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[Postdoc and Ph.D. positions available in both experimental and theoretical physics.]

Phonon-polaritons

Strong coupling between an electromagnetic wave and a solid-state polar excitation yields a hybrid bosonic quasiparticle known as a polariton. The mixed light-matter character of polaritons yields a range of generative characteristics with remarkable consequence for controlling electromagnetic interactions. In particular, phonon-polaritons (phPols), resulting from the coupling of a polar transverse optical phonon with a photon, in highly anisotropic nanostructures- such as van der Walls (vdW) crystals– exhibit a hyperbolic dispersion relation at frequencies where permittivity tensor elements have opposite signs [2], providing a means to create large wavevector states not bounded by frequency, and thereby enabling extreme energy concentration, large enhancements of the local density of states, sub- diffractional imaging, propagation control, extreme non-linearity, and topologically protected states. This rich array of functionalities is presently being developed to implement infrared nanophotonic circuits.

New regimes of light-matter interactions

Our interest in phPol is motivated by their capacity to provide access to a new regime of light-matter interactions, where intrinsically weak processes, such as two-photon emission or quadrupolar transitions, become dominant radiative emission channels.

New and powerful approach to studying and engineering phonon-polaritons

Despite their considerable interest, the science of phPols is still in its early stages and the set of tools available to engineer their properties is limited. Until present, the exploration of phPol has relied extensively on absorption probed through scattering-type scanning near-field optical microscopy (s-SNOM). This advanced technique provides detailed images of propagating phPol as a function of frequency, and in turn the omega-k polariton dispersion relation. Crucially, this technique for investigating phPols require specialized, sophisticated and expensive, instruments. In part, this can be explained by the large wavevector mismatch between free-space photons and confined polaritons, which requires a special means (prisms, gratings, or metalized tips) to couple far field instrumentation such as light sources and detectors in these absorption-based experiments.

Very recently, we have demonstrated that Raman spectroscopy can also be an extremely powerful technique for probing phPols in non-centrosymmetric vdW crystals. Owing to the relaxation of selection rules and the deep sub-wavelength confinement in thin samples, dispersion curves, confinement, and interactions with excitonic resonances can be studied in a convenient backscattering configuration without the need for any wavevector matching strategies. Moreover, Raman spectroscopy maps low energy excitations (5-150 meV) into the visible range, where thermal background is negligible and where detection with single-photon sensitivity is readily available.<\p>

Objective

Our research objective is to further push Raman spectroscopy as a transformational and practical tool for the investigation of PhPols and exploit it to acquire the knowledge and expertise to engineer confined phPol that will enable novel light sources whose emission process is not limited to the electric dipolar operator. Please contact us!

Postdoc and Ph.D. positions available in experimental physics.

All-enabling entanglement

Entanglement is the exceptional quantum resource upon which most quantum technologies are built. To generate entanglement, quantum emitters rely on a quantum memory (a spin for example) which must exhibit exceptional coherence. This preserves spin-photon entanglement over long times and allows generating the long chains of entangled photons required for measurement-based quantum computing, quantum repeaters, quantum spectroscopy and precision metrology. Unfortunately, fluctuations and disorder present in materials randomize the phase of the quantum memory, compromise its coherence time and prevent multipartite entanglement.

Ultra-quiet solid-state environment

Hence, it is important to develop materials that suppress all processes limiting quantum emitter coherence, which include charge noise, spin noise, and phonon induced dephasing. In terms of coherence, Si is one of the most desirable material platforms: it is one most perfected material ever synthesized and isotopic selection is possible. These aspects explain the record-breaking coherence of electron spin (several minutes) and nuclear spins (39 minutes at 300K!). However, Si is an indirect gap material and its capacity to emit light is dismal.

Objective

The objective of this research trust is to develop highly coherent and bright quantum emitters based on isotopically engineered and direct gap germanium.

Rational

Just like Si, Ge is one of the highest purity materials. Relative impurity concentration of less than 1e9 reduces charge noise to the lowest levels. Isotopic enrichment above 99.9% suppresses mass disorder and mitigates decoherence processes mediated by phonons. By selecting an isotope with no net nuclear spin (74Ge), we get rid of nuclear spin disorder and fluctuations, eliminating the most important decoherence mechanisms found in most semiconductors. Hence, Ge should provide coherence times approaching that of Si.

Quantum emitters

Quantum emitters require localized electronic states. Similarly to many major material platforms (diamond, SiC, BN and other Van der Walls materials), we intend to use defects or impurities deliberately introduced in germanium as quantum emitters. Some of them have been shown to be some of the most efficient emitters in indirect gap materials with radiative lifetimes of less than 1 μs. If their quantum efficiency is excellent (5 to 90%), this emission rate is clearly insufficient. To rival with the brightness of III-V quantum emitters, we propose to make Ge a direct gap material using mechanical strain. The germanium direct gap is very close and is positioned only 140 meV above the indirect gap. By applying a modest tensile strain, the dispersion of the conduction band is dramatically reshaped and the gap becomes direct, singularly enhancing radiative efficiency of both the gap and defect states. Hence, Ge is a compelling material platform for the development of IR quantum light sources in the range from 2 to 3 μm. The long coherence of spin states will be instrumental to the emission of long chains of mutually entangled photons, a fundamental requirement and building block for several quantum technologies. Please, contact us.