Nanoslit Arrays Research Articles

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

A waveguide metasurface based quasi-far-field transverse-electric superlens

The imaging capability of conventional lenses is mainly limited by the diffraction of light, and the so-called superlens has been developed allowing the recovery of evanescent waves in the focal plane. However, the remarkable focusing behavior of the superlens is greatly confined in the near-field regime due to the exponential decay of evanescent waves. To tackle this issue, we design a waveguide metasurface-based superlens with an extraordinary quasi-far-field focusing capability beyond the diffraction limit in the present work. Specifically, we analyze the underlying physical mechanism and provide experimental verification of the proposed superlens. The metasurface superlens is formed by an array of gradient nanoslits perforated in a gold slab, and supports transverse-electric (TE) waveguide modes under linearly polarized illumination along the long axis of the slits. Numerical results illustrate that exciting such TE waveguide modes can modulate not only optical phase but also evanescent waves. Consequently, some high-spatial-frequency waves can contribute to the focusing of the superlens, leading to the quasi-far-field super-resolution focusing of light. Under 405 nm illumination and oil immersion, the fabricated superlens shows a focus spot of 98 nm (i.e. <em>λ</em>/4.13) at a focal distance of 1.49 μm (i.e. 3.68<em>λ</em>) using an oil immersion objective, breaking the diffraction limit of <em>λ</em>/2.38 in the quasi-far field regime. The developed metasurface optical superlens with such extraordinary capabilities promises exciting avenues to nanolithography and ultra-small optoelectronic devices.

Self-referencing biosensors using Fano resonance in periodic aluminium nanostructures.

Surface plasmon resonance (SPR) is an important technique for real-time and label-free detection of specific binding biomolecules. However, conventional SPR signals come from both the surface binding biomolecules and the variation in the bulk refractive index. This work demonstrates that Fano resonance in an aluminum capped nanoslit array has the ability to remove the signal of bulk refractive index changes from the SPR signal. As compared to gold nanostructures, the aluminum nanostructure provides an asymmetrical Fano resonance with clear peak and dip wavelengths. The peak wavelength is close to the grating resonance condition. The evanescent depth at the peak wavelength is up to several microns. The dip wavelength comes from the SPR effect. The evanescent depth at the dip wavelength is about 300 nm. By simultaneously measuring the shifts of peaks and the dip wavelengths, the variation in the bulk refractive index can be removed and only the biolayer thickness is measured. The finite-difference time-domain calculation shows that the 470 nm-period nanoslit array with 90 and 70 nm slit depths has the optimal thickness sensitivity. In this experiment, a simple multispectral imaging system is developed for multiple bio-interaction measurements. The measured results verify that the bulk refractive index changes can be removed and the surface biomolecular interactions can be directly obtained without the need of a reference channel.

Dual dielectric cap gold nanoslits array optical resonance filter with large figure-of-merit.

In this work, we investigate a gold nanoslits array optical transmission filter with dual dielectric cap layers on top of the metal nanoslits. By integrating a low index of refraction dielectric layer between a high index of refraction dielectric cap layer and the gold nanoslits, a narrow spectral linewidth optical filter with a transmission peak far away from the Rayleigh anomaly wavelength is shown. Furthermore, we propose a figure-of-merit as the ratio of the spectral distance between a transmission peak and the Rayleigh anomaly over the spectral linewidth to characterize the performance of gold nanoslits optical filters. It is shown that dual dielectric cap gold nanoslits array optical filters have significantly larger figure-of-merits than that of traditional single dielectric cap gold nanoslits array optical filters.

Characterization of near Infrared Light Transmission in Periodic Subwavelength Nanoslit Array by Using Effective Medium Theory

Characterization of near Infrared Light Transmission in Periodic Subwavelength Nanoslit Array by Using Effective Medium Theory

Ion-beam-etching based lift-off for reliable patterning of dense and inverse metallic nanostructures towards 10-nm scale

Reliable fabrication of metallic patterns at nanoscale is of great interest for various practical applications in nanodevices. Electron-beam-lithography based lift-off is the most often used method to define exquisite metal nanostructures at the 10-nm scale. In a common wet lift-off process, the metal material on top is lifted-off and washed away together with the sacrificial resist layer below. However, the wet lift-off process is sometimes ultrasonic agitation and/or heating-dependent and limited by the probable remained debris problem. In this work, we present an alternative lift-off process to obtain metallic patterns via removing the unwanted parts by using ion beam etching. We demonstrated that this ion-beam-etching based lift-off process is effective for isolated exposed negative-tone resist to fabricate nanoholes and nanoslits array and for dense networked exposed resist to fabricate high-density packed nanostructures. SERS measurements were conducted to demonstrate the molecular detection capability of the nanostructures with tiny gaps fabricated by this ion-beam-etching based lift-off process.

High-Throughput and Dynamic Study of Drug and Cell Interactions Using Contrast Images in Aluminum-Based Nanoslit Arrays

High-throughput and dynamic measurement for living cell activities can benefit biological research and drug development. A low-cost metallic nanostructure-based surface plasmon resonance (SPR) imaging platform, comprising multiple aluminum nanoslit arrays and a color image device, is proposed for label-free study of cell and drug interactions. The multiple nanoslit sensing arrays were fabricated using the compression-injection molding process. These sensing chips showed a detectable depth of 600 nm and refractive index resolution of ∼5 × 10-5 refractive index unit (RIU) by using a self-referenced two-color analysis. Two examples of kinetic studies of living cells under various doses of drugs are presented. The focal adhesion kinases inhibitor (FAKi 14) and cell interactions show exponential changes of cellular adhesion and time constants for different concentrations of antiadhesion drugs. The anticancer drug (doxorubicin (DOX))-treated cells show slow increases of SPR signals in the first 2 h due to the nucleus swelling. The DOX-treated cells further process plasma membrane disruption and become floating cells and debris in the medium, resulting in rapid drops of the SPR signals.

Metasurface with metallic nanoantennas and graphene nanoslits for sensing of protein monolayers and sub-monolayers.

Biomolecule sensing plays an important role in both fundamental biological studies and medical diagnostic applications. Infrared (IR) spectroscopy presents opportunities for sensing biomolecules as it allows their fingerprints to be determined by directly measuring their absorption spectra. However, the detection of biomolecules at low concentrations is difficult with conventional IR spectroscopy due to signal-to-noise considerations. This has led to recent interest on the use of nanostructured surfaces to boost the signals from biomolecules in a method termed surface enhanced infrared spectroscopy. So far, efforts have largely involved the use of metallic nanoantennas (which produce large field enhancement) or graphene nanostructures (which produce strong field confinement and provide electrical tunability). Here, we propose a nanostructured surface that combines the large field enhancement of metallic nanoantennas with the strong field confinement and electrical tunability of graphene plasmons. Our device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. We perform systematic electromagnetic simulations to quantify the sensing performance of the proposed device and show that it outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized. These investigations consider the model system of a representative protein-goat anti-mouse immunoglobulin G (IgG) - in monolayer or sub-monolayer form. Our findings provide guidance for future biosensors for the sensitive quantification and identification of biomolecules.

Teaching optics to a machine learning network.

In this Letter, we demonstrate how harmonic oscillator equations can be integrated in a neural network to improve the spectral response prediction for an optical system. We use the optical properties of a one-dimensional nanoslit array for a practical implementation of the study. This method allows to build more generalizable relations between the input parameters of the array and its optical properties, showing a 20-fold improvement for parameters outside the range used for the training. We also show how this model generates the output spectrum from phenomenological relationships between the input parameters and the output spectrum, indicating how it grasps the physical mechanisms of the optical response of the structure.

Plasmonic Nanoslit Arrays Fabricated by Serial Bideposition: Optical and Surface-Enhanced Raman Scattering Study.

Recently, studies have been carried out to combine surface-enhanced Raman spectroscopy substrates that are based on either localized surface plasmon or surface plasmon polariton structures. By combining these two systems, the individual drawbacks of each can be overcome. However, the manufacturing methods involved so far are sophisticated, labor-intensive, expensive, and technically demanding. We propose a facile method for the fabrication of a flexible plasmonic nanoslit surface-enhanced Raman scattering (SERS) sensor. We utilized the pattern on periodic optical disks as an inexpensive substitute for printing the periodic pattern on polydimethylsiloxane with soft imprint lithography. The Ag nanoslits were fabricated by serial bideposition using the dynamic oblique angle deposition technique. The nanoslit structures were physically and optically characterized, and the experimental results were compared to the results of the numerical simulation: Monte Carlo and finite-difference time-domain simulation. The AgNS samples showed excellent SERS performance with an enhancement factor of ∼105 and a limit of detection of 5 × 10-7 g/mL for a Rhodamine 6G solution. Their biosensing capability was demonstrated by the sensing of bilirubin.

Polarized resonant emission of monolayer WS2 coupled with plasmonic sawtooth nanoslit array

Transition metal dichalcogenide (TMDC) monolayers have enabled important applications in light emitting devices and integrated nanophotonics because of the direct bandgap, spin-valley locking and highly tunable excitonic properties. Nevertheless, the photoluminescence polarization is almost random at room temperature due to the valley decoherence. Here, we show the room temperature control of the polarization states of the excitonic emission by integrating WS2 monolayers with a delicately designed metasurface, i.e. a silver sawtooth nanoslit array. The random polarization is transformed to linear when WS2 excitons couple with the anisotropic resonant transmission modes that arise from the surface plasmon resonance in the metallic nanostructure. The coupling is found to enhance the valley coherence that contributes to ~30% of the total linear dichroism. Further modulating the transmission modes by optimizing metasurfaces, the total linear dichroism of the plasmon-exciton hybrid system can approach 80%, which prompts the development of photonic devices based on TMDCs.

Fabrication of the Au Nano-Aperture Array Platform for Single Molecule Analysis

We have fabricated the Au nano-aperture array platforms on the Au films for single molecule analysis. Previously, we reported fabrication of the 200 nm wide double slits with various separations between two nanoslits, and the optical characteristics. In this paper, we will address the optical characteristics of various Au aperture platfoms; the double slits with a 50 nm opening width, the nanoslit array with its opening width less than 100 nm, and the circular type nano-aperture by using 30 keV focused Ga ion beam techniques, along with Au nanopore formation under the high energy electron beam irradiations at 200 keV. For the slit width less than 100 nm, we observed the surface plasmon polariton (SPP)-mediated intraband transmission peak at ∼500 nm, and the SPP-coupled peak around ∼650 nm. The optical intensities are measured to be dependent upon the size of the nano-aperture, the opening width of the slit, and the thickness of the Au film. For a 30 nm slit width, the broadband emission ranging from ∼600 nm to 800 nm were also observed. The fabricated Au plasmonic platforms can be utilized as single molecule bio-sensor.

A Compound Phase-Modulated Beam Splitter to Distinguish Both Spin and Orbital Angular Momentum

Based on a phase-modulated metallic nanoslit array, an angular momentum (AM) beam splitter has been demonstrated to distinguish both spin and orbital components carried by the light beam. With such a device, the AM modes could be coupled into nondiffracting surface plasmonic beams propagating toward different directions. According to the experimental results, the extinction ratio for spin AM beam splitting is larger than 10 and the spatial interval of adjacent orbital AM modes (with a topological charge interval of 2) is more than 1.1 μm. We believe that such a device would have great potential to achieve a highly compact photonic integrated circuit with the plasmonic beam.

A Concave Blazed-Grating-Based Smartphone Spectrometer for Multichannel Sensing

A compact imaging smartphone spectrometer utilizing a lightweight aspheric concave blazed-grating is presented for multichannel spectral detection. The lightweight concave diffraction grating is fabricated on a plastic substrate using a two-step thermal-embossing method in a cost-effective and mass-productive way, which enables a large-area, smooth and aspheric concave surface while preserving a blazed-grating structure. The blazed-grating serves as a key single element for aberration-free light focusing and highly-efficient light diffraction. It thus eliminates extra moving optics in the spectrometer and improves the optical characteristics and system integration, which are proven by both simulation and experiments. Spectral linewidth of 1 nm is achieved by combining the concave grating with a smartphone camera. This compact imaging smartphone spectrometer is only about the size of a match-box. Spectra can be obtained directly from the detected images using a developed application (APP) interface. High-throughput plasmonic resonance sensing using microfluidic silver nanoslit arrays under various glycerin environments is demonstrated, and the detected resonance wavelength response is comparable with that detected by a commercial compact spectrometer.

Resonances of the magneto-optical intensity effect mediated by interaction of different modes in a hybrid magnetoplasmonic heterostructure with gold nanoparticles.

Here we demonstrate a novel magnetoplasmonic heterostructure for efficient control of light. It consists of gold nanoparticles embedded in a thin magnetic film covered with a gold layer pierced with periodic nanoslit array. Unique feature of the proposed structure is that it supports four different types of optical modes in the same frequency range including localized and propagating surface plasmons along with waveguide modes. A peculiar magneto-optical response appears at the frequencies of the mode hybridization. The most important result comes from hybridization of the localized and propagating plasmons leading to a significant increase of the magneto-optical effect intensity.

Large‐Area and Low‐Cost Nanoslit‐Based Flexible Metasurfaces for Multispectral Electromagnetic Wave Manipulation

AbstractManipulating multispectral electromagnetic (EM) waves has drawn great attention due to its potential applications in EM compatibility, radiative cooling, and multispectral imaging. Metasurfaces are intensively investigated to control EM waves and achieve versatile functions. However, the design of multispectral manipulation and the fabrication of multiscale metasurfaces remain a great challenge. Here, a multiscale flexible metasurface with nanometer‐sized characteristic dimensions and a square centimeter sized area is designed and fabricated for the simultaneous manipulation of infrared and terahertz waves. A simple yet powerful analytic method derived from catenary field theory is proposed to guide the design. A metasurface consisting of 2D nanoslit array is fabricated onto a flexible polyimide substrate by combining shadow mask fabrication with surface plasmon lithography to achieve nanoscale, large‐area, and repeatable fabrication. The experimental results show that the proposed metasurface possesses high reflectance exceeding 95% within the 8–12 µm wavelength range, indicating low thermal emissivity. Additionally, the metasurface exhibits low reflection in the THz regime and is capable of implementing integrated functions in conjunction with THz devices. Furthermore, the significant advantage of flexibility affords the metasurface excellent compatibility with irregular surfaces. This work may provide important guidance for the design and fabrication of large‐scale multispectral metasurfaces.

Polymer LEDs with improved efficacy via periodic nanostructure-based aluminum.

Periodic aluminum-capped nanoslit arrays were produced on a polycarbonate plastic substrate by rapid hot embossing nanoimprint lithography and thermal evaporation, and they were used as a transparent window for blue-emitting polymer light-emitting diodes (PLEDs). The external quantum efficiency of blue-emitting PLEDs was enhanced by thesurface plasmon polaritons of the periodic aluminum-capped nanoslit arrays. A maximum current efficiency of 4.84cd/A was achieved for the proposed PLED, which was over 2.2 times that of the reference PLED (2.18cd/A). These results demonstrate that periodic nanostructure can assist in the simple and low-cost fabrication of high-performance polymer optoelectronic devices.

Plasmonic Multiplexing Extraordinary Optical Transmission Sensor With Enhanced Sensitivity Through Grooves in Metal Film

To achieve significantly improved sensitivity, the author propose plasmonic sensors based on extraordinary optical transmission incorporating a periodic nanoslit array in metal film with additional grooves. The author show that the peak wavelength shift in the transmission spectrum due to biomolecular film adsorption can be boosted by using the nanofocusing of light in the grooves. The sensitivities to biomolecular film of the sensor prove to be extremely improved in comparison with those of conventional sensors with the same structure and materials, but without grooves with 1.3 and 8.4 times at the top and the wall of the silt, respectively. The sensor performances for the plasmonic structures with symmetric and asymmetric grooves are investigated and the proposed sensors are shown to be able to operate for spatial multiplexing biomolecular detection.

Polarization-Selective Holographic Metasurface For Creating Cylindrical Vector Beams

Generalized cylindrical vector beams (GCVB) are spatially polarization inhomogeneous. It could be considered as a combination of a radially polarized beam and an azimuthally polarized beam with different weighting factors. Here, we report a holographic metasurface patterned on a gold film layer for generating a GCVB. The nanostructure is a superposition of an individually oriented nanoslit array that determines the polarization distribution of the GCVB and a binary fork hologram that eliminates the additional Pancharatnam–Berry phase resulted from the circularly polarized incident plane beam. The behavior of a single nanoslit with specific parameters as a nanopolarizer and that of the overall holographic metasurface when illuminated by a circularly polarized plane beam are simulated and verified. A GCVB with plane wavefront and doughnut-shaped intensity distribution is experimentally produced using this holographic metasurface. The polarization state of the GCVB is also detected and confirmed.

Manipulating surface plasmon polaritons with M-shaped nanoslit array via polarized incident waves

Surface plasmonic bending beams (SPBs), which preserve their spatial shapes while propagating along the desired curved trajectories in a metal-dielectric interface, offer important applications in the fields of fiber sensors, optical trapping, and micro-nano fabrication. In this work, the straight-lined, single-curved and double mirror-symmetric curved distributions of M-shaped nanoslit arrays are designed on a metal film to generate SPBs. These SPBs include unidirectional coupling of SPPs, single arbitrary bending beams, and double arbitrary bending beams. The electric-field intensity and the propagation directions of these SPBs are controlled by the polarization angle of input light waves. Especially, the reverse propagation of unidirectional coupling of arbitrary SPBs is generated by the polarization angles 45° and 135°. These findings provide guidance in the design and optimization of plasmonic bending beam generators.