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Guided-mode waves structure of electric and magnetic dipole resonances in a metamaterial slab

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Abstract

The electric dipole (ED) and magnetic dipole (MD) resonances in a slab of dielectric nanostructures induced by oblique light incidence were analyzed in terms of guided-mode wave theory for transverse electric (TE) and transverse magnetic (TM) polarization. Owing to the symmetry breaking in TE and TM polarizations at oblique incidences of light, the resonance modes produced different features, which were analyzed with comprehensive studies of energy fluxes associated with the resonance modes. The MD resonances are excited by the guided-mode wave with an energy flux in the opposite direction in the incident plane for TM polarization and excited by two guided-mode waves with energy fluxes orthogonal to the incident plane for TE polarized light. Meanwhile, the ED resonances are excited by the guided-mode waves with energy fluxes in both directions, incident plane, and out of the incident plane for both TE and TM polarizations. The proposed analyses provide clear explanations of the stability or vanishing of resonance modes, with variations in the inter-distance between nanostructures near the Rayleigh anomaly condition, which were verified by the experimental measurements performed with 2D arrays of Si nanostructures for various incident angles with TE and TM polarizations.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Dielectric metamaterials consisting of periodic nanoparticles or subwavelength structures on or within dielectric materials have drawn significant attention in the last decade [113] owing to their extraordinary features, such as negative index [13], directional scattering [46], ultra-thin flat lenses [710], and perfect mirrors [11,12], which cannot be realized with ordinary materials in nature. Unlike metals, dielectric materials with a high refractive index and a low extinction coefficient suppress light dissipation and exhibit strong resonances in arrays of dielectric nanoparticles, depending on the period of lattice or polarization of the incident light, leading to fluorescence enhancement [4,13], optical sensors [1416], or color filters [1720].

The electric and magnetic resonances in a spherical dielectric particle are calculated using the Mie theory from which electromagnetic fields are expressed as a series of electric dipole (ED), magnetic dipole (MD), and higher-order excitations [21] and can be approximated to dipole (or quadrupole) terms for nanoparticles [2224]. The periodic arrays of spherical nanoparticles can be rationally treated as a set of dipole radiators, and the total dipole moments of the arrays are obtained by the electromagnetic fields contributed by each dipole, known as the coupled-dipole method [25]. Meanwhile, the resonance can also be interpreted as leaky Bloch modes at which the incident light is coupled with the guided mode under resonance conditions, known as guided-mode or leaky-mode resonance [26,27].

To the best of our knowledge, the first experimental demonstration was reported in [18,19], where the square lattice of Si nanostructures on a quartz substrate exhibited reflective color filters with high angular tolerance, and the resonance effects were described in terms of guided-mode waves [28]. The optical resonances in nanoparticle arrays have led to a great interest in the area of dielectric metamaterials from which the various optical properties induced by the ED and MD resonances were studied in accordance with the types of particle arrays, period of nanoparticles, and light polarization [2427,2932].

The resonance conditions of nanoparticle arrays with a square lattice can be tuned by varying the periodicities of nanoparticles at which significant lattice resonances occur near the wavelength of diffraction, known as a Rayleigh anomaly (RA) [24,27,32]. For normal incident light, each dipole mode (ED or MD) is separately coupled with different periodicities in the x- and y-axis directions, and the far-field resonance effects are explored with respect to the RA conditions of the grating [24,32]. The resonances can also be achieved by changing the incident angle with a constant period over the lattice, where the same ED or MD resonances are induced at certain incident angles of polarized light [27].

Herein, we have discovered the internal structures of guided-mode waves related to ED and MD resonances in a slab of dielectric nanostructures induced by polarized light at different incident angles. We also explored the conditions of the resonance appearance or disappearance in the slab through a theoretical analysis of the energy fluxes associated with the modes. The analysis provided clear interpretations of unique optical properties in a slab of dielectric nanostructures, where the symmetry of resonance modes contributes to significant effects in the stability of the resonance modes at the oblique incident of light and the stability could be analyzed by the components of Poynting vector associated with the resonance modes near the RA conditions. Specifically, we revealed that the MD resonance excited by light with TM polarization was attributed to the guided-mode wave with the energy flux in the opposite direction of the incident light, which could be effectively interpreted as a negative refractive index. Meanwhile, the MD resonance excited by light with TE polarization was attributed to the guided-mode wave with energy fluxes of two components that were almost orthogonal to the incident plane. The ED resonance was attributed to the guided-mode waves with energy fluxes in both directions––that is, in-plane and out-of-plane, for both TE and TM incident lights. The results were supported by experimental demonstrations of optical effects by RA conditions for various incident angles of TE- and TM-polarized light. These results imply the potential applications for designing of optical metasurfaces, color filters in display applications, optical switches, and high-performance sensors utilizing the polarizations.

2. Guided-mode waves in two dimensional arrays on a substrate

The arrays of nanoparticles or nanolayers with a high refractive index can be considered a wave-guiding layer with an effective refractive index neff. When neff is larger than the refractive indices of both the upper layer no and the bottom substrate ns, the incident light can be trapped and guided in the layer (see Fig. 1(a)) because of the diffraction and excitation of the guided mode propagating inside the slab. The guided-mode wave with propagation vector q close to $2\pi /L$ (where L is the period of the array) can be excited by the first-order diffraction of incident light at normal incidence. The excitation of the guided modes and the trapping of light into the slab result in resonances in the reflection and transmission spectra, which can be used for various applications [1420].

 figure: Fig. 1.

Fig. 1. Dispersion curves of ω(q) for a propagating vector q and guided modes in a planar layer (a) and in periodic arrays (b), where L, no, ns, nH, and nL are the period and the refractive indices of the air, substrate, and high and low index media, respectively. The modes near q∼2π/L can be excited by the incident light through first-order diffraction.

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The symmetry condition for the forward and backward waves induces a bandgap, as shown in Fig. 1(b). For lower band energy, a light field is accumulated in a material with a high refractive index [33], which promotes electric or magnetic field enhancement [18]. As the period of L increases, the guided-mode waves can leak out to the substrate when $|q |\le ({2\pi /\lambda } ){n_s}$, which leads to the critical distance of the mode disappearance

$$L \ge \lambda /{n_s},$$
which is known as the RA condition [24,32].

The guided-mode waves in a slab of 2D arrays of nanoparticles have more complex structures than those of 1D linear arrays [18,19]. For an oblique incident angle, the guided modes may propagate not only in the forward and backward directions but also in orthogonal directions, resulting in resonances in the reflection and transmission spectra with several unexpected features. In particular, 2D arrays of dielectric particles with a high refractive index exhibit bandpass resonances in the visible wavelength with high angular tolerance [18,19] at which the guided modes associated with the ED and MD resonances can explain the reasons for the high incident angle tolerance and provide more knowledge on optical energy flows propagating inside a slab.

Let us consider 2D arrays of dielectric particles with a high refractive index on a substrate with different periods in the x- and y-directions: Lx and Ly (Fig. 2). The incident light at an angle θ generates electric and magnetic fields in the slab, which can be represented as a combination of spatial harmonics.

$$\begin{array}{l} \overrightarrow E (x,y,z) = \sum\nolimits_{m,m^{\prime}} {{{\overrightarrow E }^{(m,m^{\prime})}}(z){e^{iq_x^{(m)}x}}} {e^{iq_y^{(m^{\prime})}y}}\\ \overrightarrow H (x,y,z) = \sum\nolimits_{m,m^{\prime}} {{{\overrightarrow H }^{(m,m^{\prime})}}(z){e^{iq_x^{(m)}x}}} {e^{iq_y^{(m^{\prime})}y}} \end{array}$$
with wave vectors q both having x- and y-axis components:
$$\begin{array}{l} q_x^{(m)} = {k_\parallel } + m\frac{{2\pi }}{{{L_x}}}\\ q_y^{(m^{\prime})} = m^{\prime}\frac{{2\pi }}{{{L_y}}} \end{array}$$
where ${k_\parallel } = {k_o}{n_o}\sin (\theta )$ is a wave vector component parallel to the slab of the incident light, ${k_o} = 2\pi /\lambda $, no is the refractive index of the upper medium (air), and m, m’ are integer numbers (m and m’ count harmonics along x and y directions, respectively).

 figure: Fig. 2.

Fig. 2. (a) Schematic of 2D arrays of a nanostructure on a substrate illuminated by incident light. The wavy arrows illustrate guided-mode waves in a slab of nanostructures. Lx and Ly are the periods of the nanostructures. The TM and TE polarizations are defined as in (b) and (c).

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If the refractive index of the nanoparticles is slightly larger than that of the ambient medium, the slab of nanoparticles can be considered a medium with low perturbation of the refractive index $\varDelta n/n < < 1$. The electric and magnetic fields generated by incident light in slab ${\overrightarrow E ^{(m,m^{\prime})}}(z)$ and ${\overrightarrow H ^{(m,m^{\prime})}}(z)$ can be expressed as a combination of forward and backward waves:

$$\begin{array}{l} {\overrightarrow E ^{(m,m^{\prime})}}(z) = \overrightarrow E _ + ^{(m,m^{\prime})}{e^{i{\kappa _z}z}} + \overrightarrow E _ - ^{(m,m^{\prime})}{e^{ - i{\kappa _z}z}}\\ {\overrightarrow H ^{(m,m^{\prime})}}(z) = \overrightarrow H _ + ^{(m,m^{\prime})}{e^{i{\kappa _z}z}} + \overrightarrow H _ - ^{(m,m^{\prime})}{e^{ - i{\kappa _z}z}} \end{array}$$
where $\kappa _z^{(m,m^{\prime})} = \sqrt {k_o^2n_{eff}^2 - {{(q_x^{(m)})}^2} - {{(q_y^{(m^{\prime})})}^2}}$ is defined as the effective refractive index of the medium [22]. The top and bottom boundary conditions in the slab with a fixed thickness h separate the discrete resonance modes with the propagation constant $\kappa _z^{(m,m^{\prime})}$
$$\kappa _z^{(m,m^{\prime})}h + {\phi _1} + {\phi _2} = \pi N$$
which can be excited in the slab by coupling with incident light due to diffraction in the (m,m’) order. Here ϕ1 and ϕ2 are the phases of light reflected from the bottom and top slab surfaces. The resonance modes propagate throughout the slab––that is, become the guided mode when the wave vector components $q_x^{(m)}$ and $q_y^{(m^{\prime})}$ satisfy Snell’s law for total internal reflection.
$$\begin{array}{l} |{q_x^{(m)}} |\ge \max \{{{n_o},{n_s}} \}\frac{{2\pi }}{\lambda }\\ |{q_y^{(m^{\prime})}} |\ge \max \{{{n_o},{n_s}} \}\frac{{2\pi }}{\lambda } \end{array}$$
For nanoparticles with a high refractive index, the low perturbation approximation is not valid, and the guided-mode waves have more complex structures. Maxwell’s equations are written for the electric field components Ex and Ey as follows:
$$\begin{array}{l} \partial _z^2{E_x} = \left( { - k_o^2\varepsilon - \partial_y^2 - {\partial_x}\frac{1}{\varepsilon }{\partial_x}\varepsilon } \right){E_x} + \left( {{\partial_x}{\partial_y} - {\partial_x}\frac{1}{\varepsilon }{\partial_y}\varepsilon } \right){E_y}\\ \partial _z^2{E_y} = \left( {{\partial_x}{\partial_y} - {\partial_y}\frac{1}{\varepsilon }{\partial_x}\varepsilon } \right){E_x} + \left( { - k_o^2\varepsilon - \partial_x^2 - {\partial_y}\frac{1}{\varepsilon }{\partial_y}\varepsilon } \right){E_y} \end{array}$$
where r denotes the coordinate derivative rE = ∂E/∂r, with r = x, y, and z. The electric field vector $\overrightarrow E = ({E_x},{E_y})$ with only two x and y components in Eq. (6) can be written in the operator form:
$$\partial _z^2\overrightarrow E = \widehat L\{{\overrightarrow E } \}$$
where $\widehat L\{A \}$ is the linear operator defined in Eq. (6) and can be written in matrix form for the amplitude harmonics of the electromagnetic field [34]. The eigenmodes ${\overrightarrow E _p}$ of Eq. (7) are defined as follows:
$$\widehat L\{{{{\overrightarrow E }_p}} \}={-} \kappa _p^2{\overrightarrow E _p}$$
Thus, the waves existing in the slab are linear combinations of eigenmodes propagating in the positive and negative z directions with a propagation constant κp:
$${\overrightarrow E} _{p} = {\overrightarrow E}_{p}^{( + )}{e^{i{\kappa _p}z}} + {\overrightarrow E}_{p}^{( - )}{e^{ - i{\kappa _p}z}}$$
The amplitudes of the forward “+” and backward “–” eigenmodes can also be represented as a linear combination of spatial harmonics:
$${\overrightarrow E}_{p}^{({\pm} )} = \sum\nolimits_{m,m^{\prime}} {\{{{\overrightarrow E}_{p,x}^{({\pm} )(m,m^{\prime})} + {\overrightarrow E}_{p,y}^{({\pm} )(m,m^{\prime})}} \}{e^{iq_x^{(m)}x}}{e^{iq_y^{(m^{\prime})}y}}}$$
which can be obtained from the boundary conditions for electric and magnetic field components at z = 0 and z = h. Compared to the case of the slab with a low refractive index perturbation, the slab of dielectric nanoparticles with a high refractive index has several spatial harmonics $\overrightarrow E _p^{({\pm} )(m,m^{\prime})} = (E_{p,x}^{({\pm} )(m,m^{\prime})},E_{p,y}^{({\pm} )(m,m^{\prime})})$ of eigenmode $\overrightarrow E _p^{({\pm} )}$.

The vector q = $(q_x^m,q_y^{m^{\prime}})$ is the wave vector that defines the direction of the phase propagation of each eigenmode of (m,m’) and does not need to coincide with the direction of energy propagation in the medium. Indeed, the energy flux of the guided-mode wave is a combination of fluxes of the waves propagating along the forward “+” and backward “–” waves along the z-direction and is defined as the Poynting vector:

$$\overrightarrow S _p^{({\pm} )} = \frac{c}{{8\pi }}[{\overrightarrow E_p^{({\pm} )} \times \overrightarrow H_p^{({\pm} )\ast }} ]$$
The Poynting vector averaged over the unit cell of Lx${\times}$Ly, $\left\langle {\overrightarrow S_p^{({\pm} )}} \right\rangle $ can be expressed as the sum of the spatial harmonics (m,m’) components:
$$\left\langle {\overrightarrow S_p^{({\pm} )}} \right\rangle = \sum\nolimits_{m,m^{\prime}} {\overrightarrow S _p^{({\pm} )(m,m^{\prime})}},$$
where
$$\overrightarrow S _p^{( \pm )(m,m^{\prime})} = \frac{c}{{8\pi }}\left\langle {[{\overrightarrow E_p^{( \pm )(m,m^{\prime})} \times \overrightarrow H_p^{( \pm )(m,m^{\prime})\ast }} ]} \right\rangle $$

Thus, the eigenmode ${\overrightarrow E _p}$ is contributed to by partial waves with various wave vectors ${\overrightarrow q ^{(m,m^{\prime})}}$ with $q_x^{(m)}$ and $q_y^{(m^{\prime})}$ as defined by Eq. (2): however, the energy flux of each partial component is defined as ${\overrightarrow S ^{({\pm} )(m,m^{\prime})}}$. Moreover, the eigenmode ${\overrightarrow E _p}$ in Eq. (9) becomes a guided-mode wave when the ${\overrightarrow q ^{(m,m^{\prime})}}$ in the plane components satisfies the total internal reflection condition.

$$|{{{\vec{q}}^{({m,m^{\prime}} )}}} |\;>\; \frac{{2\pi }}{\lambda }\textrm{max}\{{{n_o},{n_s}} \}$$
The analysis of the components of guided-mode waves provides us with a useful qualitative method that increases the understanding of the optical resonances in the slab. Let us first consider the wave vectors ${\overrightarrow q ^{(m,m^{\prime})}}$ of the guided-mode wave, which is excited by the first-order diffraction, as shown in Fig. 3. The blue (red) arrows represent the components of guided-mode waves that are excited in the x (y) direction by ±1 order diffraction of incident light. The wave vectors ${q^{({\pm} ,0)}} = {k_\parallel } \pm 2\pi /{L_x}$ then correspond to the components of the forward and backward propagation waves. The components become guiding modes in the slab when $|{{q^{({\pm} ,0)}}} |> ({2\pi /\lambda } ){n_s}$ in the assumption ns > no, which are represented in Figs. 1(a) and (b) as dashed lines. At $|{{q^{({\pm} ,0)}}} |\le ({2\pi /\lambda } ){n_s}$, the wave vectors ${q^{({\pm} ,0)}}$ leak into the substrate, which is achieved when the period Lx exceeds $RA_x^{({\pm} )}$ as follows:
$$RA_x^{({\pm} )} = \frac{\lambda }{{[{{n_s} \mp {n_o}\sin (\theta )} ]}}$$

 figure: Fig. 3.

Fig. 3. Propagation vectors q of guided-mode waves which can be excited in the slab by incident light due to first order of diffraction. The blue and red arrows correspond to the waves excited in the incident plane due to diffraction in the x-direction (i.e., m = ±1, m′ = 0) and in the y-direction (i.e., m = 0, m′ = ±1), respectively

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Thus, the guided-mode waves having partial components (m=±1, m’ = 0) (we will use further notation (±,0) for short) disappear for Lx above $RA_x^{( + )}$ (or $RA_x^{( - )}$). Moreover, if Lx exceeds RA, then

$$RA_{xo}^{({\pm} )} = \frac{\lambda }{{[{{n_o} \mp {n_o}\sin (\theta )} ]}}$$
The guided-mode waves having a partial component (±,0) also leaked out to the upper medium. With the conditions in Eq. (14) and (15), the presence of partial components (±,0) in the guided-wave modes could be justified at resonance in the slab excited by various incident angles of light with different polarizations, even without solving Maxwell's equations of Eq. (6).

Similar RA conditions for the (0,±) partial components of the wave vectors, ${q^{(0, \pm )}} = {k_\parallel } \pm 2\pi /{L_y}$ propagating in the orthogonal y-direction are obtained from the relations of Eq. (13) as follows:

$$R{A_y} = \frac{\lambda }{{\sqrt {n_s^2 - n_o^2{{\sin }^2}(\theta )} }}$$
$$R{A_{oy}} = \frac{\lambda }{{\sqrt {n_o^2 - n_o^2{{\sin }^2}(\theta )} }}$$
Thus, the components (0,±) satisfying the period Ly below the RA conditions of Eq. (16) and (17) contribute to the optical resonances in the 2D slab of the nanoparticles. Notably, the RA conditions for higher-order partial components can also be found similarly, but it will not be considered in this work because the Lx and Ly satisfying the relations of Eq. (16) and (17) are sufficiently large.

Let us further consider the guided-mode waves associated with MD and ED resonances in detail. Figure 4 shows the simulation results of reflectance intensity maps in the visible wavelength with respect to the variations of Lx and Ly, which were obtained using the rigorous coupled-wave analysis (RCWA) method [35]. TM-polarized light was set to illuminate the 2D periodic nanoparticle arrays at an incident angle of 10°. The geometries and refractive indices of the materials are described in the figure captions. In Fig. 4(a), the resonance modes of ED and MD were identified at ∼550 nm and ∼650 nm, respectively and were affected by the $RA_x^{( - )}$ and $RA_x^{( + )}$ lines of Eq. (14). Notably, the resonance intensity of the MD mode was completely suppressed above the $RA_x^{( - )}$ line, whereas the resonance intensity of the ED mode was totally suppressed immediately above the $RA_x^{( - )}$ line but existed again in the position between the $RA_x^{( - )}$ and $RA_x^{( + )}$ lines. This difference implies that the internal structures of the resonance modes of ED and MD at oblique incidences of light differed completely from those in the normal incident case. The higher excitation modes of the electric quadrupole (EQ) and magnetic quadrupole (MQ) also existed near λ = 450–500 nm: these modes were not considered in this work but rather focused on the ED and MD modes.

 figure: Fig. 4.

Fig. 4. Reflectance intensities in 2D arrays on a substrate with variations of period, Lx in (a) and Ly in (b), while the other period of orthogonal axis is fixed to 300 nm. The TM polarized light is illuminated with an incident angle of θ = 10°. The periodic pattern sizes are d = h = 150 nm with a refractive index of n = 4. The refractive index of the substrate is ns = 1.45. Colored dashed lines represent RA conditions defined in Eqs. (14)–(17).

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Figure 5 represents the partial components of Poynting vectors for MD [(a) and (b)] and ED [(c) and (d)] resonances, which were estimated at the positions of white dots with λ = 658 nm, Lx = 300 nm and λ = 545 nm, Lx = 300 nm, respectively, as shown in Fig. 4(a). The magnitudes of the components of Poynting vectors, averaged to $S_x^{(m,m^{\prime})} = ({S_x^{( + )(m,m^{\prime})} + S_x^{( - )(m,m^{\prime})}} )/2$ and $S_y^{(m,m^{\prime})} = ({S_y^{( + )(m,m^{\prime})} + S_y^{( - )(m,m^{\prime})}} )/2$ are denoted as points with different sizes. The relative strengths can also be referred to with the color legend bar in each plot. In Figs. 5(a) and (b), the principal components of the Poynting vectors for the MD resonance are $S_x^{({ - ,0} )}$ = -0.62So and $S_x^{({ + ,0} )}$ = 0.24So, where So is the Poynting vector of the incident light. The other (m,m’) partial components are insignificant owing to the multiple scattering of the light inside the slab. This indicates that the guided-mode associated with MD resonance propagates in the x-direction and thus the resonance can be excited only at the condition of Lx below$\; RA_x^{(- )}$, but independent of the period of Ly as strongly supported by the 2D maps in Figs. 4(a) and (b). Meanwhile, Figs. 5(c) and (d) show that the ED resonance has several principal components of the Poynting vectors, ${\vec{S}_1} = S_x^{({ - ,0} )}{\vec{e}_x}$ along x direction with relative amplitude $S_x^{({ - ,0} )} ={-} 0.8{S_o}$, being the opposite in-plane direction to the incident light, and two partial components being almost orthogonal to the incident plane ${\vec{S}_2} = S_x^{({0,1} )}{\vec{e}_x} + S_y^{({0,1} )}{\vec{e}_y}$ and ${\vec{S}_3} = S_x^{({0, - 1} )}{\vec{e}_x} + S_y^{({0, - 1} )}{\vec{e}_y}$ with relative amplitudes of $S_x^{({0, \pm 1} )} = 0.04{S_o}$ and ${\; }S_y^{({0, \pm 1} )} = 0.31{S_{o,}}$ respectively. Thus, the ED mode could be excited in both x and y-directions under the conditions of Lx below$\; RA_x^{(- )}$, and Ly below RAy, as represented in Figs. 4(a) and (b).

 figure: Fig. 5.

Fig. 5. Components of Poynting vectors normalized to the incident light, $S_x^{(m,m^{\prime})}$ in (a), (c) and $S_y^{(m,m^{\prime})}$ in (b), (d) of guided-mode waves excited in the slab by the incident wave of TM polarization, which are calculated at the wavelengths λ = 658 nm (MD mode) in (a) and (b), λ = 545 nm (ED mode) in (c) and (d) with Lx = Ly = 300 nm. The incident angle of light is θ = 10°. Each component is normalized to the incident wave flux. The sizes of the circles correspond to the intensity of $S_x^{(m,m^{\prime})}$ and $S_y^{(m,m^{\prime})}$. Geometrical parameters of the slab are the same as Fig. 4.

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The internal structures of the resonance modes corresponding to the three white dots in Fig. 4 are further examined in Fig. 6, where optical energy flow at the resonance in [(a), (d), (g)], 3D cross-sectional plots in [(b), (e), (h)], and schematics of the principal component plots in [(c), (f), (i)] are compared. All the energy flows and 3D field plots were obtained with the RCWA method [35], where the electric and magnetic fields in a unit cell were calculated and converted to field vectors and magnitude plots. Figures 6(a)–(c) show the MD resonance at λ = 658 nm with Lx = Ly = 300 nm. Figures 6(d)–(f) show the ED resonance at λ = 545 nm with Lx = Ly = 300 nm. Figures 6(g)–(i) show the ED resonance at λ = 560 nm with Lx = 413 nm, Ly = 300 nm, and right below $RA_x^{( + )}$ line in Fig. 4(a). The H and E fields are localized at the center of the nanostructure in Figs. 6(a), (d), and (g), which demonstrate the resonance MD and ED modes. The optical energy flow $\vec{S}$ (averaged over y-direction) of the guided- mode wave associated with MD and ED resonances below the $RA_x^{(- )}$ line shows in the opposite direction to the parallel component of the wave vector, ${k_\parallel }$, which is consistent with the analysis of partial components in Fig. 5. Excitation of these guided-mode waves in the slab in the opposite direction of energy flow can be interpreted as negative refractions in metamaterials of nanostructures. The principal partial components of excited guided-mode waves associated with these resonances are schematically indicated by the red waved arrows in Figs. 6(c) and (f).

 figure: Fig. 6.

Fig. 6. Optical energy flow maps in [(a), (d), (g)], cross-sectional plots of 3D field intensities normalized to incident light in [magnetic field (b), electric field (e), (h)], and principal components of guided-mode waves in [(c), (f), (i)] of resonance modes in the slab excited by incident light with TM polarization. Parameters in (a)–(c) correspond to MD resonance at λ = 658 nm (Lx = Ly = 300 nm). Parameters in (d)–(f) correspond to ED resonance at λ = 545 nm (Lx = Ly = 300 nm). Parameters in (g)–(i) correspond to ED resonance at λ = 560 nm (Lx = 413 nm, Ly = 300 nm). The incident angle of light is θ = 10°. Geometrical parameters of the slab are the same as Fig. 4.

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Figure 6(h) represents the optical energy flow $\left\langle {\overrightarrow S } \right\rangle $ averaged over the y-direction of the guided-mode wave associated with the ED resonance at $RA_x^{( - )}$ < Lx < $RA_x^{( + )}$. Because Lx lies above $RA_x^{( - )}$, the ED mode could not hold the (-1,0) component in the structure, resulting in the disappearance of the ED resonance right above the $RA_x^{( - )}$ line. However, as Lx increases further, the ED resonance appears immediately below the $RA_x^{( + )}$ line, which corresponds to guided-mode waves with a (+,0) component in the structure. Thus, this resonance is the result of excitation of the guided-mode wave flowing in the same direction as the parallel wave vector, called the “direct mode”, as represented in Fig. 6(g). Whereas the resonance modes of ED and MD in Figs. 6(a) and (d) are excitations of the guided-mode wave flowing in the opposite direction as the parallel wave vector, called “opposite modes”.

Figure 7 shows the simulation results of the reflectance intensity maps for TE-polarized light, where the same geometries and parameters in Fig. 4 were utilized. The ED and MD resonances exist near 550 nm and 650 nm, respectively, similar to the results in Fig. 4. However, owing to the orthogonal relations between polarizations (TE and TM) and resonance modes (ED and MD), the reflectance intensities with respect to the variation of periods, Lx and Ly were opposite. The MD mode is independent of the $RA_x^{({\pm} )}$ lines and the ED mode is suppressed above $RA_x^{(+ )}$ in Fig. 7(a), whereas both the ED and MD mode are suppressed above RAy line in Fig. 7(b).

 figure: Fig. 7.

Fig. 7. Reflectance intensities in the 2D arrays on a substrate with variations of period, Lx in (a) and Ly in (b), while the other period of orthogonal axis is fixed to 300 nm. The TE-polarized light is illuminated with an incident angle of θ = 10°. The periodic pattern sizes are d = h = 150 nm with a refractive index of n = 4. The refractive index of the substrate is ns = 1.45. Colored dashed lines represent RA conditions defined in Eqs. (14)–(17).

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These results are explained by the partial components of guided-mode waves associated with the MD and ED resonances in Fig. 8, which were estimated at the positions of the white dots in Fig. 7(a), at λ = 650 nm, Lx = 300 nm, and λ = 550 nm, Lx = 300 nm, respectively. The MD resonance at λ = 650 nm and Lx = Ly = 300 nm is excited by the guided-mode waves of partial components ${\vec{S}_1} = S_x^{({0,1} )}{\vec{e}_x} + S_y^{({0,1} )}{\vec{e}_y}$ and ${\vec{S}_2} = S_x^{({0, - 1} )}{\vec{e}_x} + S_y^{({0, - 1} )}{\vec{e}_y}$ with the relative amplitudes of $S_x^{({0, \pm 1} )} = 0.03{S_o}$ and ${\; }S_y^{({0, \pm 1} )} = 0.38\,{S_o}$, which represents the propagation in almost y direction and less sensitive to variation of period Ly as shown in Fig. 7(a).

 figure: Fig. 8.

Fig. 8. Components of Poynting vector normalized to the incident light $S_x^{(m,m^{\prime})}$ in (a), (c) and $S_y^{(m,m^{\prime})}$ in (b), (d) of guided-mode waves excited in the slab by the incident wave with TE polarization, which are calculated at the wavelengths λ = 650 nm (MD mode) in (a) and (b), λ = 550 nm (ED mode) in (c) and (d) with Lx = Ly = 300 nm. The incident angle of light is θ = 10°. Each component is normalized to the incident wave flux. The sizes of the circles correspond to the intensity of $S_x^{(m,m^{\prime})}$ and $S_y^{(m,m^{\prime})}$. Geometrical parameters of the slab are the same as Fig. 4.

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However, the ED resonance at λ = 550 nm, Lx = Ly = 300 nm is excited by the guided-mode waves of several partial components${\; }{\vec{S}_1} = S_x^{({ - 1,0} )}{\vec{e}_x}$, ${\vec{S}_2} = S_x^{({1,0} )}{\vec{e}_x}$, ${\vec{S}_3} = S_y^{({0,1} )}{\vec{e}_y}$ and ${\vec{S}_4} = S_y^{({0, - 1} )}{\vec{e}_y}$ in x and y directions, where the relative amplitudes are indicated in Fig. 8(c) and (d). Thus, the ED resonance disappears above RAy line in Fig. 7(b) and decreases above $RA_x^{(- )}$ line in Fig. 7(a). The existence of ED resonance in the range $RA_x^{(- )} < {L_x} < \; RA_x^{(+ )}$ is ensured by the nonzero component ${\vec{S}_2} = S_x^{({1,0} )}{\vec{e}_x}$ which satisfies the condition of propagation of Eq. (14) for Lx in this range.

3. Experimental result

The above analyses were applied to 2D arrays of Si nanostructures, which were patterned with a size of dx = dy = 120 nm, height of h = 105 nm, and periods of Lx= Ly = 240 nm on a quartz substrate, as shown in Fig. 9.

 figure: Fig. 9.

Fig. 9. Scanning electron microscope image of 2D arrays of Si nanostructures, fabricated on a quartz substrate. (a) top view, and (b) side view images

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The nanostructure arrays have been verified as reflective color filters with high angular tolerances of up to 45° and analyzed by the guided-mode resonance theory in [18,19]. Herein, the changes in the resonant spectra for various incident angles θ and polarization of incident light were reinterpreted using the analytic method in Sec. 2.

Figures 10(a)–(d) show the calculation results of the reflectance intensity of the 2D Si arrays applied in Fig. 9 at an incident angle of θ = 20°. The refractive index of crystalline Si was used in [36]. The reflectance spectra appear similar to the non-absorbing 2D arrays in Fig. 4 and exhibit all features described in Section 2, except that the $RA_{ox}^{( - )}$ line is mid-positioned between the $RA_x^{( + )}$ and $RA_x^{( - )}$ lines. Owing to the smaller pattern size of 120 nm, the ED and MD modes were found near 475 nm and 525 nm, respectively, whereas no resonance modes of EQ and MQ were found [22].

 figure: Fig. 10.

Fig. 10. Reflectance intensities in the 2D arrays on a quartz substrate with variations of period, Lx in (a), (c) and Ly in (b), (d) while the other period of orthogonal axis is fixed to 240 nm. The polarized light is illuminated with an incident angle of θ = 20°. The periodic pattern sizes are dx = dy = 120 nm, and h = 105 nm. Colored dashed lines represent RA conditions as defined in Eqs. (14)–(17).

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Figure 11 shows the reflectance spectra of the Si nanostructures at incident angles of up to 60°, where the measured (solid lines) and simulated (dashed lines) reflectance spectra are in good agreement for both (a) TE and (b) TM polarization. The reflectance profile in the range of 450–550 nm is superposed with ED mode (∼475 nm) and MD mode (∼525 nm), and the modes exhibit different angle dependences. In TE polarization, the peak intensities of the ED mode are approximately 60%, which is 10–15% lower compared to the MD mode at incident angles of θ = 0–20°. However, at an angle above θ = 50°, the peak intensities of the two modes were similar (∼80%). Meanwhile, in TM polarization, both the ED and MD modes decrease with an increase in the incident angle from θ = 0 to 20°: thus, the two peak intensities decrease to ∼20% at incident angles above 50°. Considering that the intensity of the resonance mode is substantially changed near the RA lines, and the RA lines are functions of the incident angle, as shown in Eq. (14)–(17), the different angle dependences can be explained.

 figure: Fig. 11.

Fig. 11. Reflectivity spectral data for different incident angles of TE light (a) and TM incident light (b). The solid (dashed) lines represent the measured (calculation) results.

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Figure 12 shows the calculation results of the reflectance intensities of 2D Si arrays with respect to the variations in Lx at the incident angle of θ = 50°. The orthogonal period Ly was fixed at 240 nm. The period of Lx, corresponding to the RA conditions at the ED and MD resonances, is ∼200–300 nm in Fig. 12, which is comparable to the sample period of Lx = 240 nm. According to the results in Figs. 11 and 12, the changes in the reflectance spectra are consistent with the behaviors of the guided-mode waves of the ED and MD resonances described in Sec. 2. Here, the guided-mode wave associated with MD resonance for TE polarization contains (0,+) and (0,-) components (see Fig. 8(b)), which is not affected by $RA_x^{( - )}$ condition as represented in Fig. 12(a), while the guided-mode wave associated with MD resonance for TM polarization contains only the (-,0) component (see Fig. 5(а)), which disappears at the Lx above $RA_x^{( - )}$ condition. The guided-mode wave associated with ED resonance for TM polarization is a “direct mode” in the range of $RA_x^{( - )}$ < Lx < $RA_{ox}^{( - )}$ and is represented as the marginal intensities of the ED mode (∼450 nm) in the range of 200 nm < Lx < 250 nm in Fig. 12(b). The results are also verified by the experimental results in Fig. 11(b), where the reflectance spectra for incident angles θ > 50° show non-zero intensities at ∼475 nm.

 figure: Fig. 12.

Fig. 12. Reflectance intensities in the 2D Si arrays on a quartz substrate with variations of period Lx. The other period of orthogonal axis is fixed to 240 nm. The light is illuminated with an incident angle of θ = 50° for TE (a) and TM (b). The periodic pattern sizes are d = 120 nm, h = 105 nm with the refractive index of crystalline Si. The refractive index of quartz substrate is ns = 1.5. Colored dashed lines represent the RA lines of Eq. (14) and (15).

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4. Conclusions

We analyzed the internal structures of guided-mode waves associated with ED and MD resonances excited in a layer of dielectric nanoparticles with a high refractive index under the incident light. The guided-mode wave associated with MD optical resonances are induced by the guided-mode wave with energy flux in the opposite direction, parallel to the in-plane direction of incident light for TM polarization, and by two guided-mode waves with energy fluxes orthogonal to the in-plane direction for TE polarization. Owing to the relevant components of Poynting vectors propagating in a close orthogonal direction for TE polarization, the resonance modes are significantly stable up to a large incident angle. Meanwhile, the guided-mode waves associated with ED resonance consist of partial components propagating in both in-plane and orthogonal directions of the incident light and are thus affected by RA conditions due to the diffraction in both directions. This results in the suppression of resonances at specific periods of Lx and Ly or certain incident angles with constant periods. The analyses of guided-mode waves associated with ED and MD resonances successfully explain the stability or vanishing of resonance modes with variations in the inter-distance between nanostructures near the RA condition. The aforementioned theoretical analyses were experimentally verified with changes in the reflectance spectra of Si arrays on a quartz substrate at an oblique incident angle. These works could be used to the potential applications for designing of optical devices, structural color filters, optical switches, and high-performance sensing devices.

Funding

Korea Institute of Energy Technology Evaluation and Planning (20204030200070).

Acknowledgments

This work was supported by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and financial resources were provided by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20204030200070).

Disclosures

The authors declare that there are no conflicts of interest related to this article.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Figures (12)

Fig. 1.
Fig. 1. Dispersion curves of ω(q) for a propagating vector q and guided modes in a planar layer (a) and in periodic arrays (b), where L, no, ns, nH, and nL are the period and the refractive indices of the air, substrate, and high and low index media, respectively. The modes near q∼2π/L can be excited by the incident light through first-order diffraction.
Fig. 2.
Fig. 2. (a) Schematic of 2D arrays of a nanostructure on a substrate illuminated by incident light. The wavy arrows illustrate guided-mode waves in a slab of nanostructures. Lx and Ly are the periods of the nanostructures. The TM and TE polarizations are defined as in (b) and (c).
Fig. 3.
Fig. 3. Propagation vectors q of guided-mode waves which can be excited in the slab by incident light due to first order of diffraction. The blue and red arrows correspond to the waves excited in the incident plane due to diffraction in the x-direction (i.e., m = ±1, m′ = 0) and in the y-direction (i.e., m = 0, m′ = ±1), respectively
Fig. 4.
Fig. 4. Reflectance intensities in 2D arrays on a substrate with variations of period, Lx in (a) and Ly in (b), while the other period of orthogonal axis is fixed to 300 nm. The TM polarized light is illuminated with an incident angle of θ = 10°. The periodic pattern sizes are d = h = 150 nm with a refractive index of n = 4. The refractive index of the substrate is ns = 1.45. Colored dashed lines represent RA conditions defined in Eqs. (14)–(17).
Fig. 5.
Fig. 5. Components of Poynting vectors normalized to the incident light, $S_x^{(m,m^{\prime})}$ in (a), (c) and $S_y^{(m,m^{\prime})}$ in (b), (d) of guided-mode waves excited in the slab by the incident wave of TM polarization, which are calculated at the wavelengths λ = 658 nm (MD mode) in (a) and (b), λ = 545 nm (ED mode) in (c) and (d) with Lx = Ly = 300 nm. The incident angle of light is θ = 10°. Each component is normalized to the incident wave flux. The sizes of the circles correspond to the intensity of $S_x^{(m,m^{\prime})}$ and $S_y^{(m,m^{\prime})}$. Geometrical parameters of the slab are the same as Fig. 4.
Fig. 6.
Fig. 6. Optical energy flow maps in [(a), (d), (g)], cross-sectional plots of 3D field intensities normalized to incident light in [magnetic field (b), electric field (e), (h)], and principal components of guided-mode waves in [(c), (f), (i)] of resonance modes in the slab excited by incident light with TM polarization. Parameters in (a)–(c) correspond to MD resonance at λ = 658 nm (Lx = Ly = 300 nm). Parameters in (d)–(f) correspond to ED resonance at λ = 545 nm (Lx = Ly = 300 nm). Parameters in (g)–(i) correspond to ED resonance at λ = 560 nm (Lx = 413 nm, Ly = 300 nm). The incident angle of light is θ = 10°. Geometrical parameters of the slab are the same as Fig. 4.
Fig. 7.
Fig. 7. Reflectance intensities in the 2D arrays on a substrate with variations of period, Lx in (a) and Ly in (b), while the other period of orthogonal axis is fixed to 300 nm. The TE-polarized light is illuminated with an incident angle of θ = 10°. The periodic pattern sizes are d = h = 150 nm with a refractive index of n = 4. The refractive index of the substrate is ns = 1.45. Colored dashed lines represent RA conditions defined in Eqs. (14)–(17).
Fig. 8.
Fig. 8. Components of Poynting vector normalized to the incident light $S_x^{(m,m^{\prime})}$ in (a), (c) and $S_y^{(m,m^{\prime})}$ in (b), (d) of guided-mode waves excited in the slab by the incident wave with TE polarization, which are calculated at the wavelengths λ = 650 nm (MD mode) in (a) and (b), λ = 550 nm (ED mode) in (c) and (d) with Lx = Ly = 300 nm. The incident angle of light is θ = 10°. Each component is normalized to the incident wave flux. The sizes of the circles correspond to the intensity of $S_x^{(m,m^{\prime})}$ and $S_y^{(m,m^{\prime})}$. Geometrical parameters of the slab are the same as Fig. 4.
Fig. 9.
Fig. 9. Scanning electron microscope image of 2D arrays of Si nanostructures, fabricated on a quartz substrate. (a) top view, and (b) side view images
Fig. 10.
Fig. 10. Reflectance intensities in the 2D arrays on a quartz substrate with variations of period, Lx in (a), (c) and Ly in (b), (d) while the other period of orthogonal axis is fixed to 240 nm. The polarized light is illuminated with an incident angle of θ = 20°. The periodic pattern sizes are dx = dy = 120 nm, and h = 105 nm. Colored dashed lines represent RA conditions as defined in Eqs. (14)–(17).
Fig. 11.
Fig. 11. Reflectivity spectral data for different incident angles of TE light (a) and TM incident light (b). The solid (dashed) lines represent the measured (calculation) results.
Fig. 12.
Fig. 12. Reflectance intensities in the 2D Si arrays on a quartz substrate with variations of period Lx. The other period of orthogonal axis is fixed to 240 nm. The light is illuminated with an incident angle of θ = 50° for TE (a) and TM (b). The periodic pattern sizes are d = 120 nm, h = 105 nm with the refractive index of crystalline Si. The refractive index of quartz substrate is ns = 1.5. Colored dashed lines represent the RA lines of Eq. (14) and (15).

Equations (19)

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L λ / n s ,
E ( x , y , z ) = m , m E ( m , m ) ( z ) e i q x ( m ) x e i q y ( m ) y H ( x , y , z ) = m , m H ( m , m ) ( z ) e i q x ( m ) x e i q y ( m ) y
q x ( m ) = k + m 2 π L x q y ( m ) = m 2 π L y
E ( m , m ) ( z ) = E + ( m , m ) e i κ z z + E ( m , m ) e i κ z z H ( m , m ) ( z ) = H + ( m , m ) e i κ z z + H ( m , m ) e i κ z z
κ z ( m , m ) h + ϕ 1 + ϕ 2 = π N
| q x ( m ) | max { n o , n s } 2 π λ | q y ( m ) | max { n o , n s } 2 π λ
z 2 E x = ( k o 2 ε y 2 x 1 ε x ε ) E x + ( x y x 1 ε y ε ) E y z 2 E y = ( x y y 1 ε x ε ) E x + ( k o 2 ε x 2 y 1 ε y ε ) E y
z 2 E = L ^ { E }
L ^ { E p } = κ p 2 E p
E p = E p ( + ) e i κ p z + E p ( ) e i κ p z
E p ( ± ) = m , m { E p , x ( ± ) ( m , m ) + E p , y ( ± ) ( m , m ) } e i q x ( m ) x e i q y ( m ) y
S p ( ± ) = c 8 π [ E p ( ± ) × H p ( ± ) ]
S p ( ± ) = m , m S p ( ± ) ( m , m ) ,
S p ( ± ) ( m , m ) = c 8 π [ E p ( ± ) ( m , m ) × H p ( ± ) ( m , m ) ]
| q ( m , m ) | > 2 π λ max { n o , n s }
R A x ( ± ) = λ [ n s n o sin ( θ ) ]
R A x o ( ± ) = λ [ n o n o sin ( θ ) ]
R A y = λ n s 2 n o 2 sin 2 ( θ )
R A o y = λ n o 2 n o 2 sin 2 ( θ )
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