Silicon (Si) nanostructures are emerging in the fields of metasurfaces and nanophotonics, while aluminum (Al) is a plasmonic material that is active in the ultraviolet (UV) region. While Si is active in the visible range, its performance in the UV region is not well understood. Here, we discuss our experimental results of the confinement effect in the UV region of Si and Al nanostructures. We prepared Si and Al nanocylinder arrays with a periodicity in the UV range, so that UV light is diffracted coherently and trapped in the plane of the array. We deposited a UV absorbing film on top of the arrays and examined the UV confinement effect by measuring the photoluminescence (PL) intensity from the film. The PL intensity from the film on the Al nanocylinder array was found to be higher than it was from the Si array, showing that the confinement effect is more pronounced in the Al array in the UV region. The result is useful for selecting a constituent material when fabricating nanostructures that are active at specific wavelengths.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Plasmonic technologies give a tool for localizing light energy at the nanoscale via excitation of surface plasmon polaritons, offering a platform on which strong light-matter interactions can occur. Metallic nanostructures are often utilized to tune the spectral and spatial configurations of light confinement. Of particular interest are diffractive arrays where lattice plasmons, i.e., hybridized mode of localized surface plasmon resonances (LSPRs) with in-plane diffraction, are supported [1,2]. In such arrays, the light is trapped in the plane of the array via the excitation of the lattice plasmons. Lattice plasmons have been studied in diffractive arrays consisting of a variety of metals, including Au [3–5], Ag [6,7], and of conducting oxides  and nitrides [9,10]. Their interactions with light at various frequencies (from ultraviolet (UV) to near infrared) have also been explored. Al is a plasmonic metal that possesses a high plasma frequency in the UV region [11–13]. Diffractive arrays of Al nanocylinders excite lattice plasmons over a wide range of frequencies from the UV to the visible regions .
Si nanophotonics also offers the opportunity to manipulate light in the visible and near infrared regions [15,16]. Si is transparent in the near infrared at wavelengths (λ) longer than λ > 1100 nm, which is below the bandgap of Si ( = 1.1 eV). Even in the visible region, Si is not a strong absorber because of its indirect bandgap. Figure 1 shows the refractive indices (n) and extinction coefficients (k) (versus wavelength) of the polycrystalline Si used in this study. The data were deduced from the fit to the spectroscopic ellipsometry data. The k of Si is negligible in the visible region, while n is high compared to those of high-refractive-index dielectric oxides such as TiO2. The magnitude of the extinction in Si is more easily quantified via the absorption length ℓa, i.e., the distance over which light propagates before the intensity becomes 1/e of the original value. This parameter is inversely proportional to the absorption coefficient α ( = 4πk/λ). Figure 1(b) shows that ℓa varies over four orders of magnitude as λ increases from 300 to 800 nm, i.e., ℓa increases from 6 to 3 × 104 nm. This shows that visible light travels some distance through the Si nanostructure before being fully absorbed. For example, ℓa ∼ 1000 nm at λ = 600 nm. This is why Si metasurfaces are utilized as enhancers for photoluminescence (PL) [17–19] and magneto-optical effect [20,21], and as pixels for color-printing [22–27] in the visible portion of the spectrum.
In contrast to the absorbing properties of Si in the visible region, its absorption in the UV region is serious. This is because of the inter-band intrinsic transition of Si at the center of the Brillouin zone, which is a direct transition without a wavevector mismatch and the probability of which is higher compared to the transition through the indirect bandgap at 1.1 eV [28,29]. Although the ℓa of Si is smaller in the UV region than it is in the visible region and its performance in light confinement may be less efficient, n is higher in the UV region than it is in the visible range; thus, it is not clear how well Si nanostructures trap UV light waves.
In this paper, we discuss our experimental results of the light confinement effect in Si nanostructures in the UV region. To begin, we prepare Si and Al nanocylinder arrays of identical design and compare the UV confinement in the Si array with that in the Al array. Si and Al nanocylinder arrays are designed to support hybridized modes of in-plane diffraction with Mie resonances and LSPRs, respectively. The extent of light confinement is verified via PL intensity. A luminous film absorbing UV and emitting red PL with a high quantum yield is deposited on the arrays. We select tris(hexafluoroacetylacetonato) europium bis(triphenylphosphine oxide) (Eu(hfa)3(TPPO)2) as an emitter [30–32]. The large Stokes shift (1.6 eV) of Eu(hfa)3(TPPO)2 allows us to design the arrays to tune the spectral position of the lattice plasmon to the excitation wavelength while the arrays being virtually transparent at the PL wavelength. The PL intensities of the Si and Al nanocylinder arrays are compared with the aid of numerical simulations.
2. Results and discussion
Figures 2(a) and (b) show the SEM images of the triangle arrays of the Al and Si nanocylinders, respectively, with a lattice constant a = 200 nm. The cylinder diameters are approximately 80 nm and the heights of the Al and Si cylinders are 100 and 90 nm, respectively. We also prepared Al and Si triangle arrays with a = 250 and 300 nm (see Fig. 9 in Appendix for the SEM images).
Figures 3(a) and (b) show the transmittance spectra for the Al and Si nanocylinder arrays with a = 200 nm after the deposition of the emitter film. The spectra are plotted as a function of the angle of incidence (θin) and are shown as a colormap. The spectrum of the Al nanocylinder array (Fig. 3(a)) shows two dips at λ = 306 and 360 nm. These dips reflect the excitation of the LSPRs hybridized to in-plane diffraction with different spatial distributions of light energy, as shown in Fig. 4 using simulated data. The dips are redshifted as the incident angle deviates from θin = 0 °; this tendency coincides with the white dotted lines that correspond to the conditions at which in-plane diffraction, where the incident light is scattered in the plane of the array (i.e., the Rayleigh anomaly) occurs. Because the parallel component of the wave vector in the array plane is conserved, the Rayleigh anomaly conditions satisfy the relation: kout = kin ± G, where kout and kin are the diffracted and incident wave vectors, respectively. G = (m1b1, m2b2) is the reciprocal lattice vector, which can be expressed for the triangle lattice as :
Figure 3(c) compares the transmittance between the Al and Si nanocylinder arrays at θin = 0 °. Two dips are noticeable in each spectrum. It is noted that at longer wavelengths, typically λ > 500 nm, the arrays are transparent. Given that the main PL peak of Eu3+ is around λ = 617 nm, both the arrays are transparent around the spectral range in which PL occurs. The peak at λ = 580 nm is an experimental artifact from the deuterium lamp.
The simulated optical transmittance data are presented in Figs. 4(a) and (b). In the simulation, we varied the diameter of the cylinders around 80 nm, the value obtained from the SEM image. Finally, we employed 60 nm for both the Al and Si arrays, which gave the best agreement with the experiment. The experimental data are qualitatively reproduced by the simulations. At θin = 0 °, two dips in the spectrum for both Al and Si nanocylinder arrays are visible (see Fig. 4(c)). The arrays are transparent at λ > 500 nm.
To understand the origin of the dips, we plotted in Figs. 4(d) – 4(g) the optical energy distribution calculated under the illumination conditions corresponding to the two dips. Note that the light is incident from the substrate side, in accordance with the experimental configuration. For the Al nanocylinder array, the dips at λ = 306 and 362 nm represent the LSPRs hybridized with the in-plane diffractions. The field distribution shows that the extinction at λ = 306 nm (Fig. 4(d)) is primarily due to the LSPRs at the substrate/Al interface, and the reflection is observed to be large. The energy distribution at λ = 362 nm (Fig. 4(e)) is similar to that at λ = 306 nm, but the energy is more focused in the vicinity of the Al nanocylinder (note the difference in the scales of the two colormaps). For the Si nanocylinder array, the effect of in-plane diffraction is less notable, as shown by the weaker angular dependence of the optical resonance (Figs. 3(b) and 4(b)). It is noted that at λ = 306 nm, the light energy does not penetrate the Si nanocylinder (Fig. 4(f)) because of the strong absorption at this wavelength, as shown in Fig. 1; ℓa = 6 nm at λ = 306 nm, which is far smaller than any of the dimensions of the Si nanocylinder (diameter = 60 nm and height = 90 nm). By contrast, at λ = 412 nm, the light energy is inside the nanocylinder (Fig. 4(g)), indicating the presence of Mie-type resonance. The simulations for the arrays with a = 250 and 300 nm (Appendix Figs. 12 and 13, respectively) show that as a increases, the diffraction conditions shift to longer wavelengths where the absorption of Si is smaller. Consequently, excitation of in-plane diffraction in the Si array becomes stronger and the spatial distribution of light energy becomes close to that of the Al array.
For the PL measurement, a λ = 325 nm line from the He-Cd laser was used. The simulated light energy distribution at λ = 325 nm and θin = 0°, corresponding to the excitation condition used in the PL measurement, is shown in Appendix Fig. 14. For the Al array, the optical energy distribution is similar to that corresponding to the conditions λ = 306 nm and θin = 0°; however, the light energy is concentrated in the emitter film. We calculated the average light energy contained in the emitter film (see sec. 4.4 for calculation detail) and found that there was 1.31 times more energy stored in the emitter film on the Al array than in the emitter film on the flat SiO2 glass substrate, which was used as a reference. In contrast, for the Si array, no light energy accumulation was observed inside the emitter film at the illumination conditions λ = 325 nm and θin = 0°. The energy stored in the film was 0.85 times that stored in the emitter film of the reference.
Figure 5 shows the PL spectra for the emitter films on the Al and Si nanocylinder arrays with a = 200 nm. The emitter shows red PL, typical of the Eu3+. The main peak at λ = 617 nm corresponds to the 5D0-7F2 transition. PL from the Al array is 1.58 times that from the emitter film of the reference, while the PL from the Si array is the same as that of the reference. The spectral shape does not change. We also compare these PL results with those obtained for the arrays with a = 250 and 300 nm (Appendix Fig. 15). There is also more intense PL from the emitter film on the Al array than from the emitter film on the Si array.
Among the many plasmon-related optical phenomena that have caught the attention of scientists in the field of plasmonics, the interaction of light emitters with metallic nanostructures has been a topic [34–36]. The effect of a metallic nanostructure on PL can be illustrated by considering three optical phenomena: absorption, quantum yield (the ratio of the radiative decay rate with the nonradiative one), and outcoupling (a process where the PL in the film is diffracted out in a direction defined by the periodicity) . Plasmonic diffractive arrays contribute to PL enhancement by tuning the lattice geometries, such as the pattern or the lattice constant, so that they can support the lattice plasmons in the spectral ranges of absorption  and PL [38,39]. In the present study, the array is transparent in the spectral range in which PL occurs, because the diffraction does not occur in this spectral range. In this portion of the spectrum, the reason for the increased PL intensity from the Al array is absorption enhancement. We note that the 1.58 times increase in the magnitude of the PL enhancement, shown in Fig. 5, is similar to the simulated enhancement of light energy confined to the emitting film (1.31 times), supporting the speculation that absorption enhancement is the principal cause of the observed PL enhancement. The possibility of a change in quantum yield will be examined via PL lifetime measurement later.
Figure 6 compares the experimentally obtained increase in the intensity of the PL from the emitting film on the Al and Si arrays with a = 200, 250, and 300 nm and the simulated energy of excitation light (λ = 325 nm and θin = 0°) confined inside the emitting film. A correlation is found between them, confirming that the PL enhancement comes from an increase in the absorption. Note that for the array with a > 300 nm, the diffraction shifts to the visible and there is no excitation of fundamental diffraction in the UV region .
We also measured the PL decay to assess the PL quantum yield of the emitters on the arrays. Figure 7 shows the PL decay curves for the Eu(hfa)3(TPPO)2 films on the arrays, highlighting the decay of the main PL peak at λ = 617 nm. The PL from the reference film can be fitted by a single exponential with a lifetime = 779 µs. This long lifetime is due to the forbidden nature of the f - f transition, which results from the spatial symmetry of the initial and final states of the transition. The PL decays more quickly for the films on the Al and Si arrays than it does for the reference. The PL lifetimes, deduced from the single exponential fit to the decay curve, are 577 and 621 µs for the Al and Si nanocylinder arrays with a = 200 nm, respectively. The PL lifetimes for all the arrays measured are summarized in Table 1. The change in the PL decay could come from the change in radiative and/or nonradiative decay rates. The radiative decay change occurs with the change in the optical density of the states, i.e., the Purcell effect, at the wavelengths of PL. We assume that the Purcell effect is small in the present system, because both the Al and Si arrays are transparent at λ = 617 nm, as shown in Fig. 3. Therefore, we assume that the lifetime change occurs owing to the increase in nonradiative decay. This nonradiative decay results from the energy transfer from the emitter to the Al or Si and results in energy dissipation to heat and a decrease in PL intensity. The faster decay for the Al array indicates that the energy transfer is larger for the Al array. Even with the rapid energy transfer, the Al array manifests a higher PL enhancement.
At the present excitation wavelength (λ = 325 nm), the increase in PL intensity from the Al array is higher than it is from the Si array. However, this relationship could change at different wavelengths. In Fig. 8, we plot the light energy stored in the emitter film as a function of the wavelength of the incident light for the arrays with a varying from 200 to 400 nm. For both the Si and Al arrays, the wavelength at which maximum confinement occurs tends to redshift with an increase in a, indicating the presence of in-plane diffraction (denoted by the vertical dotted lines). For all a values, the Al arrays show a higher confinement effect in the UV region (λ < 400 nm). This indicates that the absorption by Si in the UV region is so strong that the Si arrays cannot confine the UV light in the plane of the array. At a = 350 and 400 nm, however, the Si arrays show a higher confinement effect at λ = 480 and 535 nm for a = 350 and 400 nm, respectively. These results confirm that Si arrays trap visible light more effectively than the Al arrays with identical designs under resonant condition, which is in accordance with the experimental work that reports a higher PL enhancement from the Si array with a = 400 nm than that from the Al array due to light trapping of the visible light .
We have fabricated periodic Al and Si nanocylinder arrays with identical designs and compared the PL intensities from the Eu(hfa)3(TPPO)2 films on the arrays. The PL from the Al array is enhanced because of the confinement of UV light to the emitting film. By contrast, the PL from the emitter on the Si array is not enhanced. The simulation shows that the incident UV light does not accumulate in the film on the Si array because of the large optical absorption of Si in the UV spectral region. The results verify the superiority of Al diffractive nanocylinder arrays to confine UV light.
4.1 Array fabrication
An aluminum thin film (thickness: 100 nm) was grown on a SiO2 glass substrate using electron beam deposition. The thin film was then patterned using a combination of nanoimprint lithography (EntreTM3, Obducat) and reactive ion etching (RIE) (RIE-101iPH, Samco) to fabricate the Al nanocylinder arrays. First, the resist (TU2-170, thickness: 200 nm) was coated onto the Al thin film and prebaked for 5 min at 95 °C. As a master mold for the nanoimprint lithography, Si molds consisting of triangle arrays of nanopillars were fabricated using electron-beam lithography (F7000s-KYT01, Advantest) and Si deep etching (RIE-800iPB-KU, Samco). Next, the surface structures of the Si molds were transferred to the resist by nanoimprint lithography. The triangle arrays of Al nanocylinders were structured by RIE under a gas flow of N2 and Cl2. The Si nanocylinder arrays were fabricated using a combination of electron beam lithography and Si deep etching. A 90-nm-thick polycrystalline Si film was deposited on a SiO2 glass substrate using a low-pressure chemical vapor deposition at 600 °C under the flow of He and SiH4 gases. The resist (NEB22A2, Sumitomo) was coated onto the Si thin film and patterned by electron-beam lithography (F7000s-KYT01, Advantest). After development, the thin film was patterned by Si deep etching (RIE-800iPB-KU, Samco). Finally, the resist residues were removed by O2 ashing (RIE-10NR-KF, Samco).
4.2 Preparation of the emitter film
Eu(hfa)3(TPPO)2 films on the substrates were prepared by vacuum evaporation. A powder sample of Eu(hfa)3(TPPO)2 was mounted in a holder and placed in a vacuum chamber (SVC-700, Sanyu Electron) under a pressure of 7 × 10-4 Pa. The powder was sublimated by heating up to 338 °C and deposited onto the Al and Si nanocylinder arrays. The thickness of the film was estimated with a surface stylus profiler (Alpha-Step IQ, KLA Tencor). The dielectric function of the film was determined by spectroscopic ellipsometry (FE-5000, Otsuka Electron).
4.3 Optical characterization
Zeroth-order optical transmission spectra (s-polarized component) were measured as a function of the angle of incidence θin. For the measurement, the sample was placed on a rotation stage and white light from a deuterium lamp was incident from the backside (substrate side). The zeroth-order optical transmission spectra were obtained by normalizing the transmission of the incident light through the sample to that through the glass substrate. PL measurements were performed by illuminating the samples with a He–Cd laser (excitation wavelength λ = 325 nm, s-polarized) at θin = 0° from the backside. The PL was measured from the opposite side at the angle from the surface normal (θem = 10°) using a fiber-coupled spectrometer (USB4000, Ocean Optics) mounted on a computer-controlled rotation stage. The stage could be rotated around the excitation spot. As a reference, we measured the PL of an Eu(hfa)3(TPPO)2 thin film on a flat SiO2 glass substrate. The PL decay at the main luminescence peak (λ = 617 nm) was measured using a time-correlated single-photon counting module (Quantaurus-Tau, Hamamatsu Photonics) equipped with a pulsed flash lamp (temporal resolution of 0.5 µs) with a band-pass filter (center wavelength: 320 nm, full width at half maximum: 40 nm) as an excitation source.
The optical characteristics of the arrays were simulated using the finite-element method (COMSOL Multiphysics). Three-dimensional models were used with periodic boundary conditions on the lateral coordinates to model the triangle lattice with a periodicity of 200, 250, and 300 nm. The simulated structures consisted of a SiO2 glass substrate and Si or Al nanocylinder with an Eu(hfa)3(TPPO)2 thin film on the top of it. The film thickness obtained from the surface stylus profiler measurement varied; we used representative values of 125 nm for the Si and Al nanocylinder arrays. The height of the nanocylinders were 100 and 90 nm for Si and Al, and the diameters were 60, 70, and 90 nm for the Al nanocylinders (with a = 200, 250, and 300 nm), and 60, 80, and 90 nm for the Si nanocylinders (with a = 200, 250, and 300 nm). The refractive indices (n) and extinction coefficients (k) were deduced from the fits to the spectroscopic ellipsometry data for Al, Si, and the Eu(hfa)3(TPPO)2 thin film ; the n of the SiO2 glass substrate was set to 1.46. A plane wave with an electric field oscillating in the y-direction (see the inset in Fig. 2 for the configuration) was incident from the top boundary at a defined θin to investigate the optical response of the model.
The average light energy contained in the emitter film was calculated as the squared magnitude of the electric field normalized to that of the reference, |Efilm|2/|Efilmref|2, integrated over the film.
Ministry of Education, Culture, Sports, Science and Technology (MEXT) (16H04217, Nanotech Cupal); Iketani Science and Technology Foundation; Asahi Glass Foundation.
The authors thank Dr. Takayuki Nakanishi (Tokyo University of Science) and Prof. Yasuchika Hasegawa (Hokkaido University) for the synthesis of Eu(hfa)3(TPPO)2. This work was partly supported by the Nanotechnology Hub, Kyoto University and the National Institute for Material Science (NIMS) Nanofabrication Platform in the “Nanotechnology Platform Project,” sponsored by MEXT, Japan.
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