Tunable giant Purcell enhancement of quantum light emitters by means of acoustic graphene plasmons

TL;DR

This work tackles the challenge of achieving large, tunable emission-rate enhancements for quantum emitters at telecom and mid-infrared wavelengths. It introduces an acoustic graphene plasmon (AGP) cavity formed by a graphene sheet, a metallic nanocube, and an ultrathin dielectric spacer in a hBN/WS2/hBN heterostructure to couple emitters to highly confined AGP modes. Finite-difference time-domain simulations predict giant Purcell factors up to $F \sim 10^{6}$ in the mid-IR and $F \sim 10^{4}$ at $\lambda = 1.55\,\mu\mathrm{m}$, with QE up to $0.95$ (mid-IR) and $0.89$ (telecom); multipolar transitions ($E1/E2/E3$) and nonlinear 2PSE reach enhancements up to $F_{E1} \sim 10^{4}$, $F_{E2} \sim 10^{7}$, $F_{E3} \sim 10^{9}$, and $F_{2PSE} \sim 10^{9}$, and entangled-photon emission at $1.55\,\mu\mathrm{m}$ achieves $QE \approx 0.79$. The AGP resonances are tunable via gate-controlled $E_F$, enabling real-time on/off control, and a concrete Er$^{3+}$-doped WS$_2$ example demonstrates telecom compatibility. These results point to electrically tunable, CMOS-friendly quantum-light sources for quantum communication and processing, with broad applicability to SPEs in 2D materials and rare-earth centers.

Abstract

Inspired by the remarkable ability of plasmons to boost radiative emission rates, we propose leveraging acoustic graphene plasmons (AGPs) to realize tunable, giant Purcell enhancements for single-photon, entangled-photon, and multipolar quantum emitters. These AGPs are localized inside a cavity defined by a graphene sheet and a metallic nanocube and filled with a dielectric of thickness of a few nanometers and consisting of stacked layers of 2D materials, containing impurities or defects that act as quantum light emitters. Through finite-difference time domain (FDTD) calculations, we show that this geometry can achieve giant Purcell enhancement factors over a large portion of the infrared (IR) spectrum, up to 6 orders of magnitude in the mid-IR and up to 4 orders of magnitude at telecommunications wavelengths, reaching quantum efficiencies of 95\% and 89\%, respectively, with high-mobility graphene. We obtain Purcell enhancement factors for single-photon electric dipole (E1), electric quadrupole (E2), and electric octupole (E3) transitions and two-photon spontaneous emission (2PSE) transitions, of the orders of $10^{4}$, $10^{7}$, $10^{9}$, and $10^9$, respectively, and a quantum efficiency of 79\% for entangled-photon emission with high-mobility graphene at a wavelength of $λ=1.55$ $μ$m. Importantly, AGP mode frequencies depend on the graphene Fermi energy, which can be tuned via electrostatic gating to modulate fluorescence enhancement in real time. As an example, we consider the Purcell enhancement of spontaneous single- and two-photon emissions from an erbium atom inside single-layer (SL) WS$_2$. Our results could be useful for electrically tunable quantum emitter devices with applications in quantum communication and quantum information processing.

Figures (10)

• Figure 1: Schematic of proposed AGP cavity and heterostructure. Dipole emitter is located within the hBN layer, with the perpendicular orientation shown in blue and the parallel orientation shown in green. The acoustic graphene plasmons (AGPs) are localized in the region between the nanocubes and the graphene layer(s).
• Figure 2: Schematic diagram showing Er (green ball) doping in tungsten (dark blue ball) disulphide (mustard color ball). A layer of SL WS$_2$ is sandwiched between layers of hexagonal BN (pink and blue balls). Graphene layer (grey balls) is also shown at the bottom.
• Figure 3: $E_X$ field of the AGP at the center of the heterostructure cavity with respect to the z-axis for the case when the dipole is oriented parallel to the graphene layer and placed both a) on resonance at x = 0 nm and b) off resonance at x = 18 nm. A large Purcell enhancement is seen c) on resonance and also d) off resonance. We see that the radiative enhancement e) on resonance is significantly larger than f) off resonance as well. This is because when the dipole is on resonance, it is located near the maximum value of $E_X$, whereas when off resonance it is located near a local minima of $E_X$.
• Figure 4: $E_Z$ field of the AGP at the center of the heterostructure cavity with respect to the z-axis for the case when the perpendicular dipole is placed both a) on resonance at $x$ = 18 nm and b) off resonance at $x$ = 0 nm. A large Purcell enhancement is seen for both c) off resonance and d) on resonance with both having a similar magnitude. However, we see that the radiative enhancement is much larger e) on resonance while radiative enhancement is much lower f) off resonance as the monopole excited by the dipole at $x$ = 0 nm does not couple strongly to the far field while the dipole resonances excited by the dipole at $x$ = 18 nm couples efficiently to the far field. This shows that the efficiency of the Purcell enhancement is highly dependent on the location of the emitter relative to the electric field distribution of the AGP.
• Figure 5: Near square root relationship between the frequency of the excited AGP in the cavity as a function of a) number of graphene layers intercalated with FeCl3 ($C_S$), b) hBN layer thickness ($h$), and c) graphene Fermi energy ($E_F$) for the perpendicular dipole orientation. The radiative enhancement factor as a function of wavelength is shown for d) multiple values of $E_F$ as well to clearly demonstrate the electrostatic tunability of the AGPs. When not the variable of interest, we set $C_S$ = 1, $E_F$ = 1.0 eV, and $h$ = 5 nm.