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 • n=c/v or n=(εμ)^0.5 • c is the speed of light in vacuum, v is the speed of light in a medium, ε is permittivity and μ is permeability • ε 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 wave’s 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 • I’m 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 n<0 with the light getting focused. While the blue dotted are for n>0 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: • ε’μ”+ ε”μ’<0 (’ is real and ” is complex part) • If ε’<0 or μ’<0 we have a single-negative NIM (SN-NIM) • If ε’<0 and μ’<0 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 < 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