The field of thin-film optics is an important area in optical technologies. A huge variety of filters and optical devices are produced and used in numerous industries and for a wide range of applications. Most of these thin-film optical structures, assemblies and devices consist of homogeneous layers deposited with precisely controlled thicknesses and material parameters and they are most often realized in a low pressure atmosphere needing a sophisticated technology. Examples of devices using such multilayer filters include, but are not limited to, antireflection filters, low-and high pass filters, phase plates, flat beamsplitters, polarization filters, micromirrors. In several of these devices thin film optical structures may be arranged so that their combination widens the potential use of these devices.
A significant drawback associated with these thin-film optical devices is that very often a large number of layers are needed to obtain a significant optical effect. It is not exceptional that the required number of stacked layers is higher than 50. These optical devices function mainly by multiple reflections between the interfaces of the different stacked layers of the device. The complexity of the technology to master the optical quality of such devices can be considerably high and is thus expensive. Also, adhesion and stability problems associated with multiple stacked layers may be a problem. Scattering effects and unwanted reflections inside the optical stack are a current problem especially if special optical effects are to be obtained such as occurring in high quality filters, interferometry or high power laser applications where any stray light may be a limiting factor.
Gratings have also been used widely as devices to disperse and filter optical beams. The combination of gratings and waveguides has been proposed to make optical structures and devices with unique properties such as filters having very narrow bandwidths. More particularly a considerable amount of development work has been made in the field of resonating waveguide gratings as they allow producing particularly interesting optical effects that cannot be realized with classical optical components.
A resonant waveguide grating, also called guided-mode resonance filter, consists of a combination of a sub-wavelength grating and a thin film waveguide. Such structures have a multilayer configuration and a basic arrangement comprises a substrate, a thin dielectric or semiconductor waveguide layer and possibly an additional layer in which a grating is formed. A so-called resonance occurs when the incident light is diffracted by the grating and matches a mode of the waveguide. As most of the spectrum does not couple into the waveguide, strong spectral changes are observed in reflection and transmission. The existence of such resonances has been discovered in the earlier stages of grating developments (R. W. Wood, Phil. Mag. vol 4, pp. 396-402, 1902). These resonances belong to one type of the anomalous diffraction phenomena in grating structures and imply a rapid variation in the external observable diffracted orders with respect to physical parameters such as the angle and/or the wavelength of the incident wave. In the early stages of grating manufacturing the abrupt change of reflection could not be explained. Hessel and Oliner (Appl. Optics, vol. 4, pp. 1275-1297, 1965) pointed out that there are basically two types of grating anomalies. One is called the Rayleigh type, which is the classical Wood's anomaly, and another is called the resonance type. The Rayleigh-type anomaly is owing to the energy of higher diffracted order transferred to an evanescent wave.
The resonance anomaly in diffraction gratings, being of particular interest in the context of the current patent application, is due to the coupling process of externally incident wave to a surface guided wave which is supported by the structure. Such grating anomalies can be divided into two types in function of the type of the structure and accurate results can be obtained by using the Fourier-Rayleigh approximation. This method cannot be applied in the case of deep grating grooves. Several authors investigated the reflection from weakly corrugated waveguides. The convergence problems of deep grating grooves could be relaxed by using the rigorous simulation methods such as the Fourier-Modal-Method (FMM) or the Rigorous Coupled-Wave Analysis (RCWA). With these new mathematical tools, many devices have been proposed and developed the last decade.
One of the main applications of guided mode resonance structures has been the design of filters with very narrow spectral linewidths in reflection and transmission. The bandwidth can be designed to be extremely narrow and of the order of 0.1 nm and may be tuned by parameters such as the grating depth, the duty cycle and the thickness of the waveguide layer. Magnusson proposed wavelength selective reflection filters and investigated their line shapes (R. Magnusson and S. S. Wang: “New principle for optical filters”, Appl. Phys. Lett., vol. 61, pp. 1022-1024, 1992). Also, a systematic analysis of resonant grating waveguide structures has been published by Rosenblatt and Sharon:                D. Rosenblatt et al., “Resonating grating waveguide structures”, IEEE J. Quantum Electron., vol. 33, nr. 11. pp. 2038-2059, 1997;        A. Sharon et al.: “Resonating grating-wavegudie structures for visible and near-infrared radiation:”, J. Opt. Soc. Am., vol. 14, nr. 11, pp. 2985-2993, 1997.        
Rosenblatt and Sharon explained in these papers that the efficient transfer of wave energy between forward and backward propagations at resonance is due to the relative phase-shift between the incident and the diffracted waves, resulting in destructive and constructive interference of forward and backward propagating waves.
Limitations of resonating waveguide structures are particularly linked with the fact that specular reflection phenomena reduce the performances of filters. These specular reflection effects may also limit the performance of resonating waveguide structures when they are used to produce specific colors. The color hue is limited by internal reflection effects and/or by effects due to specular reflection.
Guided mode resonance devices may also be used as components in sensors. By applying a substance such as a gas or a bio-chemical layer in contact with the resonating waveguide, these substances may be detected. A limitation of guided mode resonance devices in sensors is that the interaction length of the waves interacting with said substances is limited and thus the effects obtained are small.
An example of use of a resonating waveguide-grating as a sensor to detect the presence of a gas is described in the article of L. Davoine et al.: “Resonant absorption of a chemically sensitive layer based on waveguide gratings”, Applied Optics, pp. 340-349, vol. 52, nr. 3, 2013. In such a device the major drawback is the inherent leakage of light along the waveguide, therefor the resonant light cannot be absorbed completely. In addition a delicate trade-off has to be chosen between a possible absorption enhancement and the resonance bandwidth of the resonating waveguide-structure.
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