Plasmon-related optical properties of unpenetrated metallic periodic structures

Optical Materials 29 (2006) 211–215 www.elsevier.com/locate/optmat

Plasmon-related optical properties of unpenetrated metallic periodic structures Xiangang Luo a

a,c,* ,

Junxian Ma b, Terry Ishihara

c

Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China b College of Information Engineering, Shenzhen University, Shenzhen 518060, China c Frontier Research System, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan Received 14 May 2005; accepted 1 September 2005 Available online 14 October 2005

Abstract Plasmonic nano-structures are one of the most exciting topics in the field of nano-materials. The fabrication process and the plasmonrelated optical properties of unpenetrated metallic films with periodic rectangular hollows were studied in this paper. The results show that for frequencies close to the surface plasmon band, a high transmission of optical waves is possible even in an unpenetrated metallic structure. The near-field studies show that the field enhancement due to metallic nano-structures play a key role for high transmission in the case of unpenetrated metallic films with periodic rectangular hollows. The corrugation on the input side transfers light into surface plasmon, which results in high intensity of electric field located in the grooves, and the periodic metals on the exit side are responsible for scattering of plasmon into transmitted light. The optical properties of the unpenetrated metallic films with periodic rectangular hollows can be used in many fields such as nano-optics devices, biological sensors, chemical reactors, etc. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Diffraction; Nano-optics; Near-field optics; Metallic periodic structure; Plasmonics

1. Introduction In recent years, there has been much interest in the properties of light transmission through periodic arrays of subwavelength apertures perforated in metallic films. This increased interest was stimulated by a report of unexpectedly large optical transmission through two dimensional metallic subwavelength size holes by Ebbesen et al. [1]. An important finding is that the transmission can be up to 2–3 orders of magnitude larger than that predicted by conventional aperture theory [2]. This surprising discovery has attracted much interest in both physics [3–8] and potential applications [9–11] in many other fields, such as optical devices, biology, optical displays, near-field microscopy, perfect lens [12], and photolithography [13], etc. *

Corresponding author. Address: Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China. E-mail address: [email protected] (X. Luo). 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.002

While debate still continues about the detailed interpretation of the effect [14,15], it is generally admitted that the enhanced transmission is related to resonance that can localize a large amount of electromagnetic energy near holes or slits with dimensions much smaller than the optical wavelength [16–19]. A key point actively discussed so far is the role played by surface plasmon polariton (SPP) on the enhanced transmission [20,21]. At high transmission frequencies, electromagnetic radiation penetrates through holes or slits in a metallic structure [22,23]. In additional to the ideas about metallic structures with multi-holes or a single hole, the optical properties through thin metallic films without holes have also been studied extensively. However, in previous publications, most of these studies have been focused on the SPP-assisted fluorescence [24–28]. Only a few works discussed the transmission through sinusoidally corrugated opaque silver films [29–32]. As Nicolas Bonod pointed out [31], it is necessary to remark that for continuous films, the corrugation plays

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no significant role in the light transmission through the film, but only serves as a periodical perturbation to ensure the coupling between the incident wave and the plasmons. When compared to grating hole array, high transmittance through continuous films can be obtained only for smaller thickness. In this paper, we focus our attention on the optical properties of unpenetrated metallic films with periodic rectangular hollows (we refer to it as UMFPRH in this paper). The distinguishable physical characteristic of our structure is that all the holes or slits are sealed with metal. The experimental measurements show that the far-field transmission through an unpenetrated metallic structure rectangular in shape does not change appreciably compared with that of the perforated metallic structures with the same periodicity and thickness when an appropriate amount of metal is filled in the location of the grooves. This paper is organized as follows: first, we introduce the fabrication process of periodic rectangular grooves and discuss the experimental results of far-field transmission. Then we analyse the optical properties of the structures.

of the groove, K is the period, H is the depth of the groove and t is the thickness of the center layer. In Fig. 2, solid curves show the far-field transmission spectra through a one-dimensional UMFPRH for different incident angles. The parameters of the UMFPRH are as follows: H = 50 ± 0.2 nm, t = 40 ± 0.2 nm, K = 700 nm and D = 140 nm. The total area occupied by the grooves is just 20% of the whole area of the metallic structure because the width of the groove is just one fifth of the period. For normal incidence, the maximum far-field transmission through the unpenetrated periodic structure is nearly 30% of the incident radiation, with a peak around 760 nm, which is almost the same as the transmission from the metallic film with slits reported by Sun et al. [33]. Even more surprising is that the maximum absolute transmission, defined as the fraction of transmitted light divided by the fraction of the surface area occupied by the grooves, is around 1.5 even though the center thickness of the grooved parts is 40 ± 0.2 nm, which is larger than the skin depth [34]. In order to understand the physical origin of high transmission, numerical calculation is performed in the following sections.

2. Experimental

3. Numerical calculation

The fabrication process of the metallic structures is shown in Fig. 1. We prepared resist gratings on glass substrate by using a conventional scheme of electron beam lithography. After developing the resist, we dry etched the patterns into quartz substrate with CF4. Then a Ag film was deposited on the substrate by evaporating process in millennium chamber, which has a controlling accuracy about ±0.2 nm. After resist liftoff using acetone, a structure consisting of rectangle transparency slits was formed on glass. Finally, another Ag layer was deposited on the top of the liftoff-patterns to form unpenetrated metallic structures rectangular in shape. The inset in Fig. 1 shows the schematic picture of the metallic structure. D is the width

The electromagnetic properties of one-dimensional unpenetrated structures have been analyzed by means of frequency-dependent finite-difference time-domain ((FD)2TD) [35–38] with the absorptive boundary condition [39]. The (FD)2TD allows the calculation of both reflection and transmission properties of the system. To implement the (FD)2TD approach, the calculated space was discretized into a mesh of square cells, and MaxwellÕs equations in differential form were solved directly. A polynomial fitted to experimental data [40] is used throughout this work to describe the frequency-dependent dielectric function of silver. The real part and imaginary part of the dielectric constant are [41], respectively,

Fig. 1. Schematic description of the fabrication process. Inset is the schematic picture of an unpenetrated periodic structure in Ag film. D is the width of the groove, K is the period, t is the thickness of the center layer and H is the depth of the groove.

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1.2

Experimental results

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Fig. 2. Experimental transmission spectra (solid lines) and calculated transmission spectra (dashed lines) recorded at various incident angles in the far field for the structure (K = 700 nm, t = 40 nm, D = 140 nm, H = 50 nm). The structure is illuminated at different incident angles with p-polarized collimated light. The spectra were measured using a winspec microscope coupled to an Acton monochromator and a Princeton Instruments CCD (charge-coupled device).

13

1500

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where x is the angular frequency of the incident light. The calculated transmission spectra (dashed lines) of the same structure as experiment are shown in Fig. 2. The calculated values of the transmission and the positions of the peaks agree very well with the experimental results. The position of the transmission peaks change in a marked way even for very small angles, as illustrated in Fig. 2. The peaks change in intensity and split into two peaks that move in opposite directions with an increase of incident angles, which is exactly the behavior observed when light couples with SPP [42]. 4. Optical properties 4.1. Surface plamon induced transparency We consider a flat Ag layer 40 nm in thickness on a glass plate. Fig. 3 shows that less than 5% of the incident light (solid line) is transmitted through the sample into the farfield. Thus, this film is fairly opaque to visible light. However, when we periodically add a metal strip which has a height of 10 nm and a width of 360 nm to the both sides of the flat metallic film with a period of 500 nm (inset of Fig. 3), the maximum transmission of visible light increases to 55% at the photon energy of 2350 meV (dot-dashed line). The transmission spectra show that the maximum transmission efficiency of a thicker metallic film with additional rectangle metallic layers is larger than that of a thinner flat film around 2350 meV. Usually, a SPP may not be

Fig. 3. Far field transmissions of thinner a flat film (40 nm) and a thicker structure (t = 40 nm, H = 50 nm, K = 500 nm, D = 140 nm) calculated by FDTD.

optically excited at a planar metal surface by incident plane waves without matching the wave number of the incident light with the wave number of the SPP. When a corrugation is added to the surfaces, the periodic corrugated structure can provide in-plane momentum to make coupling of the incident light with surface plasmon. The relationship between the periodicity and SPP can be expressed in the following way: k sp ¼ k 0 sin h  2pn=K

ð3Þ

where ksp is the wave number of SPP, n is integer, K is the period of the grooved structure, k0 is the incident wave number and h is the incident angle. On the opposite surface, with the aid of the grating momentum, the transmitted light is produced due to scattering by the corrugation structure on the back surface, which causes high transmission. 4.2. Near field distribution In this part, we study the near-field distribution information in an array of unpenetrated grooves which has parameters as follows: K = 700 nm, t = 40 nm, H = 50 nm and D = 140 nm. The reflection spectra are shown in Fig. 4a. At normal incidence, there are two dips in the reflected spectra. One is located at 1640 meV, another is located at 1182 meV. At oblique incidence, the dip at 1640 meV splits into two dips which move in opposite directions with an increase of the incident angle, while the dip positon at 1182 meV does not change, which is independent of the incident angle. Fig. 4b shows the near field distribution at 1640 meV. In this case, the SPP on both sides are excited [9], the coupling between the modes on opposite sides becomes strong (we refer to it as coupled surface plasmon polariton (CSPP)), so the tunneling probability is increased. Fig. 4c shows near field distribution at 1182 meV. The near-field distribution at 1182 meV is highly concentrated in the grooves on both sides of the film (we refer to it as localized plasmon (LP)). Excitation of an

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Fig. 4. (a) Reflection spectra of unpenetrated structures (t = 40 nm, H = 50 nm, K = 700 nm, D = 140 nm) for different incident angles: (b)–(d) shows the near-field distribution for three different cases; (b) CSPP at 1640 meV, (c) LP at 1182 meV and (d) SPP at 1677 meV.

LP mode in the illuminated surface grooves results in strong field enhancement. This in turn excites the LP mode in the grooves on the opposite surface by tunneling and builds a strong electromagnetic field, which subsequently is scattered into the air. Fig. 4d shows the near field distribution at 1677 meV. Due to the coupling of incident light with SPP, the electric field is enhanced in the grooves on the input surface, thus the enhanced field can increase the tunneling process. Though the electric fields are built up in grooves at LP, CSPP, and SPP conditions, different modes have distinguishable features. Actually, these modes on unpenetrated metallic structures have similar properties as these on perforated metallic structures: all the modes are leaky and lossy modes. Because of the leaky properties of the plasmonic modes, the intensity of the confined near-field is just enhanced 14 times or so, which are shown in the color bar in Fig. 4a–c. The transmission efficiency is dependent on the lifetime of these modes [6,7]. The width of the reflection peak is determined by the lifetime of the SPP mode on the input side, whereas the width of the transmission peak is deter-

mined by the lifetime of the SPP mode on the exit side. To reach higher transmission, following conditions should coincide: (1) the narrow width of the reflection peak; (2) the narrow width of the transmission peak; (3) efficient coupling of the two SPP modes, which is determined by the center-thickness. In the case of LP, the reflection peak is broad and the lifetime is shorter. In the case of SPP on the illuminated side, the coupling with the opposite side is weak for unpenetrated structures. Thus, the transmission becomes smaller compared with those of CSSP. 5. Conclusion To summarize, we have presented a study on the plasmon-related optical properties of unpenetrated metallic nano-structures rectangular in shape. The results show that even in an unpenetrated structure a high transmission of optical waves is possible. Our results not only give insight into the different physical mechanisms of the unexpected strong light transmission through metallic nano-structures, but they may also lead to further development of micro- and nano-photonics

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