Enhancing Second Harmonic Generation in Gold Nanoring

Letter pubs.acs.org/NanoLett

Enhancing Second Harmonic Generation in Gold Nanoring Resonators Filled with Lithium Niobate Dennis Lehr,*,† Jörg Reinhold,† Illia Thiele,† Holger Hartung,† Kay Dietrich,† Christoph Menzel,† Thomas Pertsch,† Ernst-B. Kley,† and Andreas Tünnermann†,‡ †

Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-University Jena, Max-Wien-Platz 1, 07743 Jena, Germany Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Straße 7, 07745 Jena, Germany

‡

ABSTRACT: Plasmonic nanorings provide the unique advantage of a pronounced plasmonic field enhancement inside their core. If filled with a polarizable medium, it may significantly enhance its optical effects. Here, we demonstrate this proposition by filling gold nanorings with lithium niobate. The generated second harmonic signal is compared to the signal originating from an unpatterned lithium niobate surface. Measurements and simulation confirm an enhancement of about 20. Applications requiring nanoscopic localized light sources like fluorescence spectroscopy or quantum communication will benefit from our findings. KEYWORDS: Lithium niobate, nanorings, second harmonic generation, ion beam enhanced etching, plasmonics

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the substrate is larger than the index of the surrounding medium, the first order low-frequency resonance of the metallic nanostructure exhibits a field enhancement in close vicinity to the particle−substrate interface. Hence, the nonlinear field generated in that region will be strong compared to the nonlinear response of the same substrate without the resonant nanostructure.3,14,15 However, two important aspects have to be kept in mind. First, for an optimum performance the plasmonic resonator has to be carefully designed, where it is obviously advantageous to embed the resonator into the nonlinear material or to place the nonlinear material inside the resonator to maximize the overlap between plasmonically enhanced field and the nonlinear material.12,16 Second, for any kind of application and potential mass fabrication we have to rely on efficient and reproducible fabrication techniques. Selforganization processes to fabricate core−shell particles filled with a nonlinear medium have been developed and demonstrated for this purpose.17−20 Our approach goes beyond by relying on a unique combination of heavily standardized wafer-level processes and electron beam lithography. As nanoparticles, we propose the use of gold nanorings, which are very similar to core−shell particles exhibiting a strong field inside their core.21 Plasmonic nanorings for application at near-infrared (NIR) frequencies can be generated deterministically with high quality and throughput by means of character projection (CP)22 electron beam lithography (EBL) and double patterning (DP),23,24 that is, pattern definition with an area of multiple dm2 is possible

econd harmonic generation (SHG), that is, doubling the frequency of light by interaction with a nonlinear material1 is an optical process with great technological importance. As the required interaction length between light and the nonlinear material is large, devices employing this process tend to be bulky. Integration in switches and optical modulators requires miniaturization of optical components. Designs that are more compact are achieved by optical waveguides. They enhance the nonlinear interaction length through lateral field confinement and, hence, increase SHG compared to bulky systems.2 However, strong nonlinear signals are generated just upon propagation over distances large compared to the wavelength. For a range of recent applications, this lateral miniaturization is not sufficient. Second harmonic light sources within threedimensionally confined volumes at the subwavelength scale are required.3 They have been proposed as nonbleaching localized probes for fluorescence spectroscopy4,5 or as integrated single photon sources for quantum communication.6 Furthermore, such structures provide more flexibility when designing devices because phase matching does not play any role. To realize efficient generation of second harmonics at subwavelength volumes, light confinement and field enhancement as provided by nanoscale resonators is required. Obvious choices for such resonators are metallic nanostructures exploiting plasmonic resonances.7−9 Placing metallic nanostructures on top of nonlinear substrates or embedding them into a material with high intrinsic optical nonlinearity enables high nonlinear responses at moderate power; in the case of second order nonlinearity, lithium niobate (LN) is an excellent choice.3,10−12 In fact, any finite resonant metallic nanostructure simply placed on top of a nonlinear material can provide an enhanced nonlinear response.13 Assuming the linear refractive index of © XXXX American Chemical Society

Received: October 9, 2014 Revised: January 8, 2015

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DOI: 10.1021/nl5038819 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters within hours. By prepatterning the crystalline LN substrate and subsequent fabrication of the metallic nanorings, we obtain nanorings filled with crystalline LN. Within this Letter, we describe the implemented fabrication process to efficiently realize the proposed LN-filled nanorings utilizing the methods mentioned above. To evaluate our samples we performed measurements of the linear as well as the nonlinear properties and compare the generated second harmonic signal to an unpatterned LN substrate. The experiment is complemented with numerical finite difference time domain simulations supporting the experimental results. To confirm our approach and to identify suitable geometrical ring parameters with respect to their linear and their nonlinear optical properties we performed numerical simulations with the free finite difference time domain implementation meep.25 The simulations are performed in three steps. First, the field at the fundamental frequency (FF) is calculated applying a monochromatic plane wave excitation. Second, the nonlinear polarization is calculated using this fundamental field.26 Eventually, the propagation of the second harmonic (SH) fields is simulated upon excitation of the system by the nonlinear polarization. This stepwise calculation of the SH fields is applicable only if the evolution of the fundamental fields can be decoupled from all other harmonics. The decoupling is achieved in the undepleted-pump approximation (UPA)27 neglecting the feedback of higher harmonics on the linear polarization, that is, the nonlinear polarization has to be small compared with the linear polarization at the FF. The UPA is commonly assumed for plasmonically enhanced optical nonlinearities28−30 due to the small volumes of nonlinear interaction and valid within the power range considered here. The hereby studied filled gold nanorings are periodically arranged on a square lattice in the y−z-plane on top of a crystalline LN substrate (x-cut) where tabulated data for the permittivity of gold is taken from ref 31. The rings are characterized by their inner diameter (80 nm), outer diameter (120 nm), height (100 nm), and period (260 nm). The chosen period prevents the occurrence of diffraction orders outside the substrate throughout our study. This is particularly valid for the SH frequencies such that transmission and reflection are fully characterized by the zeroth diffraction order for all frequencies. The nanorings are illuminated preferably with light polarized in z-direction. Here, the electric field of the incoming light and, hence, the dipolar moment of the localized surface plasmon polariton resonance is aligned to the largest principal diagonal element of the second-order susceptibility tensor χ(2) zzz = −8.34 × 10−11 (m/V). This will generate the largest second harmonic signal. The results of the simulation are presented in Figure 1. The reflection obtained for plane wave excitation at the FF (Figure 1b) indicates a pronounced plasmonic resonance at 315 THz (952 nm wavelength). At normal incidence, about 50% of the incoming light is reflected. At this fundamental resonance, the field is mainly concentrated at the nanoring-substrate interface, where a considerable portion of the field is inside the core as well (Figure 1a). The z-component of the electric field, which is dominating the contribution to the SH process, is increased by a factor of 4 to 8 inside the core and peaks at 12 near the lower boundary of the gold ring (Figure 1a). To evaluate the enhancement independently from the illumination intensity, we calculated the SH enhancement12 τ that compares the second harmonic intensity reflected by the gold ring array Irings to the

Figure 1. Simulated linear and nonlinear optical properties of periodically arranged gold nanorings with LN inside their core for zpolarized plane wave excitation (E0). (a) Z-component of the electric field at resonance frequency normalized to electric field of the illuminating wave. The cross sections intersect the center point of the unit cell. Incident frequency, 315 THz; launch angle, 0°. (b) Reflection spectrum for normal and oblique incidence. (c) SH enhancement factor τ for normal and oblique incidence, as well as with and without nonlinearity inside the core.

intensity reflected by the unpatterned surface of the substrate Isub (Figure 1c). Irings τ= Isub Within our simulations, we calculated the SH intensity reflected from the unpatterned substrate analytically.27 We chose the bare, unpatterned LN surface instead of the structured surface, that is, LN pillars without Au rings, as reference of choice as it allows us to determine the B


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Figure 2. Sketches of intermediate fabrication steps for the fabrication of LN-filled nanorings. (a) Resist pillars after EBL. (b) Cr/SiO2 pillars after ICP-RIE as mask for IBEE. (c) Pillars of crystalline LN. (d) LN pillars coated with gold. (e) LN pillars encapsulated with a gold ring after removing gold from all horizontal surfaces by IBE.

Figure 3. (a) SEM Images showing the mask for IBEE (Cr/SiO2 pillars). (b) LN pillars after IBEE. (c) Final pattern: LN pillars encapsulated with a gold ring.

(χ(2) = 0). In this case, the SH enhancement τ breaks down from almost 60 to below 15. This confirms our presumption that filling the rings with LN is crucial for getting a significant enhancement. Note that the results are shown for normal incidence as well as for oblique incidence in TE-configuration (electric field vector parallel to the sample surface). The latter are required for comparison with the experiment because the experimental measurements are performed at oblique incidence to spatially separate light reflected from the front- and backside of the substrate. The resonance position is independent from the angle of incidence and, hence, the resonance is clearly a localized surface plasmon polariton resonance. From our point of view, an efficient and reproducible fabrication technique is a necessity for a real world application. We therefore developed a fabrication process aiming toward this objective. Main steps within our process comprise the creation of crystalline LN pillars by CP-EBL and ion beam enhanced etching (IBEE). CP-EBL ensures high-throughput and pattern accuracy. IBEE allows selective etching of LN while preserving the crystallinity of LN.33 Afterward, DP is utilized, which allows metallization of the pillars to form the nanorings. As the first step within the process chain, the LN substrate is consecutively coated with 250 nm silicon dioxide (SiO2), 20 nm chromium (Cr), and 110 nm chemically amplified negative tone resist (TOK OEBR-CAN034). Resist pillars with a period of 260 nm and a diameter of 120 nm are patterned by EBL (Figure 2a). This is a serial process and requires rasterizing the electron beam across the sample to define the desired pattern. If the patterned area is increased, it easily becomes the most time-consuming and limiting process step within a lithographic process chain. We therefore utilize a Vistec SB350 OS EBL system capable of CP, that is, imaging of a fixed mask into the electron beam resist. This increases the raster size to the size of

enhancement of surface SHG by applying a resonant nanopattern. The physical interpretation of our results would not change by using LN pillars as reference. In fact, the SH enhancement would even become larger, because the SH signal obtained from LN pillars is smaller than the SH signal obtained from the unpatterned surface by roughly a factor of 4. This decrease may be explained by the antireflective behavior of nonresonant subwavelength nanostructure. Please note that the lines connecting the individual monochromatic simulations in Figure 1 are just guides to the eye with unspecified line shapes. Clearly, the SH enhancement τ peaks at the fundamental plasmonic resonance and reaches values of up to 60 for normal incidence. In fact, the resonance position estimated from the linear reflection is at slightly larger frequencies compared to the peak in τ. Furthermore, the resonance width in the linear reflectance spectrum (Figure 1b) is larger than the width of the SH enhancement τ (Figure 1c). However, both the shift as well as the change in width can be attributed to the nonlinear response and the shift between the spectral near field and far field intensities.7,32 The scattering of the data points around the guide to the eye lines close to 300 THz is due to a localized higher order resonance of the LN filled rings at 600 THz. This dip in the enhancement occurs for all four scenarios shown in Figure 1c, that is, at normal as well as oblique incidence. The quite sharp, resonant absorption at 600 THz leads to this decrease of the SH signal and, hence, the SH enhancement, which can be observed solely from the simulations. This as well as other higher order resonances are broadened and eventually invisible in the experiment due to deviations of the fabricated rings from the idealized shape. To investigate the contribution of LN inside the rings to the second harmonic signal, we performed identical simulations but replaced the material inside the rings with an artificial material having identical linear but vanishing nonlinear properties C


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Nano Letters the mask (typically 2.5 × 2.5 μm2) and reduces the overall writing time by up to 3 orders of magnitude compared to classical EBL.22 In our case, a writing time in the order of dm2 in hours is achieved. Next, the resist pattern is transferred into the Cr layer and the underlying SiO2 layer by inductively coupled plasma reactive ion etching (ICP-RIE) using the etchants chlorine and fluoroform, respectively (Figures 2b and 3a). The Cr layer primarily serves as hard etch mask for the SiO2 layer. The resulting SiO2 pattern is transferred into the LN substrate by IBEE. The sample was therefore irradiated with Argon ions with energies of 120 and 60 keV and a fluence of 1015 cm−2 and 1.2 × 1015 cm−2, respectively. The pillar-shaped SiO2 mask is protecting the underlying LN crystal from irradiation damage. In the unmasked region, this irradiation leads to a damage of the crystalline structure from the surface down to a depth of 100 nm. After irradiation, the mask is wetchemically removed. By wet etching of the damaged LNVolume in 50% aqueous KOH at a temperature of 65 °C LN pillars are formed34 (Figures 2c and 3b). To yield gold nanorings the LN pillars are coated with 40 nm gold by means of physical vapor deposition. To coat the pillars homogeneously the sample is tilted by 45° and continuously rotated during evaporation (Figure 2d). Finally, the gold is removed from all horizontal surfaces by ion beam etching (Figures 2e and 3c). With respect to the comparability between our theoretical study and our sample, the dimensions of the fabricated sample were chosen to match the dimensions of the theoretical model. We demonstrated the process on 100 mm fused silica wafer with a patterned area of one cm2. For applications requiring larger areas, our described method and the used tools easily allow the fabrication of multiple dm2 sized substrates. To characterize the fabricated samples and to verify the proposed enhancement scheme we performed optical measurements of the linear reflection spectra and the SH enhancement τ (Figure 4). For the SH enhancement, we measured the SH signal from the nanopatterned surface and compared it to the measured SH signal of the unpatterned substrate. We utilized a

tunable Spectra-Physics Mai Tai Ti:sapphire Laser source. In the appropriate wavelength range, the laser delivers pulses with a duration of about 300 fs. In the whole wavelength range, the average power was set to 10 mW. The beam was focused on the surface using a lens with a focal length of 30 cm. The resulting spot size (full width half-maximum) was about 0.1 mm2. This corresponds to a maximum intensity of 4 GW/cm2 and an average fluence of 10 μJ/cm2. The electric field vector was parallel to the z-axis of the LN crystal, which is the same configuration as in the simulation. We tilted the sample by 45° to measure the signal originating from the nanostructured surface (denoted as “FF1 + SH1” in Figure 4) and separate it from the bulk signal. At this angle, the bulk signal obtained upon propagation through the substrate and possible multiple backreflections at the substrate−air interfaces is bypassing the nanoring array (inset Figure 4). This ensures that no additional SH signals are superimposing the SH signal originating from the nanostructured surface. The FF signals for the linear reflection spectrum and the SH signals for the SH enhancement were measured with the same setup, in particular with the same detector, the same laser source, and the same input intensity, except for the optical filters, which are placed between the sample and the detector. For detecting FF signals, we used neutral density filters just to reduce the detection signal. For detecting the SH signals, chromatic filters are used to block all wavelengths except the corresponding SH wavelength. As a proof we checked the input power dependence of the signals for each wavelength: The FF signals were proportional to the input power and the SH signals increased quadratically by increasing the input power. The input beam passed an optical chopper. The choppered signal was detected using a Hamamatsu Photosensor Module H5784-20 and the Stanford Research Lock-In Amplifier SR850. The spectra (Figure 4) were measured for discrete wavelengths by scanning the center wavelength of the laser pulse with a 5 nm step size, which is slightly more than the spectral bandwidth of the laser. The measured linear spectrum indicates a plasmonic resonance at a frequency of 365 THz (820 nm wavelength). At this resonance, 55% of the incoming light is reflected. This value is close to the simulated spectrum. The resonance position is slightly shifted by about 40 THz to higher frequencies. This minor shift may be attributed to geometrical and dispersive deviations to the model used in our theoretical study. Most notably, the real shape of the fabricated sample does not perfectly match the ideal shape of the model. For example, the footing, which is clearly visible in Figure 3b, is not considered in our simulations. The SH enhancement in the spectral range of the local plasmon resonance, which was found in the simulation, is experimentally confirmed. At 351 THz, the SH enhancement reaches a maximum of τmax = 18, which is close to the expected value of 23 determined by our simulations. These deviations may also be attributed to geometrical properties of the fabricated sample, in particular, sharp edges that are assumed in simulation but chamfered in the sample produce stronger local field enhancement. Thus, a higher SH enhancement is expected from the simulation. Furthermore, the small shift of 10 THz between the maxima in the linear reflection and in the SH enhancement factor, which was observed in the simulation, appears in the experiment as well. From our experimental and theoretical results, we can conclude that the crystallinity of LN inside the rings is in fact

Figure 4. Measured SH enhancement factor and linear reflection spectrum of the fabricated sample. The inset depicts the chosen measurement geometry. The illuminating beam is impinging at an angle of 45°. This way, the surface reflection containing the SH signal of the nanostructure is spatially isolated from the bulk signal that is contained in higher order backside reflections bypassing the nanoring array. D


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preserved during the process. Otherwise, τmax is expected to be less than five because the nonlinearity is decreased considerably upon transition from crystalline to the noncrystalline regime. We also verified that the introduced and performed process is adequate for encapsulation of LN with gold nanorings to enhance SHG plasmonically. In conclusion, we proposed gold nanorings filled with crystalline LN as a nano-optical device to enhance SHG at NIR frequencies. Like self-organized core−shell particles, these nanorings provide the unique advantage of a pronounced field enhancement inside their core, which significantly increases SHG. We developed a fabrication process to provide an efficient and reproducible fabrication technique, a necessity for many applications, which allows the deterministic fabrication of LN-filled nanorings. This is achieved by utilizing methods like character projection electron beam lithography, ion beam enhanced etching, and double patterning. Through our theoretical study, we confirmed our presumption that filling the nanorings with LN is crucial for getting a significant enhancement. Measurements of the linear reflection spectra as well as of the SH signal were performed. They are in good agreement with the numerical simulations, hence, verifying our numerical results and the adequateness of the fabrication process. The measurements of the SH enhancement confirmed the theoretically predicted τmax of about 20 for tilted illumination. At normal incidence, one can expect τmax to reach values of 50 to 60. Moreover, the implemented fabrication process is well suited to perform an effective fabrication with an efficiency of dm2 in hours and allows tuning of the geometrical parameters over a wide range.23,24 Even fabrication of nanorings with spatially varying geometrical properties side-by-side is generally possible. We are confident that our results will directly contribute to the development of efficient nanoscopic localized light sources employing enhanced SHG in subwavelength scale volumes. Beyond that, it is possible to fill the rings with other polarizable materials than lithium niobate, which we assume allows enhancing light-matter interactions at the nanoscale for a broad range of applications.

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ABBREVIATIONS LN, lithium niobate; SHG, second harmonic generation; NIR, near-infrared; CP, character projection; EBL, electron beam lithography; DP, double patterning; IBEE, ion beam enhanced etching; τ, SH enhancement factor; SiO2, silicon dioxide; Cr, chromium; ICP-RIE, inductively coupled plasma reactive ion etching; FF, fundamental frequency; SH, second harmonic

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors contributed equally. Funding

The authors gratefully acknowledge support by the German Federal Ministry of Education and Research (PhoNa 03IF2101A, OpMiSen 16SV5577) and by German Research Foundation (PE 1524/5-2). C.M. gratefully acknowledges financial support by the Carl Zeiss Stiftung. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank D. Voigt, N. Sergeev, W. Gräf, M. Banasch, H. Schmidt, H.-J. Fuchs, W. Rockstroh, F. Schrempel, R. Geiss, and T. Käsebier for their assistance during sample fabrication. E


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Enhancing Second Harmonic Generation in Gold Nanoring

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