Showing papers on "Diffraction grating published in 2013"

Label-free biodetection using a smartphone

Abstract: Utilizing its integrated camera as a spectrometer, we demonstrate the use of a smartphone as the detection instrument for a label-free photonic crystal biosensor. A custom-designed cradle holds the smartphone in fixed alignment with optical components, allowing for accurate and repeatable measurements of shifts in the resonant wavelength of the sensor. Externally provided broadband light incident upon an entrance pinhole is subsequently collimated and linearly polarized before passing through the biosensor, which resonantly reflects only a narrow band of wavelengths. A diffraction grating spreads the remaining wavelengths over the camera's pixels to display a high resolution transmission spectrum. The photonic crystal biosensor is fabricated on a plastic substrate and attached to a standard glass microscope slide that can easily be removed and replaced within the optical path. A custom software app was developed to convert the camera images into the photonic crystal transmission spectrum in the visible wavelength range, including curve-fitting analysis that computes the photonic crystal resonant wavelength with 0.009 nm accuracy. We demonstrate the functionality of the system through detection of an immobilized protein monolayer, and selective detection of concentration-dependent antibody binding to a functionalized photonic crystal. We envision the capability for an inexpensive, handheld biosensor instrument with web connectivity to enable point-of-care sensing in environments that have not been practical previously.

Silicon nitride CMOS-compatible platform for integrated photonics applications at visible wavelengths

Abstract: Silicon nitride is demonstrated as a high performance and cost-effective solution for dense integrated photonic circuits in the visible spectrum. Experimental results for nanophotonic waveguides fabricated in a standard CMOS pilot line with losses below 0.71dB/cm in an aqueous environment and 0.51dB/cm with silicon dioxide cladding are reported. Design and characterization of waveguide bends, grating couplers and multimode interference couplers (MMI) at a wavelength of 660 nm are presented. The index contrast of this technology enables high integration densities with insertion losses below 0.05 dB per 90° bend for radii as small as 35 µm. By a proper design of the buried oxide layer thickness, grating couplers with efficiencies above 38% for the TE polarization have been obtained.

Ultrahigh-efficiency apodized grating coupler using fully etched photonic crystals

Abstract: We present an efficient method to design apodized grating couplers with Gaussian output profiles for efficient coupling between standard single mode fibers and silicon chips. An apodized grating coupler using fully etched photonic crystal holes on the silicon-on-insulator platform is designed, and fabricated in a single step of lithography and etching. An ultralow coupling loss of -1.74 dB (67% coupling efficiency) with a 3 dB bandwidth of 60 nm is experimentally measured.

Multibeam diffraction grating-based backlighting

Abstract: Multibeam diffraction grating-based backlighting includes a light guide and a multibeam diffraction grating at a surface of the light guide. The light guide is to guide light from a light source. The multibeam diffraction grating is to couple out a portion of the guided light using diffractive coupling and to direct the coupled out portion away from the light guide as a plurality of light beams with different principal angular directions.

Integral Equation Analysis of Plane Wave Scattering by Coplanar Graphene-Strip Gratings in the THz Range

Abstract: The plane wave scattering and absorption by finite and infinite gratings of free-space standing infinitely long graphene strips are studied in the THz range. A novel numerical approach, based on graphene surface impedance, hyper-singular integral equations, and the Nystrom method, is proposed. This technique guarantees fast convergence and controlled accuracy of computations. Reflectance, transmittance, and absorbance are carefully studied as a function of graphene and grating parameters, revealing the presence of surface plasmon resonances. Specifically, larger graphene relaxation times increases the number of resonances in the THz range, leading to higher wave transmittance due to the reduced losses; on the other hand an increase of graphene chemical potential up-shifts the frequency of plasmon resonances. It is also shown that a relatively low number of graphene strips ( >10) are able to reproduce Rayleigh anomalies. These features make graphene strips good candidates for many applications, including tunable absorbers and frequency selective surfaces.

Long-Wavelength VCSEL Using High-Contrast Grating

Abstract: Recent advances in high-contrast grating (HCG) vertical-cavity surface-emitting lasers (VCSEL) emitting at 1550 nm is reported in this paper. The novel near-wavelength HCG has an ultrathin structure and broadband reflectivity. It enables a monolithic, simple fabrication process for realizing InP-based VCSELs emitting at ~1550 nm. We report 2.4-mW single-mode output under continuous-wave operation at 15°C. We show that, despite broadened by the Brownian motion, the HCG-VCSEL has a total linewidth of 60 MHz or a coherent length of 5 m in air, and an intrinsic linewidth <;20 MHz. Transmission of directly modulated 10 Gbps over 100-km dispersion-compensated single-mode fiber is demonstrated. Tunable HCG-VCSEL is demonstrated with the HCG integrated with a micro-electro-mechanical structure. Continuous wavelength tuning as wide as 26.3 nm is achieved. The tunable VCSEL was used as a source for external modulation for 40-Gbps differential-phase-shift-keyed signal and transmitted over 100-km dispersion-compensated link with negligible power penalty.

Highly efficient color filter array using resonant Si3N4 gratings.

Abstract: 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.

Electromagnetically induced grating in asymmetric quantum wells via Fano interference

Abstract: We propose a scheme for obtaining an electromagnetically induced grating in an asymmetric semiconductor quantum well (QW) structure via Fano interference. In our structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced. The diffraction efficiency of the grating can be controlled efficiently by tuning the control field intensity, the interaction length, the coupling strength of tunneling, etc. This investigation may be used to develop novel photonic devices in semiconductor QW systems.

Experimental demonstration of surface and bulk plasmon polaritons in hypergratings

Abstract: Hyperbolic metamaterials (HMMs) represent a novel class of fascinating anisotropic plasmonic materials, supporting highly confined bulk plasmon polaritons in addition to the surface plasmon polaritons. However, it is very challenging to tailor and excite those modes at optical frequencies using prism coupling technique because of the intrinsic difficulties to engineer non-traditional optical properties using artificial nanostructures and the unavailability of high refractive index prisms for matching the momentum between the incident light and the guided modes. Here, we experimentally demonstrate the excitation of both surface and bulk plasmon polaritons in a HMM through a grating coupling technique of surface plasmon excitation that makes use a hypergrating, which is a combined structure of metallic diffraction grating and HMM. Initially, we propose an optical hyperbolic metamaterial based on Au/TiO2 multilayers and confirm the hyperbolic dispersion, and the presence of high-k modes in the fabricated HMM. Reflection measurements as a function of incident angle and excitation wavelength show the existence of both surface and bulk plasmon polaritons inside the hypergrating. The proposed configuration is expected to find potential applications in bio-chemical sensors, integrated optics and optical sub-wavelength imaging.

Packaging Process for Grating-Coupled Silicon Photonic Waveguides Using Angle-Polished Fibers

Abstract: A novel process for fiber-packaging submicrometer silicon waveguides is presented. The process uses fibers polished at an angle to reflect light between a horizontal core and the slightly off-vertical input and output path of a grating coupler. The necessity for a reflective coating on the fiber facet is overcome through the use of total internal reflection and a novel technique of epoxy distribution based on capillary action. Simulations of alignment tolerance are presented, along with measurements confirming the applicability of passive alignment. A peak coupling efficiency within 0.2 dB of the theoretical maximum for the grating coupler is achieved.

Asymmetric transmission of terahertz radiation through a double grating.

Abstract: We report on experimental evidence of unidirectional transmission of terahertz waves through a pair of metallic gratings with different periods. The gratings are optimized for a broadband transmission in one direction, accompanied with a high extinction rate in the opposite direction. In contrast to previous studies, we show that the zero-order nonreciprocity cannot be achieved. Nonetheless, we confirm that the structure can be used successfully as an asymmetric filter.

Self-diffraction of continuous laser radiation in a disperse medium with absorbing particles

Abstract: We study the self-action of light in a water suspension of absorbing subwavelength particles. Due to efficient accumulation of the light energy, this medium shows distinct non-linear properties even at moderate radiation power. In particular, by means of interference of two obliquely incident beams, it is possible to create controllable phase and amplitude gratings whose contrast, spatial and temporal parameters depend on the beams’ coherence and power as well as the interference geometry. The grating characteristics are investigated via the beams’ self-diffraction. The main mechanism of the grating formation is shown to be thermal, which leads to the phase grating; a weak amplitude grating also emerges due to the particles’ displacements caused by the light-induced gradient and photophoretic forces. These forces, together with the Brownian motion of the particles, are responsible for the grating dynamics and degradation. The results and approaches can be used for investigation of the thermal relaxation and kinetic processes in liquid suspensions.

Off-axis setup taking full advantage of incoherent illumination in coherence-controlled holographic microscope

Abstract: Coherence-controlled holographic microscope (CCHM) combines off-axis holography and an achromatic grating interferometer allowing for the use of light sources of arbitrary degree of temporal and spatial coherence. This results in coherence gating and strong suppression of coherent noise and parasitic interferences enabling CCHM to reach high phase measurement accuracy and imaging quality. The achievable lateral resolution reaches performance of conventional widefield microscopes, which allows resolving up to twice smaller details when compared to typical off-axis setups. Imaging characteristics can be controlled arbitrarily by coherence between two extremes: fully coherent holography and confocal-like incoherent holography. The basic setup parameters are derived and described in detail and experimental validations of imaging characteristics are demonstrated.

Numerical solution of an inverse diffraction grating problem from phaseless data

Abstract: This paper is concerned with the numerical solution of an inverse diffraction grating problem, which is to reconstruct a periodic grating profile from measurements of the phaseless diffracted field at a constant height above the grating structure. An efficient continuation method is developed to recover the Fourier coefficients of the periodic grating profile. The continuation proceeds along the wavenumber and updates are obtained from the Landweber iteration at each step. Numerical results are presented to show that the method can effectively reconstruct the shape of the grating profile.

3‐D optical modeling of thin‐film silicon solar cells on diffraction gratings

Abstract: An effective rigorous 3-D optical modeling of thin-film silicon solar cells based on finite element method (FEM) is presented. The simulation of a flat single junction thin-film silicon solar cell on thick glass (i.e., superstrate configuration) is used to validate a commercial FEM-based package, the High Frequency Structure Simulator (HFSS). The results are compared with those of the reference software, Advanced Semiconductor Analysis (ASA) program, proving that the HFSS is capable of correctly handling glass as an incident material within very timely, short, and numerically stable calculations. By using the HFSS, we simulated single junction thin-film silicon solar cells on glass substrates textured with one-dimensional (1-D) and two-dimensional (2-D) trapezoid-shaped diffraction gratings. The correctness of the computed results, with respect to an actual device, is discussed, and the impact of different polarizations on spectral response and optical losses is examined. From the simulations carried out, optimal combinations for period and height in both 1-D and 2-D grating configurations can be indicated, leading to short-circuit current percentage increase with respect to a flat cell of, respectively, 25.46% and 32.53%. With very limited computer memory usage and computational time in the order of tens of minutes for a single simulation, we promote the usage of 3-D FEM as a rigorous and efficient way to simulate thin-film silicon solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

High 90% efficiency Bragg gratings formed in fused silica by femtosecond Gauss-Bessel laser beams

Abstract: Direct laser write of volume Bragg gratings with diffraction efficiency (absolute) ∼90% is demonstrated using Gauss-Bessel laser beams in fused silica glass. Axial multiplexing of ∼ 90 μm long segments of modified optical material was demonstrated and thick Bragg gratings of aspect ratio depth/period ≈234 were achieved with period d = 1.5 μm. Typical fabrication scanning speeds were up to 50 mm/s for gratings with cross sections up to five millimeters made within 1 h time. Potential applications of high efficiency Bragg gratings in a low nonlinearity medium such as silica are discussed.

Spontaneous emission and collection efficiency enhancement of single emitters in diamond via plasmonic cavities and gratings

Abstract: We demonstrate an approach, based on plasmonic apertures and gratings, to enhance the radiative decay rate of single nitrogen-vacancy (NV) centers in diamond while simultaneously improving their collection efficiency. Our structures are based on metallic resonators formed by surrounding sub-wavelength diamond nanoposts with a silver film, which can enhance the spontaneous emission rate of an embedded NV center. However, the collection efficiency of emitted photons remains low due to losses to surface plasmons and reflections at the diamond-air interface. In this work, we mitigate photon losses into these channels by incorporating grating structures into the plasmonic cavity system.

Contact image sensor using switchable bragg gratings

Abstract: There is provided a contact image sensor comprising: an illumination; a first SBG array device; a transmission grating; a second SBG array device; a waveguiding layer comprising a multiplicity of waveguide cores separated by cladding material; an upper clad layer; and a platen The sensor further comprises: means for coupling light from said illumination means into the first SBG array; means for coupling light out of the cores into output optical paths coupled to a detector comprising at least one photosensitive element.

Experimental high numerical aperture focusing with high contrast gratings

Abstract: We demonstrate high aperture (up to NA∼0.64) three-dimensional focusing in free space based on wavefront-engineered diffraction gratings. The grating lens’ optical response is tailored by spatially varying the grating ridge and groove width in two dimensions to achieve focal lengths of order 100 μm that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2π phase jumps and was obtained by applying an algorithm for finding the optimal path for both phase and amplitude. Experimental measurements reveal a lateral spot size of 5 μm that is close to the size of a corresponding Airy disk.

CMOS-Compatible Integrated Spectrometer Based on Echelle Diffraction Grating and MSM Photodetector Array

Abstract: We demonstrate an integrated spectrometer-on-a-chip composed of an echelle diffraction grating (EDG) and metal-semiconductor-metal (MSM) waveguide photodetector array based on silicon-on-insulator (SOI). In the passive section, silicon oxynitride (SiON) is chosen as the material for the waveguide core and is deposited after selectively removing the top silicon layer of the SOI wafer. The buried silicon dioxide layer of the SOI wafer functions as the lower cladding for the SiON core waveguide. In the active section, the MSM photodetector array is fabricated on the top silicon layer of the SOI waveguide with a pitch width of 7.5 μm. With the butt-coupling structure, a responsivity of 0.41 A/W is obtained at 850 nm. Based on the CMOS-compatible fabrication process, we have fabricated a 60-channel spectrometer with a chip size of 9 mm × 6 mm operating around 850 nm. The measured channel spacing is 0.494 nm, with an adjacent channel crosstalk around 18 dB. The channel nonuniformity is less than 1.5 dB. The CMOS-compatible spectrometer with integrated silicon photodetector array can provide a low-cost solution for high-resolution on-chip spectral analysis for visible and near-infrared light with the wavelength below 1100 nm.

Optical phased array using high contrast gratings for two dimensional beamforming and beamsteering.

Abstract: We have developed a microelectromechanical system (MEMS) optical phased array incorporating a high-index-contrast subwavelength grating (HCG) for beamforming and beamsteering in a range of ± 1.26° × 1.26°. Our approach needs only a thin single-layer HCG made of silicon, considerably improving its speed thanks to the low mass, and is suitable for high optical power applications. The measured resonant frequency of HCG is 0.32 MHz.

Grating configurations for a tiled waveguide display

Abstract: Grating configurations are described for creating time sequenced field of view (FOV) tiles for a waveguide display. Pairings of non-output diffraction gratings and output diffraction gratings are activated to create a number of FOV tiles in a time sequence, for example in a frame update period for the image. Examples of a non-output grating are an input grating and a fold grating. For a set of at least three gratings used to make the pairings, each non-output grating is paired with each output grating. The number of pairings, and so the number of FOV tiles, is equal to a product of the total number of non-output gratings and the total number of output gratings. At least one diffraction grating in the pairing is an active pairing. Also described is a multiplexed diffraction grating including multiplexed K-vectors which increases the overall angular bandwidth for both incidence and diffraction.

In-situ atomic force microscopy study of the mechanism of surface relief grating formation in photosensitive polymer films

Abstract: When photosensitive azobenzene-containing polymer films are irradiated with light interference patterns, topographic variations in the film develop that follow the local distribution of the electric field vector. The exact correspondence of e.g., the vector orientation in relation to the presence of local topographic minima or maxima is in general difficult to determine. Here, we report on a systematic procedure how this can be accomplished. For this, we devise a new set-up combining an atomic force microscope and two-beam interferometry. With this set-up, it is possible to track the topography change in-situ, while at the same time changing polarization and phase of the impinging interference pattern. This is the first time that an absolute correspondence between the local distribution of electric field vectors and the local topography of the relief grating could be established exhaustively. Our setup does not require a complex mathematical post-processing and its simplicity renders it interesting for characterizing photosensitive polymer films in general.

Display comprising an optical waveguide and switchable diffraction gratings and method of producing the same

Abstract: An apparatus is disclosed for producing an optical display comprising an optical waveguide (1) and a pair (10, 16) of switchable diffraction gratings that are switchable between a diffractive state and a non-diffractive state. A pair of non-switchable diffraction gratings (2, 14) is arranged to receive diffract light from one switchable grating for guided propagation along the optical waveguide and out to the other switchable grating for viewing. The pair of non-switchable gratings are tuned to a first operating wavelength of light, while the pair of switchable gratings are tuned to a different operating wavelength of light to diffract that light into/from a field of view in common with that of the non-switchable gratings such that light of two wavelengths occupies the same field of view.

Terahertz beam steering and variable focusing using programmable diffraction gratings.

Abstract: We propose a freely programmable THz diffraction grating based on an electrostatically actuated, computer controlled array of metallic cantilevers. Switching between different grating patterns enables tailoring spatio-temporal profiles of the THz waves. By characterizing the device with spatially resolved THz time domain spectroscopy, we demonstrate beam steering for a wide frequency band extending from 0.15 THz to 0.9 THz. The steerable range at 0.3 THz exceeds 40°. Focusing is also demonstrated by programming a chirped grating. The proposed approach could be employed to mimic arbitrary diffraction optics, enabling highly integrated and extremely flexible systems indispensable for THz stand-off imaging and communications.

Femtosecond x-ray absorption spectroscopy with hard x-ray free electron laser

Abstract: We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10−3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10−3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.

Femtosecond laser damage threshold of pulse compression gratings for petawatt scale laser systems

Abstract: Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400–800 mJ/cm2 range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.

Use of photonic crystal cavities for temporal differentiation of optical signals

Abstract: We show that general-form optical cavities are able to perform the temporal differentiation of optical signals. Analytical relationships to account for the scattering losses for such a cavity’s characteristics are deduced on the basis of temporal coupled-mode theory. A compact nanocavity-aided differentiator based on a ridge photonic crystal waveguide is designed.

Accurate Analysis of Light Scattering and Absorption by an Infinite Flat Grating of Thin Silver Nanostrips in Free Space Using the Method of Analytical Regularization

Abstract: We study the plane wave scattering and absorption by a flat grating of thin silver nanostrips located in free space, in the visible-light range. The formulation involves generalized boundary conditions imposed on the strip median lines. We use an accurate numerical solution to this problem based on the dual-series equations and the method of analytical regularization. This guarantees fast convergence and controlled accuracy of computations. Reflectance, transmittance, and absorbance as a function of the wavelength and the grating parameters are analyzed. In addition to well-known surface-plasmon resonances, sharp resonances are revealed in the H-polarized scattering near but not equal to the Rayleigh wavelengths of nonzero diffraction orders; in the E-polarized scattering these resonances are not visible. Asymptotic formulas for the frequencies and natural fields of the grating resonances are presented.

CMOS-Compatible Polarization Splitting Grating Couplers With a Backside Metal Mirror

Abstract: We present a highly efficient 1-D grating structure that serves to couple light between standard optical fibers and single-mode waveguides in the silicon-on-insulator platform and to split both orthogonal polarization states. The efficiency of the fabricated coupler is enhanced by a backside metal mirror and reaches -2.4 dB for both polarizations at 1552 nm with an extinction ratio >25 dB in a wide wavelength range. The efficiency can be theoretically improved to -1.1 dB when optimizing the number of periods and using a nonuniform grating.