Cascaded momentum-space polarization filters enabled label-free black-field microscopy for single nanoparticles analysis.

Label-free optical imaging of single-nanometer-scale matter is extremely important for a variety of biomedical, physical, and chemical investigations. One central challenge is that the background intensity is much stronger than the intensity of the scattering light from single nano-objects. Here, we propose an optical module comprising cascaded momentum-space polarization filters that can perform vector field modulation to block most of the background field and result in an almost black background; in contrast, only a small proportion of the scattering field is blocked, leading to obvious imaging contrast enhancement. This module can be installed in various optical microscopies to realize a black-field microscopy. Various single nano-objects with dimensions smaller than 20 nm appear distinctly in the black-field images. The chemical reactions occurring on single nanocrystals with edge lengths of approximately 10 nm are in situ real-time monitored by using the black-field microscopy. This label-free black-field microscopy is highly promising for a wide range of future multidisciplinary science applications.

Investigating the effect of volatility on the hygroscopicities of acetate nanoparticle aerosols by surface plasmon resonance microscopy.

Under high relative humidity (RH) conditions, the release of volatile components (such as acetate) has a significant impact on the aerosol hygroscopicity. In this work, one surface plasmon resonance microscopy (SPRM) measurement system was introduced to determine the hygroscopic growth factors (GFs) of three acetate aerosols separately or mixed with glucose at different RHs. For Ca(CH3COO)2 or Mg(CH3COO)2 aerosols, the hygroscopic growth trend of each time was lower than that of the previous time in three cyclic humidification from 70% RH to 90% RH, which may be due to the volatility of acetic acid leading to the formation of insoluble hydroxide (Ca(OH)2 or Mg(OH)2) under high RH conditions. Then the third calculated GF (using the Zdanovskii-Stokes-Robinson method) for Ca(CH3COO)2 or Mg(CH3COO)2 in bicomponent aerosols with 1:1 mass ratio were 3.20% or 5.33% lower than that of the first calculated GF at 90% RH. The calculated results also showed that the hygroscopicity change of bicomponent aerosol was negatively correlated with glucose content, especially when the mass ratio of Mg(CH3COO)2 to glucose was 1:2, the GF at 90% RH only decreased by 4.67% after three cyclic humidification. Inductively coupled plasma atomic emission spectrum (ICP-AES) based measurements also indicated that the changes of Mg2+concentration in bicomponent was lower than that of the single-component. The results of this study reveal thatduring the efflorescence transitions of atmospheric nanoparticles, the organic acids diffusion rate may be inhibited by the coating effect of neutral organic components, and the particles aging cycle will be prolonged.

Retrieving the subwavelength cross-section of dielectric nanowires with asymmetric excitation of Bloch surface waves.

Optical microscopy with a diffraction limit cannot distinguish nanowires with sectional dimensions close to or smaller than the optical resolution. Here, we propose a scheme to retrieve the subwavelength cross-section of nanowires based on the asymmetric excitation of Bloch surface waves (BSWs). Leakage radiation microscopy is used to observe the propagation of BSWs at the surface and to collect far-field scattering patterns in the substrate. A model of linear dipoles induced by tilted incident light is built to explain the directional imbalance of BSWs. It shows the potential capability in precisely resolving the subwavelength cross-section of nanowires from far-field scattering without the need for complex algorithms. Through comparing the nanowire widths measured by this method and those measured by scanning electron microscopy (SEM), the transverse resolutions of the widths of two series of nanowires with heights 55 nm and 80 nm are about 4.38 nm and 6.83 nm. All results in this work demonstrate that the new non-resonant far-field optical technology has potential application in metrology measurements with high precision by taking care of the inverse process of light-matter interaction.

Chip-based wide field-of-view total internal reflection fluorescence microscopy.

Conventional total internal reflection fluorescence (TIRF) microscopy requires either an oil-immersed objective with high numerical aperture or a bulky prism with high refractive index to generate the evanescent waves that work as the illumination source for fluorophores. Precise alignment of the optical path is necessary for optimizing the imaging performance of TIRF microscopy, which increases the operation complexity. In this Letter, a planar photonic chip composed of a dielectric multilayer and a scattering layer is proposed to replace the TIRF objective or the prism. The uniform evanescent waves can be excited under uncollimated incidence through this chip, which simplifies the alignment of the optical configurations and provides shadowless illumination. Due to the separation of the illumination and detection light paths, TIRF microscopy can have a large field-of-view (FOV).

Converting the guided-modes of Bloch surface waves with surface pattern.

The guided-modes of Bloch surface waves, such as the transverse electric modes (TE00 and TE01 modes), can simultaneously exist in a low-refractive-index ridge waveguide with subwavelength thickness that are deposited on an all dielectric one-dimension photonic crystal. By using the finite difference frequency domain method, coupled mode theory and finite-difference time-domain method, the conversion between the guided-modes has been investigated. This conversion can be realized in a broadband wavelength with surface pattern of this low-index ridge. This conversion is useful for developing lab-on-a-chip photonic devices, such as a mode converter that can maintain the output mode purity over 90% with working wavelength ranging from 590 to 680 nm, and a power splitter that can maintain the splitting ratio over 8:2 with wavelength ranging from 530 to 710 nm.

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals.

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS2) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS2 relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

In Situ Quantitative Observation of Hygroscopic Growth of Single Nanoparticle Aerosol by Surface Plasmon Resonance Microscopy.

Aerosol particle hygroscopicity is an important factor in visibility reduction, cloud formation, radiation forcing, and the global climate. The high number concentration of nanoparticles (defined as particles with diameters below 100 nm) means that their hygroscopic growth abilities and potential contributions to the climate and environment are significant. Therefore, a rapid and accurate in situ analysis method for single nanoparticle hygroscopic growth in an atmospheric environment is important to characterize the effects of the particle's physical and chemical properties in this process. In this work, surface plasmon resonance microscopy with azimuthal rotation illumination (SPRM-ARI) is used to observe the hygroscopic growth and water content of single nanoparticles in situ. The hygroscopic growth results of a single-component nanoparticle are well matched with the extended aerosol inorganic model (E-AIM) results, and the proposed method remains reliable even when the relative humidity (RH) exceeds 90%. For a bicomponent nanoparticle (with NaCl as the primary content), the presence of a component without deliquescence phase transitions under increasing humidity conditions causes the measured data to differ from both the Zdanovskii-Stokes-Robinson (ZSR) model and E-AIM predictions in the low RH range. However, because of their complete liquefaction, the growth factor (GF) variation of the bicomponent nanoparticle is close to the model predictions in the high RH range. Finally, based on the positive correlation between particle volume and the gray intensity of SPRM-ARI, GF values can be obtained from the cube root of the gray intensity and the actual water content of single nanoparticles can then be derived.

Extending the Propagation Distance of a Silver Nanowire Plasmonic Waveguide with a Dielectric Multilayer Substrate.

Chemical-synthesized silver nanowires have been proven as an efficient architecture for plasmonic waveguides, but the high propagation loss prevents their widely applications. Here, we demonstrate that the propagation distance of the plasmons along a silver nanowire can be extended if this nanowire was placed on a dielectric multilayer substrate containing a photonic band gap but not placed on a commonly used glass substrate. The propagation distance at 630 nm wavelength can reach 16 µm, even when the silver nanowire is as thin as 90 nm in diameter. Experimental and simulation results further show that the polarization of this propagating plasmon mode was nearly parallel to the surface of the dielectric multilayer, so it can be excited by a transverse-electric polarized Bloch surface wave propagating along a polymer nanowire with diameter at only about 170 nm on the same dielectric multilayer. Numerical simulations were also carried out and are consistent with the experiment results. Our work provides a platform with which to extend the propagation distance of the plasmonic waveguide and also for the integration between photonic and plasmonic waveguides on the nanometer scale.

Selective excitation of surface plasmon modes propagating in Ag nanowires.

Surface plasmon modes propagating in metal nanowires are conveniently excited by focusing a laser beam on one extremity of the nanowire. We find that the precise positioning of the nanowire inside the focal region drastically influences the excitation efficiency of the different SPP modes sustained by the plasmonic waveguide. We demonstrate a spatially selective excitation of bound and leaky surface plasmon modes with excitation maps that strongly depend on the orientation of the incident linear polarization. We discuss this modal selection by considering the inhomogeneous distribution of the field components inside the focus. Our finding provides a way to discriminate the effective indices of the modes offering thus an increased coupling agility for future nanowire-based plasmonic architectures.

Near-Infrared Circularly Polarized Light Triggered Enantioselective Photopolymerization by Using Upconversion Nanophosphors.

Circularly polarized light (CPL) is considered to be a true chiral entity and has been suggested as an explanation for the introduction of initial chiral biases into key biomolecular building blocks. CPL used recently asymmetric photochemical reactions is of wavelengths mainly in the UV and visible regions, whereas natural CPL observed in star-forming regions of the Orion constellation falls in the IR region. Whether CPL in the IR or near-IR region could be utilized to trigger asymmetric photochemical reactions remains to be determined. Herein, it is demonstrated that enantioselective photopolymerization can be realized by using λ=980 nm CPL as the only chiral source. By incorporating NaYF4 nanophosphors as the antenna species, the enantioselective photopolymerization of achiral benzaldehyde-substituted diacetylene monomer can be realized based on an upconversion mechanism upon exposure to λ=980 nm CPL. The screw direction of the helical PDA chains can be completely controlled by the handedness of incident λ=980 nm CPL.

Radiative decay engineering 8: Coupled emission microscopy for lens-free high-throughput fluorescence detection.

Fluorescence spectroscopy and imaging are now used throughout the biosciences. Fluorescence microscopes, spectrofluorometers, microwell plate readers and microarray imagers all use multiple optical components to collect, redirect and focus the emission onto single point or array imaging detectors. For almost all biological samples, except those with regular nanoscale features, emission occurs in all directions. With the exception of complex microscope objectives with large collection angles (NA ≤ 0.5), all these instruments collect only a small fraction of the total emission. Because of the increasing knowledge base on fluorophores within near-field (<200 nm) distances from plasmonic and photonic structures we can anticipate the development of compact devices in which the sample to be detected is located directly on solid state detectors such as CCDs or CMOS cameras. Near-field interactions of fluorophores with metallic or dielectric multi-layer structures (MLSs) can capture a large fraction of the total emission. Depending on the composition and dimensions of the MLSs, the spatial distribution of the sample emission results in distinct optical patterns on the detector surface. With either plain glass slides or MLSs the most commonly used front focal plane (FFP) images reveal the x-y spatial distribution of emission from the sample. Another approach, which is often used with two or three-dimensional nanostructures, is back focal plane (BFP) imaging. The BFP images reveal the angular distribution of the emission. The FFP and BFP images occur at certain distances from the sample which is determined by the details of the optical components. Obtaining these images requires multiple optical components and distances which are too large for the compact devices. For devices described in this paper, the images will be detected at a fixed distance between the sample and some arbitrary distance below the MLS which is determined by the geometry and thicknesses of the components. We refer to measurements at these locations as out-of-focal plane (OFP) imaging. Herein we describe a method to measure the optical fields at micron and multi-micron distances below the MLS, which will represent the images seen by an optically coupled array detector. The possibility of sub-surface optical images is illustrated using five different multi-layer structures. This is accomplished using an optical configuration which allows measurement at a front focal plane (FFP), back focal plane (BFP) or any OFP locations. Our OFP imaging method provides a link between the FFP images which reveals the surface distribution of fluorophores with the BFP images that reveal the angular distribution of emission. This linkage can be useful when examining structures which have nanoscale features due to fluorescence or leakage radiation from nanostructures.

Plasmonic lasing of nanocavity embedding in metallic nanoantenna array.

Plasmonic nanolasers have ultrahigh lasing thresholds, especially those devices for which all three dimensions are truly subwavelength. Because of a momentum mismatch between the propagating light and localized optical field of the subwavelength nanocavity, poor optical pumping efficiency is another important reason for the ultrahigh threshold but is normally always ignored. On the basis of a cavity-embedded nanoantenna array design, we demonstrate a room-temperature low-threshold plasmonic nanolaser that is robust, reproducible, and easy-to-fabricate using chemical-template lithography. The mode volume of the device is ∼0.22(λ/2n)(3) (here, λ is resonant wavelength and n is the refractive index), and the experimental lasing threshold produced is ∼2.70MW/mm(2). The lasing polarization and the function of nanoantenna array are investigated in detail. Our work provides a new strategy to achieve room-temperature low-threshold plasmonic nanolasers of interest in applications to biological sensoring and information technology.

Fluorescence emission difference with defocused surface plasmon-coupled emission microscopy.

A novel fluorescence emission difference method is proposed to improve the lateral resolution of SPCEM without increasing instrument complexity. We discovered the profile of transverse PSF in SPCEM will dramatically change from a hollow spot to a solid spot, when the axial position of sample varies within one wavelength in the vicinity of the focal plane. The subtraction of an image whose PSF is hollow spot and an image with solid PSF will greatly enhance the resolution and contrast of SPCEM images. The mechanism of the distinctive PSF is demonstrated through basic optics theories, and the improvement of lateral resolution is verified by theoretical simulations and experimental results. It is believed that our method will stand out for its pleasant resolution enhancement and its instruments' simplicity to facilitate many biological cellular observations.

Resolution-enhanced surface plasmon-coupled emission microscopy.

A novel fluorescence emission difference technique is proposed for further enhancements of the lateral resolution in surface plasmon-coupled emission microscopy (SPCEM). In the proposed method, the difference between the image with phase modulation by using a 0-2π vortex phase plate (VPP) along with a diaphragm and the original image obtained from SPCEM is used to estimate the spatial distribution of the analyzed sample. By optimizing the size of the diaphragm and the subtractive factor, the lateral resolution can be enhanced by about 20% and 33%, compared with that in SPCEM with a single 0-2π VPP and conventional wide-field fluorescence microscopy, respectively. Related simulation results are presented to verify the capability of the proposed method for improving lateral resolution and reducing imaging distortion. It is believed that the proposed method has potentials to improve the performance of SPCEM, thus facilitating biological observation and research.

Local spectroscopy of silver nanowire in different environments excited with a halogen lamp.

We report a propagation spectrum detection system in which one end of a plasmonic silver nanowire is locally illuminated from a normal halogen lamp and the scattered light is recorded spectroscopically at the other end. The system is applied to investigate surface plasmon polariton-Fabry-Perot (SPP-FP) modes of silver nanowires with different lengths at air-glass and oil-glass interfaces. The generalized FP model is used to analyze the spectrum, which fits well with the experimental results. The influence of nanowire length and environment on the properties of the FP resonances is discussed. The propagation spectrum detection system will find applications for integrated optical circuits and plasmonic sensing.

Extracting surface wave-coupled emission with subsurface dielectric gratings.

Conventional surface plasmons (SPs) or Bloch surface waves (BSWs) have a wave vector exceeding that of light in vacuum, and, therefore, the surface plasmon-coupled emission (SPCE) or Bloch surface wave-coupled emission (BSWCE) cannot escape from the corresponding structures. With the aid of a high-refractive-index prism or an oil-immersion objective, the SPCE or BSWCE can be coupled into free space. But the large volumes of the prism and objective are certainly unfavorable for miniaturization of the optical systems or inconvenient for applications such as the optical displays. Here we experimentally demonstrate a new method to extract the SPCE or BSWCE with a subsurface dielectric grating. The experimental results verify that the chip-like substrate with two decorated sides can bring out the directional fluorescence emission in free space. The emitting direction and emitting patterns can be tuned by the period size and dimensionality of the gratings. Our work provides a new strategy to realize free-space directional fluorescence emission at a very low cost and compact configuration, which has potential applications in fluorescence-based sensing, imaging, light-emitting diodes, optical displays, and other near-field optical devices.

Back focal plane imaging of directional emission from dye molecules coupled to one-dimensional photonic crystals.

Bloch surface waves (BSWs) on one-dimensional photonic crystals (1DPCs) have been used to beam the fluorescence emission from the dye molecules. All dielectric 1DPC displays its low propagating loss, narrow resonance and the absence of absorption or quenching. In this paper, back focal plane imaging reveals that in addition to the BSW mode, a guided mode and a cavity mode also exist in the 1DPC which all couple with the excited dye molecules. The appearance of these modes is sensitive to the wavelength of the fluorescence and alters the beaming effect by the 1DPC. Numerical simulations verify the existence of these modes which are consistent with the experimental results. Comparisons between the Bloch surface wave coupled emission and surface plasmon coupled emission are also presented for a clearer understanding of the multilayered film enabled directional emission.

Effect of metal film thickness on Tamm plasmon-coupled emission.

Tamm plasmons (TPs) are the result of trapping optical energy at the interface between a metal film and a one-dimensional photonic crystal. In contrast to surface plasmons, TPs display unique properties such as the ability to undergo direct optical excitation without the aid of prisms or gratings, being populated using both S- and P-polarized light, and importantly, they can be created with incident light normal to the surface. This latter property has recently been used to obtain Tamm plasmon-coupled emission (TPCE), which beams along a path directly perpendicular to the surface. In this paper the effects of metal film thickness on the TPCE are investigated using back focal plane (BFP) imaging and spectral resolutions. The observed experimental results are in agreement with the numerical simulations. The present work provides the basic understanding needed to design structures for TPCE, which in turn has potential applications in the fabrication of active materials for light emitting devices, fluorescence-based sensing, using microarrays, and imaging.

Dynamically generating a large-area confined optical field with subwavelength feature size.

By using a prism of high refractive index, free-space cylindrical vector beams can be selectively converted into confined optical fields with large area, such as surface plasmon polaritons or waveguide modes, whose interference will produce optical features at the nanometer scale. Due to the polarization sensitivity of these modes, the macroscopic distribution of the confined field can be dynamically manipulated through an electronically driven liquid crystal. Based on these phenomena, a promising maskless interference nanolithography is proposed and experimentally demonstrated.

High-Q/Veff gap-mode plasmonic FP nanocavity.

In this paper, a high-Q/Veff gap-mode plasmonic Fabry-Perot nanocavity, which is composed of a silver nanowire on a flat silver substrate spaced by patterned dielectric distributed Bragg gratings, is investigated both analytically and numerically. The design parameters and properties of the nanocavity are exploited with the use of generalized Fabry-Perot model. The Veff ~0.0026 (λ/n)³ and Q/Veff ~1.4 × 105/µm³ of the nanocavity can be achieved. Such a gap-mode plasmonic Fabry-Perot nanocavity design provides a promising realization for wide novel band filters and spaser.