Research Progress of Single-Photon Emitters Based on Two-Dimensional Materials

Abstract

From quantum communications to quantum computing, single-photon emitters (SPEs) are essential components of numerous quantum technologies. Two-dimensional (2D) materials have especially been found to be highly attractive for the research into nanoscale light-matter interactions. In particular, localized photonic states at their surfaces have attracted great attention due to their enormous potential applications in quantum optics. Recently, SPEs have been achieved in various 2D materials, while the challenges still remain. This paper reviews the recent research progress on these SPEs based on various 2D materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and twisted-angle 2D materials. Additionally, we summarized the strategies to create, position, enhance, and tune the emission wavelength of these emitters by introducing external fields into these 2D system. For example, pronounced enhancement of the SPEs' properties can be achieved by coupling with external fields, such as the plasmonic field, and by locating in optical microcavities. Finally, this paper also discusses current challenges and offers perspectives that could further stimulate scientific research in this field. These emitters, due to their unique physical properties and integration potential, are highly appealing for applications in quantum information and communication, as well as other physical and technological fields.

Keywords: 2D materials; Purcell enhancement; TMDs; quantum emitters; single-photon emitter.

Figure 2

SPEs of other TMDs. (a) Radiative rate in the presence of exciton localization using established theories for semiconductor nanostructures. The exciton COM wave function (1/e²) width is determined by the form of the harmonic confinement potential. The radiative rate is greater than ~1 ns⁻¹ for all plausible confinement scenarios (i.e., width ≥ 4 nm), thus exceeding that of WSe₂ QEs by 2 orders of magnitude. (b) PL spectrum from MoSe₂/WSe₂ Moiré superlattices. The blue and red regions represent the estimated PL signal from the emitter and the background. (c) Plot of g²(τ) measurements of the 1.401 emission peak in Figure 2b, using 2.3 μW CW excitation at 760 nm, show clear antibunching. The red solid line represents a fit of the experimental data, revealing a g²(0) = 0.28 ± 0.03. The black dots are the data measured by the HBT experiment and the red curve is the fitted result. The black dashed line represents the experimental limitation for g²(0) owing to the nonfiltered emission background. (d) Plot of emission energy versus gate voltage for E1, E2, and E3 peaks. Values of electrical dipole moments of ~420 meVnmV⁻¹ are determined by the linear fits. Panel (a) reprinted with permission from Ref. [57], Copyright 2021, American Chemical Society. Panels (b–d) reprinted with permission of AAAS from Ref. [63]. Copyright 2020, the authors, some rights reserved, exclusive licensee AAAS. Distributed under a Creative Commons Attribution License 4.0 (CC BY).

Figure 6

Purcell effect of SPEs in 2D materials. (a) Artistic sketch of the assumed sample configuration, featuring the monolayer wrapped over metal cones evolving on the silver–Al₂O₃ surface. (b) Fluorescence saturation curves obtained from the pristine (red open circles) and coupled (blue open squares) systems. (c) The microcavity consists of a hemispherical and flat mirror (only two stacks shown on either side). The quantum emitter hosted by hBN emits confocally with the excitation laser. A PDMS spacer sets the cavity length. To prevent the polymer from influencing the emitter, the PDMS is etched in the middle. (d) Normalized lifetime measurement on (blue) and off (gray) the CBGB. The values 0.6(2) and 6.9(4) ns indicate the extracted lifetime of the transitions, respectively. Panel (a) reprinted with permission from Ref. [102], Copyright 2018, American Chemical Society. Panel (b) reprinted with permission from Ref. [100], Copyright 2017, American Chemical Society. Panel (c) reprinted with permission from Ref. [32], Copyright 2019, American Chemical Society. Panel (d) reprinted with permission from Ref. [109], Copyright 2021, American Chemical Society.

Figure 1

SPEs of WSe₂. (a) PL intensity map of the broad spectral range 1.55–1.77 eV, (i) (ii) (iii) (iv) (v) for localized exciton luminescence appearing randomly on a monolayer WSe2 sample. (b) g²(τ) under resonant CW excitation of the BS-X. (c) Contour representation of the four peaks, after normalizing to the maximum peak intensity, yielding a fine structure splitting ~0.4 meV. (d) Contour plot of the µPL spectra of an SPE as a function of the applied electric field on the piezoelectric actuator. The electric field is reversibly swept in the range from −20 to 20 kV/cm. The observed red- and blue-shifts are due to the induced compressive and tensile strain fields by the actuator. An energy shift equal to 5.4 μeV/V is observed. Panel (a) reprinted from Ref. [42], Copyright 2015, Optical Society of America. Panel (b) reprinted from Ref. [51], Copyright 2016, Optical Society of America. Panel (c) reprinted from Ref. [52], Copyright 2016, the author(s). published by Springer Nature. Panel (d) reprinted with permission from Ref. [53], Copyright 2019, American Chemical Society.

Figure 3

SPEs of hBN. (a) Emission line shape for SE1 in hBN, as a function of the change in lattice energy during optical relaxation (ΔE = E_ZPL − E, where E is the observed photon energy). The data are binned to produce approximately uniform uncertainty, as indicated by the representative error bar. Curves show the results of a fit using the model described in the text (thick red curve), along with the ZPL and PSB components (thin curves) as indicated by the legend. (b) PL variation as a function of the relative orientation between the emitter’s optical dipoles and an in-plane B = 890 G. (c) The plot shows the scaled energy shift as a function of applied strain to the bendable substrate for three emitters with different tunability values of −3.1 meV/% (green), +3.3 meV/% (yellow), and +6 meV/% (red). Inset shows a sketch of a quadratic energy shift, ΔE, for the single-photon emission induced by intrinsic strain. (d) A gradual 17 nm red shift is observed by 200 μW, 532 nm CW illumination to the sample; there is no gate voltage applied to induce this effect. The different colored lines correspond to the PL spectral lines of the samples measured at different times. In this case, the electrons are liberated from the ionic liquid by the laser excitation alone using a λ_exc = 532 nm illumination source at 200 μW. The spectra are collected for 10 s each during a time-resolved PL measurement and offset for clarity. The time axis maps the collection time of the spectra displayed. Panel (a) reprinted with permission from Ref. [65], Copyright 2017, American Chemical Society. Panel (b) reprinted from Ref. [69], Copyright 2019, the author(s), published by Springer Nature. Panel (c) reprinted from Ref. [76], Copyright 2017, the author(s), published by Springer Nature. Panel (d) reprinted with permission from Ref. [77], Copyright 2019, American Chemical Society.

Figure 4

Defect engineer of SPEs. (a) Difference in the energy of the ZPL and PSB versus ZPL energy. (b) Spectral evolution of an individual luminescent center. The purple line shows the PL spectrum after oxygen irradiation and annealing, the blue line represents the untreated sample, and the orange line indicates the PL spectrum after only oxygen irradiation. The spectra after exfoliation and pure irradiation are multiplied by 5. All data shown are obtained from hBN flakes of batch 1 irradiated for 10 s at 240 eV. (c) Schematic illustration of the exposed MoS₂/hBN van der Waals heterostructure. (d) The left image shows the PL image of SPEs in hBN sample following irradiation by a femtosecond laser pulse. The right image details the variation in light intensity of an individual SPE as a function of the excitation power, with an excitation wavelength of 514 nm and a beam size of 1 μm. The black line represents the fitted results, The inset graph depicts the g²(τ) curve obtained from an HBT experiment for a single emitter. Panel (a) reprinted with permission from Ref. [67], Copyright 2016, American Chemical Society. Panel (b) reprinted with permission of AAAS from Ref. [86]. Copyright 2021, the authors, some rights reserved, exclusive licensee AAAS. Distributed under a Creative Commons Attribution Noncommercial License 4.0 (CC BY-NC). Panel (c) reprinted from Ref. [58], Copyright 2019, the author(s), published by Springer Nature. Panel (d) reprinted with permission from Ref. [87], Copyright 2022, American Chemical Society.

Figure 5

Strain engineer of SPEs. (a) Illustration of the fabrication method: (1) mechanical exfoliation of LM on PDMS and all-dry viscoelastic deposition on patterned substrate; and (2) deposited LM on patterned substrate. (b) Increasing nanopillar height also leads to a reduction in spectral wandering. Solid black circles represent the mean value of spectral wandering of several QEs for a given nanopillar height, while the error bars represent the standard deviation of each distribution, both extracted from time-resolved high-resolution spectral measurements. (c) Schematics of the deformed monolayer WSe₂ due to the nanogap. The saddle-shaped deformation occurs along the x-axis (y-axis) for the narrow(wide) nanogap. The exciton oscillation is aligned with the elongation direction. (d) Real space representation: A free exciton is created (dark red arrow), and the strain efficiently funnels excitons with the electron in the bright (solid black line) and dark (dashed black line) conduction band down in energy towards the strain maximum near r₀ due to the strain-dependent band gap. The red and green arrows indicate the different spin directions of the electrons. Mixing of the strain-localized dark exciton with a defect state leads to the formation of a strongly localized defect exciton. Panels (a,b) reprinted from Ref. [91], Copyright 2017, the author(s), published by Springer Nature. Panel (c) reprinted with permission from Ref. [93], Copyright 2021, American Chemical Society. Panel (d) right adapted with permission from Ref. [49], Copyright 2019, APS.