InGaAsP/InP Nanocavity for Single-Photon Source at 1.55-μm Telecommunication Band

Song et al. Nanoscale Research Letters (2017) 12:128 DOI 10.1186/s11671-017-1898-y NANO COMMENTARY Open Access InGaAsP/InP Nanocavity for Single-Photon Source at 1.55-μm Telecommunication Band Hai-Zhi Song1,2,3* , Mukhtar Hadi1, Yanzhen Zheng2, Bizhou Shen3, Lei Zhang2, Zhilei Ren2, Ruoyao Gao2 and Zhiming M. Wang1 Abstract A new structure of 1.55-μm pillar cavity is proposed. Consisting of InP-air-aperture and InGaAsP layers, this cavity can be fabricated by using a monolithic process, which was difficult for previous 1.55-μm pillar cavities. Owing to the air apertures and tapered distributed Bragg reflectors, such a pillar cavity with nanometer-scaled diameters can give a quality factor of 104–105 at 1.55 μm. Capable of weakly and strongly coupling a single quantum dot with an optical mode, this nanocavity could be a prospective candidate for quantum-dot single-photon sources at 1.55-μm telecommunication band. Keywords: Nanocavity, Single-photon source, Quantum dot, Distributed Bragg reflector PACS Codes: 78.67.-n, 78.67.Hc, 78.67.Pt Background Optical microcavities and nanocavities are widely studied for their prospects in many fields of research and technology, such as optical communication, nonlinear optics, optoelectronics, and quantum information technology [1, 2]. For solid-state quantum information processing, microcavities and nanocavities containing semiconductor quantum dots (QDs) have been demonstrated to be effective as indispensable devices such as efficient [3–5] and indistinguishable single-photon sources (SPSs) [6, 7] and coherent quantum-control devices [8, 9]. Among many cavity types, pillar cavities are advantageous for fiberbased quantum information processing owing to high coupling efficiency to fiber [10] and suitability for electrical driving [11]. For the purpose of quantum communication over a silica fiber-based network, 1.55-μm InAs/InP QDs are promising as SPSs [12] and thus pillar cavities * Correspondence: 1 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Section 2-4, Jianshebei Road, Chengdu 610054, Sichuan, China 2 School of Optoelectronic Information, University of Electronic Science and Technology of China, Section 2-4, Jianshebei Road, Chengdu 610054, Sichuan, China Full list of author information is available at the end of the article containing InP-based QDs are strongly required. Like the pillar cavities for InAs/GaAs QDs, the straight way for InAs/InP QDs might be micro- or nanopillar cavities composed of InP lattice-matched distributed Bragg reflector (DBR) layers such as InP/InGaAsP and AlInGaAs/ AlInAs, which might be monolithically fabricated. However, this kind of pillar cavity is thought to be so high, due to small refractive index contrast of ~0.2 [13] that nobody wants to try. By increasing the refractive index contrast in DBRs, people tried pillar cavities hybridizing semiconductor and dielectric materials, e.g., Ta2O5/SiO2–InP [14] and Si/SiO2–InP [15, 16]. However, the hybrid approach is not ideal due to the complicated fabrication process, defects near the light source caused by thin active layer, and mismatching thermal expansion in different materials. Consequently, a practically good pillar cavity has not been available yet as a SPS applied in 1.55-μm quantum information processing. More efforts must thus be devoted to finding methods of overcoming the above-stated problems. We are herewith considering some techniques beyond material hybrid. In the case of planar DBR cavity, an effective way to increase refractive index contrast of InP-based materials is to introduce air gaps by sacrificing some semiconductor layers [13, 17]. For pillar cavities, © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Song et al. Nanoscale Research Letters (2017) 12:128 partial air-gap layers like in GaAs/air DBR cavities [18] might be incorporated to enhance the refractive index contrast. In this work, therefore, we propose a nanopillar cavity consisting of InGaAsP/InP layers with partial air gaps, which can be monolithically fabricated. It is presented that this nanocavity has high quality (Q) factors and small mode volumes, satisfying the requirements of SPS at 1.55-μm telecommunication band. Methods The proposed cavity structure is schematically demonstrated in Fig. 1a. It shows that disk shaped (in the XY plane) and coaxially set (in the Z direction) InGaAsP and InP layers with different diameters D and d, respectively, are alternatively stacked on an InP substrate. Effectively, the small-sized InP layers are compassed by surrounding air gaps, or namely with air apertures. The InGaAsP layers are lattice matching to the InP substrate and have an energy gap larger than the photon energy of 1.3-μm wavelength, so that they are extremely transparent for ~1.55-μm light. Compared to the previous air-gap DBR cavities [13, 17, 18], in which non-air-gap regions are imperfect features or mechanical supporters, the remaining semiconductor in the partial air-gap layers here takes both the mechanically supporting and optically confining roles so that the present cavity appears completely free standing. In more detail, the top and bottom parts of the cavity are conventional DBRs composed of periodical InGaAsP and InP layers. Each InP layer in the DBRs is set as thick as t1 = λB/4, where λB is the Bragg wavelength, set to be around 1.55 μm. This thickness is actually a quarter wavelength of air because the optical media of this layer in the pillar is mainly air rather than InP. In the case of Page 2 of 7 planar air-gap DBR cavities [13, 17, 18], semiconductor layers are usually set to be three-quarter-wavelength thick, but our simulation implies that this design in our case hardly has good cavity quality. Thus, the InGaAsP layers in the DBRs are set quarter-wavelength thick, i.e., t2 = λB/(4n2), where n2 is the refractive index of InGaAsP. Inserted between the conventional DBRs are more InGaAsP/InP-air-aperture segments (pairs) as tapered DBRs on both the top and bottom sides. Here, “taper” means adiabatically deducing the layer thicknesses as the DBR extends towards the cavity center (spacer) [19, 20]. In detail, the tapered DBRs have linearly decreasing layer thicknesses t1i = t1(1−ρ(2i−1)) for InP and t2i = t2(1−2ρi) for InGaAsP, where i stands for the taper segment number and ρ is the tapering slope of layer thickness, i.e., the decreased fraction per tapered layer. In between the tapered DBRs, an InP layer is inserted as the spacer layer with thickness t0 = t1(1−2ρN), where N is the total taper segment (...truncated)