Coinage metal nanostructures support localised surface plasmons, which confine optical fields much tighter than their wavelength (1). This extreme enhancement enables vibrational spectroscopy within small volumes, even down to single molecules (2,3). For many years lateral resolution was believed to be 10 nm (4), however recent experiments resolve the atomic structure of single molecules using tipenhanced Raman spectroscopy (3) and directly sequence RNA strands (5). Atomistic simulations also suggest plasmonic confinement to atomic scales is possible (6). Here we show that light-activated mobilisation of surface atoms in a plasmonic hotspot triggers the formation of additional 'picocavities'bounded by a single gold atom. Their ultra-small light localisation alters which vibrational modes of trapped molecules are observed, due to strong optical field gradients that switch the Raman selection rules. The resulting cascaded ultra-strong plasmonic confinement pumps specific molecular bonds, thereby creating non-thermal vibrational populations, and forms a new type of optomechanical

Semiconductor microcavities are used to support freely flowing polariton quantum liquids allowing the direct observation and optical manipulation of macroscopic quantum states. Incoherent optical excitation at a point produces radially expanding condensate clouds within the planar geometry. By using arbitrary configurations of multiple pump spots, we discover a geometrically controlled phase transition, switching from the coherent phase-locking of multiple condensates to the formation of a single trapped condensate. The condensation threshold becomes strongly dependent on the programmed superfluid geometry and sensitive to cooperative interactions between condensates. We directly image persistently circulating superfluid and show how flows of light-matter quasiparticles are dominated by the quantum pressure in such configurable laser-written potential landscapes.

We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatiallytrapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two ellipticallypolarized (up to 95% circularly-polarized) states with opposite handedness of polarization. The magnetized condensate remains stable for many seconds at 5 K, but at higher temperatures it can flip from one magnetic orientation to another. We optically address these states and demonstrate the inversion of the magnetic state by resonantly injecting 100-fold weaker pulses of opposite spin. Theoretically, these phenomena can be well described as spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations.Condensation of exciton-polaritons (polaritons) spontaneously breaks the global phase symmetry [1][2][3][4][5]. Owing to their easy optical interrogation, high-speed (ps) interactions, and macroscopic coherence (over hundreds of microns) [6], polariton condensates are excellent candidates to probe and exploit for sensing [7,8], spinoptronics [9][10][11], new optoelectronic devices [12][13][14], and quantum simulators [15]. The driven-dissipative multicomponent polariton system can undergo additional bifurcations and condense into states which are not eigenstates of the single-particle Hamiltonian, but many-body states with reduced symmetry [16,17]. Thus, we should expect that two-component exciton-polariton condensates can also show spontaneous symmetry breaking bifurcations in their polarization state. Spin studies of microcavity polaritons have been of great interest in recent years [18][19][20][21][22][23][24][25][26][27][28][29]. However, spontaneous symmetry-breaking bifurcation of spin has not been observed before.Here, we demonstrate spontaneous magnetization in an exciton-polariton condensate, as a direct result of bifurcations in the spin degree of freedom. Utilizing an optically trapped geometry, condensates spontaneously emerge in either of two discrete spin-polarized states that are stable for many seconds, > 10 10 longer than their formation time. These states emit highly circularly-polarized coherent light (up to 95%) and have opposite circular polarizations. The condensate stochastically condenses in a left-or right-circularly polarized state, with an occurrence likelihood that can be controlled by the ellipticity * ho278@cam.ac.uk † jjb12@cam.ac.uk of the nonresonant pump. The two spin-polarized states can be initialized and switched from one state to another with weak resonant optical pulses. Our system has potential applications in sensing, optical spin memories and spin switches, and it can be implemented for studying long-range spin interactions in polariton condensate lattices. This article is structured as follows: in Section I we review trapped polariton condensates and the current understanding of polarization in untrapped polariton condensates. In Section ...

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.interferometer | rings | BEC | SQUID C ircular loops are a key geometry for superfluid and superconducting devices because rotation around a closed ring is coupled to the phase of a quantum wavefunction; so far, however, they have not been optically accessible, although this would enable a new class of quantum devices, particularly if room temperature condensate operation is achieved.In lasing systems with an imposed circular symmetry, an annulus of lasing spots can sometimes form along the perimeter of the structure (1-6). Such transverse modes are often referred to as "petal states" (1) or "daisy modes" (2) and are interpreted as annular standing waves (3), whispering gallery modes (4), or coherent superpositions of Laguerre-Gauss (LG) modes with zero radial index (5, 6). Their circular symmetry makes them interesting for numerous applications such as free space communication or fiber coupling (7), and their LG-type structure suggests implementations using the orbital angular momentum of light (8), such as optical trapping (9) or quantum information processing (10). Petal states have been reported for various conventional lasing systems, including electrically and optically pumped vertical cavity surface-emitting lasers (VCSELs) (2, 4), as well as microchip (6) and rod lasers (1).A fundamentally different type of lasing system is the polariton laser (11,12). Polaritons are bosonic quasiparticles, resulting from the strong coupling between microcavity photons and semiconductor excitons (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Their small effective mass (bestowed by their photonic component) and strong interactions (arising from their excitonic component) favor Bose-stimulated condensation into a single quantum state, called a polariton condensate (14,15). These full...

Practical challenges to extrapolating Moore's law favour alternatives to electrons as information carriers. Two promising candidates are spin-based and all-optical architectures, the former offering lower energy consumption, the latter superior signal transfer down to the level of chip-interconnects. Polaritons-spinor quasi-particles composed of semiconductor excitons and microcavity photons-directly couple exciton spins and photon polarizations, combining the advantages of both approaches. However, their implementation for spintronics has been hindered because polariton spins can be manipulated only optically or by strong magnetic fields. Here we use an external electric field to directly control the spin of a polariton condensate, bias-tuning the emission polarization. The nonlinear spin dynamics offers an alternative route to switching, allowing us to realize an electrical spin-switch exhibiting ultralow switching energies below 0.5 fJ. Our results lay the foundation for development of devices based on the electro-optical control of coherent spin ensembles on a chip.

We introduce and analyze a novel mean-field model for polariton condensates with velocity dependence of the effective polariton mass due the photon and exciton components. The effective mass depends on the in-plane wave vector k, which at the inflection point of the lower polariton energy branch becomes infinite and above negative. The polariton condensate modes of the new mean-field theory are now sensitive to mass variations and for certain points of the energy dispersion the polariton condensate mode represents fractional quantum mechanics. The impact of the generalized kinetic energy term is elucidated by numerical studies in 1D and 2D showing significant differences for large velocities. Analytical expressions for plane wave solutions as well as a linear waves analysis show the significance of this new model.

Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin-polarize spontaneously. We observe a crossover from an antiferromagneticto a ferromagnetic pair state by reducing the coupling barrier in real-time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.The development of spin-charge lattice models for understanding strongly-correlated states of matter is a successful theme of modern quantum physics. This has driven the desire to model and probe complex condensed matter phenomena using highly-controlled systems, such as ultracold atoms [1], photons [2,3], or superconducting junctions [4]. Exciton-polariton (polariton) lattices have emerged as an alternative system [5, 6] with unique properties. Due to their strongly dissipative and nonlinear nature, many-body polariton gases can reach steady states which are remarkably different from their equilibrium case [7]. Moreover, they have peculiar spin properties [8][9][10] and exhibit spontaneous magnetization (emitting circularly polarized light) above a critical bifurcation threshold [11], analogous to the weak lasing regime [12]. In this Letter we study the basic building block of a polariton spin lattice: two optically trapped spin-polarized condensates which are tunably coupled. We demonstrate that trapped out-of-equilibrium polariton condensates can exhibit Ising-like behavior related to spin bifurcations. The two condensate system investigated here is shown to correspond to one plaquette of a bosonic ladder [13], and allows demonstration of a crossover in the competition between Josephson coupling and spin bifurcation. These features have not been seen in any other system to date.Polariton condensates are coherent many-body states [14][15][16][17], which can be confined in potentials [18][19][20] and interact with each other via Josephson junctions [21][22][23][24]. For a pair of interacting trapped spin-polarized condensates, their polarization states are expected to couple. However, the driven-dissipative and nonlinear nature of polariton condensates makes * ho278@cam.ac.uk † jjb12@cam.ac.uk the underlying coupling mechanism considerably richer than that in the conventional Ising case, leading to exotic forms of magnetism where the orientation and strength of coupling is not determined by the sign of the interaction.We achieve tuning between ferromagnetic (F) or antiferromagnetic (AF) coupling by directly modulating the tunneling barrier, adjusting either the height of the barrier or the separation between the condensates. We show arXiv:1603.02830v1 [cond-mat.other]

Site-controlled growth of quantum dots (QDs) for single photon emitters (SPEs) is achieved applying a buried stressor approach. Theoretical and experimental analysis shows that site-controlled QD growth on buried oxide stressor-layers benefits enormously from a defect-free growth interface. Laterally modulated strain fields at GaAs(001) growth surfaces are used to tailor surface morphologies at the centre of prescribed mesa structures for subsequent QD growth. Suitable morphologies for site-controlled QD growth such as nano-hillocks and nanoholes are identified. Site-controlled QD growth appears above the boundaries between the oxidised layer and the non-oxidised semiconductor layer. Through fine tuning of wetting layer thickness and growth interruption high selectivity for QD nucleation is achieved. Thus, growth of single QDs at the centre of a current-injection limiting aperture is demonstrated. Moreover, the QD growth on a defect-free surface yields high quality optical properties in terms of narrow emission linewidth and temporal stability with no discernible difference to QDs grown on planar substrates. The technological simplicity of the buried stressor approach and the inherent integration of a current aperture for efficient carrier injection into site-selected QDs enable mass production of SPEs on large substrate sizes.