Yiwu Ding

Resonant leaky-mode spectral-band engineering and device applications.

Single-layer subwavelength periodic waveguide films with binary profiles are applied to design numerous passive guided-mode resonance elements. It is shown that the grating profile critically influences the spectral characteristics of such devices. In particular, the symmetry of the profile controls the resonance spectral density. Symmetric profiles generate a single resonance on either side of the second stopband whereas two resonances arise, one on each side of the band, for asymmetric structures. Moreover, the profiles Fourier harmonic content, along with the absolute value of the grating modulation strength, affects the resonance linewidths and their relative locations. Computed Brillouin diagrams are presented to illustrate many key properties of the resonant leaky-mode spectra in relation to modulation strength and profile symmetry at the second stopband. Associated mode plots elucidate the spatial distribution of the leaky-mode field amplitude at resonance and show that, for small modulation, the mode shape may be simple whereas at higher modulation, the shape appears as a complex mixture of modes. By computing device spectra as function of the modulation strength, the buildup of the final spectral properties is illustrated and the contributions of the various leaky modes clarified. The results presented include wavelength and angular spectra for several example devices including narrow linewidth bandpass filters with extended low sidebands for TE and TM polarization, wideband reflectors for TE and TM polarization, polarizer, polarization-independent element, and a wideband antireflector, all with only a single binary layer with one-dimensional periodicity. These results demonstrate new dimensions in optical device design and may provide complementary capability with the field of thin-film optics.

Use of nondegenerate resonant leaky modes to fashion diverse optical spectra.

In this paper, we show that bandstop and bandpass filters with versatile spectral attributes can be implemented with modulated films possessing asymmetric grating profiles. The profile asymmetry breaks the resonant leaky mode degeneracy at normal incidence thereby permitting precise spectral spacing of interacting leaky modes with interesting implications in optical filter design. Several example filters, containing only a single grating layer, are designed with this methodology to demonstrate the concept.

Resonant Photonic Biosensors with Polarization-Based Multiparametric Discrimination in Each Channel

In this paper, we describe guided-mode resonance biochemical sensor technology. We briefly discuss sensor fabrication and show measured binding dynamics for example biomaterials in use in our laboratories. We then turn our attention to a particularly powerful attribute of this technology not possessed by competing methods. This attribute is the facile generation of multiple resonance peaks at an identical physical location on the sensor surface. These peaks respond uniquely to the biomolecular event, thereby enriching the data set available for event quantification. The peaks result from individual, polarization-dependent resonant leaky modes that are the foundation of this technology. Thus, by modeling the binding event and fitting to a rigorous electromagnetic formalism, we can determine individual attributes of the biolayer and its surroundings and avoid a separate reference site for background monitoring. Examples provide dual-polarization quantification of biotin binding to a silane-coated sensor as well as binding of the cancer biomarker protein calreticulin to its monoclonal IgG capture antibody. Finally, we present dual-polarization resonance response for poly (allylamine hydrochloride) binding to the sensor with corresponding results of backfitting to a simple model; this differentiates the contributions from biolayer adhesion and background changes.

Band gaps and leaky-wave effects in resonant photonic-crystal waveguides.

The paper addresses the leaky stop bands associated with resonant photonic crystal slabs and periodic waveguides. We apply a semianalytical model pertinent to the second band to compute the dispersion curves describing the leaky stop band and verify its correctness by rigorous band computations. This approximate model provides clear insights into the physical properties of the leaky stop band in terms of explicit analytical expressions found. In particular, it enables comparison of the structure of the bands computed in complex propagation constant, implying spatially decaying leaky modes, with the bands computed in complex frequency, implying temporally decaying modes. It is shown that coexisting Braggcoupling and energy-leakage mechanisms perturb the bands in complex propagation constant whereas these mechanisms are decoupled in complex frequency. As a result, the bands in complex frequency are well defined exhibiting a clear gap. These conclusions are verified by numerical diffraction computations for both weak and strong grating modulations where the resonance peaks induced by external illumination are shown to closely track the band profile computed in complex frequency. Thus, in general, phase matching to a resonant leaky mode occurs via real propagation constant that is found by dispersion computations employing complex frequency.

Photonic devices enabled by waveguide-mode resonance effects in periodically modulated films

The chief properties and possible applications of periodic waveguides and their leaky modes are presented in this paper. After summarizing the basic physics of the guided-mode resonance, computed leaky-mode field patterns are provided to illustrate their structure and the high local focal field enhancement obtainable. An example fabricated bandstop filter is found to exhibit 90% efficiency, 1 nm linewidth, and low sidebands. Computed spectra for a single-layer bandpass filter operating at 1.55 μm wavelength yield low sidebands, extending 100 nm, and an angular aperture of ~1.7°. Resonant vertical-cavity surface-emitting lasers (VCSEL) are presented in which multilayer Bragg-stack mirrors are replaced with leaky-mode resonance layers. The use of guided-mode resonance mirrors provides optical power flow across and laterally along the laser active region. The round-trip gain is thereby increased resulting in high laser efficiency and relaxed mirror reflectivity constraints. As the GMR mirror achieves high reflectivity at resonance, the laser wavelength is locked at the resonance wavelength principally defined by the period. Example resonant VCSEL embodiments are shown along with their computed characteristics. Resonant biosensors are addressed last. The high parametric sensitivity of the guided-mode resonance effect, a potential limitation in filter applications, can be exploited for sensors as illustrated by several examples.

Silicon-Layer Guided-Mode Resonance Polarizer With 40-nm Bandwidth

Fabrication and characterization of a guided-mode resonance-based polarizer is presented. This polarizer is made by electron-beam patterning a single layer of amorphous silicon on a glass substrate. The fabricated device has high and low transmittance for transverse-electric and transverse-magnetic polarizations, respectively, over a ~ 40-nm wavelength range for normally incident light. Its experimental extinction ratio is ~ 97:1 at a central wavelength of 1510 nm.

MEMS tunable resonant leaky mode filters

A tunable double-grating resonant leaky mode microelectromechanical-type element is introduced. A significant level of tunability is demonstrated by adjusting mechanically the structural symmetry of the grating profile. Tuning is also possible by variation of the thickness of the resonant layer. For a particular example silicon-on-insulator structure treated, it is shown that the resonance wavelength can be shifted by /spl sim/300 nm with a horizontal movement of /spl sim/120 nm within the 1.4-1.7-/spl mu/m wavelength band. Additionally, the reflectance can be varied from /spl sim/10/sup -3/ to 1 at central wavelength of 1.55 /spl mu/m with a vertical movement of /spl sim/200 nm. These results demonstrate new possibilities in design of tunable optical devices.

Mapping Surface-Plasmon Polaritons and Cavity Modes in Extraordinary Optical Transmission

Transmission of light through periodic metal films with intensity considerably exceeding that predicted by aperture theory is referred to as extraordinary optical transmission (EOT). The mechanisms responsible for this effect have been investigated intensively during the past decade. Here, we show an elegant method of visualizing the operative physical mechanisms for model resonance systems. By numerically mapping the resonance loci, modal and plasmonic mechanisms emerge clearly with delineated regions of dominance. Thus, the photonic transmission resonances are parametrically correlated with localized electromagnetic fields forming pure surface-plasmon polaritons (SPPs), coexisting plasmonic and cavity-mode (CM) states, and pure CMs. This mapping method renders a consistent picture of the transitions between photonic states in terms of key parameters. It shows how the TM1 CM seamlessly morphs into the odd SPP mode as the film thickness diminishes. Similarly, the TM0 mode converts to the even SPP mode. At the intersection of these mode curves, an EOT-free gap forms due to their interaction. On account of a reflection phase shift of a slit-guided mode, an abrupt transition of the resonance loci in the SPP/CM region is observed.

Characteristics of resonant leaky-mode biosensors

This paper presents key properties and examples of applications of resonant leaky-mode biosensors operating in the subwavelength regime. The main resonance features observed under variation of input wavelength and angle are discussed. The dependence of the resonance lineshape on element design parameters is highlighted. The surface-localized power concentration at resonance is described along with the standing-wave pattern of the leaky modes obtained at normal incidence. An example fabrication process involving holographic patterning, etching, and deposition of high-index material is provided. The fabricated elements resonate well with good agreement between experiment and theory found. As examples of practical applications, experimental results on detection of proteins and bacteria are given. The tag-free resonant sensor technology demonstrated may be feasible for use in fields such as in medical diagnostics, drug development, environmental monitoring, and homeland security.

Leaky-mode resonance photonics: Technology for biosensors, optical components, MEMS, and plasmonics

Resonant leaky modes can be induced on dielectric, semiconductor, and metallic periodic layers patterned in one or two dimensions. Potential applications include bandpass and bandstop filters, laser mirrors, ultrasensitive biosensors, absorption enhancement in solar cells, security devices, tunable filters, nanoelectromechanical display pixels, dispersion/slow-light elements, and others. As there is now a growing realization worldwide of the utility of these devices, it is of interest to summarize their physical basis and present their applicability in photonic devices and systems. In particular, we have invented and implemented highly accurate, label-free, guided-mode resonance (GMR) biosensors that are being commercialized. The sensor is based on the high parametric sensitivity inherent in the fundamental resonance effect. As an attaching biomolecular layer changes the parameters of the resonance element, the resonance frequency (wavelength) changes. A target analyte interacting with a bio-selective layer on the sensor can thus be identified without additional processing or use of foreign tags. Another promising pursuit in this field is development of optical components including wideband mirrors, filters, and polarizers. We have experimentally realized such devices that exhibit a minimal layer count relative to their classical multilayer thin-film counterparts. Theoretical modeling has shown that wideband tuning of these filters is achievable by perturbing the structural symmetry using nano/microelectromechanical (MEMS) methods. MEMS-tuned resonance elements may be useful as pixels in spatial light modulators, tunable lasers, and multispectral imaging applications. Finally, mixed metallic/dielectric resonance elements exhibit simultaneous plasmonic and leaky-mode resonance effects. Their design and chief characteristics is described.