Results and Discussion…

The measurements were essentially carried out twice for each of the three samples, and the obtained spectra are rather identical, indicating a good experimental reproducibility. Figure 5-4 shows the normalized transmittance of the SRR array for geometry (c). The transmission dips correspond to the peaks of the normalized reflectance for the same SRR array and measurement geometry shown in Figure 5-7, while strong absorption occurred for frequency below about 60 THz. This might be a transmission property of the Conning glass wafer, namely, the glass is opaque for part of the MIR region. The expected magnetic resonance wavelength 6-18 μm, i.e. 16-50 THz in frequency, was blocked by this substrate absorption. Thus, we will present only the normalized reflectance spectra in the following discussion.

0 5 0 100 1 50 200 2 50

Figure 5-4 Normalized transmittance of the SRR array for geometry (c).

Figure 5-5 to 5-8 show the normalized reflectance of the SRR array for the four measurement geometries. For all measurement geometries, there is a reflection band from 24 to 36 THz, while at higher frequency, another reflection peak can be clearly seen and has a blue shift as the unit cell size decreases. Some of the reflection peaks split especially for small unit cell size (600nm and 900nm) and for oblique incidence ((a) and (b)). This might be due to the incapability of the machine to cover the entire spectral region smoothly. The orientation of the electric field influences the reflection spectra by changing the spectral configuration which can be seen when comparing (a) and (b) or (c) and (d). When the electric field was parallel to the gap-bearing side of the SRR, the reflection peak shifted to higher value.

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Figure 5-5 Normalized reflectance of the SRR array for geometry (a).

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Figure 5-6 Normalized reflectance of the SRR array for geometry (b).

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Figure 5-7 Normalized reflectance of the SRR array for geometry (c).

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Figure 5-8 Normalized reflectance of the SRR array for geometry (d).

Figure 5-9 to 5-12 show the normalized reflectance of the CRR array for the four measurement geometries. Again, the reflection band between 24-36 THz is fixed in all measurement geometries. The reflection spectra of the CRR array are quite similar to that of the SRR array for geometry (a) and (c), while there is almost no difference between (a) and (b) or (c) and (d) which implies that the orientation of the electric field does not affect the EM response of the CRR array. This is reasonable and expected since the CRR is symmetric. Because the CRR does not have a capacitance, it is assumed to exhibit only the electric response. And since the spectrum of the CRR array for geometry (a) does not remove any peak when comparing with that of the SRR array, one may conclude that the reflection peaks occur beyond 50 THz should be an electric response rather than a magnetic one.

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Figure 5-9 Normalized reflectance of the CRR array for geometry (a).

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Figure 5-10 Normalized reflectance of the CRR array for geometry (b).

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Figure 5-11 Normalized reflectance of the CRR array for geometry (c).

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Figure 5-12 Normalized reflectance of the CRR array for geometry (d).

Figure 5-13 to 5-16 show the normalized reflectance of the 2-cut SRR array for the four measurement geometries. The fixed 24-36 THz reflection band is consistent with the previous SRR and CRR case. We may conclude that this unwanted reflection band is due to the feature of the glass substrate. The spectra for geometry (a) and (c) are also similar to that of the SRR and CRR, while in (b) and (d), the spectra vary in a different way from that of the SRR. In a nut shell, the three patterns show finger print spectra result for geometry (b) and (d).

0 50 10 0 150 200 250

Figure 5-13 Normalized reflectance of the 2-cut SRR array for geometry (a).

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Figure 5-14 Normalized reflectance of the 2-cut SRR array for geometry (b).

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Figure 5-15 Normalized reflectance of the 2-cut SRR array for geometry (c).

0 50 100 150 200 2 50

Figure 5-16 Normalized reflectance of the 2-cut SRR array for geometry (d).

To examine the scaling of the EM response with the unit cell size, the electric resonance wavelength (and its corresponding frequency and wave number) of the five SRR unit cell sizes measured from geometry (c) are listed in Table 5-5 and the λp–a relation is plotted in Figure 5-17 which shows a nearly linear scaling. Here we are unable to draw the scaling of the magnetic response since it was not observed in our measurement results.

Table 5-5 The numeral data of the electric response wavelength and corresponding frequency and wave number for different unit cell sizes.

a (nm) λp (μm) fp(THz) kp (cm^-1) 600 2.37635 126.24383 4208.12764 900 3.33239 90.02539 3000.84629 1200 4.17825 71.80045 2393.34849

Figure 5-17 The scaling of the electric response wavelength with unit cell size for the SRR array measured by geometry (c).

To reveal the influence of the electric field orientation on the reflection spectrum, the measured spectra of geometry (a) and (b) were plotted in the same graph, while those of (c) and (d) were plotted in another one, i.e., with the same polarization but different beam impinging azimuths for comparison. Figure 5-18 summarizes the spectra of the three patterns for the smallest unit cell size, 600 nm. It can be clearly seen that the spectra of the CRR were not affected by the E orientation, while for SRR and 2-cut SRR, the electric resonance peaks shift to higher value in geometry (b) and (d). The degree of shift is larger for the SRR than that for the 2-cut SRR. This is expected since the 2-cut SRR is more symmetric than the SRR (but not as symmetric as the 4-cut SRR), it should be able to reduce, if not remove, the electric coupling effect.

Namely, the electric resonance peaks are less influenced by the E orientation for more symmetric ring resonators. However, the reflection peak near the magnetic resonance frequency induced by the electric coupling effect was not observed in our measurement results except for the smallest 2-cut SRR where an additional reflection peak at about 60 THz occurred as shown in Figure 5-18 (F). One possible reason may be that the magnetic response frequency is smaller than expected, so only the peak of the smallest 600nm 2-cut SRR (which has smaller capacitance and thus higher magnetic response frequency then the 600 nm SRR) can survive the blocked region of the spectra. For the 900 nm 2-cut SRR, a secondary reflection peak is also observable at about 46 THz as shown in Figure 5-19.

0 50 10 0 150 200 250

Figure 5-18 Comparison of the normalized reflectance for the (A) SRR arrays in geometry (a) and (b), (B) SRR arrays in geometry (c) and (d), (C) CRR arrays in geometry (a) and (b), (D) CRR arrays in geometry (c) and (d), (E) 2-cut SRR arrays in geometry (a) and (b), and (F) 2-cut SRR arrays in geometry (c) and (d). The unit cell size is 600nm.

0 50 100 150 200 250 0

2 4 6 8 10 12

CUT 180

Normalized reflectance (a.u.)

Frequency (THz)

c d

Figure 5-19 Comparison of the normalized reflectance for the 2-cut SRR arrays in geometry (c) and (d). The unit cell size is 900nm.

Chapter 6

Conclusion

Meta-materials were studied in this thesis work because of their resonance features.

Three types of ring resonators: SRR, CRR, and 2-cut SRR were designed with five unit cell sizes, ranging from 600 nm to 1800 nm. The success in fabrication of Au pattern arrays indicates the feasibility to make nano-scaled gold devices on the glass substrate using the e-beam lithography together with the lift-off technique.

Optical measurements were carried out using the microscoped-FTIR for four nontrivial geometries. The magnetic resonance was not observed due to the overlapping of the interested frequency region with the substrate absorption, i.e. Si-O bond vibration, around 10 μm. This implies that glass may not be a suitable substrate for optical studies at the MIR region.

However, the electric resonance of these patterns showed clear trends with reproducibility. First, the electric resonance frequency was influenced by the orientation of the resonators with respect to the electric field. When the electric field is parallel to the non-gap-bearing sides of the resonators, the reflection spectra were similar for SRR, CRR, and 2-cut SRR. In contrast, when the electric field is parallel to the gap-bearing sides of the resonators, the reflection spectra of the three types of the resonators became different. The reflection spectra of the CRR remained almost unchanged while the electric resonance frequency shifted to much higher values for the SRR (blue shift). For 2-cut SRR, only slight shift of the peaks were observed

which implied that more symmetric resonators can effectively reduce the electric coupling effect. On the other hand, a linear scaling of the electric resonance with the unit cell size has been observed.

In order to demonstrate both electric and magnetic response of the ring resonators, it is useful to utilize different ring resonator designs with various unit cell sizes and measure the optical features for all possible geometries that induce the EM response and cross-check the spectra.

To study the magnetic resonance of the ring resonators at 10 μm, either the glass substrate should be replaced or the unit cell size should be tuned. Simulations prior to the experiment by CST Microwave Studio, a state-of-the-art electromagnetic field solver [1, 2, 25, 37, 43], will provide useful information and physic insights that may help and allow more reasonable design of the device. In the future, to adhere bio-molecules such as proteins with or without metallic ions or certain DNA sequences on the metal resonators is of interest to study how these bio-matters would perturb and alter the EM responses, which maybe an enabled technique for bio-sensing and manipulation.

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Vita

Min-chen Wu was born in Hsinch, Taiwan on October 26th, 1983. She received the B.S. degree in the Electronics Engineering Department from National Chiao Tung University (NCTU) in June, 2006. She entered the Institute of Electronics, National Chiao Tung University (NCTU) in September, 2006. Her major research interests are focused on nanotechnology, meta-materials and bio-sensing. She received the M.S.

degree from NCTU in July 2008.