NANO LETTERS
Band-Selective Optical Polarizer Based on Gold-Nanowire Plasmonic Diffraction Gratings
2008 Vol. 8, No. 9 2653-2658
Xinping Zhang,* Hongmei Liu, Jinrong Tian, Yanrong Song, and Li Wang College of Applied Sciences, Beijing UniVersity of Technology, Beijing 100022, People’s Republic of China Received March 24, 2008
ABSTRACT We report a plasmonic diffraction grating device as a new kind of optical polarizer. This simple device consists of periodically distributed gold nanowires on top of a transparent glass substrate and is based on the strong polarization dependence of the particle plasmon resonance of the gold nanowires. A high-efficiency secondary diffraction in the same device enhances the polarization extinction ratio significantly. Linearly polarized spectrum in the red with a bandwidth of 53 nm is selectively picked up from the nonpolarized white light, where a polarization extinction ratio higher than 100 at about 650 nm has been achieved. The idea of plasmonic diffraction grating is important for exploiting new detection and sensor techniques.
1. Introduction. Photonic structures in the nanoscale are very important in tailoring light propagation in the visible spectral band through modifying the transmission, reflection, and polarization or through guiding the photons into designed channels. Plasmonic structures can find their extensive applications in optoelectronic device,1 optical filters,2-5 optical switch,6 negative refraction materials,7 and nonlinear optics.8 Combination of the plasmonic response and the photophysical properties of the photonic structures may be used to exploit new applications. For instance, fabricating gold nanowires periodically on top of a waveguide layer produces a kind of metallic photonic crystals, showing new polariton properties that are recognized as the strong coupling between the particle plasmon resonance of the gold nanowires and the propagation mode in the waveguide. 9-11 This unique physical property enables potential development of new biosensors. Particle plasmon resonance of the gold nanowires exhibits strong polarization dependence of the incident light, namely, only the polarization perpendicular to the nanowires (TM polarization) will excite particle plasmon resonance and experience strong optical extinction through interaction with the gold nanowires. The other polarization that is parallel to the nanowires (TE polarization) will show merely the response of the photonic structures. Thus, these polarizationdependent features may be applied in optical engineering, yielding new polarization elements. In this paper, we demonstrate a new kind of band-selective optical polarizer composed of gold nanowires that are * Corresponding author. E-mail:
[email protected]. 10.1021/nl0808435 CCC: $40.75 Published on Web 08/14/2008
2008 American Chemical Society
distributed periodically on a transparent glass substrate and are fabricated using a simple solution-processible method. We call this kind of nanostructure plasmonic diffraction gratings. The corresponding diffraction mechanism is based on the strong modulation of the light transmission or the scattering of the incident light by the particle plasmon resonance for the polarization perpendicular to the gold nanowires. Furthermore, the transparent glass substrate mediates a secondary diffraction by the gold nanowires, so that a selected band is picked up through diffraction into a separate path from the transmitted and reflected light beams, enabling practical application of this kind of device. Using a solution-processible method, we are able to fabricate largescale periodically arranged gold nanowires with excellent homogeneity and reproducibility. 2. Plasmonic Grating Device Consisting of Gold Nanowires. The gold-nanowire grating was fabricated using the solution processible method.12,13 Figure 1a gives the scanning electron microscopic (SEM) image of a finished goldnanowire grating which has been fabricated on a piece of indium tin oxide (ITO) glass. It can be seen clearly that all of the gold nanoparticles have been confined almost completely into the grating structures, forming nanowire structures. In previous works, the ITO layer is used as the waveguide for producing the waveguide mode to couple with the particle plasmon resonance. However, for the application in this work the waveguide mode in the ITO layer plays no role in the performance of the plasmonic diffraction grating. Nevertheless, we need to stress that the ITO layer is important in the high quality fabrication of the gold-nanowire
Figure 1. (a) SEM image of the gold-nanowire grating showing the large-scale homogeneity of the grating structures. (b) Enlarged SEM image showing the quality of the gold nanwires. (c) The optical extinction spectrum showing particle plasmon resonance (PPR) of the gold nanowires.
grating due to its different surface energy properties from normal glass12 and is very helpful for demonstrating the welldefined particle plasmon resonance properties through the coupling with the waveguide mode in the ITO layer. Additionally, the ITO layer is helpful for the SEM measurements. As illustrated in the enlarged image in Figure 1b, the nanowires have actually become broken into segments with a mean length of a few micrometers along the extending direction as has been expected.13 This is because of the fact that the gold nanowires are thin in width ( λC1 and excites the propagation mode in the substrate. (4) The diffracted beam C reaches the gold nanowires again after being totally reflected and is diffracted secondarily into beam D and E, which propagate parallel to the reflected beam A and the transmitted beam B, respectively, with a displacement defined by 2d·tgR, where d is the thickness of the substrate. Thus, the separation between beams D and A can be adjusted by changing the thickness of the substrate. The left-hand image in Figure 2a gives the diffraction pattern of beam F and G. From the color in the photograph, we can find that longer wavelength is diffracted with larger angle of γ and R, so that red color is closer to the surface of the substrate than the green. According to geometry in Figure 2a, it is understandable that the diffraction pattern is not Nano Lett., Vol. 8, No. 9, 2008
Figure 2. (a) Geometric configuration of the gold-nanowire structures used as a plasmonic diffraction grating. Left-hand image: the dispersed diffraction pattern of beam F and G. (b) Tuning of the spectral band of the guided propagation (beam C) inside the substrate with changing the angle of incidence (θ).
symmetric about the surface of grating. A black hole can be observed in the center of the dispersed diffraction pattern, which is exactly due to the excitation of the guided propagation (beam C) in the substrate. However, no other output has been observed at the edge surface of the device. Therefore, almost all of the energy in beam C is diffracted into beam D and E. This is because the secondary diffractions into D and E are based on the same simple diffraction condition as the primary one that produces beam C: Λ sin θ + n2Λ sin R ) λ and n2Λ sin R + n2Λ sin θt ) n2Λ sin R + Λ sin θ ) λ. The excitation of the guided propagation in the substrate has resulted from the simultaneously satisfied three conditions at the same wavelength λC1: (1) diffraction of beam F into the substrate at γC1 ) 90°, or Λ sin θ + Λ sin γC1 ) Λ sin θ + Λ sin(90°) ) Λ sin θ + Λ ) λC1. (2) Diffraction of beam G is totally reflected by the bottom interface between the substrate and the air in the case of sin RC1 ) 1/n2; and (3) the condition in 2 also implies Λ sin θ + n2Λ sin RC1 ) Λ sin θ + n2Λ(1/n2) ) Λ sin θ + Λ ) λC1. Therefore, λC1 actually set the lower limit of the spectral band for the diffraction into the guided propagation mode of beam C. Furthermore, the upper limit (λC2) of this diffraction band is set by the condition of R ) 90°. Therefore, λC2 ) Λ sin θ + n2Λ sin(90°) ) Λ sin θ + n2Λ. Thus, the selective diffraction into the guided propagation mode in the substrate 2655
take place within the spectral band defined by λC1 < λ < λC2. Therefore, the bandwidth is defined as ∆λ ) (n2 - 1)Λ, which is independent of the angle of incidence and can be controlled by varying the grating period. However, this band can be tuned by changing the incident angle according to the expressions of λC1 and λC2, as illustrated in Figure 2b. The center wavelength is tuned from 420 to 780 nm when θ is changed from 0 to 90°. 3.2. Polarization Diffraction Induced by Particle Plasmon Resonance. To characterize the polarization properties of the diffraction by the gold-nanowire grating quantitatively, we need to consider the modulation of the light field that propagates through the gold nanowires. The modulation of the amplitude or the intensity of the light field can be evaluated using the measured optical extinction spectra for both the TE and the TM polarizations, where the particle plasmon resonance enhances the optical extinction of the TM polarization (see Figure 1c) in a characteristic spectral band with respect to the TE polarization, whereas other optical modulation properties by the gold can be estimated using the optical constant given in ref 15 Furthermore, we also need to take into account the reflectivity of the device and the duty cycle of the gold-nanowire grating, and we assume no energy loss during propagation through the substrate. Using the simple diffraction theory, we derived the efficiency (η) as a function of wavelength (λ) for the diffraction into beam C: NφΛ sin2 2 φR φ(Λ - R) 4 + sin2 + η(λ)∝ 2 µ2(λ) sin2 φΛ 2 2 2 φ sin 2 φΛ φΛ φ(2R - Λ) - cos 2µ(λ) cos cos 2 2 2
{
[
]}
where φ ) 2π/λ·(sin θ + n2 sin R) and a denotes the width of the gold nanowires. µ(λ) is used to characterize the optical absorption coefficient by the gold nanowires, which is different for TM and TE polarizations. We use the spectrum of particle plasmon resonance to simulate the diffraction of TM polarization and use the baseline of the optical extinction measurement for the TE polarization as a constant absorbance in the studies of the TE diffraction. The secondary diffraction to produce beams D and E should have an efficiency proportional to η2. Although the above theory is only a rough evaluation of the diffraction properties of the plasmonic gratings, it agrees very well with the experimental observation and confirms our proposed principle solidly. Figure 3 shows the calculated intensity of the TM (ITM, black curve) and TE (ITE, red curve) polarizations after the first diffraction into beam C at an incident angle of 45° and for a detection direction assumed to be the same as the reflected beam A. The polarization extinction ratio defined as ITM/ITE for the secondary diffraction is shown by the blue curve in Figure 3, which is higher than 215 at about 636 nm. Therefore, the modulation depth through particle plasmon resonance enhances significantly the diffraction into TM polarization, and the TM polarization dominates the secondary diffraction into beam D. It will be found that the peak polarization extinction ratio shifts about 10 nm to the blue from the measured data. This might be related to inaccuracy in the reconstruction of the spectrum of the particle plasmon resonance. 2656
Figure 3. Calculated diffraction intensity of the TE (ITE, red) and TM (ITM, black) polarization and the polarization extinction ratio (p ) ITM/ITE, blue).
In summary, the principle for the band-selective and polarization-dependent diffraction is that the particle plasmon resonance induces much stronger modulation or scattering of the incident light for the TM than the TE polarization. Thus, the diffraction of the TM polarization is enhanced significantly, and it dominates the diffracted light energy after the first and secondary diffraction. This effect should take place efficiently within the spectral band of particle plasmon resonance of the gold nanowires. Overlap of the particle plasmon resonance and the spectral band of the secondary diffraction aided by the geometric configuration of the device is crucial for realizing this kind of polarizer. In fact, the above analysis can also give a straightforward physical picture for the mechanisms of the waveguide mode in the ITO layer and its coupling with the particle plasmon resonance. The only difference is that the ITO waveguide layer has a thickness of about 200 nm, so that the secondary diffraction may take place many times with different conditions (e.g., total reflection needs to take place at the ITO-substrate interface) and the spectral response takes place in an extremely narrow band. Furthermore, the secondarily diffracted beams are inside the reflected and the transmitted beams instead of propagating in separate paths. Thus, interference can take place between the secondary diffractions and the reflected and transmitted beams, inducing completely different optical properties.9-11 4. Optical and Spectral Properties. 4.1. Experimental Observations. Figure 4a shows the experimental observation of the secondary diffraction on the gold nanowires (1), the corresponding nonpolarizeried incident beam (2), and the transmitted beam that hits on a sheet of white paper placed behind the device (3). Figure 4b shows the photograph of the spot of secondarily diffracted beam (D in Figure 2a) that has propagated about 100 mm away from the device (4 in Figure 4a) and has passed through a linear polarizer with the transmission axis perpendicular to the nanowires. If the polarizer is rotated by 90°, the spot of beam D is too dim to be recognized. We can see that the secondary diffraction is almost in pure red and is comparably bright as the incident white light, implying high spectral sensitivity and high efficiency of this polarization device. Measurement showed that almost all of the light energy in beam C has been Nano Lett., Vol. 8, No. 9, 2008
Figure 4. (a) Spots of the secondarily diffracted (1) and the incident (2) beams on the front surface of the device, and that of the transmitted beam (3) hitting the surface of a sheet of white paper; the gold-nanowire device is designated by 4. (b) The spots of the reflected (beam A in Figure 2) and secondarily diffracted beam (beam D in Figure 2) observed about 100 mm away from the device and after passing through a polarizer with the transmission axis perpendicular to the gold nanowires.
diffracted into D and E with no other channels of energy loss. An incident angle of 45° has been chosen in the above measurements so that the spectrum of the guided propagation in the substrate overlaps the particle plasmon resonance, and the corresponding geometry of incidence is typical in practice. 4.2. Optical Spectral Properties. Figure 5a gives the spectral properties of the secondarily diffracted beam D (see Figure 2) at an incident angle of 45° and at different polarization directions, where the angle between the transmission axis of the polarizer placed in front of the spectrometer and the extending direction of the nanowires (β) is tuned from 0 to 90°. It can be found that the spectrum of beam D is almost completely linearly polarized perpendicular to the gold nanowires with a peak polarization extinction ratio larger than 100 at about 650 nm. These spectra have an equal bandwidth of about 53 nm at fwhm. Actually, this band can be tuned within the spectrum of particle plasmon resonance by changing the angle of incidence. The dashed curve in Figure 5a gives the spectrum of beam D when the angle of incidence is increase to 52°, where the center wavelength of the diffracted band shifts from 646 to 661 nm. For better understanding the mechanisms for producing the red-band spectrum in Figure 5a, we also show the polarization properties of the transmitted beam through the gold-nanowire grating in Figure 5b, where Iβ/Iβ-0 is plotted as a function of wavelength with Iβ defined as the transmitted light intensity at the polarization angle of β from 10 to 90° and Iβ)0 defined as the TE polarization. Three features in the spectrum can be clearly observed in Figure 5b. The spectral band extending from about 400 to 600 nm and centered at 535 nm is a result from the diffraction into the space outside the substrate (see the left-hand image in Figure 2). The signal peaked at about 800 nm results from the waveguide mode inside the ITO layer and the corresponding secondary diffractions should be within the reflected beam. Therefore, we cannot observe it in a separate light path. The spectral band centered at about 640 nm coincides very well with the linearly polarized band in Figure 5a. This means Nano Lett., Vol. 8, No. 9, 2008
Figure 5. (a) Measured polarization properties of the secondary diffraction (beam D in Figure 2) with the angle between transmission axis and the nanowires (β) tuned from 0 to 90° and the incident angle θ fixed at 45°. Dashed curve: the measured spectrum of beam D at θ ) 52° and β ) 90°. (b) Polarization properties of the transmission beam characterized by the ratio between the spectrum polarized at different values of β (10 to 90°) and that polarized parallel to the nanowires (TE polarization).
that the secondary diffraction spectrum in Figure 5a is lost in the transmitted beam. It can be estimated that the diffractions into beam D and E take more than 80% the total energy of the incident light in the spectral band shown in Figure 5a for the polarization perpendicular to the gold nanowires, implying high efficiency of this band-selective polarization diffraction device. 5. Conclusions. We introduced in this paper a new idea of plasmonic diffraction grating that can be used directly as an optical polarization device. The corresponding diffraction mechanism is based on the strong modulation of the light transmission through the gold nanowires by the particle plasmon resonance. Thus, the efficiency of the TM diffraction (polarized perpendicular to the gold nanowires) is significantly enhanced with respect to the TE diffraction (parallel to the nanowires). Using the secondary diffraction mediated by the transparent substrate, we can pick up the TM polarization in a selected band efficiently and conveniently, which propagates in the same direction as the reflected and transmitted beams but in a separate path. High efficiency, 2657
high sensitivity to the wavelength and incident angle, and simple fabrication technique with low cost enable practical application of this device in optical engineering. Acknowledgment. The authors acknowledge the HighTech Research and Development Program of China (2007AA03Z306), the Natural Science Foundation of China (10774011, 10744001), and the Beijing Educational Commission (KZ200810005004) for the financial support. References (1) Tvingstedt, K.; Persson, N.-K.; Ingana¨s, O.; Rahachou, A.; Zozoulenko, I. V. Appl. Phys. Lett. 2007, 91, 113514. (2) Rosenblatt, D.; Sharon, A.; Friesem, A. A. IEEE J. Quantum Electron. 1997, 33, 2038. (3) Sharon, A.; Rosenblatt, D.; Friesem, A. A. J. Opt. Soc. Am. A 1997, 14, 2985. (4) Fitio, V. M.; Bobitski, T. V. J. Opt. A: Pure Appl. Opt. 2004, 6, 943. (5) Wang, S. S.; Magnusson, R. Appl. Opt. 1995, 34, 2412.
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