ABSTRACT INTRODUCTION

CONCLUSION ACKNOWLEDGEMENTS & REFERENCES

BACKGROUND, APPROACH, RESULTS AND PLANNED WORK

A planar dielectric antenna for perfect single photon extractionfrom a quantum light source

J. Lewandowski, G. Ballesteros-Garcia, A. Kaczmarczyk, S. Kumar, B. GerardotHeriot-Watt UniversityEH14 4AS Edinburgh

Background:

A WSe2 monolayer flake crystal structure imperfection, such as composition fluctuation or a well width can often trap and isolate carriers, which effectively creates a quantum dot7. Unlike graphene, monolayer WSe2 shows a direct bandgap with strong photoluminesce (PL), which goes to the indirect one for more layers unless an appropriate strain is applied8.

Efficient single photon emitters are a key step required for the progress in quantum cryptography and quantum computing research. Remarkably so, Transition Metal Dichalcogenides have proved to be promising sources of single photons due to their unique opto-electronic properties. Here I present a design made to efficiently extract single photons using a tungsten diselenide (WSe2) Quantum Dot buried in a bulk with potential near-unity extraction efficiencies.

Single photon emitters are crucial in the development of quantum cryptography and computing1, and have been also considered as potential photon sources in spectroscopy and metrology2. In principle semiconductor quantum dots (QDs) can provide monochromatic single photons on demand3, though they often suffer high losses due to the low directivity of their far-field profile, unless, by burying them in a bulk, it can be modified using an appropriate micro-cavity design4. At the forefront of the buried QD research scientists around the world try to create a high-efficiency monochromatic single photon sources2,4. Remarkably, the research on transition-metal dichalcogenide monolayers (TMDMs), such as tungsten diselenide (WSe2) uncovered their unique properties including large direct bandgap and degenerate valleys5, as well as apparent immunity to photobleaching6.

Approach:

• Design and optimize structures for 3 cases in Matlab

• Exfoliate and find suitable 1L WSe2 flakes• Fabricate the structure• Characterize the performance using micro

photoluminescence at cryogenic (~4K) temperatures

The Matlab design utilized the transfer matrix method with the following transfer (1) and propagation (2) matrices for the propagating electromagnetic field used in the code3:

Results Case 1

Future Work Dry and clean

mechanical scotch-tape exfoliation method9

Transfer of flakes onto a viscoelastic PDMS (PolyDiMethylSiloxane) stamp

Fast optical identification method

Case 2

Case 3

All in all, the presented design indicates that the best extraction efficiency times the Purcell factor (of 1.153) would be achieved if the case 3 structure with Solid Immersion Lens and 0.68 Numerical Aperture collection lens was applied. Nevertheless since two additional monolayers of WSe2 flakes were found, the other two structures (case 1 and case 2) may also be fabricated and characterized.

I would like to thank Professor Brian Gerardot for support and general help, Guillem Ballesteros-Garcia for the help with the Matlab programming and general help, Santosh S. Kumar for the spin-coating tutorials, the lab presentation and general help and Artur Kaczmarczyk for the exfoliation tutorial and general help.

References:1. S. Kumar et al., Strain-induced spatial and spectral isolation of quantum emitters in mono- and bi-layer WSe2 , Nanoletters, DOI:10.1021/acs.nanolett.5b03312 (2015).

2. G. Lee et al., A planar dielectric antenna for directional single-photon emission and near-unity collection efficiency , Nature Photonics 312 (2010).

3. Yong Ma et al., Efficient photon extraction from quantum dot in a broad-band planar cavity antenna, Journal of Applied Physics 115 (2014).

4. Xue-Wen Chen et al., 99% efficiency in collecting photons from a single emitter, Optics Letters Vol.36 No.18 (2011).

5. Ajit Srivastava et al., Optically active quantum dots in monolayer WSe2, Nature Nanotechnology, Vol.10 (2015).

6. Philipp Tonndorf et al., Single-photon emission from localized excitons in an atomically thin semiconductor , Optica Vol.2, No.4 (2015).

7. M Koperski et al., Single photon emitters in exfoliated WSe2 structures, Nature Nanotechnology, Vol.10 (2015).

8. Sujay B. Desai et al., Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe2, Nanoletters 14, 4592-4597 (2014).

9. M. M. Benameur et al., Visibility of dichalcogenide nanolayers, Nanotechnology Vol.22, No.12 (2011).

Figure 1: Schematic bandstructure8 Figure 2: Considered scenarios

Figure 3: Transfer matrix formalism

Figure 4: Case 1 simulation results with the best value of 0.442 for (0.117µm,0.394µm)

Figure 5: Case 2 simulation results with the best value of 0.6236 for (0.118µm,0.410µm)

Figure 6: Case 3 simulation results with the best value of 1.153 for (0.137µm,0.300µm)

Figure 8: The best found 1L WSe2 (5.5x12µm2)

Figure 10: Antibunching

Figure 7: WSe2 on Nitto tape transmission mode

The future work comprises of the fabrication of the designed and optimised structure:

Gold depositionSpin coatingTransfer of flakeEncapsulation

Followed by the high resolution micro photoluminescence spectroscopy at cryogenic temperatures for single photon extraction characterisation.

Figure 9: Experiment set-up