We present a theory of the Fano resonance for optical resonators, based on a temporal coupled-mode formalism. This theory is applicable to the general scheme of a single optical resonance coupled with multiple input and output ports. We show that the coupling constants in such a theory are strongly constrained by energy-conservation and time-reversal symmetry considerations. In particular, for a two-port symmetric structure, Fano-resonant line shape can be derived by using only these symmetry considerations. We validate the analysis by comparing the theoretical predictions with three-dimensional finite-difference time-domain simulations of guided resonance in photonic crystal slabs. Such a theory may prove to be useful for response-function synthesis in filter and sensor applications.

/pdf/temporal-coupled-mode-theory-for-the-fano-resonance-in-2okwo9a7sx.pdf

Temporal coupled-mode theory for the Fano resonance in optical resonators

Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure "resonates"; namely, complete interference occurs and no light is transmitted. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation.

Resonant grating waveguide structures

We propose an active plasmonic device based on graphene. Highly confined plasmonic waves in monolayer graphene are efficiently excited using an etched diffractive grating on silicon. The guided-wave resonance of the combined structure creates a sharp notch on the normal-incidence transmission spectra, as the incident optical wave couples to the graphene plasmonic wave. This structure can be used as a highly tunable optical filter or a broad-band modulator because the resonant wavelength can be quickly tuned over a wide wavelength range by a small change in the Fermi energy level of the graphene. In this paper, we analyze the performance of this device with finite-difference time-domain simulations. We compare the proposed structure with recently demonstrated graphene nanoribbons based on bound plasmonic oscillations.

http://www-ece.rice.edu/ece/xugroup/Papers/Excitation%20of%20Plasmonic%20Waves%20in%20Graphene%20by%20Guided-Mode%20Resonances.pdf

Excitation of Plasmonic Waves in Graphene by Guided-Mode Resonances

Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of pho- tonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou- plers, spectrometers, high-reflectivity mirrors, athermal waveg- uides, multimode interference couplers, and dispersion engi- neered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201400083

Waveguide sub-wavelength structures: a review of principles and applications

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

/pdf/engineering-metallic-nanostructures-for-plasmonics-and-3c47ox5c4s.pdf

Engineering metallic nanostructures for plasmonics and nanophotonics

A new ray picture model based on the multiple interference of light waves in dielectric resonant grating–waveguide structures is presented. The model clearly elucidates the phase relationship between the incident plane wave and the waves diffracted from the resonant grating structure that is responsible for the interference of these waves. As a result of this interference process the incident wave can be totally reflected at a certain wavelength and orientation angle. The model is used to describe and analyze this resonance behavior of the grating–waveguide structures as a function of wavelength and incidence angle. The analysis is verified experimentally with semiconductor (InGaAsP/InP) structures at wavelengths of 1.55 μm and also with dielectric (silicon nitride/SiO2) structures at wavelengths of 0.6 μm. All of the structures were formed by electron beam lithography and chemical vapor deposition. The measured results reveal that subnanometer resonance bandwidths and finesses as large as 6000 can be achieved at contrast ratios of 50 with relatively compact structures.

Resonant grating–waveguide structures for visible and near-infrared radiation

Resonant grating–waveguide structures formed with InP/InGaAsP semiconductor materials were tested to show light modulation at a wavelength of 1.55 μm. Narrow, subnanometer resonant spectral bandwidths and a ratio of reflected intensities between resonance and away from resonance of greater than 50 were measured. For a resonant structure with an area of 3 mm × 3 mm, the modulation frequency reached 5 MHz.

Light modulation with resonant grating–waveguide structures

A new analytic model, based on the interference of waves, and numerical calculations are exploited for analyzing the spectral behavior of the transmitted and reflected intensities from metal-based grating–waveguide structures. The results reveal that strong intensity changes can occur near resonance with resonance bandwidths in the subnanometer range. These intensity changes are found to be strongly dependent on specific parameters of the structures, particularly the height of the grating and the absorption in the metallic layer. Several structures were fabricated and evaluated experimentally to demonstrate that spectral bandwidths of 0.1 nm and intensity changes as large as 60% can be achieved.

A. Sharon

Papers

Resonant grating waveguide structures

Resonant grating–waveguide structures for visible and near-infrared radiation

Light modulation with resonant grating–waveguide structures

Metal-based resonant grating waveguide structures

Spectral shifts and line-shapes asymmetries in the resonant response of grating waveguide structures