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arXiv:2006.15219 · Physical Review Letters

Two-Photon Spontaneous Emission in Atomically Thin Plasmonic Nanostructures

Yunuhen Muñiz, Alejandro Manjavacas, Carlos Farina, Diego A. R. Dalvit, Wilton J. M. Kort-Kamp

Published Jul 15, 2020• 34 citations
arXivHTMLPDF

Abstract

The ability to harness light-matter interactions at the few-photon level plays a pivotal role in quantum technologies. Single photons-the most elementary states of light-can be generated on demand in atomic and solid state emitters. Two-photon states are also key quantum assets, but achieving them in individual emitters is challenging because their generation rate is much slower than competing one-photon processes. We demonstrate that atomically thin plasmonic nanostructures can harness two-photon spontaneous emission, resulting in giant far field two-photon production, a wealth of resonant modes enabling tailored photonic and plasmonic entangled states, and plasmon-assisted single-photon creation orders of magnitude more efficient than standard one-photon emission. We unravel the two-photon spontaneous emission channels and show that their spectral line shapes emerge from an intricate interplay between Fano and Lorentzian resonances. Enhanced two-photon spontaneous emission in two-dimensional nanostructures paves the way to an alternative efficient source of light-matter entanglement for on-chip quantum information processing and free-space quantum communications.

DOI
10.1103/physrevlett.125.033601

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The ability to harness light-matter interactions at the few-photon level plays a pivotal role in quantum technologies. Single photons-the most elementary states of light-can be generated on demand in atomic and solid state emitters. Two-photon states are also key quantum assets, but achieving them in individual emitters is challenging because their generation rate is much slower than competing one-photon processes. We demonstrate that atomically thin plasmonic nanostructures can harness two-photon spontaneous emission, resulting in giant far field two-photon production, a wealth of resonant modes enabling tailored photonic and plasmonic entangled states, and plasmon-assisted single-photon creation orders of magnitude more efficient than standard one-photon emission. We unravel the two-photon spontaneous emission channels and show that their spectral line shapes emerge from an intricate interplay between Fano and Lorentzian resonances. Enhanced two-photon spontaneous emission in two-dimensional nanostructures paves the way to an alternative efficient source of light-matter entanglement for on-chip quantum information processing and free-space quantum communications.
The rate of spontaneous emission is known to depend on the environment of a emitter, and the enhancement of one-photon emission in a resonant cavity is known as the Purcell effect. Here we develop a theory of spontaneous two-photon emission for a general electromagnetic environment including inhomogeneous dispersive and absorptive media. This theory is used to evaluate the two-photon Purcell enhancement in the vicinity of metallic nanoparticles and demonstrates that the surface-plasmon resonances supported by these particles can increase the emission rate by more than two orders of magnitude. The control over two-photon Purcell enhancement given by tailored nanostructured environments could provide an emitter with any desired spectral response and may serve as an ultimate route for designing light sources with novel properties.
We develop a framework that computes two-photon spontaneous emission (TPSE) spectra of a quantum emitter near an arbitrarily shaped nanostructure. The model considers the interaction up to the electric quadrupolar order, which is relevant for nanophotonic structures sustaining strongly confined fields that are used to enhance and to tailor spontaneous emission processes. Moreover, we consider interference effects between multipolar two-photon emission channels, for the first time to our knowledge. First, we show for a s → s transition of a hydrogen atom placed under a silver plasmonic nanodisk a substantial enhancement in the photon-pair emission rates by 5 and 11 orders of magnitude for the two-electric dipole (2ED) and two-electric quadrupole (2EQ) transitions, respectively. Then for the same emitter under a plasmonic graphene nanotriangle, we demonstrate a breakdown of the electric dipole approximation in the TPSE process where the interference between the 2ED and 2EQ transitions is important, as it increases the total rate by 63 %. Third, we explore platforms where entangled photons of different energy are emitted in the far-field in different directions. In the end, our framework is a complete tool to design emitters and nanostructures for the TPSE process, leading to a rich assortment of functional nanoantennas.
We summarize our studies of the plasmonic enhancements of the spontaneous two-photon emission. In particular, we draw attention to the rigorous investigation of quantum framework for the multi-photon spontaneous decay rates in an environment with arbitrary structure of electromagnetic modes. We demonstrate that nanostructured environment can be used for tailoring the basic characteristics of the two-photon emission such as spectrum, lifetime, and entanglement. Moreover, careful engineering of the two-photon Purcell enhancement may provide any desired spectral response and may serve as an ultimate route for designing light sources with novel properties.
Abstract Single-photon source in micro- or nanoscale is the basic building block of on-chip quantum information and scalable quantum network. Enhanced spontaneous emission based on cavity quantum electrodynamics (CQED) is one of the key principles of realizing single-photon sources fabricated by micro- or nanophotonic cavities. Here we mainly review the spontaneous emission of single emitters in micro- or nanostructures, such as whispering gallery microcavities, photonic crystals, plasmon nanostructures, metamaterials, and their hybrids. The researches have enriched light-matter interaction as well as made great influence in single-photon source, photonic circuit, and on-chip quantum information.
Abstract Spontaneous two-photon emission is a process in which the transition between two quantum states happens via simultaneous emissions of two identical photons. Such second-order quantum radiation is of paramount importance in astrophysics, atomic physics, and quantum technology. In particular, on-demand spontaneous two-photon emission from single quantum emitters has long been envisioned to revolutionize photonic quantum science and technology. To date, experimental realizations of two-photon emission at the single quanta level are still highly challenging yet elusive due to their low probabilities. In this work, we explore spontaneous two-photon emission from a single semiconductor quantum dot deterministically coupled to a high-quality microcavity in which the cavity resonances greatly enhance both excitation and emission processes. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity mode drives the biexciton to the ground state and simultaneously emits two photons into the cavity mode, resulting in two-photon emissions with a record brightness comparable to the competing single-photon process and sub-nanowatt power consumption. For the first time, the quantum nature associated with spontaneous two-photon emissions in the cavity quantum electrodynamics regime has been investigated via photon statistics measurements. Furthermore, spontaneous two-photon emission is exploited to build unconventional entangled quantum light sources with the unique advantage of combining the best of two worlds: near-unity entanglement fidelity for spontaneous parametric down-conversion source and on-demand photon emissions for atomic quantum emitters. Our work provides unprecedented insights into the two-photon process in the quantum regime and opens an unexplored avenue for pursuing optically reconfigurable quantum light sources with nonlinear quantum radiations.
In applications like quantum cryptography and quantum computation, it is desirable to obtain single photon sources which can produce a train of single photons on demand at a high repetition rate, especially at or near room temperature. Such single-photon sources can be realized by tailoring the photonic environment of the quantum emitter. The photonic environment determines the local density of states, into which the photon can be emitted. A number of structures such as interfaces, cavities and waveguides have already been used to modify the spontaneous emission (SE) rate; nevertheless there is still room for improving the overall efficiency of the single-photon sources, e.g., by inventing new ways of enhancing light-matter interaction.