Near-field Enhancement Effect Research Articles

Plasmonic enhancement of third-order nonlinear optical susceptibilities in self-doped Cu_2-xS nanoparticles

Heavily doped Cu2-xS nanoparticles with hole densities of ~1021 cm−3 were chemically synthesized and their localized surface plasmon resonance (LSPR) in the near-infrared region was investigated by nonlinear optical spectroscopy. We found that their third-order susceptibility χ(3) exhibits resonant enhancement around LSPR, similar to that in plasmonic noble metal nanoparticles. It was found that the maximum χ(3) value of Cu2-xS nanoparticles was comparable to that of Au nanoparticles with the same dimensions and concentrations. Our results indicate that Cu2-xS nanoparticles can be used as nonlinear optical materials operating in the near-infrared region, even though their near-field enhancement effect is slightly weaker than that of Au nanoparticles.

Improving All-Inorganic Perovskite Photodetectors by Preferred Orientation and Plasmonic Effect.

All-inorganic perovskites have high carrier mobility, long carrier diffusion length, excellent visible light absorption, and well overlapping with localized surface plasmon resonance (LSPR) of noble metal nanocrystals (NCs). The high-performance photodetectors can be constructed by means of the intrinsic outstanding photoelectric properties, especially plasma coupling. Here, for the first time, inorganic perovskite photodetectors are demonstrated with synergetic effect of preferred-orientation film and plasmonic with both high performance and solution process virtues, evidenced by 238% plasmonic enhancement factor and 106 on/off ratio. The CsPbBr3 and Au NC inks are assembled into high-quality films by centrifugal-casting and spin-coating, respectively, which lead to the low cost and solution-processed photodetectors. The remarkable near-field enhancement effect induced by the coupling between Au LSPR and CsPbBr3 photogenerated carriers is revealed by finite-difference time-domain simulations. The photodetector exhibits a light on/off ratio of more than 106 under 532 nm laser illumination of 4.65 mW cm-2 . The photocurrent increases from 0.67 to 2.77 μA with centrifugal-casting. Moreover, the photocurrent rises from 245.6 to 831.1 μA with Au NCs plasma enhancement, leading to an enhancement factor of 238%, which is the most optimal report among the LSPR-enhanced photodetectors, to the best of our knowledge. The results of this study suggest that all-inorganic perovskites are promising semiconductors for high-performance solution-processed photodetectors, which can be further enhanced by Au plasmonic effect, and hence have huge potentials in optical communication, safety monitoring, and biological sensing.

New optical near-field nanolithography with optical fiber probe laser irradiating atomic force microscopy probe tip

ABSTRACTA new optical near-field nanolithography scheme with optical fiber probe laser irradiating atomic force microscopy (AFM) probe tip is proposed. Through optical theoretical analysis, if the metallic AFM probe tip is in the evanescent field of optical fiber probe tip, the secondary near-field enhancement effect can appear underneath the metallic AFM probe tip. Based on the independently developed AFM system and optical system, the nanolithography platform is built and nanocraters structures can be successfully created, which demonstrates that the new scheme is an effective and promising nanolithography approach for the future nanoelectronics, nanobiotechnology, nanowelding, etc.

4-fold photocurrent enhancement in ultrathin nanoplasmonic perovskite solar cells.

Although perovskite materials have been widely investigated for thin-film photovoltaic devices due to the potential for high efficiency, their high toxicity has pressed the development of a solar cell structure of an ultra-thin absorber layer. But insufficient light absorption could be a result of ultra-thin perovskite films. In this paper, we propose a new nanoplasmonic solar cell that integrates metal nanoparticles at its rear/front surfaces of the perovskite layer. Plasmon-enhanced light scattering and near-field enhancement effects from lumpy sliver nanoparticles result in the photocurrent enhancement for a 50 nm thick absorber, which is higher than that for a 300 nm thick flat perovskite solar cell. We also predict the 4-fold photocurrent enhancement in an ultrathin perovskite solar cell with the absorber thickness of 10 nm. Our results pave a new way for ultrathin high-efficiency solar cells with either a lead-based or a lead-free perovskite absorption layer.

Tuning near-field enhancements on an off-resonance nanorod dimer via temporally shaped femtosecond laser

We theoretically investigated ultrafast thermal dynamics tuning of near-field enhancements on an off-resonance gold nanorod dimer via temporally shaped femtosecond (fs) laser double pulses. The nonequilibrium thermal excitation is self-consistently coupled into a near-field scattering model for exploring the ultrafast near-field enhancement effects. It is revealed that the near electric-field localized within the gold nanorod dimer can be largely promoted via optimizing the temporal separation and the pulse energy ratio of temporally shaped femtosecond laser double pulses. The results are explained as thermal dynamics manipulation of plasmon resonances in the nanorod dimer via tailoring temporally shaped femtosecond laser. This study provides basic understanding for tuning near-field properties on poorly fabricated metallic nano-structures via temporally shaped femtosecond laser, which can find potential applications in the fields such as fs super-resolution near-field imaging, near-field optical tweezers, and fs photothermal therapy.

Simulation of molecular imaging using nanowire optical probe with flat end face

In this paper, the output near field of nanowire with flat end face is calculated, and the nanowire as light-emitting probe for near-field imaging of 30nm diameter spherical gold nanoparticles representing biomarkers for labeling biological cells or molecules is simulated by using the three-dimensional finite-difference time-domain (3D FDTD) method. Due to the near-field enhancement effect at the edge of the nanowire and the local surface plasmon enhancement effect of gold nanoparticles induced by the output near field of the nanowire, the near field at the nanoparticles has the strongest peak when the x–y positions of the nanoparticles are close to the perimeter of the nanowire. Simulation results also show that the near field at the gold nanoparticles highly depends on the polarization vector of the incident light and the nanowire can work effectively as optical probe for molecular imaging. Results presented here can offer valuable references for molecular imaging technology with nanowires as optical probes.

Nanospot Soldering Polystyrene Nanoparticles with an Optical Fiber Probe Laser Irradiating a Metallic AFM Probe Based on the Near-Field Enhancement Effect

With the development of nanoscience and nanotechnology for the bottom-up nanofabrication of nanostructures formed from polystyrene nanoparticles, joining technology is an essential step in the manufacturing and assembly of nanodevices and nanostructures in order to provide mechanical integration and connection. To study the nanospot welding of polystyrene nanoparticles, we propose a new nanospot-soldering method using the near-field enhancement effect of a metallic atomic force microscope (AFM) probe tip that is irradiated by an optical fiber probe laser. On the basis of our theoretical analysis of the near-field enhancement effect, we set up an experimental system for nanospot soldering; this approach is carried out by using an optical fiber probe laser to irradiate the AFM probe tip to sinter the nanoparticles, providing a promising technical approach for the application of nanosoldering in nanoscience and nanotechnology.

Extinction properties of gold nanorod complexes

Plasmonics with subwavelength characteristics can break the diffraction limit of light and be used to produce the sub-wavelength optoelectronic device, thus it has aroused great interest for decades. Local surface plasmon resonance of metal nanoparticles has become one of the research hotspots due to the fact it can produce extinction and near-field enhancement effect. How to achieve controllable plasmon line shape and generate strong electromagnetic field enhancement is of great significance for improving the sensing performance, nonlinear effect and surface enhanced Raman factor of metallic nanostructures. The optical properties of plasmonic oligomer clusters composed of normal and L-shaped nanrod dimers are investigated by using the finite-difference time-domain method in this paper. There are two energy modes for an L-shaped nanorod due to its shaped anisotropy, where plasmons oscillate along the arms of the L-shaped nanorod or oscillate over the whole length of the L-shaped nanorod. Therefore, two bonding resonances appear in the spectrum of an L-shaped nanorod dimer, while only one bonding resonance exists for normal nanorod dimer. When a normal nanorod dimer and an L-shaped nanorod dimer are aligned together to form a quadrumer, the three bonding resonances can be excited simultaneously and radiative damping can be suppressed effectively around the dip spectral positions. It is shown that the optical responses of quadrumer can be strongly tuned by manipulating the geometry parameters. For example, the coupling between the two dimers can be modified by adjusting the separation, and the three resonances shift toward higher energies with the increasing of the separation. In addition, the optical responses of individual nanorod depend on the corresponding arm length. As a result, the three resonances of the quadrumer can also be well tuned by adjusting the arm length. Comparing the variation of resonance peak positions between L-shaped nanorod dimer and normal nanorod dimer, we can more intuitively understand spectral lineshape variation of quadrumer. These results can be used for guiding the design of nano-photonic devices for plasmonic oligomer clusters and also for developing the application of surface-enhanced Raman scattering and biological sensing.

Nanospot welding of carbon nanotubes using near-field enhancement effect of AFM probe irradiated by optical fiber probe laser

The carbon nanotubes interconnection can be achieved by the new nanospot welding method with the near-field enhancement effect of the metallic AFM probe tip irradiated by optical fiber probe laser.

Near-Field Hot Spots in Gold Nanoplasmonic Templates and Their Use for Surface Enhanced Raman Scattering Sensing Application

Finite difference time domain (FDTD) method is adapted to investigate near-field enhancement effects on plasmonic structures (patterned in gold film) such as concentric rings with small separation, square, and rectangle. The near-fields effect on surface enhanced Raman scattering (SERS) is typically studied on square and rectangular structures. These metal structures are fabricated by laser interference lithography. Raman active molecules (Rhodamine 6G in PMMA (polymethyl methacrylate)) are spread onto patterned structure by spin coating, and Renishaw inVia Raman spectrometer was used to study SERS. Typical SERS enhancement of the order of 105 is seen for square and rectangular structures. It is observed that the corner points and edges of square and rectangular structures are most sensitive to concentrate near fields. In the case of concentric rings, huge near fields are seen to exist at the gap between the metal rings. Concentric rings are proposed to be very effective structure for SERS sensing applications such as molecular identification and biological mapping.

Visible Plasmon-Enhanced Water Splitting

The interaction between photon and molecule is weak. The probability of interaction between photon and molecule is roughly estimated as low as 10-7 because the focal spot size of visible light (~1×10-8 cm2) is 107 times larger than the general molecular absorption cross-section (~1×10-15 cm2) due to the relationship between the diffraction limit of the light and the molecular size. Thus far, the number of excited-state molecules has been increased by photon density using a laser as an excitation source, although there is a loss of energy since the probability of interaction does not change. We have recently found that metallic nanostructures exhibiting localized plasmon resonance are promising in the photochemical reaction field, which make it possible to increase the interaction between photons and molecules. As one of the most important achievements, we have observed significant photopolymerization via two-photon absorption in photoinitiator molecules of negative photoresists in the nanogaps of closely spaced gold nanoparticles irradiated by a weak incoherent light source [1]. The key technology and outstanding feature of our photochemical reaction fields is the fabrication of nanogap metallic nanostructures with nanometric accuracy and use of the localization of electromagnetic waves within nanogaps. Near-field enhancement effects localized on nanogap gold structures were studied by two-photon photoluminescence from gold [2], surface-enhanced Raman scattering [3] and plasmon-enhanced photochemical reactions [4,5]. Thus, gold nanostructures are useful in photochemical reaction fields, which make it possible to promote strong coupling between photons and molecules with the aim of effectively using photons. From this standpoint, we have recently demonstrated plasmonic photoelectric conversion from visible to near-infrared wavelengths using electrodes in which gold nanorods are arrayed on TiO2single-crystal and photocatalytic water splitting [6,7].

In Situ-Generated Nano-Gold Plasmon-Enhanced Photoelectrochemical Aptasensing Based on Carboxylated Perylene-Functionalized Graphene

A novel in situ-generated nanogold plasmon-enhanced photoelectrochemical aptasensor for Hg(2+) ions was fabricated using a perylene-3,4,9,10-tetracarboxylic acid/graphene (PTCA-GR) heterojunction. The fabricated photoelectrochemical aptasensor was based on thymine-Hg(2+)-thymine coordination chemistry and the plasmonic near-field absorption enhancement effect of the subsequent specific catalytic formation of nanogold. The energetic electrons from the surface plasmons of the nanogold were injected into the LUMO orbit of the organic PTCA semiconductor and then rapidly transferred to the electrode through GR due to the possible Hg(2+)-DNA molecular wires following irradiation with the visible light (λ > 450 nm) and at a bias voltage of 0.2 V. The fabricated aptasensor was linear in its response to the concentration of Hg(2+) ions in the range of 5-500 pmol L(-1), with a detection limit of 2 pmol L(-1). The presence of up to 200-fold greater concentrations of other common metal ions did not interfere with the detection of Hg(2+) ions in an aqueous system, and the results corresponded well with those obtained by ICP-MS. This novel plasmon-enhanced photoelectrochemical aptasensor exhibited good performance with its high sensitivity, good selectivity, low cost, and portable features. The strategy of the localized surface plasmon resonance through the in situ generation of noble metal nanoparticles paves the way for improvements in PEC aptasensor performance.

Enhanced plasmonic resonant excitation in a grating gated field-effect transistor with supplemental gates

An alternative-grating gated AlGaN/GaN field-effect transistor (FET) is proposed by considering the slit regions to be covered by a highly doped semiconductor acting as supplemental gates. The plasmonic resonant absorption spectra are studied at THz frequencies using the FDTD method. The 2DEGs, under supplemental gates, modulated by a positive voltage, can make the excitation of the higher order plasmon modes under metallic fingers more efficient in comparison to ungated regions in common slit-grating gate transistors. Moreover, the supplemental gates can confine the electric field of dipole oscillation between metallic gate fingers under THz radiation. The competition of the near-field enhancement and screening effect of the supplemental gate fingers results in the intensity of the higher order plasmon resonances being maximized at increased doping concentration. Our results demonstrate the possibility of significant improvement in the excitation of plasmon resonances in FETs for THz detection.

Time-resolved analysis of nonlinear optical limiting for laser synthesized carbon nanoparticles

Nonlinear optical limiting materials have attracted much research interest in recent years. Carbon nanoparticles suspended in liquids show a strong nonlinear optical limiting function. It is important to investigate the nonlinear optical limiting process of carbon nanoparticles for further improving their nonlinear optical limiting performance. In this study, carbon nanoparticles were prepared by laser ablation of a carbon target in tetrahydrofuran (THF). Optical limiting properties of the samples were studied with 532-nm laser light, which is in the most sensitive wavelength band for human eyes. The shape of the laser pulse plays an important role for initializing the nonlinear optical limiting effect. Time-resolved analysis of laser pulses discovered 3 fluence stages of optical limiting. Theoretical simulation indicates that the optical limiting is initialized by a near-field optical enhancement effect.

Large-area laser nano-texturing with user-defined patterns

Writing nano-sized features less than the diffraction limit of the lasers efficiently over a large area requires special technology development. This paper reports the use of a self-assembled particle lens array with near-field enhancement effect to write millions of nano-sized user-defined features, e.g. English letters, lines, curves, simultaneously by angular beam scanning. About a 5 mm × 5 mm area can be written with a single shot of a laser beam or few scans for up to 100 million identical features of nano or sub-micro scales. With the help of certain environmental conditions, such as the use of a suitable chemical solution in conjunction with the particle lens array, the characteristic of the features produced can be further controlled, including the generation of reversed (e.g. pits become hills and grooves become walls) features of laser-written patterns. The technical challenges, experimental findings and theoretical analysis/simulation are presented.

Near-field imaging of Bloch surface waves on silicon nitride one-dimensional photonic crystals

We perform a near-field mapping of Bloch Surface Waves excited at the truncation interface of a planar silicon nitride multilayer. We directly determine the field distribution of Bloch Surface Waves along the propagation direction and normally to the surface. Furthermore, we present a direct measurement of a near-field enhancement effect under particular coupling conditions. Experimental evidence demonstrates that a approximately 10(2) near-field intensity enhancement can be realistically attained, thus confirming predictions from rigorous calculations.

Evidence of near-field laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) at nanometre scale for elemental and isotopic analysis on gels and biological samples

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a powerful surface analytical method for local elemental analysis on metallic, ceramic, geological or biological sample surfaces. Here we show a new way of nanometre scale analysis of elements on sample surfaces by near-field LA-ICP-MS (NF-LA-ICP-MS). This technique uses the near-field enhancement effect on the tip of a thin silver needle in a laser beam (Nd:YAG laser, wavelength 532 nm) on the sample surface. The thin silver needle was etched electrolytically in an electrochemical cell using a droplet of citric acid as electrolyte. For nanolocal analysis by NF-LA-ICP-MS on soft matter (e.g., on 2-D gels and biological samples) a small volume transparent laser ablation chamber was constructed and coupled to a double-focusing sector field inductively coupled plasma mass spectrometer (ICP-MS). A small amount of soft sample material is ablated at atmospheric pressure by a single laser shot in the near-field of the silver tip in the defocused Nd:YAG laser beam. The ablated material is transported with argon as carrier gas into the inductively coupled plasma (ICP) ion source of the sensitive double-focusing sector field mass spectrometer with reverse Nier–Johnson geometry. By single-shot analysis on 2-D gels and biological surfaces doped with uranium in the μg g−1 range using NF-LA-ICP-MS an enhancement of ion intensities of transient signals in comparison with the background signal of up to factor 60 was observed. In gels doped with isotopically enriched 65Cu and 67Zn spikes by NF-LA-ICP-MS (single shot analysis) ion intensities up to the n × 105 cps range and isotope ratios (235U/238U, 65Cu/63Cu and 67Zn/64Zn) were measured at a lateral resolution in the nanometre scale. Using the near-field effect in LA-ICP-MS, it was demonstrated that nanolocal analysis is possible in single-shot measurements of elements on biological samples and on a gel surface with spatial resolution at the hundreds of nanometres range. This first experiment on near-field LA-ICP-MS opens up a new, challenging path for future applications in nanoimaging of elements in life science, biology and medicine, e.g., for analyses of single cells, cell organelles or biological structures at nanometre range in order to detect neurodegenerative diseases, but also in material science, nanotechnologies and nanoelectronics.

Femtosecond Laser Processing Using Subwavelength Thin Metal Slit Arrays

We have studied femtosecond laser processing using subwavelength thin metal slits patterned onto Si substrates. A thin Ta film (200 nm thickness) with a periodic-structured slit array (200–1000 nm width) was formed on the Si substrates. The ablation thresholds of the Ta and Si were investigated. Near-field enhancement effects were observed at the edge of the metal slit arrays. Ridge structures were created in the slits on the Si surfaces using a single laser shot. In addition, an interesting processing effect was observed that was dependent on the polarization of the beam; only a p-polarized beam could create grooves perpendicular to the slits on the Si substrate. The grooves were formed under the metal layer. Our experimental results concerning the enhancement of the electrical field at the edge of the slits were consistent with the results from finite-difference time-domain simulations.

Surface Plasmon Enhancement Effect and Its Application to Near-Field Optical Recordin

We studied on enhanced photon-tunneling and the near-field enhancement effect for a nano-scale aperture with a concentric surface plasmon resonance structure. About two-orders larger light transmission and an enhanced near-field were achieved by optimizing the grating structures. This enhanced near -field was applied to optical recording, and about 100 -nm recorded patterns were successfully made on phase change media. This surface plasmon optics would open up the ultra-high-density recording technology.

Near-field amplification of antithrombotic effects of dipyridamole through vessel wall cells.

Many studies have demonstrated that dipyridamole is not purely an antiplatelet agent but also exerts its antithrombotic properties through the vessel wall. Using a laboratory model in which bovine endothelial cells are seeded to form a subendothelial matrix (SEM), investigators have shown that dipyridamole enhances the antithrombotic action of the endothelium, probably by increasing intracellular levels of cAMP and cGMP through phosphodiesterase inhibition. If so, the antithrombotic action of dipyridamole should also be observable away from the endothelium itself, i.e., a near-field effect. To test this hypothesis, we reseeded the SEM with a monolayer of human umbilical vein endothelial cells (hUVECs), occluding a portion of the SEM with teflon stops. We then treated the SEM with and without hUVECs with various doses of dipyridamole and exposed them to whole blood under flow conditions. Human endothelial cells cultured on the SEM increased its ability to reduce thrombus formation in the adjacent SEM. Dipyridamole enhanced this antithrombotic effect of hUVECs in a dose-dependent manner. No increment in antithrombotic effect was seen in dipyridamole-treated SEM without hUVECs. We therefore conclude that endothelial cells have near-field antithrombotic properties that are enhanced by dipyridamole. Possible explanations for the near-field enhancement effect of dipyridamole are discussed in light of the published literature.