Optical control of room-temperature valley polaritons

Journal name:
Nature Photonics
Volume:
11,
Pages:
491–496
Year published:
DOI:
doi:10.1038/nphoton.2017.121
Received
Accepted
Published online

The formation of half-light half-matter quasiparticles under strong coupling results in properties unique from those of the constituent components. Fingerprints of both light and matter are imprinted on the new quasiparticles, called polaritons. In the context of two-dimensional (2D) materials, this opens up the possibility of exploiting the intriguing spin–valley physics of a bare semiconductor combined with the light mass of the photonic component for possible quantum technologies. Specifically, the valley degree of freedom1, 2, which remained largely unexplored until the advent of these materials, is highly attractive in this context as it provides an optically accessible route for the control and manipulation of electron spin. Here, we report the observation of room-temperature strongly coupled light–matter quasiparticles that are valley polarized because of the coupling of photons with specific helicity to excitons that occupy quantum mechanically distinct valleys in momentum space. The realization of valley polaritons in 2D semiconductor microcavities presents the first step towards engineering valley-polaritonic devices.

At a glance

Figures

  1. Schematic of the valley polariton phenomena.
    Figure 1: Schematic of the valley polariton phenomena.

    The solid (grey) curves indicate the lower polariton branch (LPB) and the upper polariton branch (UPB). The bare cavity and the exciton dispersion are shown by the black and orange dashed curves, respectively. Pump 1 is used to excite directly the exciton reservoir, whereas Pump 2 excites the lower polariton branch at specific k|| and ω. The emission is collected at smaller angles. The top inset shows the valley-polarization phenomena in 2D TMDs caused by the broken inversion symmetry. The K and K′ points correspond to the band edges separated in momentum space but energetically degenerate. The bottom inset is a schematic of the microcavity structure with silver mirrors and a SiO2 cavity layer embedded with 2D WS2.

  2. Dispersion of the microcavity.
    Figure 2: Dispersion of the microcavity.

    The left panels show the angle-resolved reflectivity, whereas the right panels show the angle-resolved PL obtained via Fourier space imaging. af, The different detunings of the microcavities are Δ = –105 meV (a and b), Δ = –60 meV (c and d) and Δ = +16 meV (e and f).

  3. Helicity of the polariton emission.
    Figure 3: Helicity of the polariton emission.

    a,b, Helicity-resolved lower polariton branch emission spectrum integrated over all angles for excitation with σ− (a) and σ+ (b) excitation at 1.98 eV, which corresponds to the bare exciton A energy (Pump 1). The peak helicities are 27 ± 2% for σ− and 27 ± 2% for σ+ excitation. c, Angle-resolved helicity for the three different detunings for the σ+ excitation, in which the slightly positive detuned cavity (Δ = +16 meV) shows increasing helicity as a function of angle, whereas the negative detuned cavities (Δ = –60 meV and Δ = –105 meV) show no observable trend within the experimental uncertainty. Error bars represent the standard deviation (+/−2) of a set of measurements.

  4. Helicity of polariton emission under the resonant pump (Pump 2).
    Figure 4: Helicity of polariton emission under the resonant pump (Pump 2).

    a,b, Helicity-resolved polariton emission spectrum for the case in which the pump is in resonance with the lower polariton branch (1.95 eV) with σ− (a) and σ+ (b) polarization. A helicity of 14 ± 2% is observed. c,d, Angle-resolved helicity at small angles (small k||) for the σ− (c) and σ+ (d) excitation polarizations. Although no clear trend in angle dependence is observed for the small angles, clearly the system shows a helicity >12% even at k||= 0. Error bars represent the standard deviation (+/−2) of a set of measurements.

  5. Theoretical simulation of the angle dependence of helicity.
    Figure 5: Theoretical simulation of the angle dependence of helicity.

    a,b, The dependence of PL intensity (a) and the helicity (b) as a function of angle, which is related to the in-plane momentum for the pump at 1.98 eV, which corresponds to the bare exciton A energy. Simulations show the angular dependence observed experimentally in which the +16 meV detuned cavity shows an increase in helicity with an increase in angle. Simulations also indicate a decrease in helicity for the most-negative detuned cavity (Δ = –105 meV), which was not observed in the experiments since the decrease is within the experimental error.

References

  1. Ohkawa, F. J. & Uemura, Y. Theory of valley splitting in an N-channel (100) inversion layer of Si. I. Formulation by extended zone effective mass theory. J. Phys. Soc. Jpn 43, 907916 (1977).
  2. Sham, L. J., Allen, S. J., Kamgar, A. & Tsui, D. C. Valley–valley splitting in inversion layers on a high-index surface of silicon. Phys. Rev. Lett. 40, 472475 (1978).
  3. Shkolnikov, Y. P., De Poortere, E. P., Tutuc, E. & Shayegan, M. Valley splitting of AlAs two-dimensional electrons in a perpendicular magnetic field. Phys. Rev. Lett. 89, 226805 (2002).
  4. Isberg, J. et al. Generation, transport and detection of valley-polarized electrons in diamond. Nat. Mater. 12, 760764 (2013).
  5. Goswami, S. et al. Controllable valley splitting in silicon quantum devices. Nat. Phys. 3, 4145 (2007).
  6. Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Spin and pseudospins in layered transition metal dichalcogenides. Nat. Phys. 10, 343350 (2014).
  7. Yao, W., Xiao, D. & Niu, Q. Valley-dependent optoelectronics from inversion symmetry breaking. Phys. Rev. B 77, 235406 (2008).
  8. Xiao, D., Yao, W. & Niu, Q. Valley-contrasting physics in graphene: magnetic moment and topological transport. Phys. Rev. Lett. 99, 236809 (2007).
  9. Xiao, D., Liu, G.-B., Feng, W., Xu, X. & Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 108, 196802 (2012).
  10. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nat. Nanotech. 7, 494498 (2012).
  11. Cao, T. et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nat. Commun. 3, 887 (2012).
  12. Jones, A. M. et al. Optical generation of excitonic valley coherence in monolayer WSe2. Nat. Nanotech. 8, 634638 (2013).
  13. Zeng, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Valley polarization in MoS2 monolayers by optical pumping. Nat. Nanotech. 7, 490493 (2012).
  14. Ye, Z., Sun, D. & Heinz, T. F. Optical manipulation of valley pseudospin. Nat. Phys. 13, 2629 (2016).
  15. Schaibley, J. R. et al. Valleytronics in 2D materials. Nat. Rev. Mater. 1, 16055 (2016).
  16. Liu, X., Galfsky, T., Sun, Z., Xia, F. & Lin, E. Strong light–matter coupling in two-dimensional atomic crystals. Nat. Photon. 9, 3034 (2015).
  17. Dufferwiel, S. et al. Exciton–polaritons in van der Waals heterostructures embedded in tunable microcavities. Nat. Commun. 6, 8579 (2015).
  18. Sidler, M. et al. Fermi polaron-polaritons in charge-tunable atomically thin semiconductors. Nat. Phys. 13, 255261 (2017).
  19. Liu, W. et al. Strong exciton–plasmon coupling in MoS2 coupled with plasmonic lattice. Nano Lett. 16, 12621269 (2016).
  20. Wang, S. et al. Coherent coupling of WS2 monolayers with metallic photonic nanostructures at room temperature. Nano Lett. 16, 43684374 (2016).
  21. Lundt, N. et al. Room-temperature Tamm-plasmon exciton–polaritons with a WSe2 monolayer. Nat. Commun. 7, 13328 (2016).
  22. Amo, A. et al. Exciton–polariton spin switches. Nat. Photon. 4, 361366 (2010).
  23. Paraïso, T. K., Wouters, M., Léger, Y., Morier-Genoud, F. & Deveaud-Plédran, B. Multistability of a coherent spin ensemble in a semiconductor microcavity. Nat. Mater. 9, 655660 (2010).
  24. Kavokin, K. V., Shelykh, I. A., Kavokin, A. V., Malpuech, G. & Bigenwald, P. Quantum theory of spin dynamics of exciton–polaritons in microcavities. Phys. Rev. Lett. 92, 17401 (2004).
  25. Liew, T., Kavokin, A. & Shelykh, I. Optical circuits based on polariton neurons in semiconductor microcavities. Phys. Rev. Lett. 101, 16402 (2008).
  26. Li, G. et al. Spin-enabled plasmonic metasurfaces for manipulating orbital angular momentum of light. Nano Lett. 13, 41484151 (2013).
  27. Shelykh, I. A. et al. Polariton polarization-sensitive phenomena in planar semiconductor microcavities. Semicond. Sci. Technol. 25, 13001 (2010).
  28. Leyder, C. et al. Observation of the optical spin Hall effect. Nat. Phys. 3, 628631 (2007).
  29. Lee, J., Mak, K. F. & Shan, J. Electrical control of the valley Hall effect in bilayer MoS2 transistors. Nat. Nanotech. 11, 421425 (2016).
  30. Raja, A. et al. Coulomb engineering of the bandgap in 2D semiconductors. Nat. Commun. 8, 15251 (2017).
  31. Maialle, M. Z., de Andrada e Silva, E. A. & Sham, L. J. Exciton spin dynamics in quantum wells. Phys. Rev. B 47, 1577615788 (1993).
  32. Yu, H. et al. Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides. Nat. Commun. 5, 35 (2014).
  33. Qiu, D. Y., Cao, T. & Louie, S. G. Nonanalyticity, valley quantum phases, and light-like exciton dispersion in monolayer transition metal dichalcogenides: theory and first-principles calculations. Phys. Rev. Lett. 115, 176801 (2015).
  34. Wu, F., Qu, F. & MacDonald, A. H. Exciton band structure of monolayer MoS2. Phys. Rev. B 91, 75310 (2015).
  35. Shelykh, I. A., Kavokin, A. V. & Malpuech, G. Spin dynamics of exciton polaritons in microcavities. Phys. Status Solidi 242, 22712289 (2005).
  36. Tassone, F., Piermarocchi, C., Savona, V., Quattropani, A. & Schwendimann, P. Bottleneck effects in the relaxation and photoluminescence of microcavity polaritons. Phys. Rev. B 56, 75547563 (1997).

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Author information

  1. Present address: Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.

    • Xiaoze Liu

Affiliations

  1. Department of Physics, City College, City University of New York, New York 10031, USA

    • Zheng Sun,
    • Jie Gu,
    • Areg Ghazaryan,
    • Zav Shotan,
    • Christopher R. Considine,
    • Michael Dollar,
    • Biswanath Chakraborty,
    • Xiaoze Liu,
    • Pouyan Ghaemi &
    • Vinod M. Menon
  2. Department of Physics, Graduate Center, City University of New York, New York 10016, USA

    • Zheng Sun,
    • Jie Gu,
    • Xiaoze Liu,
    • Pouyan Ghaemi &
    • Vinod M. Menon
  3. Department of Engineering Physics, École Polytechnique de Montréal, Montréal, Quebec H3T 1J4, Canada

    • Stéphane Kéna-Cohen

Contributions

V.M.M., Z.Sun and X.L. initiated the project. Z.Sun, S.K.-C. and V.M.M. designed the experiments. Z.Sun, C.R.C. and M.D. fabricated the microcavity samples, Z.Sun, J.G., B.C. and Z.Shotan collected the optical characterization data, and Z.Sun, J.G. and B.C. analysed the data. A.G., P.G. and S.K.-C. carried out the theoretical modelling. All the authors contributed to the discussion of the results and writing the manuscript.

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The authors declare no competing financial interests.

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