P. G. Savvidis

A GaAs polariton light-emitting diode operating near room temperature

The increasing ability to control light–matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons—such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity—are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.

Sculpting oscillators with light within a nonlinear quantum fluid

Polaritons—quasiparticles made up of a photon and exciton strongly coupled together—can form macroscopic quantum states even at room temperature. Now these so-called condensates are imaged directly. This achievement could aid the development of semiconductor-based polariton-condensate devices.

Polariton-mediated energy transfer between organic dyes in a strongly coupled optical microcavity

Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light-matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes.

Polariton condensate transistor switch

A polariton condensate transistor switch is realized through optical excitation of a microcavity ridge with two beams. The ballistically ejected polaritons from a condensate formed at the source are gated using the 20 times weaker second beam to switch on and off the flux of polaritons. In the absence of the gate beam the small built-in detuning creates a potential landscape in which ejected polaritons are channelled toward the end of the ridge where they condense. The low-loss photonlike propagation combined with strong nonlinearities associated with their excitonic component makes polariton-based transistors particularly attractive for the implementation of all-optical integrated circuits.

Coupling Quantum Tunneling with Cavity Photons

Tunneling Through with a Light Touch Quantum tunneling underpins a host of essential techniques, such as scanning tunneling microscopy and quantum cascade lasers, as well as chemical reactions. The tunneling particles are normally electrons, and control of the tunneling process has generally been by electric fields. By coupling tunneling electrons with cavity photons trapped inside a semiconductor microcavity, Cristofolini et al. (p. 704, published online 5 April; see the Perspective by Szymańska) produced mixed states that then allowed direct optical control of the tunneling process. Such an optical-based approach to manipulating and controlling the tunneling process may find applications in quantum information science. Optical coupling is used to control the tunneling of electrons between a pair of quantum wells. Tunneling of electrons through a potential barrier is fundamental to chemical reactions, electronic transport in semiconductors and superconductors, magnetism, and devices such as terahertz oscillators. Whereas tunneling is typically controlled by electric fields, a completely different approach is to bind electrons into bosonic quasiparticles with a photonic component. Quasiparticles made of such light-matter microcavity polaritons have recently been demonstrated to Bose-condense into superfluids, whereas spatially separated Coulomb-bound electrons and holes possess strong dipole interactions. We use tunneling polaritons to connect these two realms, producing bosonic quasiparticles with static dipole moments. Our resulting three-state system yields dark polaritons analogous to those in atomic systems or optical waveguides, thereby offering new possibilities for electromagnetically induced transparency, room-temperature condensation, and adiabatic photon-to-electron transfer.

All-dielectric GaN microcavity: Strong coupling and lasing at room temperature

The strong light-matter coupling regime and lasing in a GaN microcavity fabricated by incorporating a high optical quality GaN membrane inside an all-dielectric mirror cavity is demonstrated at room temperature. A nonlinear increase of the emission and line narrowing marks the onset of polariton lasing regime with significantly reduced threshold compared with previous reports for bulk GaN microcavity. This combination of low lasing thresholds and ease of fabrication allows incorporation of quantum wells and electrical contacts into the active region, paving the way for electrically driven room temperature (RT) polariton laser devices.

Nonlinear optical spin hall effect and long-range spin transport in polariton lasers

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.

Bragg polaritons: strong coupling and amplification in an unfolded microcavity.

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 band gap, 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.

Polariton condensation in an optically induced two-dimensional potential

We demonstrate experimentally the condensation of exciton polaritons through optical trapping. The nonresonant pump profile is shaped into a ring and projected to a high quality factor microcavity where it forms a two-dimensional repulsive optical potential originating from the interactions of polaritons with the excitonic reservoir. Increasing the population of particles in the trap eventually leads to the emergence of a confined polariton condensate that is spatially decoupled from the decoherence inducing reservoir, before any buildup of coherence on the excitation region. In a reference experiment, where the trapping mechanism is switched off by changing the excitation intensity profile, polariton condensation takes place for excitation densities more than two times higher and the resulting condensate is subject to much stronger dephasing and depletion processes.

Terahertz circular dichroism spectroscopy of biomolecules

Biopolymers such as proteins, DNA and RNA fold into large, macromolecular chiral structures. As charged macromolecules, they absorb strongly in the terahertz due to large-scale collective vibrational modes; as chiral objects, this absorption should be coupled with significant circular dichroism. Terahertz circular dichroism (TCD) is potentially important as a biospecific sensor, unobscured by spectral features related to abiological material. We have constructed atomistic simulations and elastic continuum models of TCD. These models estimate the magnitude of the TCD and the relation between TCD spectroscopic signatures (zero crossings) and the structure, charge distribution and mechanical properties of biomaterials. A broad band TCD spectrometer based on a polarizing interferometer is developed to explore TCD in biomolecules in aqueous solution. Preliminary results on TCD in lysozyme in water at several terahertz frequencies is presented.