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Abstract
We systematically analyzed the magnetodielectric resonances of Se colloids for the first time in an attempt to utilize them as building blocks for all-dielectric optical metafluids. By taking advantages of the synergistic properties of Se colloids, including their (i) high-refractive-index at optical frequencies, (ii) unprecedented structural uniformity, and (iii) ready availability, we were able to observe Kerker-type directional light scattering, resulting from the efficient coupling between strong electric and magnetic resonances, directly from Se colloidal suspensions. Thus, the use of Se colloids as a generic magnetodielectric building block suggests the opportunity for the production of fluidic low-loss optical antennas, which can be processed via spin-coating and painting.
© 2017 Optical Society of America
1. Introduction
Over the last decade, all-dielectric metamaterials have held a special position among known nanophotonic systems because strong electric and magnetic resonances can be simultaneously induced from their simply-shaped, individual meta-atoms (e.g., spheres and cylindrical posts) [1–26]. More importantly, as depending on the dielectric Mie resonance within the high-refractive-index structure, the optical loss can be remarkably reduced. In stark contrast, their plasmonic counterparts, which are considered as another promising material for nano-optics, requires complexed or structured meta-atoms (e.g., split ring resonators or plasmonic clusters), especially to obtain magnetic resonances at optical frequencies. Furthermore, they face unavoidable and significant Ohmic loss problems [25,26]. In the early stages, the lithographic definition of subwavelength-scaled monolithic structures made of silicon (Si), germanium (Ge), and gallium arsenide (GaAs) were a representative class of methods in the tool set of optical engineers; indeed, the subsequent realization of various intriguing and low-loss properties, including index-near-zero, high-refractive-index, directional scattering, Fano resonance, and magnetic mirror effects have all benefitted from recent advances in top-down lithography [1,5,10,11,16–20,23,27].
The rapidly growing field of all-dielectric nano-optics could be further advanced by expanding the range of accessible structural motifs and fabrication methods. As a representative example, the use of high-refractive-index colloidal nanospheres (NSs) as magnetodielectric building blocks has been viewed as a versatile, yet pivotal strategy for low-loss optical magnetism [3,4,6,9,13–15,21,22,24]. For example, individual Si or boron (Br) NSs themselves can exhibit Kerr-type directional scattering due to the strong coupling between their electric and magnetic resonance modes [4,8,9,13–24,25,27]. More importantly, a self-assembled Si NS cluster with an ultra-small gap (less than 5 nm), which is a structure that is difficult to achieve via top-down lithography, was found to be essential for highly-directional Fano resonances and electromagnetically-induced transparency (EIT) [21,22,24]. In this case, the strongly-confined electric dipole in the gap between Si NSs should interact with the induced magnetic dipoles within each Si NS [22]. However, contrary to low-refractive-index NSs (e.g., silica and polymer), high-refractive-index NSs have proven to be very difficult to attain by means of chemical synthesis [14,15]. Alternatively, physical synthesis via femtosecond laser ablation has promised compelling advantages in obtaining high-refractive-index NSs; a proof-of-concept demonstration of low-loss magnetodielectric resonances has been reported with this method [4,9,13,21,22,24]. Nevertheless, both the intrinsic irregularity in the obtainable NPs and an inability to generate NPs on a large scale remain as obstacles that prevent widespread usage of the laser ablation method. Very recently, L. Shi et al, successfully synthesized relatively uniform Si colloids by vapor decomposition combined with high-temperature annealing [14,15]. However, the vapor decomposition method resulted in the hydrogenation of amorphous Si (a-Si:H) rather than pure Si [28]; consequently, the a-Si:H NSs tend to have 50 − 70% the refractive index of pure Si NS. Of course, high-temperature annealing allows the refractive index of a-Si:H NSs to be enhanced (e.g., up to 75 − 90% the refractive index of pure Si) [15]. However, a post annealing process will give rise to volume shrinkage of the a-Si:H NSs, which degrade the structural fidelity of the self-assembled Si NS cluster or superlattice [15]. Also, this solid-state annealing process can hinder the large-scale generation of uniform Si NSs in a fluid phase (colloidal suspension). Thus, it is difficult to generalize this strategy for immediate practical utility.
In this work, we propose that selenium (Se) colloidal nanoparticles (NPs) can be used as a truly uniform and efficient magnetodielectric building block for optical metamaterials. It is well known that highly-smooth and uniform Se NSs can be synthesized on a large scale by a one-step solution reaction at room temperature [29]. More importantly, the refractive index of Se (i.e., ~3.0 at the visible domain) is sufficient to induce profound optical magnetism. Simultaneously, the optical loss is negligible, especially at wavelength above 600 nm [30]. Nevertheless, there have been no reports investigating the use of the chemically synthesized Se NSs as low-loss magnetodielectric building blocks. Even if Mie resonance of Se NPs obtained by the laser ablation was studied, these Se NPs were not so spherical and uniform [31]. Here, we utilized the synergistic advantages of Se NSs to address the challenge of obtaining highly-uniform and low-loss magnetodielectric NPs and widen the scope of optical metamaterials. In essence, the unprecedented uniformity of Se NSs, which are homogeneously dispersed in water with a high volume fraction of at least 1% allowed us to observe strong electric and magnetic resonances, exactly matching the theoretical predictions. Thus, we found that Se NS suspensions can act as all-dielectric optical metafluids. Furthermore, highly-directional scattering was observed directly from this Se colloidal metafluid, thereby providing a platform for a liquid-state, low-loss optical antenna; this can be solution-processed via spin-coating and painting.
2. Results and discussion
2.1 Merits of Se colloids as a building block for all-dielectric metafluids
Figure 1(a) compares the refractive indices between crystalline Si, amorphous Se, and thermally-annealed a-Si:H. Here, we dealt with the refractive index of annealed a-Si:H, because this material has recently become a representative colloidal NP material which exhibits strong optical magnetism [15,25,26]. According to the previous works reported by L. Shi et al., high-temperature annealing of Si0.75H0.25 and Si0.60H0.40 at 600 °C increases the refractive indices up to 90% and 75% that of pure crystalline Si [15]. As shown in Fig. 1(a), the refractive index of Se can be sufficiently high to induce strong electric and magnetic resonances, even if it is slightly less than that of annealed a-Si:H NSs. Also, Se exhibits negligible optical loss at wavelengths above 600 nm, while the real part of its refractive index is non-disperse [30]. Thus, Se could be a promising material for direct use as a low-loss magnetodielectric building block.
Fig. 1 (a) Real and imaginary parts of the refractive indices for crystalline Si, amorphous Se, and annealed amorphous hydrogenate Si (a-Si:H). The refractive indices of amorphous Se and annealed a-Si:H were obtained from ref [15]. and ref [30], respectively. (b) Scanning electron microscopy (SEM) image of 230 nm Se nanospheres (NSs). The two inset images correspond to the magnified SEM image of 230 nm Se NSs and macroscopic images of the 190 nm, 230 nm, and 300 nm Se colloidal suspensions. (c)-(k) A collective set of optical microscopy images for (c)-(e) 190 nm, (f)-(h) 230 nm, and (i)-(k) 300 nm Se NSs. Here, Se NSs were placed onto a solid-state substrate (glass). Each row consists of reflective dark field (DF), reflective bright field (BF), and transmissive BF optical microscopy images (from left to right).
More importantly, highly-uniform and smooth Se NSs can be chemically synthesized on a large scale by a one-step, full-solution process at room temperature; this process also allows for exquisite control over the NS size (hereafter, size indicate diameter rather than radius) [29]. In this work, we used the controlled reduction of selenious acid with the assistance of hydrazine at room temperature; amorphous Se colloids were synthesized by precipitations of the reduced Se (Appendix 1). The size of the synthesized Se colloids ranged from 190 nm to 350 nm. The as-synthesized Se colloids were purified with deionized water via repetitive centrifugation and washing.
Figure 1(b) shows a representative scanning electron microscopy (SEM) image of 230 nm Se NSs together with macroscopic images of the 190 nm, 230 nm, and 300 nm Se colloidal suspensions (SEM images for 190 nm and 300 nm are included in Appendix 2); we figured out that highly-uniform Se NSs with smooth surfaces were indeed obtained (size distribution was less than 3%). Evenly distributed scattering colors from differently-sized Se NSs were observed in optical microscopy images as shown in Figs. 1(c)-1(k). A collective set of reflective dark field (DF), reflective bright field (BF), and transmissive BF optical microscopy images for 190 nm (see Figs. 1(c)-1(e)), 230 nm (see Figs. 1(f)-1(h)), and 300 nm Se NSs (see Figs. 1(i)-1(k)) are included as representative examples.
This versatile access to large amounts of high-quality Se NSs is in stark contrast to the accessibility of a-Si:H NSs. In general, a-Si:H NSs colloids are synthesized by the decomposition of Si3H8 under high-temperature and supercritical conditions [14,15,28], which are not easy to access (especially at universities). More importantly, the as-synthesized a-Si:H colloids need to be annealed at high temperatures (e.g., 600 C°) to increase their refractive index; this solid-state annealing process severely restricts their available quantities. The volume shrinkage of a-Si:H NSs is another challenge caused by the solid-state annealing process, because this results in the deformation of the self-assembled colloidal structure [15].
The unprecedented uniformity and smoothness of colloidal Se NSs can be further elucidated by UV/VIS absorption spectroscopy. As this analysis reflects the aqueous ensemble response of the Mie resonance [32], the uniformity of our Se NSs can be spectrally quantified. The left panels of Figs. 2(a)-2(c) correspond to the numerically-predicted Mie resonant behaviors of 190 nm, 230 nm, and 350 nm Se NS suspensions; it is clear that the UV/VIS absorption spectra for each Se NS suspension exactly matches with these theoretical predictions (i.e., the width and peak position), as shown in the right panels of Figs. 2(a)-2(c). Particularly, all hallmarks of the Mie resonances, including the magnetic dipole (MD), electric dipole (ED), magnetic quadrupole (MQ), and electric quadrupole (EQ), were clearly visible. The modal characteristics of the MD, ED, and MQ resonances of 230 nm Se NS are summarized in Fig. 2(d). The good agreement between the UV/VIS absorption spectra and the numerical simulation results provides strong evidence of the high uniformity and smoothness of colloidal Se NSs. The high quality of these materials affords the opportunity to use Se colloids as alternative but highly-efficient magnetodielectric building blocks.
Fig. 2 (a)-(c) Magnetodielectric resonances of (a) 190 nm, (b) 230 nm, and (c) 350 nm Se NSs; all of the NSs are dispersed in water. In each part, the left panels correspond to the theoretical analyses done by numerical simulation, while the right panels indicate the results of UV/VIS absorption spectroscopy. (d) Modal analysis of the magnetic dipole (MD), electric dipole (ED), and magnetic quadrupole (MQ) for 230 nm Se NSs dispersed in water. The position of the ED can be found in Fig. 6(b). Here, light was assumed to be illuminated along the z-axis (from top to bottom).
2.2 Degradation of Se NSs during solid-state optical characterizations
It is not surprising that the relatively low glass transition temperature (Tg) of Se (~31 °C) [29] may result in degradation of the structure during optical characterizations. Because high-refractive-index nanocavities such as Se NSs can be effectively heated up via strong Mie resonance, the temperature of a nanocavity undergoing resonance can be increased beyond the Tg of Se. The absorption and scattering spectra of an aqueous Se NS suspension were found to be unchanged during repetitive optical measurements (e.g., UV/VIS spectroscopy). It seems that liquid water can serve as a cooler. However, it was turned out that the Se NSs placed onto the solid substrate (i.e., lossy glass) were degraded during reflective DF spectroscopic measurements (see Fig. 3(a) and Appendix 3). In our common measurement (where the intensity of the light source was 10 mW/m2), the DF scattering spectrum of an individual 230 nm Se NS, obtained at the first measurement (see 0 min in Fig. 3(a)), was already deformed compared to the numerical prediction (i.e., Fig. 3(b)). As the light illumination time increased, these DF spectra along with the DF scattering colors were further deformed. After 5 hours, the intensity of the spectral peak at 650 nm (originating from magnetic resonances) was significantly reduced. In contrast, the DF scattering spectra of an individual gold (Au) NS were not significantly changed after long illumination times, as shown in Figs. 3(c)-3(d): some distributions of ED resonant peak height originated from vibrational errors. From these results, we concluded that the use of Se NSs as a magnetodielectric building block for all-dielectric metamaterials should be limited to a fluidic platform, collectively referred to as metafluids [32,33].
Fig. 3 (a) DF scattering spectra of 230 nm Se NSs as function of the light illumination time. Insets indicate DF optical microscopy images. (b) Theoretical analyses of DF scattering for 230 nm Se NSs placed on a solid-state glass substrate. (c) DF scattering spectra of 75 nm Au NS as a function of the light illumination time. Inset indicates DF optical microscopy image of 75 nm Au NSs placed on a solid-state glass substrate. (d) Theoretical analyses of the scattering cross section (SCS) for 75 nm Au NS placed on a solid-state glass substrate. The contributions of the ED, MD, and EQ to the scattering cross section are numerically analyzed.
2.3 Se NS suspension for an all-dielectric optical metafluid exhibiting directional light scattering
Despite their intrinsic limitations in solid-state nanophotonic devices, Se colloids are still attractive due to their high uniformity and ready access to copious quantities; this makes them promising materials for use as magnetodielectric building blocks in all-dielectric optical metafluids. To exploit this possibility in greater detail, we made highly-concentrated 230 nm Se NS solutions in water and measured the DF and BF scattering spectra (see Figs. 4(a)-4(c)). In Visualization 1 and Visualization 2, we can see the distinct Brownian motions of each individual Se colloid. As was also observed in the DF optical microscopy images of dried Se NSs (see Figs. 1(c)-1(k)), nearly the same scattering colors in the reflective and transmissive DF optical microscopy images were observed across all of the Se NSs moving in water (see Figs. 4(a)-4(c)). Thus, a more detailed study on the magnetodielectric resonances of Se NS (e.g., directional light scattering) can be achieved by measuring the ensemble optical response of Se colloidal suspensions, as follows.
Fig. 4 (a)-(b) Reflective and (c) transmissive DF images of 230 nm Se NSs dispersed in water (230 nm Se NSs metafluids). (d) Schematic for the measurement setup of the BF transmission spectra. (e)-(f) Transmissive BF spectra of 230 nm Se NSs metafluids (e) without and (f) with a cross-analyzer. (g) Schematic for the measurement setup of the DF transmissive (i.e., forward scattering) and reflective (i.e., backward scattering) spectra. (h) Numerically-simulated scattering cross section (SCS) and (i) experimentally-measured forward/backward scattering spectra of 230 nm Se NSs metafluids. These spectra were obtained from an ensemble of numerous 230 nm Se NSs dispersed in water. (j) Theoretically-analyzed far-field scattering pattern as a function of the wavelength. Here, the source light was assumed to be illuminated along the z-axis (from right to left).
In principle, the trend observed in the BF transmission spectrum of the magnetodielectric structure (see Fig. 4(d)) is inversely proportional to that of the total extinction cross section [15]; indeed, the measured BF transmission spectrum of the 230 nm Se colloidal suspension showed the expected trend and included the hallmarks of an MD (i.e., a dip at 665 nm) and MQ (i.e., a dip at 570 nm), as presented in Fig. 4(e). The insertion of a cross-analyzer in the BF transmissive pathway minimizes the electric responses [34]; isolating the magnetic resonances in the BF transmission spectrum. As shown in Fig. 4(f), one distinct peak at 665 nm was observed after inserting the cross-analyzer; this matches well with the theoretically-predicted MD scattering cross section (see Appendix 4). Evidence of the MQ was not visible in the cross-analyzer-filtered BF transmission spectrum due to its significant loss at 570 nm (see Fig. 2(b) and Appendix 4). Therefore, we can conclude that the Se colloidal suspension itself can act as an all-dielectric metafluid, showing strong magnetism at optical frequencies.
In addition, the directivity of light scattering from the 230 nm Se colloidal suspension can be quantized by comparing the DF scattering spectra in the transmission and reflection modes (see Fig. 4(g)) [13]. For Si NSs, the wavelengths of the electric and magnetic resonances were relatively far from each other, which is mainly due to the high refractive index over 3.5 at the wavelength of interest (see Appendix 5) [13]. Thus, the directivity of light scattering from Si NSs is not so high. In the case of our Se NSs, the MD and ED can be less confined compared with those of Si NSs, as shown in Appendix 5. In other words, both the MD and ED resonances become more leaky and broad. Thus, the MD and ED resonance modes can be well overlapped (see Fig. 6), which allows light scattering from Se NSs to be highly directional, as predicted by numerical simulations (see Fig. 4(h)) [24]. The experimental comparison between the transmissive (forward) and reflective (backward) DF scattering spectra validated our theoretical analysis (see Fig. 4(i)). In this measurement, we confirmed that the intensities of incident light in both forward and backward directions were same. The numerically-analyzed far-field scattering pattern shown in Fig. 4(j) confirmed the experimental spectral results of directional scattering. To the best of our knowledge, this is the first time that directional light scattering has been observed directly from a magnetodielectric colloid suspension (i.e., an optical metafluidic antennas).
3. Conclusion
We reimagined Se colloids to explore the possibility of using them as truly uniform and low-loss magnetodielectric building blocks for fluidic metamaterials at optical frequencies (i.e., optical metafluids). By taking advantage of the exotic properties of Se colloids, strong magnetism and Kerker-type highly-directional light scattering were measured directly from Se colloidal suspensions. Thus, the use of Se NSs as magnetodielectric building blocks can be generalized for all-dielectric optical metafluids; the soft fluidity, which is a primary feature of metafluids, can facilitate the realistic translation of optical metamaterial into the immediate practical applications.
Appendix 1
Synthesis of Se colloids: All chemical reagents, including selenious acid (Aldrich, 99.999%), hydrazine hydrate (N2H4∙H2O, Aldrich, 55% N2H4), and ethylene glycol (Aldrich, anhydrous), were used as received. 5 ml of a hydrazine hydrate solution (0.35 M) in ethylene glycol was added to 20 ml of ethylene glycol under a stirring rate of 350 rpm. After 10 minutes, 0.07 M H2SeO3 dissolved in ethylene glycol was added to the hydrazine hydrate solution. To control the size of Se NSs, we varied the ratio of H2SeO3 to hydrazine hydrate (see Fig. 1(b) and Fig. 5). During the reaction, the initially-transparent solution gradually turned into a reddish solution within ~ 15 min; the reaction was further proceeded for about 2 hours to ensure complete reduction of H2SeO3.
Numerical simulations: Theoretical analyses were carried out by the finite element method (FEM) and finite-difference time-domain (FDTD) simulations. Here, FEM and FDTD were supported by COMSOL Multiphysics 5.0 and CST microwave studio 2014, respectively. FEM was used mainly for the numerical calculation of the extinction cross section, while FDTD was employed for the calculation of the far-field scattering pattern. The refractive indices of Se and Si used in the numerical simulations (Fig. 1(a)) were obtained from ref [15]. and ref [30], respectively.
Optical measurement: UV/VIS absorption spectroscopy (Shimadzu) was used to experimentally measure the extinction cross section of the Se colloidal suspension. To measure the forward and backward scattering spectra, we used our home-built DF spectroscope, which was integrated with an optical microscope (Nikon Eclipse series), CCD (PIXIS 400B, Princeton Instruments), and spectrometer (IsoPlane, Princeton Instruments) are integrated. The NA of the objective lens for the measurement of the transmission and reflection DF spectra was 0.9.
Synthesis of Au NSs: 75 nm-sized, highly-uniform Au NSs were synthesized by iterative reduction of seed and subsequent growth [35]. First, we synthesized Au nanorods. Then, the two vertices of Au nanorods were selectively etched by a controlled reduction. We repeated reductive etching and further growth of Au until the seed AuNSs became highly-uniform and smooth. Finally, AuNS seeds were further grown into 75 nm AuNSs.
Appendix 2
SEM images for 190 nm and 300 nm Se NSs are presented in Fig. 5.
Appendix 3
Degradation of Se colloids during solid-state optical measurement is indicated in Fig. 6.
Appendix 4
Contribution of electric and magnetic resonances to light scattering of Se colloid is detailed in Fig. 7.
Fig. 7 Contribution of magnetic dipole (MD), electric dipole (ED), and electric quadrupole (EQ) resonances to light scattering of Se colloids. (a) 190 nm, (b) 230 nm, and (c) 350 nm.
Appendix 5
Mode comparison between 230 nm Si and Se nanospheres is included in Fig. 8.
Funding
Samsung Research Funding Center for Samsung Electronics (SRFC-MA1402-09).
Acknowledgments
The author wish to thank Dr. S.J. Yoo for fruitful discussion.
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