- Email: [email protected]
2D plasmonic gold nano-patches for linear and nonlinear applications
Microelectronic Engineering 111 (2013) 234–237
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
2D plasmonic gold nano-patches for linear and nonlinear applications M. Grande a,⇑, G.V. Bianco b, M.A. Vincenti c, D. de Ceglia c, V. Petruzzelli a, M. Scalora d, G. Bruno b, A. D’Orazio a, M. De Vittorio e,f,g, T. Stomeo g a
Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, Via Re David 200, 70125 Bari, Italy Institute of Inorganic Methodologies and Plasmas, IMIP-CNR, via Orabona 4, 70126 Bari, Italy c AEgis Technologies Inc., 410 Jan Davis Dr., Huntsville 35806, AL, USA d Charles M. Bowden Research Center, RDECOM, Redstone Arsenal, AL 35898-5000, USA e Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Via Arnesano, 73100 Lecce, Italy f National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, Via Arnesano, 73100 Lecce, Italy g Center for Bio-Molecular Nanotechnology, Istituto Italiano di Tecnologia (IIT), Via Barsanti, 73010 Arnesano (Lecce), Italy b
a r t i c l e
i n f o
Article history: Available online 17 April 2013 Keywords: Plasmonics Electron beam lithography Bio-sensors SERS
a b s t r a c t We demonstrate the fabrication of a 2D periodic arrangement of gold square nano-patches on both silicon and borosilicate glass substrates whose optical properties can be exploited for both linear and nonlinear applications. The comparison between numerical and experimental results, combined with highresolution scanning electron microscopy images highlights the importance of the geometry and the fine shape in the determination of the square gold nano-patches optical properties. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, the interaction of light with metallic nanoparticles or nanostructures has attracted a great interest, leading to the development and progress of Plasmonics [1–5]. Plasmons are essentially coupled oscillations of light and free electrons at the metal-dielectric interface. These coupled oscillations, named Surface Plasmon Polaritons (SPPs), are well confined propagating modes, can be efficiently generated at optical frequencies (from the visible to near-infrared range), and are tremendously sensitive to the surrounding environment changes. All these features make them of great relevance for detection and sensing applications [6–14]. The properties of metallic nanostructures include the optical excitation of Localized Surface Plasmon Resonances (LSPRs), strong localization of the energy at the nanometer scale, enhancement of electric fields and resonance wavelength tunability [2,15,16]. The influence of morphology on the optical properties of small metallic nanoparticles has been studied both theoretically and experimentally by several groups [15,17,18–25]. Due to their optical properties, plasmonic nanostructures have been extensively exploited in different fields including photovoltaics [26], extremely sensitive chemical and biological sensing [10,27–29], near-field scanning optical microscopy [29], high-resolution imaging [30,31], Raman spectroscopy, Surface Enhancement Raman Spectroscopy (SERS) [32] and nonlinear effects with rela⇑ Corresponding author. Tel.: +39 (0)805963532. E-mail address: [email protected] (M. Grande). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.172
tively high conversion efficiency [33–34]. In all these applications the plasmonic nanostructures are defined on semiconductor substrates. Recently, there has been an increasing demand on incorporating engineered plasmonic nanostructures that support LSPR in transparent substrates-based photonic devices such as light-emitting diodes (LEDs), semiconductor lasers and photodetectors, to control, enhance and improve their performance [35–43]. The fabrication of these plasmonic nanostructures is typically carried out using electron beam lithography (EBL), which allows realizing nanostructures with very-high resolution, controllability and reproducibility. Well-controlled pattern features are necessary in order to get a plasmonic device exhibiting the desired functions. While e-beam writing is one of the most powerful tool for nanoscale fabrication on semiconductor materials such silicon, its applicability to insulating substrates is a more challenging task because of surface charging effects. Unlike patterning on conducting substrates that dissipate excess charge as the beam passes through the resist, charge is trapped near the surface when the substrate is insulating. This charging causes an unbalanced surface potential of the resist that deflects the beam resulting in pattern distortion and alignment errors. As a consequence of this effect, it is very difficult to get high resolution and high density patterns on insulating substrates such as borosilicate glass substrate. In this paper we demonstrate the fabrication of a 2D periodic arrangement of gold square nano-patches on both silicon and borosilicate glass substrates whose optical properties have been used to implement plasmonic devices for both linear and nonlinear applications.
M. Grande et al. / Microelectronic Engineering 111 (2013) 234–237
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samples have been characterized in both linear and nonlinear regimes. 2. 2D square plasmonic gold nano-patches: fabrication
Fig. 1. Sketch of the 2D square plasmonic gold nano-patches probed by focused light: p, a and w indicate the periodicity, the slit aperture and the metal thickness, respectively.
Two different anti-charging schemes that efficiently prevent charge accumulation during EBL exposures of periodic nanostructures on borosilicate glass substrate and silicon substrates have been exploited. The first scheme consists in the growth of a Chrome (Cr) layer between e-beam resist and borosilicate glass insulating substrate. We used polymethyl-methacrylate (PMMA/MMA) bilayer resist for writing the 2D periodic nanostructures using Raith 150 system with 30 kV acceleration voltage and 10 lm beam aperture that leads to a beam current of 18 pA. The second scheme allows to define 2D periodic square nano-patches on silicon substrate: this structure has been used as a bio-sensing platform to realize plasmonic devices in both linear and nonlinear regime. In this case, a 700 nm-thick PMMA/MMA bilayer resist on silicon substrate has been employed and the e-beam writing has been carried out using the same previous exposure parameters. The quality of the fabricated arrays of nano-patches has been analyzed by means of scanning electron microscope (SEM) inspection. Finally the fabricated
The sketch of the fabricated square plasmonic gold nanopatches is shown in Fig. 1 where p, a and w indicate the periodicity, the slit aperture and the metal thickness, respectively. We fabricated square nano-patches in a 2D square array since this configuration makes the device insensitive to the polarization as reported in Ref. [25]. Moreover the periodicity p is properly chosen in order to design a plasmonic resonance at 633 nm for the following nonlinear measurements. This condition can be achieved fixing the periodicity p equal to 630 nm. The fabrication procedure of the 2D arrangement of gold square nano-patches involved three main processing steps and each one required an optimization and check process. The first step consisted of the optimization of the e-beam writing process on both silicon and borosilicate glass substrates. To define the geometry of our 2D pattern we have used a double layer PMMA/MMA resist, which is a positive resist and allows achieving a high quality lift-off process needed for defining metal nanopatches on both the samples. In particular, we used a thickness of 700 nm of bi-layer resist (PMMA/MMA). Since the borosilicate glass is an insulating substrate, before the deposition of the bilayer resist, we covered it with a few nanometers of Chrome layer in order to avoid the charging effect which causes the deflection of the e-beam during the exposure leading up to the distortion of the pattern. The choice of the Cr layer instead of a different metal is due to the fact that the first one improves the adherence properties of gold layer on the glass. It is worth pointing out that the impact of the Cr adhesion layer on the gold patch optical response is only related to a negligible red-shift of the overall resonance with the increase of the Cr thickness. The 2D plasmonic pattern was written by using a Raith150 e-beam lithography system (equipped with a Gemini Column) operating at 30 kV. A preliminary dose-test was performed to define the optimum layout since the actual size of the pattern is influenced by the electron dose. A proximity error correction (PEC) was also applied to accomplish this target and the final dose was determined through Scanning Electron Microscope (SEM) inspections at 10 kV. Subsequently the sample was developed in a Methyl–Isobutyl–Ketone (MIBK) solution and then rinsed in an Isopropyl alcohol–Methanol mixture. Isopropyl alcohol (IPA) solution was used as stopper for the development. Subsequently the 200 nm-thick gold layer was evaporated by means of a thermal evaporator with a current of 300 A and a deposition rate of 2 Å/s. Finally, a lift-off process in an acetone bath was employed to
Fig. 2. SEM micrograph for the 2D gold nanopatch array on (a) borosilicate glass and (b) silicon substrate. In (b) the resist-evaporated gold layer peeling off is shown during the lift-off process.
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of the SEM has been set equal to 5 kV since the borosilicate glass substrate gives rise to charge effects that limit the inspection time. In Fig. 2(b) the resist-evaporated gold layer peeling off is shown during the lift-off process of the 2D gold nano-patches. 3. 2D square plasmonic gold nano-patches: linear and nonlinear characterization
Fig. 3. Comparison between experimental measurements (purple solid line) and the numerical results for the 2D array on borosilicate glass substrate when the slit aperture a is equal to 120 nm (dashed red line) and 200 nm (dashed-dot blue line), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Reflection spectra when the 2D array is covered by air (cyan) and by IPA (purple). The insets refer to the dark field measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
remove the resist regions thus revealing that the two-dimensional nano-patch array dimensions correspond to the nominal numerical results. Fig. 2(a) and (b) shows the SEM micrographs of the final 2D gold nano-patches on borosilicate glass and silicon, respectively. The inspection of the images reveals square gold nano-patches with desired geometrical parameters. In Fig. 2(a), the accelerating voltage
The fabricated devices have been characterized in the linear regime by means of an optical setup in the VIS-NIR range. The optical setup is equipped with a white light source that is focused on the sample by a 5 microscope objective. An optical iris filters the area corresponding to the 2D nano-patch arrays. Then the reflected light is sent to an integrated spectrometer (Ocean Optics) through a multimode fiber. Fig. 3 shows a good agreement between the simulated and measured reflection spectra of the device on borosilicate glass with p = 630 nm and a = 200 nm. The measurements reveal a dip at about 640 nm that is slightly shifted with respect to the numerical findings. This discrepancy can be related to the rounded corners of the single nano-patch (as shown in Fig. 2(a) and to the gold film roughness that tends to smooth and broaden the resonances of the structure [42]. Moreover the experimental results reveal the presence of the Fabry–Perot (FP)-like state corresponding to the dip at about 840 nm. The simulations also show that the reflection behavior is almost the same when the slit aperture a is shrunk down to 120 nm. This shows that the device is not extremely sensitive to fabrication defects or errors. Fig. 4 shows the reflection spectra of the sample with p = 630 nm and a = 120 nm realized on the silicon substrate (Fig. 2(b)) when the array is covered by air (cyan) and by Isopropyl alcohol (IPA) (purple). The uncovered device shows a plasmonic resonance wavelength (Full Width Half Maximum, FWHM) equal to about 640 nm (20 nm) that shifts at about 870 nm (30 nm) when the IPA is considered. This measured shift allows to calculate a sensitivity value equal to 630 nm/RIU (nIPA = 1.37) and a figure of merit (FOM) equal to 21 nm/RIU. However we also calculated the potential sensitivity of the device by evaluating the spectral shift when an extremely small change of the refractive index is induced at the top surface of the sensor. Our simulations predict sensitivity values up to 1000 nm/ RIU and a corresponding figure of merit (FOM) of 222 RIU1 (FWHM of the resonance is only 4.5 nm) [42], values representing a significant advancement in the improvement of plasmonic sensor devices. Further experimental proof of the sensing capabilities of this device has been provided by the observation of color variations in the diffracted field when a small amount of IPA is introduced
Fig. 5. (a) Raman signal for the sample reported in Fig. 2(b); (b) enhancement factor vs the numerical aperture. The red dashed curve refers to the enhancement factor of the reference structure based on random gold nano-particles as reported in [32]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
M. Grande et al. / Microelectronic Engineering 111 (2013) 234–237
on top of the grating. These measurements have been carried out in the dark field configuration and the images visualized in the eyepiece have been acquired by means of a digital camera. The insets in Fig. 4 confirm the variation of the 2D nano-patch array color from cyan (air) to purple (IPA). Similar reflection spectra and color observation may be also calculated a priori allowing the tailoring of structures that target specific molecules based on the induced refractive index [43]. Finally, we assessed the nonlinear response of the plasmonic devices when the gold nano-patch 2D array is functionalized by means of rigid thiol molecules (4-methoxy-terphenyl-400 -methanethiol, TPMT). Fig. 5(a) shows the Raman signal collected when the 2D array is probed with a 633 nm laser source with a microscope objective having a Numerical Aperture (NA) equal to 0.2. Surface Enhanced Raman Scattering enhancement factors for different NAs are reported in Fig. 5(b). The details on the enhancement factor can be found in reference [32]. As may be inferred from the plot, the SERS enhancement factor decreases when the numerical aperture is narrowed, showing a knee for NA = 0.75. 4. Conclusion Tailoring the optical properties of plasmonic nanostructures requires a complete understanding of the physical parameters that may influence the plasmonic resonances. In this work, we showed that, among the wide variety of plasmonic nanostructures, 2D periodic nano-patches in a square form are very interesting because they can be exploited in linear and nonlinear applications. In particular, in the linear regime, the device can be efficiently used as a sensor with high sensitivity and figure of merit. At the same time, the array can be employed as engineered substrate for Surface Enhanced Raman Scattering measurement achieving an enhancement factor of 2 105 with a 10-fold improvement with respect to other plasmonic nanostructures [32]. We also demonstrated that the 2D plasmonic nano-patches have been realized on both semiconductor (silicon) and transparent (borosilicate glass) substrates. In the last case, by growing a few nanomters-thick Chrome layer between e-beam resist and borosilicate glass insulating substrate, we optimized an anti-charging scheme that efficiently prevents charge accumulation during e-beam exposure of periodic nanostructures on this kind of substrate. The inspection of the images by high-resolution scanning electron microscopy highlighted square gold nano-patches with desired geometrical parameters as required by the design. Acknowledgment M.G. acknowledges partial financial support from the Army Research Office (W911NF-11-10284_1490-AM-01). References [1] H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Springer-Verlag, Berlin, 1998.
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Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
2D plasmonic gold nano-patches for linear and nonlinear applications M. Grande a,⇑, G.V. Bianco b, M.A. Vincenti c, D. de Ceglia c, V. Petruzzelli a, M. Scalora d, G. Bruno b, A. D’Orazio a, M. De Vittorio e,f,g, T. Stomeo g a
Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, Via Re David 200, 70125 Bari, Italy Institute of Inorganic Methodologies and Plasmas, IMIP-CNR, via Orabona 4, 70126 Bari, Italy c AEgis Technologies Inc., 410 Jan Davis Dr., Huntsville 35806, AL, USA d Charles M. Bowden Research Center, RDECOM, Redstone Arsenal, AL 35898-5000, USA e Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Via Arnesano, 73100 Lecce, Italy f National Nanotechnology Laboratory, Istituto Nanoscienze-CNR, Via Arnesano, 73100 Lecce, Italy g Center for Bio-Molecular Nanotechnology, Istituto Italiano di Tecnologia (IIT), Via Barsanti, 73010 Arnesano (Lecce), Italy b
a r t i c l e
i n f o
Article history: Available online 17 April 2013 Keywords: Plasmonics Electron beam lithography Bio-sensors SERS
a b s t r a c t We demonstrate the fabrication of a 2D periodic arrangement of gold square nano-patches on both silicon and borosilicate glass substrates whose optical properties can be exploited for both linear and nonlinear applications. The comparison between numerical and experimental results, combined with highresolution scanning electron microscopy images highlights the importance of the geometry and the fine shape in the determination of the square gold nano-patches optical properties. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, the interaction of light with metallic nanoparticles or nanostructures has attracted a great interest, leading to the development and progress of Plasmonics [1–5]. Plasmons are essentially coupled oscillations of light and free electrons at the metal-dielectric interface. These coupled oscillations, named Surface Plasmon Polaritons (SPPs), are well confined propagating modes, can be efficiently generated at optical frequencies (from the visible to near-infrared range), and are tremendously sensitive to the surrounding environment changes. All these features make them of great relevance for detection and sensing applications [6–14]. The properties of metallic nanostructures include the optical excitation of Localized Surface Plasmon Resonances (LSPRs), strong localization of the energy at the nanometer scale, enhancement of electric fields and resonance wavelength tunability [2,15,16]. The influence of morphology on the optical properties of small metallic nanoparticles has been studied both theoretically and experimentally by several groups [15,17,18–25]. Due to their optical properties, plasmonic nanostructures have been extensively exploited in different fields including photovoltaics [26], extremely sensitive chemical and biological sensing [10,27–29], near-field scanning optical microscopy [29], high-resolution imaging [30,31], Raman spectroscopy, Surface Enhancement Raman Spectroscopy (SERS) [32] and nonlinear effects with rela⇑ Corresponding author. Tel.: +39 (0)805963532. E-mail address: [email protected] (M. Grande). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.172
tively high conversion efficiency [33–34]. In all these applications the plasmonic nanostructures are defined on semiconductor substrates. Recently, there has been an increasing demand on incorporating engineered plasmonic nanostructures that support LSPR in transparent substrates-based photonic devices such as light-emitting diodes (LEDs), semiconductor lasers and photodetectors, to control, enhance and improve their performance [35–43]. The fabrication of these plasmonic nanostructures is typically carried out using electron beam lithography (EBL), which allows realizing nanostructures with very-high resolution, controllability and reproducibility. Well-controlled pattern features are necessary in order to get a plasmonic device exhibiting the desired functions. While e-beam writing is one of the most powerful tool for nanoscale fabrication on semiconductor materials such silicon, its applicability to insulating substrates is a more challenging task because of surface charging effects. Unlike patterning on conducting substrates that dissipate excess charge as the beam passes through the resist, charge is trapped near the surface when the substrate is insulating. This charging causes an unbalanced surface potential of the resist that deflects the beam resulting in pattern distortion and alignment errors. As a consequence of this effect, it is very difficult to get high resolution and high density patterns on insulating substrates such as borosilicate glass substrate. In this paper we demonstrate the fabrication of a 2D periodic arrangement of gold square nano-patches on both silicon and borosilicate glass substrates whose optical properties have been used to implement plasmonic devices for both linear and nonlinear applications.
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samples have been characterized in both linear and nonlinear regimes. 2. 2D square plasmonic gold nano-patches: fabrication
Fig. 1. Sketch of the 2D square plasmonic gold nano-patches probed by focused light: p, a and w indicate the periodicity, the slit aperture and the metal thickness, respectively.
Two different anti-charging schemes that efficiently prevent charge accumulation during EBL exposures of periodic nanostructures on borosilicate glass substrate and silicon substrates have been exploited. The first scheme consists in the growth of a Chrome (Cr) layer between e-beam resist and borosilicate glass insulating substrate. We used polymethyl-methacrylate (PMMA/MMA) bilayer resist for writing the 2D periodic nanostructures using Raith 150 system with 30 kV acceleration voltage and 10 lm beam aperture that leads to a beam current of 18 pA. The second scheme allows to define 2D periodic square nano-patches on silicon substrate: this structure has been used as a bio-sensing platform to realize plasmonic devices in both linear and nonlinear regime. In this case, a 700 nm-thick PMMA/MMA bilayer resist on silicon substrate has been employed and the e-beam writing has been carried out using the same previous exposure parameters. The quality of the fabricated arrays of nano-patches has been analyzed by means of scanning electron microscope (SEM) inspection. Finally the fabricated
The sketch of the fabricated square plasmonic gold nanopatches is shown in Fig. 1 where p, a and w indicate the periodicity, the slit aperture and the metal thickness, respectively. We fabricated square nano-patches in a 2D square array since this configuration makes the device insensitive to the polarization as reported in Ref. [25]. Moreover the periodicity p is properly chosen in order to design a plasmonic resonance at 633 nm for the following nonlinear measurements. This condition can be achieved fixing the periodicity p equal to 630 nm. The fabrication procedure of the 2D arrangement of gold square nano-patches involved three main processing steps and each one required an optimization and check process. The first step consisted of the optimization of the e-beam writing process on both silicon and borosilicate glass substrates. To define the geometry of our 2D pattern we have used a double layer PMMA/MMA resist, which is a positive resist and allows achieving a high quality lift-off process needed for defining metal nanopatches on both the samples. In particular, we used a thickness of 700 nm of bi-layer resist (PMMA/MMA). Since the borosilicate glass is an insulating substrate, before the deposition of the bilayer resist, we covered it with a few nanometers of Chrome layer in order to avoid the charging effect which causes the deflection of the e-beam during the exposure leading up to the distortion of the pattern. The choice of the Cr layer instead of a different metal is due to the fact that the first one improves the adherence properties of gold layer on the glass. It is worth pointing out that the impact of the Cr adhesion layer on the gold patch optical response is only related to a negligible red-shift of the overall resonance with the increase of the Cr thickness. The 2D plasmonic pattern was written by using a Raith150 e-beam lithography system (equipped with a Gemini Column) operating at 30 kV. A preliminary dose-test was performed to define the optimum layout since the actual size of the pattern is influenced by the electron dose. A proximity error correction (PEC) was also applied to accomplish this target and the final dose was determined through Scanning Electron Microscope (SEM) inspections at 10 kV. Subsequently the sample was developed in a Methyl–Isobutyl–Ketone (MIBK) solution and then rinsed in an Isopropyl alcohol–Methanol mixture. Isopropyl alcohol (IPA) solution was used as stopper for the development. Subsequently the 200 nm-thick gold layer was evaporated by means of a thermal evaporator with a current of 300 A and a deposition rate of 2 Å/s. Finally, a lift-off process in an acetone bath was employed to
Fig. 2. SEM micrograph for the 2D gold nanopatch array on (a) borosilicate glass and (b) silicon substrate. In (b) the resist-evaporated gold layer peeling off is shown during the lift-off process.
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of the SEM has been set equal to 5 kV since the borosilicate glass substrate gives rise to charge effects that limit the inspection time. In Fig. 2(b) the resist-evaporated gold layer peeling off is shown during the lift-off process of the 2D gold nano-patches. 3. 2D square plasmonic gold nano-patches: linear and nonlinear characterization
Fig. 3. Comparison between experimental measurements (purple solid line) and the numerical results for the 2D array on borosilicate glass substrate when the slit aperture a is equal to 120 nm (dashed red line) and 200 nm (dashed-dot blue line), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Reflection spectra when the 2D array is covered by air (cyan) and by IPA (purple). The insets refer to the dark field measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
remove the resist regions thus revealing that the two-dimensional nano-patch array dimensions correspond to the nominal numerical results. Fig. 2(a) and (b) shows the SEM micrographs of the final 2D gold nano-patches on borosilicate glass and silicon, respectively. The inspection of the images reveals square gold nano-patches with desired geometrical parameters. In Fig. 2(a), the accelerating voltage
The fabricated devices have been characterized in the linear regime by means of an optical setup in the VIS-NIR range. The optical setup is equipped with a white light source that is focused on the sample by a 5 microscope objective. An optical iris filters the area corresponding to the 2D nano-patch arrays. Then the reflected light is sent to an integrated spectrometer (Ocean Optics) through a multimode fiber. Fig. 3 shows a good agreement between the simulated and measured reflection spectra of the device on borosilicate glass with p = 630 nm and a = 200 nm. The measurements reveal a dip at about 640 nm that is slightly shifted with respect to the numerical findings. This discrepancy can be related to the rounded corners of the single nano-patch (as shown in Fig. 2(a) and to the gold film roughness that tends to smooth and broaden the resonances of the structure [42]. Moreover the experimental results reveal the presence of the Fabry–Perot (FP)-like state corresponding to the dip at about 840 nm. The simulations also show that the reflection behavior is almost the same when the slit aperture a is shrunk down to 120 nm. This shows that the device is not extremely sensitive to fabrication defects or errors. Fig. 4 shows the reflection spectra of the sample with p = 630 nm and a = 120 nm realized on the silicon substrate (Fig. 2(b)) when the array is covered by air (cyan) and by Isopropyl alcohol (IPA) (purple). The uncovered device shows a plasmonic resonance wavelength (Full Width Half Maximum, FWHM) equal to about 640 nm (20 nm) that shifts at about 870 nm (30 nm) when the IPA is considered. This measured shift allows to calculate a sensitivity value equal to 630 nm/RIU (nIPA = 1.37) and a figure of merit (FOM) equal to 21 nm/RIU. However we also calculated the potential sensitivity of the device by evaluating the spectral shift when an extremely small change of the refractive index is induced at the top surface of the sensor. Our simulations predict sensitivity values up to 1000 nm/ RIU and a corresponding figure of merit (FOM) of 222 RIU1 (FWHM of the resonance is only 4.5 nm) [42], values representing a significant advancement in the improvement of plasmonic sensor devices. Further experimental proof of the sensing capabilities of this device has been provided by the observation of color variations in the diffracted field when a small amount of IPA is introduced
Fig. 5. (a) Raman signal for the sample reported in Fig. 2(b); (b) enhancement factor vs the numerical aperture. The red dashed curve refers to the enhancement factor of the reference structure based on random gold nano-particles as reported in [32]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
M. Grande et al. / Microelectronic Engineering 111 (2013) 234–237
on top of the grating. These measurements have been carried out in the dark field configuration and the images visualized in the eyepiece have been acquired by means of a digital camera. The insets in Fig. 4 confirm the variation of the 2D nano-patch array color from cyan (air) to purple (IPA). Similar reflection spectra and color observation may be also calculated a priori allowing the tailoring of structures that target specific molecules based on the induced refractive index [43]. Finally, we assessed the nonlinear response of the plasmonic devices when the gold nano-patch 2D array is functionalized by means of rigid thiol molecules (4-methoxy-terphenyl-400 -methanethiol, TPMT). Fig. 5(a) shows the Raman signal collected when the 2D array is probed with a 633 nm laser source with a microscope objective having a Numerical Aperture (NA) equal to 0.2. Surface Enhanced Raman Scattering enhancement factors for different NAs are reported in Fig. 5(b). The details on the enhancement factor can be found in reference [32]. As may be inferred from the plot, the SERS enhancement factor decreases when the numerical aperture is narrowed, showing a knee for NA = 0.75. 4. Conclusion Tailoring the optical properties of plasmonic nanostructures requires a complete understanding of the physical parameters that may influence the plasmonic resonances. In this work, we showed that, among the wide variety of plasmonic nanostructures, 2D periodic nano-patches in a square form are very interesting because they can be exploited in linear and nonlinear applications. In particular, in the linear regime, the device can be efficiently used as a sensor with high sensitivity and figure of merit. At the same time, the array can be employed as engineered substrate for Surface Enhanced Raman Scattering measurement achieving an enhancement factor of 2 105 with a 10-fold improvement with respect to other plasmonic nanostructures [32]. We also demonstrated that the 2D plasmonic nano-patches have been realized on both semiconductor (silicon) and transparent (borosilicate glass) substrates. In the last case, by growing a few nanomters-thick Chrome layer between e-beam resist and borosilicate glass insulating substrate, we optimized an anti-charging scheme that efficiently prevents charge accumulation during e-beam exposure of periodic nanostructures on this kind of substrate. The inspection of the images by high-resolution scanning electron microscopy highlighted square gold nano-patches with desired geometrical parameters as required by the design. Acknowledgment M.G. acknowledges partial financial support from the Army Research Office (W911NF-11-10284_1490-AM-01). References [1] H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Springer-Verlag, Berlin, 1998.
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