Plasmonic metamaterial as broadband absorptive linear polarizer

Transcription

1 Plasmonic metamaterial as broadband absorptive linear polarizer Chunrui Han and Wing Yim Tam* Department of Physics and William Mong Institute of Nano Science and Technology The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, China Abstract We demonstrate the broadband and huge transmittance difference between orthogonally linear polarizations in the visible range through a novel plasmonic nanowire array made by shadowing vapor deposition method. The broadband transmittance difference is due to the selective absorption of particular polarization, up to 60% in preferential direction averaged from 500 to 850 nm, by plasmonic excitations at distinct wavelengths in different metallic elements of Ag short-long nanowire array. The most significant resonance from the bottom Ag strip can contribute ~ 90% absorption around the resonance frequency which simultaneously leads to a reflection valley. By manipulating the separation between the two short wires, the resonances as well as the optical properties can be tuned. The plasmonic nanostructure has potential applications as broadband linear polarizer or anti-reflective coating in both optics and photovoltaic fields. Keywords: broadband polarizer, plasmonics, shadowing vapor deposition, antireflection. PACS: Ek, Xj, Pt * Corresponding Author: phtam@ust.hk

2 Plasmonic metamaterials are artificial metallic/dielectric nanostructures with features smaller than the wavelength of the operating light, engineered to achieve unusual optical properties that are not available in nature. The unique property of plasmonic material is the localized surface plasmon resonance (LSPR), i.e. the collective oscillations of conductive electrons in metallic nanoparticles when interacting with an incident light at resonance frequency [1]. Such LSPR has the characteristics of strong absorption and enhancement of local electromagnetic fields which are the basis of many fascinating applications, such as dual/broad band absorbers [2, 3], thermal emitter [4], optical sensors [5], antennas [6], and near field scanning optical microscopy [7]. Most of the applications utilize the significant absorption that induced by LSPR which strongly depends on the size, shape, orientation and the surrounding dielectric environment of the metallic nanostructures. Accordingly, different nanostructures have been designed to optimize the LSPR signal at one particular frequency [8] or multiple distinct frequencies [9], such as metallic disk, square, T shape, nanostrips and so on, with either one size/layer or multi-sizes/layers [10-12]. The widely used designs are a thick metallic layer at the bottom, the patterned metallic nanostructures on the top with the dielectric layer in between. Although such configuration has the advantage to excite magnetic resonance between top and bottom layers, the thick bottom metal layer limits applications to absorption only. Moreover, the devices are rarely to work in the visible range. The idea to explore the transmitted and reflected signals from the plasmonic metasurface have been proposed in recent years and adopted to design optical devices such as wave plates [13-16] and polarizers [17-20]. The unique functionalities result from the resonant responses of anisotropic nanostructures, such as nanowires or slits, which are able to control the amplitude and phase of two orthogonal linear polarizations. In particular, the broadband response is

3 accessible based on the polarization dependent charge oscillations in the metallic elements of the nanostructure which can serve as broadband optical component in the spectroscopy and imaging systems. Here, we use plasmonic nanowire arrays to demonstrate the broadband optical responses in both experiment and simulation, and its function as broadband linear polarizer in the visible range. The polarizers, according to the working principle, are usually divided into two types: absorptive type and reflective type. A typical absorptive type polarizer called polaroid is made of polyvinyl alcohol plastic with an iodine doping [21] in which the incident light polarized parallel to the polymer chains is absorbed. For metallic absorptive polarizer, the elongated Ag nanoparticle array is one choice [17] due to the different resonances modes excited along the two axis of the elongated nanoparticle. In contrast, the reflective type polarizers, represented by the metallic wire-slit array, relying on the high reflectance of polarized light in particular direction. Especially, recent study reveals that the broadband transmission of the linearly polarized light with electric field orthogonal to the metallic wire is due to the significantly reduced impedance mismatch by diminishing the slit resonance (Fabry-Perot resonance) in the metallic wire-slit array [22]. Therefore, it is easy to get high and flat transmission for light field orthogonal to the wire as well as high and flat reflection for light field parallel to the wire, when the period of metallic wire-slit array is less than ~ 300 nm and the thickness of the metallic wire is more than 100 nm. However, the broadband absorption is difficult to attain due to the wavelength dependent resonance of the metallic element. Hence, here we show a new route to realize broadband absorption and transmission by precise arrangement of the metallic elements in the plasmonic nanostructure and open an option for broadband linear polarizer in the visible range.

4 Specifically, we introduce a novel plasmonic nanostructure consisting of Ag short-long nanowire array which exhibits broadband transmittance and absorption differences between orthogonal linear polarizations. The simulations using a commercial finite-integration timedomain algorithm (CST Microwave Studio) show ~ 80% transmittance difference and 60% absorption difference averaged from 500 to 850 nm. Additionally, the broadband absorption originates from the collective electron oscillations along X polarization in different wavelengths for distinct Ag elements and consequently leads to the low reflection level of X polarization. Especially, around the resonance frequencies, almost zero reflection is obtainable which may open a new application for the structure as antireflective material in the photovoltaic devices. Moreover, the optical responses are tunable by controlling the width between two short wires. To illustrate the principle more clearly, a structure composed of Ag wire-slit array which show broadband reflection of X polarization is also designed and used as a control experiment. Figs. 1a and 1b are the schematic view of the Ag short-long wire array and wire-slit array, respectively. The corresponding SEM images are shown in Figs. 1c and 1d. The fabrication is straightforward. Firstly, the 2D templates (dark green region) consisting of PMMA short-long strips in Fig. 1a and long strips (60 m) in Fig. 1b were prepared in advance by standard e- beam lithography, which are followed by shadowing depositions twice [23], with deposition direction indicated by the orange arrows of Fig. 1a in Y-Z plane. 30 nm of Ag is coated on the top and 15 nm on the side wall of the PMMA strips. The width of the resultant top Ag wire is a=50 nm and the depth of the side wall Ag (Z direction) is h 1 =85 nm. The dramatic difference between the two configurations lies in the addition of Ag strip along Y direction on the bottom ITO layer as shown in Fig. 1a which is due to the short length of one wire thus the Ag vapor

5 can go deep into the bottom during 45 o directional deposition. Such a strip is actually two Ag strips (from two times depositions) touching each other and connecting with the side wall Ag at two ends. We calculate the optical responses of the two configurations in Fig. 1 using orthogonally linear polarized incident light. The dielectric constant of PMMA and ITO is the same as Ref. 24 and the complex dielectric constant of shadowing deposited Ag is extracted following the same procedures. We measure the transmission and reflection spectra using microscope based optical setup under 20X objective, which could produce stable linear polarized light from 500 nm to 850 nm. The absorption is calculated as 1-T-R. In Fig. 2, the first and third columns show the calculated transmittance, reflectance and absorption spectra of the Ag short-long nanowire array and wire-slit array, respectively. The corresponding experimental results are depicted in the second and fourth columns which show good agreement with the simulations. The incident light is linearly polarized under normal incidence from substrate side and propagates along Z direction. Black and purple curves denote that the polarization (direction of electric field vector) of incident light is along X and Y directions, respectively. The metal wire is parallel to the X polarization. In Fig. 2a, the transmittance of X polarization (black curve) is as low as 5% averaged across the whole spectra from 500 to 850 nm, with a transmittance dip at 636 nm (blue arrow). In comparison, the transmittance of Y polarization in Fig. 2a (purple curve) is high and flat (80% averaged from 500 to 850 nm) which means that the Ag short-long wire array could perform as broadband linear polarizer. The low transmittance of X polarization is correlated with the absorption indicated by the black curve in Fig. 2c which keeps a high level across the broad bandwidth with a remarkable peak at ~ 600 nm. The averaged absorption is ~ 61% with the

6 peak value ~ 0.9 and FWHM 149 nm. The correlation will be discussed carefully in Fig. 3. The simultaneously generated reflection valley is shown in Fig. 2b (black curve) with minimum value 0.08, FWHM 116 nm. It is notable that in Fig. 2b, the reflectance of Y-polarized light (purple curve) is already very low for the whole broad bandwidth and thus the additional reflection valley from X-polarization leads to one more potential application of the short-long Ag nanowire array as anti-reflective material. For Ag wire-slit array the experimental results in Fig. 2j agree very well with the simulations in Fig. 2g, which shows remarkably large, flat and broadband transmittance difference between X and Y polarizations. The broadband nature of Ag wire-slit array is due to the different impedance responses to the orthogonally polarized incident light. The impedance mismatch appears at the interface of two materials with different dielectric constants [25]. In visible regime, the large contrast of refractive index would cause large impedance mismatch and hence high reflection, which is exactly the reason why Ag film could be used as reflective mirror. For X polarization, the electric field oscillates along the metal wire, in which the free electrons oscillate in the same way as the incident electric field. Therefore, the metal wires looks like a metal film which induce large impedance mismatch to the incident polarization and result in high reflectance (Fig. 2h black curve). Along Y polarization, the weak slit resonances dominate and effectively reduce the impedance mismatch so the reflectance is low and broadband [22] as shown by the purple curve in Fig. 2h. As a result, the broadband transmittance difference in Fig. 2g results from the large and flat reflectance difference as shown in Fig. 2h, and hence the Ag wire-slit array can be treated as broadband reflective type polarizer. Different from the short-long wire array, it is difficult to find an obvious transmittance or reflectance dip for Ag wire-slit array due to the weak

7 resonance property of the structure. The origin of the broadband absorption of the short-long wire array will be revealed in Fig. 3. There are four configurations used in numerical simulation shown in Fig. 3, in which configuration I and IV have been realized in the experiment. Configurations II and III are two middle states for the purpose to identify the role of the absorption step by step. The mechanism of the configuration I performed as broadband reflective polarizer has been explained in the above. For configuration II, one wire is shortened by 85 nm along X direction in a unit cell. The total absorption (area under the red curve) in Fig. 3c is 1.3 times higher than that in Fig. 3a with the maximum absorption of value 0.3 located at 803 nm. The absorption loss occurs almost on the top and side wall of the Ag short wire (red region in configuration II) where the currents oscillate with the incident X polarization at resonance wavelength ~ 803 nm. Thus, the increased absorption of configuration II comes from the plasmonic excitation of the short wire. For configuration III, where the longer Ag wire is connected to the bottom ITO glass, the total absorption in Fig. 3e increases significantly which is 1.84 times higher than that in Fig. 1b with an obvious absorption peak ~ 0.56 at 647 nm. Importantly, the dramatic increase of absorption comes from the strong current oscillation on the side wall of the longer wire, the particular region which connects the top layer Ag with the bottom ITO glass (red region connecting the long wire of configuration III). Based on the size and the vertical arrangement of the metals in this particular region, the resonance occurs at 647 nm. For configuration IV, when the bottom Ag strip is inserted, the absorption in Fig. 3g (red curve) increase substantially from 500 to 650 nm with a remarkable absorption peak at ~ 600 nm. The total area under red curve in Fig. 3g is 1.47 times larger than that under the red curve in Fig. 3e. The additional absorption is essentially due to the collective oscillation of the electrons excited in the bottom Ag strip along

8 X polarization which is ultra-strong with FWHM 149 nm. For Y polarization, as shown by the blue curve in Fig. 3h, the transmittance of short-long wire array is high and broadband. The reason is similar to the wire-slit array case which is due to weak slit resonance. One may consider that the bottom Ag strip along Y direction in short-long wire array could induce large reflection to Y polarization. However, as shown in Fig. 3h, the reflectance (green curve) is proved to be low and flat across the whole spectra. The reason is the thickness of the Ag strip is only 15 nm, too thin to induce large impedance mismatch for Y polarized incident light. Thus the reflectance is low and the transmittance of the Y polarization is still high and broadband (blue curve in Fig. 3h). Finally, in Fig. 3g, the X-polarized incident light is 61% absorbed, 34% reflected and 5% transmitted averaged from 500 to 850 nm. Therefore, the short-long nanowire array could be classified as absorptive type polarizer. To examine the effective function as linear polarizer, the two samples in Figs. 1c and 1d were characterized by angular dependence transmittance measurement as shown in Fig. 4 at the extinction wavelengths, i.e. 636 nm for Ag short-long nanowire array (black squares) and 702 nm for Ag wire-slit array (red dots). The polarization of the incident light is fixed along Y direction. The rotation angle is between parallel wires and the X-axis. Thus, the minimum transmission is observed when the wire is parallel with the Y polarization with rotation angle at 90 o. The minimum transmission of short-long nanowire array is 0.05 which is much lower than that of wire-slit array 0.16, even though the maximum transmittances of them are comparable at 0 o /180 o. Hence, the short-long wire array has larger extinction ratio (T min /T max ) than wire-slit array. The averaged transmittance of X polarization is comparable between the two configurations as shown in Figs. 2d and 2j.

9 It is well known that the behavior of the plasmonic resonance has direct dependence on the size of the metal. As shown in Fig. 3g, the dramatic oscillation occurs in the bottom Ag strip with the frequency of the absorption peak determined by the width of the strip. Thus, it is an effective and simple way to control the resonance by tuning the width d of the bottom Ag strip, i.e. the separation between two short wires. Here, the optical responses as a function of d are shown in Fig. 5. The transmission, reflection and absorption averaged from 500 to 850 nm for X (black) and Y (purple) polarizations, respectively, are shown in Fig. 5a-c. Firstly, for X polarization, the averaged transmission is very low and slightly increase from d=50 (0.01) to 105 nm (0.12) as shown by the black squares in Fig. 5a. The increase is relatively large from d=80 to 105 nm because of the decrease of the averaged reflection and absorption in the same range, as shown by the black triangles and dots in Figs. 5b and 5c, respectively. The decrease of the averaged reflection (black triangles) from d=50 (0.54) to 85 nm (0.34) is obvious as shown in Fig. 5b which is due to the increase of the absorption in the same range (black dots in Fig. 5c). Importantly, the averaged absorption first increases from 0.44 (d = 50 nm) to 0.61 (d = 85 nm), then decreases to 0.57 (d = 105 nm) along the increase of d. Since absorption is determined by the collective electric resonances of different metal elements including the bottom Ag strip, the side wall Ag connecting the Ag long wire and the Ag short wire as introduced in Fig. 3, the change of d will cause the size variations of the above three metal elements. Thus, it is expected that the highest absorption can be reached when the combination of the multiple oscillations are optimized. Here, it is clear that d = 85 nm is an optimized parameter which has maximum averaged absorption 0.61 as shown in Fig. 5c (black dots). Secondly, for Y polarization, the averaged transmission (purple square in Fig. 5a), reflection (purple triangle in Fig. 5b), and absorption (purple dots in Fig. 5c) don t show dramatic

10 changes along the increase of d, with the transmission keeping high (~80%) and both the reflectance and absorption very low. As a result, to perform as a good broadband absorptive polarizer for Ag short-long wire array, the preferential magnitude of d should range from 50 to 85 nm, for which the averaged transmission of X polarization is low and the absorption keeps increasing. Moreover, it is valuable to discuss the spectra of the reflection. As shown in Fig. 5d the reflection dips (green triangles) are very small and almost zero from d = 50 to 75 nm which are determined by the absorption peaks (red dots). In other word, for each d, the reflection dip is attributed to the absorption peak and both have the same wavelength (black squares) at the resonance of the bottom Ag strip. In addition, the resonance red shifts along the increase of d. It is notable that in Fig 5b, the averaged reflectance of X polarization (black triangles) becomes almost flat (decrease slower) from 85 to 105 nm. The reason behind is that along the increase of d, the decrease of reflections from 600 to 850 nm cancels the increase from 500 to 600 nm due to the shortening of the Ag short wire. However, it is difficult to make the averaged reflection for X polarization as low as that of the Y polarization (purple triangles of Fig. 5b) because the reflection from the Ag long wire of the short-long wire array can t be eliminated. Finally, using shadowing vapor deposition we have fabricated two plasmonic nanostructures with distinct optical behaviors. The linear polarization conversions of these structures are almost zero. The resonances of the metallic elements with preferential responses to X polarization in the short-long wire array lead to the broadband absorption, and consequently contribute to the low and broadband transmission. Because of the high and flat transmission to Y polarization, the structure can be used as broadband absorptive linear polarizer. Moreover, the significant collective oscillation from bottom Ag strip is ultra-strong which induces high and broad absorption peak with value ~90% and FWHM 149 nm and consequently leads to a

11 reflection valley around the resonance. As a result, the short-long wire array has the additional potential to be an antireflective material. Furthermore, the shadowing deposited wire-slit array as broadband reflective type polarizer has its economic advantages than conventional metallic wire-slit array. For example, as shown in Fig. 3a, the reflectance of X polarization can be 80% by the structure of configuration I with 30 nm Ag on the top and 15 nm thick Ag on the side wall. However, for conventional metallic wire-slit array, the same reflectance is accessible only when the metal wires are very thick (>100 nm). Hence, using shadowing vapor deposition, part of the metallic material can be effectively replaced by the dielectric material without degrading the final performance. In conclusion, two types of plasmonic nanostructures have been designed to demonstrate broadband and huge transmittance difference between orthogonally linear polarized light. The bandwidth covers all of the measured spectra from 500 to 850 nm. For Ag short-long wire array, the broadband absorption is due to the polarization dependent plasmonic excitations of the metallic elements at distinct wavelengths. The optical properties are adjustable by controlling the size of metal elements. For wire-slit array, as broadband polarizer, the sample made by shadowing vapor deposition is more economic than the conventional one. Our designs can be applied in the optics and photovoltaic fields.

12 ACKNOWLEDGMENT Support from Hong Kong RGC grants FSGRF13SC26, FSGRF14SC30, HKUST2 CRF 11G and AoE P-02/12 is gratefully acknowledged. The technical support of the Raith-HKUST Nanotechnology Laboratory for the electron-beam lithography facility at MCPF (SEG_HKUST08) is hereby acknowledged. The simulations were carried out using the server in the Key Laboratory of Advanced Micro-structure Materials, Ministry of Education, China; and also the School of Physics Science and Engineering, Tongji University in collaboration with Prof. Y. Li in Tongji University.

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16 Figures Fig. 1 a,b Sketches of the short-long wire array and wire-slit array which are PMMA (green) supported Ag (orange) on ITO substrate (blue) after Ag shadowing vapor deposition twice, respectively. Orange arrows indicate the directions of Ag flux. l 1 =280 nm, l 2 =140 nm, l 3 =195 nm a=50 nm, d=85 nm, h 1 = 85 nm, h 2 =130 nm, = 45 o. c,d The corresponding SEM images by tilt view. Blue scale bars are 200 nm.

17 Fig. 2. Transmittance, reflectance and absorption spectra of the Ag short-long wire (first and second columns) array and wire-slit array (third and fourth columns). Black and purple colors indicate the polarization of the incident light as X and Y direction, respectively. The first and third columns are the simulation results and the second and fourth columns are the experimental results.

18 Fig. 3. Calculated transmittance (blue), reflectance (red) and absorption (green) spectra of four configurations whose models are shown on the right-hand side based on Ag shadowing deposition twice. The polarization configurations of the incident light in X and Y directions are

19 shown on top of the two columns by black and purple arrows, respectively. In a and b, the configuration I is Ag wire-slit array, the same as Fig. 1b; in c and d, one wire is shortened by 85 nm. In e and f, longer Ag wire is connected to the bottom ITO layer (blue). In g and h, a rectangular Ag strip on the bottom connecting with the longer Ag wire. The Ag elements which have large absorption of X polarization are painted as red.

20 Fig. 4. Angular dependence transmittance of Ag short-long wire array (black squares) and wireslit array (red dots) measured by rotating the sample from 0 o to 180 o, with Y polarized incident light at 636 and 702 nm, respectively.

21 Fig. 5 Simulated optical properties as a function of d, the width of the bottom Ag strip. a-c Average value of transmission, reflection and absorption from 500 to 850 nm, for X (black) and Y (purple) polarizations, respectively. d The reflection dip (green triangle) and absorption peak (red circle) and the wavelength (black square) of the dip/peak positions as a function of d.