Loss Compensation in Optical Metamaterials Using Gain Media

7/28/2019 Loss Compensation in Optical Metamaterials Using Gain Media

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LOSS COMPENSATION IN OPTICALMETAMATERIALS USING GAINMEDIA

Summer internship 2010

Submitted by-

Apoorv Balwani (071505)

PST IV Yr

Under the guidance of

Dr. Giusseppe Strangi


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AGENDA

Objective of todays presentation is to affiliate

the audience with the basic project

undertaken by the trainee during the summer

term 2010.

Discussions will be done on optical

metamaterials, the various associated

phenomenon and the polymer scienceaspect attached to it.


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ABOUT EMPLOYER

The University of Calabria, situated in Southern Italy,is a public institution established in 1972. It is amedium-sized university with about 36,000 students,800 teaching and research staff and about 700

administrative staff. It has six fully functional facultiesof Economics, Engineering, Philosophy, Sciences,Pharmacy and Political science.

LICRYL originates from the Liquid Crystal Group

which operates since 1980 at the University ofCalabria in the field of soft matter, being one of thepioneers in the field of material science.


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ABOUT THE PROJECT

The Licryl lab is involved in a European collaboration, Metachemwhich aims at creation and development of metamaterials.

The role of Licryl is to conduct practical experiments on samplessent and suggest feedbacks for improvement in the materials.

At present, the focus is on reducing the optical losses faced ingold nanoparticles to produce metamaterials with unusually highlevels of transparency. This is to be done by use of gain media,which in this case are fluorescent dyes which can transfer energyat optical wavelengths to the gold nanoparticles.

The aim of this project was to learn and conduct these

experiments in the lab and also attempt to transfer a uniform layerof these nanoparticles on polymeric substrates to obtain a thinfilm for testing in solid phase.


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LOSS COMPENSATION IN OPTICALMETAMATERIALS USING GAIN

MEDIALoss compensation

(and resulting

enhancement oftransmission and

scattering) by using

gain media

Creation of gold

nanoparticles-polymersystems and

preliminary study of

loss compensation


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LOSS COMPENSATION (AND RESULTING ENHANCEMENT OFTRANSMISSION AND SCATTERING) BY USING GAIN MEDIA Ethanol, toluene or chloroform based solution of gold

nanoparticles was used as a primitive metamaterialistic system.

Samples were received from various labs and had variedparameters in terms of size, shape and composition.

Nanoparticle solution was washed with fluorescent dyes and the

sample was subjected to a pump-probe system of Nd-YAG lasersto test the magnitude of loss compensation and activationenergies for various FRET phenomena.

Probes used were both monochromatic light as well asbroadband light. The underlying theory was confirmed with thedata.

While Bari samples showed higher magnitude of enhancements,the threshold energy in case of Bordeaux samples was lesser.

In effect, both samples showed at least 3-4 times increase inintensity of transmitted light for very miniscule quantities ofenergy pumped into the system.


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OPTICAL METAMATERIALS

Ever since the emergence of Veselagos paperon left handed materials in 1968[1], whichexplored the extraordinary scenario, where thevectors E, H and kform a left handed system,and presented the required parameters for amaterial to achieve such systems; the scientific

community has been abuzz with curiosity anddeeper research in the field of what is nowcalled Metamaterials.

A metamaterial is an artificially structuredmaterial which attains its properties from theunit structure rather than the constituent

materials. A metamaterial has aninhomogeneity scale that is much smaller thanthe wavelength of interest, and itselectromagnetic response is expressed interms of homogenized material parameters. Cai & Shelev

An engineered 3-D

metamaterial that can

reverse and bend the

natural direction ofvisible and near-infrared

light, a development

that could help form the

basis for higher

resolution optical

imaging and

nanocircuits for high-

powered computers


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OPTICAL METAMATERIALS

The scientific community effectively converges

on the following basics points which describe

the nature of metamaterials:

they have properties unlike any natural substances they are synthesized by human beings

they exhibit exotic electromagnetic properties at

optical frequencies which can be tailored as per wish

the order of inhomogeneity in the material is smaller

than the wavelengths of interest


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EXPERIMENTAL SETUP


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THEORY

When a chromophore is excited, it emits energy in form oflight (radiative transmission). This chromophore, whenplaced in vicinity of another chromophore (capable ofaccepting the energy) at distances less than thewavelength of light emitted, transfers energy to the

adjacent particle in a non-radiative manner through dipolecoupling. This phenomenon is known as ForstersResonance Energy Transfer (FRET) and forms thefoundation of this experiment.

The basic idea behind the experiment is to use dyes with

emission bands/spectra that considerably overlap theSurface Plasmon absorption spectra of the goldnanoparticles, so that at distances in the order ofnanometers, FRET occurs in the most efficient manner.


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THEORY

Step 1: The donor chromophore-fluorescent dye chosen was C-500(Cumarin 500) which has a fluorescentemission peak at around 506 nm andactivation energy at around 320 nm. Thisvalue was close to the plasmon resonance

peak of gold nanoparticles (nanospheres),which was around 520 nm.

Probe light was shined at 532 nm (greenlight). The spectroscopic results were noted.In theory, it was expected that a fraction of

light was absorbed by the goldnanoparticles to form localized Plasmonwaves. Thus, both transmitted as well asscattered light was of lower intensity thanthat expected from a totally transparentmaterial.

450 480 510 540 570 600 630 660 690 720 750 780 810

0

1

2

3

4

5

Abs_sa

mple

_S1

Fluorescence

(arb.units)

Wavelength (nm)

506 nm


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THEORY

Step 2: Next, the pump beam at 320 nm was directed atthe sample. This caused the dye to activate, and emitfluorescent light at 506 nm. Due to FRET, this energy wasabsorbed by the nanoparticles at LSPR (localized surfacePlasmon frequency), which lead it to transmit more

energy at than wavelength. This theory was consolidatedby the various experiments carried out on thenanoparticles which have been described in laterchapters with results.

Thus by using a system in which a dye activated at

certain energy and cause FRET at a different energy, wesee that the value of transmitted light as well as scatteredlight observed had increased considerably. In otherwords, the particles had become more transparent.


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OBSERVATIONS- BARI SAMPLES

The samples received from Bari were

cylindrical nanorods. Thus in effect, they had

LSPR on two different surfaces (radial &

cylindrical) with different resonancefrequencies


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The dye was mixed in the solution and the

dye particles thus transmitted energy both

radiatively as well as through FRET. This

resulted in a graph of intensity v/swavelength observed on the spectrometer,

wherein a distinct, sharp peak at probe

wavelength was observed in a background ofwhat is the fluorescent spectroscopy of the

dye particles (due to residual fluorescence).


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This meant that the fluorescence of the sample would decreasewhen the pump was switched on (with probe beam off), provingthat this fluorescent energy was utilized for FRET. This effect isknown as quenching and the graphs depicting the effect are givenon the following page.

In this certain quenching experiment, quenching effect was seenby measuring fluorescence of solution containing only dyeparticles, and then fluorescence of solution containing both dyeand nanorods (B2) was noted at the same energy. The reductionin intensity at every wavelength (or quenching) was also plotted.

The quenching effects at each energy was noted and plotted

using two curves on the following pages. It was seen than theeffect of quenching increased with increasing pump energy.


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The quenching effect, measured by subtracting the intensities offluorescence in dyes and B2 is seen to follow a slightlyexponential curve, while the % quenched values vary linearly withthe energy. This suggests that while the increase in quenchingwith energy is rapid, the increase in fluorescence of the dye isalso rapid enough to prevent an exponential rise in transmission.

The quenching effect graph is seen to closely match the trends ofintensity v/s pump energy graphs obtained by the researchers forRayleigh scattering and transmission experiments on the samesample prior to commissioning of the internship.

Also the maximum quenching is seen at around 497-500 nm. As

shown before, this is close to the emission spectrum of the dye inuse.


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Quenching of dyes


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Quenching of dyes


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Quenching of dyes


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Quenching of dyes


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Quenching of dyes


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Quenching effect dependence on pump

energy


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Quenching effect dependence on pump

energy


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Broadband measurements


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Broadband measurements


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Broadband measurementsAs can be seen from the graph, the efficiency of

the dye seems to vary with varying wavelengths.

It is typically found to be more efficient for lowerwavelengths, thus highlighting the significance ofoverlapping fluorescence spectrum with theabsorption spectrum of sample.

The threshold energy also seems to decreasewith increasing probe wavelength, which thetrainee (self) was unable to account for or relateto.


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Conclusion

Thus the experiments yielded positive results

in increasing transparency of the sample,

with enhancements of as much as 8 timeswitnessed due to FRET.


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OBSERVATIONS- BORDEAUX SAMPLES

These samples

consisted of gold

nanoparticles envelopedby a silica shell. The dye

particles were embedded

inside the Si shell, thuspreventing any residual

fluorescence.

C500

in shell

Au Core

12 2 nm

Silica Shell

12 5 nm


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The absorption spectroscopy of ethanol

solution of these particles showed a single

peak at around 520 nm.

450 480 510 540 570 600 630 660 690 720 750 780 810

0

1

2

3

4

5

Abs

_sam

ple

_S1

Fluorescenc

e

(arb.units)

Wavelength (nm)

506 nm


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Rayleigh scattering Similar trends as in bari samples were obtained.

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

2

4

6

8

10

12

14

16

18

20Enhancement of SPs

of gold Core-Shell Nps

with C500 in the shell

sample S1

(threshold 0.75mJ)

ScatteredIn

tensity(arb.unit

s)

Pump Energy (mJ)


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Rayleigh scattering

Comparisons with previous study however found that

the threshold energy in this case was significantly

higher.

Also, results varied significantly on various samples of

the same Bordeaux make, which varied only in

exposure to room temperatures.


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Rayleigh scattering In the two compared graphs shown, S2 was a relatively fresh sample, while

S1 had a more prolonged exposure to room temperatures

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

5

10

15

20

25

30

35

Enhancement of SPs

of gold Core-Shell Nps


sample S2

(threshold 0.66mJ)

Scattered

Intensity(arb.units)

Pump Energy (mJ)-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

2

4

6

8

10

12

14

16

18

20 Enhancement of SPsof gold Core-Shell Nps


sample S1

(threshold 0.75mJ)

Scattered

Intensity

(arb.units

)

Pump Energy (mJ)


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Rayleigh scattering Transmission studies showed a similar curve, but with a steeper slope, and

consequentially, lower threshold energy. This was again conducted on a freshsample.

0.0 0.5 1.0 1.5 2.0

4

6

8

10

12

14

Peak of transmission of a

probe beam @ 532nm

sample S2

(threshold 0,25 mJ)

Intensity

(arb.units)

Pump Energy (mJ)

The enhancement in

transmission

however was no

more than 3-4 times,

considerably lesser

magnificationcompared to Bari

samples.


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Broadband experiments

0 1 2 3 4 5 6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4 @ 490

@ 500

@ 510@ 520

@ 540

Intensity

(arbit)

Energy (mJ)


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Conclusion

Thus we observe a similar trend in the Bordeaux

samples as that obtained in the Bari samples. The

inclusion of analysis of Intensity jump (similar to the

study of quenching effect in Bari samples), however,

throws more light upon the nature of enhancement.

The behavior and trend of increase in enhancement

of intensity can be treated analogous to what weobserved in quenching of the fluorescent dyes.

C ti f ld ti l l t


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The preliminary tests of the gold nanoparticles-solvent systems provedsuccessful in terms of loss compensation and enhancement of opticalproperties.

However, for practical purposes, it can be well seen that solid systemsalone can provide the durability and portability for the applications inoptical fields.

Hence, transfer of gold nanoparticles over solid substrates in forms ofthin films was attempted and the functionality of the samples tested.

The methods of transfer were pretty much primitive, however the resultsrevealed the possibility of a successful solid substrate based goldnanoparticle system.

The substrates used were made of polymer or glass. Preliminary tests

revealed that a silicone- PDMS made as a successful base for thenanoparticles.

Further tests on the optical characteristics are ongoing.

Creation of gold nanoparticles-polymer systems

and preliminary study of loss compensation

C ti f ld ti l l t


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Experiments: The part two of project aimed atcreating a very thin film of nanoparticles on a solidsubstrate. The experiments conducted to find theoptimum solid substrate-nanoparticle system variedin the material used for the substrate, and themethods of film creation. Based on these parameters,the following experiments were conducted: Solvent evaporation

on glass

on PDMS

Spin coating with NOA 61

Plasma polymerization

Sandwich samples



C ti f ld ti l l t


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Solvent evaporation: Ethanol is a very

volatile solvent. At the same time Bordeaux

samples came with a coating of silica around

the particles. This brought a huge possibility(by forming Si-O-Si bonds) for the gold

nanoparticles to adhere to the surface of

other silica based substances. For thisreason, Glass and PDMS were used.



C ti f ld ti l l t


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On glass

Although SEM results were non-

confirmatory on glass, we did obtain a

uniform but low density image on highcontrast background ITO. We also

obtained an absorption spectroscopy that

confirmed presence of gold nanoparticles(also confirmed by the reddish tinge on

the glass surface).



C ti f ld ti l l t


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On glass



Creation of gold nanoparticles polymer systems


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On glass





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On PDMS Poly (dimethylsiloxane) or PDMS is a type of transparent

silicone which is UV resistant and has many favorableproperties for optical and laser applications. Four 1 mm by1mm cross sections were made with varying depths ( .25 mm,

.5 mm and 1 mm) and filled with Serge samples and left to dry. A very uniform and dense layer was obtained on the

sample, as confirmed by the SEM images. The uniformlayers were however discontinuous and mostly clustured.

The preliminary transmission tests also to an extentreplicate the GNP-solvent results, with the exception of

destabilizing the system at higher energies. Theseinstabilities can be a result of laser ablation of goldnanoparticles. A prolonged exposure to high energy laserwas also found to permanently damage the system.





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On PDMS





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On PDMS





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On PDMS



The preliminary transmission

tests also to an extent

replicate the GNP-solvent

results, with the exception ofdestabilizing the system at

higher energies. These

instabilities can be a result of

laser ablation of gold

nanoparticles. A prolonged

exposure to high energy laser

was also found to

permanently damage the

system.



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Spin Coating

Spin coater is a device that is used to spread a

liquid layer of the solution on a surface using the

principle of centripetal force.The coating thickness can be varied by varying

the angular speed of the coaters.

To eliminate the problem of discontinuous non-

uniform layers, spin coater was used on previous

experiments, and also with NOA 61





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NOA 61 is an optical adhesive consisting mainly of poly-mercaptoesters and was used as a polymer linker as well as toprovide a substrate for the gold nanoparticle system.

First, three parts by volume of NOA 61 were mixed with one partof gold nanoparticles solution with ethanol.

This mixture was then sonicated at elevated temperatures todecrease the viscosity of the emulsion as NOA 61 is thixotropic innature.

This emulsion was used to coat four different samples of ITOglass which differed in the angular speed and the time of spinningto give spin coats of different thicknesses.

Absorption spectroscopy suggests the optimum results beingobtained with spin coating values near that of sample C as seenin the next slide.



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Spin coating



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The globules obtained are those of gold

particles, now agglomerated into micron-

particles, covered on the sides by PANI.The test is inconclusive in the sense of

obtaining nanoparticulate behavior of

gold. However, the PANI casing on thespherulites proves the ability of the

process of forming stable systems.



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Sandwich samples

The

preliminary

tests onabsorption

spectroscopy

came out to

be positive fora successful

preparation of

GNP-solid

substrate

systems.



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Conclusion

As shown, various GNP-substrate systems were

created and found to be stable to various

degrees. Preliminary tests also supported thesuccess of the experiment. Further tests are

necessary to see if all the criteria of the tests are

met and the systems can act as metamaterials.


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Loss Compensation in Optical Metamaterials Using Gain Media

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