Single Photons, Phonons and Valley-Locked Spins in Atomically Thin WSe2 Público

Chen, Xiaotong (Fall 2019)

Permanent URL: https://etd.library.emory.edu/concern/etds/4m90dw69p?locale=pt-BR
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Abstract

Single-photon emitters or optically active quantum dots (QDs) in atomically thin WSe2, comprised of localized bound electron-hole pair, exhibit a variety of optical properties, such as the single-photon emission and the spin-valley degree of freedom which may find potential application in quantum information processing, quantum sensing and exploring fundamental many-body quantum physics. In this dissertation, we explore the interaction between single QDs and phonons or charge carriers in the monolayer WSe2 at low temperature down to 4 K and high magnetic up to 8 T. Experimentally, we observed an entanglement state between phonons carrying pseudo angular momentum (chiral phonons) and single photons arising due to unique electronic and phononic properties. We also investigated an optical initialization of a single spin-valley state which can be exploited in quantum information processing. In addition, by studying the phonon sideband, the intrinsic properties of QDs, such as the size and interaction type with phonons are revealed. 

The first part demonstrates the entanglement between the single photons and chiral phonons. The doubly degenerate chiral phonons interacting with the neutral QDs lead to phonon replica peaks at lower energy. The phonon replica and its parent neutral QD originate from the same QD as they exhibit an identical spectral jittering pattern. However, the emission from neutral QD is linear polarized whereas the phonon replica exhibits an unexpected random polarization at zero magnetic fields. As the phonon replica results from a coherent phonon scattering process, the unpolarization at zero magnetic fields is intriguing. This behavior arises due to the entanglement state formed by two indistinguishable scattering paths of the single photons and chiral phonons. Under the perpendicular magnetic field, because the indistinguishability is lost and the entanglement state is destroyed, the phonon replica and parent neutral QD both recover the circular polarization.

In the second part, we studied the optical initialization of a single spin-valley state in a single positively-charged QD. The singlet QD has a similar spectral jittering as a neutral QD indicating the same origination and exhibits spectral features consistent with a single positively-charged QD. Under a perpendicular magnetic field, the emission polarization of the charged QD can be selectively controlled by choosing the helicity of the excitation laser but this phenomenon is absent in neutral QD. In other words, the spin-valley state of the excess hole is optically initialized by the helicity of the excitation laser as its spin-valley state is opposite to the recombined electron-hole pair in the single positively-charged QD. We explain the spin-valley initialization as a result of the quenching of exchange interaction in single positively-charged QDs.

Finally, after calculating and fitting the phonon sideband, we estimate the intrinsic properties of QDs, such as the size and coupling type with phonons in monolayer WSe2. The spectral calculation of QDs is conducted within the independent boson model and under different coupling types between QDs and phonon modes. Our spectral calculations exhibit a good fitting to the experimental results, especially for the phonon sideband at the low energy side. The size of QDs is estimated at about 4 nm and the coupling type is the deformation coupling between QDs and acoustic phonon modes. The fidelity of the fitting is about 60% to 70% at different temperatures and it can be enhanced by improving the fitting effect of phonon sideband at high energy side. 

Table of Contents

1 Introduction 1

1.1 Transition metal dichalcogenides (TMDC) . . . . . . . . . . . . 1

1.2 Exciton and quantum dot in monolayer WSe2 . . . . . . . . . . 3

1.3 Exciton valley depolarization.................... 6

1.4 Chiral phonons in monolayer hexagonal lattice . . . . . . . . . . 9

2 Experimental Methods 14

2.1 Device fabrication.......................... 14

2.2 Magneto-optical PL measurement................. 17

3 Entanglement between single photon and chiral phonon in mono- layer WSe2 21

3.1 Samplereview............................ 21

3.2 Phonon replica of neutral quantum dot . . . . . . . . . . . . . . 23

3.3 Power dependence and Zeeman splitting . . . . . . . . . . . . . 28

3.4 Polarization measurements..................... 32

3.5 Single photon and chiral phonon entanglement . . . . . . . . . . 35

3.6 Polarization recovery under magnetic field . . . . . . . . . . . . 38

3.7 PLE & localization of QDs & Raman symmetry analysis . . . . 41

3.7.1 PLE............................. 41

3.7.2 Localization of QDs..................... 41

3.7.3 Raman symmetry analysis ................. 42

3.8 Conclusion and Perspectives.................... 44

4 Optical initialization of quantum dot with a single spin-valley 45

4.1 Single spin-valley control mechanism . . . . . . . . . . . . . . . 45

4.2 Positively charged quantum dot .................. 48

4.3 Polarization resolved magneto-optical measurement . . . . . . . 53

4.4 Single spin-valley initialization................... 56

4.5 Conclusion and Perspectives.................... 60

5 Phonon sideband in QD spectra 61

5.1 Electron-phonon interaction .................... 61

5.2 Estimation of the size of QDs ................... 63

5.3 Conclusion and Perspectives.................... 69 

Bibliography .................... 70 

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