Mie-resonances in vertical, small aspect-ratio and subwavelength silicon nanopillars are investigated using visible bright-field µ-reflection measurements and Raman scattering. Pillar-to-pillar interactions were examined by comparing randomly to periodically arranged arrays with systematic variations in nanopillar diameter and array pitch. First- and second-order Mie resonances are observed in reflectance spectra as pronounced dips with minimum reflectances of several percent, suggesting an alternative approach to fabricating a perfect absorber. The resonant wavelengths shift approximately linearly with nanopillar diameter, which enables a simple empirical description of the resonance condition. In addition, resonances are also significantly affected by array density, with an overall oscillating blue shift as the pitch is reduced. Finite-element method and finite-difference time-domain simulations agree closely with experimental results and provide valuable insight into the nature of the dielectric resonance modes, including a surprisingly small influence of the substrate on resonance wavelength. To probe local fields within the Si nanopillars, µ-Raman scattering measurements were also conducted that confirm enhanced optical fields in the pillars when excited on-resonance.
© 2013 Optical Society of America
Optical resonances, such as those described for spherical particles by Mie in his seminal manuscript , can occur when free-space propagating light interacts with sub-wavelength dielectric cavities or plasmonic nanoparticles. The optical properties of these nanoparticles are largely determined by their size, geometry and composition and thus can be tuned to achieve photonic devices with selective spectral properties. Chemical sensors [2–6], optical filters , polarizers  and nanoantennas [9,10] are only a few applications that benefit from such tunable optical properties. Historically, a great deal of attention has been directed toward plasmonic nanoantennas because they can resonate at sizes much less than λ/2, and toward larger whispering-gallery mode optical resonators because of their high quality-factor Q [11,12]. More recently, dielectric-cavities with dimensions ~λ/2n have attracted increasing attention for their ability to resonate at the nanoscale when the refractive index n is large, and for their compatibility with semiconductor processing [6,13–18]. Potential applications include sub-wavelength lasers [19,20], enhancement of the spontaneous emission of optical emitters  embedded in the cavity through a strong Purcell factor, and improved coupling to photovoltaic devices [14,22]. In addition, as we report, the reflectance from arrays of nanopillars that are only ~200 nm tall can in principle reach sub-percentage values, suggesting potential use in “near-perfect absorber” applications.
While many reports are concerned with single nanoparticles or random arrangements of nanoparticles, periodic arrays provide a means for studying the optical properties in well controlled systems [23–27], and enable the investigation of near- and far-field interactions between adjacent nanostructures, which may lead to the controlled manipulation of optical signals and light-matter interactions . Array periodicity provides an additional dimension for improving the performance of optical and optoelectronic devices through phase-coherent coupling of the localized electromagnetic fields between adjacent nanoparticles or through multiple scattering or grating-diffraction effects. For instance, within specific wavelength regions, periodic arrays of nanoparticles embedded in photovoltaic devices of simple architectures have been reported to produce higher energy harvesting efficiencies than their nanoparticle-free counterparts [14,22,29–31].
While the properties of macroscale dielectric resonators are well recognized , many aspects regarding the nature of nanoscale dielectric cavities, such as the modal characteristics, interparticle interactions within an array, and design guidelines for various properties and applications, have not been clearly established. To help address these deficiencies, we have investigated the behavior of the Mie resonances in vertically-oriented, cylindrical Si nanopillars as a function of pillar diameter (68 - 260 nm) and pitch (200 - 500 nm). The pillars were etched into a Si substrate to heights of 178 to 272 nm as a square array with 100 pillars along each array edge. Hence, unlike previous studies of the modes in longer nanowires [33,34], this work focuses on nanopillars that are truly nanoscale in all three dimensions (~λ/2n). Furthermore, we examine coupling effects in periodic arrays of these dielectric nanoresonators, which have so far been discussed primarily only in theoretical terms [35–37].
We find that short, vertical nanopillars exhibit intense Mie resonances in the visible spectral region governed by a transverse-mode eigencondition that reliably predicts the resonant wavelength. This resonant wavelength essentially scales linearly with pillar diameter and is nearly independent of pillar height within the range studied here. Consistent with finite-difference time-domain (FDTD) computations of the resonantly strengthened internal optical fields in the pillars, we measure an enhanced Si Raman line from the arrays, and find that the enhancement exceeds that reported from taller Si nanowires . We also find that the resonant wavelength is modulated by the density of the nanoarray through both air- and substrate-mediated interpillar interactions. Although similar pitch-induced effects have been thoroughly investigated for arrays of plasmonic nanoantennas [26,27,39–41], to our knowledge, this is the first experimental report of pitch-induced shifts in the optical resonances of arrays of dielectric nanocavities.
The arrays were characterized experimentally by integrating bright-field reflection (BFR) spectroscopy and µ-Raman spectroscopy. FDTD computations reproduce our measured spectra with superb fidelity, lending confidence in the computed field distributions that provide detailed and distinguishing mode characteristics critical for their identification. The BFR spectra exhibited sharp dips due to an intense Mie resonance (mode 1), which FDTD computations showed to be confined within a pillar and to bear similarities to the lowest-order hybrid mode in microwave pillboxes . For larger diameters, a second order resonance (mode 2) appears at bluer wavelengths. Simulations show that the same modes are excited for pillars in free space or affixed to a substrate as in the experiments, with the substrate providing a surprisingly small influence on the resonance wavelength. Although excited at normal incidence with polarization parallel to the substrate (in the x-direction), it is Ez, the field parallel to the cylinder axis, that achieves greatest electromagnetic field magnitude for our geometry. The field intensity within a pillar is computed to exhibit substantial enhancements (|E/E0| ~3), which leads to enhancements in the Si 520 cm−1 Raman line when the cavity resonance spectrally coincides with the Raman pump laser wavelength. This is consistent with work by Cao et al. , where Raman enhancements within single, isolated Si nanowires were observed. In addition, the Raman enhancements measured in this work are two times larger than those obtained for periodic arrays of longer (~0.8 – 1.1 µm) Si nanopillars under longer wavelength excitation (632.8 nm), as reported by Khorasaninejad et al. , and ~three times the maximal Raman enhancement reported for longer Si nanopillars (~0.4 – 2.3 µm in length), also with 632.8 nm excitation, as reported by Wells et al. .
Remarkably, the simulations show that the resonant wavelength of the fundamental mode (mode 1) is essentially independent of pillar height for short pillars (over a range of 100~215 nm), despite a strong z-dependence to the field amplitudes within a pillar. Indeed, we measure that the resonant condition is determined essentially by the pillar diameter, given by a relation nka = κ, where k is the free space wavevector, a is the pillar radius, and the eigenvalue κ is ~2.6 for mode 1, and 5.6 for mode 2, similar to specific leaky transverse modes in a single semiconductor nanowire [33,43,44]. However, the resonant wavelength is modulated by the density of nanopillars within the array, blue-shifting the resonant wavelength by up to ~15% as the pitch is reduced from 3 to 1.5 times the diameter, which we attribute to interparticle coupling through air . Furthermore, the shift exhibits weak oscillations as the pitch is varied, which are attributable to a substrate mediated interaction.
The demonstration of the resonant confinement of the incident laser light within shallow nanostructures patterned directly on a substrate of equal refractive index suggests that this approach provides potential for enhancing optical processes in patterned nanostructures atop both active and passive devices in a controlled manner, in support of the work of Spinelli et al. .
2. Experimental details
The nanopillar arrays (100 x 100 pillars) were fabricated in silicon via electron-beam lithography and a standard liftoff process, whereby square periodic arrays of Cr nanodots were created and used as a hard mask for reactive ion etching (RIE). During the RIE process, a 3:4 ratio of CHF3 and SF6, respectively, for 14.5 min resulted in Si nanopillars of approximately 217 nm in height. Additional information concerning the fabrication of similar nanopillars can be found in the literature [25,45,46]. A SEM micrograph of an array of ~209 nm diameter nanopillars on a 300 nm pitch, imaged at 45°, is presented as an inset in Fig. 1(b). Each array was designed to have a different nanopillar diameter and/or pitch, with diameters ranging from ~115 to 260 nm and gaps between pillar walls ranging from ~60 to 165 nm, resulting in periodicities ranging from 175 to 425 nm. The overall dimensions of the arrays ranged from ~17 to ~42 μm square.
For the BFR measurements, two microspectroscopy systems were used to record the reflectance spectra strictly from individual arrays without signal contamination from the surrounding substrate. In the first, a quartz-tungsten-halogen lamp fully illuminated an individual array through a microscope objective (20X, NA = 0.45). We employed standard Koehler illumination, but apertured the system to restrict incoming radiation to ± 10° about the sample normal. The same objective collected the reflected and scattered light, which was then focused onto the entrance slit of an imaging spectrometer (300 mm focal length, 50 groove/mm grating) mounted directly onto a side port of an inverted microscope. Use of an imaging spectrometer equipped with a charge-sensitive device camera permitted recording spectra only from within the array under study. The raw spectra were normalized by the spectrum reflected from nearby portions of the unpatterned silicon substrate, which is featureless in the spectral band of interest. Hence our reported reflectance percentages are relative to a silicon wafer. A second, commercial system (Craic Technologies), was used to acquire spectra at wavelengths ranging from ~400 to 1700 nm. As above, a single objective (40X, NA = 0.60) illuminated and collected the light, but here the incident angles were unrestricted, and a square aperture in the image plane selected the region of interest within an array. The two systems produced very similar spectra where their wavelength ranges overlapped.
µ-Raman spectra of each nanopillar array were collected using a custom-built experimental setup. The excitation of 8 mW CW at 514 nm from an argon ion laser was focused to a diameter of ~1 μm at the sample surface through a microscope objective (100X, NA = 0.70). The back-scattered light was collected through the same objective and coupled with optical fiber to an Ocean Optics QE6500 thermoelectrically cooled CCD array detector.
Finite-difference time domain (FDTD) as well as Finite Element Method (FEM) simulations were performed using the LumericalTM and COMSOL MultiPhysicsTM software packages, respectively. The two packages solve Maxwell’s equations in either the time or the frequency domains, respectively. Both methods produced nominally identical results and thus only the results from the FDTD simulations are presented unless otherwise noted. In all cases, we used optical constants from Palik  and the geometries and boundary conditions were constructed in such manner as to eliminate spurious reflections.
3. Results and discussion
3.1 The Mie resonance in nanopillar arrays: overview
Figure 1 displays representative BFR spectra collected from arrays of nanopillars of 217 nm height with (a) varying nanopillar diameter D, at a constant pitch P = 300 nm, and (b) with varying array pitch at a constant 133 nm pillar diameter. The remarkably close correspondence between these measured spectra and our FDTD simulations are evident by comparing the computed spectra in Figs. 1(c) and 1(d). It should be noted that quantitatively comparable spectra (not shown) were also computed using the full-wave FEM capability of COMSOL. In all cases, a pronounced reflectance resonance can be identified (mode 1) that disperses strongly to longer wavelengths as pillar diameter increases. For larger diameters, a higher order mode (mode 2) appears at bluer wavelengths, and also strongly disperses with diameter. In addition, mode 1 is seen in Figs. 1(b) and 1(d) to comprise more than one nearly degenerate submode component, which appear as a resolved doublet at the smallest pitch of 225 nm. Hereafter, we define the resonant wavelength λr as the minimum in the reflectance spectrum of this complex of modes, which FDTD computations show is associated with a mode internal to the nanopillars, as discussed below.
The quality of a resonance mode can be defined as Q = λ/Δλ, where λ and Δλ are the wavelength and FWHM, respectively, of the resonance band obtained from a Lorentzian fit. On one hand, mode 1 exhibits a quality factor Q that remains roughly constant with increasing diameter and resonant wavelength, Fig. 1(a), with a value of ~5, which must be considered a lower bound due to possible broadening by unresolved components, and due to the inevitable inhomogeneities in nanopillar diameter within a given nanopillar array that would artificially broaden the resonance as well. On the other hand, mode 2 is narrower, with a Q of ~10. This suggests that high-Q, sub-wavelength resonators may be achievable for larger diameterstructures through the introduction of higher order modes, while gaining spectral bandwidth through a lower-Q fundamental mode shifted to longer wavelengths. This is one of the reasons behind the broadband absorption response recently demonstrated by Spinelli et al. , and clearly has practical implications for broadband UV-Vis-NIR absorbers, enhanced photovoltaics, and optical emitters and detectors, as the authors discussed. It is also notable that the computed on-resonance reflectance for mode 1 can acquire sub-percent values, which are nearly reached experimentally. Hence nanopillar arrays of this nature may prove fruitful as an alternative approach to achieving “near-perfect absorbers” [48–51].
The pitch dependences of λr for a number of representative pillar diameters with a height of 217 nm are summarized in Fig. 2(a), together with linear least-squares fits. The non-negligible effect due to the interactions among pillars (discussed below) is evident for each diameter by the observed slope in the pitch dependence. Figure 2(b) displays the diameter dependence of the mode 1 and 2 resonant wavelengths λr by plotting λr/nSi versus diameter D for a series of arrays with pillars of 272 nm height and with a constant gap of 188 nm between the circumferential walls of the pillars. Note that P = D + gap. The size of this gap was kept constant to avoid complications from possible interpillar interactions at small pitch, as mentioned above. Normalizing λr by the wavelength dependent refractive index nSi  (which gives the wavelength in Si) removes the effects of refractive index dispersion. The resonant frequency for both modes is seen to conform to a linear dependence on diameter, as demonstrated by the linear least-squares fits (solid lines) in Fig. 2(b). The linearity of λr/nSi with diameter suggests casting data in a “universal” plot as displayed in Fig. 3, which graphs(λr/nSi)/D versus P/(λr/nSi) (i.e., the ordinate is the resonant wavelength in Si divided by pillar diameter, and the abscissa is the array pitch in units of the wavelength in Si). The normalizations cause all mode 1 data of Fig. 2 to cluster in revealing ways (the symbols are retained between the two figures). Also shown in Fig. 3 is an inset plotting (λr/nSi)/D versus D for pillars arranged at random on the Si substrate, which have been used to reveal the resonant properties of arrayed plasmonic nanostructures independent of effects induced by periodicity [26,27]. The dashed lines in both the inset and main figure mark the average value of 1.2 for (λr/nSi)/D as determined from the random arrays. This value indicates that the resonance condition within the silicon nanopillars for non-periodic arrays occurs at wavelengths that are 1.2 times the nanopillar diameter. This value is also approached in the periodic arrays (filled symbols) in Fig. 3 as pitch increases. In addition, the open circles in Fig. 3 show that the mode 1 resonance for pillars separated by a constant gap between pillar walls appears to oscillate about this same limiting value. These facts taken together suggest that the interpillar interactions play an important role at small pitches, leading to a substantial blue shift in the resonant wavelength.
While dielectric cavities are not in general amenable to simple theoretical treatments , the close linear dependence of λr/nSi on diameter in Fig. 2 suggests a practical guideline for predicting the Mie resonances of pillars such as these in the form of an eigenvalue expression analogous to that used to describe leaky-mode scattering resonances in infinite cylinders [33,53,54] or confined modes in a cylinder with highly reflecting endfaces  kra=a[(nSik)2–kz2]1/2=κ, where kr is the radial wavevector, k is the free space wavevector 2π/λr, a is the cylinder radius, and kz is the wavevector describing the variation in field amplitudes along the cylinder axis (as ordinarily imposed by an external boundary condition). The eigenvalue κ depends on the nature of the mode. For our pillars, kz can evidently be neglected, so that nSika = κ pertains. Evaluating κ from the fits of modes 1 and 2 in Fig. 2(a) gives κ1 = 2.6 and κ2 = 5.6, respectively. Interestingly, these eigenvalues compare to those for two low-order leaky-mode resonances that are excitable in infinite cylinders illuminated at normal incidence, for which kz = 0, and for which κ1 = 2.31 and κ2 = 5.45 for Si in this wavelength range . Each of these particular infinite-cylinder eigenvalues apply to both an accidentally degenerate transverse magnetic (TM) and a transverse electric (TE) mode, respectively. As indicated by FDTD simulations, discussed next, a near-degeneracy for analogous TM and TE modes in our finite-height pillars may play a role in the doublet observed for mode 1.
3.2 Modal character: FDTD simulations
To more fully characterize the nature of the resonant modes observed in the optical spectra, we examined field patterns computed with FDTD methods. Figures 4(a) and 4(b) show |E|2 for the short- and long-wavelength modes, respectively, of the doublet observed at the tightest pitch of 225 nm in the spectra of Figs. 1(b) and 1(d) for 133 nm diameter pillars. Additional simulations for larger pitch arrays (not shown) show that the deepest reflection feature is always associated with the pattern of Fig. 4(a), where fields are concentrated within the pillar. Below we discuss how this mode bears strong similarities to the HEM11 magnetic-dipole mode known for dielectric cavities at microwave frequencies. The fields associated with the longer-wavelength component of the doublet predominantly reside exterior to the pillar as plotted in Fig. 4(b), and approximately conform to the field pattern of an electric dipolar mode. Additional computations (not shown) of field patterns for mode 2 reveal that it derives from the same type of internal mode as the short-wavelength component of mode 1, Fig. 4(a), but with an extra node in the radial direction, consistent with the linear dependence on diameter of the resonant frequency as plotted in Fig. 2(b).
To clarify the role of the substrate, we also computed the resonant properties of nanopillar arrays suspended in air. Figure 5(a) shows far-field FEM simulation results for arrays with diameters ranging from 100 to 200 nm (at a constant pitch of 300nm) and Fig. 5(b) presents results for arrays with pitches ranging from 225 to 425 nm (at a constant diameter of 125 nm). The resonant modes of these substrate-free arrays are similar to those of arrays anchored to a substrate in terms of: (i) the resonant wavelength and its strong dependence on diameter, Fig. 5(a); (ii) the occurrence of a submode sensitive to pitch, Fig. 5(b); and (iii) the field patterns, Figs. 4(c) and 4(d). However, the two cases differ in that for the substrate-free array, it is the longer wavelength component of the doublet that has electric-field strength predominantly within the pillar, and it is at larger pitches that the splitting is best resolved. Despite these interesting differences, Fig. 5(b) shows that a substrate-free array exhibits a doublet with acentral wavelength very near the resonant wavelength of pillars of the same diameter and approximate height arrayed on a substrate. For example, the resonance wavelength computed for a 125 nm pillar in air, Fig. 5(b), is centered near 600 nm, while that of a pillar with nearly equal diameter (133 nm) on a substrate is computed to be quite similar, as seen in Fig. 4(a) and 4(b), where the doublets appear at 597 nm and 640 nm. Similarly, in Fig. 2(b), a 125 nm diameter pillar is seen to resonate at λ/nSi~150 nm, which corresponds to λ~617 nm, again very close to the air simulation for a pillar of the same diameter. Evidently, the substrate has only a minor effect on the resonance wavelength.
As noted above, the resonant wavelength of mode 1 can be approximated as nSika = κ, where κ = 2.6, which is near the value of 2.31 that characterizes transverse-modes in infinite cylinders . Examination of the optical field amplitudes of mode 1 within the pillar (not shown) finds that Ex~J0(nSikr) dominates for the redder component of the doublet in the xz plane, while for the bluer component it is Ez~J1(nSikr)cos(ϕ) that dominates, where ϕis the azimuthal angle measured from an axis parallel to the incident polarization along the x-axis (see Fig. 4 for the axes orientations). These field patterns are consistent with the two degenerate modes associated with the eigenvalue of 2.31  for an infinite cylinder excited at normal incidence. This suggests a possible relation between our observed doublet and these eigenmodes, but with the degeneracy lifted such that our bluer component [Fig. 4(a)] is associated with a TM mode and the redder with a TE mode. However, upon closer inspection of the field patterns for the bluer component, an alternative description of this dominant resonance arises by observing that Ex and Ez together comprise a circular pattern as indicated by the dashed arrow in Fig. 4(a). Such a pattern suggests that this mode could be related to the HEM11 dielectric cavity mode of an isolated cylinder [55,56] and in fact the same field pattern occurs within pillars in a substrate-free array, Fig. 4(d). This is the lowest order hybrid mode, so named because neither E nor H is purely transverse to the cylinder axis (i.e., both have axial components). For an isolated cavity, the HEM11 mode can be viewed as a magnetic dipole in mid-plane, consistent with Hy computed for a pillar on a substrate in Fig. 4(e). The mode radiates energy along to the cylindrical axis  and hence should couple to a plane wave incident along the axis as in our geometry. It is therefore likely that this mode is analogous to the magnetic-dipole Mie resonance of high refractive-index spheres that has recently been reported for Si nanoparticles [17,18].
To further test the similarity of our mode to an HEM11 cavity mode, we plot in Fig. 2(b) the resonant wavelength predicted by an empirical formula from the literature that was derived by fitting the resonance wavelength obtained with numerically computed surface-integral methods . This empirical prediction for an isolated cavity falls surprisingly close to our data for cavities on a substrate, and is also essentially linear with the same slope. Furthermore, a weak height dependence is predicted, consistent with our measurements. For example, we find no identifiable influence of height on resonance wavelength in Fig. 3, which compiles data for pillars that varied in height from 178 to 272 nm. Additional FDTD computations (not shown) reveal that the resonant wavelength of these arrays remains remarkably constant, shifting less than ~10 nm even down to pillar heights of ~100 nm (the shortest examined).
As noted above, the simulations surprisingly find that the substrate does not significantly affect the resonance wavelength observed in our nanopillars. However, the substrate clearly plays a significant role in the overall reflective response, as the simulations also show that an array in air produces reflective peaks through resonant scattering, while pillars on the Si substrate produce reflective dips at resonance. For resonant particles on a high index substrate, a contributing cause of this reduction in back-reflected power is the strong forward coupling into the high index substrate, as recently shown by Spinelli et al. . The substrate also affects the lifting of the degeneracy of the submodes comprising mode 1, as shown by comparing |E|2 for substrate-anchored and substrate-free pillars in Fig. 4. While the strongly confined internal mode appears to be pulled toward the substrate, Fig. 4(a), and no doubt radiates in the forward direction into the higher index substrate [13,14,31,35], evidently the internal junction between the pillar and the substrate presents a boundary sufficiently reflective to support a dielectric cavity mode quite similar to the lowest hybrid mode in an isolated cylinder. In fact, it appears that interpillar interactions are at least as strong as any influence from the substrate, especially for arrays with smaller pitch. In addition, the field profiles in Figs. 4(a)-4(d) suggest that the substrate plays a role in enhancing of the confinement of the mode, either internally or externally to the nanopillar. The substrate-free modes appear somewhat less localized than the modes on the substrate.
While we have drawn connections between the character of our observed resonances and the character of various known resonant modes in dielectric structures such as magnetic Mie resonances, transverse modes in infinite-cylinders, and microwave cavities, we note that further investigation is needed to clarify more completely how best to describe the set of nearly degenerate components comprising mode 1. Such analysis is beyond the scope of the present manuscript.
3.3 Interparticle interactions: effects of array density
Interparticle interactions in arrays of nanoplasmonic resonators have drawn widespread interest [27,57–62], but have received relatively little attention in arrays of dielectric nanoresonators. In plasmonic arrays, the interaction is usually treated by considering, at the location of any given particle, the total self-consistent field produced by both the incident radiation and the sum of all fields emanating from the surrounding array of resonators. The net effect, which can be coherently enhanced in periodic arrays, is to produce shifts in the resonant frequency and alterations of the quality factor Q. In analytical treatments, the resonators themselves are approximated as radiating dipoles, which can be especially suitable for plasmonic particles because, in principle, the dipole resonance is supported even as the particle size approaches zero. In contrast, dielectric resonators require a minimum size ~λ/2n to support an internal standing wave. Nonetheless, for high-index materials like silicon, where λ/2n can be considerably less than the free space wavelength, pitch-dependent effects akin to plasmonic arrays have recently been predicted for periodic arrays of dielectric spheres in air and periodic arrays of tellurium nanocubes on BaF2 substrates  resonating in the lowest magnetic- and electric-dipole Mie modes . For our arrays, the criterion allowing a strict dipole approximation is likely not met , and in any event a detailed quantitative analysis is beyond the scope of this work. Here we qualitatively describe the measured effects on resonant wavelength induced by array density.
As noted above, significant effects on the spectra of our nanopillar dielectric resonances [Fig. 1(b)], occur as pitch is varied. In addition to changes in resonant lineshape, the resonant wavelength λr for a given pillar diameter shifts blue by up to 15-20% as pitch is reduced, as is evident in Fig. 2(a). When plotted as in the universal graph of Fig. 3, the shift appears to accelerate as the array density increases. Such a blue shift is characteristic of periodic arrays when in the “evanescent” regime, which occurs when radiation at the resonant wavelength diffracts into a propagating wave in-plane, with evanescent tails out of plane [60–62]. Since, for normal incidence, this diffraction condition is given by P≤λr/n, it can in principle pertain either to interactions mediated through air (n = 1), or to interactions mediated through both air and Si. For all arrays investigated here, for which λr/nSi<P<λr/nair, the evanescent regime condition is met for any interactions mediated through air. Note that the constant gap data presented in Fig. 3 (open circles) appears immune to the overall blue-shift observed at small pitch for constant diameter data (filled symbols), which suggests that the overall blue shift might be dominated by a gap-mediated effect (as opposed to a specific periodicity effect), as would be the case if coupling originated with lateral evanescent fields that arise from satisfying boundary conditions in Maxwell’s equations, or from laterally confined cavity modes. However, our study does not permit unambiguously distinguishing periodic- from gap-mediated interactions.
In addition to the gradual blue shift with reducing pitch, inspection of the mode 1 resonant wavelengths of Figs. 2(a) and 2(b) shows a possible oscillatory variation about the straight lines fits for a number of data sets. Similarly, close inspection of Fig. 3 raises the possibility that the oscillations are in phase when plotted as a function of pitch normalized by the wavelength in Si. This hypothesis is confirmed in Fig. 6, where deviations from straight-line fits to selected data from the universal plot of Fig. 3 are plotted versus P/ /(λr/nSi), and fitted to a sine function. The weak but discernible oscillations, amounting to several percent of the resonant wavelength, tend to peak when the array pitch matches an integer number of resonant wavelengths in the substrate, independent of the pillar diameter. Related oscillatory variations have been observed in periodic arrays of plasmonic resonators, and attributed to radiative interparticle coupling when the array is in the “radiative” regime, i.e., when P>λr/n [60–62]. Similar pitch-dependent oscillatory interactions should arise for radiative interactions among dielectric resonators that can be described by lattice sums , and have been predicted in general for particle pairs  and observed between pairs of Si nanowires . For our range of pitches, the radiative regime occurs only for substrate mediated interactions. Theoscillating resonant frequency about a linear trend, extracted in Fig. 6, are attributable to a coherent enhancement of coupling when the pitch matches an integer number of wavelengths. Hence these data provide evidence for a substrate mediated interaction in the radiative regime. This implies that, within the arrays investigated, both the radiative and evanescent coupling regimes are active and contribute to the pitch-induced shifts in the spectra and are mediated separately by the correspondingly high (Si) and low (air) index materials surrounding the nanopillars.
Finally, as noted previously, we have discovered that array density affects differently the resonant properties of each submode comprising mode 1. For example, as seen in Figs. 1(b) and 1(d), the submode resonant wavelengths and amplitudes disperse differently with pitch, leading to a resolved doublet at the smallest pitch investigated. The substrate evidently plays a role as well, since the simulations for substrate-free pillars show an overlap of submodes at the smallest pitch, Fig. 5(b). A role of the substrate in moderating air-mediated interactions is not surprising, given the report of a gradual suppression of diffractive coupling between periodically-arrayed plasmonic Au nanoparticles  as the distance was reduced between the particles and a higher index substrate underneath. Therefore, it is anticipated that interparticle coupling effects are also directly tied to the asymmetry of the refractive index of the surrounding environment, and thus must be taken into account for any device design.
3.4 µ-Raman signal enhancements
The reflectance spectra discussed above reveal the far-field optical response to cavity resonances in the nanopillar arrays. In order to probe local field intensities, we measured the ~521 cm−1 Si µ-Raman scattering intensity as a function of nanopillar diameter at 514 nm excitation, as presented in Fig. 7(a), and at 785 nm excitation as displayed in Fig. 7(b). These measurements employed the same array as used for Fig. 2(b), with the interpillar gap fixed at 188 nm and a nanopillar height of 272 nm. In Fig. 7(a), the Raman scattering enhancement (blue circles) exhibits a distinct maximum for a nanopillar diameter of ~109 nm, where the Mie resonance wavelength of mode 1 (red circles) coincides with the pump wavelength (horizontal dashed line). We attribute this enhancement to the resonant increase in the internal optical fields in the nanopillars. This is clearly indicated by field intensity distributions from the simulations under 514 nm incident, shown in Fig. 7(c), corresponding to arrays of nanopillars with diameters of 109 nm (left, for mode 1) and 200 nm (right, for mode 2), a height of 272 nm and a gap of 190 nm. On resonance, a ~30-fold overall enhancement of the Si µ-Raman scattering signal intensity is observed at 514 nm incidence, relative to the intensity measured from the unpatterned substrate. This enhancement is approximately 3 times the enhancement obtained off-resonance, i.e. from nanopillar arrays for which the resonant mode is not supported near the 514 nm incident laser wavelength. The enhancement from non-resonant arrays may arise from an improved escape probability for photons originating in the substrate between pillars and scattered by the nanopillar texture , or originating within the pillars themselves and guided preferentially away from the substrate . For larger diameters, 190 – 200 nm, the Raman intensity increases 15-fold relative to the surrounding unpatterned silicon, due to the coincidence of the laser wavelength with the resonance condition of mode 2. This corresponds to a 2x increase in Raman intensity over that which was observed from arrays with resonances spectrally detuned from the incident pump laser wavelength. The resonant enhancements in the Raman signals for both, modes 1 and 2, are attributable to the resonantly enhanced field strengths within the nanopillars, as indicated by the corresponding plots of |E|2 obtained from FDTD field simulation, Fig. 7(c). It is important to make a distinction between this mechanism and that reported in Ref . for periodic arrays of Si nanowires (0.8-1.1 µm in height), where the increase in enhancement from larger diameter nanopillars was attributed to interference effects within the arrays and not due to a higher order cavity resonance as reported here.
While there have been several previous reports of enhanced Raman signals in Si nanostructures, including the dependence on diameter for nanowires and longer cylinders [6,38,42,66], in this work we emphasize the direct correlation between the Raman enhancements and the independently measured optical resonances of the nanostructures, and we unambiguously identify through simulations the modal characteristics of these resonances. This approach is general and can be applied to understand the Raman signal dependence reported elsewhere. For example, using somewhat longer Si nanowires, Khorasaninejad et al.  reported that the maximum Raman signal excited with 633 nm excitation is observed when the diameter is 135 nm for pitches ranging from 400 to 800 nm. This diameter that yielded their maximum Raman signal is quantitatively consistent with our results, which indicate a resonance near 630-650 for 133 nm diameter nanopillars as illustrated by the resonances observed in Fig. 1(b) and 1(d), as well as Fig. 2(b). Furthermore, we observe a 17–fold enhancment in the overall Si Raman intensity excited at 785 nm incidence from arrays of ~184 nm diameter nanopillars, Fig. 7(b). In this case also, the enhancement occurred when the Mie resonance wavelength (mode 1) of the nanopillar array spectrally coincided with the Raman pump laser wavelength.
In conclusion, we have shown that periodic arrays of Si nanopillars interact with incident electromagnetic radiation to produce Mie resonances that are consistent with dielectric nanocavities. This leads to the confinement of a large portion of the incident EM field within the subwavelength Si nanopillars as evidenced by FDTD and FEM simulations as well as µ-Raman measurements. The resonances are observed to blue-shift linearly with decreasing nanopillar diameter, and less so with decreasing array pitch. The strong dependence of the resonance wavelength on the diameter of the Si nanopillars indicates that the local Mie resonance and the resonance-enhanced extinction in the high index Si nanopillars dominate the overall optical properties of the arrays. The pitch-induced shifts can be considered as the relatively weak, coherent coupling between these Si nanopillar Mie resonators, similar to effects studied widely for arrays of plasmonic particles. In addition, the dependence of these resonances on the nanoparticle and array parameters demonstrates that these resonances can be tuned to preferred wavelengths with specific linewidths and bandwidth for a given application. This can be useful in the development of photonic and plasmonic/photonic hybrid devices such as higher-efficiency photovoltaic solar cells and enhanced optical emitters and detectors.
Support for this work was provided through the Naval Research Laboratory's Nanoscience Institute. Support for F.J.B. was provided through the ASEE-NRL Postdoctoral Fellowship Program. J. G. acknowledges the support from the ASEE-Naval Research Office Summer Faculty Fellowship Program. The authors would like to thank Prof. Linyou Cao for his insights into these systems. Electron beam lithography was performed at the Center for Nanoscale Technology at the National Institute for Standards and Technology (NIST) in Gaithersburg, MD.
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