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During surface plasmon polariton (SPP) excitation, the imaging of the interaction between the DC current and the Transverse Mode (TM) optical field is reported for the first time. Using an optical microscope, we have successfully captured images of electro-optic interaction during plasmonic demodulation phenomena. A significant response is observed when images of the transmitted light – which represents SPP excitation – become less bright in the presence of an electric field when the thin-film metal thickness is approximately equivalent to the skin depth of gold (30 nm). The synchronization achieved between the optical reflectance analysis and the SPP imaging shows that maximum interaction is achieved when the optical reflectance change, ΔR is 0.0530.
© 2014 Optical Society of America
1. Introduction
Plasmonic research that is utilized to examine the optical properties of materials has many applications in scientific and technological fields. Today, we have witnessed a rapid development in the scope of plasmonic applications, particularly in the area of sensing [1–3]. The ability to interact with electronics demonstrates plasmonics’ versatility for developing active devices [4,5]. By considering the coupling of surface plasmons undergoing electronic transport, Noginova et al. [6] explored the possibility of controlling surface plasmon polaritons (SPPs) externally by applying an electrical current propagating in metallic films. However, active plasmonic technology not been fully explored because of researchers’ tendency to exclusively investigate its sensing characteristics. The development of active plasmonic devices is still in its infancy, and no comprehensive study has been executed thus far. This absence of research has motivated the present study, in which the electro-optic effect experienced in thin film metal strips during SPP excitation is explored. More interestingly, in the event of SPP excitation (introduced as a plasmonic demodulation phenomenon), the interactions between the DC current and the propagating Transverse Mode (TM) optical wave are captured by observing changes in image intensities.
In this study, we used gold metal strip-thin films fabricated on glass substrates to successfully develop a simple and low-cost plasmonic demodulator. This paper discusses an exciting research breakthrough in the field of active plasmonics in which (to the authors’ knowledge) the imaging of electro-optic interaction is reported for the first time. To compare the imaging results, optical reflectance analysis is implemented, and this analysis exhibits similar and corresponding results.
2. Methodology
An important requirement for generating SPPs is the use of metal strip-thin films. Because of its stabilization and oxidization resistance [7–9], gold metal strip-thin film is employed in this experiment. To produce a significant electro-optic effect, the metal strip-thin film is fabricated using a metal lift-off technique. Four steps are involved in producing the gold metal strip-thin film: preparing both a glass substrate and the mask design, transferring the mask onto the surface of a glass slide, forming a thin gold film on the glass slide using a direct current (DC) sputtering process, and removing the photoresist using the metal lift-off technique. An advantage of using a glass slide as a substrate is that a glass slide does not absorb heat when the DC current is applied across the metal strip. Once the metal strip-thin film is fabricated on the glass slide, it is attached with a half-cylindrical prism under the Kretschmann configuration in order to enhance the wave vector of light, kx, which passes through the prism to be matched with the SPP wave vector, kspp [10].To study the interactions resulting from the electro-optic effect, the thickness t of the gold metal strip-thin films was varied to 30 nm, 50 nm, and 100 nm; meanwhile the laser power levels P were adjusted to 0.5 mW, 1.0 mW, and 1.5 mW.
SPP excitation is performed using an angular interrogation technique [11] in which the incident angles are increased from 30° to 70° as the intensities of the light reflected from the gold-coated glass slide are measured using a silicon photodetector. Figure 1(a) shows the experimental setup for SPP imaging conducted at room temperature of 27°C. Dimas Software Version 4.0 Professional Edition was used to analyze the SPP images. The use of this technique proves that the SPP excitation phenomenon can be detected via visual representation of SPP intensity in a non-quantifiable manner. The electro-optical response is studied in the presence of a 5-V cm−1 electric field across the metal strip at the time of SPP excitation where the DC current flows in a perpendicular motion with respect to the propagation of the SPP since this combination leads to a strong variation of the transmission coefficient. Figure 1(b) shows the electric circuit for the current flow along the gold metal strip-thin film where the total resistance of the circuit is given by the resistance of the external control circuit; the resistance of this film is calculated by R = Rexternal control resistance + Rfilm. Hence, the film resistance Rfilm determines the total resistance of the circuit. An applied DC current of I0 = 51.372 mA is supplied for each metal strip-thin film with length of 10 mm, width of 0.5 mm and various thicknesses of 30 nm, 50 nm, and 100 nm. For these metal strip dimensions, the dissipated Joule heat in the thinnest gold thin film can be considered to be small and negligible. Meanwhile, the beam-spot diameter is set approximately equal to the metal strip’s width (0.5 mm) which – considering that SPP excitations occur along the entire width of the metal strip – contributed to a very small thermal effect when the DC current was turned on. For each measurement, the metal strip was cooled down for a certain period of time before a new measurement is taken to ensure thermal effects can be reduced. Hence, the observed electro-optic effect can be considered not to be related to the heating of the gold metal film. Using an optical microscope and a digital camera (Rax Vision S35796), an electro-optic interaction imaging system is implemented, in which visual images of the light transmitted from the surface of the gold metal strip-thin film are captured during SPP generation under an electric field. For verification purposes, the reflectance values Rmin recorded by the silicon photodetector [9–11] were compared with the imaging results.
Fig. 1 (a) A schematic diagram of electro-optic interaction imaging using an optical microscope. (b) Electrical circuit used to observe the flow of current in a gold metal-strip thin film during SPP excitation.
3. Results and Analysis
3.1 Interactions between the DC current and the optical reflectance during SPP excitation
This section reports the results of the interactions between the DC current and the optical reflectance values during SPP excitation. Figure 2(a) shows the electro-optical response in a gold metal strip while varying the laser power levels to P = 0.5 mW, P = 1.0 mW, and P = 1.5 mW when the film thickness is fixed at 30 nm. A DC current of I = Io = 51.372 mA was applied along the metal strip. To provide a clearer picture, an enlarged view of Fig. 2(a) that focuses on the resonant angle has been provided in Fig. 2(b).
Fig. 2 Illustration of reflectance curves resulting from the interaction between SPP excitation and the DC current in the gold metal strip-thin film with thickness t = 30 nm. (a) Range of incident angles between 30° and 70°. (b) Range of incident angles between 48° and 54°.
We discovered that resonance occurred at an incident angle of θi = 50.5° for all three levels of laser power. The solid blue, green, and red lines represent the reflection curves at the various laser power levels (P = 0.5 mW, P = 1.0 mW, and P = 1.5 mW, respectively) in the absence of DC current (I = 0). Meanwhile, the dashed lines represent the reflection curves in the presence of DC current (I = Io). The usage of a low laser power level (P = 0.5 mW) produced poor electro-optical responses, for which the minimum reflection point Rmin increased from 0.1711 to 0.1899 (with a small difference ΔRmin of 0.0188). Enhancing the laser power level to P = 1.0 mW led to an increase in Rmin, from 0.1089 to 0.1409, which resulted in ΔRmin = 0.0320. The use of high laser power level P = 1.5 mW produced a strong electro-optical response, with the increment of ΔRmin to 0.0530, for which the values of Rmin increased from 0.0514 to 0.1044.
Figure 3(a) displays an overview of the reflection curves for a 50-nm thick gold metal strip-thin film with incident angles ranging between 30° and 70°. For the purpose of clarification, Fig. 3(b) shows the blown-up view of the reflection curves for incident angles between 47° and 52°. The position of the minimum reflectance Rmin increases with the increase in the thickness of the thin film, compared with that in Fig. 2. The position of the resonant angles shifted from 50.5° to 49.0° for the three laser power levels as the film thicknesses increased to 50 nm. The use of low laser power levels P = 0.5 mW exhibited a poor electro-optical response, in which ΔRmin increased by 0.0077, from 0.3193 to 0.3270. Once the laser power level was raised to P = 1.0 mW, Rmin increased from 0.0107 to 0.2493, which resulted in ΔRmin = 0.2386. Enhancing the power level to P = 1.5 mW resulted in an improved ΔRmin (about 0.0194) in which the values of Rmin were shifted from 0.1734 to 0.1928. Changes in Rmin become more evident with the increase in laser power levels. The electro-optical response is more tangible when the thin film thickness is t = 30 nm rather than t = 50 nm.
Fig. 3 Illustration of reflectance curves resulting from the interaction between SPP excitation and the DC current in the gold metal strip-thin film with thickness t = 50 nm. (a) Range of incident angles between 30° and 70°. (b) Range of incident angles between 47° and 52°.
Figure 4(a) indicates the effect of laser power levels upon the optical reflectance of the gold metal strip-thin film of thickness t = 100 nm with incident angles ranging between 30° and 70°. Again, Fig. 4(b) shows the blown-up view of the incident angles ranging between 46° and 52°. However, for this thickness, SPP excitation nearly failed to occur because of the loss factor experienced by the SPPs. The use of lasers with low power levels (P = 0.5 mW and P = 1.0 mW) resulted in the absence of SPP excitation. As the laser power level was raised to P = 1.5 mW, poor SPP excitation occurred with Rmin equaling 0.790. For all three laser power levels, the presence of the DC current did not affect the value of the optical reflectance, which proves the non-existence of the electro-optic effect at t = 100 nm.
Fig. 4 Illustration of reflectance curves resulting from the interaction between SPP excitation and the DC current in the gold metal strip-thin film with thickness t = 100 nm. (a) Range of incident angles between 30° and 70°. (b) Range of incident angles between 46° and 52°.
3.2 Electro-optics Interaction Imaging
Interactions between the optical and electrical fields in gold metal strip-thin films can be seen by observing changes in the transmitted light images in the presence of a DC current. Figure 5 displays imaging of electro-optic interactions when the thin film thickness was held constant at t = 30 nm. Figure 5(a)(i) exhibits images of light transmitted at an incident angle of 42.0°. The reddish captured images do not represent SPP excitation but instead represent the low intensity of the light transmitted when the incident angle is less than the critical angle, which is 44.0°. We emphasize at this point that SPP excitation occurs only during the phenomenon of total internal reflection. This criterion must be met to generate sufficient energy for exciting SPPs. Based on the image intensity portrayed in Fig. 5(a)(i), it is evident that the electro-optic reaction was observed neither in the absence of an electric field (I = 0) nor in the presence of an electric field (I = Io). As the angle of incidence was increased to 50.5°, the resulting image [in the absence of DC current (I = 0)] displayed a greenish-white bright light with the red color present at the image’s edges, as captured in Fig. 5(a)(ii) . Based on observations made over the course of the entire experiment, we conclude that a nearly white transmitted light represents SPP excitation; meanwhile the red light is simply common transmitted light. By imposing a DC current on the metal strip (I = Io), the intensities of the transmitted light decreased, which proves that SPP excitations are weaker in the presence of an electric field. This phenomenon reduces the role of SPPs in electro-optics interaction [4]. SPP excitation occurs not only at resonant angles but is also experienced at any point along the resonant curves. However, maximum excitation is obtained at resonant angles, where the strongest optical power absorption occurs in the metal strip-thin film. As the incident angle approaches the non-resonant angle of 54.0° [Fig. 5(a)(iii)], the greenish-white color of SPP excitation is reduced, because compared with the image in Fig. 5(a)(ii), the electro-optic effect is not as significant. Furthermore, at the non-resonant incident angle of 57.0° [as illustrated in Fig. 5(a)(iv)], the intensity of both transmitted light and SPP excitation decreased, which represents weak SPP excitation.
Fig. 5 Transmitted light images of gold metal strip-thin film with thickness t = 30 nm in the absence of DC current, I = 0 mA, and in the presence of DC current, I = I0 = 51.372 mA, using various laser power levels: (a) P = 1.5 mW, (b) P = 1.0 mW, and (c) P = 0.5 mW.
Overall, light intensities decreased when the DC current passed through the metal strip. While the location of the incident angles was within the resonant curve, SPP excitations of different intensity strength still occurred. As the incident angle approached the resonant angle, stronger excitation was produced. It can be concluded that, as the incident angles’ distance from the SPP resonant point increases, weaker SPP excitation occurs, which explains the phenomena in Fig. 5(a)(iii) and Fig. 5(a)(iv), where SPP excitations still occur but not at the resonant angles. The location of the incident angle also affects the electro-optic interactions, such that these reactions were seemingly observed at resonant angles rather than other incident angles. As the incident angles approach the resonant angles, maximum electro-optic interaction is observed. Figure 5(b) features transmitted light images from the surface of the gold metal strip (with thickness t = 30 nm) when the strip is subjected to a laser with power level P = 1.0 mW. Prior to the phenomenon of the total internal reflection (specifically, at the critical angle of 42.0°), both SPP excitation and the electro-optic effect did not occur, as shown in Fig. 5(b)(i). The greenish-white spots at an incident angle of 50.5° indicate the occurrence of SPP excitation [Fig. 5(b)(ii)]. As DC current passes through the metal strip, the intensities of the transmitted light become weaker because of plasmonic demodulation. In support of this conclusion, Fig. 5(b)(iii) and Fig. 5(b)(iv) show similar patterns as Fig. 5(a)(iii) and Fig. 5(a)(iv), in which less pronounced electro-optics interaction is displayed. Low laser power levels affect the amount of SPP excitation. In comparison with Fig. 5(a) and Fig. 5(b), poor SPP excitations were observed in Fig. 5(c)(i), Fig. 5(c)(ii), and Fig. 5(c)(iii) as the laser power decreased to P = 0.5 mW.
Based on optical reflectance analysis, increasing the thin film thicknesses from t = 30 nm to t = 50 nm reduced the amount of SPP excitation and shifted the location of the resonant angles from 50.5° to 49.0° [7–9]. Figure 6 depicts the microscope images of the transmitted light as the angle interrogation technique was performed on the gold metal strip with thickness t = 50 nm. Note that neither SPP excitation nor the electro-optic effect occurred prior to reaching the critical angle. Compared with the resonant incident angle of 49° [captured in Fig. 6(a)(ii)], poor SPP excitation and electro-optic interaction were clearly observed at a non-resonant incident angle of 54°, as displayed in Fig. 6(a)(iii). Figure 6(a)(iv) shows the microscope images when the strip is subjected to an incident angle of 57.0°. In reference to Fig. 3(a), the location of this incident angle was at the edge of the resonant curve, which explains the occurrence of weak SPP excitation. The darker image obtained results from the presence of an applied electric field, which indicates the onset of plasmonic demodulation at an incident angle of 57.0°. Figure 6(b) features microscope images of the gold metal strip-thin film at thickness t = 50 nm when the strip is subjected to incident light with laser power level P = 1.0 mW. Overall, the transmitted light images did not exhibit high intensities in any of the four figures. Similarly, for plasmonic demodulation phenomena, no changes in image intensities were observed as DC current passed along the metal strip. In reference to Fig. 3(a), SPP excitation still occurs at an incident angle of 49.0°, as this angle is still located along the resonant curve. Nevertheless, the microscope image portrayed in Fig. 6(b)(i) was clearly unable to identify either SPP excitation or plasmonic demodulation because of the optical microscope’s limited resolution. Figure 6(c) captures images of the transmitted light with low laser power level P = 0.5 mW. For all figures displayed above electro-optical responses were very weak when changes in image intensities due to the presence of the electric field were less pronounced.
Fig. 6 Transmitted light images from the gold metal strip-thin film with thickness t = 50 nm in the absence of current, I = 0 mA, and in the presence of current, I = I0 = 51.372 mA, using various laser power levels: (a) P = 1.5 mW, (b) P = 1.0 mW, and (c) P = 0.5 mW.
Figure 7 depicts microscope images from resonant angle 49.0° for the gold metal strip-thin film with thickness t = 100 nm subjected to varying laser power levels of P = 0.5 mW [Fig. 7(a)], P = 1.0 mW [Fig. 7(b)], and P = 1.5 mW [Fig. 7(c)]. Other incident angles are not discussed, as no SPP excitation occurred, as explained in the optical reflectance analysis for Fig. 4. As the laser power levels were set at P = 0.5 mW and P = 1.5 mW, the figures seemingly indicate that the intensities of the transmitted light did not experience any change when the DC current passed along the gold strip, which proves the non-existence of plasmonic demodulation phenomena. Figure 7(c) indicates weak SPP excitation for a gold metal strip with thickness t = 100 nm subjected to a high-intensity laser. Nevertheless, electro-optics interactions failed to occur because of the limited amount of SPPs, which were inadequate to react with the DC current.
Fig. 7 Transmitted light images of the gold metal strip-thin film with thickness t = 100 nm in the absence of DC current, I = 0 mA, and in the presence of DC current, I = I0 = 51.372 mA, using various laser power levels: (a) P = 1.5 mW, (b) P = 1.0 mW, and (c) P = 0.5 mW.
5. Discussion
SPP excitations can only be observed under the TM polarization state which is normal to the metal surface [12]. The presence of SPPs affect the circuit’s DC current. These electro-optic effects are greatly influenced by the direction of the DC current with respect to the SPP propagation, for which both directions are always perpendicular to each other. Stronger light transmissions are observed as the DC current decreases, which results from the increment in SPP excitations. As the DC current is turned on, the changes in the minimum optical reflectance values increase accordingly; a similar increase was observed in the transmitted light intensity images with the enhancement of the laser power level. These observations prove the existence of the electro-optic effects in which the interactions between the DC current and SPPs occur without the influence of thermal effect or Joule heating. These interactions can be observed by studying the image intensities in the presence of a DC current. Images that becomes less bright when DC current passes through the metal strip result from the decrease in SPP density. The purpose of implementing the optical reflectance analysis was to support the visually obtained results.
At t = 30 nm, the maximum electro-optic effect occurs, in which the visual images and the optical reflectance values exhibit synchronized outcomes. The use of a high laser power level produces an optimal electro-optical response due to maximum SPP excitation, which thus creates strong interaction between optical and electrical parameters, as captured in Fig. 8(a). As the laser power is increased from P = 0.5 mW to P = 1.5 mW, the gap between the optical reflectance values when I = 0 and I = Io becomes larger. At t = 50 nm, the DC current dominates the metal strip meanwhile the metal strip’s balance region is comprised of SPPs. Experimentally, this situation is illustrated in Fig. 6, where the electro-optic effect can still be observed. The increment in film thickness to t = 100 nm shows the non-existence of the electro-optic effect, a situation that has been captured in Fig. 4 and Fig. 7. Weak SPP excitation at high laser power levels proves the existence of small amounts of free electron oscillations. However, these SPPs are inadequate to interact with the DC current in order to create the electro-optic effect on the metal strip. Relying on such observations, we do not discuss the effect of thin film thickness t = 100 nm in Fig. 8(a) due to the absence of plasmonic demodulation phenomenon.
Fig. 8 (a) Relationship between the optical reflectance R and laser power level P. (b) Effects of gold metal strip- thin film thicknesses t on the values of optical reflectance R.
Figure 8(b) displays the influence of metal thin film thicknesses on optical reflectance changes. An exponential relationship – in which ΔR is the optical reflectance change and t refers to the gold metal strip-thin film thickness – is exhibited between both parameters. These relationships are expressed in Eq. (1), Eq. (2), and Eq. (3) with laser power levels P = 0.5 mW, P = 1.0 mW, and P = 1.5 mW, respectively.
A greater optical reflectance change results in better electro-optics interaction. Previous work by Noginova et al. [6] exhibits the same interaction behavior, in which the maximum DC current signal corresponds to the minimum optical reflectance. It is seemingly observed that the maximum optical reflectance change is produced when the metal thin film thickness is set to t = 30 nm at laser power P = 1.5 mW, for which ΔR is 0.0530. As the thickness is increased to t = 50 nm, ΔR drops by about 40%, and the difference in SPP excitation is due to the electro-optic effect becoming less significant. The same patterns are observed at P = 1.0 mW and also at P = 0.5 mW.
6. Conclusion
In this work, we proposed the creation of an optimal electro-optic effect for plasmonic demodulation using a gold film with a thickness of t = 30 nm and a high laser power level of P = 1.5 mW. Instead of employing a common thin film with rectangular dimensions, a metal strip structure was used to ensure that the apparent interactions between the SPP and DC current could be studied. We believe this finding – in which plasmonic demodulation phenomena during SPP excitation are visually proven that agree with optical reflectance analysis – will serve as a starting point for future investigations into active plasmonics.
Acknowledgment
The authors would like to acknowledge the support of the Universiti Kebangsaan Malaysia and the Malaysian Ministry of Higher Education (MOHE) for funding this work under grant DIP-2012-17 and ERGS/1/2012/STG02/UKM/02/2 respectively. We would also like to thank Encik Ahmad Makarimi bin Abdullah and Dr. Mohamad Zahid Abdul Malek from Advanced Materials and Research Institute AMREC, Kedah, Malaysia for their technical support, and we would additionally like to thank Universiti Sains Islam Malaysia (USIM) for the SLAI/KPT scholarship awarded to the first author.
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