Wavelength-dependent Measurements Research Articles

Plasmonic nanoparticle networks formed using iron porphyrin molecular bridges

The adsorption orientation of the iron(III) tetrakis(1-methyl-4-pyridyl)porphine (FeTMPyP) μ-oxo-bridged dimer species located in the inter-particle gap of solid gold nanosphere (SGN) and hollow gold nanosphere (HGN) networks was studied using a combination of linear extinction and surface-enhanced Raman Scattering (SERS) measurements. Nanoparticle aggregation was accomplished by μM additions of iron porphyrin to the colloidal SGN and HGN solutions. Aggregation was monitored by measuring the UV-Visible-NIR extinction spectra of the nanoparticle systems; a broadened and red-shifted localized surface plasmon resonance (LSPR), relative to the LSPR of the isolated nanoparticles, indicated aggregate formation. In conjunction with the LSPR extinction measurements, wavelength-dependent SERS measurements were used to determine the orientation of the FeTMPyP with respect to the SGN and HGN surfaces.

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Ablation efficiency and relative thermal confinement measurements using wavelengths 1,064, 1,320, and 1,444 nm for laser-assisted lipolysis

Laser-assisted lipolysis is routinely used for contouring the body and the neck while modifications of the technique have recently been advocated for facial contouring. In this study, wavelength-dependence measurements of laser lipolysis effect were performed using different lasers at 1,064, 1,320, and 1,444 nm wavelengths that are currently used clinically. Fresh porcine skin with fatty tissue was used for the experiments with radiant exposure of 5–8 W with the same parameters (beam diameter = 600 μm, peak power = 200 mJ, and pulse rate = 40 Hz) for 1,064, 1,320 and 1,444 nm laser wavelengths. After laser irradiation, ablation crater depth and width and tissue mass loss were measured using spectral optical coherence tomography and a micro-analytical balance, respectively. In addition, thermal temporal monitoring was performed with a thermal imaging camera placed over ex vivo porcine fat tissue; temperature changes were recorded for each wavelength. This study demonstrated greatest ablation crater depth and width and mass removal in fatty tissue at the 1,444 nm wavelength followed by, in order, 1,320 and 1,064 nm. In the evaluation of heat distribution at different wavelengths, reduced heat diffusion was observed at 1,444 nm. The ablation efficiency was found to be dependent upon wavelength, and the 1,444 nm wavelength was found to provide both the highest efficiency for fatty tissue ablation and the greatest thermal confinement.

Intercoupling surface plasmon resonance and diffusion reflection measurements for real‐time cancer detection

Spatial diffusion reflection (DR) measurements of gold nanorods (GNR) were recently suggested as a simple and highly sensitive non-invasive and non-ionizing method for real-time cancer detection. In this paper we demonstrate that wavelength dependent DR measurements enable the spectral red-shift observation of highly concentrated GNR. By conjugating targeting moieties to the GNR, large density of GNR can specifically home onto cancer cells. The inter-particle plasmon resonance pattern of the highly concentrated GNR leads to an extension and a red-shift (Δλ) in the absorption spectrum of the concentrated GNR. Dark-field microscopy was used in order to measure the expected Δλ in different GNR concentrations in vitro. Double-wavelength DR measurements of tissue-like phantoms and tumor bearing mice containing different GNR concentrations are presented. We show that the DR profile of the highly concentrated GNR directly correlate with the spectral extension and red-shift. This presented work suggests that wavelength dependent DR method can serve as a promising tool for real-time superficial tumor detection.

Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics

Silicon nanowires (NWs) could enable low-cost and efficient photovoltaics, though their performance has been limited by nonideal electrical characteristics and an inability to tune absorption properties. We overcome these limitations through controlled synthesis of a series of polymorphic core/multishell NWs with highly crystalline, hexagonally-faceted shells, and well-defined coaxial (p/n) and p/intrinsic/n (p/i/n) diode junctions. Designed 200-300 nm diameter p/i/n NW diodes exhibit ultralow leakage currents of approximately 1 fA, and open-circuit voltages and fill-factors up to 0.5 V and 73%, respectively, under one-sun illumination. Single-NW wavelength-dependent photocurrent measurements reveal size-tunable optical resonances, external quantum efficiencies greater than unity, and current densities double those for silicon films of comparable thickness. In addition, finite-difference-time-domain simulations for the measured NW structures agree quantitatively with the photocurrent measurements, and demonstrate that the optical resonances are due to Fabry-Perot and whispering-gallery cavity modes supported in the high-quality faceted nanostructures. Synthetically optimized NW devices achieve current densities of 17 mA/cm(2) and power-conversion efficiencies of 6%. Horizontal integration of multiple NWs demonstrates linear scaling of the absolute photocurrent with number of NWs, as well as retention of the high open-circuit voltages and short-circuit current densities measured for single NW devices. Notably, assembly of 2 NW elements into vertical stacks yields short-circuit current densities of 25 mA/cm(2) with a backside reflector, and simulations further show that such stacking represents an attractive approach for further enhancing performance with projected efficiencies of > 15% for 1.2 μm thick 5 NW stacks.

Absorption of Light in a Single-Nanowire Silicon Solar Cell Decorated with an Octahedral Silver Nanocrystal

In recent photovoltaic research, nanomaterials have offered two new approaches for trapping light within solar cells to increase their absorption: nanostructuring the absorbing semiconductor and using metallic nanostructures to couple light into the absorbing layer. This work combines these two approaches by decorating a single-nanowire silicon solar cell with an octahedral silver nanocrystal. Wavelength-dependent photocurrent measurements and finite-difference time domain simulations show that increases in photocurrent arise at wavelengths corresponding to the nanocrystal's surface plasmon resonances, while decreases occur at wavelengths corresponding to optical resonances of the nanowire. Scanning photocurrent mapping with submicrometer spatial resolution experimentally confirms that changes in the device's photocurrent come from the silver nanocrystal. These results demonstrate that understanding the interactions between nanoscale absorbers and plasmonic nanostructures is essential to optimizing the efficiency of nanostructured solar cells.

Plasmon-induced local photocurrent changes in GaAs photovoltaic cells modified with gold nanospheres: A near-field imaging study

Modification of photovoltaic devices with metallic nanoparticles is expected to be one of the key methods for development of high performance devices as a future energy source. To clarify the mechanism of photocurrent changes induced by surface plasmon resonances of metal nanoparticles, we measured near-field photocurrent excitation images for a GaAs photodiode modified with gold nanospheres (diameter 100 nm) with a spatial resolution higher than 100 nm. The relationship between the photovoltaic efficiency and the plasmons of the gold nanospheres was investigated through the measurements of incident wavelength and polarization dependence of the near-field photocurrent images. Isolated nanospheres deposited on the GaAs active surface caused local photocurrent suppressions at the plasmon resonance wavelengths. In the case of assemblies (dimers and trimers) of the spheres, a remarkable decrease of photocurrent at the gap site between the spheres was observed. From the results, it turned out that the enhanced optical fields created via the plasmons on the metal nanostructures do not improve the photovoltaic efficiency and that forward scattering of photons by the gold nanoparticles is considered to be more important than the enhanced field effect at the particles for the GaAs photovoltaic device studied.

Wavelength-dependent backscattering measurements for quantitative monitoring of apoptosis, Part 2: early spectral changes during apoptosis are linked to apoptotic volume decrease

Elastic scattering spectroscopy (ESS), in the form of wavelength-dependent backscattering measurements, can be used to monitor apoptosis in cell cultures. Early changes in backscattering upon apoptosis induction are characterized by an overall decrease in spectral slope and begin as early as 10 to 15 min post-treatment, progressing over the next 6 to 8 h. The timescale of early scattering changes is consistent with reports of the onset of apoptotic volume decrease (AVD). Modeling cellular scattering with a fixed distribution of sizes and a decreasing index ratio, as well as an increased contribution of the whole cell to cellular scattering, resulting from increased cytoplasmic density, is also consistent with observed spectral changes. Changes in ESS signal from cells undergoing osmotically-induced volume decrease in the absence of apoptosis were similar, but smaller in magnitude, to those of apoptotic cells. Further, blockage of Cl(-) channels, which blocks AVD and delays apoptosis, blocked the early scattering changes, indicating that the early scattering changes during apoptosis result, at least partially, from AVD. Work continues to identify the additional sources of early spectral scattering changes that result from apoptosis induction.

Improved negative bias illumination instability of sol-gel gallium zinc tin oxide thin film transistors

We investigated the Ga doping effect on the performance and negative bias illumination instability of sol-gel zinc tin oxide (ZTO) thin film transistors (TFTs). The performance of the Ga doped ZTO (GZTO) TFTs is controlled and optimized by the concentration of Ga ions, which suppress the formation of oxygen vacancies. The negative bias illumination instability of the devices with a sol-gel hybrid material passivation layer is compared through a time-evolution stress analysis and illumination wavelength dependence measurements. The GZTO TFT exhibits improved stability relative to the ZTO TFT, because Ga ions effectively decrease charge trapping sites originating from oxygen vacancies.

Visualizing Core–Shell Structure in Substituted PPV Oligomer Aggregates Using Fluorescence Lifetime Imaging Microscopy (FLIM)

The use of fluorescence lifetime imaging microscopy (FLIM) is introduced as a means of directly imaging core–shell structured organic aggregates through the gradient observed in their emission wavelength and lifetime as a function of distance from their center to their exterior. The aggregates studied consist of alkoxy-substituted oligomeric PPVs (OPPVs) 7 and 13 rings in length that are formed via reprecipitation in a mixture of methyl tetrahydrofuran (MeTHF) and methanol (MeOH). Prior bulk fluorescence spectroscopy and wavelength-dependent lifetime measurements on these aggregates (J. Phys. Chem. C2009, 113, 18851–18862) showed that their properties are consistent with the presence of two types of emitters, one that behaves identically to the monomer with the other having the longer emission wavelengths and shorter lifetimes characteristic of aggregated chains. These two emitters were postulated to be the components of “core-shell”-like structures in which the core consists of aggregated chains and the ...

Wavelength-dependent backscattering measurements for quantitative monitoring of apoptosis, Part 1: early and late spectral changes are indicative of the presence of apoptosis in cell cultures

Apoptosis, a form of programmed cell death with unique morphological and biochemical features, is dysregulated in cancer and is activated by many cancer chemotherapeutic drugs. Noninvasive assays for apoptosis in cell cultures can aid in screening of new anticancer agents. We have previously demonstrated that elastic scattering spectroscopy can monitor apoptosis in cell cultures. In this report we present data on monitoring the detailed time-course of scattering changes in a Chinese hamster ovary (CHO) and PC-3 prostate cancer cells treated with staurosporine to induce apoptosis. Changes in the backscattering spectrum are detectable within 10 min, and continue to progress up to 48 h after staurosporine treatment, with the magnitude and kinetics of scattering changes dependent on inducer concentration. Similar responses were observed in CHO cells treated with several other apoptosis-inducing protocols. Early and late scattering changes were observed under conditions shown to induce apoptosis via caspase activity assay and were absent under conditions where apoptosis was not induced. Finally, blocking caspase activity and downstream apoptotic morphology changes prevented late scattering changes. These observations demonstrate that early and late changes in wavelength-dependent backscattering correlate with the presence of apoptosis in cell cultures and that the late changes are specific to apoptosis.

Synthesis and Characterization of White-Emitting CdS Quantum Dots Stabilized with Polyethylenimine

We report on the synthesis and optical and structural characterization of ultrasmall (<2 nm) CdS nanoparticles with a narrow size distribution of 10% prepared in aqueous and alcohol solutions of polyethyleneimine (PEI). The PEI-stabilized CdS nanoparticles reveal a structured absorption band with the first excitonic maximum at 3.5 eV and broad-band photoluminescence with quantum yields of 12−14% in water, 18−20% in ethanol (80 vol %), and up to 60−70% in solid PEI films at room temperature. The nature of the photoluminescence was studied by using the time- and wavelength-dependent emission measurements. The role of precursor cadmium(II)−PEI complex in the formation of uniform and ultrasmall luminescent CdS nanoparticles, as well as the dynamic emission quenching by water, are discussed. A study of the photochemical properties of PEI-stabilized CdS nanoparticles both under continuous and nanosecond pulse illumination showed excellent stability of solid PEI films incorporating CdS nanoparticles toward UV il...

First Hyperpolarizability Dispersion of the Octupolar Molecule Crystal Violet: Multiple Resonances and Vibrational and Solvation Effects

The first hyperpolarizability (β) dispersion curve is measured for the first time for an octupolar nonlinear optical (NLO) molecule (crystal violet, CV) and modeled theoretically, yielding an in-depth understanding of the electronic structure and vibronic and solvation effects on such octupolar conjugated systems. Tunable wavelength hyper-Rayleigh scattering (HRS) measurements were performed on this prototypical octupolar molecule in the broad fundamental wavelength range of 620-1580 nm, showing significant shortcomings of the commonly used β dispersion models. Three well-separated β resonances involving the lowest-energy state and several higher excited states are clearly observed, including a significant contribution from a nominally one-photon forbidden transition. The experimental results for second-harmonic wavelengths above 330 nm are successfully modeled by means of a vibronically coupled essential-state description for octupolar chromophores, developed by Terenziani et al. (J. Phys. Chem. B 2008, 112, 5079), which takes into account polar solvation effects. The relative intensities of the various resonances, including the one below 330 nm, are also quantified by quantum chemical calculations. Furthermore, interesting effects of inhomogeneous broadening due to polar solvation of the two-dimensional chromophore are recognized in both linear and nonlinear spectra, allowing us to quantitatively address the long-standing problem of the band shape of the linear absorption spectrum of CV. This clearly demonstrates that extensive wavelength-dependent HRS measurements, as presented in this work, are essential to the characterization and design of NLO materials and represent a powerful tool to gain valuable information on molecular excitations and environmental effects in general.

Nonlinear birefringence and time-resolved Kerr measurement of spin lifetimes in (110)GaAs/AlyGa1−yAsquantum wells

We report a study of the nonlinear birefringence in undoped (110)-oriented GaAs/AlGaAs quantum wells using time-resolved pump-probe Kerr spectroscopy. Due to the optical anisotropy of the (110) quantum well plane, photoexcited carriers can give rise to a nonlinear birefringence and so cause probe polarization rotation independent of the pump polarization, i.e., independent of spin orientation. We develop a methodology for accurate determination of electron-spin lifetimes using the Kerr technique which takes account of this phenomenon and present room-temperature measurements of wavelength and power density dependence of the spin-relaxation rate.

Two-component Doppler-shift fluorescence velocimetry applied to a generic planetary entry probe model

This study discusses the development and application of planar laser-induced fluorescence of nitric oxide (NO PLIF) to measure velocities in an axisymmetric hypersonic near-wake flow field around a model planetary-entry vehicle configuration. Shapes and positions of NO spectral lines at every location in the flow are determined over several successive shock tunnel runs. The lines experience Doppler shifts proportional to the local flow velocity component in the direction of the fluorescence-generating laser. A Gaussian line shape function is then fitted to the acquired wavelength-dependent fluorescence measurements, the line center of which is correlated to the time-averaged velocity at each pixel location. The flow field is probed successively by a laser in two orthogonal directions, which yields the velocity magnitude and direction everywhere in the illuminated plane. The accuracy of the measurement technique is discussed, and various strategies to characterize systematic errors are presented. The variation of random uncertainties in different regions of the flow field provides information about the local steadiness of the flow. To the authors’ knowledge, the measurements represent the first two-component velocity map of a hypersonic near-wake flow.

Analysis of Electronic and Photocatalytic Properties of Semiconductor Powders through Wavelength-Dependent Quasi-Fermi Level and Reactivity Measurements

Wavelength-dependent measurements of the quasi-Fermi level of electrons by the suspension photovoltage method allows locating intra-bandgap states involved in interfacial electron transfer. Employing in these experiments reducing agents with current amplification properties may lead to false results due to the strongly reducing properties of oxidation intermediates. The observation that upon UV excitation holes oxidize water or an other donor instead of relaxing to nearby intra-bandgap states suggests an only weak electronic coupling between these and valence band states. This interpretation is corroborated by the wavelength-dependent reactivity of the two nitrogen-modified anatase photocatalysts TiO2−N and TiO2−N,C prepared from NH3 and urea, respectively. Although both materials have a very similar energy level scheme, the former is inactive in visible light mineralization of formic acid but the latter is very active. In TiO2−N,C the photogenerated hole can be stabilized through delocalization within an...

In-fiber polarimeters based on hollow-core photonic bandgap fibers

In-fiber polarimeters or polarization mode interferometers (PMIs) are fabricated by cascading two CO2-laser-induced in-fiber polarizers along a piece of hollow-core photonic bandgap fiber. Since the two interfering beams are the orthogonal polarizations of the fundamental mode, which are tightly confined to the core and have much lower loss than higher order modes, the PMIs can have either short (e.g., a few millimeters) or long (tens of meters or longer) device length without significantly changing the fringe contrast and hence provide design flexibility for applications required different device lengths. As examples of potential applications, the PMIs have been experimentally demonstrated for wavelength-dependent group birefringence measurement; and for strain, temperature and torsion sensors. The PMI sensors are quite sensitive to strain but relatively insensitive to temperature as compared with fiber Bragg grating sensors. The PMIs function as good directional torsion sensors that can determine the rate and direction of twist at the same time.

Observation of persistent photoconductance in single ZnO nanotube

Vertically aligned ZnO nanotube fabricated on an indium tin oxide substrate is found to exhibit strong persistent photoconductivity (PPC). Excitation wavelength-dependent conductance measurement on individual ZnO nanotube reveals the presence of defect states at 240 meV above the valence band edge, which are directly associated with the PPC effect. Our observations are consistent with the hypothesis that double ionization of defect-localized states is responsible for the PPC effect.

Probe of flexibility and conformational heterogeneity in Zn-cytochrome c (Zn-cyt c) folding by Three-Pulse Photon Echo Peak Shift (3PEPS) Spectroscopy

We investigated the dynamics and conformational heterogeneity of Zn-cyt c with 3PEPS, a type of femtosecond nonlinear spectroscopy that is used to quantify the spectrum of sub-nanosecond protein motions and the inhomogeneous broadening of the system. Wavelength-dependent measurements were performed on the Soret transitions of the folded state, the guanidine hydrochloride unfolded state (4.5 M), and the state at the midpoint (2.5 M) of the denaturant titration. The measurements resolve a substantial increase in the inhomogeneous broadening of the Soret band upon unfolding, as expected due to an increase in conformational diversity upon unfolding.