A diamond nanowire singlephoton source nature nanotechnology, 2010, 5, 195-199 IIDA Atsushi Miyasaka lab. Single-molecule detection Dye molecule Quencher(消光剤) Fluctuation Single-molecule detection Ensemble measurement Direct observation of dynamical state changes Single-molecule detection can provide the information which cannot be obtained by ensemble measurements Single-photon source Photon number Radiation process 1 0 0 Time Only one photon can be detected at one time. We consider that a single molecule is a single photon emitter. Application The high secure communication such as Quantum cryptography (量子暗号) Requirement 1. Emission efficiency should be high. kf >> kn quantum dots, fluorescence dyes kf kn kf: radiation rate kn: nonradiation rate 2. Detection efficiency should be high. Free-space Waveguide, Nanowire Only two directions Using a detector positioned above optical structure Photons are emitted to all directions. Motivation To realize highly efficient single photon emitting source at room temperature • Fabrication of a free-standing diamond nanowire including nitrogen vacancy • Comparison of the efficiency between diamond nanowire and bulk diamond crystal. Contents • Introduction Single-molecule spectroscopy Single-photon source Requirement for single photon source Motivation • Experiment Nitrogen vacancy (N-V) center Sample • Result＆Discussion Confocal microscopy Photon anti-bunching Photon correlation Comparison between nanowire and bulk diamond crystal • Conclusion Nitrogen-vacancy (N-V) center Diamond: Ⅰa ,Ⅰb, Ⅱa, Ⅱb A two point defect in the diamond lattice 1. Substitutional nitrogen atom 2. Vacancy (missing carbon atom) Most of the artificial diamond are this type. yellow Room temperature operation !! High photostability No-photobleach(光退色) Quantum efficiency (量子収率） ≈ 1 Short decay time at excited state FDTD calculation Finite Difference Time Domain method (時間領域差分法) Maxwell’s equation The rate of the leaks to the substrate is large low collection efficiency Nanowire geometry provides an order of magnitude improvement. Sample Reactive-ion etching O2 positive electrode prasma + + + negative electrode E-beam lithography provide ordered arrays. Etching direction is only perpendicular. Sample Straight, smooth sidewall Diameter=260 nm Height=1.9 μm Confocal microscopy 5μm Photon anti-bunching Beam splitter (50:50) Detector 1 A molecule emits one photon from its one excited state. Detector 2 One photon can not be divided. If you detect photons from a single molecule, there is no possibility to detect two photons by the detector 1 and 2 at the same time. Phenomenon that multiple photons do not exist at the same time. 12 Photon correlation Cross-correlation function (相関関数) τ1 τ 2 τ3 Photodetector 1 Photodetector 2 Coincidence counts τ4 τ5 τ6 1 τ4 0 τ1 τ5 τ2 delay time τ τ6 τ3 τ=0 Photon correlation Anti-bunching N-V center in diamond nanowire can operate as “single-photon source”. The fitting function of decay rate; exp(-(r+Γ)|τ|) r; excitation rate excited power (P) Γ; decay rate from excited state = 1/ lifetime The value in the limit of zero excited power Life-time 14.6±1.9ns Photon correlation High excitation power metastable state (dark state) ・Probability of exciting a molecule again The molecule in the metastable state cannot be excited. Comparison between nanowire and bulk diamond crystal nanowire bulk I; number of photon counts per second (cps) P; the power used to saturate the N-V center response ISat (kcps) PSat (μW) nanowire 168±37 58±37 bulk 21±2 990±540 In the case of nanowire; The collection efficiency is the order of magnitude larger. Conclusion • Large number of ordered arrays of diamond nanowire can be fabricated. • Photon correlation establishes N-V center embedded in nanowire is considered as single-photon source. • The detection efficiency of nanowire is much higher than that of bulk crystal.
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