Nanophotonics II

• Plasmonics • Biophotonics• Exotics

Transverse EM wave coupled to a plasmon (wave of charges on a metal/dielectric interface)

= SPP (surface plasmon polariton)

Polariton – any coupled oscillation of photons and dipoles in a medium

Note: the wave has to have the component of E transverse to the surface (be TM-polarized).

What is a surface plasmon polariton?

Surface plasmons

Barnes et al. Nature 2003

Plasmons can be confined to nanoscale and propagate along nanostrips, through nanoholes, etc.

See derivation of plasmon dispersion on white board

Calculated dispersion of surface plasmon-polaritons propagating at a Ag/air, Ag/glass, and Ag/Si interface, respectively.

Surface Plasmon Resonance (SPR) in different materials

Plasmon resonance frequency strongly depends on geometry

Plasmon absorption by metallic nanoparticles in stained glass windows, glass cups, ceramic pots

The shape of the nanoparticle extinction and scattering spectra, and in particular the peak wavelength λmax, depends on nanoparticle composition, size, shape, orientation and local dielectric environment.

Effect of size and shape on LS PR extinction spectrum for silver nanoprisms and nanodiscs formed by nanosphere lithography. The high-frequency signal on thespectra is an interference pattern from the reflection at the front and back surfaces of the mica.

Anker et al., Nature Mat. 2008

Halas, OPN 2002

Nanoshells: control of SPR wavelength over a broad range

H. Atwater, Scientific American 2007

Calculated dispersion of surface plasmon-polaritons propagating at a Ag/air, Ag/glass, and Ag/Si interface, respectively.

Maier & Atwater, JAP 2005

Note: we cannot excite SPP by simply illuminating the surface!

ki kSPP

i

SPPii kk sinExcitation condition

Impossible to satisfy!ki is always less than kSPP

Excitation of SPP:Kretschmann configuration

Note: these SPP are not particularly small-size

Chem-Bio Sensing in the Kretschmann configuration

Nevertheless, this technique is simple and can be used when we don’t care about having short SPP wavelength

Example of SPR spectrum

Note: the angle is in the TIR range!

Integrated biosensor (Cambridge Consultants Ltd)

Example of kinetic measurement enabled by spatial imaging SPR. A moving front of betamercaptoethanol binding to gold (a). The top image is taken a few seconds after the bottom one (b).

SPR systems can detect kinetic information, such as the rate of complex formation and disintegration of biological species.

Cambridge Consultants Ltd.

Exciting SPP (or any mode of your choice) by scattering light off grating

d

i

Grating changes longitudinal wave vector of a photon by

,...2,1;2 m

dmK g

kSPPki

Coupling to SPP is achieved when SPPgii kKk sin

This is effectively a (quasi-)momentum conservation

Grating can be also used to extract SPPs:

Bozhevolnyi 2007

Photon momentum conservation in photonic crystals

d

kin

,...2,1,2 m

dmK g

=kout

+Kg

When Kg = 2kin: incoming wave is reflected

Localized (near-field) excitation of SPPs by a metallic tip of a near-field microscope illuminated by laser light

Atwater et al. 2007

Detection of SPPs by a tip of near-field microscope

Sondergaard & Bozhevolnyi 2007

Detection of SPPs with photon scanning tunneling microscope (PSTM)

Imaging SPP with PSTM

Zia et al, Mat. Today 2006

Weeber et al. 2007

Propagation of a SP along a 40-nm thick, 2.5-m wide gold stripe, imaged by PSTM …

… and through the right-angle bend

Elements of integrated photonic chips based on SPP

Plasmon waveguides made from chains of nanoparticles

Hohenau et al. 2007

SPP in periodic structures:Merging plasmonics with photonic crystals

Barnes, Nat 2003

Experimental observation of SPP photonic bandgap

Transmission through 2D SPP photonic crystal waveguides

Sondergaard & Bozhevolnyi 2007

Gold scatterers on the gold surface

More of the same

Sondergaard & Bozhevolnyi 2007

Transmission through arrays of subwavelength holes

Wavelength of transmitted light depends on the hole diameter and array period

Barnes, Nat 2003

Plasmonic switches (“plasmonsters”)

H. Atwater, Sci. Am. 2007

THE DREAM: Plasmonic chips

Slot waveguide

Invent your own technique for excitation, detection, waveguiding

of plasmons!

Biophotonics

Evanescent field sensors with substrate sensitized to a specific molecule

Adsorbed molecules change the excitation angle of EM mode

Monitoring of three-step oligonucleotide hybridization reaction

PSTMSNOM

Tip illuminates the sample;Scattered light is collected

Tip collects the evanescent light created by laser illuminating the sample from the back

Near-field microscopy for imaging nanoobjects and single molecules

Note: your tool (PSTM, NSOM etc.) can strongly perturb your sample and distort its properties.

When you do experiment, make sure you understand what you measure.

Always have a reference case to compare with and a control case for which you know what results you should obtain.

Nano-Biosensors based on localized plasmons

• Luminescence of sensitized metal nanoparticles

• Surface enhanced Raman scattering and CARS

Light incident on the nanoparticles induces the conduction electrons in them to oscillate collectively with a resonant frequency that depends on the nanoparticles’ size, shape and composition. As a result of these LSPR modes, the nanoparticles absorb and scatter light so intensely that single nanoparticles are easily observed by eye using dark-field (optical scattering) microscopy.

This phenomenon enables noble-metal nanoparticles to serve as extremely intense labels for immunoassays, biochemical sensors and surface-enhanced spectroscopies.

The shape of the nanoparticle extinction and scattering spectra, and in particular the peak wavelength λmax, depends on nanoparticle composition, size, shape, orientation and local dielectric environment.

Effect of size and shape on LS PR extinction spectrum for silver nanoprisms and nanodiscs formed by nanosphere lithography. The high-frequency signal on thespectra is an interference pattern from the reflection at the front and back surfaces of the mica.

Anker et al., Nature Mat. 2008

When molecules bind to a nanoparticle, the SPR peak wavelength is shifted:

What to observe?? (a) shift of the SPR spectrum

Anker et al., Nature Mat. 2008

Anker et al., Nature Mat. 2008

What to observe?? (b) increase in temperature caused by optically heating the nanoparticle and its environment

You can track these particles by scattering the probe beam off a thermally induced change in the refractive index!

Anker et al., Nature Mat. 2008

How to identify molecules?

Couple SPR shift measurement with SERS!

Tuning the LSPR to maximize the SERS signal. a, SERS spectrum of benzenethiol on AgFONs with varying nanosphere diameters and corresponding resonances: at 532 nm, sphere diameter D = 390 nm (green), at 677 nm, D = 510 nm (orange), and at 753 nm, D = 600 nm (red). T he reflection spectrum is shown in the insets, with minimal reflection corresponding to maximum LSPR induced absorbance and scattering.

Anker et al., Nature Mat. 2008

SERS: Surface enhanced Raman spectroscopy

Raman spectrum of liquid 2-mercaptoethanol (above )and SERS spectrum of 2-mercaptoethanol monolayer formed on roughened silver (below).

Raman scattering Coherent anti-Stokes Raman Scattering(CARS)

lase

r

lase

r

lase

r

Stok

es

Stok

es

Ant

i-Sto

kes

Usually signal is very weak, but it gets greatly enhanced near SPR!

Measured quantity: Raman shift laser- Stokes

Molecularvibrations

Biochip for multiplexed SPR detection

Anker et al., Nature Mat. 2008

The first in vivo SERS implantable glucose sensor. a, Experimental setup used for in vivo SERS measurements in rats. b, Fabrication and functionalization of SERS -active surfaces: formation of a nanosphere mask, silver deposition resulting in formation of the silver film over nanospheres (AgFON) surface, incubation in decanethiol, and incubation in mercaptohexanol. c, Atomic-force micrograph of a typical AgFON surface. d, Reflection spectrum of AgFON optimized for in vivo experiments.

Anker et al., 2008

Left-handed materials

HEk

EHk

0

0 ,

L. Mandelshtamm, 1944

1>0

2>0

1>0

2<0

2

22

ck

Recent review: Physics of Negative Refraction (Eds. C.M. Krowne, Y Zhang) (Springer, 2007).

See derivation of light propagation in LHM on white board

“Superlens” and its challenges

Zhang & Liu, Nat. Mat. 2008

Zhang & Liu, Nat. Mat. 2008