Chapter 1 Introduction
1.4 Thesis Structure
On the basics of the previous discussion, the main purpose of this work is to investigate the possibility of hypersonic array. In order to enhance the detection sensitivity, behaviors of SPPs and LSPs in the system as well as their interaction with the hypersonic pulse are also needed to be studied. Our investigation first started from simple 1-D nanograting structure, and then we extended the structure to 2-D nanodisk array.
In the following chapter, the behaviors of plasmons in the nanostructure (ex. 1-D nanograting and 2-D nanodisks array) are first studied. Then the interaction between
5
plasmons and hypersonic pulse is investigated to verify the possibility of the proposed hypersonic sensor. Finally the effect of the periodicity of the hypersonic array is discussed. Based on the understanding of these critical issues, hypersonic array system with both high longitudinal and lateral spatial resolution may be realized in the future.
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Chapter 2
Plasmonic Excitation for Hypersonic Imaging
2.1 Introduction
Recently SPPs and LSPs have attracted many attentions due to the fact that well-confined plasmonic field is very sensitive to the environmental disturbance [1].
Since hypersonic pulse modulates the refractive index of the studied sample, SPPs and LSPs are thus proposed as an effective hypersonic sensor [2-7]. In this chapter, the plasmonic behavior in metallic nanostructure (1-D gold nanogratings and 2-D gold nanodisks array) is first investigated before applying the plasmonic field to the acoustic detection.
2.2 Excitation of Surface Plasmon Polaritons
SPPs are collective electron resonance at the interface between metal and dielectric medium. The dispersion curve of SPPs excited by the TM wave can be expressed as the following equation [8]: dispersion curve between the ordinary incident light (/c) and the SPPs. Due to the fact that the light line of the incident light is not able to intersect with the dispersion curve of SPPs, SPPs thus cannot be excited by the ordinary incident light. However the periodic
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nanostructure could provide extra momentum to modify the wave vector of the incident light, which leads the energy coupling between incident light and SPPs become possible.
In the following section, periodic nanostructures such as 1-D gold nanograting or 2-D gold nanodisk array are going to be investigated
Figure 2.1 The dispersion curves of SPPs and ordinary incident light.
2.2.1 Surface Plasmon Polaritons in 1-D Gold Nanogratings
First we studied the behavior of SPPs in 1-D metallic nanograting. Our studied sample consists of a 1-D gold grating on the plane GaN substrate with a periodicity varying from 590 nm to 650 nm, the heights and the widths of slits are both around 70 nm. Fig. 2.2(a) shows the transmission spectra of the studied sample calculated by rigorous coupled-wave analysis (RCWA) algorithm [9]. We can observe that the wavelength of extraordinary transmission (EOT) shifts as the periodicity of grating increases. Theoretically there are two different types of resonances which are in charge of this behavior. One is SPPs resonance; the other one is cavity mode (CM) resonance. Here
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we take the grating with 590 nm periodicity as an example to explain these two resonances. Fig. 2.2(b) shows the optical energy field distribution at and away from the wavelength of EOT, which is calculated by the finite-difference time-domain (FDTD) algorithm [10]. The refractive index values of gold and GaN were taken from [11] and [12], respectively. With an incident wave from the substrate side polarized parallel to the sample surface but perpendicular to nanogratings (x-direction in Fig. 2.2(b)), the Ex field intensity image shows the interaction result between the incident light wave and the scattered light wave. The field pattern below the nanoslit is the result of interference from these two waves. High field intensity can be observed inside the nanoslit and this phenomenon is attributed to the different effective refractive indices between the upper and lower interfaces of the slit, which causes the so-called "cavity mode." [13] Since the incident wave is only x-polarized which propagates along the z-axis, thus the Ez field intensity image reflects the induced electric energy field by the incident wave. For the field distribution at the peak wavelength, the induced Ez field intensity can be found to be well-confined at the gold/GaN interfaces. Since it is well-known that the SPPs field is well confined at the metal/dielectric interface and exponentially decays in the z-direction into the substrate, we thus refer this field as the SPP field [14]. On the other hand, this field is not able to be observed for the wavelength far away from the peak wavelength. Fig. 2.2(c) shows the poynting vector of the simulated energy field distribution at the EOT wavelength. The propagated energy direction of CM resonance and SPPs are vertical and parallel to the sample surface respectively. The energy of SPPs is thus confined well at the gold/GaN interface, only the energy of CM resonance can propagate to the far field. Fig. 2.2(c) also indicates that the energy can transfer from SPPs to CM resonance near the slit edge. Therefore optical energy absorbed by SPPs at
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gold/GaN interface is possible to couple to CM resonance, which results in EOT.
Consequently, changing the periodicity of the grating would not only modify the behavior of SPPs but also shift the wavelength of EOT.
Figure 2.2 (a) The normalized simulated transmission spectra for 1-D gold nanograting on the plain GaN substrate with different periodicities. (b) The simulated energy field distribution of 1-D gold nanograting at and far away from the wavelength of EOT (c)
The poynting vector of the energy field at the wavelength of EOT.
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Based on the simulation, we fabricated the sample by e-beam lithography. We first grew a 3.7-m-thick GaN single wurtzite crystal film with 10% thickness fluctuation by Metal-Organic chemical Vapor Deposition (MOCVD) on a double-side polished c-plane sapphire substrate with a 350 m thickness. A 140-nm-thick E-beam resist layer was coated on the GaN film to define the desired pattern by E-beam lithography. The E-beam resist was Zep520A and the electron dose time was 0.35 s/dot. After developing the resist by a developer, a 70-nm-thick gold film was then coated on the GaN by thermal evaporation. We finally lifted off the remaining resist and finished the sample processing. The fabricated nanostructure covered an area of 300 m×300 m.
Fig. 2.3(a) depicts the example of scanning electron microscopy (SEM) image of 1-D gold nanogratings on a GaN substrate. Periodicity of nanogratings is 590 nm while the height and width of the slit are both 70 nm. Fig. 2.3(b) shows the experimental transmission spectra of the 1-D gold nanograting with different periodicity after normalizing the peak value. The EOT wavelength is red-shift by increasing the periodicity of the grating; furthermore the measured resonant wavelengths of EOT with different grating periodicities show a good agreement with the simulation in Fig. 2.2(a).
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Figure 2.3 (a) The SEM image of 1-D gold nanogratings on a GaN substrate with 590 nm periodicity. (b) The normalized experimental transmission spectra of the 1-D
gold nanograting with different periodicity.
2.2.2 Surface Plasmon Polaritons in 2-D Gold Nanodisks Array
Although SPPs in 1-D gold nanograting may be served as a hypersonic sensor, this structure however limits the detection only in one dimension. Therefore this is very
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important for hypersonic imaging purpose to extend the structure into 2-D metallic nanodisk array. Our studied sample is gold nanodisks arrays on plain GaN substrate with 150 nm diameter. The periodicity of the array is varied from 250 nm to 400 nm.
Fig. 2.4(a) shows the simulated extinction spectra of the studied sample, peak wavelength of the spectrum indicates the resonant wavelength SPPs. It is obvious that the resonant wavelength of SPPs of nanodisk arrays is red-shift by increasing the periodicity of the array. This phenomenon is called the dipolar coupling effect [15-17].
When light excite the plasmon resonance in the gold nanodisk, each nanodisk can be referred as a dipole. The relation between the polarization of the dipole and incident electric field can be express as the following equation:
E P (1)
Where P and are the polarization and the polarizability of the dipole, while E is the incident electric field. For the array structure, these dipoles can couple to each other and the collective polarizability ( collective) should be modified:
collective S polarization vector and the vector from nanodisk. Therefore changing the period will modify the coupling strength between nanodisks and affect the collective polarizability.
Due to the fact that the extinction cross-section (Cext) is related to the imaginary part of the polarizability, the resonant wavelength of SPPs is thus strongly period dependent.
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Fig. 2.4(b) shows our fabricated samples with 150 nm disk diameter and different periodicities. In all samples, we first grew a 3.4-m-thick GaN single wurtzite crystal film with 10% thickness fluctuations by the MOCVD technique on a double-side polished c-plane sapphire substrate with a 350 m thickness. A 300-nm-thick E-beam resist layer was then spin-coated on the GaN film to define the pattern by E-beam lithography. The E-beam resist was Zep520A and the electron dose time was varied from 0.7~1.1 s/dot, which depends on the periodicities of the nanodisk arrays. After developing the resist by a developer, a 50-nm-thick gold film was then coated on the GaN sample surface by thermal evaporation. We finally lifted off the remaining resist and finished the sample processing of gold nanodisks on top of plain substrates. Fig.
2.4(c) shows the experimental extinction spectra after normalizing the peak value, the extinction coefficient (ext) can be calculated from I-I0 = -ext NI0, where I0 is the intensity of the incident light, I is the intensity of the transmitted light and N is the density of nanodisks in the pattern [18]. The bandwidth of the measured spectrums are slight broader compared to the simulation due to the size inhomogeneity of the fabricated gold nanodisks. However the resonant wavelengths of SPPs in gold nanodisks arrays with different periodicities still show the good agreement with the simulation.
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Figure 2.4 (a) The normalized simulated extinction spectra. (b) The SEM images and (c) the normalized experimental extinction spectra of 2-D gold nanodisks arrays
with different periodicities.
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2.3 Localized Surface Plasmons in 2-D Gold Nanodisks on Nanorod
Although the 2-D gold nanodisks array is able to expand the acoustic detection into another dimension, the dipolar coupling effect among nanodisks would delocalize the plasmonic field intensity and degrade the lateral resolution. Furthermore, the resonant wavelength of SPPs in a 2-D nanodisk array varies with incident light angles. Since most applications of opto-acoustic detection prefer stable and repeatable systems that do not strongly depend on the incident light angle, it is therefore necessary to excite LSPs in a 2-D nanodisk array to eliminate the coupling between neighboring nanodisks and to enhance the angle tolerance of incident light.
Here we propose to modify the plain GaN substrate to nanorod substrate for exciting LSPs [19, 20]. These high-refractive-index nanorods could localize the plasmonic field within the rods, while the air gaps restrict the coupling between gold nanodisks. By suppressing the coupling effect, hypersonic detection using a single nanodisk will not be affected by adjacent nanodisks; i.e., each gold nanodisk can be considered as an independent acoustic sensor. At the same time, the intensity of the plasmonic field can be increased due to the localization of the plasmonic field. This high intensity field can further increase the detection sensitivity [21].
First 2-D gold nanodisk arrays on GaN nanorod arrays are fabricated based on the same sample processing procedure, which is the same as fabricating the 2D gold nanodisk on plain GaN substrate, but here we coated a 150-nm-thick chromium (Cr) film as an etching mask after coating the 50-nm-thick gold film. After removing the remaining resist and using the Cr/Au nanodisks as the etching mask, an inductive coupled plasma reactive ion etching (ICPRIE) system was utilized to etch the uncovered GaN substrates as nanorod substrates with different rod-lengths, which were determined
20
by the etching depths. Finally we removed the chromium masks by chromium etchant (Cr7). Fig. 2.5 shows SEM image of our fabricated sample with different periodicities and rod lengths, while the diameter of gold nanodisks are all closed to 150 nm.
Figure 2.5 SEM images of the fabricated gold nanodisk arrays on top of GaN nanorod substrates with different periodicities and rod lengths.
To understand how the length of the nanorod affects the plasmonic coupling between gold nanodisks, the extinction spectra of the fabricated samples were measured.
As shown in Fig. 2.6(a)-(c), the red-shift effect of the SPP-resonant wavelength by
21
increasing periodicity is decreased when the length of the GaN nanorod array was increased to 50 nm. This result suggests that nanorod substrate begin to suppress the dipolar coupling effect; however, the suppression is mild. By increasing rod length to 120 nm and 220 nm (Figs. 2.6(b), (c)), the trend of the wavelength shift, either an increase or decrease with periodicity, was no longer observed. This result indicates that the dipolar coupling effect among nanodisks is fully suppressed by the long-nanorod substrate, and LSPs are thus excited. It is worth noting that the plasmon-resonant wavelength of the nanodisk array on a nanorod array is blue shifted in contrast to its counterpart on a plain substrate. This effect can be explained by the fact that the nanorod structure reduces the effective refractive index of the substrate, and therefore, results in the blue-shifted resonant wavelength [22].
22
Figure 2.6 Experimentally measured normalized extinction spectra of the gold nanodisk arrays with different periods (a) on 50 nm, (b) 120 nm, and (c) 220 nm long
GaN nanorod arrays.
To double confirm that the coupling between each disk is fully suppressed by the nanorod, we calculated the optical energy field distribution in the nanodisk arrays both on plain substrate and nanorod substrate. However, we only present the simulation of
23
the array with 250 nm periodicity as it is the shortest period among all our cases, which leads to the strongest coupling. If suppression of the dipolar coupling can be observed on the nanorod array with this period, we can then safely assume that the dipolar displayed as it is associated with plasmonic field [14]. It can be seen that the Ez intensity in the gold nanodisk array on a plain substrate is weak since the excited field below the gold nanodisk can extend through the substrate between disks. This effect induces the dipolar coupling, which leads to the strong period dependency. When the rod length is increased from 0 to 50 nm, part of the SPP field is confined inside the nanorod because of an abrupt change in the refractive index at the interface between GaN nanorods and air. Such a change can prevent field penetration through the nanorod/air interface in the lateral direction. However, part of the field still reaches the substrate and thus contributes to the dipolar coupling. Therefore, the length of the rod needs to be sufficiently long to prevent energy-field leakage through the substrate. After increasing the rod length to 120 nm and 220 nm, most of the field is now confined in the nanorod, and the period dependency is thus strongly suppressed. Our simulation thus indicates that the nanorod array with a sufficiently long rod length can effectively confine the light field inside the nanorod both in the lateral and longitudinal directions.
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Figure 2.7 Simulated energy field distributions of gold nanodisk arrays with a 250 nm period on nanorod substrate with different rod lengths.
Furthermore, we measured the extinction spectra of gold nanodisk arrays on the nanorod substrate (L= 220 nm) and plain substrate at different incident angles of light to verify the LSP dominant behavior in long nanorods. Fig. 2.8 summarizes the peak wavelengths of the measured extinction spectra as a function of incident angle. The SPP-resonant wavelength on a plain substrate shifted from 814 nm to 850 nm when the angle of incident light was changed. However, the resonant wavelength of the gold nanodisk on top of the 220 nm-nanorod substrate remained constant at around 778 nm.
Since the group velocity of LSPs is zero, this angularly independent characteristic further confirms the excitation of LSPs inside the nanorod [23, 24].
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Figure 2.8 Peak wavelengths of measured extinction spectra of gold nanodisk array on plain substrate and nanorod substrate at different incident angles of light.
In this chapter, we discussed the plasmonic behavior from 1-D gold nanograting to 2-D gold nanodisk on plain substrate and on nanorod substrate. Our results indicate that gold nanodisk on top of GaN nanorod could confine the plasmonic field and induce LSPs. Such LSPs field may increase the detection sensitivity due to the intense plasmonic field. Furthermore each gold nanodisk acts like an independent detector by suppressing the coupling effect, which suggests that 2-D gold nanodisk array on top of GaN nanorod substrate as a great candidate for future hypersonic imaging system.
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Chapter 3
Confined Acoustic Vibration of Gold Nanodisk
3.1 Introduction
In previous chapter, we have investigated the plasmonic behavior in 1-D gold nanograting, 2-D gold nanodisks array on GaN plain substrate and nanorod substrate.
Our results indicate that gold nanodisk on top of GaN nanorod array may be served as a
Our results indicate that gold nanodisk on top of GaN nanorod array may be served as a