Spectral Blue Shift Research Articles

Analysis of nanogap-induced spectral blue-shifts of plasmons on fiber-integrated gold, silver and copper nanowires

We analyze the properties of HE2 surface plasmon polaritons on fiber-integrated gold, silver and copper nanowires and find a systematic blue-shift of the measured resonance wavelengths which we attribute to the emergence of a narrow nanometer-sized gap between the nanowire surface and the surrounding silica cladding. Our analysis relies on the determination of the nanogap width from the experimentally measured phase-matching wavelength by comparison with numerical simulations, revealing that these gaps are much smaller than expected from the bulk material considerations. This implies a diminished coefficient of thermal expansion along the radial direction that we believe results from the domination of interfacial van der Waals forces at high temperatures. These results are important for future fiber designs involving nanowires fabricated by pressure-assisted melt filling.

Implications for the Light-Driven Chloride Ion Transport Mechanism of Nonlabens marinus Rhodopsin 3 by Its Photochemical Characteristics.

Several new retinal-based photoreceptor proteins that act as light-driven electrogenic halide ion pumps have recently been discovered. Some of them, called "NTQ" rhodopsins, contain a conserved Asn-Thr-Gln motif in the third or C-helix. In this study, we investigated the photochemical characteristics of an NTQ rhodopsin, Nonlabens marinus rhodopsin 3 (NM-R3), which was discovered in the N. marinus S1-08T strain, using static and time-resolved spectroscopic techniques. We demonstrate that NM-R3 binds a Cl- in the vicinity of the retinal chromophore accompanied by a spectral blueshift from 568 nm in the absence of Cl- to 534 nm in the presence of Cl-. From the Cl- concentration dependence, we estimated the affinity (dissociation constant, Kd) for Cl- in the original state as 24 mM, which is ca. 10 times weaker than that of archaeal halorhodopsins but ca. 3 times stronger than that of a marine bacterial Cl- pumping rhodopsin (C1R). NM-R3 showed no dark-light adaptation of the retinal chromophore and predominantly possessed an all-trans-retinal, which is responsible for the light-driven Cl- pump function. Flash-photolysis experiments suggest that NM-R3 passes through five or six photochemically distinct intermediates (K, L(N), O1, O2, and NM-R3'). From these results, we assume that the Cl- is released and taken up during the L(N)-O1 transition from a transiently formed cytoplasmic (CP) binding site and the O2-NM-R3' or the NM-R3'-original NM-R3 transitions from the extracellular (EC) side, respectively. We propose a mechanism for the Cl- transport by NM-R3 based on our results and its recently reported crystal structure.

Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials.

The transport and photophysical properties of organic molecular aggregates, films, and crystals continue to receive widespread attention, driven mainly by expanding commercial applications involving display and wearable technologies as well as the promise of efficient, large-area solar cells. The main blueprint for understanding how molecular packing impacts photophysical properties was drafted over five decades ago by Michael Kasha. Kasha showed that the Coulombic coupling between two molecules, as determined by the alignment of their transition dipoles, induces energetic shifts in the main absorption spectral peak and changes in the radiative decay rate when compared to uncoupled molecules. In H-aggregates, the transition dipole moments align "side-by-side" leading to a spectral blue-shift and suppressed radiative decay rate, while in J-aggregates, the transition dipole moments align "head-to-tail" leading to a spectral red-shift and an enhanced radiative decay rate. Although many examples of H- and J-aggregates have been discovered, there are also many "unconventional" aggregates, which are not understood within the confines of Kasha's theory. Examples include nanopillars of 7,8,15,16-tetraazaterrylene, as well as several perylene-based dyes, which exhibit so-called H- to J-aggregate transformations. Such aggregates are typically characterized by significant wave function overlap between neighboring molecular orbitals as a result of small (∼4 Å) intermolecular distances, such as those found in rylene π-stacks and oligoacene herringbone lattices. Wave function overlap facilitates charge-transfer which creates an effective short-range exciton coupling that can also induce J- or H-aggregate behavior, depending on the sign. Unlike Coulomb coupling, short-range coupling is extremely sensitive to small (sub-Å) transverse displacements between neighboring chromophores. For perylene chromophores, the sign of the short-range coupling changes several times as two molecules are "slipped" from a "side-by-side" to "head-to-tail" configuration, in marked contrast to the sign of the Coulomb coupling, which changes only once. Such sensitivity allows J- to H-aggregate interconversions over distances several times smaller than those predicted by Kasha's theory. Moreover, since the total coupling drives exciton transport and photophysical properties, interference between the short- and long-range (Coulomb) couplings, as manifest by their relative signs and magnitudes, gives rise to a host of new aggregate types, referred to as HH, HJ, JH, and JJ aggregates, with distinct photophysical properties. An extreme example is the "null" HJ-aggregate in which total destructive interference leads to absorption line shapes practically identical to uncoupled molecules. Moreover, the severely compromised exciton bandwidth effectively shuts down energy transport. Most importantly, the new aggregates types described herein can be exploited for electronic materials design. For example, the enhanced exciton bandwidth and weakly emissive properties of HH-aggregates make them ideal candidates for solar cell absorbers, while the enhanced charge mobility and strong emissive behavior of JJ-aggregates makes them excellent candidates for light-emitting diodes.

LSPR Biosensor Based on Nanostructured Gold Films: Detection of Mycotoxins

This work focuses on the development of optical sensors based on the LSPR phenomenon in nano-structured gold films suitable for detection of mycotoxins. A simple technology of annealing thin gold films was utilized for the formation of gold nano-islands exhibiting the LSPR effect. The morphology of gold nano-structures produced was studied with SEM and AFM while their optical properties were analysed with UV-vis absorption spectroscopy and spectroscopic ellipsometry (SE). A blue spectral shift of LSPR band caused by changes in the refractive index of medium constitutes the main principle of LSPR sensing. Bio-sensing tests were attempted using SE in total internal reflection mode (TIRE); a noticeable spectral shift was recorded on course of immune binding of aflatoxin B1 to specific antibodies immobilized on the surface of gold.

Probing the exciton-phonon coupling strengths of O-polar and Zn-polar ZnO wafer using hard X-ray excited optical luminescence

The temperature-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the optical properties of O and Zn polarity of a c-plane single crystal ZnO wafer. By analyzing the XEOL and XRD, we found an unprecedented blue shift of the free exciton transition with increasing the excited carrier density as tuning the X-ray energy across the Zn K-edge, and the O-polar face possesses better crystal structure than the Zn-polar one. This spectral blue shift is attributed to the Coulomb screening of the spontaneous polarization by the excited free carriers that result in decreasing the exciton-phonon Fröhlich interaction to reduce exciton binding energy.

Preparation and broadband white emission of Eu-doped thin films based on SiAlON

The optical performance of europium (Eu) doped SiAlON thin films and its preparation by pulsed laser deposition are studied. The undoped SiAlON films show a high refractive index (2.34) close to that of silicon nitride. After doping with Eu the oxygen content increases in the films and leads to the formation of an oxynitride with a refractive index of 1.87 and excellent transparency in the visible-near infrared. Upon excitation at 355nm the films show a broad band (FWHM of 200nm) emission in the visible range (400–750nm) that is associated to the emission of the Eu in the 2+ oxidation state. Enhancement of the emission together with a blue spectral shift has been found when the films are annealed at low temperature (600°C). These films show promising properties for the development of Si compatible thin films integrated emitters.

Fusion of regularized femtosecond filaments in air: far field on-axis emission

The fusion of several coherent 800 nm femtosecond filaments is induced experimentally and numerically by transmitting a beam through a mask with circular apertures followed by the focusing lens. The far-field image of the four-filament fusion region reveals bright on-axis maximum and differs drastically from the diffraction pattern of a low energy beam propagating through the mask in the linear regime. In 3D+time numerical simulations with the carrier wave resolved we show a factor-of-5 saturable growth in the peak plasma density with successive increase in the number of mask openings. An overall spectral blueshift of the fundamental and the third harmonics follows the plasma density increase. The simulated far-field on-axis emission agrees with the experiment and serves as the indication of nonlinear interaction in the fusion region.

Sub-Picosecond Auger-Mediated Hole-Trapping Dynamics in Colloidal CdSe/CdS Core/Shell Nanoplatelets.

Quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a class of semiconductor nanomaterials whose atomically precise monodisperse thicknesses give rise to narrow absorption and emission spectra. However, the sub-picosecond carrier dynamics of NPLs at the band edge remain largely unknown, despite their importance in determining the optoelectronic properties of these materials. Here, we use a combination of femtosecond transient absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate the early time carrier dynamics of CdSe/CdS core/shell NPLs. Band-selective probing reveals sub-picosecond Auger-mediated trapping of holes with an effective second-order rate constant of 3.5 ± 1.0 cm2/s. Concomitant spectral blue shifts that are indicative of Auger hole heating are found to occur on the same time scale as the sub-picosecond trapping dynamics, whereas spectral red shifts that emerge at low excitation densities furnish an electron-cooling time scale of 0.84 ± 0.09 ps. Finally, nonadiabatic molecular dynamics simulations relate the observed sub-picosecond Auger-mediated hole-trapping dynamics to a shallow trap state that originates from the incomplete passivation of dangling bonds on the NPL surface.

Nitrogen-rich graphitic carbon nitride: Controllable nanosheet-like morphology, enhanced visible light absorption and superior photocatalytic performance

Herein, we reported the synthesis of nitrogen-rich graphitic carbon nitride nanosheets (NRCNNS) material by an NH4Cl assisted chemical blowing approach, where the released gas from the decomposition of NH4Cl prohibited the further polymerization of CN framework and separated the CN layers. A typical two-dimensional morphology with high aspect-ratio was demonstrated by scanning electron microscopy and transmission electron microscopy observation, and the excess amino group on the skeleton was proved by Fourier transform infrared and X-ray photoelectron spectra technologies. Different from the spectral blue-shift in other carbon nitride nanosheets, an enhanced absorption in visible light region was achieved in our NRCNNS due to the N,O-doping level in the band gap. The optimal NRCNNS material with an NH4Cl/melamine mass ratio of 4.0 exhibited a promoted activity (5 times higher than that of the bulky carbon nitride) and reusability for degradation of organic dye under visible light irradiation. Moreover, a possible mechanism was proposed based on the results of valence band spectra and the quenching experiments of active species.

Influence of strain on emission from GaN-on-Si microdisks

Strain-relaxation effects in AlN-buffered GaN/InGaN microdisks pivoted on Si posts of varying radii have been studied by micro-Raman spectroscopy and scanning near-field optical spectroscopy (SNOS). With increasing undercut beneath the microdisks by chemical wet-etching, the mitigation of biaxial tensile stress is found to be dependent on the contact areas between the Si posts and GaN microdisks. Strain-relaxation reduces the quantum-confined Stark effect (QCSE) in the quantum wells (QWs), leading to an 18.3% enhancement in InGaN QW internal quantum efficiency (IQE). Light out-coupling is also improved in the suspended regions owing to reduced optical absorption at AlN/Si interface compared to the central region. Meanwhile, spectral blue-shifts of ~45.6 meV are observed from the near-field photoluminescence (nf-PL) spectrum towards the edge of the microdisk. Such localization of strain relaxation can be exploited for precise strain engineering of the microdisks. The emission wavelengths of the microdisks can be stabilized by balancing strain-relaxation effects with thermal effects.

Flexible Free-Standing III-Nitride Thin Films for Emitters and Displays.

The majority of a GaN light-emitting diode (LED) is released from its sapphire substrate through selective-area laser lift-off to form a freely suspended light emitter. By virtue of being suspended in air without supporting substrates, the ultrathin crystalline and crack-free film possesses flexibility and bendability. The free-standing LEDs benefit from significant relaxation of strain, evident from red-shifting of the E2(high) phonon frequencies as measured by Raman spectroscopy toward those of strain-free free-standing GaN substrates. The phonon frequencies remain invariant upon bending of the film; this indicates that the properties of the flexible device will not be dependent on the bending curvatures. The observation of pronounced spectral blue-shifts from the photoluminescence (PL) spectrum from the flexible regions further confirms the occurrence of strain relaxation in the quantum wells. Being free-standing and thus lacking a direct heat-sinking pathway, emissions from the different regions of the suspended film can be affected by thermal effects to different extents, which are investigated by long-wave infrared thermometry. Heat accumulation is determined to be most severe at the far end of the flexible stripe at higher currents, leading to reduced efficiencies and electroluminescence (EL) spectral red-shifts. Based on this architecture, a monolithic 3 × 4 dot-matrix microdisplay prototype is demonstrated, comprising three adjacent flexible stripe emitters with four individually addressable pixels on each stripe. This proof-of-concept demonstration opens up new opportunities for GaN optoelectronics for a wide range of flexible display and visual applications.

Intercalation of solid hydrogen into graphite under pressures

We present the Raman spectral evidence of pressure-induced intercalation of solid hydrogen into graphite to 60 GPa. The intercalation is evident by the emergence of two characteristic Raman bands of hydrogen (νo1 and νo2), which appear upon the solidification of hydrogen and disappear as all sp2-hybridized graphitic carbons convert to sp3-hybridized hexagonal diamond at 57 GPa. The νo1 and νo2 frequencies of intercalated hydrogen, 4250 and 4270 cm−1 at 10 GPa, are substantially higher than the νo of bulk hydrogen, 4228 cm−1 at the same pressure, indicating the presence of strong repulsive interactions between intercalated hydrogen molecules and graphite layers and, thereby, strong internal chemical pressures. Based on the spectral blue shift of intercalated hydrogen vibrons, we estimate the internal pressure to be ∼1 GPa at 10 GPa and ∼10 GPa at 50 GPa.

Chirp-dependent spectral distribution for few-cycle pulses propagating through nano-semiconductor devices

The propagation of an initially chirped incident few-cycle pulse through an ensemble of quantum wells is numerically investigated. It is found that the distribution characteristic of the transmitted spectrum sensitively depends on the incident laser parameters, especially its positive or negative chirp property. As for the incident pulse with a positive initial chirp, beyond the obvious spectral blue-shift, the transmitted spectral distribution is discrete. In contrast, as for a negative initial chirp, the spectral distribution is continuous instead. In addition, the insensitivity of chirp-dependent spectral distribution to medium symmetry character is also tested and the intensity of high-frequency spectral components enhances obviously due to the nonlinear propagation effects.

Lunar iron and optical maturity mapping: Results from partial least squares modeling of Chang'E-1 IIM data

Iron and optical maturity (OMAT) are two key geological marks of the Moon that closely related to its geochemical evolution and interactions between surface and space environment. We apply Partial Least Squares (PLS) regression to Chang'E-1 Imaging Interferometer (IIM) (32 bands between 480 and 960nm) in mapping lunar global FeO and OMAT, and the FeO and OMAT values are derived based on reasonable spectral parameters (absorbance, band ratios, TiO2 and maturity sensitive parameters, etc.). After been calibrated by the FeO map from Lunar Prospector Gamma-Ray Spectrometer (LP-GRS), the global FeO map derived from PLS modeling shows a quantitatively more reasonable result consistent with previous remote sensing results (LP) as well as lunar feldspathic meteorite studies and Chang'E-3 landing site. Based on the new FeO map by Chang'E-1, we discover a compositional inhomogeneity across lunar highland regions, which has not been suggested by previous datasets (e.g., Clementine UVVIS). Furthermore, we suggest that at least part of the FeO enrichments in highlands would be caused by mixing of highland and mare materials. The IIM derived OMAT map does not suggest a dichotomy of the lunar highlands and mare regions, implying the compositional differences between those two terrains have been suppressed. We further check the maturity effect for the young mare basalts (<3.0Ga), and find that (1) the OMAT values of the young basaltic units with medium and high FeO and TiO2 show a linear decrease with ages; (2) units with ultrahigh-FeO (>20wt%) and ultrahigh-TiO2 (>10wt%) tend to have greater OMAT values and vary little with ages; (3) this may be due to the distinct optical maturity effects of ultramafic minerals (i.e., ultrahigh Fe and Ti) and/or the spectral blue shifts of abundant ilmenite.

Synthesis of fused 2-(2′-hydroxyphenyl)benzoxazole derivatives: the impact of meta-/para-substitution on fluorescence and zinc binding

A fused bis[2-(2′-hydroxyphenyl)benzoxazole] bis(HBO) 3 was synthesized via reaction of 4,6-dihydroxyisopthalic acid with 2-amnophenol in the presence polyphosphoric acid (PPA). In bis(HBO) 3, two benzoxazol-2-yl groups are connected via meta-phenylene, in comparison with para-phenylene in its isomer 5, in order to examine the effect of regiochemistry on optical properties. UV–vis and fluorescence spectra show that bis(HBO) 3 gave emission nearly exclusively from its keto tautomer, indicating more effective ‘excited-state intramolecular proton transfer (ESIPT)’ than mono HBO. Bis(HBO) 3 gave blue ESIPT fluorescence (λem≈480nm), while 5 gave orange-red emission at 597nm, showing a large effect of regiochemistry on emission. The effect was further evaluated through binding with Zn2+ cation, with 3 showing spectral blue shift upon zinc binding while 5 did not. In addition, 3 revealed ability to detect different stages of zinc binding, giving different spectral response to (3)2–Zn (with 2:1 ligand-to-metal ratio) and 3–Zn species (with 1:1 ligand-to-metal ratio).

Broadband Dielectric Spectroscopy on Lysozyme in the Sub-Gigahertz to Terahertz Frequency Regions: Effects of Hydration and Thermal Excitation.

We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ∼20 ps and ∼100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water-protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron-scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.

Optical absorption spectrum of the N3 solar cell sensitizer by second-order multireference perturbation theory

Here we report second-order n-electron valence state perturbation theory excited state calculations on the low-lying transitions of the [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] complex, better known as the N3 dye, both in vacuo and water solution. The present results provide unprecedented insights into the crucial role of the solvation effects, previously reported at density functional theory (DFT) and time-dependent DFT (TDDFT) level of theory, to adequately reproduce the optical absorption spectrum. We find that the NEVPT2 results in gas phase, even if slightly blueshifted, already provide a reliable description and assignation of the electronic structure and low-lying excited states of N3, whereas these are achieved in a DFT/TDDFT approach only by means of the solvent, which properly polarizes the ground-state frontier molecular orbitals. Then, inclusion of the solvent at the NEVPT2 level does not substantially modify the ground-state electronic structure as well as the excited states picture found in vacuo, solely yielding the expected spectral blueshift.

Electrooptic Kerr effect of porphyrin H-aggregates in polymer films: Polymer specific spectral blue shift

J- and H-aggregates of porphyrin molecules (TPPS4) in spin-coated polymer films have been studied by electroabsorption and circular dichroism (CD) spectroscopy. A spectral blue shift of the H-band due to the electrooptic Kerr effect was observed for the first time. This occurs only for a polyvinylpyrrolidone (PVP) film, with negligibly small spectral shift observed in polyvinyl alcohol (PVA), polyvinyl sulfate (PVS), and polyacrylic acid (PAA) films, in contrast to the red shift of the J-band which is commonly observed for any host polymers. Accordingly, the CD activity in both of J- and H-bands is more enhanced in PVP films than in PVA films. The mechanism of the blue and red shifts of the respective H- and J-bands is discussed by invoking a helical structure in micro-aggregates, which is compatible with the CD spectra, based on the molecular rearrangement model. It is proved that blue- and red-shifts occur evenly to cancel each other in the H-band if a simple helical structure is assumed, in good agreement with no spectral shifts in the H-band in PVA, PVS, and PAA films.

Defect-mediated spontaneous emission enhancement of plasmon-coupled CuInS_2 and CuInS_2/ZnS

The studies of plasmon-coupled excitons at the surface-/interface-, shallow-, and deep-trapped states of copper-indium-disulfide (CIS) with/without zinc-sulfide (ZnS) shell revealed the defect-mediated spontaneous emission enhancement. The PL enhancement with spectral blue-shift of plasmon-coupled excitons in CIS quantum dots (QDs) indicates the large reduction of nonradiative decay at the surface- and shallow-trapped states with strong spectral overlapping. The PL enhancement with spectral red-shift of plasmon-coupled excitons in CIS/ZnS QDs is accredited to the defect-mediated PL enhancement by the higher fractional amplitude at the interface-trapped state around the longer spectral region. The spontaneous emission enhancement of plasmon-coupled CIS QDs were ~2.1, ~2.2, and ~2.8-folds compared to the decay rates of CIS, and those of plasmon-coupled CIS/ZnS QDs were ~24.1, ~32.8, and ~24.9-folds compared to the decay rates of CIS/ZnS at shorter, intermediate, and longer spectral regions due to relatively stable charge carriers and close to the surface plasmon resonance. The PL enhancements of plasmon-coupled CIS at room temperature and 6 K were two-fold and three-fold compared to the integrated CIS PLs, and the PL enhancements of plasmon-coupled CIS/ZnS at room temperature and 6 K were five-fold and eight-fold compared to the integrated CIS/ZnS PLs. The large PL enhancement is attributable to the plasmon-exciton coupling through Coulomb interaction and the local field enhancement. The larger PL enhancement of plasmon-coupled CIS/ZnS compared to that of plasmon-coupled CIS is accredited to the larger spontaneous emission enhancement.

SN 2014J at M82 – I. A middle-class Type Ia supernova by all spectroscopic metrics

We present the intensive spectroscopic follow up of the type Ia supernova (SN Ia) 2014J in the starburst galaxy M82. Twenty-seven optical spectra have been acquired from January 22nd to September 1st 2014 with the Isaac Newton (INT) and William Herschel (WHT) Telescopes. After correcting the observations for the recession velocity of M82 and for Milky Way and host galaxy extinction, we measured expansion velocities from spectral line blueshifts and pseudo-equivalent width of the strongest features in the spectra, which gives an idea on how elements are distributed within the ejecta. We position SN 2014J in the Benetti (2005), Branch et al. (2006) and Wang et al. (2009) diagrams. These diagrams are based on properties of the Si II features and provide dynamical and chemical information about the SN ejecta. The nearby SN 2011fe, which showed little evidence for reddening in its host galaxy, is shown as a reference for comparisons. SN 2014J is a border-line object between the Core-normal (CN) and Broad-line (BL) groups, which corresponds to an intermediate position between Low Velocity Gradient (LVG) and High Velocity Gradient (HVG) objects. SN 2014J follows the R(Si II)-\Delta m15 correlation, which confirms its classification as a relatively normal SN Ia. Our description of the SN Ia in terms of the evolution of the pseudo-equivalent width of various ions as well as the position in the various diagrams put this specific SN Ia into the overall sample of SN Ia.