Program Goals

1. Reflection Fluorescence Ag NW Photocurrent Ge NW NIRT: Photon and Plasmon Engineering in Active Optical Devices Based on Synthesized Nanostructures Marko Lončar1, Mikhail Lukin2 and Hongkun Park3 1Harvard Electrical Engineering, 2Harvard Physics, 3Harvard Chemistry Emission from QDs coupled to nanowire, collected through an optical fiber Ag nanowireplasmon waveguide SiNx waveguide Contacts • Program Goals • Understanding and engineering of fundamental properties of light generation and control in active optical nanostructures • Development of robust and practical devices and systems for optical and quantum optical communication and information processing (e.g. single photon sources, low-power/single-photon switches, nano-lasers). • Answer important questions that pertain to hybrid nanostructures: integration of different fabrication techniques, integration across different length-scales, efficient information exchange between nano-structures and macro-world, light-matter interaction on a nanoscale. • Approach • Combination of bottom-up synthesized nanoscale light emitters and metallic (Ag, Au) nanowires with top-down nanofabricated advanced structures for light localization, such as nano-scale surface plasmons and photonic crystals. • Bottom-up synthesized nanocrystal quantum dots (QDs) offer number of advantages over conventional epitaxially grown QDs, including better uniformity, ease of fabrication and integration with passive optical platforms, and multi-wavelength operation. • Synthesized metallic nanowires can be crystalline, and are superior to top-down fabricated metallic waveguides (lower loss) • Photonic crystal cavities can enhance radiation from QDs due to large Purcell factor enabled by their large quality factor and small mode volume. Tapered fiber SiNx waveguide Ge nanowire(detector) Aligned Ag nanowire Quantum Plasmonics new approach to light-matter interface based on sub-wavelength localization and guiding of optical radiation on metallic nanowires Ag nanowire ends Hybrid Nanophotonic-Plasmonic Platform for Efficient Generation and Extraction of Photons Anti-bunching & single photon emission Second-order self-correlation function G(2)(τ ) of QD fluorescence. The number of coincidences at τ = 0 goes almost to zero, confirming that the QD is a single-photon source. The width of the dip depends on the total decay rate Γtotal and the pumping rate R. Second-order cross-correlation function between fluorescence of the QD and scattering from the NW end. This data was taken by looking at coincidences between photon emission from the QD (red circle) and NW end (blue circle). Electrical Detection of Plasmons The detection of plasmons is achieved by generation of electron-hole pairs within a semiconductor (Ge or InAs) nanowires followed by charge separation induced by a local electric field. (Tapered) Optical fiber Coupler SiNx waveguide Ag nanowire SiO2 bottom cladding Quantum dot • Zero-bias efficiency :1~10% (i.e. 0.01 ~ 0.1 electrons/plasmon) • Bias: efficiency ~10 electrons/plasmon. The intrinsic gain mechanism is due to transiently trapped charges modulating the conductivity of the Ge nanowire. Composite image: SEM micrograph and detected current when laser beam is scanned across the structure. Red spots on Ag nanowire ends correspond to 20pA current signal when laser is focused on Ag nanowire ends. Falk, Koppens et al. (2008) Waveguide couplers Coupling of CdSe quantum dots to surface plasmons supported on Ag NW. The red circle corresponds to the position of the QD coupled to nanowire. Ch III: excitation laser was focused on the circled QD. The largest bright spot corresponds to the QD fluorescence, while two smaller spots correspond to SPs scattered from the NW ends. Blue circle indicate farthest end of the wire, used for photon cross-correlation measurements.A.Akimov et al., Nature, 450, 402 (2007) Single-plasmon detector: The Ge-nanowire can detect a signal from CdSe quantum dots coupled to the Ag nanowire. • Broader Impact • Powerful and unique educational opportunities for students • interdisciplinary nature of our NIRT exposes students to theoretical work, nanostructure synthesis, device physics and engineering, nanofabrication and optical characterization. • team members co-advise students and hold bi-weekly joint group meetings • undergraduate students and minorities participate in the efforts of our NIRT through the NSF supported Research Experience for Undergraduates program. • The team members give public lectures and organize science projects at local public schools, mentor high school students and work with high school teachers (NSF RET) • The team members participate in ongoing Harvard outreach programs, as well as engage the business-oriented public (e.g. Harvard Nanotechnology & Business Forum, Harvard Industrial Outreach Program). • The knowledge and techniques developed in this program will find application in other fields, including life sciences (e.g. surface-plasmon enhanced sensing techniques), advanced photolithography, particle manipulation (tweezing), etc. Future Directions Single-photon transistor. In the storage step, a gate pulse consisting of zero or one photon is split equally in counter-propagating directions and coherently stored using an impedance-matched control field (t ). The storage results in a spin flip conditioned on the photon number. A subsequent incident signal field is either transmitted or reflected depending on the photon number of the gate pulse, owing to the sensitivity of the propagation to the internal state of the emitter.D. E. Chang et al., Nature Physics, 3, 807 (2007) Strong-coupling in SiNx nanocavities embedded with diamond nanocrystals: We designed a photonic crystal nanocavity with a quality factor Q>106, a mode volume of Vmod=0.78(λ/n)3, and an operating wavelength of λ=637 nm in SiNx (n=2). Strong coupling between a nanocrystal with an embedded nitrogen-vacancy color-center and the cavity mode is achievable for a range of cavity dimensions. M. W. McCutcheon & M. Loncar, Optics Express (in press) Ultra-high Q cavities based on semiconductor nanowires: By defining one-dimensional photonic crystal at nanowire ends cavities with Q~106and Vmode <0.2(λ/n)3 have been designed. Our cavities are well-suited for the realization of nanowire-based low-threshold lasers, single-photon sources and quantum optical devices that operate in the strong-coupling limit. Y. Zhang & M. Loncar, Optics Express, 16, 17401 (2008)