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Subwavelength resonant lattices provide a host of interesting spectral expressions on broadside illumination. The resonance mechanism is based on generation of lateral Bloch modes phase matched to evanescent diffraction orders. The leaky mode structure and mode count determine the spectra and the number of resonance states. Here, we study band flips and bound-state transitions in guided-mode resonant structures supporting multiple resonant modes. We present theoretical simulations and experimental results for a subwavelength silicon-nitride lattice integrated with a liquid film with adjustable boundary. The relatively thick liquid waveguiding region supports additional modes such that the first four transverse-electric (TE) leaky modes are present and generate observable resonance signatures. By varying the duty cycle of the basic lattice in experiment, the 4 bands undergo band transitions and band closures as quantified herein. The experimental results taken in the 1400-1600 nm spectral region agree reasonably well with numerical analysis.

Subwavelength resonant lattices offer a wide range of fascinating spectral phenomena under broadside illumination. The resonance mechanism relies on the generation of lateral Bloch modes that are phase matched to evanescent diffraction orders. The spectral properties and the total number of resonance states are governed by the structure of leaky modes and the mode count. This study investigates the effect of interface modifications on the band dynamics and bound-state transitions in guided-mode resonant lattices. We provide photonic lattices comprising rectangular Si 3 N 4 rods with a liquid film with an adjustable boundary. The band structures and band flips are examined through numerical simulations using the rigorous coupled-wave analysis (RCWA) method and analyzing the zero-order spectral reflectance as a function of the incident angle. The band structures and band flips are examined through numerical simulations, and the influences of the refractive index and the thickness of the oil layer on the band dynamics are investigated. The results reveal distinct resonance linewidths corresponding to different refractive indices of the oil layer. Furthermore, the effect of the oil thickness on the band dynamics is explored, demonstrating precise control over the number of propagating modes within the lattice structure. Theoretical simulations and experimental results are presented for a subwavelength silicon-nitride lattice combined with a liquid film featuring an adjustable boundary. The presence of a relatively thick liquid waveguiding region enables the emergence of additional modes, including the first four transverse-electric (TE) leaky modes, which produce observable resonance signatures. Through experimental manipulation of the basic lattice's duty cycle, the four bands undergo quantifiable band transitions and closures. The experimental results obtained within the 1400-1600 nm spectral range exhibit reasonable agreement with the numerical analysis. These findings underscore the significant role played by the interface in shaping the band dynamics of the lattice structure, providing valuable insights into the design and optimization of photonic lattices with adjustable interfaces.

We describe a robust dynamic spectroscopic imaging ellipsometer (DSIE) based on a monolithic Linnik-type polarizing interferometer. The Linnik-type monolithic scheme combined with an additional compensation channel solves the long-term stability problem of previous single-channel DSIE. The importance of a global mapping phase error compensation method is also addressed for accurate 3-D cubic spectroscopic ellipsometric mapping in large-scale applications. To evaluate the effectiveness of the proposed compensation method for enhancing system robustness and reliability, a whole thin film wafer mapping is conducted in a general environment where various external disturbances affect the system.

We demonstrate band flip in one-dimensional dielectric photonic lattices presenting numerical and experimental results. In periodic optical lattices supporting leaky Bloch modes, there exists a second stop band where one band edge experiences radiation loss resulting in guided-mode resonance (GMR), while the other band edge becomes a nonleaky bound state in the continuum (BIC). To illustrate the band flip, band structures for two different lattices are provided by calculating zero-order reflectance with respect to wavelength and incident angle. We then provide three photonic lattices, each with a different fill factor, consisting of photoresist gratings on Si 3 N 4 sublayers with glass substrates. The designs are fabricated using laser interferometric lithography. The lattice parameters are characterized and verified with an atomic force microscope. The band transition under fill-factor variation is accomplished experimentally. The measured data are compared to simulation results and show good agreement.

A dynamic spectroscopic imaging ellipsometer (DSIE) employing a monolithic polarizing interferometer is described. The proposed DSIE system can provide spatio-spectral ellipsometric phase map data Δ(λ, x) dynamically at a speed of 30 Hz. We demonstrate the ultrafast mapping capability of the spectroscopic ellipsometer by measuring a patterned 8-inch full wafer with a spatial resolution of less than 50 × 50 µm 2 in an hour.

We discuss the design, fabrication, and characterization of silicon-nitride microring resonators for nonlinear-photonic and biosensing device applications. The first part presents new theoretical and experimental results that overcome highly normal dispersion of silicon-nitride microresonators by adding a dispersive coupler. The latter parts review our work on highly efficient second-order nonlinear interaction in a hybrid silicon-nitride slot waveguide with nonlinear polymer cladding and silicon-nitride microring application as a biosensor for human stress indicator neuropeptide Y at the nanomolar level.

Resonant periodic nanostructures provide perfect reflection across small or large spectral bandwidths depending on the choice of materials and design parameters. This effect has been known for decades, observed theoretically and experimentally via one-dimensional and two-dimensional structures commonly known as resonant gratings, metamaterials, and metasurfaces. The physical cause of this extraordinary phenomenon is guided-mode resonance mediated by lateral Bloch modes excited by evanescent diffraction orders in the subwavelength regime. In recent years, hundreds of papers have declared Fabry-Perot or Mie resonance to be the basis of the perfect reflection possessed by periodic metasurfaces. Treating a simple one-dimensional cylindrical-rod lattice, here we show clearly and unambiguously that Mie resonance does not cause perfect reflection. In fact, the spectral placement of the Bloch-mode-mediated zero-order reflectance is primarily controlled by the lattice period by way of its direct effect on the homogenized effective-medium refractive index of the lattice. In general, perfect reflection appears away from Mie resonance. However, when the lateral leaky-mode field profiles approach the isolated-particle Mie field profiles, the resonance locus tends towards the Mie resonance wavelength. The fact that the lattice fields "remember" the isolated particle fields is referred here as "Mie modal memory." On erasure of the Mie memory by an index-matched sublayer, we show that perfect reflection survives with the resonance locus approaching the homogenized effective-medium waveguide locus. The results presented here will aid in clarifying the physical basis of general resonant photonic lattices.

Periodic guided-mode resonance structures which provide perfect reflection across sizeable spectral bandwidths have been known for decades and are now often referred to as metasurfaces and metamaterials. Although the underlying physics for these devices is explained by evanescent-wave excitation of leaky Bloch modes, a growing body of literature contends that local particle resonance is causative in perfect reflection. Here, we address differentiation of Mie resonance and guided-mode resonance in mediating resonant reflection by periodic particle assemblies. We treat a classic 2D periodic array consisting of silicon spheres. To disable Mie resonance, we apply an optimal antireflection (AR) coating to the spheres. Reflectance maps for coated and uncoated spheres demonstrate that perfect reflection persists in both cases. It is shown that the Mie scattering efficiency of an AR-coated sphere is greatly diminished. The reflectance properties of AR-coated spherical arrays have not appeared in the literature previously. From this viewpoint, these results illustrate high-efficiency resonance reflection in Mie-resonance-quenched particle arrays and may help dispel misconceptions of the basic operational physics.

In the physical description of photonic lattices, leaky-mode resonance and bound states in the continuum are central concepts. Understanding of their existence conditions and dependence on lattice parameters is of fundamental interest. Primary leaky-wave effects are associated with the second stop band at the photonic lattice Γ point. The pertinent band gap is defined by the frequency difference between the leaky-mode band edge and the bound-state edge. This paper address the polarization properties of the band gaps resident in laterally periodic one-dimensional photonic lattices. We show that the band gaps pertinent to TM and TE leaky modes exhibit significantly differentiated evolution as the lattice parameters vary. This is because the TM band gap is governed by a surface effect due to the discontinuity of the dielectric constant at the interfaces of the photonic lattice as well as by a Bragg effect due to the periodic in-plane dielectric constant modulation. We find that when the lattice is thin (thick), the surface (Bragg) effect dominates the Bragg (surface) effect in the formation of the TM band. This leads to complex TM band dynamics with multiple band closures possible under parametric variation. In complete contrast, the TE band gap is governed only by the Bragg effect thus exhibiting simpler band dynamics. This research elucidates the important effect of polarization on resonant leaky-mode band dynamics whose explanation has heretofore not been available.

Polarizers serve many application fields such as imaging, display technology, and telecommunications. Focusing on the visible spectral region, we provide the design and fabrication of compact high-efficiency resonant polarizers in the crystalline silicon-on-quartz material system. We experimentally verify the improved efficiency attained by a cascaded dual-module polarizer assembled with building blocks of elemental subwavelength grating structures. We obtain a measured extinction ratio (ER) of ∼3000 in a 2 mm thick stacked prototype device across a bandwidth of ∼110 n m in the 570-680 nm spectral domain. The ridge width of the constituent nanograting is ∼84 n m . Computed results show a high ER in spite of the lossy nature of crystalline silicon in the visible region, enabling cascaded metasurfaces while preserving high transmission.

Periodic diffractive elements known as metasurfaces constitute platform technology whereby exceptional optical properties, not attainable by conventional means, are attained. Generally, with increasing unit-cell complexity, there emerges a wider design space and bolstered functional capability. Advanced devices deploying elaborate unit cells are typically generated by electron-beam patterning which is a tedious, slow process not suitable for large surfaces and quick turnaround. Ameliorating this condition, we present a novel route towards facile fabrication of complex periodic metasurfaces based on sequential exposures by laser interference lithography. Our method is fast, cost-effective, and can be applied to large surface areas. It is enabled by precise control over periodicity and exposure energy. With it we have successfully patterned and fabricated one-dimensional (1D) and two-dimensional (2D) multipart unit cell devices as demonstrated here. Thus, zero-order transmission spectra of an etched four-part 1D grating device are simulated and measured for both transverse-electric (TE) and transverse-magnetic (TM) polarization states of normally incident light. We confirm non-resonant wideband antireflection (∼800 nm) for TM-polarized light and resonance response for TE-polarized light in the near-IR band spanning 1400-2200 nm in a ∼100 mm 2 device. Furthermore, it is shown that this method of fabrication can be implemented not only to pattern periodic symmetric/asymmetric designs but also to realize non-periodic metasurfaces. The method will be useful in production of large-area photonic devices in the realm of nanophotonics and microphotonics.

Assessing levels of neuropeptide Y (NPY) in the human body has many medical uses. Accordingly, we report the quantitative detection of NPY biomarkers applying guided-mode resonance (GMR) biosensor methodology. The label-free sensor operates in the near-infrared spectral region exhibiting distinctive resonance signatures. The interaction of NPY with bioselective molecules on the sensor surface causes spectral shifts that directly identify the binding event without additional processing. In the experiments described here, NPY antibodies are attached to the sensor surface to impart specificity during operation. For the low concentrations of NPY of interest, we apply a sandwich NPY assay in which the sensor-linked anti-NPY molecule binds with NPY that subsequently binds with anti-NPY to close the sandwich. The sandwich assay achieves a detection limit of ~0.1 pM NPY. The photonic sensor methodology applied here enables expeditious high-throughput data acquisition with high sensitivity and specificity. The entire bioreaction is recorded as a function of time, in contrast to label-based methods with single-point detection. The convenient methodology and results reported are significant, as the NPY detection range of 0.1-10 pM demonstrated is useful in important medical circumstances.

Polarizers are universal components deployed in diverse application fields including imaging, display, microscopy, interferometry, ellipsometry, and instrumentation. Here, we demonstrate design and fabrication of a new class of polarizers that are extremely compact and efficient. Based on an elemental low-loss single-resonant grating, we develop multilayer modules providing ultrahigh extinction ratio polarizers. The elemental polarizer contains a subwavelength periodic pattern of crystalline silicon on a quartz substrate. A stack of two dual-grating modules exhibits a measured extinction ratio (ER) of ∼100,000 in a sparse 2-mm-thick device across a bandwidth of ∼50  nm in the telecommunications spectral region. Theoretical computations indicate that extreme values of extinction are possible. Further development of the basic concepts explored herein may lead to a new class of practical polarizers with excellent attributes.

In photonic lattices with thin-film geometry, TE modes possess an in-plane electric field component parallel to the film surface, whereas TM modes have a magnetic field component similarly oriented. This study reports the essential properties of, and differences between, TE and TM band gaps induced by laterally periodic thin-film photonic lattices at the first Bragg condition. Because TE and TM guided waves obey different wave equations, TE and TM band gaps exhibit different evolution as the film thickness varies. The first TM band exhibits both band gap closure and band flips wherein the symmetry properties of the band-edge modes are reversed by variation of film thickness. In the first TE band, in contrast, there is neither band gap closure nor band flip. The work provides an insightful semianalytical formulation whose results are verified by rigorous computations.

External waves incident on a periodic metamaterial lattice couple to it at frequencies corresponding to the leaky, or second, stop band. The resulting leaky-mode or guided-mode resonance effects are useful in device design and spectral manipulation. Indeed, some of the most important properties of metamaterials are associated with the leaky stopband. Thus motivated, we treat the band dynamics of leaky-mode resonant photonic lattices. In particular, properties of the band gap and conditions for band closure and band flips under multimode conditions are quantified. For a symmetric lattice, the nonleaky band edge hosts a bound state in the continuum whose band transition reverses the modal symmetry of the band edge modes. The leaky edge supports a guided-mode resonant radiative peak that also undergoes band flip upon band closure. We analyze a canonical one-dimensional lattice with exact numerical methods and a semianalytical formulation modified to handle the multimodal case. We show that the band dynamics of the various leaky modes present differ appreciably with, for example, the band associated with the fundamental TE 0 and the first higher order TE 1 modes closing at differing values of dielectric-constant modulation. We compare the thin-film lattice with an infinite lattice and find an approximate analytical condition for band closure that we verify with rigorous computations.

Fabrication and sensor application of a simple plasmonic structure is described in this paper. The sensor element consists of nano-patterned gold film brought about from two-dimensional periodic photoresist templates created by holographic laser interference lithography. Reflectance spectroscopy revealed that the sensor exhibits significant refractive index sensitivity. A linear relationship between shifts in plasmonic resonances and changes in the refractive index were demonstrated. The sensor has a bulk sensitivity (S B ) of 880 nm/refractive index unit and work under normal incidence conditions. This sensitivity exceeded that of many common types of plasmonic sensors with more intricate structures. A modeled spectral response was used to study the effect of its geometrical dimensions on plasmonic behavior. A qualitative agreement between the experimental spectra and modeled ones was obtained.

Guided-mode resonant (GMR) thin films integrated on fiber tips are known to realize compact filters and sensors. However, limited progress in experimental realization has been reported to date. Here we provide a considerable advance in this technology, as we experimentally demonstrate efficient fiber-facet mounted device prototypes. To retain a large aperture for convenient coupling, we design and fabricate silicon nitride-based resonators on the tip of a multimode fiber. We account for light propagation along the multimode fiber with exact numerical methods. This establishes the correct amplitude and phase distribution of the beam incident on the tip-mounted GMR element, thus enabling us to properly predict the resonance response. To fabricate the integrated GMR structures on the tips of fibers, we employ standard microfabrication processes, including holographic interference lithography and reactive-ion etching. The experimental results agree with simulation with an example device achieving high efficiency of ∼77% in transmission. To investigate fiber sensor operation, an etched silicon nitride fiber tip filter is surrounded with solutions of various refractive indices, yielding an approximate sensitivity of 200 nm/RIU.

Resonant nanogratings and periodic metasurfaces express diverse spectral and polarization properties on broadside illumination by incident light. Cooperative resonance interactions may yield shaped spectra for particular applications, in contrast to a multilayer dielectric mirror. Here, we provide guided-mode resonance filters with flat-top spectra suitable for wavelength division multiplexing systems. Applying a single one-dimensional grating layer sandwiched by two waveguides, we theoretically achieve high-efficiency flat-top spectra in the near-infrared region. This result is obtained by inducing simultaneous nearly degenerate resonant modes. The resonance separation under this condition controls the width of the flat-top spectrum. This means we can implement spectral widths ranging from a sub-nanometer to several nanometers applying fundamentally the same device architecture.

We propose a free-space electro-optic transmission modulator based on multiple p-n-junction semiconductor subwavelength gratings. The proposed device operates with a high-Q guided-mode resonance undergoing electro-optic resonance shift due to direct electrical control. Using rigorous electrical and optical modeling methods, we theoretically demonstrate a modulation depth of 84%, on-state efficiency 85%, and on-off extinction ratio of 19 dB at 1,550 nm wavelength under electrical control signals within a favorably low bias voltage range from -4 V to +1 V. This functionality operates in the transmission mode and sustainable in the high-speed operation regime up to a 10-GHz-scale modulation bandwidth in principle. The theoretical performance prediction is remarkably advantageous over plasmonic tunable metasurfaces in the power-efficiency and absolute modulation-depth aspects. Therefore, further experimental study is of great interest for creating practical-level metasurface components in various application areas.

We demonstrate unpolarized wideband reflectors fashioned with orthogonal serial resonant reflectors. Unpolarized incident light generates internal TM- and TE-polarized reflections that are made to cooperate to extend the bandwidth of the composite spectral reflectance. The experimental results presented show ~42% band extension by a two-grating module. In addition, good angular tolerance is found because the orthogonal arrangement simultaneously supports classical and fully conic mountings at oblique angles. The resulting spectra form contiguous zero-order reflectance across wide spectral/angular regions. Furthermore, using a multimodule device with serial reflectors fabricated with silicon-on-quartz wafers with different device layer thicknesses, extreme band extension is achieved providing ~56% fractional bandwidth with reflectance exceeding 98%. These results imply potential for developing lossless unpolarized mirrors operating in diverse spectral regions of practical interest.

Narrow bandpass filters are applied in laser systems, imaging, telecommunications, and astronomy. Traditionally implemented with thin-film stacks, there is recent interest in alternative means incorporating photonic resonance effects. Here, we demonstrate a new approach to bandpass filters that engages the guided-mode resonance effect working with a cavity-based Fabry-Perot resonance to flatten and steepen the pass band. Both of these resonance mechanisms are native to simple resonant bandpass filters placed in a cascade. Numerical examples provide quantitative spectral properties including pass-band shape and sideband levels. Thus, we compare the spectra of single-layer 1D and 2D resonant gratings with the dual-cascade design incorporating identical gratings. Two- and three-cavity designs are measured against a classic multi-cavity thin-film filter with 151 layers. Whereas these initial results show comparable and improved results achieved with sparse structures, the challenge remains of developing a suitable fabrication technology to capitalize on this promise.

Bandpass filters based on subwavelength dielectric gratings are grounded in physical principles that are totally distinct from their thin-film counterparts. Ease in fabrication, design scalability, material sparsity, and on-chip integration compatibility makes them a promising alternative especially for long-wavelength applications. Here we demonstrate the interesting attribute of resonant bandpass filters of high angular stability for fully conical light incidence. Fashioning an experimental bandpass filter with a subwavelength silicon grating on a quartz substrate, we show that fully conical incidence provides an angular full width at half-maximum linewidth of ∼9.5° compared to a linewidth of ∼0.1° for classical incidence. Slow angular variation of the central wavelength with full conical incidence arises via a corresponding slow angular variation of the resonant second diffraction orders driving the pertinent leaky modes. Moreover, full conical incidence maintains a profile with a single passband as opposed to the formation of two passbands characteristic of resonant subwavelength gratings under classical incidence. Our experimental results demonstrate excellent stability in angle, spectral profile, linewidth, and efficiency.

There is immense scientific interest in the properties of resonant thin films embroidered with periodic nanoscale features. This device class possesses considerable innovation potential. Accordingly, we report unpolarized broadband reflectors enabled by a serial arrangement of a pair of polarized subwavelength gratings. Optimized with numerical methods, our elemental gratings consist of a partially etched crystalline-silicon film on a quartz substrate. The resulting reflectors exhibit extremely wide spectral reflection bands in one polarization. By arranging two such reflectors sequentially with orthogonal periodicities, there results an unpolarized spectral band that exceeds those of the individual polarized bands. In the experiments reported herein, we achieve zero-order reflectance exceeding 97% under unpolarized light incidence over a 500 nm wide wavelength band. This wideband represents a ∼44% fractional band in the near infrared. Moreover, the resonant unpolarized broadband accommodates an ultra-high reflection band spanning ∼85  nm and exceeding 99.9% in efficiency. The elemental polarization-sensitive reflectors based on one-dimensional (1D) resonant gratings have a simple design and robust performance, and are straightforward to fabricate. Hence, this technology is a promising alternative to traditional multilayer thin-film reflectors, especially at longer wavelengths of light where multilayer deposition may be infeasible or impractical.

This Letter describes a Stokes vector measurement method based on a snapshot interferometric common-path spectropolarimeter. The proposed scheme, which employs an interferometric polarization-modulation module, can extract the spectral polarimetric parameters Ψ(k) and Δ(k) of a transmissive anisotropic object by which an accurate Stokes vector can be calculated in the spectral domain. It is inherently strongly robust to the object 3D pose variation, since it is designed distinctly so that the measured object can be placed outside of the interferometric module. Experiments are conducted to verify the feasibility of the proposed system. The proposed snapshot scheme enables us to extract the spectral Stokes vector of a transmissive anisotropic object within tens of msec with high accuracy.

Applying numerical modeling coupled with experiments, we investigate the properties of wideband resonant reflectors under fully conical light incidence. We show that the wave vectors pertinent to resonant first-order diffraction under fully conical mounting vary less with incident angle than those associated with reflectors in classical mounting. Therefore, as the evanescent diffracted waves drive the leaky modes responsible for the resonance effects, fully-conical mounting imbues reflectors with larger angular tolerance than their classical counterparts. We quantify the angular-spectral performance of representative resonant wideband reflectors in conic and classic mounts by numerical calculations with improved spectra found for fully conic incidence. Moreover, these predictions are verified experimentally for wideband reflectors fashioned in crystalline and amorphous silicon in distinct spectral regions spanning the 1200-1600-nm and 1600-2400-nm spectral bands. These results will be useful in various applications demanding wideband reflectors that are efficient and materially sparse.

Background . The injured anterior cruciate ligament (ACL) has a limited healing capacity leading to persisting instability. Hypothesis/Purpose . To study if the application of a brace, producing a dynamic posterior drawer force, after acute ACL injury reduces initial instability. Study Design . Cohort study. Methods . Patients treated with the ACL-Jack brace were compared to controls treated with primary ACL reconstruction und controls treated nonsurgically with functional rehabilitation. Measurements included anterior laxity (Rolimeter), clinical scores (Lysholm, Tegner, and IKDC), and MRI evaluation. Patients were followed up to 24 months. Results . Patients treated with the ACL-Jack brace showed a significant improvement of anterior knee laxity comparable to patients treated with ACL reconstruction, whereas laxity persisted after nonsurgical functional rehabilitation. The failure risk (secondary reconstruction necessary) of the ACL-Jack group was however 21% (18 of 86) within 24 months. Clinical scores were similar in all treatment groups. Conclusion . Treatment of acute ACL tears with the ACL-Jack brace leads to improved anterior knee laxity compared to nonsurgical treatment with functional rehabilitation., Competing Interests: The authors declare that they have no competing interests.

We study spectral singularities and critical field enhancement factors associated with embedded photonic bound states in subwavelength periodic Si films. Ultrahigh-Q resonances supporting field enhancement factor exceeding 10(8) are obtained in the spectral vicinity of exact embedded eigenvalues in spite of deep surface modulation and vertical asymmetry of the given structure. Treating relations between the partial resonance Q and field enhancement factors with an analytical coupled-mode model, we derive a general strategy to maximize the field enhancement associated with these photonic bound states in the presence of material dissipation. The analytical expression for the field enhancement quantitatively agrees with rigorous numerical calculations. Therefore, our results provide a general knowledge for designing practical resonance elements based on optical bound states in the continuum in various applications.

Engaging both theory and experiment, we investigate resonant photonic lattices in which the duty cycle tends to zero. Corresponding dielectric nanowire grids are mostly empty space if operated as membranes in vacuum or air. These grids are shown to be effective wideband reflectors with impressive polarizing properties. We provide computed results predicting nearly complete reflection and attendant polarization extinction in multiple spectral regions. Experimental results with Si nanowire arrays with 10% duty cycle show ~200-nm-wide band of high reflection for one polarization state and free transmission for the orthogonal state. These results agree quantitatively with theoretical predictions. It is fundamentally extremely significant that the wideband spectral expressions presented can be generated in these minimal systems.

Resonant periodic surfaces and films enable new functionalities with wide applicability in practical optical systems. Their material sparsity, ease of fabrication, and minimal interface count provide environmental and thermal stability and robustness in applications. Here, we report an experimental bandpass filter fashioned in a single patterned silicon layer on a quartz substrate. Its performance corresponds to bandpass filters requiring 15 traditional Si/SiO(2) thin-film layers. The feasibility of sparse narrowband high-efficiency bandpass filters with extremely wide, flat, and low sidebands is thereby demonstrated. This class of devices is designed with rigorous solutions of Maxwell's equations while engaging the physical principles of resonant waveguide gratings. An experimental filter presented exhibits a transmittance of ∼72%, bandwidth of ∼0.5  nm, and low sidebands spanning ∼100  nm. The proposed technology is integration-friendly and opens doors for further development in various disciplines and spectral regions where thin-film solutions are traditionally applied.

Optical devices incorporating resonant periodic layers constitute an emerging technological area. Recent advances include spectral filters, broadband mirrors, and polarizers. Here, we demonstrate concurrent spatial and spectral filtering as a new outstanding attribute of this device class. This functionality is enabled by a unique, near-complete, reflection state that is discrete in both angular and spectral domains and realized with carefully-crafted nanogratings operating in the non-subwavelength regime. We study the pathway and inter-modal interference effects inducing this intriguing reflection state. In a proof-of-concept experiment, we obtain angular and spectral bandwidths of ~4 mrad and ~1 nm, respectively. This filter concept can be used for focus-free spectral and spatial filtering in compact holographic and interferometric optical instruments.

A guided-mode resonance mirror (GMRM) consists of a waveguide grating integrated on an optical buffer layer on a high-reflection substrate. An incident free-space wave at the resonance wavelength is once coupled by the grating to a guided mode and coupled again by the same grating back to free space. The reflection characteristics of a GMRM are numerically calculated and theoretically analyzed. It is predicted that notch filtering or flat reflection spectra are obtained depending on the optical buffer layer thickness. Design of short cavities using a GMRM is discussed for potential application in surface-mount packaging of diode lasers onto a photonic circuit board.

According to the American Cancer Society, more than 200,000 women will be diagnosed with invasive breast cancer each year and approximately 40,000 will die from the disease. The human leukocyte antigen (HLA) class I samples peptides derived from proteasomal degradation of cellular proteins and presents these fragments on the cell surface for interrogation by circulating cytotoxic T lymphocytes (CTL). Generation of T-cell receptor mimic (TCRm) monoclonal antibodies (mAbs) which recognize breast cancer specific peptide/HLA-A*02:01 complexes such as those derived from macrophage migration inhibitory factor (MIF19-27) and NY-ESO-1157-165 enable detection and destruction of breast cancer cells in the absence of an effective anti-tumor CTL response. Intact class I HLA/peptide complexes are shed by breast cancer cells and represent potentially relevant cancer biomarkers. In this work, a breakthrough biomarker screening system for cancer diagnostics incorporating T-cell receptor mimic monoclonal antibodies combined with a novel, label-free biosensor utilizing guided-mode resonance (GMR) sensor technology is presented. Detection of shed MIF/HLA-A*02:01 complexes in MDA-MB-231 cell supernatants, spiked human serum, and patient plasma is demonstrated. The impact of this work could revolutionize personalized medicine through development of companion disease diagnostics for targeted immunotherapies.

The spectral properties of high-reflection mirrors imbued with two-dimensional (2D) subwavelength periodicity are investigated. The reflectors are designed in a silicon-on-glass film that is partially etched to implement a zero-contrast interface between the grating pillars and the sublayer, thereby annulling the local reflections and phase changes associated with hard interfaces. This approach has shown impressive results for 1D polarized reflectors; here we strive to discover analogous wideband unpolarized reflectors. Using particle swarm optimization, we report wideband unpolarized reflectors in the 1.4-2.0 μm wavelength band. A 2D reflector with square pillars exhibits 99% reflectance across a bandwidth exceeding 350 nm and possesses tolerance against angular deviations. The complementary structure with rectangular periodic voids achieves a bandwidth of 370 nm. A comparable, optimized 2D high-contrast grating reflector with grating pillars residing directly on the substrate yields a 99% bandwidth of 240 nm.

We design and fabricate efficient, narrow-band, transmission color filters whose operating principle resides in a narrow-band guided-mode resonance associated with a surface-plasmon resonance. The fundamental device consists of an aluminum grating over a 200-nm-thick aluminum oxide film on a glass substrate. Numerical simulations show a sharp resonance-derived spectral profile that is additionally shaped by a neighboring Rayleigh anomaly. Besides the Rayleigh effect, we show numerically that the narrow bandwidth is predominantly due to the low refractive-index contrast between the waveguide film and the substrate. Red, green, and blue filters are fabricated using ultraviolet holographic lithography followed by a lift-off process. The experimental spectral efficiency in transmission exceeds 80% with full-width-at-half-maximum linewidths near 20 nm. We provide color images of the zero-order transmitted spectra, and illustrate the pure colors associated with the modal resonance extracted as side-coupled output light.

We propose a mechanism of ultrahigh-Q metallic nanocavity resonances that involves an efficient loss-compensation scheme favorable for room-temperature operation. We theoretically show that surface plasmon-polaritons excited on the entrance and exit interfaces of a metallic nanocavity array efficiently transfer external optical gain to the cavity modes by inducing resonantly-amplified intracavity feedback. Surprisingly, the modal gain in the nanocavity with the externally amplified feedback is inversely proportional to the cavity length as opposed to conventional optical cavity amplifiers requiring longer cavities for higher optical gain. Utilizing this effect, we numerically demonstrate room-temperature nanocavity resonance Q-factor exceeding 10(4) in a 25-nm-wide silver nanoslit array. The proposed mechanism provides a highly efficient plasmonic amplification process particularly for subwavelength plasmonic cavities which are essential components in active nanoplasmonic devices.

We study theoretically modal properties and parametric dependence of guided-mode resonance bandpass filters operating in the mid- and near-infrared spectral domains. We investigate three different device architectures consisting of single, double, and triple layers based on all-transparent dielectric and semiconductor thin films. The three device classes show high-performance bandpass filter profiles with broad, flat low-transmission sidebands accommodating sharp transmission peaks with their efficiencies approaching 100% with appropriate blending of multiple guided modes. We present three modal coupling configurations forming complex mixtures of two or three distinct leaky modes coupling at different evanescent diffraction orders. These modal compositions produce various widths of sidebands ranging from ~30 nm to ~2100 nm and transmission peak-linewidths ranging from ~1 pm to ~10 nm. Our modal analysis demonstrates key attributes of subwavelength periodic thin-film structures in multiple-modal blending to achieve desired transmission spectra. The design principle is applicable to various optical elements such as high-power optical filters, low-noise label-free biochemical sensor templates, and high-density display pixels.

We present wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material, thereby eliminating local reflections and phase changes. This critical interface thus possesses zero refractive-index contrast; hence "zero-contrast gratings." We design reflectors with zero-contrast gratings and high-contrast gratings and compare the results. For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors. An example silicon-on-glass reflector exhibits a 99% reflectance bandwidth of ∼700  nm for zero refractive-index contrast Δn=0, whereas a high-contrast device with Δn=2 yields a bandwidth of ∼600  nm. It follows that local Fabry-Perot modes residing in the grating ridges and reflecting off a high-contrast interface are not the root cause of wideband reflection.

This paper describes a Stokes vector measurement method based on a snapshot polarization-sensitive spectral interferometry. We measure perpendicular linearly polarized complex wave information of an anisotropic object in the spectral domain from which an accurate Stokes vector can be extracted. The proposed Stokes vector measurement method is robust to the object plane 3-D pose variation and external noise, and it provides a reliable snapshot solution in numerous spectral polarization-related applications.

Metallic nanostructures are of immense scientific interest owing to unexpectedly strong interaction with light in deep subwavelength scales. Resonant excitations of surface and cavity plasmonic modes mediate strong light localization in nanoscale objects. Nevertheless, the role of surface plasmon-polaritons (SPP) in light transmission through a simple one-dimensional system with metallic nanoslits has been the subject of longstanding debates. Here, we propose a unified theory that consistently explains the controversial effects of SPPs in metallic nanoslit arrays. We show that the SPPs excited on the entrance and exit interfaces induce near-total internal reflection and abrupt phase change of the slit-guided mode. These fundamental effects quantitatively describe positive and negative effects of SPP excitation in a self-consistent manner. Importantly, the theory shows excellent agreement with rigorous numerical calculations while providing profound physical insight into the properties of nanoplasmonic systems.

We present the design and fabrication of guided-mode resonant broadband reflectors operating in transverse electric (TE) polarization. The structure consists of a subwavelength one-dimensional grating with a two-part period and a nanometric homogeneous layer of amorphous silicon on a quartz substrate. A representative reflector exhibits 99% reflectance over a 380-nm spectral range spanning 1440-1820 nm. The fabrication involves thin-film deposition, interferometric lithography, and reactive ion etching. Experimental reflectance greater than 90% is achieved over a ~360-nm bandwidth. The spectral bandwidths demonstrated exceed formerly reported results for two-part periodic resonators working in TE polarization.

We demonstrate efficient guided-mode resonant polarization-controlled tunable color filters. The devices consist of subwavelength gratings that are partially etched into a thin silicon-nitride film deposited on a glass substrate. Two color filters with grating periods of 300 nm and 370 nm are designed and fabricated. The 300-nm device exhibits green and blue colors and the 370-nm device generates red and yellow colors for TE and TM polarization, respectively. The pixels have a spectral bandwidth of ~12 nm with efficiencies exceeding 90% for TE polarization and 80% for TM polarization. The devices may find application in displays, image sensors, and biomedical imaging technologies.

We present the design, fabrication, and characterization of guided-mode resonance (GMR) linear polarizers that operate in the optical communications C-band near a wavelength of 1550 nm. We provide theoretical and experimental spectra using resonant elements fashioned in three material systems. In particular, we investigate silicon nitride resonant gratings and titanium dioxide gratings on glass substrates as well as silicon-on-quartz gratings. These materials exhibit very low losses and are capable of high diffraction efficiencies and extinction ratios; thus, high-power laser applications may be enabled. We present the methods applied to fabricate these GMR devices as well as means to ascertain their fabricated physical parameters. We quantify increased polarizer bandwidth with increased grating refractive-index modulation. The numerical results obtained with the fabricated-device parameters agree well with the experimental measured spectra.

This Letter describes a universal calibration theory by which conventional interferometry can be extended to vibration robust snapshot polarization-sensitive spectral reflectometry without any complicated optical components or active devices. Experiments for verifying the proposed calibration theory have been conducted by using a Michelson-interferometer-based normal incidence spectroellipsometric system, and also some key system design considerations for object 3D pose tolerant measurement capability have been drawn. The proposed solution enables us to extract the spectroscopic ellipsometric parameter Δ(k) of an anisotropic object within 10 ms with high accuracy.

We provide a modified Fano resonance formula applicable to dissipative two-port resonance systems. Based on a generic coupled-resonator model, the formula embodies loss-related correction terms and fundamental resonance parameters that can be determined by an analytic method or experimentally as opposed to finding phenomenological parameters by fitting to numerical results. The theory applies physically meaningful resonance parameters including resonance frequency, total decay rates, and partial radiation probabilities. For example, it shows that the classic Fano shape parameter q is given directly in terms of the phase difference between the resonant and non-resonant transmission pathways. Our new resonance formula quantitatively expresses the resonance spectra pertaining to modal nanophotonic and surface-plasmonic thin-film structures as verified by comparing with exact numerical models.

A single-step, low-cost fabrication method to generate resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. A guided-mode resonant structure is obtained by subsequent deposition of thin films of transparent conductive oxide and amorphous silicon on the imprinted area. Referenced to equivalent planar structures, around 25% and 45% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. The fabricated elements provided have 300-nm periods. Thermally imprinted thermoplastic substrates hold potential for low-cost fabrication of nano-patterned thin-film solar cells for efficient light management.

We demonstrate the design and fabrication of a highly efficient guided-mode resonant color filter array. The device is designed using numerical methods based on rigorous coupled-wave analysis and is patterned using UV-laser interferometric lithography. It consists of a 60-nm-thick subwavelength silicon nitride grating along with a 105-nm-thick homogeneous silicon nitride waveguide on a glass substrate. The fabricated device exhibits blue, green, and red color response for grating periods of 274, 327, and 369 nm, respectively. The pixels have a spectral bandwidth of ~12 nm with efficiencies of 94%, 96%, and 99% at the center wavelength of blue, green, and red color filter, respectively. These are higher efficiencies than reported in the literature previously.

We introduce the concept of Rayleigh reflectors where extremely sharp angular spectra with unity reflectance are obtained across considerable angular extents. Enabling the device is a rapid, high-efficiency transition from an evanescent to a propagating first-order substrate wave occurring at the Rayleigh angle. A high-index nanolayer located adjacent to the substrate is seen to critically affect the resulting spectra. Away from the Rayleigh anomaly, the device is a basic guided-mode resonance (GMR) high reflector. Hence, this device connects the fundamental concepts of GMR and the Rayleigh anomaly with interesting possible applications, including filters and couplers.

Off-axis digital holography generally uses a 2D-FFT based spatial filtering method to extract the complex object wave from an off-axis hologram. In this paper, we describe a novel single exposure complex object wave extraction method which can provide a faster solution than the FFT based spatial filtering approach while maintaining the reconstructed phase image quality. And also, we show that the proposed direct filtering scheme can provide more robust filtering capability to the off-axis spatial carrier frequency variation than the spatial filtering method.

We demonstrate the plasmonic analogue of a coherent photonic effect known as coherent perfect absorption. A periodically nanopatterned metal film perfectly absorbs multiple coherent light beams coupling to a single surface plasmon mode. The perfect absorbing state can be switched to a nearly perfect scattering state by tuning the phase difference between the incident beams. We theoretically explain the plasmonic coherent perfect absorption by considering time-reversal symmetry of surface plasmon amplification by stimulated emission of radiation. We experimentally demonstrate coherent control of the plasmonic absorption in good agreement with a coupled-mode theory of dissipative resonances. Associated potential applications include absorption-based plasmonic switches, modulators, and light-electricity transducers.

We present the spectral properties of multiline guided-mode resonance filters designed with extremely thick dielectric films. We treat a dielectric membrane in air with a subwavelength grating inscribed into one surface. As the film is very thick on the scale of the wavelength, it supports a large number of resonant modes. In general, the resonant modes yield a dense reflectance spectrum with irregular appearance. We show that by placing an antireflection layer on the backside of the slab, the interference between the directly transmitted zero order and the diffracted order generating the waveguide modes is eliminated. Thus, a well-shaped, unperturbed comb-like spectrum is realized. A titanium dioxide membrane that is 500 μm thick generates a spectrum with more than 1000 channels separated by ~0.8 nm near the 1.55 μm wavelength.