Phonon induced line broadening and population of the dark exciton in a deeply trapped localized emitter in monolayer WSe_2

Abstract

Abstract:We study trapped single excitons in a monolayer semiconductor with respect to their temperature stability, spectral diffusion and decay dynamics. In a mechanically exfoliated WSe 2 sheet, we could identify discrete emission features with emission energies down to 1.516 eV which are spectrally isolated in a free spectral range up to 80 meV. The strong spectral isolation of our localized emitter allow us to identify strong signatures of phonon induced spectral broadening for elevated temperatures accompanied by temperature induced luminescence quenching. A direct correlation between the droop in intensity at higher temperatures with the phonon induced population of dark states in WSe 2 is established. While our experiment suggests that the applicability of monolayered quantum emitters as coherent single photon sources at elevated temperatures may be limited, the capability to operate them below the GaAs band-edge makes them highly interesting for GaAs-monolayer hybrid quantum photonic structures. [2] F. Diedrich and H. Walther, "Nonclassical radiation of a single stored ion," Phys. Rev. Lett. 58, 203-206 (1987). [6] M. Koperski, K. Nogajewski, A. Arora, V. Cherkez, P. Mallet, J.-Y. Veuillen, J. Marcus, P. Kossacki, and M. Potemski, "Single photon emitters in exfoliated W Se 2 structures," Nature nanotechnology 10, 503-506 (2015).[7] C. Chakraborty, L. Kinnischtzke, K. M. Goodfellow, R. Beams, and A. N. Vamivakas, "Voltage-controlled quantum light from an atomically thin semiconductor," Nature nanotechnology 10, 507-511 (2015).[8] A. Srivastava, M. Sidler, A. V. Allain, D. S. Lembke, A. Kis, and A. Imamoglu, "Optically active quantum dots in monolayer W Se 2 ," Nature nanotechnology 10, 491-496 (2015).[9] P. Tonndorf, R. Schmidt, R. Schneider, J. Kern, M. Buscema, G. A. Steele, A. CastellanosGomez, H. S. J. van der Zant, S. M. de Vasconcellos, and R. Bratschitsch, "Single-photon emission from localized excitons in an atomically thin semiconductor," Optica 2, 347-352 (2015).[10] X. Xu, W. Yao, D. Xiao and T. F. Heinz, "Spin and pseudospins in layered transition metal dichalcogenides," Nature Phys 10, 343-350 (2014). [18] E. J. Sie, J. W. Mclver, Y. H. Lee, L. Fu, J. Kong and N. Gedik, "Valley-selective optical Stark effect in monolayer W S 2 ." Nature Mater. 14, 290-294 (2015).[19] J. Kim, X. Hong, C. Jin, S. Shi, C. Chang, M. Chiu, L. Li and F. Wang, "Ultrafast generation of pseudo-magnetic field for valley excitons in W Se 2 monolayers." Science 346, 1205-1208 (2014).[20] S. Kumar, A. Kaczmarczyk, and B. D. Gerardot, "Strain-induced spatial and spectral isolation of quantum emitters in mono-and bi-layer W Se 2 ," Nano letters 15, 7567-7573(2015).[21] J. Seufert, R. Weigand, G. Bacher, T. Kümmell, A. Forchel, K. Leonardi, and D. Hommel, "Spectral diffusion of the exciton transition in a single self-organized quantum dot," Applied Physics Letters 76, 1872-1874 (2000).[ [24] B. Pietka, J. Suffczynski, M. Goryca, T. Kazimierczuk, A. Golnik, P. Kossacki, A. Wysmolek, J. A. Gaj, R. Stepniewski, and M. Potemski...

Substrate engineering for high-quality emission of free and localized excitons from atomic monolayers in hybrid architectures

“…Figure 4 shows a typical photoluminescence spectrum from such a localized exciton in a WSe 2 monolayer ontop of a SiO 2 /Si substrate, which was excited by a continuous-wave(CW) 532 nm laser at an excitation power of 30 µW and a nominal sample temperature of 4.2 K. The photoluminescence spectrum consists of several sharp peaks with linewidths of 2 meV, centered at 1.52 eV. Such a spectral feature, which is red-shifted 180 meV from the WSe 2 free valley exciton(1.7eV), is comparable to previously reported localized emission signals in WSe 2 monolayers [21]. Compared to the weak, broad PL spectrum from the localized exciton in the WSe 2 monolayer exfoliated on the SiO 2 /Si substrate, several bright, spectral resolution limited (70 µeV) PL peaks were observed from WSe 2 sheets transferred onto the InGaP/GaAs substrate (temperature of 4.5 K).…”

Optical fingerprint of bright and dark localized excitonic states in atomically thin 2D materials

“…In particular, we predict pronounced phonon side bands from the energetically lowest KK' exciton involving TO/LO phonons (around 130 meV) and TA/LA phonons (around 115 meV). Our findings support the hypothesis of recent experimental findings, predicting photoluminescence resonances stemming from dark localized excitons [58,59].…”

“…Our calculations show (i) a temperature-independent contribution due to radiative decay and (ii) an linear increase in the linewidth due to the enhanced exciton-phonon scattering. The radiative part is determined by the wavefunction overlap and we find γ 0 ≈ 0.5 − 1.0 meV, which is in good agreement with the experimental obtained value of 0.9 meV [59]. The increase with temperature stems from the temperature dependence of the phonon-driven capture rate, cf.…”

Spontaneous Emission Enhancement in Strain-Induced WSe2 Monolayer-Based Quantum Light Sources on Metallic Surfaces

“…We believe, that the contribution of a localized dark exciton, which was recently observed in W Se 2 based quantum emitters, could yield such a behavior. 57 In addition, we furthermore believe that the presence of such long lived characteristic timescales in our photon statistics excludes very strong non-radiative recombination as a sole source of signicant lifetime shortening in our experiment. The extracted second order correlation function at zero delay, g 2 (0) is 0.25, adds evidence that our system acts as a single photon source.…”