao, Tingge, Hatzopoulos, Z, Savvidis, P.G, Baumberg, J.J, Tosi, G, Christmann, G, Berloff, NG, Tsotsis, P, Gao, Tingge, Hatzopoulos, Z, Savvidis, P.G, and Baumberg, J.J

Abstract

Macroscopic quantum states can be easily created and manipulated within semiconductor microcavity chips using exciton-photon quasiparticles called polaritons. Besides being a new platform for technology, polaritons have proven to be ideal systems to study out-of-equilibrium condensates. Here we harness the photonic component of such a semiconductor quantum fluid to measure its coherent wavefunction on macroscopic scales. Polaritons originating from separated and independent incoherently pumped spots are shown to phase-lock only in high-quality microcavities, producing up to 100 vortices and antivortices that extend over tens of microns across the sample and remain locked for many minutes. The resultant regular vortex lattices are highly sensitive to the optically imposed geometry, with modulational instabilities present only in square and not triangular lattices. Such systems describe the optical equivalents to one- and two-dimensional spin systems with (anti)-ferromagnetic interactions controlled by their symmetry, which can be reconfigured on the fly, paving the way to widespread applications in the control of quantum fluidic circuits.

Published
2012

11. Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

Author

Corfdir, P., Levrat, J., Rossbach, G., Butte, R., Feltin, E., Carlin, J.-F., Christmann, G., Lefebvre, P., Ganiere, J.-D., Grandjean, N., and Deveaud-Pledran, B.

Subjects
Condensed Matter::Quantum Gases, Suppression, Condensed Matter::Other, Physics::Optics, Field, Exciton-Polaritons, Binding, Condensed Matter::Mesoscopic Systems and Quantum Hall Effect, Bottleneck, Semiconductor Microcavities, Scattering, Condensed Matter::Materials Science, Localization, Amplifier, Radiative Lifetimes
Abstract

We report on the direct observation of biexcitons in a III-nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.

12. High Mobility IWO for Improved Current in Heterojunction Technology Solar Cells

Author

Christmann, G

Subjects
7. Clean energy, IWO, TCO, heterojunction solar cell
Abstract

Silicon heterojunction technology (HJT) enables record efficiencies for silicon-based solar cells [1]. The main advantage is the excellent surface passivation offered by hydrogenated amorphous silicon (a-Si:H), enabling record open circuit voltages. On the other hand, the need to use transparent conducting oxides for contacting and current spreading combined with a-Si:H leads to parasitic absorption and thus sub optimal short circuit current (JSC) for both side contacted solar cells. As a result, the advantage of this technology versus competing ones is particularly evident for back contacted solar cells [1]. However, thanks to the reduced complexity, and thus lower cost per watt, both side contacted solar cells have a much higher market penetration. It is therefore highly desirable to reduce as much as possible this parasitic absorption. In this context, high mobility TCOs like hydrogen doped indium oxide (IO:H) offers mobilities in excess of 100 cm2/(V.s) and enables higher JSC than use of the industrial standard indium tin oxide (ITO) [2]. Process control, in particular the incorporation of hydrogen, is however challenging with this material. We propose here the use of indium tungsten oxide (IWO) deposited by RF sputtering as a high mobility TCO for use in HJT solar cells. Preparing this TCO in standard conditions, i.e. with a base pressure below 5 E-7 mbar in our Oerlikon Clusterline tool already leads to mobilities higher than our best ITO process (>50 vs ~30 cm2/(V.s)) for conditions compatible with HJT solar cell processing. Furthermore, by controlling the chamber base pressure and using only argon and oxygen as reactive gases we are able to tailor the material properties to even higher mobilities (>80 cm2/(V.s)) without needing a water source or hydrogen line for doping. These layers were successfully included in HJT solar cells as evidenced by the current and efficiency gains associated with the use of this TCO.

14. Coupled counterrotating polariton condensates in optically defined annular potentials.

Author

Dreismann A, Cristofolini P, Balili R, Christmann G, Pinsker F, Berloff NG, Hatzopoulos Z, Savvidis PG, and Baumberg JJ

Abstract

Polariton condensates are macroscopic quantum states formed by half-matter half-light quasiparticles, thus connecting the phenomena of atomic Bose-Einstein condensation, superfluidity, and photon lasing. Here we report the spontaneous formation of such condensates in programmable potential landscapes generated by two concentric circles of light. The imposed geometry supports the emergence of annular states that extend up to 100 μm, yet are fully coherent and exhibit a spatial structure that remains stable for minutes at a time. These states exhibit a petal-like intensity distribution arising due to the interaction of two superfluids counterpropagating in the circular waveguide defined by the optical potential. In stark contrast to annular modes in conventional lasing systems, the resulting standing wave patterns exhibit only minimal overlap with the pump laser itself. We theoretically describe the system using a complex Ginzburg-Landau equation, which indicates why the condensate wants to rotate. Experimentally, we demonstrate the ability to precisely control the structure of the petal condensates both by carefully modifying the excitation geometry as well as perturbing the system on ultrafast timescales to reveal unexpected superfluid dynamics.

Published
2014
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