Nanophotonic switch using ZnO nanorod double-quantum-well structures

Abstract

The authors report on time-resolved near-field spectroscopy of ZnO / ZnMgO nanorod double-quantum-well structures ͑DQWs͒ for a nanometer-scale photonic device. They observed nutation of the population between the resonantly coupled exciton states of DQWs. Furthermore, they demonstrated switching dynamics by controlling the exciton excitation in the dipole-inactive state via an optical near field. The results of time-resolved near-field spectroscopy of isolated DQWs described here are a promising step toward designing a nanometer-scale photonic switch and related devices. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2743949͔ Systems of optically coupled quantum dots ͑QDs͒ should be applicable to quantum information processing. 1,2 Additional functional devices ͑i.e., nanophotonic devices 3-6 ͒ can be realized by controlling the exciton excitation in QDs. ZnO is a promising material for room-temperature operation, owing to its large exciton binding energy 7-9 and recent achievements in the fabrication of nanorod heterostructures. 10,11 This study used time-resolved near-field spectroscopy to demonstrate the switching dynamics that result from controlling the optical near-field energy transfer in ZnO nanorod double-quantum-well structures ͑DQWs͒. We observed nutation of the population between the resonantly coupled exciton states of DQWs, where the coupling strength of the near-field interaction decreased exponentially as the separation increased.To evaluate the energy transfer, three samples were prepared ͓Fig. 1͑a͔͒: ͑1͒ single-quantum-well structures ͑SQWs͒ with a well-layer thickness of L w = 2.0 nm ͑SQWs͒, ͑2͒ DQWs with L w = 3.5 nm with 6 nm separation ͑1-DQWs͒, and ͑3͒ three pairs of DQWs with L w = 2.0 nm with different separations ͑3, 6, and 10 nm͒, where each DQW was separated by 30 nm ͑3-DQWs͒. These thicknesses were determined by the transmission electron microscopy ͑TEM͒ measurement. ZnO / ZnMgO quantum-well structures ͑QWs͒ were fabricated on the ends of ZnO nanorods with a mean diameter of 80 nm using catalyst-free metal organic vapor phase epitaxy. 10 The average concentration of Mg in the ZnMgO layers used in this study was determined to be 20 at. %.The far-field photoluminescence ͑PL͒ spectra were obtained using a He-Cd laser ͑ = 325 nm͒ before detection using near-field spectroscopy. The near-field photoluminescence ͑NFPL͒ spectra were obtained using a He-Cd laser ͑ = 325 nm͒, collected with a fiber probe with an aperture diameter of 30 nm, and detected using a cooled chargecoupled device through a monochromator. Blueshifted PL peaks were observed at 3.499͑I S ͒, 3.429͑I 1D ͒, and 3.467 ͑I 3D ͒ eV in the far-and near-field PL spectra ͓Fig. 2͑a͔͒. We believe that these peaks originated from the respective ZnO QWs because their energies are comparable to the predicted ZnO well-layer thicknesses of 1.7͑I S ͒, 3.4͑I 1D ͒, and 2.2 ͑I 3D ͒ nm, respectively, calculated using the finite square-well potential of the quantum confinement effect in ZnO SQWs. 10 To confirm the near-field energy trans...

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