Broad-band nano-scale light propagation in plasmonic structures

1. Broad-band nano-scale light propagation in plasmonic structures Shanhui Fan, G. Veronis Department of Electrical Engineering and Ginzton Laboratory Stanford University In collaborations with Professors Mark Brongersma and Peter Peumans Supported by the Stanford-GCEP, and NSF-NIRT

2. Organic Solar Cell • Silicon and Compound Semiconductor Cells • High efficiency (~30%), but high cost. • Organic Solar Cell • Low cost, but low efficiency (<5%)

3. Operational Principle of Organic Solar Cells D A Photon absorption Exciton diffusion Charge-transfer Charge-separation Charge-collection

4. D Optical absorption ~ 100nm Exciton Diffusion ~ 10nm A Exciton Diffusion Process • Deliver light directly to the DA interface. • Enhance light absorption at the DA interface. • Efficiently extract carriers once they are generated.  Nanoscale manipulation of light and electrons using metals.

5. From single-wavelength to deep sub-wavelength scale 1 m Scale of SOI waveguide Vlasov et al, IBM, 2004 Scale of a transistor, < 100nm Core of a single mode fiber: ~ 10 m Kobrinsky et al, Intel, 2004

6. The need for nano-photonics in optical interconnect Micron scale dielectric waveguide Nanoscale photodetector or latch Stanford MURI on Plasmonics (Brongersma, Miller, Fan) • The relevant length scales here: • modal diameter ~ 50-100 nanometer; propagation distance ~ 10 micron • Broadband width.

7. Two-conductor configuration: perfect metal d Frequency /p  Perfect metal Air Wavevector k/kp

8. Two-conductor configuration: plasmonic metal d Band 2 Frequency /p  Band 1 Plasmonic metal Air Wavevector k/kp E. N. Economu, Physical Review B, 182, 539 (1969)

9. Band 1 Magnetic field Electric field Low Frequency Intermediate Frequency (infrared and visible) High Frequency (ultra-violet)

10. Band 1 Magnetic field Electric field Low Frequency Intermediate Frequency (infrared and visible) High Frequency (ultra-violet)

11. Plasmonic slot waveguide air (n=1) metal metal SiO2 (n=1.5) slot dimension: 50~100 nm • The corresponding microwave structure does not support a true bound mode in this asymmetric geometry. • Intermediate regime showing both microwave and plasmonic behaviors. G. Veronis and S. Fan, Optics Letters, 30, 3359 (2005)

12. Bound-mode in plasmonic slot waveguides w = 50 nm  =1550nm  =1550nm • Calculated using tabulated experimentally determined dielectric function of silver at all frequencies. • True bound mode. • Guiding bandwidth exceeding 100THz.

13. Modal Diameter << Wavelength y  =1550nm • Mode diameter is small even when the phase index approaches that of silica. • Mode diameter ~ 90 nm at 1.55 micron wavelength. • Mode diameter weakly dependent upon frequency. G. Veronis and S. Fan, Optics Letters, 30, 3359 (2005)

14. Far field v.s. near field y • Modal size determined by the near field. • Exponential decay only appears far from waveguide, where the field amplitudes are already negligible.

15. Nano-scale waveguide bends Ag air Ag 50 nm • Complete transmission through sharp bends from microwave to optical wavelength range G. Veronis and S. Fan, Applied Physics Letters, 87, 131102 (2005).

16. Coupling between dielectric and MDM guide • Direct butt coupling. • 70% Coupling efficiency. • Non-adiabatic taper. • Designed with micro-genetic algorithm. • 93% Coupling efficiency. G. Veronis and S. Fan, Optics Express (submitted)

17. Summary • Proper design of metallic nano-structures leads to sub-wavelength propagating modes with very broad bandwidth. • Such modes might be exploited for nano-scale manipulation of light in energy and information applications. • Plasmonic crystals may also be used to substantially modify optical absorption and thermal emission properties.