Negative Index of Refraction

1
Negative Index of Refraction

• By Jason Kaszpurenko
• Journal Club
• 1/16/09

2
Overview

• Both articles that I read were from a Materials
  Research Society October 2008 Bulletin
• General Overview about negative index materials
• What is ti
• What properties does it have
• What possible applications
• Making Negative Index of Refraction materials
• Two types of Negative Index Materials
• Attempts to get into the optical range
• Questions (Yours and mine)

3
Overview

• Negative index of refraction was first theorized
  by Victor Veselago in 1968
• The idea that a material could have both negative
  permittivity and permeability
• If it had both of these it would not violate the
  laws of physics
• First confirmed by J. Pendry in 2000

4
Overview

• Index of refraction is normally defined by
• nc/v or n(eµ)0.5
• c is the speed of light in vacuum, v is the speed
  of light in a medium, e is permittivity and µ is
  permeability
• e can be found negative naturally in several
  metals such as gold and silver but µ needs to be
  engineered artificially to be negative
• The shortest wavelength observed with this
  property is 710 nm

5
Overview

• In a normal material the k, E and H of the
  material right handed set (good old right hand
  rule)
• In negative index materials (NIM) the k, E and H
  form a left handed set (your students were doing
  it for negative index of materials)
• This causes the waves phase front to move in the
  opposite direction of the wave itself
• The energy of the wave is associated with the
  group velocity
• To the right we have an example of this. The
  Gaussian wave packet moves to the right while the
  wave front, (red point) moves to the left
• W. Park, J. Kim, MRS bulletin Oct 2008

6
Optical Properties of negative index materials

• Negative index materials can be used to make
• Electromagnetic cloaking devices
• Super lenses
• filters
• Sub wavelength waveguides and antennas
• Im going to talk about the super lenses

7
Super lenses

• In most optics the limiting factor is the
  wavelength of light
• The evanescent waves, waves which exponentially
  decay in mater, actually contain information that
  is smaller than the wavelength, but this is
  normally lost
• In negative index materials the evanescent waves
  are actually enhanced

8
Evanescent waves
Image a The red lines represent the evanescent
waves fon nlt0 with the light getting focused.
While the blue dotted are for ngt0 and the light
getting scattered. Image b is the amplitude of
the evanescent light in negative and positive
materials. Image c shows a simulation of this
phenomenon. The smaller image than source size
means enhanced evanescent waves.
W. Park, J. Kim, MRS bulletin Oct 2008
9
Elaboration of conditions needed for negative
index materials

• Originally Veselago argued that you need the real
  and complex parts of permeability and
  permittivity to be negative
• This is an over constrained condition the real
  one is
• eµ eµlt0 ( is real and is complex part)
• If elt0 or µlt0 we have a single-negative NIM
  (SN-NIM)
• If elt0 and µlt0 we have a double-negative NIM
  (DN-NIM), DN-NIM have the potential to have less
  losses and are considered better because of this

10
Making µ lt 0

• Three common types of magnetic resonators are
• Bihelix (figure a) this resonator uses two
  separate strips of the same metal
• Split-ring resonators (SRR) (figure b) uses to
  different rings and is a very common choice but
  the magnetic response becomes saturated in the
  visual regime.
• Pair of Nanorods is the last configuration, this
  was used by the authors to get into the optical
  regime

Chettiar, et all, MRS bulletin Oct 2008
11
Synthesis

• An attempt was made to synthesize nanorods with
  different deposition rates
• Al2O3 was deposited in between the layers Ag
  nanorods
• Sample A was deposited at 2 A/s while sample B
  was deposited at 0.5 A/s
• Using AFM cross sections we can see that the
  faster deposition rate created (right) a rougher
  surface than the slower deposition (lower right)

Chettiar, et all, MRS bulletin Oct 2008
12
Permittivity and Permeability
Sample A has a high deposition rate and Sample B
has a low deposition rate
Chettiar, et all, MRS bulletin Oct 2008
13
Results for different spacing

• When varying the spacing of the magnets are
  verried different frequencies of light are
  allowed to pass, but electrons view it is a metal
• Image a Transmission mode with TM polarization
• Image b Transmission mode with TE polarization
• Image c Reflection mode with TM polarization
• Image d Reflection mode with TE polarization

14
Conclusion

• Although theorized over 40 years ago NIM have
  only been made within the last decade
• NIM act in many unconventional ways, wave phase
  front moves in opposite direction of group
  velocity, evanescent waves increase.
• These properties lend themselves to making unique
  devices like super lenses that can overcome
  traditional optical limits
• The difficulty in making them comes from the
  negative permeability, which has to be
  artificially manufactured
• The optical regime is just being realized

15
Questions

• How does varying the oxide material in-between
  the nanorods effect the index of refraction
• With evanescent waves increasing in amplitude,
  how is energy being conserved?
• What attempts have there been on working on
  different materials with a negative permittivity