Nano-photonics in III-V semiconductors for integrated quantum optical circuits Nicholas Andrew Wasley Submitted for the degree of Doctor of Philosophy Department of Physics and Astronomy January 2013 University of Sheffield Abstract This thesis describes the optical spectroscopic measurements of III-V semiconduc- tors used to investigate a number of issues related to the development of integrated quantum optical circuits. The disorder-limited propagation of photons in photonic crystal waveguides in the slow-light regime is investigated. The analysis of Fabry-Perot resonances is used to map the mode dispersion and extract the photon localisation length. Anderson- localised modes are observed at high group indices, when the localisation lengths are shorter than the waveguide lengths, consistent with the Fabry-Perot analysis. A spin-photon interface based on two orthogonal waveguides is introduced, where the polarisation emitted by a quantum dot is mapped to a path-encoded photon. Operation is demonstrated by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarisations to anti-parallel waveguides, surprising for a non-chiral structure but consistent with an off-centre dot. Two dimensional photonic crystal cavities in GaInP and full control over the spon- taneous emission rate of InP quantum dots is demonstrated by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode. Fourier transform spectroscopy is used to investigate the short coherence times of InP quantum dots in GaInP photonic crystal cavities. Additional technological developments are also presented including a quantum dot registration technique, electrical tuning of quantum dot emission and uniaxial strain tuning of H1 cavity modes. ii Publications I.J.Luxmoore, E.DaghighAhmadi, N. A. Wasley, A.M.Fox, A.I.Tartakovskii, A. B. Krysa and M. S. Skolnick. “Control of spontaneous emission from InP single quantum dots in GaInP photonic crystal nanocavities”. Applied Physics Letters 97, 181104 (2010). I. J. Luxmoore, E. Daghigh Ahmadi, B. J. Luxmoore, N. A. Wasley, A. I. Tar- takovskii, M. Hugues, M. S. Skolnick and A. M. Fox. “Restoring mode degeneracy in H1 photonic crystal cavities by uniaxial strain tuning”. Applied Physics Letters 100, 121116 (2012). N. A. Wasley, I. J. Luxmoore, R. J. Coles, E. Clarke, A. M. Fox and M. S. Skol- nick. “Disorder-limited photon propagation and Anderson-localization in photonic crystal waveguides”. Applied Physics Letters 101, 051116 (2012). I. J. Luxmoore, N. A. Wasley, A. J. Ramsay, A. C. T. Thijssen, R. Oulton, M. Hugues, S. Kasture, Achanta V. G, A. M. Fox and M. S. Skolnick. “Interfacing spin in an InGaAs quantum dot to a semiconductor waveguide circuit using emit- ted photons”. Physical Review Letters 110, 037402 (2013). I. J. Luxmoore, R. Toro, O. Del Pozo-Zamudio, N. A. Wasley, E. A. Chekhovich, A. M. Sanchez, R. Beanland, A. M. Fox, M. S. Skolnick, H. Y. Liu and A. I. Tartakovskii. “III-V quantum light source and cavity-QED on Silicon”. Scientific Reports 3, 01239 (2013). iii iv Acknowledgment I have been privileged to study in the Low Dimensional Structures and Devices (LDSD) group of the Department of Physics and Astronomy at the University of Sheffield, alongside many talented individuals. I would like to thank those members of the LDSD group with whom I have shared my time during my studies, including past and present postgraduate students as well as the academic and support staff. In particular, I would like to thank my supervisor Maurice Skolnick for all his efforts as group leader and providing guidance and support throughout my time in Sheffield. I must also thank Isaac Luxmoore for his hard work, patience and the exemplary manner in which he conducts his research, from whom I have learned a lot and to whom I owe much. Finally I would like to thank my family and friends for their continued support, especially Laura who has had to suffer me the most during my studies. v vi Contents 1 Introduction 1 1.1 Integrated quantum optical circuits and networks for quantum in- formation processing . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Quantum information processing . . . . . . . . . . . . . . . 3 1.1.2 Implementation of QIP in the solid state . . . . . . . . . . . 4 1.1.3 Quantum optical circuits . . . . . . . . . . . . . . . . . . . . 5 1.2 Photonic crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.1 Photonic bandstructure of a photonic crystal . . . . . . . . . 7 1.2.2 Photonic crystal nano-cavities . . . . . . . . . . . . . . . . . 9 1.2.3 Guided modes in photonic crystal waveguides . . . . . . . . 11 1.3 Quantum dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.1 Energy states . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.3.2 QDs as qubits . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.4 Cavity QED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1 Weak Coupling: Purcell effect . . . . . . . . . . . . . . . . . 19 1.4.2 Strong Coupling: Rabi splitting . . . . . . . . . . . . . . . . 21 2 Experimental methods 23 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.2 Sample fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.1 QD growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.2 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.3 Fabrication acknowledgments . . . . . . . . . . . . . . . . . 28 2.3 Photoluminescence spectroscopy . . . . . . . . . . . . . . . . . . . . 29 2.3.1 Non-resonant photoluminescence spectroscopy . . . . . . . . 29 vii 2.4 Cryostats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.4.1 Cold finger continuous flow helium cryostat . . . . . . . . . 32 2.4.2 Helium bath cryostat . . . . . . . . . . . . . . . . . . . . . . 33 2.5 Time resolved measurements . . . . . . . . . . . . . . . . . . . . . 34 2.6 Second-order correlation measurements . . . . . . . . . . . . . . . . 35 2.7 Two colour, spatially selective collection and interference measure- ments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.7.1 Spatial selection . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.7.2 Spatial alignment . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7.3 Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.7.4 Two-colour selection . . . . . . . . . . . . . . . . . . . . . . 41 3 Disorder limited photon propagation and Anderson localisation in photonic crystal waveguides 43 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2 Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.1 Slow-light mode . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.3 Experimental geometry . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.4 PCW characterisation . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.5 Analysis of Fabry-Perot resonances in PCWs . . . . . . . . . . . . . 49 3.5.1 Group index . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.5.2 Finesse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.6 Anderson-localisation . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.7 Scatter at low group velocity . . . . . . . . . . . . . . . . . . . . . . 54 3.8 Localisation length in PCWs . . . . . . . . . . . . . . . . . . . . . . 58 3.9 Photon propagation limit for viable integrated quantum optical cir- cuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 3.9.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.10 Future directions, applications and integration of slow-light based PCWs in quantum optical circuits. . . . . . . . . . . . . . . . . . . 61 3.10.1 PCW on-chip phase shifter . . . . . . . . . . . . . . . . . . . 61 3.10.2 PCW engineering . . . . . . . . . . . . . . . . . . . . . . . . 65 3.11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 viii