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NIRT: Photon and Plasmon Engineering in Active Optical Devices based on Synthesized Nanostructures Marko Lončar

, Mikhail Lukin

and Hongkun Park

3 1

Harvard Electrical Engineering,

Harvard Physics,

Harvard Chemistry 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.

Quantum Plasmonics

new approach to light-matter interface based on sub-wavelength localization and guiding of optical radiation on metallic nanowires

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) Optical fiber Polymer core (spot-size converter) SiN x waveguide Ag nanowire Quantum dot Schematic of hybrid quantum plasmonic device that combines bottom-up synthesis and top-down nanofabrication.

SiO 2 bottom cladding Purcell factor as a function of wire diameter.  =630nm TEM and SEM images of Ag nanowires prepared by the polyol process.

Anti-bunching & single photon source

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 Γ

total 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).

A.Akimov et al, Nature,

450

, 402 (2007) Normalized energy flux for an emitter positioned (from top to bottom) at distances

k 0 d 0:002, 0.2,

and 0.7 from the nanowire. •The first plot is mostly dark and indicates that the emitter decays primarily nonradiatively.

• The middle plot demonstrates efficient excitation of guided plasmons at the final radius

• The last plot exhibits the typical lobe pattern associated with radiative decay.

D. E. Chang et al.,

PRL

, 053002 (2006)

Effective mode-index (n eff ) of Ag nanowires

(  =630nm) vs NW radius (r). Insets: mode profile for r=50nm and r=150nm. In contrast to dielectric waveguides, Ag NW supports guided mode even when r<<  . This ultra confined plasmon mode is ideal for “optical wiring” of nanoscale quantum emitters.

Optimized efficiency of single-photon generation vs R, including coupling to the dielectric waveguide.

Solid line: theoretical efficiency using a nanowire. Dotted line: theoretical efficiency using a nanotip.

Coupling of CdSe QD radiation 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)

Future Directions 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.

Schematic diagram of transistor operation involving a three-level emitter.

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)

Hybrid photonic crystal /semiconductor nanocrystal

single photon source, ultra low power switch, and quantum/ optoelectronics networks.

(a) Schematic of