Nanoscale Optical Sensors Research Articles

Dielectric Mie voids: confining light in air

Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors, and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we experimentally implement a novel strategy for dielectric nanophotonics: Resonant subwavelength localized confinement of light in air. We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties. Due to the confinement in air, the modes do not suffer from the loss and dispersion of the dielectric host medium. We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV). Furthermore, we utilize the bright, intense, and naturalistic colours for nanoscale colour printing. Mie voids will thus push the operation of functional high-index metasurfaces into the blue and UV spectral range. The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro- and nanoscale optical elements. In particular, this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material for novel sensing and active manipulation strategies.

Resonant Dielectric Metagratings for Response Intensified Optical Sensing

AbstractThe development of nanoscale optical sensors is desirable for a broad range of applications, including wearable medical‐diagnostics, biochemical detection, and environmental monitoring. Optical detection platforms based on resonant nanostructures are the golden standard for miniaturized footprint and high optical sensitivity. These sensors function by measuring a shift in resonance wavelength upon binding of analytes to their surface. However, such measurements are sensitive to intensity fluctuations of the illuminating source and its wavelength calibration, which limits their applicability. Here, a novel optical sensing concept based on diffraction measurements from resonant dielectric metagratings is proposed and experimentally demonstrated. It is shown that this approach enables the direct measurement of unknown analytes with enhanced sensitivity and without the need for intensity calibrations. The intensified sensitivity of this metagrating‐sensor is derived from combining the resonant phenomena of the nanostructures with the tailored diffraction from the metagrating, thereby providing the highest sensitivity demonstrated to date amongst grating‐based sensors. As a proof of concept, the metagrating‐sensor was validated using an antibody binding assay, achieving a femtomolar‐level limit of detection. Due to their high sensitivity and robust performance, the proposed metagrating sensors pave the way for novel miniaturized medical diagnostics and biosensing applications.

Nano-scale selective and sensitive optical sensor for metronidazole based on fluorescence quenching: 1H-Phenanthro[9,10-d]imidazolyl-calix[4]arene fluorescent probe

It is crucial to determine and control the metronidazole (MET) ingredient in food and pharmaceuticals for human health and food safety. Even though many sensors have been previously reported to detect MET, there is still a need for a highly selective and sensitive, easy, fast, cost-effective sensor in this area. Herein, we report a fluorescent calix[4]arene derivative (PIMC) for highly selective and sensitive and facile and rapid MET detection based on fluorescence (FL) quenching. The highest FL quenching occurs when PIMC is exposed to MET solution at 400 nm (λex = 340). Owing to the quenching efficacy of MET linearly up to 5.5 × 104 nM was obtained a detection limit of 2.44 nM. Besides, interferences of other pharmaceuticals and ions on probe performance were investigated. The FL probe was successful in MET detection without the assistance of any separation techniques in a pharmaceutical sample (tablet) with an acceptable recovery of 101.3%. The applicability of the current probe as a paper-based sensor to MET detection has been successfully tested. As a result, the proposed probe presents a fast and suitable strategy to sensitive and selective detect MET and proves a good potential for practical applications, especially pharmaceutical preparations.

Nanowire Waveguides and Lasers: Advances and Opportunities in Photonic Circuits.

Due to their single-crystalline structures, comparatively large aspect ratios, tight optical confinement and smooth surfaces, nanowires have increasingly attracted research interests for both fundamental studies and technological applications in on-chip photonic devices. This class of nanostructures typically have cross-sections of 2~200 nm and lengths upwards of several micrometers, allowing for the bridging of the nanoscopic and macroscopic world. In particular, the lasing behaviors can be established from a nanowire resonator with positive feedback via end-facet reflection, making the nanowire a promising candidate in the next generation of optoelectronics. Consequently, versatile nanowire-based devices ranging from nanoscale coherent lasers, optical sensors, waveguides, optical switching, and photonic networks have been proposed and experimentally demonstrated in the past decade. In this article, significant progresses in the nanowire fabrication, lasers, circuits, and devices are reviewed. First, we focus on the achievements of nanowire synthesis and introduce the basics of nanowire optics. Following the cavity configurations and mode categories, then the different light sources consisting of nanowires are presented. Next, we review the recent progress and current status of functional nanowire devices. Finally, we offer our perspective of nanowires regarding their challenges and future opportunities in photonic circuits.

Subwavelength, polarimetric color sorting by densely interleaved nano-resonators

Sorting light by spectrum in subwavelength scale emerges as important research due to its potential applications in nanoscale optical devices. Diverse research activities are explored to achieve efficient spectral sorting based on nanostructured metasurfaces by leveraging plasmonic and dielectric resonances. While they show many advantages of controlling light in nanoscale with compact form factor, practical realization of the efficient sorting of light in visible spectrum is still elusive due to the strong optical coupling and crosstalk between nanostructures that are arranged in subwavelength scale. Here, we demonstrate silicon-based ultrathin metasurfaces composed of densely interleaved one-dimensional nano-resonators (meta atoms) with three different widths that function both as efficient spectral sorter and light absorber in the visible regime. We find that the geometric aspect ratio of meta atoms and specific mode of resonance play a crucial role in realizing efficient subwavelength spectral splitting with significantly suppressed crosstalk. The sorting efficiency up to 78% is reported by optimally designing the metasurface. We believe that suggested designing scheme paves an important way to realize ultrathin, subwavelength spectral sorters and to develop nano-scale optical sensors.

Nanophotonic Device Based on Fano Resonance in Engineered Slot Waveguide for Optical Detection of Viral Infections

A nanophotonic biosensing platform based on an engineered slot waveguide is proposed. The proposed device is based on Fano resonance which consists of a periodic arrangement of two rows of nano-disks and rectangular grating. The periodically arranged nano-disk structure in the form of engineered slot waveguide provides enhanced light-matter interaction and easy accommodation of bio-samples. The surface morphology of (3-Aminopropyl) trimethoxysilane (APTMS) as a functionalized layer on the silicon surface is experimentally analyzed. The proposed sensor device can detect very small refractive index change of 0.0008. Based on refractive index sensing, the device numerically exhibits a high sensitivity of 1463 nm/RIU for some viral infections including hepatitis B the with a Figure of Merit (FOM) of 471 for refractive index change of 0.02. The limit of detection for the proposed device is 1.02 $\times10^{-3}$ RIU (Refractive Index Unit). The proposed scheme with Fano like resonance spectrum carries a great potential for on-chip and off-chip nanoscale optical devices and sensors.

Optical Probes for Neurobiological Sensing and Imaging.

Fluorescent nanosensors and molecular probes are next-generation tools for imaging chemical signaling inside and between cells. Electrophysiology has long been considered the gold standard in elucidating neural dynamics with high temporal resolution and precision, particularly on the single-cell level. However, electrode-based techniques face challenges in illuminating the specific chemicals involved in neural cell activation with adequate spatial information. Measuring chemical dynamics is of fundamental importance to better understand synergistic interactions between neurons as well as interactions between neurons and non-neuronal cells. Over the past decade, significant technological advances in optical probes and imaging methods have enabled entirely new possibilities for studying neural cells and circuits at the chemical level. These optical imaging modalities have shown promise for combining chemical, temporal, and spatial information. This potential makes them ideal candidates to unravel the complex neural interactions at multiple scales in the brain, which could be complemented by traditional electrophysiological methods to obtain a full spatiotemporal picture of neurochemical dynamics. Despite the potential, only a handful of probe candidates have been utilized to provide detailed chemical information in the brain. To date, most live imaging and chemical mapping studies rely on fluorescent molecular indicators to report intracellular calcium (Ca2+) dynamics, which correlates with neuronal activity. Methodological advances for monitoring a full array of chemicals in the brain with improved spatial, temporal, and chemical resolution will thus enable mapping of neurochemical circuits with finer precision. On the basis of numerous studies in this exciting field, we review the current efforts to develop and apply a palette of optical probes and nanosensors for chemical sensing in the brain. There is a strong impetus to further develop technologies capable of probing entire neurobiological units with high spatiotemporal resolution. Thus, we introduce selected applications for ion and neurotransmitter detection to investigate both neurons and non-neuronal brain cells. We focus on families of optical probes because of their ability to sense a wide array of molecules and convey spatial information with minimal damage to tissue. We start with a discussion of currently available molecular probes, highlight recent advances in genetically modified fluorescent probes for ions and small molecules, and end with the latest research in nanosensors for biological imaging. Customizable, nanoscale optical sensors that accurately and dynamically monitor the local environment with high spatiotemporal resolution could lead to not only new insights into the function of all cell types but also a broader understanding of how diverse neural signaling systems act in conjunction with neighboring cells in a spatially relevant manner.

Subcellular Optical pH Nanoscale Sensor

AbstractCoated gold nanoparticles bearing a pH‐sensitive molecule serve as nanoscale optical sensors for non‐invasive pH quantification of their endocytosis through surface‐enhanced Raman scattering. Our sensor consists in colloidal gold spheres coated either with (polyethylene glycol) PEG molecules or a silica shell. They carry a sensor molecule that specifically recognize protons. The read out for monitoring changes in the pH is the Raman shift of the sensor molecule that is enhanced on the surface of the plasmonic spheres. Sensing was performed along the way of internalization from the extracellular site through different endo/lysosomal compartments where they are closely packed. The creation of hot spots favored by particle agglomeration inside cells was responsible for the enhancement of changes in signal intensity and was dependent on the surface chemistry. We establish a correlation between the physicochemical properties of the nanoscale sensor (shape, surface chemistry) and its ability to monitor the different pH along its cellular internalization. The PEGylated spheres can sensitively track the pH along their cellular internalization whereas the silica coated ones fail.

A Plasmonic Temperature-Sensing Structure Based on Dual Laterally Side-Coupled Hexagonal Cavities.

A plasmonic temperature-sensing structure, based on a metal-insulator-metal (MIM) waveguide with dual side-coupled hexagonal cavities, is proposed and numerically investigated by using the finite-difference time-domain (FDTD) method in this paper. The numerical simulation results show that a resonance dip appears in the transmission spectrum. Moreover, the full width of half maximum (FWHM) of the resonance dip can be narrowed down, and the extinction ratio can reach a maximum value by tuning the coupling distance between the waveguide and two cavities. Based on a linear relationship between the resonance dip and environment temperature, the temperature-sensing characteristics are discussed. The temperature sensitivity is influenced by the side length and the coupling distance. Furthermore, for the first time, two concepts—optical spectrum interference (OSI) and misjudge rate (MR)—are introduced to study the temperature-sensing resolution based on spectral interrogation. This work has some significance in the design of nanoscale optical sensors with high temperature sensitivity and a high sensing resolution.

Formation of Carbonized Polystyrene Sphere/hemisphere Shell Arrays by Ion Beam Irradiation and Subsequent Annealing or Chloroform Treatment.

Heat-resistant two-dimensional (2D) sphere/hemisphere shell array is significant for the fabrication of novel nanostructures. Here large-area, well-ordered arrays of carbonized polystyrene (PS) hollow sphere/hemisphere with controlled size and morphology are prepared by combining the nanosphere self-assembly, kV Ag ion beam modification, and subsequent annealing or chloroform treatment. Potential mechanisms for the formation and evolution of the heat-resistant carbonized PS spherical shell with increasing ion fluence and energy are discussed. Combined with noble metal or semiconductor, these modified PS sphere arrays should open up new possibilities for high-performance nanoscale optical sensors or photoelectric devices.

Plasmonic/Nonlinear Optical Material Core/Shell Nanorods as Nanoscale Plasmon Modulators and Optical Voltage Sensors

AbstractHerein, we report the design and synthesis of plasmonic/non‐linear optical (NLO) material core/shell nanostructures that can allow dynamic manipulation of light signals using an external electrical field and enable a new generation of nanoscale optical voltage sensors. We show that gold nanorods (Au NRs) can be synthesized with tunable plasmonic properties and function as the nucleation seeds for continued growth of a shell of NLO materials (such as polyaniline, PANI) with variable thickness. The formation of a PANI nanoshell allows dynamic modulation of the dielectric environment of the plasmonic Au NRs, and therefore the plasmonic resonance characteristics, by an external electrical field. The finite element simulation confirms that such modulation is originated from the field‐induced modulation of the dielectric constant of the NLO shell. This approach is general, and the coating of the Au NRs with other NLO materials (such as barium titanate, BTO) is found to produce a similar effect. These findings can not only open a new pathway to active modulation of plasmonic resonance at the sub‐wavelength scale but also enable the creation of a new generation of nanoscale optical voltage sensors (NOVS).

Plasmonic/Nonlinear Optical Material Core/Shell Nanorods as Nanoscale Plasmon Modulators and Optical Voltage Sensors

Herein, we report the design and synthesis of plasmonic/non-linear optical (NLO) material core/shell nanostructures that can allow dynamic manipulation of light signals using an external electrical field and enable a new generation of nanoscale optical voltage sensors. We show that gold nanorods (Au NRs) can be synthesized with tunable plasmonic properties and function as the nucleation seeds for continued growth of a shell of NLO materials (such as polyaniline, PANI) with variable thickness. The formation of a PANI nanoshell allows dynamic modulation of the dielectric environment of the plasmonic Au NRs, and therefore the plasmonic resonance characteristics, by an external electrical field. The finite element simulation confirms that such modulation is originated from the field-induced modulation of the dielectric constant of the NLO shell. This approach is general, and the coating of the Au NRs with other NLO materials (such as barium titanate, BTO) is found to produce a similar effect. These findings can not only open a new pathway to active modulation of plasmonic resonance at the sub-wavelength scale but also enable the creation of a new generation of nanoscale optical voltage sensors (NOVS).

Fabrication and optical properties of controlled Ag nanostructures for plasmonic applications

Noble metal nanoparticles exhibit unique plasmon resonances compared to bulk metal that depends on the nanoparticle size, shape, and interparticle distance. Towards this goal, the paper describes a simple and low-cost method for producing large area arrays of ordered silver nanoparticles with morphologies such as dots, rings, and bowls, based on the nanosphere lithography by combining evaporation and sputtering techniques. Pronounced, we demonstrate here a facile fabrication strategy that nanosphere monolayer could serve as a sacrificial mask access to generate silver nanobowl arrays. The experimental and simulated results show that the Ag nanodot arrays exhibit a satisfactory surface-enhanced Raman spectroscopy effect and adjustable frequency. These approaches will surely facilitate further exploration of metal nanostructures for applications in nanoscale optical sensors.

A Possible Use of Fiber-Optic Ring Resonator as a Nano-Scale Optical Sensor for Detection of Physical Quantities

This paper presents a proposed fiber-optic ring resonator with a resonance loop made of a conventional single-mode optical fiber as a nano-scale sensor for measurement of physical quantities, such as pressure and temperature. The operational theory of the resonator as an optical sensor is presented, where the effects of characteristic parameters of the resonator on the sensor response is investigated. It is shown that the behavior of sensor response with respect to some particular physical quantities is linear and is capable of detecting measurand variations of the order of 100 nanometers. The proposed sensor is suitable for design of hydrophones used in submarine co mmunicat ions.

Bypassing the diffusion limit

The detection of ultralow concentrations of molecules using nanoscale optical sensors is hindered by the difficulty in bringing the two into contact, where diffusion acts on impractical timescales. Fortunately, introducing plasmonic structures to super-hydrophobic surfaces may offer a way around this problem.

Strong Modification of Quantum Dot Spontaneous Emission via Gap Plasmon Coupling in Metal Nanoslits

A metal−dielectric−metal (MDM) waveguide with a nanoscale gap supports highly confined surface plasmon−polariton modes, termed gap plasmons. The spontaneous emission of an emitter placed in such a metal nanogap is expected to be strongly modified due to coupling to gap plasmons. We investigate the light emission properties of semiconductor quantum dots (QD) in a metal nanoslit, which is a truncated MDM waveguide. More specifically, we measure both the lifetime and the state of polarization of the out-coupled QD emission from a metal nanoslit. We observe clear lifetime and polarization changes of QD emission. As the slit width gets smaller, the QD exciton lifetime gradually decreases, and its emission becomes polarized normal to the slit, as expected for gap plasmon coupled light emission. We also find that the polarization of the collected QD emission is flipped (i.e., becomes parallel to a slit) when the excited emitters are located just outside the slit. We have conducted dipole emission calculations in metal nanoslits, and these explain the experimentally observed lifetime and polarization changes well. These findings may have novel applications in nanoscale optical sources, sensors, and active devices.

Fabrication of large area ordered metal nanoring arrays for nanoscale optical sensors

In contrast to metal nanodots, nanoring structures are accessible to various sensing and spectroscopy applications at nanometer scale due to their tunable plasmon resonances and large empty volume confined that provides more spaces for molecular attachment. In almost all the applications, fabrication of large area ordered metal nanoring arrays represents the first and one of the most significant challenges. This paper describes a simple and low-cost method of producing large area ordered metal nanoring arrays based on self-assembled monolayers of monodisperse colloidal spheres by combining reactive ion etching, sputtering and ion polishing techniques. Inner diameter, wall thickness and wall height of nanorings are controllable by varying the process conditions of reactive ion etching, sputtering, ion polishing or any combination thereof while their period can be changed by using colloidal spheres of various sizes. This new approach will surely facilitate further exploration of metal nanorings for applications in nanoscale optical sensors.