Vapor-deposited thin films with negative real refractive index in the visible regime J. J. Yi, A. Lakhatakia, W. Y. Ching, T.L. Chin Optics Express Vol.

Vapor-deposited thin Vapor-deposited thin films with negative realfilms with negative realrefractive index in the refractive index in the

visible regimevisible regimeJ. J. Yi, A. Lakhatakia, W. Y. J. J. Yi, A. Lakhatakia, W. Y.

Ching, T.L. ChinChing, T.L. ChinOptics Express Vol 17, No 10 May 11 2009Optics Express Vol 17, No 10 May 11 2009

Introduction

• Metamaterials– artificial composite materials

that, by their microstructure, exhibit properties not exhibited by their component materials

Introduction

•Negatively refracting metamaterials–Consists of coupled, metallic, subwavelength elements that simulate electric and magnetic dipoles.

Introduction

• In such metamaterial, an EM wave propagates so that the direction of its energy flow is opposed to its phase velocity, a condition captured by the real part of the refractive index being negative

Introduction

• Negative refractive index can be made available by the fabrication of thin films using Oblique angle deposition techniques.

• These films are optically anisotropic, like biaxial crystal.

OAD (e-beam)

Silver

Electron gun

Substrate

θ

Theory

• In general, the equivalent relative permittivity tensor ε and the equivalent relative permeability tensor μ must have the same set of three eigenvalues.

• When the film is illuminated normally, different combinations of the eigenvalues of these tensors appear for the two polarization states (p- and s- polarized)

TheoryRefractive index

Relative Intrinsic Impedance

where v=p,s depending on the polarization

Theory

• Both the nv and ηv can be determined by measuring the reflection coefficient rv and the transmission coefficient τv of the film of thickness d.

Methodology

• 2-inch square substrates of fused silica (glass)

• Electron beam deposition• Base pressure: 4x10-6 Pa• Deposition Rate: 0.3 nm/s• Deposition angle: 86°

Methodology

• The reflection and transmission coefficients were measured at specific λ’s using an ellipsometer and a walk-off interferometer.

Methodology

• Diode lasers of λ=532 nm, 639 nm and 690 nm were used as light sources and the ellipsometer used was a PSA (Polarizer-Sample-Analyzer) system

• This yields the ration τp/τs

Methodology

• The walk-off interferometer was used to measure τs and both reflection coefficients

• “the incident laser beam is separated into two beams––one s-polarized and the other p-polarized.”

Methodology

• “One of the polarized beams is normally incident on the sample (silver thin film) and the other polarized beam is incident on the bare substrate; the two reflected and transmitted beams combine and produce interference.”

Methodology

• “The polarization state of the combined beam yields the absolute phase of the reflection coefficient or the transmission coefficient associated with a specific polarization state.”

Results

• Film thickness: 240 nm• Nanorods angle: 66°±5°• Average nanorod length: 650 nm• Average nanorod diameter: 80 nm

Results

Results

Discussion• S-polarized

– The real parts of ε, μ and n are all positive at all three wavelengths

• P-polarized– The real part of μ > 0 while the real part of ε

< 0 at all three wavelengths.– Both the real and the imaginary part of ε

increase with wavelength.• Similar to that predicted by the plasmonic-type

permittivity model for composites containing thin-wire metal inclusions

Discussion

• P-polarized– The imaginary part of n > 0, while the

real part of n < 0 at all three wavelengths.

– -np’/np” lies between 0.3 and 0.65 over the 532-690 nm

λ np’

532 nm -0.705

639 nm -0.476

690 nm -0.552

Conclusion

• A well-established thin-film technique, Oblique Angle Deposition, can be used to deposit thin films that refract light negatively.

• OAD is stable for depositing multilayered stacks used in optical filters and mirrors.

• Layers of a gain medium can be incorporated to offset any attenuation using