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«by Princess MyCia Cox A dissertation submitted to the faculty of The University of North Carolina at Charlotte in partial fulfillment of the ...»

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DESIGN AND FABRICATION OF LOW LOSS AND LOW INDEX

OPTICAL METAMATERIALS

by

Princess MyCia Cox

A dissertation submitted to the faculty of

The University of North Carolina at Charlotte

in partial fulfillment of the requirements

for the degree of Doctor of Philosophy in

Nanoscale Science

Charlotte

Approved by:

______________________________

Dr. Michael Fiddy

______________________________

Dr. Patrick Moyer ______________________________

Dr. Marcus Jones ______________________________

Dr. Brigid Mullany ii ©2013 Princess MyCia Cox

ALL RIGHTS RESERVED

iii

ABSTRACT

PRINCESS MYCIA COX. Design and fabrication of low loss and low index optical metamaterials. (Under the direction of DR. MICHAEL FIDDY) The study of optics has changed over the past 3000 years. We have developed beyond the early lens technologies of quartz and other types of glass materials and new materials are being engineered to enhance the properties of naturally occurring materials through metamaterials. Scattering and propagation of light through subwavelength scale structures can alter the bulk electromagnetic permittivity and permeability of the constituent materials incorporating these nanostructures. Such metamaterials can be modeled and designed to create desired electromagnetic responses such as having a refractive index less than unity. This is the focus of this research using nanoscale lowloss metallic and semiconductor materials. We investigate a new rigorous scattering model for sub-wavelength sized particles that provides a guide that allows bulk materials with reduced refractive index to be made. We show numerical and experimental results supporting this goal.

Using nanoscale low loss semiconductor materials we investigate this scattering model for sub-wavelength sized particles. This approach could lead us to the “Holy Grail” in optical science which is a negative or near zero index material that operates at visible wavelengths.

iv

ACKNOWLEDGEMENTS

I would like to thank my committee chair and advisor Dr. Michael Fiddy for his dedication to this project and for helping me strive to continue this endeavor to the very end. I thank him for his continual spirit of positivity and enthusiasm over this body of work.

I would like to thank my committee members for supporting me through this process and encouraging me to think beyond and go beyond the call of duty as a graduate student.

I would like to thank the professors and graduate students at the University of North Carolina at Charlotte for their insights and support. I would like to thank the Fiddy Research Group for help with numerical simulations, and Dr. Jeffrey Tharp for training and his expertise on HFSS software.

Finally, I would like to thank my mother Dr. R.Cox and my sisters and brothers who have encouraged me and cheered for me every step of the way. This dissertations is dedicated to them and my father Leroy Cox Jr, who has since passed on who taught me to

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TABLE 1: Theoretical steps for finding the concentration of particles 47 required for a given target index.

TABLE 2: UV-VIS spectrograph measured data from the Al-Doped ZnO 62 during synthesis using the ethanol wash method.

TABLE 3: UV-VIS spectrograph measured data from the Al-Doped ZnO 65 during synthesis using the water wash method.

TABLE 4: Dynamic Light Scattering measured data of Al-Doped ZnO 66 during synthesis in relationship to particle growth using the ethanol wash method.

TABLE 5: Dynamic Light Scattering measured data of Al-Doped ZnO 67 during synthesis in relationship to particle growth using the water wash method.

TABLE 6: Emission spectrograph measured data from the Al-Doped ZnO 70 during synthesis using the ethanol only wash method.

TABLE 7: Emission spectrograph measured data from the Al-Doped ZnO 71 during synthesis using the water wash method.

TABLE 8: Size of AZO nanoparticles using ethanol wash synthetic 77 technique after adhesion of octadecylamine.

TABLE 9: UV-VIS spectrographs of AZO nanoparticles using the 79 ethanol wash synthetic technique before and after ligand adhesion of octadecylamine.

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FIGURE 10: Material parameter space characterized by electric permittivity 39 and magnetic permeability (Liu, 2010).

FIGURE 11: Index change of gold nanoparticles dependent on volume fraction 40 of gold nanoparticles relative to the host medium (Kubo, 2007).

FIGURE 12: Experimental measurements of the dielectric constants for 40 silver particles (Pei,2010)

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FIGURE 15: Matlab numerical prediction of the target index as a function 51 of the log of the number of particles per liter for gold nanoparticles between 1017 to 1020,values for the target index from Johnson and Christy, 1972.

FIGURE 16: Matlab numerical prediction of the target index as a function 52 of the log of the number of particles per liter for gold nanoparticles values from Stoller, 2006.

FIGURE 17: Matlab numerical prediction of the target index as a function 52 of the log of the number of particles per liter for gold nanoparticles between 1017 to 1020, index values from Stoller, 2006.





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FIGURE 22: UV-Vis spectrograph of Al-Doped ZnO with variation in the 63 mole percentage of aluminum. The growth of the nanoparticles is seen with variation with time using the ethanol wash method.

FIGURE 23: UV-VIS spectrograph of Al-Doped ZnO with variation in the 65 mole percentage of aluminum. The growth of the nanoparticles is seen with variation of time using the water wash method.

FIGURE 24: Dynamic Light Scattering histogram of 1% Al-Doped ZnO 67 during synthesis in relationship to particle growth using the ethanol wash method.

FIGURE 25: Dynamic Light Scattering histogram of 3% Al-Doped ZnO 68 during synthesis in relationship to particle growth using the ethanol wash method.

–  –  –

during synthesis in relationship to particle growth using the ethanol wash method.

FIGURE 27: Emission spectrographs of Al-Doped ZnO with variation in the 70 mole percentage of aluminum. The growth of the nanoparticles is seen with variation of time using the ethanol wash method.

FIGURE 28: Emission spectrographs of Al-Doped ZnO with variation in the 72 mole percentage of aluminum. The growth of the nanoparticles is seen with variation of time using the water wash method.

–  –  –

FIGURE 31: Dynamic Light Scattering histograms of AZO nanoparticles 77 size using ethanol wash synthetic technique after adhesion of octadecylamine. (A) 1% Al-doped (B) 3% Al-doped (C) 6% Al-doped.

FIGURE 32: UV-VIS spectrographs of AZO nanoparticles using ethanol 78 wash method before and after octadecylamine ligand adhesion.

FIGURE 33: Infrared spectrograph of 1% AZO nanoparticles using ethanol 80 wash method after adhesion of octadecylamine.

FIGURE 34: Infrared spectrograph of 1% AZO nanoparticles using ethanol 80 wash method before ligand adhesion.

FIGURE 35: Photograph of AZO nanoparticles evaporated on to the surface 81 of the IR detector.

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FIGURE 40: COMSOL numerical predictions of the real part of the index for 84 100 nm AZO nanoparticles with interparticle spacing between 0.5-100 nm.

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FIGURE 42: COMSOL numerical predictions of the imaginary part of the 86 index for AZO nanoparticles with interparticle spacing between 0.5-100 nm.

FIGURE 43: Index of refraction of silicon wafer using UV-NIR Ellipsometer. 87

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1.1 A Background to Optical Metamaterials Metamaterials are artificially engineered structures that are designed to change the propagation of electromagnetic waves. People have been altering electromagnetic waves for centuries based upon material composition; this is commonly seen in lens and mirror technologies. A significant difference in metamaterials is that they are specifically designed to exhibit unusual electromagnetic properties and are endeavoring to exhibit controlled responses. To understand this concept of unusual electromagnetic properties we need to define those properties as they relate to permittivity and permeability.

Permittivity is the measure of the response of the medium to the electric field and permeability is the measure of the material’s response to the magnetic field. For metamaterials this response can be changed depending on the orientation of the subwavelength-scaled features used to build a metamaterial, namely “meta atoms”. The meta atoms are engineered elements that have designed electromagnetic resonances and then are assembled into periodic or random arrays to give composite materials. These meta atoms have a resonance response since near resonance, dispersion relations dictate that a significant change in the effective permittivity and /or permeability of the medium can be found.

In 1927 the Nobel prize was awarded for the significance of defining these dispersion relations, known as the Kramers-Kronig relations. Any variation of the real parts of the permittivity and/or permeability (and hence refractive index which is just the square root of the product of these two parameters) with frequency is accompanied by variation in the imaginary part.1 It thus connects the absorption or gain of a resonance or material’s electromagnetic response to the refractive index. This is a fundamental consequence of the fact that the polarizability of any material is a causal function.

Based on an appreciation of how to engineer structures exhibiting such resonances, researchers have conceptualized and realized some unusual electromagnetic properties such as magnetism at optical frequencies, negative refractive index, large positive refractive index, zero reflection through impedance matching, perfect absorption, giant circular dichroism and enhanced nonlinear optical properties.2

1.2 Applications of optical metamaterials The potential applications of optical metamaterial are still being revealed since the potential to make materials not found in nature is almost limitless. For example, using metamaterial surfaces that exhibit optical magnetic responses or which behave as perfect absorbers have been recently proposed. For the latter, light is neither reflected nor transmitted over certain frequency bands for a broad range of angles, and this could have significant contributions for light harvesting and hence to the solar cell industry, (Soukoulis, 2011).

The metamaterial research community is actively exploring metamaterials whose dielectric constants are engineered or even dynamically tuned, to provide, for example bulk negative index materials. In these both the permittivity and permeability are designed to be negative. Researchers have discovered numerous applications for negative index materials such as superscatterers, high resolution imaging systems, optical concentrators and also cloaking technologies based on transformation optics. When a structure is covered or coated by a low index material and then illuminated by an incident wave, one can realize a reduction in the scattering cross section of that structure. The coating reduces the reflections over a wide band of angles and frequencies. The scattered field can be reduced and applications in optics, infrared, THz and radar technology are very exciting. 3 The application of high resolution imaging of subwavelength features using optical metamaterials is developing quickly and holds enormous promise for biological and medical research since one could then avoid invasive techniques for discovering cellular processes.

1.3 Challenges Even though these applications are intriguing and the implications are vast, there are some challenging questions that must be addressed in their design and fabrication.

These resonant meta atoms needs to be smaller than a wavelength and ordered in an array with variation in geometric orientation to exhibit these electromagnetic properties for these optical metamaterials. Lithographic techniques have been used to design and fabricate these materials but these processes are expensive. The other challenge is fabricating these structures on a large scale in 3D. The next question is how can we overcome these challenges?

–  –  –

Nanoscale science, nanotechnology and nanoscale materials emerged and became well established research themes in the latter half of the 20th century. Advances haves been rapid, driven by new cutting edge synthesizing techniques for creating nanostructures such as carbon nanotubes, metal oxides, and metallic and bimetallic, and various nanoarchetypes.

A key reason the scientific community has devoted so much effort to the creation of nanomaterials is the enormous potential for these materials within applied sciences and commercial development. For example, the work done by Louis E. Brus who discovered colloidal semiconductors known as “quantum dots”, has transformed research in transistors, light emission diodes (LEDs), solar cells and diode lasers4, 5.

Now researchers are combining nanoscale materials with metamaterials science.

Scientists are using nanoscale materials to form new types of metamaterials. Some of these metamaterials contain conducting elements and can be modeled as inductance (L) and capacitance (C) circuits to predict the bulk properties of metamaterials that use nanoscale materials.6 It has been shown that the influence of metal nanoparticles or semiconductor nanoparticles can have a significant effect on the electromagnetic response of such metamaterials.

However, our interest relies on the most fundamental nanoscale materials which are metal colloids, semiconductors, and colloidal gold. By using nanoscale materials as a component of a nanoscale composite material we plan to control the permittivity and permeability of the material and thus alter its bulk index of refraction.



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