12 research outputs found
Collective coherence in planar semiconductor microcavities
Semiconductor microcavities, in which strong coupling of excitons to confined
photon modes leads to the formation of exciton-polariton modes, have
increasingly become a focus for the study of spontaneous coherence, lasing, and
condensation in solid state systems. This review discusses the significant
experimental progress to date, the phenomena associated with coherence which
have been observed, and also discusses in some detail the different theoretical
models that have been used to study such systems. We consider both the case of
non-resonant pumping, in which coherence may spontaneously arise, and the
related topics of resonant pumping, and the optical parametric oscillator.Comment: 46 pages, 12 figure
Sculpting oscillators with light within a nonlinear quantum fluid
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 Physic
Coexistence of low threshold lasing and strong coupling in microcavities
We report the coexistence of low threshold lasing and strong coupling in a high-quality semiconductor microcavity under near-resonant optical pumping. A sharp laser mode splits from the lower-polariton branch and approaches the bare cavity mode frequency as the pump power increases. The lasing is produced by low density localized exciton states, which are weakly coupled to the cavity mode. The appearance of this lasing mode distinguishes quantum-well excitons into those which are strongly or weakly coupled with the cavity mode
Tuning the energy of a polariton condensate via bias-controlled Rabi splitting
We introduce an electrically driven scheme to tune the polariton condensate energy in a high-finesse GaAs microcavity. In contrast to the conventional redshift observed in semiconductor quantum wells (QWs) under applied electrical bias arising from the quantum-confined Stark effect (QCSE), we report here the blueshift of a polariton condensate caused by controlled reduction of the Rabi splitting due to tunneling-induced charge buildup and fractional bleaching of QWs. At larger electrical bias, the QCSE becomes dominant, leading to a redshift in the linear regime, while in the nonlinear regime to the eventual quenching of the condensate emission. This ability to tune the polariton condensate energy brings within reach the realization of voltage-controlled polariton condensate devices and variable-wavelength sources of coherent light
Teaching polaritons new tricks
Semiconductor microcavities have attracted much recent interest because they utilize simultaneously 2D confinement of both excitons and photons in the same heterostructure. Strong coupling of these two states produces unique dynamics that can be well described in a quasiparticle state, the cavity polaritons. Their dispersion relation is dramatically modified with an apparent trap in k-space offering exciting new possibilities for tailoring nonlinear optical properties in microcavities. The reduced density of states inside the trap allows the macroscopic occupancy of polaritons producing much of the new physics. This paper describes recent work on nonlinear effects in semiconductor microcavities including stimulated scattering, parametric oscillation and non-equilibrium phase transition. By teaching polaritons new tricks, both fundamental questions about their bosonic nature can be answered and practical applications in a variety of optoelectronic and interferometric devices can be found
Stimulated spin dynamics of polaritons in semiconductor microcavities
Time-resolved polarization spectroscopy of polariton pair scattering in semiconductor microcavities enables complete measurement of the polariton spin dynamics. In addition to spin-preserving interactions previously reported, we observe two additional even stronger scattering processes, which mix polaritons of opposite spin. Because of the polaritons' bosonic character, this results in the stimulation of spin flips. Such mechanisms should allow realization of spin-sensitive interferometers