Integration of ingaasgaas QW with surface plasmon and photonic bandgap structure on its PL emission

Integration of InGaAs/GaAs Quantum well with Surface Plasmon and Photonic Bandgap Structure and their effect on its PL Emission GAO HONGWEI M.Eng, DaLian University of Technology, Chi

Integration of InGaAs/GaAs Quantum well with Surface

Plasmon and Photonic Bandgap Structure and their effect

on its PL Emission

GAO HONGWEI

(M.Eng, DaLian University of Technology, China)

A THESIS SUB MITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER

ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2014

DECLARATION

I hereby declare that the thesis is my original work and it has been written by me

in its entirety I have duly acknowledged all the sources of information which

have been used in the thesis

This thesis has also not been submitted for any degree in any university

previously

Gao Hongwei

15 May 2014

Acknowledgements

First of all, I want to thank my supervisor, Professor Chua Soo Jin, for his

guidance It is a great honor to be his PhD student I learnt the importance of

attitude towards research and even towards life in general His academic advice

both in doing experiment and in organizing research structure is highly

appreciated

I would like to appreciate the help and support from Dr Xiang Ning, who offered

me the opportunity to work on exciting and interdisciplinary topics and provided

me the wonderful chance to finish my PhD degree I appreciate all the advices,

time and ideas she contributed to help me finish my journey as a PhD student

I gratefully acknowledge Dr Teng Jinghua for offering me the opportunity to

learn and use the fabrication and characterization equipment in the Institute of

Materials Research and Engineering I also thank Dr Lu Jun and Mr Tung Kar

Hoo Patrick for providing MBE grown samples

I also want to express my appreciation to the support given by Mr Tan Beng

Hwee and Ms Musni our helpful laboratory officers

Finally, I would like to thank my parents for their constant encouragement and

support during the course of this work

Table of Contents

DECLARATION I Acknowledgements .II Summary V List of Figures VII List of Tables .XI List of Publications .XII List of Abbreviations XIV

Chapter one: Introduction 1

1.1 General introduction of Plasmonics 1

1 2 Introduction of photonic bandgap 13

1.3 Research motivation 17

1.4 Scope of thesis 19

Chapter 2: Theory 22

2.1 Principle of Surface Plasmon 22

2.11 Non-localized surface plasmon 29

2.12 Localized surface plasmon 30

2.2 Principle of Photonic bandgap structures 34

Chapter 3: Study on tuning SPs resonance wavelength by metallic nanohole structure and metallic nanoparticle structure 45

3.1 Introduction 45

3.2 Tuning SPs resonance wavelength by Ag nanohole structure 45

3.2.1 Sample preparation 46

3.2.2 Results and discussion 47

3.2.3 Conclusions 52

3.3 Tuning SPs resonance wavelength by metallic nanoparticle structures 52

3.3.1 Introduction 52

3.3.2 Sample preparation 53

3.3.3 Results and discussion 56

3.3.4 Conclusions 67

3.4 Summary 67

Chapter 4: Coupling of SPR to InGaAs QW 69

4.1 Introduction 69

4.2 Coupling of Surface Plasmon with InGaAs/GaAs Quantum Well Emission by Gold Nanodisk Arrays 70

4.2.1 Introduction 70

4.2.2 Sample preparation 72

4.2.3 Results and discussion 75

4.2.4 Conclusions 81

4.3 Enhancement of GaAs/InGaAs Quantum Well Emission by disordered Gold Nanoparticle Arrays 81

4.3.1 Introduction 81

4.3.2 Sample preparation 82

4.3.3 Results and Discussion 85

4.3.4 Conclusions 90

4.4 Summary 91

Chapter 5: Coupling of SPR and Photonic Bandgap to InGaAs QW Emission 93

5.1 Introduction 93

5.2 Sample preparation 94

5.3 Results and discussion 97

5.4 Conclusions 109

Chapter 6: Conclusion and future work 111

6.1 Conclusions 111

6.2 Suggestion of future work 113

References 115

Summary

Surface plasmon resonance (SPR) excited at the metal-dielectric interface has

been investigated for various applications in the Optoelectronics Enhancing

photoluminescence intensity of InGaAs/GaAs quantum well (QW) system in the

near infrared (NIR) range by SPs is demonstrated for the first time

In order to overcome the fabrication challenge of putting metal nanoparticles

close to the quantum well layer without affecting its quality, a 50 nm thin SiO2

was introduced between the Au nanodisk arrays and GaAs surface We fabricated

an ordered array of Au nanostructures with relatively large features to match the

InGaAs/GaAs QW emission wavelength Without the SiO2 layer and its lower

refractive index compared to GaAs, the Au nonodots would have to be much

smaller By overlapping the SPs resonant wavelength with that of the QW

emission, a strong coupling was demonstrated, and more than 4-fold enhancement

of the PL intensity was achieved To match the longer QW emission wavelength

and to further make the fabrication process easier, we studied the irregular array

of Au nanodisks on the InGaAs/GaAs QW system with the 50 nm SiO2 layer By

introducing the irregularity, the number of SPs modes was increased, which

induced a larger exit angle for coupling the light out However, it also resulted in

poorer coupling of the field distribution with the Quantum Well, resulting in only

a 2-fold enhancement in the photoluminescence intensity obtained

To achieve a stronger SP-QW coupling effect, a thin quantum well barrier is

desirable to allow the confined electromagnetic field caused by SP to couple more

strongly with the QW However, a thin quantum well barrier layer leads to a

poorer QW emission performance To solve this problem, we report on a photonic

bandgap structure patterned on the thick quantum well barrier The array of Au

nanodisk is placed into the holes of the photonic bandgap structure filled with a

15 nm SiO2 layer Thus the Au nanodisks are placed close to the InGaAs active

layer without sacrificing the thickness of the GaAs quantum well barrier layer,

which is very important for a practical device With this design, a maximum

7.6-fold enhancement in the photoluminescence intensity has been obtained

All the experimental results were verified by numerical simulations

List of Figures

Figure 1.1Number of articles varying with year 4 Figure 1.2 Demonstration of generating SP with prism 5 Figure 1.3 Demonstration of generating SP with grating 5 Figure 1.4 (a) Sample structure of InGaN/GaN QW and excitation/emission of PL measurement (b) PL spectra of InGaN/GaN QWs coated with Ag, Al, and Au The PL peak intensity of uncoated InGaN/GaN QW at 470 nm was normalized to 1 9 Figure 1.5 (a) PL enhancement ratios at several wavelengths for the same sample

as in Figure 1.4(b) (Inset) Dispersion diagrams of surface plasmons generated on Ag/GaN, Al/GaN, and Au/GaN surfaces (b) Integrated PL enhancement ratios for samples with Ag, Al, and Au are plotted against the thickness of GaN spacers The solid lines are the calculated values by the penetration depths 10 Figure 1.6 (a) Sample structure of dye doped polymer with both pump light and emission light configurations (b) PL spectra of Coumarin 460 on Ag, Au, and quartz The PL peak intensity of Coumarin 460 on quartz was normalized to 1 11 Figure 1.7 (a) Sample structure of CdSe nanocrystals on Au-coated quartz chips (b) PL spectra for CdSe nanocrystals on Au and quartz (Qz) 12 Figure 1.8 (a) Sample structure of Si nanoparticles dispersed in SiO2 media and excitation/emission configuration of PL measurement (b) PL spectra of Si/SiO2with Au, Al, and no metal layer 13 Figure 2.1 Definition of a planar waveguide geometry The waves propagate along the x-direction in a cartesian coordinate system 24 Figure 2.2 Geometry for SPP propagation at a single interface between a metal and a dielectric 27 Figure 2.3 Sketch of a homogeneous sphere placed into an electrostatic field 31 Figure 2.4 One-dimensional photonic crystal made of an infinite number of planar layers of thickness d 35 Figure 2.5 Band diagram for one-dimensional photonic crystal The shaded areas are the allowed bands The diagram represents both TE and TM modes For a 1D photonic crystal, there are no complete bandgaps, i.e there are no frequencies for which propagation is inhibited in all directions Values used: ɛ1=2.33 (SiO2),

ɛ2=17.88 (InSb) 38

Figure 2.6 The photonic band structure for the lowest- frequency modes of a square array of dielectric (ɛ=8.9) vein (thickness 0.165a) in air The blue lines are

TM bands and the red lines are TE bands The left inset shows the high-symmetry points at the corners of the irreducible Brillouin zone (shaded light blue) The right inset shows a cross-sectional view of the dielectric function 40 Figure 2.7 Displacement fields of X-point TM modes for a square array of dielectric (ɛ=8.9) veins in air The color indicates the amplitude of the displacement field, which is oriented in the z direction (out of the page) The dielectric band is on the left, and the air band is on the right 42 Figure 2.8 Magnetic fields of X-point TE modes for a square array of dielectric (ɛ=8.9) veins in air The green dashed lines indicate the veins, and the color indicateds the amplitude of the magnetic field, which is oriented in the z direction The dielectric band is on the left, and the sir band is on the right 42 Figure 3.1(a) SEM image of monolayer nanosphere on substrate 48 Figure 3.1(b) SEM image of monolayer na nosphere on substrate after dry etching 48 Figure 3.1(c) SEM image of Ag hole arrays after removing the nanosphere 48 Figure 3.2 Experimental reflectance spectra for three samples Triangular-shaped black curve is for Ag holes with diameter of 375 nm on Si substrate; Dot shaped red curve is for Ag holes with diameter o f 340 nm on Si; Cross- line blue curve is for Ag hole with diameter of 310 nm on Si 49 Figure 3.3 Simulated reflectance spectra for three samples Triangular shaped black curve is for Ag holes with diameter of 375 nm based on Si substrate; Dot shaped red curve is for Ag holes with diameter of 340 nm; Cross- line shaped blue curve is for Ag holes with diameter of 310 nm 50 Figure 3.4 Simulated SPs resonance wavelength for various diameter of Ag hole arrays Periodicity is 600 nm 52 Figure 3.5 Schematic structure of Sample A, B, C, and D 56 Figure 3.6 SEM image of Ag nanoparticle arrays on GaAs substrate in low amplitude Inset: SEM image of Ag nanoparticle arrays in high amplitude Periodicity is 430 nm, particle diameter is around 260 nm 57 Figure 3.7 Measured (above) and simulated (below) reflectance spectrum of Sample A Periodicity is 430 nm, the diameter is around 260 nm 58 Figure 3.8 Measured (above) and simulated (below) reflectance spectrum of Sample B Periodicity is 430 nm, diameter is around 260 nm 60 Figure 3.9 Measured (above) and simulated (below) reflectance spectrum of Sample C Periodicity is 430 nm, diameter is around 260 nm 61

Figure 3.10 Measured (above) and simulated (below) reflectance spectrum of Sample D Periodicity is 430 nm, diameter is around 260 nm 62 Figure 3.11 Dipole mode plasmonic resonance wavelength vs periodicity (radius

of circle-shape Au nanoparticle is 60 nm) 66 Figure 3.12 Dipole mode plasmonic resonance wavelength vs radius of circle-shape Au nanoparticle arrays (periodicity if 430 nm) 67 Figure 4.1 Schematic illustration of ordered Au nanodisk arrays on InGaAs/GaAs

QW sample fabrication process 74 Figure 4.2 SEM image of Au nanodisk arrays formed on top of QW for Sample A1 75 Figure 4.3 PL spectra of as- grown sample, Sample B1 (dished curve) and sample with SiO2/Au nanodisk arrays, Sample A1 (solid curve) 76 Figure 4.4 Reflectance spectrum of Sample A1: Au nanodisk arrays on SiO2 on top of as-grown sample (PL ~945 nm) 78 Figure 4.5 Schematic illustration of simulated structure Electric dipole source (center at 945 nm, FWHM of 60 nm) is 20 nm below GaAs surface SiO2thickness is 50 nm, Au nanodisk arrays period is 280 nm, radius is 70 nm, and thickness is 10 nm. 80 Figure 4.6 Spectrum of E intensity variations with wavelength for Sample A1 80 Figure 4.7 SEM image of irregular Au nanodisk arrays for Sample B2 84 Figure 4.8 SEM image of ordered Au nanodisk arrays for Sample C2 84 Figure 4.9 PL spectra of Sample A2, as grown sample, (blue dotted curve), Sample B2, irregular Au structure/SiO2 film on the as grown sample (black dashed curve), and Sample C2, ordered Au structure/SiO2 film on the as grown sample (red solid curve), all three curves normalized to the its substrate peak 85 Figure 4.10 Reflectance spectra of irregular Au nanostructure, Sample B2 (black dashed curve) and of ordered Au nanodisk, Sample C2 (red solid curve) 88 Figure 4.11 Spectra of electric field intensity as a function of wavelengths in the plane of irregular Au nanodisks (black dashed curve) and 100 nm above the irregular Au nanodisks (red solid curve) for Sample B2 Inset shows the far field projection for Sample B2 90 Figure 4.12 Spectra of electric field intensity as a function of wavelengths in the plane of ordered Au structure (black dashed curve) and 100 nm above the ordered

Au structure (red solid curve) for Sample C2 Inset shows the far field patterns for Sample C2 90

Figure 5.1 Schematic illustration of Samples A, B, and C fabrication process 97 Figure 5.2 PL peak intensity varying with etch depths at around 940 nm from Samples A, B, C, and D 99 Figure 5.3 PL spectra for Samples A, B, and C at etch depth of 80 nm compared with Sample D. 100 Figure 5.4a PL spectra varying with etching depth for Sample A compared with Sample D, inset is the structure of Sample A 100 Figure 5.4b PL spectra varying with etching depth for Sample B compared with Sample D, inset is the structure of Sample B 101 Figure 5.4c PL spectra varying with etching depth for Sample C compared with Sample D, inset is the structure of Sample C 101 Figure 5.5 Reflectance spectrum of Sample A 105 Figure 5.6 Schematic illustration of simulated structure, Samples A, B, C, and D, the etch depth d are 50 nm, 60 nm, 70nm, 80 nm, and 90 nm SiO2 thickness is 15

nm Au thickness is 10 nm Hole periodicity is 280 nm, radius is 70 nm 107 Figure 5.7a Simulated far field electrical intensity at 940 nm for Sample A with different etch depth, the magnitude is represented by the color scale The hole etch depths in Sample A1, A2, A3, A4, and A5 are 90 nm, 80 nm, 70 nm, 60 nm and 50 nm respectively The schematic of structure is shown in Figure 5.6 107 Figure 5.7b Simulated far field electrical intensity at 940 nm for Sample B with different etch depth, the magnitude is represented by the color scale The hole etch depths in Sample B1, B2, B3, B4, and B5 are 90 nm, 80 nm, 70 nm, 60 nm and 50 nm respectively The schematic of structure is shown in Figure 5.6 108 Figure 5.7c Simulated far field electrical intensity at 940 nm for Sample C with different etch depth, the magnitude is represented by the color scale The hole etch depths in Sample C1, C2, C3, C4, and C5 are 90 nm, 80 nm, 70 nm, 60 nm and 50 nm respectively The schematic of structure is shown in Figure 5.6 108 Figure 5.7d Simulated far field electrical intensity at 940 nm for Sample D, the magnitude is represented by the color scale The schematic of structure is shown

in Figure 5.6. 109 Figure 5.8 Simulated PL peak intensity varying with etch depth at around 940 nm for Samples A, B, C and D 109

List of Tables

Table 2.1 Concentration factors for the lowest two bands of the square lattice of veins at the X point 41

Table 3.1 Comparison between experimental and simulation results of Ag holes

on Si with diameter of 375 nm, 340 nm, and 310 nm 50

List of Publications

Journal Papers

1 Hongwei Gao, Jinghua Teng, SooJin Chua, Ning Xiang, "Enhancement of

GaAs/InGaAs Quantum Well Emission by disordered Gold Nanoparticle Arrays",

Applied physics A In press

2 Hongwei Gao, Kar Hoo Patrick Tung, Jinghua Teng, Soo Jin Chua, Ning

Xiang, "Coupling of surface plasmon with InGaAs/GaAs quantum well emission

by gold nanodisk arrays" Applied optics, Vol 52, No 16(1), June (2013)

3 Hongwei Gao, Jinghua Teng, SooJin Chua, Ning Xiang, "Study on PL

enhancement of InGaAs/GaAs QW emission by gold nanoparticle arrays",

Journal of Molecular and Engineering Materials, submitted

4 Hongwe i Gao, Kar Hoo Patrick Tung, Jinghua Teng, SooJin Chua, Ning Xiang,

"Incorporation of SPR with Photonic Bandgap Structure to enhance InGaAs/GaAs QW emission", to be submitted

5 Wang Benzhong, Gao Hongwe i, Lau Jun, Chua Soo Jin, "Investigation of

transmission of Au films with nanohole arrays created by nanosphere

lithography", Applied Physics A, Vol 107 Issue 1, p139, Apr (2012)

6 K H P Tung, H W Gao and N Xiang, "Time evolution of self-assembled

GaAs quantum rings grown by droplet epitaxy" Journal of Crystal Growth 371

(0), 117-121 (2013)

Conference Presentations

7 Hongwei GAO, Ning XIANG, Benzhong WANG, Jinghua TENG, Soo Jin

CHUA, "Study on Light Transmission Through Silicon Covered by Ordered Metal Particle Arrays" International Conference on Materials for Advanced

Technologies, 26th Junto 1st Jul 2011, Singapore

8 Hongwei GAO, Jun Lu, Jing Hua Teng, Soo Jin Chua, Ning Xiang, "Coupling

of Surface Plasmon with Gaas/AlGaAs Quantum Well Emission by Gold

Nanoparticle Arrays", International Conference of Young Researchers on

Advanced Material, July 1st-6th, 2012, Singapore

9 Hongwei GAO, Jinghua Teng, Soo Jin Chua, Ning Xiang, "Study on PL

enhancement of GaAs/InGaAs Quantum Well Emission by Gold Nanoparticle

Arrays", META’13 CONFERENCE, 18th–22nd

, March,2013,SHARJAH– UNITED ARAB EMIRATES

10 Hongwei GAO, Jinghua Teng, SooJin Chua, Ning Xiang, "Enhancement of GaAs/InGaAs Quantum Well Emission by disordered Gold Nanoparticle Arrays",

– 22nd March,2013,SHARJAH–UNITED ARAB EMIRATES

11.Jian Huang, Hongwei GAO, Jun Lu, Ning Xiang, Aaron J Danner, Jinghua Teng, "Photoemission Enhancement by Wavelength-Tunable Surface Plasmon Excitation of Gold Caps on an AlGaAs Quantum Disk Array", International Conference of Young Researchers on Advanced Material, July 1st-6th,2012, Singapore

12 Jian HUANG, Hongwei GAO, Kar Hoo Patrick TUNG, Aaron DANNER,

Jing Hua TENG, Ning XIANG, "Enhanced Photoluminescence by Surface Plasmon Excitation in Gold Capped InGaAs Quantum Disk Array",7th International Conference on Materials for Advanced Technologies, 30th June to 5thJuly 2013, Singapore

13 K H Tung, J Lu, H W GAO and N Xiang, "Fabrication and

characterization of III-V nanostructures on ordered hexagonal through pore SiO2thin film", International Conference of Young Researchers on Advanced Material, July 1st-6th, 2012, Singapore

14 K H Tung, H W GAO, J Lu and N Xiang, ''Fabrication of ordered

hexagonal through pore SiO2 thin film using porous alumina film", MRS-S, 2012

15 Kar Hoo Patrick TUNG, Hongwe i GAO, Ning XIANG, "Morphological and Optical Emission Correleation of Gaas Quantum Rings Grown by Droplet Epitaxy",7th International Conference on Materials for Advanced Technologies,

30th June to 5th July 2013, Singapore

List of Abbreviations

EBL: Electron beam lithography

FDTD: Finite-difference time-domain

ICP: Inductively coupled plasma

LIL: Laser interference lithography

LSPR: Localized surface plasmon resonance

MBE: Molecule beam epitaxy

Chapter one: Introduction

1.1 General introduction of Plasmonics

Metal nanostructures had already been employed to generate beautiful colors

in glass artifacts and in windows of churches by artists before their unique optical

properties were studied The most famous example is the Lycurgus cup which

appeared in 4th century AD Then from the beginning of 20th century, some

scientific studies were carried out in which surface plasmons were observed

Firstly, Robert W Wood1 reported unexplained features in optical reflection

measurements on metallic gratings in 1902 Later, employing the Drude theory of

metals and the electromagnetic properties of small spheres as derived by Lord

Rayleigh, Maxwell Garnett describes the bright colors observed in metal doped

glasses2 in 1904 Later in 1908, the famous and widely used Mie theory was

developed by Gustav Mie, which described the light scattering by spherical

particles3 In 1956, David Pines attributed the energy losses while electrons travel

through metals to the collective oscillations of free electrons in the metal4, and

gave it the name “plasmons” Robert Fano called the coupled oscillation of bound

electrons and light inside transparent media as “polariton”5 in the same year The

first theoretical description of surface plasmon was reported in 1957 with the

publication by Rufus Ritchie on electron energy losses in thin films In this paper,

plasmon modes which can exist near the metal surface were demonstrated6 In

1968, the behavior of metal gratings7 where surface plasmon resonances were

excited was described by Ritchie In the same year, surface plasmon was optically

excited on metal film8 by Andreas Otto and Erich Kretschmann, which can be

regarded as a major improvement in the study of surface plasmon With these

methods, surface plasmons were easily generated by many researchers

Till this time, although the properties of surface plasmon were well observed,

the relation between surface plasmon and the optical properties of metal

nanoparticles was not known yet In 1970, the first study describing optical

properties of metallic particles9 from the surface plasmon aspect was reported by

Uwe Kreibig and Peter Zacharias In their report, the electronic response as well

as optical response of Au and Ag was compared In 197410, the term of surface

plasmon polariton (SPP) was introduced by Stephen Cunningham and his

colleagues Meanwhile, strong Raman scattering from pyridine molecules which

are located close to a roughened Ag surface11 was observed by Martin

Fleischmann and coworkers, and this opened the door for the research of Surface

Enhanced Raman Scattering (SERS)

The previous studies had built the fundamental understanding of surface

plasmon, however, there was no practical applications yet With high density

electronics approaching the fundamental physical limits, researchers started to

seek solutions to overcome the challenge Due to the fast-developing

nanofabrication techniques, small metallic nanostructure can be realized, which

resulted in a wide range applications12 Nanosized metallic structures with

promising optical properties can be integrated with electronic devices, which

significantly improve the performance of original electronic devices According to

the records, various passive waveguides and biosensors were successfully

fabricated based on the unique properties of surface plasmon Nanofabrication

techniques led to a storm of research in metal-based optics and nanophotonics,

which can be reflected in the number of scientific papers published each year

These numbers are displayed in Figure 1.1 It is seen that the numbers almost

doubled every 5 years from 1990 onwards This is because since 1990,

researchers could make use of commercial electromagnetic simulation codes to

design proper metallic structures; nanofabrication techniques are available to

make the desired structures, while physical analysis technique allows the analyses

of the optical properties of the metallic structures Surface plasmon resonance

sensors were the majority among these researches, almost occupying 50% of all

publications As time passed, besides applications in sensors, surface plasmons

were proposed to guide and manipulate light at nanometer scale, generate

extraordinary optical transmission through subwavelength metal apertures, and

perform as perfect lens13-15 using a thin metallic film All these applications are

reported in the articles16,17

Figure 1.1 Number of articles var ying with ye ar 62 .

Theoretically, surface plasmons (SPs) are a kind of electromagnetic field

which exists at the interface between metal and dielectric At this interface,

coherent electron oscillations happen when the real part of the dielectric function

changes sign across the interface SPs can be excited by both electrons and

photons Firing electrons into bulk metal, the electrons will be scattered During

this process, energy can be transferred into the bulk plasma As long as the

scattering vector has a component parallel to the surface, SPs are created

Coupling of photons into SPs is not as straightforward It cannot be achieved

unless a coupling medium, such as a prism or grating is used to match the wave

vector between photons from free space and wave vector of surface plasmon

propagating at the interface of metal-dielectric There are two types of structures

making use of prisms to generate SPs One is by putting a prism against a thin

metal film, which is considered as Kretschmann configuration (Figure 1.2a) The

other method is to locate the prism very close to a metal surface, as in Otto

configuration (Figure 1.2b) Besides using a prism, use of a grating is another

practical tool to compensate the incident wave vector parallel to the grating by an

amount related to its periodicity, as can be seen from Figure 1.3

Figure 1.2 De monstration of gener ating SP wi th prism

Figure 1.3 De monstration of gener ating SP wi th grating

Besides using the prism and grating, surface plasmons can also be generated by

metallic nanoparticles with size comparable to or smaller than the incident light

wavelength Unlike generated from a metal film, this kind of plasmon cannot

Incident Beam

k x

K surface plasmon

propagate along the surface of metal, and is known as localized surface plasmon

(LSP) The LSP has two important effects One is enhancing the electric field near

the particle's surface, which decreased quickly with distance from the surface The

other is the maximum absorption of light at LSPs resonant frequency For noble

metal nanoparticles, like silver and gold, this occurs mostly in uv-visible range

The recent catalyst for the great interest in plasmonics is the extraordinary

optical transmission (EOT)18, which was reported in 1998 Literally, EOT means

that the transmitted light is beyond the expected amount Theoretically, it occurs

in periodic subwavelength holes milled in thin metallic films Through this kind

of structures, the detected normalized transmission of light exceeds by orders of

magnitude more than the classical prediction made by Bethe19 at certain

wavelength Bethe's prediction that the transmission should depend on the

diameter of subwavelength hole d and the wavelength λ with a scaling factor of

(d/λ)4

To explain the origins of EOT, Ebbisen pointed out that the resonant

excitation of SPs near the surface of metal was the reason for the enhanced light

transmission According to his explanation, the incident light first excited SPs of

periodic metallic hole structure and thus enhanced the electromagnetic field at the

metal aperture and consequently, enhanced the transmission This point of view

has since been verified

According to publications, both the periodicity and the size of holes have

important consequences on the plasmonic effect, which have been observed by

changing different incident angles of the excitation light for various

reciprocal-lattice directions20,21 The effect caused by periodicity is very obvious As for the

size of holes, it was reported that both the intensity and the linewidth of

transmission peaks were increased with increase in the hole diameter 22,23 Hole

size can also affect the location of transmission peaks Besides the periodicity and

size of the circular holes, different shapes of hole were studied, for example the

periodic subwavelength diamond-shaped24 hole arrays and triangles hole

arrays25,26 In addition, combining the shape, array symmetry and periodicity

provided a powerful tool to fabricate terahertz filters27 Double hole arrays with a

slight overlap between two holes generated an additional localized field

enhancement near the resulting cusps28,29, was reported to be used to enhance

nonlinear phenomena

Besides EOT being caused by an array of metallic subwavelength-diameter

holes, LSP generated by metallic nanoparticles (NPs) is another interesting field

It can confine largely enhanced electromagnetic field near the surface of NPs at

resonance wavelength, which is an important aspect of LSPs The term

'localization' means that the high spatial resolution (subwavelength), is only

limited by the size of NPs These promising properties make LSP popular for

various applications For instance, strong EM fields can be employed in

enhancing spectral information for surface-enhanced Raman spectroscopy (SERS),

metal-enhanced fluorescence (MEF), plasmon resonance energy transfer

(PRET)28,30, and nanoplasmonic molecular rulers31,32 Importantly, the largely

enhanced electromagnetic field only exists around LSP resonance wavelength

Thus, in order to utilize the strong electromagnetic field near the surface of NPs

effectively, tuning LSP resonance to the desired wavelength becomes crucial

Researchers have devoted great effort to achieve this goal It is reported that the

LSP resonance wavelength can be adjusted by varying the type of material and

surrounding medium, size or shape of metallic NPs, and the arrangement of

metallic nanostructures

Among these wide research fields related to LSPs, we review only the

plasmonic effect on the photoluminescence (PL) from solid-state light emitting

materials, including InGaN/GaN quantum well, organic light emitting diodes,

CdSe quantum dots, and Si photonics

By coating a metal layer on InGaN/GaN material, large photoluminescence

(PL) enhancement was reported by Koichi Okamoto33 They grew InGaN/GaN

single QW (3 nm) structures on sapphire substrates with metal organic chemical

vapor deposition (MOCVD) technique After the growth of single QW, metal was

deposited on it Three metals were used They were silver, aluminum, and gold

The thickness of each metal was chosen to be 50 nm The sample structure is

shown in Figure 1.4a The QWs were excited from the bottom, using a 406 nm

cw-InGaN diode laser The PL signals were also collected from the substrate side

PL emission spectrum of each sample is plotted in Figure 1.4b Comparing PL

peak intensity from Ag-coated sample with that from an uncoated sample, a

14-fold enhancement was observed around 470 nm The enhancement factor is as

high as 17 fold for the integrated PL intensity However, there was no

enhancement caused by Au coating As for the Al-coated sample, the peak

intensity is enhanced by a factor of 8 while the integrated intensity is enhanced by

a factor of 6 The authors explained their experimental results using the coupling

effect between SPs and QW emission The excited SPs intensified the

electromagnetic field near the QW located close to the metal film, which

increased the density of state as well as the spontaneous emission rate of QW

This effect resulted in the enhancement of light emission As the SPs resonance of

Au occurred at a longer wavelength than the QW emission wavelength (470 nm),

no PL enhancement was obtained from samples coated with Au

Figure 1.4 (a) Sample structure of InGaN/ GaN QW and excitati on/e mission of PL

me asureme nt (b) PL spec tra of InGaN/ GaN QWs coate d wi th Ag,Al, and Au The PL pe ak intensity of uncoate d InGaN/ GaN QW at 470 nm was nor malize d to 1 33

The PL enhancement varying with wavelength with a spacer layer of 10 nm

between metal layer and QWs, is shown in Figure 1.5a At shorter wavelengths,

the enhancement ratio from Ag sample was increased As for Al samples, the

enhancement ratio was almost constant, independent of wavelength PL intensities

varying with the distance between QWs and metal layers is shown as F igure 1.5b

The spacer thickness of 10 nm, 40 nm and 150 nm was used between QW emitter

and metal coating PL intensity from Al and Ag samples was found to decrease

exponentially with the increase in spacer thickness, but there is no such behavior

for Au-coated QWs as shown in Figure 1.5a

Figure 1.5 (a) PL e nhance me nt r ati os at se veral wavelengths for the same sample as in Figure 1.4 (b) (Inset) Dispersion diagr ams of surface plas mons generate d on Ag/ GaN, Al/GaN, and Au/ GaN surfaces (b) Integrate d PL enhanceme nt ratios for samples wi th Ag,

Al, and Au are pl otte d against the thickness of GaN s pacers The solid lines are the calculate d val ues by the pene tration de pths 33 .

11-fold enhancement of light emission from organic thin films using

SP-OLED34 coupling was reported by Neal T D Half of substrate was metalized

with either Ag or Au Then the substrates were coated with a dye layer to a

thickness of ~ 200 nm The dye used was Coumarin 460 With a half metalized

substrate and half bare substrate, a fast and direct comparison can be observed

between polymer emission on top of Au/Ag layer and the polymer emission on

the bare quartz substrate The sample structure and PL spectra are shown in

Figures 1.6a and 1.6b respectively It is seen that an 11-fold PL intensity

enhancement is obtained from the dye layer on Ag film, compared to that on a

bare quartz substrate This is attributed to the coupling effect of SPs generated by

Ag with the dye emission Since the SPs resonance wavelength from Ag film is

close to the dye emission wavelength, they can be coupled However, as for the

Au film, whose SPs resonance wavelength is much longer than the dye emission

wavelength, no significant PL enhancement was observed from dye emission on

Au film compared with that on bare quartz substrate

Figure 1.6 (a) Sample structure of dye dope d polymer with both pump light and e mission light configurations (b) PL s pectr a of Coumarin 460 on Ag, Au, and quar tz The PL pe ak intensity of Coumarin 460 on quartz was nor malize d to 1 34

Luminescence enhancement of CdSe quantum dots by SP coupling was

investigated by several groups 5-fold PL enhancement was obtained by

Kulakovich et al.35 through coupling SPs generated from Au colloids with

CdSe/ZnS quantum dot emission Later, as large as 50-fold PL enhancement was

observed by Song36 et al with CdSe/ZnS QDs and nanoperiodic silver arrays

Another direct observation between SPs with CdSe quantum dot spontaneous

emission was reported by Okamoto's group37 They first deposited a 50 nm Au

layer on half of a quartz substrate Quantum dots, 5 nm in diameter, emitting at

620 nm wavelength were put on the two different parts of the substrate The

sample structure is shown in Figure 1.7a The PL spectra of two samples are

plotted in Figure 1.7b With the 50 nm Au film under the CdSe quantum dots, the

excitation

emission

excitation

emission

PL intensity emitted from these QDs increased 30-times compared with the

emission from CdSe QDs on quartz The large enhancement was explained to be

due to the coupling of SP with QDs The authors believed the coupling can

happen as long as the energies between SP wavelength and QD emission

wavelength were matched The intrinsic IQE values of material did not affect this

process

Figure 1.7 (a) Sample structure of CdSe nanocrystals on Au -coate d quartz chi ps (b) PL spectra for CdSe nanocr ystals on Au and quar tz (Qz) 37

Coupling of SP with Si photonics was also investigated and PL enhancement

was achieved by a few groups38-41 Scherer et al deposited Au and Al thin layers

on top of SiO2, in which Si nanocrystals were embedded42 The structures can be

seen from Figure 1.8a PL spectra from these three samples were reported in

Figure 1.8b For the Al coated sample, there is a 2-fold enhancement in PL

intensity compared with that of the original sample at the peak wavelength of 600

nm For the Au coated sample, an almost 70-fold increase in PL enhancement was

observed at the longer wavelength of 750 nm It was proposed that with carefully

optimized SP coupling, the emission rate as well as excitation densities of Si

nanocrystals can be increased Using this concept to design the structure, Si LED

can be cheaply fabricated

Figure 1.8 (a) Sample structure of Si nanopar ticles dis persed in SiO 2 me di a and excitation/emission c onfigur ati on of PL measure ment (b) PL spec tra of Si/SiO 2 with Au, Al, and no me tal l ayer 42 .

Based on the pioneered study, we can also apply SPs coupling effect in

InGaAs/GaAs quantum system

1 2 Introduction of photonic bandgap

Photonic crystals (PhCs), in which the refractive index varies periodicity

between high index material and low index material, were first predicted in 1987

by Eli Yablonovitch from Bell Communications Research in Red Bank and by

New Jersey et al from the University of Toronto independently43,44 The

circumstance of photons encountered in this periodic variation between high

index to low index of PhCs, is similar to electrons confined in a periodic atomic

potential in a semiconductor Due to this similarity, PhCs are regarded as

'semiconductors for photons'45 Under certain conditions of refractive index

variations, a complete photonic bandgap can be realized Any light with a

frequency within this gap will be forbidden from propagating in all directions

inside the PhC Yablonovite46 was the first to demonstrate microwaves from

propagating in any direction in a PhC The structure was built by milling an array

of 1 mm holes into a slab, whose refractive index is 3.6 The distance between

holes was made equal to the wavelength of light divided by the material's

refractive index Limited by the level of fabrication technique at the time, creating

PhCs with high refractive index material was impossible This challenge was not

overcome until 1997, when nanofabrication techniques became available and

much effort were devoted to fabricate PhCs and applying them in various fields

PhCs can be designed and created by controlling the refractive index contrast, the

filling factor of each material in the structure and the arrangement of high- and

low- index materials Employing this rule, any PhCs with desired properties could

be created They also have potentials to lead to optical integration, just like

semiconductors leading to electronic integration

Till now, the most successful applications of PhCs is in the fiber It was

proposed to carry high power light with PhCs fibers The structure for a

traditional fiber is a lower refractive index cladding surrounding a higher

refractive index glass core Although the contrast of refractive index was small, it

is enough to concentrate the light in the core by total internal reflection47,48

However, confining high power light within a very small region of space in the

core, results in Raman scattering This Raman scattering can corrupt the light

signal or even damage the fiber The first photonic crystal fiber consists of a

hexagonal array of air holes running the entire length of the fiber Within this

structure, the power carried by the light was still limited By removing the central

core, leaving a diameter of 15 μm hole which acts as a waveguide, light is

trapped by the photonic crystal structure and only some of the light can propagate

along the fiber

The amazing properties of PhCs makes them a promising candidate to achieve

high performance in sensing applications, because they can strongly confine light

to a very small volume, resulting in the ability to characterize chemical species in

the nanometer scale Recently, various photonic architectures have been widely

fabricated and employed for sensing applications, such as the ring resonator,

microdisks, and microspheres Ultra compact sensor chips with very high

performance can be created by using advanced chemical surface functionalization

techniques and integration with micro fluidic systems Scullion et al reported that

using functionalized slotted PhC cavities with an integrated microfluidics device,

low concentration of dissolved avidin, approximately 1μm/ml, can be detected49

Besides high sensing performance in detecting chemical solutions, detecting gas

is another important application According to the initial spectra of gas, a good

gas sensor could be used in the mid infrared range, and PhCs are proposed as

candidate in this gas sensor field Moreover, temperature, pressure and humidity

were also investigated using PhCs sensors50

Recent studies showed that PhCs can also be applied to optoelectronic devices,

both active and passive They can enhance the spontaneous emission and light

extraction efficiency, although the spontaneous emission rates were once

considered an intrinsic property of material51, which cannot be changed Because

of the work of Purcell, it is realized that these properties can be optimized by

varying the environment He predicted the radiation rate of an atom in a

wavelength-size cavity was much faster than that of an atom in the free space

This prediction was later verified by Haroche et al. 52, 53 These led to design of

various structures to modify the environment of an emitter to obtain a relatively

high Purcell factor But most of them were focused on a single emitter or a

molecular, which improved the theoretical understanding but not so useful for

practical applications Instead, semiconductor material, regarded as consisting of

thousands atoms in a small volume, is of importance in applications, such as light

emitting diodes and laser for telecommunication applications Researchers tried to

generate PhCs effect with semiconductor materials, and they observed the similar

effect as experienced by a single atom Under these circumstances, vacuum

fluctuations were strongly modified by PhCs54 This modified vacuum fluctuation

will slow down or speed up the decay of emitted light

Since the spontaneous emission can be modified by PhCs, a PhC can be

designed to enhance the spontaneous emission of a semiconductor The value of

the enhancement factor can be roughly estimated by the ratio of the available

electromagnetic modes density for the emitted photon to the density of

free-photon states Q/8π(V/λ3), where Q is the quality factor of the cavity, V is the

effective volume of the resonance, and λ is the wavelength of light55 Analytic

solutions56 can be found in a simple structure, for example a planar microcavity or

a cylindrical waveguide, where the relative changes of the spontaneous lifetime

were estimated by simply taking the ratio of permitted solid angles in k space to

the total solid angle 4π The dipole spontaneous emission rate can be calculated

by summing up the radiation rate for each allowed mode at a given frequency

As discussed previously, light-emitting diodes (LED) are important

applications of semiconductor in daily life However, their light extraction

efficiency of LED is relatively low57,58, only around 4% light can be emitted out

because most photons are trapped in the high index semiconductor active layer by

total internal reflection This low efficiency has been improved by PhCs effect

For example, as high as 70 fold enhancement was achieved by Zelsmann et al by

introducing 2D photonic band gap structures to silicon-on - insulator LED

devices59

In the similar way, PhCs can also be used in our semiconductor quantum

emission system, InGaAs/GaAs

1.3 Research motivation

It has been known that how to enhance internal quantum yields and convert

them to external efficiency are main challenges for III-V quantum systems

InGaAs/GaAs is an important member in III-V quantum group, which has wide

applications in daily life Theoretically, in a normal planar InGaAs/GaAs QW,

there is only about 4% of the light emitted can be extracted from the top surface

Most of them are trapped in high index active layer So, the need for improving its

extraction efficiency is greatly important, and this is our goal

SPs, a hot topic recently, are capable to confine dramatic strong local

electromagnetic field in subwavelength structures at their resonance wavelengths

Based on Purcell's theory, putting a nano-scale dipolar emitter, such as

electron-hole in QW, close to a metal surface in the nanometer range, a strong

electromagnetic field confined by SPs from metal will cause a strong influence on

dipole emission from QW In turn, the affected dipole radiative emission energy

will be coupled to SP modes, and they are subsequently released to free space As

a result, an increased radiative decay (decreased lifetime) and modified emission

efficiency of the QWs can be realized If SPs can be coupled with the

InGaAs/GaAs QW properly, the enhanced emission efficiency will significantly

improve the emission intensity of the optoelectronic devices It also provides new

avenues in creating novel SP optoelectronic devices Thus studying the LSP

coupling with InGaAs/GaAs QW is of great significance Making the resonant

wavelength of LSP to overlap with emission wavelength from QW is a basic

requirement for coupling effect of LSPs with QW emission Thus it is important

to design proper metallic structures to match this condition Moreover, the

distance between metal and QW emitter also plays an important role in

determining whether there is an enhancement or quenching based on LSPR So it

is more crucial in optimizing metallic structures to achieve high PL enhancement

factors for InGaAs/GaAs QW

To enhance the light extraction efficiency of InGaAs/GaAs QW to benefit the

optoelectronic device, PhC is also one of the best choices Therefore, in this thesis,

we incorporate SPs with photonic bandgap structure to enhance InGaAs/GaAs

QW emission The experimental results and discussion are included in the

following chapters of this thesis

1.4 Scope of thesis

The related theories, both SPs, and photonic bandgap were discussed first to

facilitate a better understanding of the research carried out Surface plasmon is a

kind of electromagnetic (EM) wave, which propagates at the metal-dielectric

interface We can understand this EM wave by solving Maxwell’s Equations with

the proper boundary conditions By solving the Maxwell’s Equations, it is noticed

that only TM mode can excite SPs For practical applications, metallic nanohole

arrays and metallic nanoparticle arrays were studied SPs generated by nanohole

arrays can propagate along the film and solutions can be found by solving

Maxwell’s equations with planar boundary conditions in cartesian coordinate

system As with the case of SPs excited from arrays of metallic nanoparticle, Mie

theory is used, which solve Maxwell’s equations in the cylindrical coordinate

system The details are provided in Chapter 2

Materials that are periodically structured with respect to refractive indices on a

scale comparable to the wavelength of light can prohibit photons from travelling

in specific directions Inside such a structure, reflections from the periodic

refractive index interfaces can cause constructive and destructive interferences

Photons react to the refractive index contrast in an analogous manner to the way

electrons react in an environment of a periodic potential generated by ions Each

results in a range of allowed energies and a band structure was characterized by

an energy gap or photonic band gap The details are also included in Chapter 2

To generate the coupling between QW emission and SPs, tuning SPs resonance

wavelength is of great importance Both metallic nanohole arrays and metallic

nanoparticle arrays were investigated, in which nanosphere lithography and laser

interference lithography techniques were employed to serve as low cost, highly

efficient nanofabrication methods The morphology was observed through

scanning electron microscopy (SEM) To observe the SPs resonance wavelength,

reflectance spectra were measured The results show that the surface plasmon

resonance wavelength can be tuned by varying the periodicity, size of hole and

particle, and surrounding medium refractive index The details are discussed in

Chapter 3

Further research was carried out to study the coupling effect of SPs with

InGaAs/GaAs QW emission To make the coupling effect happen, proper metallic

structures were designed to overlap SPs resonance wavelength with that of the

QW emission Both ordered Au nanodisk arrays and irregular Au nanodisk arrays

were studied Introducing a thin SiO2 layer between Au nanodisk arrays and QW

surface overcomes the fabrication challenge With the insertion of SiO2 of low

refractive index, a relatively large period of Au nanostructure on InGaAs/GaAs

QW was achieved which enhanced the coupling effect Introducing irregularity to

Au nanodisk arrays, the exit angle of SP bright modes was enlarged Subsequently,

the degree of coupling energy emitting to the outside world was enhanced, which

enhances the out coupling of PL from InGaAs/GaAs QW Theoretical simulations

agree reasonably well with the experimental result The details can be found in

Chapter 4 of this thesis

Since the photonic crystal also can be used for increasing extraction efficiency

of semiconductor quantum system, SPs was incorporated with photonic bandgap

structure to enhance InGaAs/GaAs QW emission By varying the thickness of the

insertion layer between Au nanodisk and QW emitter, we separated the effect on

InGaAs/GaAs QW caused by photonic bandgap with that from SPs Simulations

based on our samples were also carried out and confirmed the experimental

results The details can be found in Chapter 5 In Chapter 6, a summary of all the

accomplishments and the proposed future work are presented

Chapter 2: Theory

2.1 Principle of Surface Plasmon

Surface plasmon is a kind of electromagnetic (EM) wave, which propagates at

the metal-dielectric interface We can understand this EM wave by solving

Maxwell’s Equations with proper boundary conditions Aiming to address the

problem easily, we consider materials to be homogeneous continuum, which can

be described using a frequency-dependent relative permittivity60

Here, we start from basic Maxwell’s equations of macroscopic

electromagnetism in the following form:

where D is the dielectric displacement, E is the electric field, H is the magnetic

field intensity, and B is the magnetic flux density, ρext is the external charge and

J ext is the current density

We first apply the Maxwell’s Equations (2.1) to the flat interface between a

metal and a dielectric to investigate the basic physical properties of surface

plasmon polaritons (SPPs) As we have seen, without external charge and current

densities, we can combine the curl equations (2.1c), and (2.1d) to

Mathematically, E, , and there is no external stimuli, which means equation (2.2) can be rewritten as

where is the wave vector of the propagating wave in vacuum

Now, we define the propagation geometry (see Figure 2.1) To simplify the

problem, we assume ɛ depends only on one spatial coordinate, which means the

waves propagating along the x-direction, and without spatial variation in-plane direction, therefore We take the plane z = 0 as the interface sustaining the propagating waves, then electric field can be written as ,

y-where β=k x is the wave vector in the direction of propagation Inserting this

expression into (2.5), we can obtain the desired form of the wave equation

Figure 2.1 Defini tion of a pl anar waveg ui de geome try The waves propag ate along the directi on i n a c artesian c oor di nate syste m 60 .

x-Similarly, we can also obtain the equation for the magnetic field H

We now use the curl equations (2.1c), and (2.1d) to find explicit expressions

for the different field components of E and H For harmonic time dependence

, we arrive at the following equations

For propagation along the x-direction

and homogeneity in the y

direction

, this set of equation can be simplified to

From the above equations, we know two sets of self- consistent solutions with

different polarization properties of the propagating waves will be allowed One is

the transverse magnetic (TM or p) mode, where only the field components E x , E z

and H y are nonzero, and the other is the transverse electric (TE or s) modes, with

only H x , H z and E y being nonzero

For TM modes, equations (2.8) can be reduced to