near-infrared active metamaterials and their applications ...qihuagroup/data/xiong/papers/xlwen...

Near-infrared active metamaterials and their applications in tunable surface-enhanced Raman

scattering Xinglin Wen,1 Qing Zhang,1 Jianwei Chai,2 Lai Mun Wong,2 Shijie Wang,2

and Qihua Xiong1,3,* 1Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological

University, Singapore 637371, Singapore 2Institute of Materials Research & Engineering, Agency for Science, Technologies and Research, 3 Research Link,

Singapore 117602, Singapore 3NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang

Technological University, Singapore, 639798, Singapore *[email protected]

Abstract: By utilizing the phase change properties of vanadium dioxide (VO2), we have demonstrated the tuning of the electric and magnetic modes of split ring resonators (SRRs) simultaneously within the near IR range. The electric resonance wavelength is blue-shift about 73 nm while the magnetic resonance mode is red-shifted about 126 nm during the phase transition from insulating to metallic phases. Due to the hysteresis phenomenon of VO2 phase transition, both the electric and magnetic modes shifts are hysteretic. In addition to the frequency shift, the magnetic mode has a trend to vanish due to the fact that the metallic phase VO2 has the tendency to short the gap of SRR. We have also demonstrated the application of this active metamaterials in tunable surface-enhanced Raman scattering (SERS), for a fixed excitation laser wavelength, the Raman intensity can be altered significantly by tuning the electric mode frequency of SRR, which is accomplished by controlling the phase of VO2 with an accurate temperature control.

©2014 Optical Society of America

OCIS codes: (220.1080) Active or adaptive optics; (300.1030) Absorption; (300.6450) Spectroscopy, Raman.

References and links

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#205426 - $15.00 USD Received 24 Jan 2014; accepted 25 Jan 2014; published 31 Jan 2014(C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002989 | OPTICS EXPRESS 2989

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1. Introduction

Plasmonics and metamaterials have attracted numerous interests in imaging, energy harvesting, light manipulation, sensing [1–5]. Unfortunately the properties of the plasmonic and metamaterials structures are fixed at the moment the fabrication process was done. The ability to engineer the properties of the fabricated structures is in demand, which can make the plasmonics and metamaterials structure active even after fabrication. Active devices are of great importance in the practical application of plasmonics and metamaterials structures. Recently many active devices were demonstrated, such as the reconfigurable devices realized by altering the shape and the coupling strength of the structure units [6–8], the semiconductor based active devices realized by changing the carrier density of the substrate [9–12], the liquid-crystal based active devices accomplished by changing the effective index of the liquid-crystal with electric and magnetic field [13, 14], and the electrically controlled active devices [15–17]. It was also demonstrated that the phase-change materials (PCM) are very good candidates to make active metamaterials devices [18–23]. The phase transition of PCM can introduce significant change of optical and electrical properties, which can be utilized to construct active and tunable device, and the phase transition can be induced by external stimulus such as temperature, light, electric field and magnetic field [24–26]. VO2 is a typical and important phase change medium which exhibits insulating-metallic transition at 340 K on a sub picosecond timescale [27]. This kind of transition is initiated by the growing of metallic puddles in the insulating host and the transition is reversible when the stimulus is removed, as such the control of the VO2 phase in real time is possible [28]. Hybridizing VO2 with metamaterials has drawn many interests due to the dramatic change of the refractive index and conductivity during the phase transition [18, 20, 23, 29, 30]. However, most of the previous work was demonstrated in THz range, the report on the visible-near IR range was limited. Here, we have fabricated SRR arrays on 100 nm VO2 film with electron beam lithography (EBL), the electric and magnetic resonances are around 900 nm and 1600 nm respectively. We observe that the electric mode is blue-shift (~73 nm) while the magnetic mode is red-shift (~126 nm) during the phase transition induced by heating, and the peak shifts of electric and magnetic modes are hysteretic which emerges from the hysteresis nature of the VO2 phase transition. We also show that we can tune the SERS intensity by controlling the phase of VO2 with a temperature controller, which suggests huge potential as an active sensing device.

2. Sample preparation

The VO2 films were grown by DC unbalanced magnetron sputtering by using vanadium metal target with purity of 99.9%. The base pressure of system is 7 × 10−7 mbar. The growth temperature was 600 degree with argon and oxygen gas ratio 6:1. The sputtering power was 275 W and the growth time was 30 mins. We use standard four probe method to measure the resistance by using a physical property measurement system (PPMS) apparatus. Figure 1(a) shows the temperature dependent resistance of the VO2 film on sapphire, the resistance difference is nearly four orders of magnitude between two different phases, it is about 2 Ohms in metallic phase while as high as 10,000 Ohms in insulating phase, suggesting that the fabricated VO2 film is in high quality. In addition, the resistance exhibits a hysteresis effect which is the nature of VO2 phase transition. Figure 1(b) is the Raman spectra of the VO2 film at two different phases excited by a 532 nm laser with a power of 1.25 mW. We can observe a series of vibrational modes at the insulating phase at room temperature and the corresponding Raman shifts are labelled in Fig. 1(b). Those modes vanished at metallic phase at 80 °C,


which is consistent with a previous report [31]. The Raman study is also evidence that the phase changing indeed occurs when we heat the sample up to 80 °C.

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SRRs metamaterials were fabricated on 100 nm VO2 by electron beam lithography (EBL). A layer of electron beam resist 950 PMMA A4 (MICROCHEM, USA) was spin-coated on the VO2/sapphire substrate, and then we use a scanning electron microscope (SEM) (JEOL 7001F) equipped with a nanometer pattern generation system (NPGS) to define the SRRs arrays. After developing the exposed patterns, 30 nm Au with a 2 nm Cr adhesion layer were deposited on the substrate with a thermal evaporator (Elite Engineering, Singapore) and followed by a lift-off procedure in acetone. Figure 2(a) shows the dimension of the SRR structure and we will use the arm width w to denote the size of the whole structure. Figure 2(b) shows the SEM image of the SRRs array with 40 nm arm width, it can be seen that the shape and size of these patterns are uniform. We define the SRR size by shrinking the arm width as shown in Fig. 2(a). It has been demonstrated by our group that it was capable of making the arm width down to 25-30 nm, whose optical responses were located at visible-NIR range [32].

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Fig. 2. Split ring resonators (SRRs) on VO2. (a) dimension of a single SRR, Lx = Ly = 4w, h = 1.6w, Px = Px = 6w. (b) SEM image of SRR array on VO2 film, the arm width w is 40 nm.

3. Tunable electric and magnetic resonances

Split ring resonators (SRRs), which have both electric and magnetic resonances, are a typical metamaterials component and widely used in the metamaterials devices designs due to the ease of fabrication and simulation [33–37]. The electric mode arises from the electric dipole of the bars of SRR, while the magnetic mode is initiated by the circulating current excited by the incident light when the polarization is parallel to the split gap. In a simple way, the magnetic resonance of SRR can be modeled by a “LC” circuit, where the whole SRR loop contributes to the inductance L and the gap contributes to the capacitance C, and the


resonance frequency is proportional to1/2( )LC −

[38]. The electric resonance frequency could be tuned by engineering the refractive index around the SRR in the same way as all the plasmonics structures did. For the magnetic resonance frequency, it also can be tuned by manipulating the inductance L and the capacitance C of SRR. In our experiment, we can tune the electric and magnetic resonances frequency simultaneously by hybridizing the SRR metamaterials with the phase change materials VO2.

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Fig. 3. Temperature-dependent transmission spectra. (a) Transmission during the heating process from 30 °C to 80 °C, the electric mode blue shifts while the magnetic mode red shifts. (b) Transmission during the cooling process from 80 °C to 30 °C, the electric mode red shifts while the magnetic mode blue shifts. (c) and (d) are extracted electric and magnetic modes resonances wavelength in one heating/cooling cycle, respectively.

Temperature dependent transmission spectra of the as-fabricated SRR/VO2 were measured to demonstrate the tunable and active metamaterials devices. The transmission spectra were measured by a commercial microspectrophotometer (Craic 20/20), and a heating stage was integrated to the Craic 20/20 system to implement the temperature dependent transmission measurement. The heating stage is equipped with a controller with a 0.1 K accuracy. In order to observe the magnetic mode, the polarization of the incident light is set to parallel to the split gap. Figure 3(a) shows the transmission of SRR with 40 nm arm width when the sample temperature is controlled from 30 to 80 °C, the electric and magnetic resonance wavelengths are 889 and 1580 nm at 30°C, respectively. When the temperature was increased to 55 °C, the VO2 started undergoing the phase transition from insulating phase to metallic phase and the electric resonance frequency started blue-shifting while the magnetic resonance red-shifting. These two resonances would not shift further when heated to 80 °C and the electric and magnetic resonances are 816 and 1706 nm respectively. Subsequently, we cooled the substrate from 80 °C and the VO2 started undergoing phase transition from metallic phase to insulating phase at 70 °C as shown in Fig. 3(b). During the cooling process, the electric frequency red-shift while the magnetic frequency blue-shift and they returned to the initial status when the sample was cooled to 30 °C. We extracted the electric and magnetic resonance wavelength and plotted them in Fig. 3(c) and 3(d), respectively. It can be clearly seen that both the electric and magnetic resonances have the hysteresis effect which is consistent with the hysteresis nature of the VO2 phase transition. The resonance peak around 600 nm is the high-order plasmon frequency arising from the width of the SRR bar and it


would not shift with the VO2 phase transition [35]. It is also interesting to notice that the electric resonance frequency and magnetic resonance frequency shift in an opposite trend during the phase transition. The red shift of the magnetic resonance frequency emerges from the increase of the capacitance between the SRR gap when VO2 transits from insulating phase to metallic phase [18, 23]. On the contrary, the blue shift of the electric resonance frequency is because of the decrease of the complex refractive index of the VO2 when it undergoes the transition from insulating phase to metallic phase [27]. From Fig. 3(a) and 3(b) we can also see that the magnetic mode is becoming broader with increasing temperature, this is because the metallic phase VO2 has the tendency to short the SRR gap and render the magnetic mode having a trend to vanish, which was also observed in some active THz metamaterials [23, 25].

4. Tunable surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is a powerful technique in molecule diagnosis, which can even enable single molecule detection [39–42]. The dramatic Raman signal enhancement mainly arises from the electromagnetic enhancement, in which the plasmon can enhance the incidence and scattering light intensity simultaneously. Based on the electromagnetic enhancement mechanism, we have to optimize the frequency matching between the incidence light, plasmon resonance and the Raman scattering light frequency to obtain the highest SERS enhancement. Unfortunately the plasmon resonance frequency is fixed for usual plasmonic devices and we have to change the incidence laser wavelength to optimize the frequency match, which is not that convenient and limited by the laboratory resources. Herein, our tunable and active metamaterials devices offers more advantages in the frequency matching, for instance, for a fixed laser excitation we can tune the SERS enhancement by tuning the plasmon frequency.

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Fig. 4. Tunable SERS. (a) resonance wavelength at insulating phase (black) and metallic phase (red) respectively. (b) Raman spectra of monolayer 2-naphthalenethiol molecules attached to the SRR fabricated on VO2 film, the Raman intensity at 30 °C (black), at 80 °C (green) and when it cool back to 30 °C (red). Please be noted that the excitation laser wavelength is 785 nm.

We demonstrate the tunable SERS concept using 2-naphthalenethiol molecules. A monolayer of 2-naphthalenethiol molecules can be formed on Au surface by utilizing the S-Au bonding and 2-naphthalenethiol has a characteristic Raman shift at 1379 cm−1 due to the ring stretching mode [43]. Raman signal was measured by using a micro Raman spectrometer (Horiba-JY T64000) with a 785 nm solid state laser as the incidence light. In order to make the electric resonance frequency of SRR closer to the excitation laser, we choose the SRR with 30 nm arm width for demonstration. As Fig. 4(a) shows, the electric resonance wavelength is 763 nm at 30 °C and it blue-shifts to 753 nm when heating to 80 °C, at the mean time Fig. 4(b) shows the Raman intensity at 1379 cm−1 of initial 30 °C, heating to 80 °C and cooling back to 30 °C. From the Fig. 4(b) we can see that the Raman intensity is about 2.5 times stronger at 30 °C which is because the resonance wavelength at 30 °C (763 nm) is


closer to the excitation laser wavelength (785 nm) compared with the resonance wavelength at 80 °C (753 nm). Please be noted that it only blue-shifts 10 nm for SRR with 30 nm arm width (Fig. 4(a)) while the blue-shift is as large as 70 nm for SRR with 40 nm arm width (Fig. 3(a)), this is because the refractive index change of VO2 during the phase transition is more pronounced at longer wavelength while it is becoming less obvious in shorter wavelength especially when it comes to the visible range [27]. Based on the detection purpose and the excitation laser wavelength, we can also alter the SRR size to change its resonance frequency, making it more suitable for some specific applications.

5. Conclusion

Near Infrared SRR metamaterials have been fabricated on a VO2 film to demonstrate the active tuning. Both the electric and magnetic resonances frequency of SRR can be tuned simultaneously by utilizing the phase transition of VO2. When VO2 undergoes the transition from insulating to metallic phase, the electric resonance frequency blue shifts because of the decreasing refractive index, while the magnetic resonance frequency red shifts due to the increasing capacitance in the SRR gap. We can also make these resonance frequencies return to its initial status by inducing the transition from metallic back to insulating phase so that we can control the resonance frequency in real time. What’s more, the magnetic mode has a trend to vanish when the VO2 is in metallic phase, which suggests considerable application in switch design. Active tuning devices provide more choices to tune the SERS intensity, we demonstrated we could tune the Raman intensity of 2-naphthalenethiol utilizing the tunable electric resonance frequency. The bigger resonance frequency shift can induce larger Raman intensity difference. Presumably, one can even make the device “Raman ON” in one phase and “Raman OFF” in another phase by optimizing the SRR resonance frequency and the excitation laser wavelength. Besides metamaterials, many other plasmonics structures can also be hybridized to VO2 to implement the active tuning. Here we use the temperature as the external stimulus to induce the phase transition of VO2, while other stimuli like light, electric and magnetic field are also possible, which can broaden the applications of these VO2-based active devices.

Acknowledgments

The author Q.X. is thankful for the strong support from Singapore National Research Foundation through a Fellowship Grant (NRF-RF2009-06) and a Competitive Research Program (NRF-CRP-6-2010-2), Singapore Ministry of Education via two Tier2 grants (MOE2011-T2-2-051 and MOE2011-T2-2-085). He also acknowledges very strong support from Nanyang Technological University via start-up grant (M58110061) and New Initiative Fund (M58110100).


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