(4-year PhD project with full EU funding (€18,500 p.a. + EU student fees))

Prof Ortwin Hess [OH], Prof J Donegan [JD] in collaboration with Prof Bert Hecht [BH] (University of Würzburg, Germany)

Please send any questions, expressions of interest and/or your cv to: Professor Ortwin Hess (ortwin.hess@tcd.ie).

Background. Plasmonic nanomaterials have the unique ability to confine light in extremely sub-wavelength volumes and massively enhance electromagnetic fields. For high enough field enhancement, one enters the strong-coupling regime, where the energy exchange between the excited states of molecules/materials and plasmons is faster than the de-coherence processes of the system. As a result, the excitonic state of the molecule becomes entangled with the photonic mode, forming hybrid excitonic-photonic states. These hybrid-states are part light, part matter and allow for the characteristic Rabi oscillations of the atomic excitations to be observed. Until recently, the conditions for achieving strong-coupling were most commonly met at cryogenic temperatures such that de-coherence processes are suppressed. As a major step forward, we have recently demonstrated room-temperature strong coupling single emitters1 to ultra-confined light fields in plasmonic resonators2 at ambient conditions. The fact that strong-coupling conditions may be reached at room temperature is of immense interest because it represents a clear route to a practical implementation of true quantum behaviour in nanophotonic systems.

Innovation. (I) In strongly ultra-confined fields couplings can become very strong. This can lead to extremely fast external energy transfer rates allowing to access e.g. the trion state in single quantum dots, which is usually quenched via the Auger effect, can be made radiative by coupling to a plasmonic nanoresonators2. This is particularly interesting for materials with highly mobile excitons that can easily be quenched at rims or defect states, e.g. small patches of 2D semiconductors, preparing the grounds for the development of ultrafast sources of indistinguishable photons if timescales of dephasing can be approached enabling room-temperature high-speed linear optical quantum computation. (II) The ultra-confinement of the optical near-field also fundamentally changes the very nature of how and on which scales strong coupling works: In classical far-field strong coupling the cavity mirrors are unaffected by the fields emitted by the atom. In near-field strong coupling, however, the resonator itself is strongly polarized by the emitter’s near-field due to the ultra-close proximity of the emitter and the cavity material.

Collaboration. The PhD project will embrace theory and simulation and will through collaboration be closely linked to nanophotonic and ultrafast photonics experiments conducted by BH’s group in Würzburg and JD’s group at TCD3 involving regular joint meetings and collaborative visits.

Program and Methodology. (1) Ultrafast quantum nanoplasmonic theory will use a full-wave Maxwell- Bloch approach4, embracing the (spatio-temporal) simulation of the ultrafast quantum dynamics of a single emitter which is self-consistently combined with a three-dimensional ultrafast spatio-temporal simulation of the optical fields on the basis of the (full-wave) Maxwell equations (on sub-nm and dynamics on sub-fs scales). Initially the simulation will involve a spatially resolved density-matrix approach. Subsequently, we will explore e.g. a self-consistent solution of the time-dependent Schrödinger equation linked to a spatio-temporal simulation of Maxwell’s equations. (2) Experiments. Experimental work performed in Würzburg in BH’s group will be linked with experiments at TCD and embrace scanning probe and ultrafast optics technology which will use specifically designed, low-radiative-loss plasmonic resonators as scanning probes.

References

1. Chikkaraddy, R. et al. Single-molecule strong coupling at room temperature in plasmonic nanocavities. Nature 535, 127–130 (2016).
2. Groß, H., Hamm, J. M., Tufarelli, T., Hess, O. & Hecht, B. Near-field strong coupling of single quantum dots. Science Advances 4, eaar4906 (2018).
3. Caridad, J. M. et al. Control of the plasmonic near-field in metallic nanohelices. Nanotechnology 29, 325204 (2018).
4. Kongsuwan, N. et al. Suppressed Quenching and Strong-Coupling of Purcell-Enhanced Single-Molecule Emission in Plasmonic Nanocavities. ACS Photonics 5, 186–191 (2018).

(4-year PhD project with full EU funding (€18,500 p.a. + EU student fees))

Prof Ortwin Hess [OH], Prof J Donegan [JD] in collaboration with Prof Bert Hecht [BH] (University of Würzburg, Germany) and Prof Franco Nori [FN] (RIKEN, Japan).

Please send any questions, expressions of interest and/or your cv to: Professor Ortwin Hess (ortwin.hess@tcd.ie).

Background. Ever since Richard Feynman’s suggestion that quantum systems could properly only be modelled by quantum systems/computers1 significant efforts have been made to computationally mimic quantum systems via algorithmic quantum simulation. However, real quantum systems are often too complex to be simulated on a classical computer. Quantum simulators, although not as general as a quantum computer (which would, of course, be an ideal quantum simulation platform), mimic a particular quantum system and will play a pivotal role in the study of quantum many-body physics. Indeed, it has been suggested that a ‘quantum computer’ (i.e. an analogue quantum simulator) can be physically implemented with cold ions confined in a linear trap interacting with laser beams2.

Innovation. Here, the project aims to innovatively conceive a nanoplasmonic Paul Trap for single electrons, propelling the original ultra-cold trapped-ion-based concept4 and new related realisations5 to room-temperatures and sub-wavelength (nano-) scales suitable for integration. Trapped electrons (not atoms, ions, etc) could then be used as tuneable elements in quantum computing schemes.

Collaboration. The project involves developing the theory and simulation, expanding on current finite-difference time-domain (FDTD) simulations of electron-beam spectroscopy of plasmonic nanoparticles6 and collaborate with FN on quantum simulators7. The nano-cross antenna (nXA) geometry8 will be realised using advanced nanostructure facilities at Würzburg. The project will be linked with nanophotonic and e-beam characterisation at TCD.

Program and Methodology. Planned theory/simulation will be closely linked with nano-technological realisation and experiments in Würzburg/Germany and at TCD. We will set up our theory on different levels: (I) effective semiclassical methods, (II) direct solution of the time-dependent Schrödinger equation, (III) quantum description with Janes-Cunnings Hamiltonian. Experiments at TCD and in Würzburg involve and profit from state-of the art clean-room nanofabrication and advanced electron microscopy techniques to manufacture and probe the plasmonic nanostructures. (1) Design and calculation/measurement of optical scattering cross-section and optical properties of a nXA via FDTD and frequency-domain methods. Determination of the electrical pseudo potential. (2) Experimental verification of the cross-antenna performance e.g. by time-resolved photo-electron emission. (3) Injection and manipulation of free electrons via applied DC voltages (field emission). (4) Demonstration that a Paul trap made up of a nano-cross antenna is capable of trapping a single electron thereby forming a “photonic quantum dot” with well-defined electronic quantum states. Electrons trapped in such optical potentials could be used as tuneable elements in quantum computing schemes.

References

1. Feynman, R. P. Simulating physics with computers. International Journal of Theoretical Physics 21, 467–488 (1982).
2. Cirac, J. I. & Zoller, P. Quantum Computations with Cold Trapped Ions. Physical Review Letters 74, 4091– 4094 (1995).
3. Acín, A. et al. The quantum technologies roadmap: a European community view. New Journal of Physics 20, 080201 (2018).
4. Lloyd, S. Universal Quantum Simulators. Science 273, 1073–1078 (1996).
5. Gross, C. & Bloch, I. Quantum simulations with ultracold atoms in optical lattices. Science 357, 995–1001 (2017).
6. Crai, A., Demetriadou, A. & Hess, O. Electron Beam Interrogation and Control of Ultrafast Plexcitonic Dynamics. ACS Photonics (2019) doi:10.1021/acsphotonics.9b01338.
7. Buluta, I. & Nori, F. Quantum Simulators. Science 326, 108–111 (2009).
8. Biagioni, P., Huang, J. S., Duò, L., Finazzi, M. & Hecht, B. Cross Resonant Optical Antenna. Physical Review Letters 102, (2009).

(4-year PhD project with full EU funding (€18,500 p.a. + EU student fees))

Prof Ortwin Hess [OH] and Prof J Donegan [JD] in collaboration with Prof Diana Huffaker [DH] (Cardiff University, UK).

Please send any questions, expressions of interest and/or your cv to: Professor Ortwin Hess (ortwin.hess@tcd.ie).

Background. Quantum entanglement occurs in compound quantum systems when spatially separated emitters share the same quantum state. The delocalization of entangled quantum states is key to unleashing the power of quantum technologies, e.g., it underlies superdense coding1 and reduces the need for quantum communication lines in quantum cryptography and quantum teleportation. Several key components for quantum technologies have been implemented by making use of enhanced interactions between photons and atoms in cavities with a high cooperativity, among which qubit state generators, qubit memories, and quantum gates relying on multiple excitation states. To scale the number of qubits up without losing efficiency or fidelity, there now is hightened interest in cavities that contain emitter pairs or emitters with multiple excitation pathways4 and atoms/molecules have been steadily replaced by solid-state emitters such as quantum dots5 and vacancy centres. Innovation. The project involves designing nanoplasmonic metal- semiconductor-metal (MIM) waveguides to demonstrate that slow-light physics can improve designs of (1) nanolasers and (2) on the basis of dissipation-driven entanglement6 store and entangle multiple qubits. Building on Maxwell-Bloch simulations of stopped-light lasing7 we will exploit ideas related to spin-momentum locking8, loss compensation, energy transfer among multiple emitters10, and of insights about the physics of tapered waveguides.

Collaboration. The PhD project will embrace theory and simulation on ‘rainbow-trapping’11 and stopped-light lasing7; it will through collaboration be closely linked to experiment12; and materials nanotechnology13.

Objectives and Methodology. The project involves design and modelling of nanophotonic quantum devices using ‘time-cavity’ concepts via slow/stopped light singularities together with solid-state emitters at room temperature based on metal- insulator-metal (MIM) waveguides. There are three possible foci which may be addressed sequentially or in parallel: (1) Spin-momentum locking for coherent emission and lasing. The aim is to enhance the capabilities of a slow-light laser by optimisation for SPP spin- momentum locking14 which arises because of the strong confinement of electromagnetic fields near metal-dielectric interfaces. (2) Cooperativity of slow-light singularities in a ‘time- cavity’. Modelling an active open cavity to determine its cooperativity, which depends on how efficiently SPP’s are emitted (spontaneous emission of photons) and how well surface-plasmon polaritons (SPP)’s are contained by the gain medium (outcoupling). If the cooperativity is high, a three-level emitter inside the cavity could be used as a quantum memory to store incident photons using Stimulated Raman Adiabatic Passage (STIRAP)15. (3)‘Time-cavity’ quantum dynamics and dissipation-driven entanglement. Depending on their separation, emitter pairs couple mainly through Coulomb interactions or radiatively through the exchange of SPP’s10. Our aim is to identify how the energy transfer is affected by the slow-light waveguide, both for weak coupling in the Förster regime and for strong coupling. The aim is to model and shed light on the conditions to control of hybridized emitter states with an external pump beam towards a deterministic preparation of qubits and entanglement1.

References

1. Nielsen, M. A. & Chuang, I. L. Quantum computation and quantum information. (Cambridge University Press, 2010).
2. Cirac, J. I., Zoller, P., Kimble, H. J. & Mabuchi, H. Quantum State Transfer and Entanglement Distribution among Distant Nodes in a Quantum Network. Phys. Rev. Lett. 78, 3221–3224 (1997).
3. Ritter, S. et al. An elementary quantum network of single atoms in optical cavities. Nature 484, 195–200 (2012).
4. Evans, R. E. et al. Photon-mediated interactions between quantum emitters in a diamond nanocavity. Science eaau4691 (2018) doi:10.1126/science.aau4691.
5. Groß, H., Hamm, J. M., Tufarelli, T., Hess, O. & Hecht, B. Near-field strong coupling of single quantum dots. Science Advances 4, eaar4906 (2018).
6. Hou, J., Słowik, K., Lederer, F. & Rockstuhl, C. Dissipation-driven entanglement between qubits mediated by plasmonic nanoantennas. Physical Review B 89, (2014).
7. Pickering, T., Hamm, J. M., Page, A. F., Wuestner, S. & Hess, O. Cavity-free plasmonic nanolasing enabled by dispersionless stopped light. Nature Communications 5, 4972 (2014).
8. Luo, S., He, L. & Li, M. Spin-momentum locked interaction between guided photons and surface electrons in topological insulators. Nature Communications 8, 2141 (2017).
9. Pusch, A., Wuestner, S., Hamm, J. M., Tsakmakidis, K. L. & Hess, O. Coherent Amplification and Noise in Gain-Enhanced Nanoplasmonic Metamaterials: A Maxwell-Bloch Langevin Approach. ACS Nano 6, 2420–2431 (2012).
10. Andrew, P. Energy Transfer Across a Metal Film Mediated by Surface Plasmon Polaritons. Science 306, 1002–1005 (2004).
11. Tsakmakidis, K. L., Boardman, A. D. & Hess, O. ‘Trapped rainbow’ storage of light in metamaterials. Nature 450, 397–401 (2007).
12. Bello, F. et al. Combining ε -Near-Zero Behavior and Stopped Light Energy Bands for Ultra-Low Reflection and Reduced Dispersion of Slow Light. Scientific Reports 7, 8702 (2017).
13. Gao, J. et al. Strongly coupled slow-light polaritons in one-dimensional disordered localized states. Scientific Reports 3, 1994 (2013).
14. Lodahl, P. et al. Chiral quantum optics. Nature 541, 473–480 (2017).
15. Reiserer, A. & Rempe, G. Cavity-based quantum networks with single atoms and optical photons. Reviews of Modern Physics 87, 1379–1418 (2015).

(4-year PhD project with full EU funding (€18,500 p.a. + EU student fees))

Prof Ortwin Hess [OH], Prof L Bradley and Prof J Donegan [JD] in collaboration with Prof Paivi Torma [PT] (Aalto University, Helsinki, Finland).

Please send any questions, expressions of interest and/or your cv to: Professor Ortwin Hess (ortwin.hess@tcd.ie).

Background. In a topological laser, the laser light would not scatter from imperfections and sharp edges in the same way as usual laser light. This allows creating, for example, a laser cavity of any shape, and possibly of very small size. This is crucial for on-chip integrated optics in the micro- and even nano-scales. The present internet is based on optical communication; optics offers vast bandwidths for data and, in principle, over a thousand times faster processing speeds than electronics. Yet, using an optics-based internet for transmission of photonic quantum information, processing and sensing is not yet sufficiently realized because optical components and interconnects cannot be miniaturized and shield quantum information to the same extent as electronics with present technology. Novel nanomaterials and device concepts, such as, in particular, nanoplasmonic metasurface environments with topological protection for lasing and quantum photonic communication that enable miniaturized and integrated photonic and quantum photonic communication are disruptive.

Collaboration.The PhD project will embrace theory and simulation and will through collaboration be closely linked to experiments and fabricating complex hybrid nanoplasmonic arrays of varying size and gaps as well as fabrication of lasing structures made using state-of-the-art nanolithography at TCD and at Aalto University /Finland and offers frequent active interchange between the groups.

Innovation. The first topological laser was realized only very recently1 but it is not based on nanostructures and offers limited possibilities for miniaturization. Here we will pursue an alternative route by utilizing metal nanoparticles that capture light on length scales that are well below the wavelength of light forming a radiatively coupled active metasurface. Moreover, as topological properties typically apply for low-energy excitations it is thus interesting to explore how non- linear phenomena such as lasing and condensation behave in a system that is topologically non- trivial in the linear regime.

Program and Methodology. We combine theoretical analysis based on group-theory2 with full-wave Maxwell- Bloch simulations and with experiments. (1) Group theory3 for radiatively coupled metasurfaces. As our plasmonic metasurface is a radiatively coupled system where all nanoparticles feel the response of all others, forming collective modes. Commonly used tight-binding models are thus invalidated., but we can base our theoretical description on group symmetry arguments and T-matrix scattering simulations. The symmetry properties of the lattice dictate the existence of energy degenerate modes at high-symmetry points of the Brillouin zone, for which the lifting of the degeneracy by a symmetry breaking mechanism can lead to topological features. (2) Topological magnetic metasurfaces. We will model via Maxwell- Bloch simulations and via collaboration with PT experimentally create topological effects by using nanoscale magnetic materials, which will be a breakthrough in basic research but also offer application prospects beyond demonstrating the concept. (3) Quantum Metric. Following Bleu et al.4, the Berry curvature and quantum metric can be extracted out of a more complex polarization analysis. The quantum metric and Berry curvature are the real and imaginary parts of the quantum geometric tensor, and while the latter has already proven to be a central concept of modern physics, the physical significance of the former is only emerging. The simulations and the experiment proposed here would be the first ever observation of the quantum metric.

References

1. Bahari, B. et al. Nonreciprocal lasing in topological cavities of arbitrary geometries. Science 358, 636–640 (2017).
2. Saba, M., Hamm, J. M., Baumberg, J. J. & Hess, O. Group Theoretical Route to Deterministic Weyl Points in Chiral Photonic Lattices. Physical Review Letters 119, (2017).
3. Saba, M., Wong, S., Elman, M., Oh, S. S. & Hess, O. Nature of topological protection in photonic spin and valley Hall insulators. Phys. Rev. B 101, 054307 (2020).
4. Bleu, O., Solnyshkov, D. D. & Malpuech, G. Measuring the quantum geometric tensor in two-dimensional photonic and exciton-polariton systems. Physical Review B 97, (2018).