Near-field Intensity Research Articles

Identifying extreme ultraviolet lithography attenuated phase shifting mask absorber materials using effective media approximation modeling

Pattern transfer in an extreme ultraviolet lithography (EUVL) system requires reflective optical elements illuminated at oblique illumination angles. This, in combination with the three-dimensional effects at the mask, is the source of the so-called mask 3D (M3D) effects that include shadowing, best focus shifts, and contrast fading. Alternative mask absorbers at lower thickness possess the ability to extenuate the M3D effects and improve overall imaging performance. An approach to recognize candidate material combinations as alternative EUVL mask absorbers through dielectric constant modeling by using methods involving Wiener bounds and effective media approximation (EMA) is presented. Using EMA, several stoichiometrically stable binary alloys of refractory metals that may serve as attenuated phase shifting mask absorbers have been studied. The optical properties and thickness of each absorber candidate alloy in a standard EUV mask stack are optimized for imaging performance. The best performing alloys are characterized through rigorous 3D image modeling of near-field intensity and phase at varied illumination angles.

Deformation measurement by single spherical near-field intensity measurement for large reflector antenna

This paper presents a new method to obtain the deformation distribution on the main reflector of an antenna only by measuring the electric intensity on a spherical surface with the focal point as the center of the sphere, regardless of phase. Combining the differential geometry theory with geometric optics method, this paper has derived a deformation-intensity equation to relate the surface deformation to the intensity distribution of a spherical near-field directly. Based on the finite difference method (FDM) and Gauss-Seidel iteration, deformation has been calculated from intensity simulated by geometrical optics (GO) and physical optics (PO) methods, respectively, with relatively small errors, which prove the effectiveness of the equation proposed in this paper. By means of this method, it is possible to measure the deformation only by scanning the electric intensity of a single hemispherical near-field whose area is only about 1/15 of the aperture. The measurement only needs a plane wave at any frequency as the incident wave, which means that both the signals from the outer space satellite and the far-field artificial beacon could be used as the sources. The scanning can be realized no matter what attitude and elevation angle the antenna is in because the size and angle of the hemisphere are changeable.

Lattice structures and optical properties of Ce:LYSO crystal waveguides with implanted C, Si and Cu ions

We report on the lattice structures and optical properties of Ce:LYSO crystal waveguides implanted with C, Si and Cu ions at 6 MeV. SRIM software was used to reconstruct the elastic collisions between the implanted ions and target nuclei. The maximum structural changes were induced by the Cu-ion implantation, as determined by XRD and Raman spectra. Photoluminescent and absorption spectra were used to study the optical properties of Ce3+ ions in the Ce:LYSO waveguides. Prism-coupling and end-face coupling methods, respectively, were used to measure the guiding modes and the near-field intensity distribution of light at 633 nm. Typical well + barrier waveguides were formed in the Ce:LYSO crystals implanted by C, Si and Cu ions, which will be a interest application in integrated optical devices.

Photonic Gap Antennas Based on High-Index-Contrast Slot Waveguides

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\sim10^{-4}\lambda_{0}^3$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operation principle of such Photonic Gap Antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot-waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or CYTOP as the gap-material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of $\sim$1000 for air gaps and $\sim$400 for CYTOP gaps over a spectral bandwidth of $\Delta\lambda\approx300$ nm at $\lambda=1.25$ \textmu m. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of $\sim$3000 for air gaps and $\sim$1200 for CYTOP gaps.

Optical nanoantenna with muitiple surface plasmon resonances for enhancements in near-field intensity and far-field radiation.

Plasmonic nanostructures with dual surface plasmon resonances capable of simultaneously realizing strong light confinement and efficient light radiation are attractive for light-matter interaction and nanoscale optical detection. Here, we propose an optical nanoantenna by adding gold nanoring to the conventional Fano-type resonance antenna. With the help of gold nanoring, the following improvements are simultaneously realized: (1). The near-field intensity of the Fano-type antenna is further enhanced by the Fabry Perot-like resonance formed by the combination of the gold nanoring and the substrate waveguide layer. (2). Directional radiation is realized by the collaboration of the gold nanoring and the Fano-type antenna, thus improving the collection efficiency of the far-field signal. (3). The multi-wavelength tunable performance of the Fano resonance antenna is significantly improved by replacing the superradiation mode in the Fano resonance with the dipole resonance induced by the gold nanoring. The optical properties of the nanoantennas are demonstrated by numerical simulations and practical devices. Therefore, the proposed optical nanoantenna provides a new idea for further improving the performance of conventional Fano-type nanoantennas and opens new horizons for designing plasmonic devices with enhancements in both near- and far-field functionalities, which can be applied in a wide range of applications such as surface-enhanced spectroscopy, photoluminescence, nonlinear nanomaterials/emitters and biomedicine sensing.

Collapse fragility due to near-field directivity ground motions: Influence of component, rupture distance, hypocenter location

The variability in source characteristics induces spatial variability in the characteristics of near-field ground motion intensities which in turn impacts the fragility behaviour of structures. The study aims to quantify such variability in fragility curves of low-rise structures with a natural period of 0.2 sec with change in nucleation asperity, a source parameter of earthquake rupture corresponding to (Mw ⋍ 7). Four rupture scenarios are considered which differ from each other based on the position of nucleation asperity. The rupture scenarios are termed as bilateral, partial bilateral, partial unilateral, and unilateral, respectively. For bilateral rupture, the nucleation asperity is placed exactly in the middle of the fault face. In contrast, in the unilateral case, the nucleation asperity is placed at the end of the fault. Additionally, fragility curves corresponding to each horizontal component (fault-parallel and fault-normal) of ground motions at stations with varying rupture distances are also highlighted. The study is also extended to evaluate structures’ fragility behaviour with a natural period of 0.6 sec, 1 sec and 4 sec. Additionally bi-directional response to the simulated ground motions were emphasized.

Optical planar and ridge waveguides formed in Er3+-doped germanate glass by ion implantation and precise diamond blade dicing.

In this work, we report on the preparation and characterization of the planar and ridge optical waveguides in the Er3+-doped germanate glass by combining hydrogen ion implantation and precise diamond blade dicing. The nuclear energy loss and the implantation depth were calculated by the SRIM 2013 software. The refractive index profile was obtained by the reflectivity calculation method. The dark-mode spectrum and the near-field intensity distribution were measured by the prism coupling system and end-face coupling technique, respectively. This work has important reference significance for the development of Er3+-doped germanate glass active devices in the optical communication field.

Efficiency enhancement in thin-film silicon solar cells by plasmonic nanodiscs array

Plasmonic nanoparticles are promising ways for the efficiency enhancement of thin-film solar cells. We have taken into account the bare silicon wafer and the embedded thin-film silicon solar cells with Au nanodiscs array. We study these geometries of thin-film silicon solar cells and find the effect of Au nanoparticles on light transmission and reflection spectrum, power absorption, generation rate, power electric, and short-circuit current density; and for this, we have used finite difference time domain, FDTD software. We also consider the near-field electric intensity in the vicinity of nanoparticles with different sizes, which are known as a solar cells efficiency enhancement mechanism. Our study can be useful for new perspectives for antireflection coating applications and light management in silicon solar cells.

Lensless phase retrieval based on deep learning used in holographic data storage.

This paper proposes a lensless phase retrieval method based on deep learning (DL) used in holographic data storage. By training an end-to-end convolutional neural network between the phase-encoded data pages and the corresponding near-field diffraction intensity images, the new unknown phase data page can be predicted directly from the intensity image by the network model without any iterations. The DL-based phase retrieval method has a higher storage density, lower bit-error-rate (BER), and higher data transfer rate compared to traditional iterative methods. The retrieval optical system is simple, stable, and robust to environment fluctuations which is suitable for holographic data storage. Besides, we studied and demonstrated that the DL method has a good suppression effect on the dynamic noise of the holographic data storage system.

CNN-based few-mode fiber modal decomposition method using digital holography.

Modal decomposition (MD) has become an indispensable analysis approach for revealing the modal characteristics of optical fibers. A new MD approach based on the convolutional neural network (CNN) is presented to retrieve the exact superposition of eigenmodes of few-mode fibers. Using the near-field beam intensity and phase patterns obtained from digital holography, not only the amplitude of each eigenmode but also the exact phase difference between the higher-order modes and the fundamental mode can be predicted. Numerical simulations validate the reliability and feasibility of the approach. When ten modes in the few-mode fiber are considered, the similarities of the intensity and phase pattern between the reconstructed fields and the given fields can achieve to 97.0% and 85.6%, respectively.

Nanofocusing of a metallized double periodic arranged nanocone array for surface-enhanced Raman spectroscopy.

A plasmonic double periodic arranged nanocone array (DPANA) integrated by nanotips and nanogaps exhibit strong capability of light compression, and thus lead to extremely enhanced electric near-field intensity. The DPANA is fabricated by the self-assembled mask integrated with the inductively couple plasma (ICP) etching technology. Finite-difference time-domain (FDTD) simulations suggest that the metallized DPANA can generate a strong hotspot at the sharp tip apex and the nanogap between adjacent sharp tips. The electric-field enhancement characteristic is firstly verified with the help of the second-order surface nonlinear optical response of the metallized DPANA. The surface-enhanced Raman spectroscopy (SERS) examination of the metallized DPANA exhibits high sensitivity due to clearly presenting the Raman spectra of Rhodamine-6G (R6G) with concentrations down to 10 pM and has excellent uniformity, time stability, and recyclability, simultaneously. Furthermore, the principle demonstration of SERS practical application is also performed for thiram. This as-prepared SERS substrate has great potential application for trace amount detection.

Optical planar and ridge waveguides in terbium scandium aluminum garnet crystal fabricated by ion implantation and precise diamond blade dicing

Terbium scandium aluminum garnet crystal is a promising material which has excellent characteristics for magneto-optical devices. We report on the fabrication of optical planar and ridge waveguides in the terbium scandium aluminum garnet by using combination of helium ion implantation and precise diamond blade dicing. The conditions for the ion implantation are an energy of 400 keV and a fluence of 6 × 1016 ions/cm2. The rotating velocity and cutting speed of the diamond blade were set to be 20,000 rpm and 0.1 mm/s, respectively. The implantation depth and distributions of energy loss were calculated by the stopping and range of ions in matter 2013. The refractive index distribution and the dark-mode spectrum for the implanted planar waveguide were obtained by the reflectivity calculation method and the prism coupling system, respectively. The near-field intensity profiles of both the planar and ridge waveguides were measured by the end-face coupling technique. The optical guiding properties of the terbium scandium aluminum garnet waveguides suggest that they can serve as candidates for optical waveguide isolators.

Coherent control at gold needle tips approaching the strong-field regime

Abstract We demonstrate coherent control in photoemission from a gold needle tip using an ω − 2ω field composed of strong few-cycle laser pulses with a nearfield intensity of ∼4 TW/cm2. We obtain the nearfield intensity from electron energy spectra, showing the tell-tale plateau of field-driven electron rescattering at the metal surface induced by the fundamental field. Changing the relative phase between the fundamental field centered at 1560 nm and its second harmonic modulates the total emitted photocurrent with visibilities of up to 80% despite the strong and broadband excitation of the plasmonic material. Our work combines a two-color coherent control scheme and strong-field physics enabled by a nanoplasmonic emitter.

근접장 빔 포커싱을 위한 초점 가변 메타표면 렌즈 설계

In this paper, we propose a metasurface lens to adjust the focal length for near-field beam focusing by using mechanical rotation. The proposed metasurface is implemented with two lenses using unit cell geometries, where the transmit phase distributions of the proposed metasurface lens are calculated based on the Moire lens theory. The lenses with dimensions of 0.48 m × 0.48 m are modeled using the FEKO EM simulation tool, and the near-field distributions and focal lengths are obtained in accordance with the rotating angle of the upper lens. When the upper lens is rotated from θ=0º to 60º, the focal length can be changed from a distance of 0.86 m to 0.68 m, and the maximum normalized nearfield intensity is achieved at a focal length of 0.68 m.

Spectral tuning of double resonant nanolaminate plasmonic nanoantennas with a fixed size

Multiresonant plasmonic nanoantennas can enhance nanolocalized multiphoton processes or enable wavelength-multiplexed nano-optic operations by supporting multiple spatially overlapped plasmonic modes. Nevertheless, current multiresonant plasmonic nanoantenna designs do not consider engineering multiresonant spectral responses with strict size and footprint constraints. Developing a strategy to engineer fixed-size nanoantennas with tunable multiresonant responses is highly desirable for maintaining controlled cellular responses at the nano-bio interface and achieving seamless integration with other nanodevices with predefined footprints. Here, we report that fixed-size tapered nanolaminate nanoantennas (TNLNAs) can achieve a wide double-resonance spectral tunability by only changing the metal-to-insulator thickness ratio (t/h). Three separate TNLNAs' samples (8/38 , 20/20, and 28/8 nm) with a nominal total height of ∼100 nm are created from a high-throughput nanofabrication technique. Specifically, we fabricated TNLNAs' samples by exploiting a nanohole array membrane from soft interference lithography as a deposition mask for electron-beam evaporation of alternating Au and SiO2 layers. Transmission and dark field scattering measurements show that TNLNAs support two distinct resonant features with t/h-dependent tunable resonant wavelengths in the range of 730–850 and 840–1050 nm, respectively. Numerical simulations reveal that (i) a bianisotropy-induced magnetoelectric response in top and bottom nanogaps due to the asymmetric tapered shape can enhance light trapping and achieve optical near-field intensity enhancements up to 1000-fold and (ii) while TNLNAs consisting of thin Au nanodisks at low t/h primarily support spatial overlap between modes with enhanced electric polarizability, TNLNAs consisting of thick Au nanodisks at high t/h support spatial overlap between modes with enhanced magnetic polarizability, evoking higher-order multipolar behaviors.

Beam wavefront retrieval by convoluted spatial spectral benchmark

We propose a method for retrieving a beam wavefront from its near-field intensity distribution after a 4f system by simply inserting a benchmark at the Fourier plane. Through a convolution of the mark-blocked spatial frequency component and the original optical field with the 4f system, the separation between the focus of any sub-aperture and the benchmark can be determined to reconstruct the beam wavefront. Theoretical and experimental studies demonstrate the validity of this method, which has potential applications in real-time wavefront sensing.

Active Individual Nanoresonators Optimized for Lasing and Spasing Operation.

Plasmonic nanoresonators consisting of a gold nanorod and a spherical silica core and gold shell, both coated with a gain layer, were optimized to maximize the stimulated emission in the near-field (NF-c-type) and the outcoupling into the far-field (FF-c-type) and to enter into the spasing operation region (NF-c*-type). It was shown that in the case of a moderate dye concentration, the nanorod has more advantages: smaller lasing threshold and larger slope efficiency and larger achieved intensities in the near-field in addition to FF-c-type systems’ smaller gain and outflow threshold, earlier dip-to-peak switching in the spectrum and slightly larger far-field outcoupling efficiency. However, the near-field (far-field) bandwidth is smaller for NF-c-type (FF-c-type) core–shell nanoresonators. In the case of a larger dye concentration (NF-c*-type), although the slope efficiency and near-field intensity remain larger for the nanorod, the core–shell nanoresonator is more advantageous, considering the smaller lasing, outflow, absorption and extinction cross-section thresholds and near-field bandwidth as well as the significantly larger internal and external quantum efficiencies. It was also shown that the strong-coupling of time-competing plasmonic modes accompanies the transition from lasing to spasing occurring, when the extinction cross-section crosses zero. As a result of the most efficient enhancement in the forward direction, the most uniform far-field distribution was achieved.

Optical band engineering via vertical stacking of honeycomb plasmonic lattices

Inspired by recent advances in atomic homo and heterostructures, we consider the vertical stacking of plasmonic lattices as a new degree of freedom to create a coupled system showing a modified optical response concerning the monolayer. The precise design of the stacking and the geometrical parameters of two honeycomb plasmonic lattices tailors the interaction among their metallic nanoparticles. Based on the similarity of the lattice symmetry, analogies can be drawn with stacked atomic crystals, such as graphene. We use the multipolar spectral representation to study the plasmonic vertical stack's optical response in the near-field regime, emphasizing symmetry properties. The strong coupling of certain optical bands shows up as anticrossings in the dispersion diagram, resulting in the polarization exchange of the interacting bands. By leveraging these effects, we engineer the near-field intensity distribution. Additionally, lifting band degeneracy at specific points of the Brillouin zone is obtained with the consequent opening of minigaps. These effects are understood by quantifying the multipolar coupling among nanospheres belonging to the same and different sublattices, as well as the interlayer and intralayer nanoparticle interactions. Differences with the atomic case are also analyzed and explained in terms of the stack's interaction matrix. Finally, we predict the absorption spectrum projected on the two orthogonal linear polarizations.

Direct imaging of fluorescence enhancement in the gap between two gold nanodisks

We present an analysis of the optical coupling between two gold nanodisks by near-field fluorescence microscopy. This is achieved by simultaneously scanning and measuring the light emitted by a single Er3+/Yb3+ doped nanocrystal glued at the end of an atomic force microscope tip. The excitation of the nanocrystal was performed at λ = 975 nm via upconversion, and fluorescence was detected in the visible part of the spectrum at λ = 550 nm. For an isolated nanodisk, the near-field presents a two-lobe pattern oriented along the direction of the incident polarization. For two nanodisks with a sizable separation distance (385 nm) illuminated with the polarization along the interparticle axis, we observe a negative effect of the coupling with a slight decrease in fluorescence in the gap. For smaller gap values (195, 95, and 55 nm), a strong increase in fluorescence is observed as well as a reduced spatial localization of the field as the distance decreases. Finally, when the disks touch each other (0 nm), the dipolar–dipolar interaction between them disappears and no fluorescence enhancement occurs. A new plasmon mode is created at another wavelength. Our experimental results are in good agreement with numerical simulations of the near-field intensity distribution at the excitation wavelength on the surface of the structures. Combining fluorescence mapping and far-field scattering spectroscopy should be of strong interest to develop bio-chemical sensors based on field enhancement effects.

Bioinspired nanoantennas for opsin sensitization in optogenetic applications: a theoretical investigation

Opsins with high sensitivity are desired to reduce dependence on optical fibers and enable deep-brain optogenetic stimulation through the intact scalp and skull, while minimizing brain tissue heating and the associated biasing of neural activity. While optimized opsin engineering has produced ultrasensitive and red-shifted opsins suitable for transcranial optogenetic stimulation, further improvements in sensitivity are throttled by biological limitations. Nanostructures are capable of generating near-field intensity enhancements of over 104, but thus far nanomaterials have not been applied to amplify local light intensity for optogenetic applications. In this manuscript, we propose the use of bowtie nanoantennas for local enhancement of 470 nm light to sensitize channelrhodopsin (ChR2) to low light intensities. We begin with a comparison of the near-field intensity enhancement offered by different metals at 470 nm, before selecting aluminum as the optimal material. Next, we tune the geometric parameters of aluminum bowtie nanoantennas to maximize the intensity enhancement at 470 nm. We further optimize enhancement by constructing bowtie nanoantenna arrays inspired by patterns occurring in biology, obtaining intensity enhancements up to a factor of 5000. Monte Carlo simulations suggest that transcranial 470 nm illumination of only 50 mW mm−2 is capable of stimulating bowtie-sensitized ChR2 in the deep brain (∼5 mm) in mice, enabling minimally invasive deep-brain stimulation with opsins found in the traditional optogenetic toolbox. This computation-guided optical antenna engineering approach opens opportunities for designing multifunctional materials for enhancing the efficiency of optogenetic neuromodulation, optical neural activity imaging, and highly localized electrical microstimulation in the brain.