Complexation of bovine serum albumin and gold nanorods revealed by plasmon-enhanced light scattering spectroscopy

Complexation of bovine serum albumin (BSA) and gold nanorods has been investigated by plasmon-enhanced light scattering spectroscopy under external stimuli such as the changes in hydrogen ion concentration value and ionic strength, with addition of ethanol. The results indicated that the BSA chains tend to adopt a coil conformation by decreasing hydrogen ion concentration value. Scattering intensity of the gold nanorods–BSA system gradually decreases because BSA chains tend to adopt a coil conformation with addition of sodium chloride. This work will further extend the application range of this method due to its simplicity, rapidity, and sensitivity.     

Strong enhancement of spontaneous emission in amorphous-silicon-nitride photonic crystal based coupled-microcavity structures

We investigated photoluminescence (PL) from one-dimensional photonic band gap structures. The photonic crystals, a Fabry–Perot (FP) resonator and a coupled-microcavity (CMC) structure, were fabricated by using alternating hydrogenated amorphous-silicon-nitride and hydrogenated amorphous-silicon-oxide layers. It was observed that these structures strongly modify the PL spectra from optically active amorphous-silicon-nitride thin films. Narrow-band and wide-band PL spectra were achieved in the FP microcavity and the CMC structure, respectively. The angle dependence of PL peak of the FP resonator was also investigated. We also observed that the spontaneous emission increased drastically at the coupled-cavity band edge of the CMC structure due to extremely low group velocity and long photon lifetime. The measurements agree well with the transfer-matrix method results and the prediction of the tight-binding approximation. Received: 8 March 2001 / Accepted: 17 March 2001 / Published online: 23 May 2001     

Surface plasmon-enhanced dual-band infrared absorber for VO_x-based microbolometer application

We propose a periodic structure as an extra absorption layer(i.e., absorber) based on surface plasmon resonance effects, enhancing dual-band absorption in both middle wavelength infrared(MWIR) and long wavelength infrared(LWIR)regions. Periodic gold disks are selectively patterned onto the top layer of suspended SiN/VO_2/SiN sandwich-structure.We employ the finite element method to model this structure in COMSOL Multiphysics including a proposed method of modulating the absorption peak. Simulation results show that the absorber has two absorption peaks at wavelengths λ =4.8 μm and λ = 9 μm with the absorption magnitudes more than 0.98 and 0.94 in MWIR and LWIR regions, respectively. In addition, the absorber achieves broad spectrum absorption in LWIR region, in the meanwhile, tunable dual-band absorption peaks can be achieved by variable heights of cavity as well as diameters and periodicity of disk. Thus, this designed absorber can be a good candidate for enhancing the performance of dual band uncooled infrared detector, furthermore, the manufacturing process of cavity can be easily simplified so that the reliability of such devices can be improved.     

Minimizing structural deformation of gold nanorods in plasmon-enhanced dye-sensitized solar cells

Plasmonic metal nanoparticles have shown great promise in enhancing the light absorption of organic dyes and thus improving the performance of dye-sensitized solar cells (DSSCs). However, as the plasmon resonance of spherical nanoparticles is limited to a single wavelength maximum (e.g., ~ 520 nm for Au nanoparticles), we have here utilized silica-coated gold nanorods (Au@SiO₂ NRs) to improve the performance at higher wavelengths as well. By adjusting the aspect ratio of the Au@SiO₂ NRs, we can shift their absorption maxima to better match the absorption spectrum of the utilized dye (here we targeted the 600–800 nm range). The main challenge in utilizing anisotropic nanoparticles in DSSCs is their deformation during the heating step required to sinter the mesoporous TiO₂ photoanode and we show that the Au@SiO₂ NRs start to deform already at temperatures as low as 200 °C. In order to circumvent this problem, we incorporated the Au@SiO₂ NRs in a TiO₂ nanoparticle suspension that does not need high sintering temperatures to produce a functional photoanode. With various characterization methods, we observed that adding the plasmonic particles also affected the structure of the produced films. Nonetheless, utilizing this low-temperature processing protocol, we were able to minimize the structural deformation of the gold nanorods and preserve their characteristic plasmon peaks. This allowed us to see a clear redshift of the maximum in the incident photon-to-current efficiency spectra of the plasmonic devices (Δλ ~ 14 nm), which further proves the great potential of utilizing Au@SiO₂ NRs in DSSCs.

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Graphical Abstract Undeformed gold nanorods provide an enhanced performance of dye-sensitized solar cells at high wavelengths

     

Surface and edge emission in organic light emitting devices

We report the studies on photoluminescence (PL) and electroluminescence (EL) spectra of organic light emitting diodes (OLED) based on most widely used light emitting material, i.e., Tris-(8-hydroxyquinoline) aluminum (Alq₃). PL studies from the edges and top surface were carried out on different thicknesses of single layer of Alq₃ coated on glass substrate and the PL intensity from the edges was found to be more than that from the top surface. On the other hand EL emission intensity from the surface was found to be larger than that from the edges of the OLED device. The discrepancy in the PL and EL emission from edge and the top surfaces is discussed. The effect of the thickness of Alq₃ layer on the PL intensity and the emission spectra are also investigated.     

Surface plasmon polariton waveguides with subwavelength confinement

Surface plasmon polaritons(SPPs) are evanescent waves propagating along metal-dielectric interfaces, which provide an effective way to realize optical wave guiding with subwavelength confinement. Metallic nanostructures supporting SPPs,that is, plasmonic waveguides, are considered as required components to construct nanophotonic devices and circuits with a high degree of miniaturization and integration. In this paper, various types of plasmonic waveguides operating in the visible, infrared, and terahertz regions are reviewed, and the status of the research on their fundamentals, fabrications,and applications is provided as well. First, we discuss the mechanisms of SPPs beyond the diffraction limit, and their launching methods. Then, the characteristics of SPPs on various plasmonic waveguides are reviewed, including top-down and bottom-up fabricated types. Considering applications, certain prototypes of plasmonic devices and circuits constructed by plasmonic waveguides for bio/chemo sensing, router, and light modulation are demonstrated. Finally, a summary and future outlook of plasmonic waveguides are given.     

Slow light with persistent spectral hole burning in waveguides

We model numerically the reshaping of a weak probe pulse propagating in an absorptive, optically dense, persistent spectral hole burning medium under conditions of slow group velocity. Saturated holes are burned in waveguide geometry by illumination in the transverse direction with low absorption, whereas the probing is carried out in longitudinal wave guiding directions with high absorption. We show that by choosing optimum hole spectrum, the Gaussian probe pulse may be delayed by several times its original duration, while overall pulse shape distortion is less than 1% and average energy loss is less than 10%.     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

Fresnel light modulator with an array of channel waveguides

One-dimensional focusing and optical modulation have been achieved using a Fresnel light modulator (FLM) based on a Fresnel lens consisting of Ti-diffused channel waveguides on LiNbO₃. By this device, the height of diffraction pattern near the focal point is modulated with an applied voltage while the focusing effect is almost kept. The experimental results obtained from the FLM with 13 zones at 633 nm are as follows: the focal length is 12.4 cm, the full width of beam at half power 47 μm, the half-wave voltage 28 V, the extinction ratio 10.9 dB and the insertion loss 11.6 dB.     

Effect of multiple tips on light emission induced by STM from gold nanostructures

Light emission induced by scanning tunneling microscope on gold islands grown on MoS₂ surfaces has been investigated. Surface geometry and roughness show that different apexes of the same tip can modify the energy of photons emitted in the tunneling junction. Comparisons of topography and photon map are used to locate islands imaged twice and to represent approximately the tip shape used. Light emission spectroscopy on the same island with two apexes of the multiple tip reveals variations of emission properties according to the apex used, showing the importance of tip geometry in the emission process induced by tip induced plasmon modes.     

金原子的放大自发辐射脉冲的弛豫振荡

首次报道了由金原子发出的波长627.8nm的放大自发辐射脉冲具有弛豫振荡.得出其在时间域和频率域的规律.这种振荡可能是高增益脉冲辐射系统的一种普遍现象.     

Resolution improvement of surface plasmon-enhanced, liquid crystal spatial light modulator: Simulation studies

Spatial resolution is an important performance characteristic of spatial light modulators (SLM). One of the key factors affecting the spatial resolution of liquid crystal (LC)-based SLM is the fringing field effect. This effect can be reduced in thin LC cells with corresponding reduction in the electro-optical response. A strong electro-optic response in thin LC layer can be attained using the surface plasmon resonance (SPR) phenomenon. While SPR-based LC SLMs were already demonstrated about 15 years ago, their development has been hampered in part by low resolution, due to the finite propagation length of the surface plasmons (SPs). A fine patterning of the metal layer supporting the propagation of SPs is studied as a possible solution for reducing the spatial blurring associated with the long propagation length of SPs. The results of detailed computer simulations showing improved resolution SPR-LC-SLM are presented.     

Surface emitting characteristics of silicon waveguides

This paper concerns the surface emitting characteristics of silicon waveguides in the millimeter-wave frequency band. The waveguides used in the experiment are rectangular slabs of high resistivity silicon (30,000 ohm-cm). A series of periodic perturbations on the waveguide surface provide a radiating antenna. A rectangular grating with a period of =1.8 mm, a height of 0.35 mm, and a duty cycle of 0.46 was sawn into the top surface of a silicon waveguide with a width of 3 mm and a height of 1.41 mm. Experiments were performed to measure the attenuation, dispersion and the radiation characteristics of the waveguides. The test setup was used to monitor the frequency, radiation angle, and the radiated power. Measurements are made over a band of frequencies around the second Bragg frequency. The detector was scanned from 88–95 GHz and changes were observed in the attenuation constant, dispersion relation and the far field radiation pattern. From these results we were able to verify the grating theory.Supported in part by the Army Research Office.     

Bombardment-induced light emission

An inescapable consequence of the bombardment of a solid with atomic particles in the energy range above a few tens of electron volts is the emission of particles from the solid via sputtering. A fraction of the sputtered flux emerges from the solid surface in an electronically excited state, and gives rise to the emission of light in a volume outside the surface. In this article, the experimental observations on this bombardment-induced light emission are reviewed together with the analytical applications of the technique and the present status of the mechanism leading to the excitation.     

Polarization conversion of light in thin-film waveguides

Polarization conversion in optical waveguides on birefringent substrates is discussed with the aid of a coupled wave model. Structures are described which tolerate large thickness variations (up to 45%) while achieving the phase matching necessary for high conversion efficiency. Polarization conversion is experimentally demonstrated in two such arrangements using ADP substrates and organic polymer films. Conversion efficiencies in excess of 98% have been obtained.     

Three-dimensional light bullets in arrays of waveguides

We report the first experimental observation of three-dimensional light bullets, excited by femtosecond pulses in a system featuring quasi-instantaneous cubic nonlinearity and a periodic, transversally modulated refractive index. Stringent evidence of the excitation of light bullets is based on time-gated images and spectra which perfectly match our numerical simulations. Furthermore, we reveal a novel evolution mechanism forcing the light bullets to follow varying dispersion or diffraction conditions, until they leave their existence range and decay.     

Two-dimensional light confinement in cross-index-modulation plasmonic waveguides

We report a numerical study of plasmonic waveguides that localize light in two dimensions at a cross section of 4.2 nm × 2.1 nm with the propagation length of 38 μm. By varying the geometrical parameters, strong mode confinements (range from λ(2)/3352 to λ(2)/2557525) are achieved with controllable propagation distance (44.68-40.988 μm), and mode size below 1 nm(2) has been demonstrated for the first time. Furthermore, a cross-index-modulation mechanism is proposed to explain the strong field localization behavior, providing guidelines for future waveguide designs.     

Unexpected light behaviour in periodic segmented waveguides

In this article, it is shown that multimode periodic segmented waveguides (PSW) are versatile optical systems in which properties of wave chaos can be highlighted. Numerical wave analysis reveals that structures of quantum phase space of PSW are similar to Poincare? sections which display a mixed phase space where stability islands are surrounded by a chaotic sea. Then, unexpected light behavior can occur such as, input gaussian beams do not diverge during the propagation in a highly multimode waveguide.     

Collinear cluster tri-partition with light ion emission

The model of collinear cluster tri-partition with light ion (Be-Ne) emission discovered in the ²³⁵U(n_th, f) reaction is discussed. Masses, velocities, energies, and nuclear charges, specific ionization losses of reaction products known from experiment allow us to assume that the process is two-stage, where at the first stage the system forms into two magic clusters, and at the second stage a light ion is detached from the light cluster, forming a magic remainder.