Wavelength-dependent Emission Research Articles

Synchrotron infrared reflectivity measurements of iron at high pressures

The infrared reflectance of iron was studied using high-pressure synchrotron radiation methods up to 50 GPa at room temperature in a diamond anvil cell of 1000-8000 cm(-1) (1.25-10 microm). The magnitude of the reflectivity shows a weak pressure dependence up to the transition from the body centered cubic (alpha) to hexagonal close packed (epsilon) phase transition, where a discontinuous change in both the slope and magnitude of the reflectivity was observed. Reflectance spectra were corrected for diamond absorption and treated with a Kramers-Kronig analysis to extract the optical constants; the emissivity of iron was derived from Kirchoff's law. The pressure and wavelength dependence of the emissivity is characterized by an empirical function for 1.5-1.9 microm; this wavelength range is useful for spectroradiometric temperature measurements from 1000 K up to approximately 2500 K. Alpha-Fe is a nonideal emitter; however, epsilon-Fe behaves as an almost perfect greybody in the infrared up to the highest pressures of the measurements. Temperature measurements based on the spectroradiometry of iron samples should take into account the wavelength dependent emissivity below the alpha-epsilon phase transition at approximately 13 GPa.

Enhancement of upconversion luminescence of Er doped Al2O3 films by Ag island films

Strong enhancement of upconversion photoluminescence (PL) of Er3+ in the visible range was achieved by placing it near rough Ag island films. In order to understand the mechanism of the enhancement, PL was excited by 978 and 488nm light and the enhancement factors were obtained in a wide spectral range (520nmto1.6μm). The emission and excitation wavelength dependence of the enhancement factors revealed that both the incident electric fields and the radiative decay rate of Er3+ are enhanced due to the excitation of surface plasmons supported by Ag islands. It was found that the upconversion PL is more sensitive to the enhancement of the incident electric field than the downconversion PL because of the multiphoton absorption process. The present result indicates that the upconversion PL obtains more benefit from the metal-enhanced fluorescence technique than the downconversion PL.

How Does Water Affect the Dynamics of the Room-Temperature Ionic Liquid 1-Hexyl-3-methylimidazolium Hexafluorophosphate and the Fluorescence Spectroscopy of Coumarin-153 When Dissolved in It?

Molecular dynamics simulations of mixtures of 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM+][PF6-]) and water have been performed in order to investigate how small amounts of water affect the translational and rotational dynamics of this ionic liquid (IL). We find that water is closely associated with the anions and that its presence enhances both the translational and rotational dynamics of the IL. In agreement with experiments, we find that the fluorescence spectra of Coumarin-153 is red-shifted because of the presence of water. Small amounts of water enhance the speed of relaxation of the solvent surrounding the solute probe after photoexcitation, but only at a "local environment" level. Interconversion between environments still occurs on a long time scale compared with the fluorescence lifetime of the probe. Excitation wavelength-dependent emission is observed both in the neat IL and in the IL+water mixture.

Photophysical Properties of Acene DCDHF Fluorophores: Long-Wavelength Single-Molecule Emitters Designed for Cellular Imaging

We report the solvatochromic, viscosity-sensitive, and single-molecule photophysics of the fluorophores DCDHF-N-6 and DCDHF-A-6. These molecules are members of the dicyanomethylenedihydrofuran (DCDHF) class of single-molecule emitters that contain an amine electron donor and a DCDHF acceptor linked by a conjugated unit; DCDHF-N-6 and DCDHF-A-6 have naphthalene- and anthracene-conjugated linkers, respectively. These molecules maintain the beneficial photophysics of the phenylene-linked DCDHF (i.e., photostability, emission wavelength dependence on solvent polarity, and quantum yield sensitivity to solvent viscosity), yet offer absorption and emission at longer wavelengths that are more appropriate for cellular imaging. We demonstrate that these new fluorophores are less photolabile in an aqueous environment than several other commonly used dyes (rhodamine 6G, Texas Red, and fluorescein). Finally, we image single copies of the acene DCDHFs diffusing in the plasma membrane of living cells.

Fourier transform emission lifetime spectrometer

We report a rapid and low cost Fourier transform spectrometer that uses a path length modulated Michelson interferometer to simultaneously measure excitation spectra and excitation wavelength-dependent emission lifetimes. Excitation spectra and lifetimes of excited tris(2,2'-bipyridyl) ruthenium(II) measured using this technique corresponded to values known in the literature. Excitation-dependent lifetimes of porous silicon measured with this technique suggest the influence of quantum confinement effects. This method may be useful for measuring mixtures of emitting species with closely spaced lifetimes as well as studying excitation wavelength-dependent emission phenomena.

Mn 2+-doped Ba 2ZnS 3 phosphor as a potential luminescent material for white LEDs

Mn 2+-doped Ba 2ZnS 3 phosphors for white light emitting diodes (LEDs) have been prepared by high temperature solid-state reaction and their photoluminescence properties are reported. Diffuse reflection spectra of Ba 2ZnS 3 host and synthesized phosphors have been measured. The excitation spectrum extends a broad range of wavelength and is consistent with the diffuse reflectance spectra. The emission bands are due to 4T 1( 4G) → 6A 1( 6S) transition of the Mn 2+ ion and there exists efficient energy transfer from host to Mn 2+ ions. The dependence of emission wavelength on Mn 2+ concentration is studied. The decay curve of Mn 2+ ions luminescence in Ba 2Zn 0.995S 3:0.005 Mn 2+ is also measured.

Fast surface temperature measurement of Teflon propellant-in-pulsed ablative discharges using HgCdTe photovoltaic cells

High-speed mercury cadmium telluride photovoltaic detectors, sensitive to infrared emission, are investigated as a means of measuring surface temperature on a microsecond time frame during pulsed ablative discharges with Teflon™ as the ablated material. Analysis is used to derive a governing equation for detector output voltage for materials with wavelength dependent emissivity. The detector output voltage is experimentally calibrated against thermocouples embedded in heated Teflon. Experimental calibration is performed with Teflon that has been exposed to ∼200 pulsed discharges and non-plasma-exposed Teflon and is compared to theoretical predictions to analyze emissivity differences. The diagnostic capability is evaluated with measurements of surface temperature from the Teflon propellant of electric micropulsed plasma thrusters. During the pulsed current discharge, there is insufficient information to claim that the surface temperature is accurately measured. However, immediately following the discharge, the postpulse cooling curve is measured. The statistical spread of postpulse surface temperature from shot to shot, most likely due to arc constriction and localization, is investigated to determine an operational envelope for postpulse temperature and mass ablation. This information is useful for determining postpulse ablation contributions to mass loss as well as evaluation of theoretical discharge models currently under development.

New method of high-precision thermometry

Measurements of temperature using infrared radiation have become common in industry. They are used in process control for materials ranging from steel to glass to silicon, in machinery to monitor component temperature and performance, and to predict component failure. We developed a rapid, noncontact method of accurately measuring the temperature and the spectral emissivity of a surface. Inclusion of the emissivity in the measurement makes it possible to achieve high-precision results. In our method, the power spectrum emitted by a surface is measured first by a multichannel spectrometer; the surface is then illuminated by a full-spectrum light source with a known spectrum, and the sum of the emitted and reflected power from the surface is measured. The data from the two measurements and the governing relations for the radiation and reflection processes are then combined to generate a set of curves that intersects near a solution point, which yields the temperature and the emissivity in the range of wavelengths of one of the channels. The emissivity in the other channels is readily calculated and yields the wavelength-dependent emissivity of the surface. The accuracy of the method for very rapid (submicrosecond) measurements is approximately 1% or less. We named this method the spectrum method because it uses the spectrum of both the emitted and reflected light to make the calculation. The method is more accurate than a grey body calculation because the wavelength-dependent emissivity is an integral part of the calculation of the surface temperature. The accuracy of the temperature and emissivity measurements can be calculated for a system containing five or more channels. We discuss the sources of errors and quantitatively assess their effect. Relative as well as absolute values of the channel emissivities have a significant effect on errors in the emissivity and temperature measurements. This temperature diagnostic has been tested on a tabletop device that allows us to make quick and reproducible measurements.

Emission wavelength dependence of fluorescence lifetimes of bacteriological spores and pollens

Concern about biological terrorism has greatly increased in the 21st century, and correspondingly, so has the need for accurate detection and identification of biological hazards, such as Bacillus anthracis. Optical techniques have been shown to be useful for this purpose. Use of fluorescence lifetimes as a function of emission wavelength for different materials using point- detection methods appears to be an additional viable option. Although the lifetimes range only between 2 and 6 ns, most biological materials tested in this study were distinguishable. A preliminary database has been compiled for use in a possible future detection system.

A Study of the Time-Resolved Fluorescence Spectrum and Red Edge Effect of ANF in a Room-Temperature Ionic Liquid

In a recent article, we have analyzed using molecular dynamics simulations the steady-state red edge effect (REE) observed by Samanta and co-workers when the fluorescent probe 2-amino-7-nitrofluorene (ANF) is photoexcited at different wavelengths in 1-butyl-3-methylimidazolium ([BMIM+]) hexafluorophosphate ([PF6-]). In this letter, we predict the time- and wavelength-dependent emission spectra of ANF in the same ionic solvent. From the analysis of our simulated data, we are able to derive an approximate time scale for reorganization of the solvent around the solute probe. The effect that slow varying local liquid environments have on the overall time-dependent signal is also discussed.

Dependence of Tryptophan Emission Wavelength on Conformation in Cyclic Hexapeptides

The wavelength of maximum emission of tryptophan depends on the local electrostatic environment of the indole chromophore. The time-resolved emission spectra of seven rigid cyclic hexapeptides containing a single tryptophan residue were measured. The emission maxima of the three decay-associated spectra for the seven peptides ranged from 341 to 359 nm, suggesting that different tryptophan rotamers have different emission maxima even in the case of solvent-exposed tryptophans. This conclusion is supported by quantum mechanical/molecular dynamics simulations of the six canonical side chain rotamers of tryptophan in solvated hexapeptides. The calculated range of emission maxima for the tryptophan rotamers of the seven peptides is 344-365 nm. The precision of the wavelength calculations and the peptide, water, and charged side chain contributions to the spectral shifts are examined. The results indicate that the emission maxima of decay-associated spectra can aid in the assignment of fluorescence lifetimes to tryptophan rotamers.

Heterogeneity in a room-temperature ionic liquid: persistent local environments and the red-edge effect.

In this work, we investigate the slow dynamics of 1-butyl-3-methylimidazolium hexafluorophosphate, a very popular room-temperature ionic solvent. Our study predicts the existence of heterogeneity in the liquid and shows that this heterogeneity is the underlying microscopic cause for the recently reported "red-edge effect" (REE) observed in the study of fluorescence of the organic probe 2-amino-7-nitrofluorene. This theoretical work explains in microscopic terms the relation between REE and dynamic heterogeneity in a room-temperature ionic liquid (IL). The REE is typical of micellar or colloidal systems, which are characterized by microscopic environments that are structurally very different. In contrast, in the case of this room-temperature IL, the REE occurs because of the long period during which molecules are trapped in quasistatic local solvent cages. This trapping time, which is longer than the lifetime of the excited-state probe, together with the inability of the surroundings to adiabatically relax, induces a set of site-specific spectroscopic responses. Subensembles of fluorescent molecules associated with particular local environments absorb and emit at different frequencies. We describe in detail the absorption wavelength-dependent emission spectra of 2-amino-7-nitrofluorene and show that this dependence on lambda(ex) is characteristic of the IL and, as is to be expected, is absent in the case of a normal solvent such as methanol.

Efficient rainbow color luminescence from InxGa1−xN single quantum wells fabricated on {112¯2} microfacets

Rainbow color luminescence from InxGa1−xN single quantum wells (SQWs) is achieved and almost covers the entire visible range when the layers are fabricated on {112¯2} facets with a few micron-width using a regrowth technique on striped GaN templates. These facets are tilted 56° with respect to the (0001) facets and border the (0001) and {112¯0} facets. The emission wavelength on the {112¯2} facets is redshifted from the {112¯0} side to (0001) side due to the variations of the In composition, which leads to the color contrast with the rainbow geometry. The temperature dependence of the photoluminescence intensity shows that the internal quantum efficiency at room temperature is 33% due to the very small internal electric fields and a small threading dislocation density compared to that in conventional (0001) InxGa1−xN SQWs. Since the emission efficiency does not show a noticeable emission wavelength dependence, this type of structure has potential as light-emitting devices with multiwavelengths that perform numerous color controllability such as pastel and white colors.

Laser and gain parameters at 2.7μm of Er3+-doped oxyfluoride transparent glass–ceramics

Abstract The room temperature emission spectrum at about 2.7 μm corresponding to the laser transition 4I11/2 → 4I13/2 in Er3+-doped nano-scaled transparent oxyfluoride glass–ceramic has been measured and stimulated emission cross-section for the transition has been calculated. The intensity of the transition has been found to be 40 times stronger and lifetime 50 times longer in the glass–ceramics compared to the precursor glass, which we show to be due to a change of frequency of the phonon involved in non-radiative de-excitation of the 4I11/2 level from 900 cm−1 in the precursor glass to 240 cm−1 in the ensuing glass–ceramics. The absorption cross-section for the excited state absorption 4I13/2 → 4I11/2 has been calculated based on the experimental reciprocal emission spectrum and wavelength dependence of the gain cross-section for the lasing transition 4I11/2 → 4I13/2 vs population inversion has been derived. The lasing/optical amplification gain parameters, such as population inversion, pump saturation intensity and product of emission cross-section and fluorescence lifetime have been obtained at the 2.7 μm wavelength. A noteworthy result is that laser action at 2.7 μm is possible in these Er3+-doped glass–ceramics, already not taking into account energy transfer or up-conversion processes, related to the 4I13/2 level, which favour the population inversion.

Edge excitation red shift and energy migration in quinine bisulphate dication

Photoinduced excited state dynamical processes in quinine sulphate dication (QSD) have been studied over a wide range of solute concentrations using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The edge excitation red shift (EERS) of emission maximum, emission wavelength dependence of fluorescence lifetimes and the time dependence of emission maximum are known to occur due to the solvent relaxation process. With increase in solute concentration, the emission spectrum shifts towards the lower frequencies accompanied with decrease in fluorescence intensity, however, absorption spectrum remains unchanged. A decrease in EERS, fluorescence lifetimes, time dependent fluorescence Stokes shift (TDFSS), fluorescence polarization and the solvent relaxation time ( τ r) is observed with the increase in solute concentration. The process of energy migration among the QSD ions along with solvent relaxation has been found responsible for the above experimental findings.

Correlation between current waveforms of electrostatic discharges and optical signatures for several selected wavelengths

The temporal variation of the optical intensity at several optical frequencies generated by electrostatic discharges was measured simultaneously with the discharge current waveform. Temporal variation of certain spectral lines closely followed that of the discharge current pulse, suggesting the possibility of accurately inferring some of the properties, such as the rise time, peak value, duration, etc., of the currents of electrostatic discharges without having to measure them directly. At the same time, the temporal variation of some other wavelengths had no resemblance to the shape of the current pulse. A strongly wavelength dependent optical emission in the ultraviolet region, possibly corresponding to pre-breakdown process was also found.

Temperature gradients, wavelength-dependent emissivity, and accuracy of high and very-high temperatures measured in the laser-heated diamond cell

We have spectroradiometrically measured a suite of temperatures between 761 ± 128 and 7307 ± 958 K in the laser-heated diamond cell using Br2 sample material. Critical evaluation of each spectrum and the dataset as a whole in comparison with synthetic data confirms the high temperatures and demonstrates the observable effects of temperature gradients and wavelength-dependent emissivity or absorbance on a single spectrum. In addition, we quantitatively demonstrate the use of a sliding two-color pyrometer analysis to produce a consistent and meaningful estimate of uncertainty of spectral temperature measurements.

Imaging protoplanetary disks with a square kilometer array

The recent detections of extrasolar giant planets has revealed a surprising diversity of planetary system architectures, with many very unlike our Solar System. Understanding the origin of this diversity requires multi-wavelength studies of the structure and evolution of the protoplanetary disks that surround young stars. Radio astronomy and the square kilometer array (SKA) will play a unique role in these studies by imaging thermal dust emission in a representative sample of protoplanetary disks at unprecedented sub-AU scales in the innermost regions, including the “habitable zone” that lies within a few AU of the central stars. Radio observations will probe the evolution of dust grains up to centimeter-sized “pebbles”, the critical first step in assembling giant planet cores and terrestrial planets, through the wavelength dependence of dust emissivity, which provides a diagnostic of particle size. High resolution images of dust emission will show directly mass concentrations and features in disk surface density related to planet building, in particular the radial gaps opened by tidal interactions between planets and disks, and spiral waves driven by embedded protoplanets. Moreover, because orbital timescales are short in the inner disk, synoptic studies over months and years will show proper motions and allow for the tracking of secular changes in disk structure. SKA imaging of protoplanetary disks will reach into the realm of rocky planets for the first time, and they will help clarify the effects of the formation of giant planets on their terrestrial counterparts.

Spectral Irradiance Calibration in the Infrared. XV. Absolute Calibration of Standard Stars by Experiments on theMidcourse Space Experiment

Calibration experiments were conducted with the SPIRIT III infrared instrument on the Midcourse Space Experiment (MSX) against a number of infrared standard stars and five emissive reference spheres that were ejected at various times during the mission. The physical properties of the 2 cm diameter spheres, such as size and emissivity, were precisely measured in the laboratory. The energy balance equation between the total flux absorbed and that emitted by the sphere is solved to obtain the time-dependent temperature of the sphere under the assumption that the sphere radiates as a blackbody with the measured wavelength-dependent emissivity. The estimated uncertainties in the modeling of the sphere are about 1 K in the thermal component and 3% for the geometric contribution. MSX also measured over 150 mean fluxes for eight standard infrared calibration stars during the 10 month mission. The measurements were scaled to the absolute fluxes that Cohen et al. adopt for α CMa (Sirius). The measured spectral energy distributions of the calibration stars relative to Sirius are within the uncertainties that Cohen et al. assign to the absolute fluxes from these stars, with a few exceptions. However, the MSX measurement uncertainties are generally much smaller, and the mission-averaged fluxes reveal statistically significant deviations from the Cohen et al. values. Of the calibration stars, only β Peg was found to be variable. MSX also measured excess fluxes for α Lyr (Vega) in the 12.1, 14.7, and 21.3 μm spectral bands; the excesses in the latter two bands are consistent with the published thermal model for the dust ring around this star. The absolute calibration of the fluxes of the stellar standards based on the average of the measurements of the spheres over all MSX bands and the five experiments agrees with those predicted to within the 1.4% MSX measurement uncertainties. The zero-magnitude absolute fluxes proposed by Cohen et al. are validated if the flux from Sirius is increased by 1%.

Asymmetric Bragg mirrors for the reduction of emission wavelength dependence on the viewing angle in organic microcavity light emitting diodes

Asymmetric Bragg mirrors for the reduction of emission wavelength dependence on the viewing angle in organic microcavity light emitting diodes