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Using low-loss phase-change materials for mid-infrared antenna resonance tuning Ann-Katrin Ursula Michel, Dmitry N Chigrin, Tobias Wilhelm Mass, Kathrin Schoenauer, Martin Salinga, Matthias Wuttig, and Thomas Taubner Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl4006194 • Publication Date (Web): 06 Jun 2013 Downloaded from http://pubs.acs.org on June 13, 2013

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Using low-loss phase-change materials for midinfrared antenna resonance tuning Ann-Katrin U. Michel,§ Dmitry N. Chigrin, § Tobias W. W. Maß, § Kathrin Schönauer, §,† Martin Salinga, § Matthias Wuttig§ and Thomas Taubner*,§ §

I. Institute of Physics (IA), RWTH Aachen University, 52056 Aachen, Germany

*To whom correspondence should be addressed. E-mail: [email protected] KEYWORDS Phase-change materials, infrared nanoantennas, resonance tuning, active plasmonics, active metamaterials, Fano resonance. ABSTRACT We show tuning of the resonance frequency of aluminum nanoantennas via variation of the refractive index n of a layer of phase-change material. Three configurations have been considered, namely with the antennas on top of, inside and below the layer. Phase-change materials offer a huge index change upon the structural transition from the amorphous to the crystalline state, both stable at room temperature. Since the imaginary part of their permittivity is negligibly small in the mid-infrared spectral range, resonance damping is avoided. We present resonance shifting to lower as well as to higher wavenumbers with a maximum shift of 19.3% and a tuning figure of merit, defined as the resonance shift divided by the FWHM of the resonance peak, of 1.03.

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TEXT Plasmonic nanoantennas made from metallic nanostructures have gained tremendous interest during the last years [Lal2007, Neubrech2008, Novotny2009, Altug2009]. If resonantly excited by light, these plasmonic antennas show a strong localized field enhancement in the vicinity of the metal surface and a distinct spectral response [Maier2007]. However, once the antenna geometry, material and substrate are chosen, its resonance wavelength is fixed [Novotny2007]. Active plasmonics and active metamaterials allow for a modulation of optical properties even after the nanostructures have been fabricated [Zheludev2010, Sámson2010_2, Shalaev2012]. Modulating the amplitude of an antenna resonance has been shown by applying a bias voltage to large-area graphene [Emani2012] for example. Different approaches to influence the resonance frequency of plasmonic nanoantennas and split-ring resonators have been presented – e.g. by varying the antenna thickness [Dicken2009, Driscoll2009], by influencing the coupling of the nanostructures [Pryce2010] and by changing the dielectric environment of the nanostructures [Jun2012, Sámson2010_1]. A concept presented in this letter can offer an interesting opportunity of reversibly manipulating the optical response of various nanostructures in a non-volatile way. We propose to change the refractive index of a cover layer to shift the resonance wavelength of aluminum nanoantennas over a spectral range of up to 610 cm-1 in the mid-infrared (mid-IR) spectral range, more precisely from 3766 to 3156 cm-1, and a tuning figure of merit (resonance shift over FWHM) of 1.03. We use nanoantenna arrays below, in between and on top of thin-films of two different materials with the purpose to show the resonance tuning to higher as well as to lower wavenumbers, both with low losses in the amplitude of the resonance. One of these materials, germanium antimony


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telluride (Ge3Sb2Te6 or short GST), is a typical phase-change material (PCM). The second investigated material, indium antimonide (InSb), which is used as a super resolution near field structure (Super-RENS) as a mask layer [Thomson2009], shows characteristics similar to those observed for many PCMs. Recently, PCMs, which are used in rewriteable DVDs [Shi2009] or in Random Access Memories (PCRAM) [Breitwisch2009], have gained interest for active plasmonics [MacDonald2009, Sámson2010_2], active photonics [Pernice2012] and metamaterials [Zheludev2010]. In this letter, we investigate GST or InSb with plasmonic nanoantenna arrays on top, in between or below the PCM layer, schematically shown in Figure 1. To avoid damping of the resonance signal as much as possible, we demonstrate the use of these materials in the mid-IR spectral range, where their absorption losses are negligibly small. Basically, PCMs are characterized by at least one amorphous and one crystalline (meta-) stable phase, whereas the transition between these states can be triggered thermally by an electrical or an optical pulse [Lencer2011] or by thermal annealing. In case of electrical switching, the reversible phase change can occur on a sub-nanosecond timescale, which allows for ultrafast switching [Bruns2009, Loke2012]. Besides, PCMs offer a unique combination of physical properties depending on their phase: a huge contrast of the optical reflectivity as well as the electrical resistivity between the amorphous and the crystalline. A fingerprint for PCMs is the significant change in the bonding situation through phase transition. Upon crystallization the covalent bonding situation in the amorphous phase is replaced by resonant bonding [Shportko2008, Lencer2008]. It seems that the high optical contrast in PCMs seems to have its origin in the resonant bonding of the crystalline phase. The presence of this bonding situation seems to coincide with a material qualifying as PCM. In this sense InSb does not qualify as a PCM. However, due to the thermally triggered phase


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transition from amorphous InSb to its crystalline state and the corresponding changes in the permittivity, in the mid-IR spectral range InSb behavior is comparable to many PCMs. In what follows, we will call InSb a “non-standard PCM”. In this letter we show that InSb could be a very interesting alternative for the application for active plasmonics. Thus, for this purpose, we call InSb a “non-standard PCM” here. Figure 2 displays the dielectric functions for amorphous “a-” and crystalline “c-”GST and InSb. In Figures 2A and 2B it can be seen, that for both materials the real part ε1 is large in the investigated mid-IR spectral range compared to values typically achieved for PCMs in the visible spectral range [Shportko2008]. Furthermore, the mid-IR spectral range is below the optical bandgap EG observed for these materials, which leads to a negligibly small imaginary part ε2 in the investigated frequency range. In Table 1 characteristic values for ε1 and ε2 at 3000 cm-1 can be found. Using these dielectric properties we show distinct resonance frequency shifting of plasmonic nanoantenna arrays in direct contact with thin-films of InSb and GST when changing the state of the material by thermal annealing. The plasmonic nanoantenna arrays employed in this letter were engineered to support plasmon excitations at mid-IR frequencies. Antenna length and lattice period have been chosen in a way providing spectrally narrow collective excitation resonance due to the Fano interference between antenna resonances and Wood’s anomaly of the lattice [Teperik2012] (cf. SI). We fabricated arrays of antennas with different length L (600 nm, 500 nm and 400 nm), whereas each antenna array consists of 3200 nanorods. The distances between antennas were optimized to achieve sharpest plasmonic resonances. Geometrical dimensions of the nanoantennas are summarized in Supporting Figures I and III. Thin-films of a-InSb and a-GST (both 50 nm thickness) were sputtered onto the silicon wafers, which are structured before, after or between sputtering (pressure: 5.3 * 10-3 mbar)


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depending on the sample layouts shown in Figure 1A. We chose to use aluminum nanorods, which were fabricated via electron beam lithography and thermal evaporation (cf. SI). For crystallization the samples are annealed under cleanroom conditions on a hot plate (under ambient air). Since the phase transition temperature for GST is about 160°C [Sittner2012], the thin-films are crystallized for 30 min at 180°C to follow the same treatment as for the thin-film samples from which the dielectric data has been determined. The same annealing conditions are used for InSb, which crystallizes at about 172°C [Herpers2009]. In the literature gold is often used for plasmonic nanoantennas [Neubrech2008, Altug2009, Weber2011]. As shown in Figure 1, the phase-change layers are in direct contact with the nanorods. Strong gold diffusion into the PCM layer during thermal crystallization justifies the use of aluminum as an alternative nanoantenna material, since negative bonding enthalpy has been found for AuX2 compounds (X = In, Sb, e.g.) [Boyen1995]. Furthermore, aluminum in plasmonics has a high potential regarding industry compatibility, since it is the most commonly used metal in silicon-based very-large-scale integration (VLSI) [Johnson2006]. We used 50 nm thin cover layers to avoid the occurrence of thin-film interference effects, which originate from the high refractive index of the PCMs. Thin-film interferences are already strong for film thickness larger than 100 nm [Sámson2010_1]. Such effects could distort the width and shape of the reflectance peaks and will be discussed elsewhere. In Figure 3 experimental Fourier-transform infrared (FTIR) and simulated spectra for both InSb and GST before and after thermal annealing are displayed. Exemplary, we show the results for nanorods with length L = 600 nm and a PCM layer thickness of 50 nm. For GST, the sample geometry matches with the one shown in Figure 1A, design 1. For InSb, the aluminum nanoantennas height is 60 nm. To obtain the individual response of the different arrays we used a conventional FTIR microscope in reflectance mode. In all spectra the


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polarization of the incident light is parallel to the long axis of the antennas. The same aperture of about (40 x 27) µm2 has been used for all measurements displayed in this letter. For a phase-transition from the amorphous to the crystalline state of InSb a clear blue-shift of the resonance peak from 2093 cm-1 to 2203 cm-1 can be detected. In contrast, the resonance frequency for GST red-shifts from 2096 cm-1 to 1869 cm-1 for antennas below GST (cf. Figure 1A, design 1). (For easier comparison with other studies all resonance frequency positions and shifts are shown in Supporting Tables I to III.) The stronger frequency shifts for GST compared to InSb is a result of the bigger contrast in ε1 for GST, which is visible in Figure 2. At about 3000 cm-1 we find ∆ε1(GST) = 27.6 and ∆ε1(InSb) = 9.0. Experimental spectra are compared with numerical simulations in Figures 3B and 3D. Calculations have been performed using finite-difference time-domain method (Lumerical Solutions) and the finite integration method (CST Microwave Studio) based on the dielectric permittivity data measured on thin-film samples (cf. SI). The simulations displayed in Figures 3B and 3D confirm the shifting direction. Deviation between measured and calculated absolute values of the resonance frequency shift originates from the simplifications of the modeled structure, simplified shape, neglected adhesive chromium layer as well as the oxide layers (silicon oxide, aluminum dioxide and the natural oxide of the PCM). Broadening of the measured spectra compared to the calculated spectra could be assigned to the samples imperfections, which include inhomogeneities in the structure dimensions. Simulations of the spectral shift demonstrate better agreement with experiment in the case of InSb structure. Regarding the GST structure, numerical data serves as an upper bound of the shift for a given geometry due to possible overestimation of the index change of the GST. Lattice disorder presented in the GST layer combined with the nanoantennas could lead to a


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considerable difference in the optical as well as electronic properties in comparison with the homogenous GST film [Sa2010, Siegrist2011]. In Figure 4 the spectral shifts of the resonance are summarized for the nanoantenna arrays of different configurations (cf. Figure 1A) for GST as PCM layer. The spectral position of the resonance shifts towards smaller wavenumbers for the antenna arrays with GST layer as compared to the array on Si substrate only. Moreover one can see a frequency shift of 228 cm-1 (12.2%), 241 cm-1 (11.8%) and 370 cm-1 (17.0%) (Figure 4.1 – 4.3) for 600 nm long antenna below, inside and on top of a 50 nm GST film respectively. The numerical results show reasonable agreement with the experimental data demonstrating systematically overestimated values of spectral shift, as discussed above. To quantify the resonance switching a tuning figure of merit (FOM) is introduced [Dicken2009]:

∆

.

The larger the FOM is, the stronger is the shift relative to the width of the resonance peak. For samples with 50 nm GST on top of aluminum antennas with a height of only 30 nm (cf. Figure 1A, sample design 1) a tuning FOM of 0.47 can be achieved for 600 nm antenna length L. Decreasing the antenna length L leads to higher FOM: for L = 400 nm a tuning FOM of 0.56 has been measured. The tuning FOM can be further improved by positioning antenna arrays inside of a 50 nm GST layer as shown in Figure 1A. Here, we find a tuning FOM of up to 0.66 for L = 400 nm. The best results have been obtained with the antennas on top of the 50 nm GST layer, were we measured a tuning FOM between 0.87 (L = 600 nm) and 1.03 (L = 400 nm). An accurate positioning of the antenna within PCM layer results in the optimal overlap of the field induced in the antenna with the PCM material leading to the


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strongest environmentally controlled resonance shift. Numerical calculations of the electric field intensity of the first resonant mode of considered nanoantennas are shown in Figure 5. One can see that moving antenna towards the top interface of the GST film leads to stronger field/PCM overlap. According to the achieved tuning FOMs, a combination of a 50 nm GST layer and an optimized antenna configuration leads to a strong shift of sharp antenna resonance peaks. Compared with the results of [Dicken2009], where the FOM of an ordinary split-ring resonator (SRR) array of 0.08 and the FOM for a nanowire coupled SRR array of 0.14 have been found, we achieved a significantly larger ratio of the tuning range to the FWHM of the resonant peaks for our sample system. In [Pryce2010] a maximum value of about 0.5 for FOM was shown for the advanced nanostructure of an array of gold bars coupled to SRRs on an elastomeric substrate under 50% strain. This emphasizes the relevance of the results for the tuning figure of merit achieved with our relatively simple structure of an aluminum nanorod array. In Figures 6 and 7 experimental resonance spectra are shown for both 50 nm InSb and GST with various antenna lengths, decreasing from panel A to C. The shorter the antennas, the higher the resonance wavenumber and the more pronounced are the frequency shifts. For L = 400 nm antenna length and 50 nm GST thickness (Figure 7C), a tremendous frequency shift of about 610 cm-1 is achieved, which corresponds to change in the resonance position of 19.3%. The best achieved shift for InSb structure is 172 cm-1 (5.9%) for L = 400 nm. In conclusion we introduced a concept, which can lead to non-volatile and reversible low-loss resonance switching of plasmonic nanorods in the mid-IR spectral range by varying the refractive index of a PCM layer next to these nanorods. GST, a standard example for PCMs, and InSb, which for our application shows similar behavior, offer a very strong contrast in the


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real part of the dielectric data due to phase transition from the amorphous to the crystalline state. Since the imaginary part of the dielectric function in both phases is very small, damping of electromagnetic waves because of absorption has been minimized. With the two exemplary materials GST and InSb we presented resonance tuning to lower as well as to higher wavenumbers with a maximum shift of about 19.3%. Additionally we achieved an extremely large ratio of the tuning range to the FWHM of the resonant peak for our sample system compared to nanowire coupled SRR arrays in the literature. The demonstrated tuning could be extended to become reversibly and ultra-fast (nanosecond timescale) [Wuttig2012] by implementing sample layouts feasible for electrical or optical switching. For the latter the layout needs to be adjusted to fit a beam diameter of the switching laser. A promising example could be a plasmonic resonator (dipole antenna, SRR) gap filled with the PCM [Padilla2006]. Electrical switching is much more challenging since appropriate optically transparent electrical contacts should be realized. For future work a detailed study of the temperature-dependent switching properties especially for c-GST is of great interest. The crystalline (meta-) stable phases of ternary PCMs can be used to switch the resonance frequency in multiple steps.

FIGURES

TOC Graphic. Graphic for the Table of Contents.


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Figure 1. A – Sample schemes with 30 nm high aluminum nanoantennas, including the 2 nm high chromium adhesive layer and a 50 nm phase-change material (PCM) layer. The thin natural oxide layers (SiO2, Al2O3, PCM-Ox) are shown as darker lines on top of all different layers. 1 – Aluminum antennas below a PCM layer. 2 – Antennas embedded in a PCM layer. 3 – Antennas on top of a PCM layer. B – Scanning electron microscopy (SEM) image of aluminum nanoantennas. Scale bar: 400 nm. C – Scheme to show the periodic distances dx and dy between the antennas in both in-plane directions.


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Figure 2. Real (solid) and imaginary part (dashed) of the dielectric function depending on the wavenumber and wavelength for GST in A and C and for InSb in B and D [Kremers2009, Herpers2009]. In the investigated mid-IR spectral range (C, D), which is marked with grey vertical lines in A and B, the difference ∆ε1 is huge, whereas ε2/ε1 is negligibly small.


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Figure 3. Antenna resonance shifting characteristics: experimental (solid) and simulated (dotted) reflectance spectra for the amorphous (black) and crystalline (red) phase of A, B GST and C, D – InSb. The antenna length L = 600 nm and the PCM layer is on top of the antennas. An arrow indicates the shifting direction. For GST a red-shift of 228 cm-1 and for InSb a blue-shift of 110 cm-1 is observed. The small absorption peak at around 2350 cm-1 is due to atmospheric CO2 absorption.


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Figure 4. The simulated (dotted) and experimental (solid) antenna resonance shifting characteristics of antenna arrays (L = 600 nm) with 50 nm a-GST (black) and c-GST (red) for different sample layouts, described in Fig. 1 for 1 - 3. With antennas on top of GST (1), the strongest shift can be detected. The small absorption peak at around 2350 cm-1 is due to atmospheric CO2 absorption.


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Figure 5. Absolute value of the first resonant mode induced in the nanoantenna array for different vertical positioning of the antenna (1- below, 2 – in between and 3 – on top GST layer). All dimensions as in Figure 1A. One can see that strongest overlap between field and PCM is achieved for the antenna on top configuration.


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Figure 6. Experimental antenna resonance shifting for various antenna lengths L: A - 600 nm, B - 500 nm and C - 400 nm of 50 nm InSb (Al nanorods with 60 nm height and 50 nm thick InSb film on top). For increasing wavenumber and decreasing antenna length, the shifting range increases. This is related to the increasing difference in ε1 between the amorphous (black) and crystalline (red) phase, as it can be seen in Fig. 2D.


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Figure 7. Experimental antenna resonance shifting for various antenna lengths L: A - 600 nm, B - 500 nm and C - 400 nm on top of 50 nm GST corresponding to the sample geometry displayed in Figure 1A, sample layout 3. The resonance shift increases for decreasing antenna length as described in Figure 6.


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TABLES

GST

InSb

εa-PCM

12.8 + 0.01i

24.8 + 0.3i

εc-PCM

40.0 + 0.8i

15.1 + 0.8i

∆ε1, a → c

27.2

9.7

(ε2/ε1)a-PCM

< 0.001

≈ 0.012

(ε2/ε1)c-PCM

≈ 0.020

≈ 0.053

Table 1. Dielectric data for GST and InSb at about 3000 cm-1.

ASSOCIATED CONTENT – SUPPORTING INFORMATION (SI) Fabrication of the aluminum nanoantenna including SEM micrographs; numerical simulations with Lumerical Solutions and CST Microwave Studio; the imaginary part of the dielectric functions of GST and InSb in the evaluated spectral range; further discussion of the spectra and wavenumbers and wavelengths for the peaks of the measured reflectance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author [email protected] Present Addresses †

PGI-3, FZ Jülich, 52425 Jülich.

ACKNOWLEDGMENT We acknowledge financial support from the Excellence Initiative of the German federal and state governments, the Ministry of Innovation of North-Rhine-Westphalia, the DFG under SFB 917 and the Jülich Aachen Research Alliance for Fundamentals of Future Information Technology JARA-FIT. We are grateful to Dr. T. Wang, M. Wimmer, M. Käs, F. Lange, P. Li, J. M. Hoffmann, B. Hauer and S. Mohrhenn for valuable discussions. The authors thank S. Kremers and A. Herpers for providing the dielectric data and S. Hermes, P. Lingnau and M. Smeets for help with the sputtering.


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