Briefly describing the history of RWGs, when Wood in 1902 observed sharp spectral variations in the intensity of a metallic grating for TM-polarized light (electric field vector is perpendicular to grating lines), which he named anomalies (Wood, 1902). A theoretical explanation of these anomalies was given by Rayleigh in 1907 as one spectral order appearing at grazing incidence and occurred at a particular wavelength called Rayleigh wavelength and found close to Wood anomalies (Hessel and Oliner, 1965). Thereby, anomalies were categorized as Rayleigh and Resonance, which both were used for filtering applications. The employment of such anomalies of grating structures in dielectric materials referred to the term: RWGs or GMRFs (Saarinen et al., 2005). A diffraction grating consists of a periodic modulation of refractive index and guided mode resonance phenomena occurs when diffracted light from a diffraction grating couples with a leaky waveguide mode and satisfying phase-matching conditions, which results in a narrow linewidth peak at a particular wavelength that depends on a selectable range of optical parameters (Avrutskii et al., 1985). In this section, we briefly discuss the role of ALD-grown thin films in the development of RWGs.

The structure of a RWG is shown in Figure 6A, which consists of a substrate with refractive index n₃, an integrated diffraction grating with refractive index distribution n₂(x) along x-direction and a medium from where light is incident (usually air) with refractive index n₁. Figure 6A shows the direction of various propagating diffraction orders and can be simply calculated from the Eq. (6) (Saleem, 2012) (6)n2sinθm=n1sinθ1+mλ∕d, where d is grating period, λ is wavelength of incident light, θ₁ is incident angle, m = 0, ± 1, ± 2, ± 3, … is the index of diffraction order, n₁ and n₂ are the refractive indices before and after the air-dielectric interface.

Resonant waveguide gratings have been extensively studied since last couple of decades and several researchers fabricated RWG structures on various substrates employing ALD films to investigate their waveguiding properties, propagation losses, and fine tune the optical spectra (Alasaarela et al., 2010, 2011b; Saleem et al., 2011a, 2012a,d, 2013a,b, 2014a,c,d; Saleem, 2012). The fabrication of silicon photonic nano-structures is established about a decade using state-of-the-art 198 or 248 nm deep UV-lithographic techniques, which limit the structure size around 100 or 160 nm, respectively. For Si-slot waveguides, the achieved feature size was below 100 nm by partial filling with ALD-grown materials as well as to reduce the propagation losses (Alasaarela et al., 2009, 2011a; Säynätjoki et al., 2011; Karvonen et al., 2014). Figure 6B shows cross-section view of SEM image of a Si-slot waveguide coated by amorphous TiO₂ film with a smooth surface showing peak-to-peak surface roughness <0.4 nm over an AFM measured area of 1 × 1 μm (Alasaarela et al., 2010).

Polycrystalline ALD-TiO₂ films grown at 350°C exhibit propagation losses due to grain boundaries that can be minimized by depositing an intermediate ALD-grown layer of Al₂O₃ as shown in Figures 6B,C. As an example of conformal ALD-growth, RWG structure of a binary grating profile in fused silica substrate is coated by amorphous film of TiO₂ as shown in Figure 6D. Figures 6E,F show SEM images of TiO₂ films on a silicon binary grating and a sinusoidal grating profile fabricated on a hydrogen bonded polymer (Azobenzene Complex) that was spin coated on a glass substrate, respectively. The grating pattern was written using circularly polarized light of 488 nm Ar⁺ laser source of 300 mW/cm² intensity to expose polymer. The fabricated polymer grating profile was coated with conformal TiO₂ thin film by ALD at 80°C to avoid thermal degradation (Alasaarela et al., 2011b). Likewise, Figures 6G,H show the ALD conformal growth of Al₂O₃ on high aspect ratio corrugated structures (Ritala et al., 1999; Weber et al., 2012a). In Figures 6I–K silicon slot waveguide patterns to be filled by ALD-grown materials and the subsequent filling by ALD-Al₂O₃ and ALD-TiO₂ in nano slots are shown, respectively. Moreover, the slots are filled as to demonstrate extremely uniform nanolaminate structure of five layers with 10 nm of each ALD-Al₂O₃ and ALD-TiO₂ with reduced propagation losses compared to crystalline ALD-TiO₂ films (Alasaarela et al., 2011a).

The optical field in the waveguide structures must be confined and guided through with minimum field loss owing to controlled sidewall surface roughness without leakage into substrate that has been accompanied through uniform surface coatings of high index material(s). Alasaarela et al. (2010, 2011a) have shown that such propagation losses due to surface roughness could be reduced by conformal coatings of ALD-TiO₂ films for which a further reduction in losses was measured by increasing film thickness. This attributes to increase in an effective index of waveguide to strongly confine the propagating waveguide modes and a significant reduction in surface roughness. Table 1 illustrates the values of waveguide losses (in decibels per centimeter) with an increase in the thickness of TiO₂ film. We recently studied, theoretically, the optical dispersion engineering properties of silicon-strip waveguides using ALD-TiO₂ films as overlayer on nano-structures (Erdmanis et al., 2012).

Low temperature ALD-growth is of much more importance for polymeric substrate waveguides in applications for various bio-molecular sensors and RWGs (Triani et al., 2010; Magnusson et al., 2011). Nanoimprint technology was proposed by (Chou et al., 1997) in 1990s to fabricate replicated nano-structures. Transparent optical polymers have emerged as potential candidates in nanophotonic functional devices through replication in a wide variety of thermo-plastics (Herzig, 1997; Jaszewski et al., 1998; Mönkkönen et al., 2002; Liou and Chen, 2006) using replication tools of high replication fidelity and resolution to fabricate high aspect ratio structures (Pietarinen et al., 2007; Siitonen et al., 2007; Cui, 2008; Worgull, 2009). Recently, we have demonstrated conformal growth of ALD technique to fill various sub-wavelength RWG structures. A silicon master stamp was fabricated using hydrogen silsesquioxane (HSQ) resist without Reactive Ion Etching (RIE) process, which employed as to replicate sub-wavelength grating structures with different periods in thermoplastic polymers as shown in Figures 7A,D (Saleem et al., 2012c). The replicated grating structures in polycarbonate substrate with periods d = 425 and 368 nm are shown in Figures 7B,E and subsequently coated by thin amorphous films of ALD-TiO₂ as shown in Figures 7C,H. Likewise, the replication of corrugated profiles in other polymers such as COC and OrmoComp and their overlayer ALD-TiO₂ coatings are shown in Figures 7F,G and 7I,J, respectively. Figure 7K shows a replicated grating profile with period d = 540 nm, coated by 60 nm of ALD-TiO₂ in applications of non-polarizing gratings (Saleem et al., 2012e). A high aspect ratio replicated structure followed by ALD-growth is depicted in Figure 7L. Saleem et al. (2011b) used such replicated polymeric gratings as athermal bio-molecular sensors within accuracy of a fraction of a nanometer. Furthermore, such sub-wavelength replicated grating structures in various polymers were compared in terms of their resonance peak stability, efficiency, full width half maximum (FWHM), ease of fabrication, and residual stresses for their use in potential applications (Saleem et al., 2013d). The interfacial adhesion of ALD-TiO₂ films on polycarbonate substrate has been demonstrated by theoretical models based on experimental measurements (Triani et al., 2005; Latella et al., 2007).

A filtering design is recently proposed by Saleem et al. (2014b) with its real experimental demonstration to present the influence of ALD-TiO₂ thickness layer on replicated nanostrcutures of RWGs. Polymeric rectangular grating profile with a ridge height h = 120 nm, period d = 425 nm, ridge width w = 268 nm, and duty cycle ff = w/d = 0.63 was covered by a thin dielectric TiO₂ cover layer of thicknesses t = 60 and 75 nm, grown by ALD. The device was illuminated by a linearly polarized plane wave (TE polarization) from air at an angle of incidence θ_i = 20° and the specularly reflection was obtained at θ_o = 20°. Figure 8A shows the calculated field at TiO₂ thicknesses of 60 and 75 nm and Figure 8B shows the simulated results by Fourier Modal Method (FMM) to predict spectral shift at (100%) maximum reflectance as a function of variation in waveguide thickness (TiO₂ layer). A spectral shift of ~28 nm was calculated theoretically with an increase in ALD-TiO₂ thickness by ~15 nm. Experimental measurements show specular reflectance resonance peaks at wavelengths 827 and 851 nm for t values of 60 and 75 nm, respectively as shown in Figure 8C. Such spectral shifts are attributed to increase in effective refractive index of waveguide, which result in an interaction of grating structure at longer wavelengths to originate resonance. Furthermore, simulated and experimentally measured results show slightly different spectral locations, which may arise due to surface roughness, true refractive indices of materials, structure linewidths, and slight variations in the periodicity after a number of fabrication steps (Saleem et al., 2011b).

ALD-grown nano-overlayers on metallic nanoplasmonic structures not only protect surface but also enhance the optical transmission. In most plasmonic devices operating at visible or infrared regimes, Ag or Au metals are used. At wavelengths below 600 nm, Ag has low ohmic losses, higher propagation lengths (low SPPs damping), higher sensitivity for biosensors, and are used in applications where a direct interface with biological molecules is not required, such as electrodes for plasmonic solar cells (Atwater and Polman, 2010). Silver Ag is used due to cost-effective solutions, better optical properties and relatively strong interfacial adhesion with glass substrates than Au (Ferry et al., 2008; Lindquist et al., 2008). Despite these advantages, Ag has lower chemical stability and readily oxidized in the air and shows a reduction in optical intensity in nanoplasmonic structures without passive layer (Im et al., 2010). While the Ag surfaces can be protected and improved in functionality by an overlayer of dielectric coating (~10–20 nm), which must be less than mean decay length of SPPs in order to annul from being absorption of SP field in thicker layers.

These ultra-thin barrier films must be dense, pinhole free, encapsulate the patterned structure entirely (Saleem et al., 2013c) and grown at a low deposition temperature to avoid degrading or oxidizing of underlying Ag film. Similarly, the use of silver nanoparticles coated by ~1 nm Al₂O₃ layer was investigated to preserve plasmon resonance peak after annealing at 500°C and have shown to degrade resonance peaks without Al₂O₃ layer even at annealing temperature of 200°C (Whitney et al., 2007). Likewise, Sung et al. (2008) showed enhanced thermal stability of Al₂O₃ coated silver nanoparticles against high power femtosecond laser. Self-assembly based plasmonic arrays tuned by minimum thickness of ALD fabricated (<20 nm) Au dots achieved enhanced visible light absorption per volume-equivalent thickness in applications including conversion of solar energy into electrical power (Hägglund et al., 2013). One of the possible reason of enhancing optical transmission is conformal and precisely uniform deposition of dielectric thin films on nano-structures is its predominance over rough topographies.

The plasmonic resonance depends on the geometric parameters, incident angle, or wavelength, surrounding environmental conditions (dielectric constants), molecular layer bindings or adsorption, surface roughness, and the thickness of an overlayer on the metallic nano-structures. A change in any one of these parameters modify the resonance properties and ultimately shifts the resonance peak. Figure 9B shows an SEM image of nanohole array structure fabricated on Au (with 5 nm Cr adhesive layer) metallic film of 200 nm thickness by Focused Ion Beam with a period of 500 nm and hole diameter of 180 nm. A schematic of a nanoplasmonic structure coated by ALD-Al₂O₃ is shown in Figure 9D while top and cross-sectional views of SEM images of ALD-Al₂O₃ overlayer on metallic nanohole array are shown in Figures 9B,C and Figure 9E, respectively. Since, the ALD-Growth fills the nanoholes conformably and gives rise resonance spectral shifts as a result of tuning effective index seen by SPs, which is approximated by a simple model (Jung et al., 1998): (7)neff=nbulk+nfilm−nbulk1−e(−2t∕l), where n_bulk and n_film are the refractive indices of the surrounding bulk materials and thin deposited film, respectively, t is the thickness of the film and l is the decay length of SPs evanescent field perpendicular to the surface. It can be seen from Eq. (7) that as the thickness t of Al₂O₃ film increases, effective index increases from n_bulk to n_film, which results in a red-shift: here n_bulk is air index and n_film is Al₂O₃ index. Thereby, by varying t one can tune the resonance frequency of SPs precisely, since ALD grows films up to angstrom scale resolution. Furthermore, an increase in the t results in higher intensity that might be attributed to low SPs damping at relatively longer wavelengths as well as an increase in the periodicity of the structure reported by Przybilla et al. (2006). Furthermore, filling of nanoholes by ALD-Al₂O₃ not only results in higher effective index and reduction in waveguide-like attenuation of sub-wavelength structures for optical transmission enhancement but also encompasses index matching on both sides of the film. Thus, making an asymmetric peak to symmetric one with a transformation of (air–metal–glass) interfaces to (air–alumina–metal–glass) that boost optical transmission at a certain Al₂O₃ thickness where SPs wave sees almost a similar index above and below metallic structures (Krishnan et al., 2001).

The effects of Al₂O₃ thickness on the spectral shifts have been investigated by theoretical calculations using 3D Finite Difference Time Domain (FDTD) simulations and compared with experimental results, not shown here [see Im et al. (2010)]. Figures 9F,G show a comparison of calculated results with ALD-Al₂O₃ deposition with a step of 6 and 10 nm on gold sample, respectively and white dashed lines depict resonance maxima and transmission minima. An increase in ALD-Al₂O₃ thickness not only results in red-shift but also increases transmission intensity by a factor of about 5 at certain thickness value. Likewise, Ag-nanohole arrays show the calculated results and their consistency with experimentally measured ones, as seen in Figures 9H,I.

3D FDTD simulations were carried out to validate the optical field distribution in each nanohole as a function of index matching on top and bottom sides of metallic films. In these simulations the bulk refractive index over nanohole arrays is varied from 1.35 to 1.55. Figure 9J shows that the maximum transmitted field intensity and output power occurs when the n_bulk above and inside of each nanohole matches to that of the glass substrate with index 1.45 and Krishnan et al. (2001) demonstrated that under such symmetric conditions optical transmission enhances by a factor of 10 compared to asymmetric interfaces. Figure 9K shows FDTD calculated results of filed distributions in nanohole arrays for different values of Al₂O₃ overlayer by considering the refractive indices of glass and alumina 1.45 and 1.65, respectively. At symmetric condition one expects that the effective index seen by the SP waves approximately matches to that of substrate (glass) and maximum electromagnetic power distribution occurs at certain thickness value of t = 60–70 nm. Hence, ALD overlayer can tune and shift transmission spectrum in a precisely and controlled manner, which is of significant importance in biochemical sensing, surface plasmon resonance imaging (Chinowsky et al., 2004), biomimetic sensing, and membrane protein research.