Theresa A. Maldonado

Fabrication and characterization of high-quality waveguide-mode resonant optical filters

Optical filters containing resonant waveguide gratings are designed and fabricated using low-loss, robust materials. The double-layer filters contain a silicon dioxide diffractive element on a hafnium dioxide waveguide deposited on a fused silica substrate. Noise-pattern formation is minimized by use of an antireflective absorption layer during holographic grating recording in photoresist. Subsequent fabrication steps include metallization, lift-off, and oxygen plasma etch to create a metal etch mask for final CF4 plasma etching of a surface-relief grating. Spectral characterization with a tunable laser shows that the resulting filter exhibits 90% efficiency, 1.2 nm linewidth, and low sidebands.

Thin-film optical filters with diffractive elements and waveguides

The chief principles and properties of optical reflection and transmission guided-mode resonance (GMR) filters are presented. These devices are based on GMR effects in dielectric structures comprising gratings and homogeneous thin films. Detailed characteristics are calculated using rigorous coupled-wave analysis for bandpass filters operating in reflection and transmission for TE- and TM-polarized incident waves. High resonance efficiency with narrow or wide linewidths is achievable with near-zero reflectance or transmittance sidebands over extended wavelength ranges. To illustrate the potential of this technology, example GMR reflection and transmission characteristics are presented for filters operating in the visible spectral region. Excellent reflection-filter features are found when antireflection conditions prevail away from the resonance wavelength. Furthermore, long-range, low sidebands are found to be obtainable for a single-layer GMR reflection filter with a TM-polarized plane wave incident at the Brewster angle. The transmission filter is optimized when the structure is highly reflective off resonance. GMR filter fabrication tolerances are discussed with examples illustrating the sensitivity of the filter center wavelength to variations in layer thickness, grating shape, and incident angle. GMR filters are found to exhibit loss-dependent wavelength shifts such that the reflection peak occurs at a different wavelength than the corresponding transmission notch. However, under antireflection conditions, the resonance location becomes insensitive to loss. Finally, reflective GMR thinfilm structures that support multiple waveguide modes are studied. These devices exhibit unique characteristic angular and spectral signatures.

Dielectric frequency-selective structures incorporating waveguide gratings

In this paper, a frequency-selective structure based on guided-mode resonance effects in all-dielectric waveguide gratings is demonstrated theoretically and verified experimentally. Reflection (band-stop) filters with high efficiency, extended low-sideband reflection, and symmetric line shapes are designed by embedding gratings in layered antireflection structures. Reflection filter examples employing common dielectric materials illustrate linewidth control by grating modulation. An additional mechanism for linewidth control is demonstrated with phase-shifted gratings. Double-line reflection filters are obtained in structures containing two gratings with different grating periods. High-efficiency transmission (bandpass) filters are demonstrated using multilayer waveguide gratings in a high-reflectance thin-film configuration with a single grating in the center layer bordered by dielectric mirrors composed of high/low quarter-wave layers. Single-layer and multilayer waveguide gratings operating as reflection and transmission filters, respectively, were built and tested in the 4-20-GHz frequency range. The presence of guided-mode resonance notches and peaks is clearly established by the experimental results, and their spectral location and line shape is found to be in excellent agreement with the theoretical predictions.

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.

Efficient nonlinear phase shifts due to cascaded second-order processes in a counterpropagating quasi-phase-matched configuration

A counterpropagating quasi-phase-matched configuration is examined that is capable of efficiently producing second-order cascaded nonlinear phase shifts with minimal power lost to the second harmonic. For all-optical switching in a nonlinear Mach-Zehnder interferometer, the calculated minimum input power needed for switching (i.e., to yield a +/-pi/2 phase shift) is 40 times smaller than the power needed in the standard typeI copropagating configuration. The throughput of this counterpropagating device is 96% at the optimum switching point.

Gaussian beam transmission and reflection from a general anisotropic multilayer structure.

Transmission and reflection of Gaussian beams from a general anisotropic multilayer structure are investigated. The principal axes of the layers are oriented arbitrarily with respect to each other and with respect to a fixed reference coordinate system. The Gaussian beam is assumed to have an arbitrary angle of incidence and linear polarization orientation. Two numerical examples are presented: a single slab of uniaxial calcite and a multilayer structure of biaxial 4-(N, N-dimethylamino)-3-acet amidonitrobenzene with antireflection coatings on the input and output faces. Results show the distortions of the beam caused by the anisotropy of the structure.

Second-harmonic generation in resonant waveguide gratings incorporating ionic self-assembled monolayer polymer films

Experimental results on resonantly excited second-harmonic generation (SHG) in a periodic ionically self-assembled monolayer (ISAM) film are reported. A double-layer guided-mode resonance filter (GMRF) structure is coated with 40 bilayers of pyrlium-based chi(2) ISAM thin film and excited with the fundamental of a Nd:YAG laser. Enhanced second-harmonic conversion in the ISAM film is achieved because of the local field enhancement associated with the fundamental resonating leaky mode. This method of SHG is particularly promising, as the ISAM films under investigation exhibit anomalous dispersion that may be applied for phase matching to improve nonlinear conversion efficiency.

Light propagation characteristics for arbitrary wavevector directions in biaxial media by a coordinate-free approach.

The general case of light propagation in lossless anisotropic media occurs in crystals that are biaxial, either naturally so or by induced means (e.g., by electrooptic effect). In these crystals the optical properties, such as the refractive indices, change with propagation direction and are conveniently described by the two-sheeted wavevector surface. Most published work treats light propagation only in the principal planes of the crystal, where the wavevector surface reduces to a circle and an ellipse and the mathematics is simplified. Commonly, however, a biaxial bulk or waveguide device, especially an active device, will be oriented so that the light propagation is not in a principal plane. A complete and concise coordinate-free approach is presented for isolating each sheet, thereby providing a convenient means for calculating the directional optical properties of the two decoupled waves for arbitrary wavevector directions and birefringence levels. The versatility of this approach coupled with available graphics software is demonstrated by displaying numerous cross sections of the wavevector surfaces.

Switching and second harmonic generation using counterpropagating quasi-phase-matching in a mirrorless configuration

Numerical solutions to the set of coupled nonlinear wave equations for a novel mirrorless counterpropagating quasi-phase-matched (c-QPM) device are presented. Mirrorless c-QPM is investigated in stand-alone and in two interferometric configurations for all-optical switching (AOS) applications. The switching intensity is found to be very low using existing materials (e.g., 500 mW in a 1-cm-long KTP waveguide with a 10 /spl mu/m/sup 2/ effective area). Additionally, parameter sets are found that minimize pulse breakup and improve the possibility of a practical implementation. Although second harmonic generation (SHG) is also investigated in the stand-alone device, the conversion efficiency is shown to be eight times less efficient than the mirrored configuration.

Thin-film multilayer optical filters containing diffractive elements and waveguides

The principles and chief properties of optical reflection and transmission filters based on guided-mode resonance (GMR) effects in multilayer structures comprising gratings and homogeneous thin films are presented. Detailed fiber characteristics (center wavelength, lineshape, and linewidth) are calculated using rigorous coupled-wave analysis for TE and TM polarized incident waves. These filters exhibit desirable characteristics such as high resonance efficiency with narrow or wide linewidths. Near- zero reflectance sidebands over extended wavelength ranges are obtainable using multilayer waveguide-grating structures. To illustrate the potential of this technology, calculated GMR reflection and transmission example characteristics are presented for filters made with common thin-film materials operating in the visible spectral region. Excellent reflection-filter features are found when antireflection conditions prevail away from the resonance wavelength. The transmission filter is optimized when the structure is highly reflective off resonance. It is found that long-range, low sidebands are obtainable for a single- layer GMR filter with a TM-polarized plane wave incident at the Brewster angle. GMR filter fabrication tolerances are briefly discussed. A calculated example illustrates the sensitivity of the filter center wavelength to variations in layer thickness. The effects of absorptive loss are treated. It is shown that, in general, GMR filters suffer loss- dependent wavelength shifts such that the reflection peak occurs at a different wavelength than the corresponding transmission notch. However, under antireflection conditions, the resonance location becomes insensitive to loss. Finally, reflective GMR thin-film structures that support multiple waveguide modes are studied. These devices exhibit characteristic angular and spectral signatures with unique appearance.