ECE280: Nano-Plasmonics and Its Applications. Week5. Extraordinary Optical Transmission (EOT)

Transcription

1 ECE280: Nano-Plasmonics and Its Applications Week5 Extraordinary Optical Transmission (EOT)

2 Introduction Sub-wavelength apertures in metal films provide light confinement beyond the fundamental diffraction limit. The transmission efficiency of sub-wavelength sized apertures decays as (d/λ)4, where d is the aperture size and λ is the wavelength of the transmitted light. It has been demonstrated that the transmission efficiency of a single hole can be greatly enhanced with the aid of surface plasmon polaritons (SPPs).

3 Resolution Improvement In 1928 E.H. Synge suggested that one could beat the diffraction limit by squeezing light through a subwavelength-size hole in an opaque screen. The light will spread quickly due to diffraction once it emerges from the other side of the hole, but if the hole is brought very near to an object to be imaged, one can illuminate that object with a light spot whose size is no larger than the hole in the screen. E.H. Synge, A suggested model for extending microscopic resolution into the ultramicroscopic region, Phil. Mag. 6 (1928), 356.

4 Light Transmission Through Deep Sub-λ Aperture Bethe, Phys. Rev. 66, 163, (1944)

5 Extraordinary Optical Transmission (EOT)

6 The Geometry 200 nm Ag film v.d. onto quartz focused ion beam lithography 150 nm holes 600 nm to micron spacing Saloman et al. Phys. Rev. Lett. 2001, 86(6), 1110

7 Ghaemi et al. Phys. Rev. B 1998, 58(11), 6779

8 Transmission Results zero-order transmission from a grating is expected to decrease monotonically at larger wavelengths. Transmission efficiency = fraction of light transmitted/ fraction of surface area holes = 2. More than twice the light that impinges on the holes is transmitted through the film Ebbesen et al. Nature 1998, 391,667

9 Transmission Mechanism? Not cavity resonance since peak position (in spectrum) does not significantly depend on hole dimensions Not waveguiding because film thickness too small (200 nm) Surface plasmon tunneling? Surface plasmon scattering? Evanescent wave tunneling and scattering?

10 Far Field Transmission Spectrum Clue 1: only metallic samples work Cr Ag Au Phys. Rev. Lett. 86, 1114 (2001) Hole spacing determines peak position Peak position independent of hole d Independent of metal (Ag, Cr, Au) Ge doesn t work

11 Far Field Transmission Spectrum Clue 2: the angular dependence of the spectra in metallic samples The zero-order transmission spectra change in a marked way even for very small angles. The peaks change in intensity and split into new peaks which move in opposite directions. This is exactly the behaviour observed when light couples with SPs in reflection gratings.

12 SP Mediated EOT k sp = k x ± ng x ± mg y How do surface plasmons help enhance the amount of light passing through a series of holes in a metal plate? 1. Some of the light incident upon the hole array becomes converted to surface plasmons The surface plasmons go through the holes. The holes serve as a resonant cavity to boost the field intensity on both surfaces of the metal On the transmission side of the metal, part of this high field intensity gets converted into freely propagating fields which creates a higher-thanexpected transmission.

13 Other Mechanisms?

14 N*N Array in Silver

15 Non-Plasmonic Samples Composite diffracted evanescent wave (CDEW) model The evanescent component of the diffracted field, which includes contributions from all in-plane k- vectors larger than k 0, can be described simply as a composite evanescent wave with propagation characteristics that are distinct from those of an SP.

16 Plasmons + Diffraction The truth of the matter is that sometimes plasmons play the dominant role, sometimes diffraction theory does, but how can one distinguish the two cases? One of the major difficulties of nano-optical research, and one of the reasons it is a field of interest, is that simple models of light interacting with subwavelength structures are not readily available. Grupp et al., Appl. Phys. Lett. 2000, 77(11), 1569

17 Diffraction Theory

18 Microscopic Picture: Elementary Process They are all associated with the scattering of an electromagnetic field by a 1D hole chain under illumination by the SPP mode (a), the fundamental Bloch mode of the hole chain (b), and an incident transverse magnetic polarized (magnetic vector along y axis) plane wave impinging with an oblique incidence defined by its in-plane wavevector component k x (c). The red and green arrows refer to the incident and scattered modes, respectively. ρ (the reflectance coefficient of the SPP mode), τ (the transmittance coefficient of the SPP mode), α (the scattering coefficient from the SPP mode to the fundamental Bloch mode and vice versa according to the reciprocity theorem), β(k x ) (the scattering coefficient from the SPP mode to the outgoing plane wave with an in-plane wavevector component kx), t(k x ) (the scattering coefficient from the fundamental Bloch mode to the plane wave) and r (the reflectance coefficient of the fundamental Bloch mode). Liu and Lalanne, Nature, 452, 728 (2008)

19 Potential Topics Generation of single optical plasmons in metallic nanowires coupled to quantum dots, Nature, 450, (2007). Planar lenses based on nanoscale slit arrays in a metallic film, Nano Letters, 9, (2009). Observation of the spin-based plasmonic effect in nanoscale structures, Physics Review Letters, 101, (2008). Plasmon-assisted transmission of entangled photons, Nature, 418, (2002). Energy-time entanglement preservation in plasmon-assisted light transmission, Physics Review Letters, 94, , (2005). Microscopic theory of the extraordinary optical transmission, Nature, 452, (2008). Energy transfer across a metal film mediated by surface plasmon polaritons, Science, 306, (2004). Two-dimensional imaging of potential waves in electrochemical systems by surface plamon microscopy, Science, 269, (1995). Nanometric optical tweezers based on nanostructured substrates, Nature Photonics, 2, (2008). Plasmonic photon sorters for spectral and polarimetric imaging, Nature Photonics, 2, (2008). Organic plasmon-emitting diode, Nature Photonics, 2, (2008). Ultrafast active plasmonics, Nature Photonics, 3, (2008). Efficient unidirectional nanoslit couplers for surface plasmons, Nature Physics, 3, (2007). Optical antennas based on coupled nanoholes in thin metal films, Nature Physics, 3, (2007). Integration of photonic and silver nanowire plasmonic waveguides, Nature Nanotechnology, 3, (2008). Far-field optical microscopy with a nanometer-scale resolution based on the in-plane image magnification by surface plasmon polaritons, Physics Review Letters, 94, (2005).