High photon-extraction efficiency is strongly required for a practical single-photon source. We succeed in fabricating metal (sliver)-embedded nanocone structure incorporating an InAs quantum dot. Efficient photon emission of $200 000 photons per second is detected and single-photon emission is demonstrated using autocorrelation measurements. The photon-extraction efficiency as high as 24.6% is obtained from the structure. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4801334]Within the past decade, much attention has been devoted to the development of single-photon sources for future applications in quantum information processing and quantum communications based on a wide variety of systems, such as single atoms, molecules, and color centers in diamonds. [1][2][3] Among these systems, single quantum dots (QDs) are potential candidates owing to their discrete levels, which offer high emission rates, narrow spectral line widths, and wide tunability of emission wavelengths.4-9 Significant progress has been made to achieve efficient single-photon sources. One of the key issues is to enhance photon-extraction efficiency, which is defined as the collection efficiency of photons emitted from a QD into the first lens in an experimental optical setup. Efficient single-photon emission has been demonstrated from single QDs in distributed Bragg reflector (DBR) microcavities with pillar structures 4,5 and inner lateral confinement, 6 photonic nanowires, 7,8 trumpet structures, 9 horn structures, 10 and so on. Coupling of photon emission from QDs to metallic nanoantennas, 11 confined plasmon modes, 12 as well as photonic waveguides 13 was also proposed. Although photonic nanowires and microcavity pillars exhibited high photon-extraction efficiencies, mechanical stability related to their high aspect ratio and their stability to couple to outer photon collection optics remain as challenging issues.Recently, we have introduced a metal-embedded GaAs pillar structure containing single InAs QDs.14,15 This structure is completely embedded in metal and is fundamentally flat and mechanically stable. Direct contact of this kind of photon sources to a single-mode fiber will provide a fiberbased photon source with long-term stability. 16 With this metal-embedded pillar structure, we have observed the photon-extraction efficiency of $8% (efficiency collected by the first lens with the numerical aperture (NA) of 0.42).14 In this case, GaAs substrates were removed by mechanical cleavage during the metal-embedding process, and hard metals were necessary for the sample preparation. Therefore, titanium (Ti) and/or niobium (Nb) hard metals were selected for the embedding metals in these samples. 15,17 However, optical reflectivity of these hard metals is generally low, and it is a drawback for efficient photon extraction. Silver (Ag) is known to have high optical reflectivity in the near-infrared spectral region although the Vickers hardness of Ag is $1/4 and $1/5 of that of Ti and Nb, respectively. In this paper, we report on the...
The microstructural evolution of twinned martensite in water-quenched Fe–1.6 C (wt.%) alloys upon in situ heating was investigated using transmission electron microscopy (TEM). In the as-quenched samples, a high density of a body-centred cubic (bcc) {112} 〈111〉 -type twinning structure exists in the martensite structure. Upon in situ heating to approximately 200–250 °C, carbides (mainly θ-Fe3C cementite) accompanying a detwinning process were observed only in the originally twinned region. The carbides were absent in the originally untwinned (twin-free) region. The experimental results have suggested that the formation of the carbides depends on the twinning-boundary ω-Fe metastable phase, which can be stabilised by interstitial carbon atoms. When the specimens were heated, the twinning-boundary ω-Fe(C) transformed into carbide (mainly θ-Fe3C cementite) particles on the original {112} twinning planes. Further heating resulted in substantial recrystallisation of α-Fe fine particles, which formed immediately after martensite transformation. The results presented here should be helpful in understanding the microstructural evolution of various carbon steels.
Lath martensite is the dominant microstructural feature in quenched low-carbon Fe-C alloys. Its formation mechanism is not clear, despite extensive research. The microstructure of an Fe-0.05 C (wt.%) alloy water-quenched at various austenitizing temperatures has been investigated using transmission electron microscopy and a novel lath formation mechanism has been proposed. Body-centered cubic {112}〈111〉-type twin can be retained inside laths in the samples quenched at temperatures from 1050 °C to 1200 °C. The formation mechanism of laths with a twin substructure has been explained based on the twin structure as an initial product of martensitic transformation. A detailed detwinning mechanism in the auto-tempering process has also been discussed, because auto-tempering is inevitable during the quenching of low-carbon Fe-C alloys. The driving force for the detwinning is the instability of ω-Fe(C) particles, which are located only at the twinning boundary region. The twin boundary can move through the ω ↔ bcc transition in which the ω phase region represents the twin boundary.
Nanosized (∼2 nm) ω-Fe3C particles with hexagonal structures have been observed only at body-centered cubic (BCC) {112}〈111〉-type twinning boundaries in twinned Fe-C martensite of the Fe-C alloy system. However, these ultrafine ω-Fe3C particles never grow large enough to be observed easily. The present structural modeling and electron diffraction calculations reveal that the formation of the new carbide (ω′-Fe3C) during coarsening of the ultrafine ω-Fe3C particles is inevitable. Coarsening or aggregation of fine ω-Fe3C particles may result in a phase transition due to the arrangement of interstitial carbon atoms. A ω-Fe3C → ω′-Fe3C transition was analyzed at the atomic scale. The ω′-Fe3C phase can exhibit an orthorhombic structure with lattice parameters aω′ = 4.033 Å, bω′ = 2.470 Å, and cω′ = 6.986 Å based on aω′ = aω, bω′ = cω, and cω′=3aω for abcc or aα-Fe = 2.852 Å (aω=2abcc, cω=3/2abcc). The simulated ω′-Fe3C electron diffraction patterns were experimentally confirmed. The ω-Fe3C → ω′-Fe3C transition can explain why the ω-Fe3C phase never becomes larger than several nanometers in carbon steel.
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