Design and fabrication of dipole antenna at optical frequency

10 Figure 1.5 Representation of biosensors configuration based on optical nano-antennas .... 44 Figure 4.3 Resonance intensity and resonance position of dipole antennas as a function o

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHONOLOGY

-

NGUYEN NGOC SON

DESIGN AND FABRICATION OF DIPOLE ANTENNA AT

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHONOLOGY

-

NGUYEN NGOC SON

DESIGN AND FABRICATION OF DIPOLE ANTENNA AT

OPTICAL FREQUENCY

Science and Engineering of Electronic Materials

MASTER THESIS OF SCIENCE MATERIALS SCIENCE

SUPERVISOR Assoc Prof Chu Manh Hoang

Hanoi – 2018

ACKNOWLEDGEMENT

Firstly, I would like to thank my supervisor Assoc Prof Chu Manh Hoang for his guidance, support, and encouragement during the time of learning and working at ITIMS

I sincerely thank all the teachers in the International Training Institute for Materials Science for their support and interest throughout the learning and implementation of this project The acknowledgment would be also sent to all the members of MEMS group for helped me in difficult conditions In addition, I would like to thank my colleagues Mr Tam, Mr Chinh, Mrs Thai, Mr Ngo Minh, Mr Hoang and Mrs Thuy for the knowledge and experience that has helped me in the last two years

Finally, I would like to thank my parents, my brother and my wife for their invaluable support and encouragement in terms of finance as well as the spirit that

is the main motivation for me to overcome all challengers

LIST OF PUBLICATIONS

1 Nguyen Van Minh, Nguyen Ngoc Son, Nghiem Thi Ha Lien, Chu Manh Hoang,

(2017), “Non-close packaged monolayer of silica nanoparticles on silicon substrate

using HF vapor etching”, IET Micro & Nano Letters, (IET, ISSN 17500443), doi:

10.1049/mnl.2016.0825, pp 656–659

2 Nguyen Ngoc Son, Nguyen Van Minh, Chu Manh Hoang, (2017), “Absorption

and scattering of goldshell semi-sphere nanoparticles”, Advances in Optics

Photonics Spectroscopy & Applications IX, Ninh Bình 7-10/11/2016,

ISBN:978-604-913-578-1, pp 385–388

3 Nguyen Ngoc Son, Chu Manh Hoang, (2017), “Modeling and simulation of

dipole nanoantenna based on semisphere nanoparticles”, Hội nghị Vật liệu và Công

nghệ nano tiên tiến, Hà Nội 14-15/8/2017 ISBN: 978-604-95-0298-9, pp 225-229

4 Nguyen Ngoc Son, Vu Thi Ngoc Thuy, Chu Manh Hoang, (2017), “Influence of

incident light on optical characteristics of the nanoparticle-based nanoantenna”, Hội

nghị toàn quốc về Vật lý chất rắn và Khoa học vật liệu lần thứ 10 (SPMS2017),

Huế 19-21/10/2017, ISBN:978-604-95-0326-9, pp 512-514

5 Nguyen Van Minh, Do Thi Hue, Nguyen Ngoc Son, Nghiem Thi Ha Lien, Chu

Manh Hoang, (2017), “Plasmonic nanostructures based on monolayer of

close-packaged silica nanoparticles”, Advances in Optics Photonics Spectroscopy &

Applications IX, Ninh Bình 7-10/11/2016, ISBN:978-604-913-578-1, pp 206-209

STATEMENT OF ORIGINAL AUTHORSHIP

I hereby declare that the results presented in the thesis are performed by the author The research contained in this thesis has not been previously submitted to meet requirements for an award at this or any higher education institutions

Date: 30/9/2018

Signature

TABLE OF CONTENTS

ABSTRACT 1

CHAPTER 1 INTRODUCTION 3

1.1 RF antennas and optical nano-antennas 3

1.2 Applications of optical nano-antennas 5

1.2.1 Optical nano-antennas as solution of nanoscale imaging and spectroscopy 5

1.2.2 Optical nano-antennas for solar energy harvesting 9

1.2.3 Optical nano-antennas for biosensors applications 11

1.3 Fabrication methods 13

1.3.1 Electron-beam lithography 14

1.3.2 Focused-ion beam milling 16

1.3.3 Self-assembly methods 17

1.4 Purpose of this thesis 19

CHAPTER 2 THEORETICAL BACKGROUND 20

2.1 Theoretical of surface plasmons 20

2.1.1 Surface plasmon polaritons 20

2.1.2 Localized surface plasmons 24

2.2 Optical characterization of nano-antennas 26

2.2.1 Far-field scattering 27

2.2.2 The near-field intensity enhancement 28

CHAPTER 3 SIMULATION AND EXPERIMENT METHODS 33

3.1 Simulation method 33

3.1.1 Modeling 34

3.1.2 Boundary conditions 35

3.1.3 Meshing 36

3.1.4 Incident light 37

3.1.5 Simulation parameter 38

3.2 Fabrication method. 39

3.2.1 Self-assembly close-packed monolayer of silica nanoparticles 39

3.2.2 Tuning size of nano-antennas 41

3.3.3 Sintering the sample 41

3.3.4 Sputtering 42

CHAPTER 4 RESULTS AND DISCUSSION 43

4.1 The influence of geometry parameters on resonance spectrum 43

4.1.1 The influence of antennas size on resonance spectrum 44

4.1.2 The influence of gap size on resonance spectrum 47

4.1.3 The influence of gold shell thickness on resonance spectrum 49

4.2 The influence of incident light on resonance spectrum 53

4.2.1 The influence of polarization angles on resonance spectrum 53

4.2.2 The influence of s-polarized light on resonance spectrum 54

4.2.3 The influence of p-polarized light on resonance spectrum 56

4.3 The influence of environment refractive index on resonance spectrum 59

4.4 Experimental results 62

4.4.1 Fabrication of a monolayer of silica nanoparticles 62

4.4.2 Controlling the antenna size by using HF vapor etching 63

CONCLUSIONS 64

SUGGESTED FUTURE WORKS 64

REFERENCES 65

LIST OF FIGURES

Figure 1.1 Working schematic of RF antennas 3

Figure 1.2 Schematics of the experimental arrangement 7

Figure 1.3 Examples of ORAs and of a stripe 8

Figure 1.4 Schematic of different types of antenna effects in photovoltaics 10

Figure 1.5 Representation of biosensors configuration based on optical nano-antennas 12

Figure 1.6 Sketch of the main steps for standard EBL and FIB nanostructuring of nano-antennas 14

Figure 1.7 SEM image of optical nano-antennas 15

Figure 1.8 Images of Au core –Ag shell nanoprisms 17

Figure 2.1 Schematic representation of SPPs propagation at a metal-dielectric interface 21

Figure 2.2 Schematic illustration of a localized surface plasmon resonance 24

Figure 2.3 Typical dark-field setup for scattering measurements 27

Figure 2.4 Sketch of a standard confocal setup 29

Figure 2.5 Sketch of a standard apertureless SNOM 30

Figure 2.6 Sketch of a standard aperture SNOM 31

Figure 3.1 The 3D view of the antenna structures 34

Figure 3.2 Schematic explanation of the boundary conditions 35

Figure 3.3 Mesh used for the antenna simulations 36

Figure 3.4 Schematic of the propagation of an s-polarized wave light and an p-polarized wave light 37

Figure 3.6 Schematic of the principle of the drop coating method 40

Figure 3.7 The sputtering system used to deposite metal film 42

Figure 4.1 Regulations on marking position in the thesis 43

Figure 4.2 Resonance spectra of dipole antennas (a) and dipole combine antennas (b) (g = 10nm, t = 10nm) with different antenna size 44

Figure 4.3 Resonance intensity and resonance position of dipole antennas as a function of the antenna size 45

Figure 4.4 Resonance intensity (a) and resonance position (b) of dipole combine antennas as a function of the antenna size (g = 10 nm, t = 10nm) 46

Figure 4.5 Resonance spectra of dipole antennas (d0 = 170 nm, t = 10 nm) (a) and dipole combine antennas (d0 = 200 nm, t = 10 nm) (b) with different gap size 47

Figure 4.6 Resonance intensity and resonance position of dipole antennas as a function of the gap size (d0 = 170 nm, t = 10 nm) 48

Figure 4.7 Resonance intensity (a) and resonance position (b) of dipole combine antennas as a function of the gap size (d0 = 200 nm, t = 10 nm) 49

Figure 4.8 Resonance spectra of dipole antennas and dipole combine antennas with

different gold shell thickness 50

Figure 4.9 The electric normalized of dipole antenna 50 Figure 4.10 Resonance intensity (a) and resonance position (b) of dipole antennas

(d0 = 170 nm, g = 10 nm) as a function of the gold shell thickness 51

Figure 4.11 Resonance intensity (a) and resonance position (b) of dipole antennas

as a function of the gold shell thickness (d0 = 200 nm, g = 0 nm) 52

Figure 4.12 Resonance spectra (a) and resonance intensity, resonance position (b)

of dipole antennas (d0 = 170 nm, g = 10 nm, t = 10 nm) with different polarization angles 53

nm) (a) and dipole combine antennas (d0 = 200 nm, g = 0 nm, t = 10 nm) (b) with different incident angles (s-polarization) 54

170 nm, g = 10 nm, t = 10 nm) as a function of incident angles (s-polarization) 55

Figure 4.15 Resonance intensity (a) and resonance position (b) of dipole antennas

as a function of incident angles (d0 = 200 nm, g = 0 nm) (s-polarization) 56

nm) (a) and dipole combine antennas (d0 = 200 nm, g = 0 nm, t = 10 nm) (b) with different incident angles (p-polarization) 56

170 nm, g = 10 nm, t = 10 nm) as a function of incident angles (p-polarization) 57

Figure 4.18 Resonance intensity (a) and resonance position (b) of dipole antennas

as a function of incident angles (d0 = 200 nm, g = 0 nm) (p-polarization) 58

nm) (a) and dipole combine antennas (d0 = 200 nm, g = 0 nm, t = 10 nm) (b) with different surrounding environment 59

170 nm, g = 10 nm, t = 10 nm) as a function of refractive index of environment 60

Figure 4.21 Resonance intensity (a) and resonance position (b) of dipole antennas )

as a function of refractive index of environment (d0 = 200 nm, g = 0 nm) 61

Figure 4.22 SEM imagines of monolayer silica nanoparticles size 235 nm with the

magnification of (a) x600 and (b) x40000 63

Figure 4.23 SEM imagines of monolayer silica nanoparticles with size 235 nm,

cross-section view(a),and cross-section view after etching HF vapor 120s (b) 63

LIST OF TABLES

Table 3.1 Parameters and materials used for simulating 37

GLOSSARY OF TERM AND ABBREVIATIONS

ABSTRACT

Optical nano-antennas are the nanostructures that can confine/enhance electromagnetic waves into sub-wavelength volumes at resonant conditions They are based on two unique properties are the surface plasmon polaritons (SPPs) and the localized surface plasmons (LSPs) The metal-dielectric interface of antennas that create the SPPs meanwhile the high electric field enhancement of LSPs was formed in the gap region which separate antenna’s arms Their various applications including biological and chemical sensing, solar cells, scanning near-field optical microscopy (SNOM) and generation of light The purpose of this thesis is to theoretically investigate and fabricate dipole optical/plasmon nano-antennas for biosensing applications in visible regime Generally, noble metals are used to fabricate optical nano-antennas However, in the frame of this thesis, we will produce dipole antennas which have a core-shell structure to enhanced electromagnetic fields The challenge in optical nano-antennas designing is understanding of the underlying physics to optimize the effectiveness of antennas

As well, the issues related to fabrication of low cost, high accuracy nanostructures

in sub-wavelength scale

The first, we focus on designing nano-antennas For theoretical analysis, the full 3-D numerical simulations have been done by using Finite Element Method (FEM) The influence of the geometrical parameters such as the radius of silica semi-sphere nanoparticles, the thickness of shell and gap distance on the resonant properties of dipole nanoantenna are studied The effects of the polarization angles and the incident angle on the resonant behavior and far-field distribution of dipole nano-antenna are predicted by simulations Besides that, we compared different kinds of antennas and the potential enhancement of dipole antennas versus arrays antennas was studied in detail

After that, we experiment to find out how to fabricate the dipole antennas that were optimized in the previous part The first step, a close-packed monolayer of silica nanospheres on silicon a substrate was fabricated by using drop coating method with supported of infrared irradiation The position of nanospheres was fixed by sintering process meanwhile the geometry parameters were controlled by

HF vapor etching Finally, a gold layer was deposited onto the nanospheres using

sputtering process to accomplished the desire nano-antennas

This thesis is organized as follows: In chapter 1, we introduce into a history

of developments, applications, and techniques to fabricate optical nano-antennas Chapter 2 presents theoretical background of plasmonic (SPPs, LSPs), optical properties of nanostructures and theoretical model of optical nano-antennas Chapter 3 includes the information about modeling steps, simulation parameters, techniques and fabrication steps Finally, in Chapter 4, we will discuss both the results of simulation and fabrication in details

CHAPTER 1 INTRODUCTION

In this chapter, we first have a discussion on radio frequency antennas (RF antennas) and optical nano-antennas to explain why we need to develop antennas in visible regime A short overview of the current state of optical nano-antennas will

be presented Then, the potential applications of nano-antennas will be presented, which show why optical nano-antennas become a hot topic for researchers over two last decades Some fabrication techniques will be explained which including high technologies and simple methods In the final section, the motivation and objectives

of this thesis are presented

1.1 RF antennas and optical nano-antennas

An RF antenna is a device which converts electrical signals into electromagnetic waves and vice versa In 1888, German physicist Heinrich Hertz built the first dipole antenna to prove the existence of electromagnetic waves predicted by Maxwell’s equations In his experiment, he placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving The research was published in Annalen der Physik und Chemie (vol 36, 1889) The basic working principle of RF antennas as shown in Figure 1.1

Figure 1.1 Working schematic of RF antennas

RF antennas are antennas working in the microwave and radio frequency domain Over the last several decades, RF antennas have been continuously investigated due to the demand of communicating over the world As a commonly

used device in modern live, RF antennas are widely used in the systems such as broadcast (audio) radio, television, mobile telephones, Wi-Fi data networks, and remote control devices among many others Various theories have been established

to analyze their properties and considerable amount of experiments have been conducted to optimize their efficiency Although RF antennas are very useful in the communications, the popular applications of RF antennas are mainly used in the radio frequency The microwave regime has been investigated so intensively that few resources of this spectrum remain for research In contrast, the infrared and visible light range are still available for further exploration

The ideal antenna was designed with total lengths L that are directly related

to the radiation wavelength λ via equation L = const × λ For example, a thin rod dipole antenna is called to be ideal in the case L = λ/2 (half-wave dipole antenna)

So that, the total length of an antenna is required to be hundreds of nanometers or smaller to operate in optical wavelength region Consequently, the first challenge of nano-antennas research is accuracy fabrication down to a few nanometers Fortunately, with the supporting of nano-fabrication tools such as E-beam lithography (EBL), Focus ion-beam lithography (FIB) or by bottom-up self-assembly techniques make this level of fabrication has become possible However, the cost of high technology is too expensive that nano-antennas are not yet suitable for commercial applications The second challenge is the difference between physical phenomena of meters and nanometers scale At optical frequencies, the radiation penetrates into the metal instead of perfect reflection from a metal’s surface [4], so that the antenna theory at radio frequencies no longer valid The high losses in visible range directly relating to another issue are the applied ability in communication decreased significantly The weak signals due to the small size of devices and high loss that make optical nano-antennas to be not a good choice for wireless technologies

1.2 Applications of optical nano-antennas

Optical nano-antennas were invented by the need for high interaction between light and materials in nanometer scale This interaction is shown through the field enhancement, field confinement, absorption and scattering cross-section, which are useful for applications such as high-resolution microscopy, spectroscopy, sensing, light emission and photovoltaics In this section, I review some highlight researches using optical nano-antennas in some emerging application areas

1.2.1 Optical nano-antennas as solution of nanoscale imaging and spectroscopy

As a counterpart of RF antennas, optical nano-antennas are able to convert the energy of free propagating radiation to localized energy and vice versa While

RF antennas were invented for use in communication applications, optical antennas were developed as solutions to nanoimaging and spectroscopy Optical microscopy (OM) is one of the popular tools in researching science The spatial resolution in OM can be given by the minimum distance ∆x between two point

nano-sources that can be unambiguously distinguished in an optical observation [50] According to the Abbe criterion for this minimum distance ∆x that can be defined as

∆x = 0.61λ/NA Where NA = n sinθ max , while n is the refractive index of the surrounding medium and θ max is the maximum collection angle of the optical system However, OM is

limited by the small range of refractive index n which depends on the transparent

materials Thus, the limited resolution of an OM is the diffraction limit λ/2 In case

of visible light, the maximum solution can reach about 200 – 300 nm that is not good enough for imaging nanostructures Thus, the scientists have developed electron microscopes to bring the spatial resolution down to the nanometer scale Utilizing electron to construct images at the nanoscale could not provide much intrinsic information about the properties of samples which may be damaged during the preparation and imaging processes, especially in biological sciences Other techniques such as scanning tunneling microscopy (STM) or Atomic force

microscopy [3, 4] were invented for collecting information at the atomic and molecular scale However, they are able to provide only topographic images of the sample Therefore, the challenge for scientists is to develop imaging techniques that achieve multiple criteria simultaneously, first working at optical frequencies (to protect the sample), deep subwavelength spatial distribution (to beyond the diffraction limit), and finally are able to collect intrinsic information about the properties of the sample

1.2.1.1 Scattering – based microscopy

When a sample is irradiated, the optical field surrounding the sample consists

of two components, the homogeneous propagating radiation, and the inhomogeneous non-propagating evanescent field The evanescent part cannot exist

in free space and remain around the sample Consequently, partial information of the sample loss in the process of image formation results in the spatial resolution limit The scattering based near-field microscopy based on an idea that is to locally convert the evanescent fields into propagating radiation by use of a scattering probe

To execute the idea about scattering – based microscopy, the scientists assumed that the incident light interacts more strongly with the surface of samples than the local metallic probe This assumption ensures that the received signal will

be the intrinsic properties of the sample and not the noise However, the sample properties always interact on the local probe and influence its properties This interaction has been recently studied in an experiment (Fig 1.2) by T Kalkbrenner

et al using a local probe in the form of a single gold nanoparticle [38] The

properties of this antenna were found to be dependent on the local environment defined by the sample properties Therefore, the antenna detuning is able to become

a local probe for investigating the intrinsic properties of the sample

Figure 1.2 (a) Schematics of the experimental arrangement (b) Plasmon spectra of

The polarization of the incident light is indicated by s and p polarizations

defined in (a) [38]

In 1995, R Bachelot et al [3] used the metal sharp tip of AFM instead of

semiconducting tips that have since become the prevalent choice The main idea of the experiment is to use the periodic vibration of a metallic apertureless tip above a diffraction spot of a laser beam focused on the sample The tip, whose point has been etched to less than 100 nm in radius, vibrates perpendicularly to the sample surface with an amplitude in range 50 – 200 nm The result allowed them to reach a lateral optical resolution of 100 nm Although the spatial resolution is still modest (𝜆/7), the limitation only by the probe size Recently, Eun-Khwang Lee et.al demonstrated full-visible-range resonant light scattering from a single dielectric optical nano-rod antenna, which has three-dimensional nano-structures under arbitrary conditions such as in non-planar substrates [44] The initial results showed the great potential of nano-antennas for the development scattering-based microscopy

1.2.1.2 Spectroscopy based on local field enhancement

As mentioned above, the loss of evanescent wave is the cause of spatial resolution limitation Evanescent wave exists only in close proximity with a light source Rather than attempting to place a primary source near the sample surface to collect evanescent waves, scientists have proposed the idea of using optical nano-antennas as secondary sources that are comparatively easy to bring close proximity with an object In this case, an effective antenna that can interacts strongly with incoming radiation and leads to a high degree of field localization and become a secondary source The localized fields have been used in several applications as excitation sources for local spectroscopy, such as IR absorption, Raman scattering [7–10] and laser-excited fluorescence [11–13]

Figure 1.3 Examples of ORAs and of a stripe (A and B) SEM images, zoom and

overview, respectively (C and D) Confocal scan images, logarithmic color code)

The nanometer-scale gold dipole antennas have been designed and

manufactured by F Martin et.al to be resonant at the visible regime After

illuminating the antennas with picosecond laser pulses, the authors obtained very interesting results The results also show that resonance at the feed gap only appears when the incident light is illuminated along the antennas' arms At the resonant wavelength, white-light supercontinuum (WLSC) radiation is generated in the gap

of the antennas This emission intensity is stronger than the three orders of magnitude when compared with the gold stripes of the same dimensions but without feed gap (Fig 1.3) In addition, the different types of discussed antennas are also offering the same confinement feature [14–16] Based on these results, the idea of using optical nano-antennas as a secondary light source is possible via the electromagnetic field confinement of nano-antennas

Amongst several techniques for researching in nanometers scale, Raman spectroscopy is one of the most frequently used methods The Raman spectrum contains intrinsic information due to the collection of both electronic and vibrational energy states However, the signal of Raman is weak that requires small scattering volume and a long time for measurement If Raman scattering is excited by evanescent light, the signal can be enhanced This technique is so-called Tip-enhanced Raman Spectroscopy (TERS), where we used a nano-antennas in form of

a metallic tip as a secondary light source to excite the sample [7–9, 65]

1.2.2 Optical nano-antennas for solar energy harvesting

During the last decades, energy demand of the world is constantly increasing and energy is still mainly dependent on fossil fuels The depletion of fossil fuels has led to a significant need for alternative clean and renewable energy sources Solar energy is one of the alternative candidates that exploit solar radiation for producing electricity The solar energy areas have grown rapidly since the first photovoltaic (PV) was created in the 1950s However, this development still can not cover the market demand because of their low efficiency and high cost To meet this

requirement, nano-antennas with the rectifier in their gap called rectifying-antenna (rectenna) They have been suggested to replace traditional PV technology, where they exhibit higher efficiency compared to the current solar cell [16]

Figure 1.4 Schematic of different types of antenna effects in photovoltaics (a)

Far-field scattering, leading to a prolonged optical path (b) Near-Far-field scattering, causing locally increased absorption, and (c) direct injection of photoexcited

carriers into the semiconductor [6]

Based on electromagnetic confined ability, the nano-antennas have at least three distinct ways to interact with a PV substrate when placed in close proximity,

as illustrated in Figure 1.4 The first mechanism based on scattering photons into the far field Plasmonic nanoparticles have a large optical cross-section that enhances interactions between light and PV substrate This leads to an increased effective optical path length and greater photon absorption probability This mechanism was

demonstrated by Howard R Stuart et.al [66] when experimented To characterize

the coupling, silver-, gold-, and copper-island layers were formed on the surface of

a thin-film photodetector fabricated in the 0.16 mm thick silicon layer of a

silicon-on-insulator (SOI) wafer K R Catchpole et.al developed an optical model for

absorption enhancement by metal nanoparticles on a silicon waveguides that demonstrated the same mechanism [12] The second mechanism is based on the spatially localized high-momentum near-field photons created in the vicinity of an

optical antenna These photons can directly excite electron-hole pairs in an indirect gap semiconductor (like silicon) even without phonon assistance [42], thereby increasing light absorption per unit thickness Last, there is the possibility of direct charge-carrier injection from the nanoparticle into the semiconductor

Making use of these three mechanisms, scientists continue to come up with different approaches to enhance the effectiveness of PV In 2007, S Pillai et.al used silver nanoparitcles for enhancing the absorbance of silicon solar cells The results show that a 7-fold enhancement for wafer-based cells at λ=1200 nm and up to 16-fold enhancement at λ =1050 nm for 1.25 m thin silicon-on-insulator (SOI) cells

[59] When investigating thick silicon further by using gold nanodisks, Hägglund et

al have found that LSPRs in particles attached to a photoactive substrate not only

enhance but also reduce photoinduced generation It depends on the wavelength and particles-substrate coupling [27] Instead of Silicon, Keisuke Nakayama used GaAs solar cells decorated with size-controlled Ag nanoparticles to improve the efficiency

of optical absorption The results show that in the absorbing layers resulting in an 8% increase in the short circuit current density of the cell Addition to, the strong scattering by the interacting surface plasmons increases the optical path of the incident light [56] Recently in 2015, a carbon nanotube optical rectenna device by engineering metal–insulator–metal tunnel diodes [64] Asha Sharma has found a coherent optical antenna field appears to be rectified directly in their devices consistent with rectenna theory Furthermore, there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating

a potential for robust operation Although still in the process of research, optical nano-antennas still produce positive results suggesting great application potential in photovoltaic applications

1.2.3 Optical nano-antennas for biosensors applications

A biosensor device is defined by its biological, or bioinspired receptor unit with unique specificities toward corresponding analytes These analytes are often of

biological origin like DNAs of bacteria or viruses, or proteins which are generated from the immune system (antibodies, antigens) of infected or contaminated living organisms [33] There are many challenges when developing high-performance biosensors such as the high sensitivity, the efficient signal capture, the non-damaged sample, etc Optical nano-antennas is considered to be one of the most promising methods in biosensor applications due to taking advantage of the interaction of light with plasmonic materials and sub-wavelength metallic structures

Figure 1.5 Representation of biosensors configuration based on

optical nano-antennas

Optical nano-antennas for biosensors applications are based on the phenomena of LSPs Localized plasmon modes are strongly dependent on the size, shape, materials of nanoparticles and refractive index of surrounding medium Therefore several structures of nano-antennas have been investigated such as nanodipole, nanobowtie, nanodimer [28, 29] and the performance of nano-antennas can be optimized by varying the size and shape of the nanostructures [15] Normally, noble metals are used to make nano-antennas Nevertheless, for biosensor applications, gold nanoparticles are mostly used because of their biocompatibility [45] Beside that their relatively simple production and modification also makes gold the best choice [8]

One of the biggest advantages of biosensors based on optical nano-antennas

is the ability to detect small biomolecules, as the dimension (2 - 20 nm) of receptors such as enzymes, antibodies, and antigens are dimensionally compatible

bio-with the antenna's structure [28] In 2007, Kim et al used gold-capped oxide

nanostructure to make LSPR coupled with the Interferometry in a porous anodic alumina (PPA) layer chip The results show that the chip is able to detect synthetic targets DNA with a detection limit of 10 pM [41] Hiep et.al used noble metal nanoparticles to develop microfluidic LSPR chip that allows studying biomolecular interactions This chip has been proposed to monitor insulin and anti-insulin antibody immunoreactions in real-time with a detection limit of 100 ng/mL for detecting insulin level that is required for a diagnosis of diabetes [32] Alzheimer disease has also been proposed to detect with LSPR biosensors, where the interaction between amyloid β-derived diffusible ligands (ADDL) and the anti-ADDL antibody are monitored by using Ag nanoparticles [36, 37] In 2014, tumor biomarker i.e., squamous cell carcinoma antigen (SCCa) has been proposed to detect with reusable and label-free LSPR based biosensor [72] Recently, Michele Dipalo et.al has reported results on three-dimensional (3D) plasmonic nanoantennas that are integrated with multielectrode arrays (MEAs) as a biosensor [17]

1.3 Fabrication methods

The resonances of optical nano-antennas strongly depend on the precision of geometry and dimensions, so optical nano-antennas require reliable and reproducible fabrication techniques Commonly, the fabrication methods should have a typical resolution below 10 nm in order to accurately define critical dimensions, such as feed-gap size or antenna arm length Nowadays, various top-down and bottom-up nanofabrication approaches have been applied to experimentally realize optical nano-antennas Top-down approaches, e.g electron-beam lithography (EBL) and focused-ion beam (FIB) milling are capable of fabricating large arrays of nearly identical nanostructures with well-defined orientations and distances On the other hand, bottom-up approaches take

advantages of chemical synthesis and self-assembly of metal nanoparticles in solution with nearly perfect symmetry and crystallization that can be put on any substrate

subjected to a solvent which removes the remaining resist and leaves the metal

structures in the voids unaffected (lift-off) [7]

Figure 1.7 SEM image of optical nano-antennas (a-b) array and single gold bowtie

nano-antennas fabricated by EBL [30], (c-f) Various nano-antennas with gap sizes

down to ~10 nm, fabricated by focused ion beam milling [57]

EBL is well known as a very effective technique in fabricating antennas because of the high spatial resolution and the ability to manufacture nano-

nano-antennas array quickly Although EBL has a high spatial resolution of up to 5 nm,

however, due to the multi crystallization of the deposited metal layer, the final structural resolution is usually not as good which is further reduced to ≈ 10nm even less after the pattern is transferred Figure 1.7(a, b) show that SEM images of the array and single gold bowtie nano-antennas Obviously, there is an instability of structure between the antennas (Figure 1.8a) and even between the two arms of nano-antennas (Figure 1.7b) when EBL was used In order to increase the stability

of the fabricated nanostructures during lift-off, adhesion layers (ITO, Cr, Ti, etc) are necessary to make metals adhere firmly to a transparent substrate However, these adhesion layers significantly increase the damping of plasmon resonance [36]

Recently, other EBL-based techniques without adhesion layer have been proposed and developed which are potential tools for effectively fabricating nano-antennas in future [41, 42]

1.3.2 Focused-ion beam milling

Another efficient machining technique for fabricating optical nano-antennas

is Focused-ion beam (FIB) milling FIB techniques use beams of ions which are accelerated and deflected in order to remove material on specific locations of the sample The accelerated ions beam is focused into a few nanometer spots and scanned over a conductive substrate to produce a desired pattern (Figure 1.6b) In the field of optical nano-antennas, FIB nanofabrication has been successfully applied in making various kinds of nano-antennas (Figure 1.7c-f) with the high resolution and the high versatility of the direct patterning approach

The main advantage of FIB milling is the broad applicability to almost any type of material Furthermore, the obtained nanostructures have the very good resolution, allowing for the fabrication of surface and gaps in ~ 10 nm or even

more In 2013, Olivier Scholder et.al successfully fabricated plasmonic

nano-antennas with 3.5 nm gap size when using Helium focused-ion beam [63] In this experiment, the range of gap sizes is controllable and reproducible with a precision

≈ 1 nm Meanwhile EBL is just useful in the case of the flat substrate, FIB milling is

a unique fabrication tool even with non-flat sample topographies In certain circumstances whenever the use of resist-based lithographies is difficult or when a resist must be avoided to allow for the epitaxial growth of single crystal metal substrates, FIB milling becomes a good choice instead of EBL For example, the Bow-tie optical antenna on top of the AFM tip was accomplished by Javad N

Farahani et.al [19] The authors demonstrate that, even in this complex geometry, it

is possible to control key antenna parameters such as overall length and width of the feed gap by focused-ion-beam milling Nevertheless, FIB milling also has certain disadvantages The first one is that in order to achieve isolated structures, a very

large area around the structure of interest has to be milled away, this step is consuming This area leads to disconnect the structure from the ground and leaves it surrounded by a large non-conductive area resulting in charging and drifts during the milling process The second problem is contamination, due to FIB milling is a sputtering process, the ions of the source will implant into the target metal film and substrate Finally, fabricating array nanostructures is a disadvantage of FIB milling

time-because of the time consuming and high cost

1.3.3 Self-assembly methods

In addition to top-down approaches, bottom-up techniques have also been widely used to obtain optical nano-antennas Self-assembly is a process where the components of a system assemble themselves to form a larger functional unit At the nanometer scale, intermolecular force holds the spontaneous gathering of molecules into a well-defined and stable structure together In chemical solutions, self-assembly is an outcome of the random motion of molecules and the affinity of their binding sites for one another In the area of optical nano-antennas, self-assembly is a simple, efficient method to obtain desire nanostructures

Figure 1.8 Images of Au core –Ag shell nanoprisms (a) [69] A image of a mixture

of nanorods and nanobipyramids with the assistance of Ag(I) (b) [48]

The advantages of nano-antennas fabricated by self-assembly methods are controlled shape, high purity, and a well-defined crystallinity With the supporting

of surfactants in the redox process, we are able to synthesize nanoparticles in various shapes and materials [47, 48] For example, the gold nanostructures were demonstrated to be strongly dependent on the gold nanocrystal structure of mediated-seed in experiments of Mingzhao Liu [48] With added silver (I) in the cetyltrimethylammonium bromide (CTAB) aqueous growth solutions, the two types

of nanoparticles either nanorods or bipyramidal can be obtained, in good yields The gaps with small size in optical nano-antennas are an essential issue due to their influence on plasmon resonance Size of gaps with on the order of less than 5 nm is very tough to achieve by the top-down approaches However, this gap size can be obtained with chemical self-assembly For example, the nanorods with 1 nm gap

size were successfully fabricated in solution by Padmanabhan Pramod et.al [60]

Since the self-assembly process is usually accomplished in solution So, how to arrange the nano-antennas in the desired position is always a difficult question Recently, variants of the self-assembly have been developed to obtain

nanostructures positioned as desired In 2001, Yadong Yin et.al combined physical

template and capillary forces to assemble nanospheres with well-controlled sizes, shapes, and structures [70] In another research, nanospheres were used as templates

to fabricate array plasmonic structures [23] Recently, Christoph Hanske et.al has successfully produced uniform arrays of micron-sized 3D pyramidal super crystals over large areas, by means of a template-assisted approach [29] The successful results of this study will open up new avenues for the fabrication of three-dimensional super crystals of plasmonic nanoparticles with more complex shapes

Based on chemical synthesis, the nanostructure made by the self-assembly methods would be more difficult to control than the physical approach However, with advantages such as low cost, no physical and material limitations, and fabricating array antennas quickly, self-assembly methods are still a good option for developing in the field of optical nano-antennas

1.4 Purpose of this thesis

Based on the overview of the previous researches, it is clear that optical nano-antennas have great potential for development and application in the coming years Because both unique optical properties are visible and sensitive to surrounding environment, the optical nano-antennas have become particularly useful in biological studies Thus, in this thesis, we offer a new optical nano-antennas structure for biosensing applications This antenna will be designed and theoretically investigated using finite element method (FEM) Self-assembly method will be used because of feasibility in fabricating array nano-antennas, which

is able to create gap sizes less than 10 nm, with high accuracy and low cost Instead

of gold, in this thesis, our nano-antennas will have core-shell structures which were demonstrated in [46] The enhancement factor of core-shell structures is much higher than solid metal

CHAPTER 2 THEORETICAL BACKGROUND

This chapter summarizes the most important theoretical approaches and techniques required to investigate optical nano-antennas First, we introduce the theoretical of Surface Plasmons In this section, the interaction and behavior of metal-dielectric interface with electromagnetic waves are investigated Second, we will review the most commonly used experimental techniques measure properties of antennas such as far-field scattering and localized intensity enhancement

2.1 Theoretical of surface plasmons

The study of plasmonics is related to the behavior of electromagnetic waves when it interacts with free conduction electrons in the metal Surface plasmons or plasmons polaritons is the oscillation of conduction electrons at the interface of metal-dielectric when excited by electromagnetic waves in optical frequency As a consequence, the electromagnetic field is highly enhanced and confined at the interface of metal-dielectric Surface plasmons can be excited both at the flat interface (Surface plasmons Polaritons-SPPs) and also at the curve surface of nanoparticles (Localized Surface Plasmons - LSPs) [49]

2.1.1 Surface plasmon polaritons

Electrons in the conduction band of metals can move freely through the lattice Those free electrons create a sea of high density (n ≈ 1023 cm−3), which called plasma electron and can be excited by electromagnetic waves The coupling

of the electromagnetic fields to oscillations of the conductor’s electron plasma leads

to the formation of SPPs This section will review the simple case of buck dielectric with a flat interface Figure 2.1(a) shows the schematic representation of SPPs at an interface between a dielectric (z > 0) with positive real dielectric constant 𝜀2 and a metal layer (z < 0) with complex dielectric functions 𝜀1 (ω) = 𝜀1′

metal-(ω) + 𝑖𝜀1′′(ω) The requirement of metallic character implies that Re [𝜀1] < 0

Figure 2.1 Schematic representation of SPPs propagation at a

In there, 𝑘𝑥,𝑧 is the x and z component of the wave vector, 𝜔 is the angular

frequency of the incident light and t is the time To solve Maxwell’s equation for

the electromagnetic wave at the interface of two materials, we using the appropriate continuity relation, the boundary conditions are:

where c is the speed of light in vacuum, k x is the same for both materials at the

interface, 𝑘0 = 𝜔/𝑐 is the wave vector in vacuum k z,i is the z component of the

wave vector in i media respectively We use equations 2.2 and 2.3 to solve for k x

𝑘𝑥 = 𝜔

𝑐√𝜀𝜀1𝜀2

For Re [𝜀1] < 0 and |Re [𝜀1]| > Im [𝜀1], we get the complex form of k x = 𝑘𝑥′ + 𝑘𝑥′′

The real and imaginary components of k x are able to calculated by following

The electromagnetic field of a SPPs approaches at its highest value at the interface

of metal and dielectric, and decays exponentially perpendicular to the interface into both media The decay of the field is characterized by the penetration depth 𝐿𝑝, which is defined as the distance of field from the interface to the position where the

amplitude of the field decreases by a factor of 1/e into the media The penetration

depth 𝐿𝑝 can be computed as:

𝐿𝑝,1 = 1

𝐿𝑝,2 = 1

We want to look for propagating wave solutions confined to the interface So that,

we use the equation set TM modes and TE modes in both half spaces yields to find out Let us first look at TM solutions [49]:

for a metal (z < 0) 𝑘𝑖 ≡ 𝑘𝑧,𝑖 (i = 1, 2) is the component of the wave vector

perpendicular to the interface in the two media Continuity of 𝐻𝑦 and 𝜀𝑖𝐸𝑧 at the interface requires that 𝐴1 = 𝐴2 and we obtain

2.1.2 Localized surface plasmons

In this section, we introduce the second fundamental excitation of plasmonics - localized surface plasmons As we know, SPPs can be excited at the flat interface meanwhile Localized Surface Plasmons (LSPs) exist at the surface of nanoparticles On the other hand, LSPs are non-propagating excitations but SPPs are propagating, dispersive electromagnetic waves The LSPs are strongly dependent on the shape, size and the dielectric property of nanoparticles Addition

to, the influence of the surrounding medium on the LSPs resonance make it become

an interested research area for sensing applications

Figure 2.2 Schematic illustration of a localized surface plasmon resonance

To understand the physics of localized surface plasmons, we using the static approximation to describes the interaction of nanoparticle with the

quasi-electromagnetic field On this assumption, the size of nanoparticles d is much

smaller than the wavelength of the incident light to make sure that the phase of the oscillating field is constant over the volume of the particles Therefore, the particle can be considered to be in an electrostatic field for calculating the spatial field distribution [49] In the presence of an external electric field, the nanoparticles will polarize itself which will create a dipole [49]

𝛼 = 4𝜋𝑎3 𝜀−𝜀𝑚

where 𝒑 is the dipole moment, 𝛼 is the polarizability, 𝑎 is the radius of a metallic

nanoparticle 𝜀 (𝜔), 𝜀𝑚 are the dielectric constant of the metal and the surrounding medium respectively The electromagnetic field coupling to plasmonic oscillations has two components, one is the radiative scattering into far-field and the other is the non-radiative absorbing component The ability of the metallic particle to transform incident light radiation into these components can be measured by the computation

of the particle’s absorption and scattering cross-sections The scattering and absorption cross-sections can be calculated via the Poynting-vector by the following equations [49]

now known as Mie theory is to expand the internal and scattered fields into a set of normal modes described by vector harmonics The quasi-static results valid for sub-wavelength spheres are then recovered by a power series expansion of the

absorption and scattering coefficients For a metallic particle with radius a, the the

polarizability α is given by [11]

𝛼𝑖 = 4

𝜀𝑚+ 𝐿𝑖(𝜀−𝜀𝑚), ∀𝑖 ∈ {1, 2, 3} (2.20) where 𝜀𝑚 corresponds to the dielectric constant of the environment meanwhile

𝜀 (𝜔) is the dielectric function of the metallic particle 𝐿𝑖 is a geometry factor equal

to 1

3 for a perfect sphere and L 1 < 1

3 for particles elongated along the x-axis The polarization is maximized when

2.2 Optical characterization of nano-antennas

Measuring experimental characteristics of optical nano-antennas is an essential step to optimize structures used in nano-antennas fabrication In fact, the properties of optical nano-antennas are usually not known precisely, making it difficult to extract reliable quantitative results The question to be addressed is how

to set up a measurement system reliably quantified, along with simulation to become effective tools in the development of the optical nano-antennas In this

section, we will review some experimental techniques suitable to characterize antenna property such as scattering, localized enhancement, and emission directivity

2.2.1 Far-field scattering

Figure 2.3 (a) Typical dark-field setup for scattering measurements (b) Dark field

scattering spectra of individual nano-antennas [50]

Firstly, we look for the scattering properties of nano-antennas The typical system used to measure scattering spectra was shown in Figure 2.3(a) To analyze the properties of single antenna resonances consistently, optical measurements need

to be performed and corroborated by high-resolution non-optical imaging techniques (e.g by SEM, TEM or AFM), to determine the actual antenna geometry

In these experiments, a white light source illuminates the sample to excite resonance

of the antenna A dark-field microscopy is used to suppress reflection or transmission background and single out scattered photons by angular separation of the illumination and collection paths The scattering light will be collected by CCD detector after passing through a spectrometer [7, 56] Figure 2.3(b) shows representative single-antenna dark-field scattering spectra, for two dipole antennas

of different length Here, the fundamental longitudinal antenna resonance dominates the scattering in the investigated spectra range As a result of increasing arm length,

the red-shift of resonance position can be seen obviously In this type of measurement, we can only collect the properties of scattering

Another type of measurement which is able to identify scattering, absorption and extinction properties simultaneously By using solutions of colloidal nano-antennas [39] or transparent substrate (e.g PDMS), the extinction spectra are measured by collecting the light that is transmitted along the forward direction The scattering spectra can be obtained by the same way as the first measurement type The pure absorption is obtained by calculating the difference between extinction and scattering However, the disadvantage of this technique is that the used substrate is limited In addition, the orientation of the NPs in solution is not well defined, so that polarization effects are averaged out

2.2.2 The near-field intensity enhancement

The ability to confine light in sub-wavelength volume is an essential attribute

of optical nano-antennas The question is "How can experimentally measure the field localization of optical antennas ?" In principle, local near-field information needs to be investigated including spatial distribution, polarization, spectra response, phase and intensity enhancement However, most of these quantities cannot be measured directly In fact, in order to get near-field information, we collect a far-field optical signal which can be traced back to the near-field information and combination with simulation In this part, we introduce confocal microscopy (far-field techniques) and scanning near-field optical microscopy (near-field techniques) which are the two most popular tool for near-field properties of nano-antennas

Although a far-field method, confocal microscopy is a very effective tool is used for antenna characteristics because of its versatility The sketch of standard confocal microscopy was shown in Figure 2.4(a) In this system, the laser source is used instead of the white light source which is used in scattering measurement The same objective is used both for illumination of the sample and for the collection of

the emitted photons Additional to, the background noises can be reduced drastically

by means of a spatial filter placed in front of the detector (typically a single-photon avalanche photodiode or a photomultiplier tube)

Figure 2.4 (a) Sketch of a standard confocal setup (b) Confocal TPL maps for a

single-crystalline Au two-wire antenna (SEM)

A near-field signal that is accessible by far-field observation is the intrinsic photoluminescence from the antenna arms This intrinsic photoluminescence is generated through single or multiphoton absorption processes In confocal microscopy, the two-photon luminescence (TPL) is the most used optical signal to characterize near-field intensity enhancement Besides that, coherent nonlinear effects can be used as well, such as SHG, third-harmonic generation, and four-wave mixing In confocal imaging process, at least three main factors contribute to the recorded far-field map The first one is the driving efficiency of the antenna mode

of interest under the specific illumination conditions of the experiment The second factor is the local field intensity enhancement and therefore the antenna resonance Finally, the antenna-mediated emission of this locally generated optical signal within the accessible spectra window [50] For example, Figure 2.4(b) indicates the confocal TPL SEM images for a single-crystalline Au two-wire antenna with different excited wavelength With 900 nm excitation, the antisymmetric mode

which resulting in a single enhancement spot When exciting the sample with a shorter wavelength, the higher order, symmetric mode comes into resonance and displays two enhancement spots Although its versatility, confocal microscopy just has a moderate resolution which is limited by diffraction limited The highest resolution that confocal microscopy can achieve is only about 200 - 300 nm On the other hand, aperture and apertureless SNOM can reach a resolution of about 50–100 and 10–20 nm, respectively

Figure 2.5 (a) Sketch of a standard apertureless SNOM, (b) Simulated near-field

distribution for a single-wire Au antenna (c) Experimental scattering map, showing the standing-wave pattern sustained by the rod (d) Phase map for the same

antenna mode [58]

Figure 2.5(a) indicates sketch of a standard apertureless SNOM An apertureless SNOM is based on a sharp tip, usually dielectric which in close proximity to the sample The tip is scanned over the sample to collect near-field information In this technique, the resolution of the image depends on the apex radius of the tip In this way, if we assume that the dielectric tip represents a small perturbation of the antenna electromagnetic response and a signal to the local field was collected The apertureless SNOM technique can be considered as the best approximation of a direct measurement of the near-fields antenna [58, 59] Another advantage of an apertureless SNOM is able to measure the phase of local fields Figure 2.5(c) and (d) show a example of amplitude and phase of a standing wave in