Orbitally resolved single-photon emission from an individual atomic vacancy center in a semiconductor

Abstract

Atomically confined spins are emerging as active components in quantum optoelectronic devices such as quantum bits and sensors. However, interrogating single spins at atomic length-scales remains a sizeable challenge, limited by diffraction in conventional optics. Here we show that the highly-local excitation provided by injecting energetic charge carriers from the atomically sharp probe of a scanning tunneling microscope can trigger single-photon emission from individual atomic vacancy centers in a layered semiconductor. With an effective spatial resolution of <1 nm, we show that the captured light closely mirrors the orbital symmetry of the bound-state wavefunction of the vacancy center while photon correlation measurements confirm single-photon emission, as reflected in clear photon anti-bunching signatures. Our results constitute an important step toward the realization of an electrically addressable single-atom quantum light source and solid-state spinphoton interface, addressed at the atomic-scale.

Figures (4)

• Figure 1: Orbitally-resolved photon emission from an individual V$_{\rm{S}}$ center in MoS$_2$.a, Schematic diagram of the experiment showing a single V$_{\rm{S}}$ center in a three layer thick MoS$_2$ crystal on a Gr/SiC substrate, probed by a STM tip. Electrons tunnel from graphene reservoir (biased) to the defect at a rate of $\Gamma_{\rm{S}}$ and from the defect to the tip (grounded) at a rate of $\Gamma_{\rm{T}}$. b, STM topographic image of the MoS$_2$ surface, recorded at $V_{\rm{b}} = 0.7$ V, showing a variety of defect species including a sulfur vacancy (V$_{\rm{S}}$), molybdenum vacancy (V$_{\rm{Mo}}$) and oxygen passivated sulfur vacancies in the top (O$\rm{_{S}^{top}}$) and sub-surface (O$\rm{_{S}^{bot}}$). c-d, Close-up topographic image of a V$_{\rm{S}}$ recorded at $V_{\rm{b}} = 0.7$ V alongside a corresponding lattice schematic. e-f, Energy-level diagram and spectroscopic measurement of the V$_{\rm{S}}$ center's in-gap electronic structure. g, Point spectrum taken atop a V$_{\rm{S}}$ reflecting three in-gap states: a, e' and e". h-i, Energy-level diagram showing resonant tunneling via the unoccupied e', the occupied e" and a defect level, respectively. Here, S and T denote the substrate and tip reservoirs, respectively. j-k, dI/dV wavefunction maps of the e' and e" states. l-m, Photon emission maps recorded at $V_{\rm{b}} = -2.6$ V and $2.6$ V, respectively, at a constant current of 10 nA, closely mirroring the defect wavefunctions in (j, k). Scale bar: 5 nm in (b) and 6 Å in (c, j-m).
• Figure 2: Atomically-resolved optical emission spectra and excitation dependence.a-b, Optical spectra at a constant excitation current of 10 nA at positive polarity (a) and negative (b) bias. c-d, Photon emission rate as a function of tunneling current for different biases at (c) positive bias polarity exhibiting linear dependence (solid traces are guides to the eye) and at (d) at negative polarity (solid traces are fits to Eq. 16 in SI). The saturation observed in the photon emission rate arises when photon emission is dominated by tunneling through discrete in-gap states of the defect. e, A fit to the two-state rate equation model (Eq. 2), we extract the charge tunnel rates $\Gamma_{\rm{S}}$ and $\Gamma_{\rm{T}}$. Inset: Energy-level diagram of the emission mechanism (see main text for a description). Grey shading in (a, b) represents lower transmission of the setup. Data in (d, e) are from separate measurements using different STM tips.
• Figure 3: Charge-state dependent photon emission.a, Charge stability diagram (Coulomb diamond) of an individual V$\rm{_S}$ center recorded in a back-gated device (refer to SI for device structure). White dashed lines indicates the alignment of the V$\rm{_S}$ electrochemical potential with the tip and graphene Fermi level allowing conduction via the in-gap states (refer SI for details). Current is Coulomb-blockaded within the dashed white boundaries at low bias. Arrow indicates the alignment of the V$\rm{_S}$ electrochemical potential $\mu_{\rm{V_{S} ^{-1}\leftrightarrow V_{S} ^{-2}}}$ with the sample Fermi-level allowing a change in the equilibrium charge state (charging peak). The data was obtained at a constant tip height with a lock-in modulation of 10 mV. The discretizations of the charging peak is due to a large $V_{\rm{g}}$ step size of 10 V. b, Mapping of the charging peak at constant tip height reveals a ring-like structure centered around the V$\rm{_S}$ whose size increases with applied bias. c-f, Simultaneously recorded d$I$/d$V$ and photon count rates at a V$\rm{_S}$ center, as a function of tip-defect lateral ($L$) and vertical ($\Delta z$) distance, at fixed tip height in (c, e) and fixed bias $V_{\rm{b}}=-2.5$V in (d, f). Scale bar in (b) is 1 nm.
• Figure 4: Single-photon and photon-pair emission.a, c, Second-order correlation function as a function of time-delay measured at high bias (a) $eV_{\rm{b}}> 2h\nu$ and low bias (c) $V_{\rm{b}}=-2.8~$V. Here, $\rm{(V_{S})_{1}, (V_{S})_{2}, (V_{S})_{3} }$ refer to three different vacancy centers. The parameters extracted from fits are $\uptau$ = 0.71, 0.23, 0.24 ns and g(2)(0) = 0.65, 0.36, 0.42, respectively for the three vacancy centers. b, d STL map (normalized photon count rate) and energy level schematic illustrating the emission mechanism corresponding to the bias regime. A vertical offset of 5 in a and 2 in c has been added for clarity. Inelastic tunneling from other in-gap states may also give rise to photon emission, however these transitions are not shown as the resultant photons may not be within the detectable energy range. Error bars in (c) are the first standard deviation.