Broad Spectral Region Research Articles

Effect of Nd concentration on structural and optical properties of Nd:Y 2O 3 transparent ceramic

Y 2O 3 transparent ceramics with different Nd concentration (0.1–7.0at%) were fabricated using ZrO 2 as additive. All the samples exhibit high transparency over a broad spectral region. The elements (Y, O and Nd) are uniformly distributed in the ceramic body, and the average grain size increases with Nd content. Based on the absorption spectrum, the Judd–Ofelt intensity parameters are calculated ( Ω 2=4.364×10 −20 cm 2, Ω 4=3.609×10 −20 cm 2 and Ω 6=2.919×10 −20 cm 2). The absorption coefficients increase linearly with Nd 3+ doping concentration. The absorption cross-section at 804 nm and stimulated emission cross-section at 1078 nm are calculated to be 1.54×10 −20 and 7.24×10 −20 cm 2, respectively. All the emission bands exhibit the highest emission intensities with 1.0at% Nd 3+ ion content, while the lifetime decreases dramatically from 321.5 μs (0.1at% Nd) to 17.9 μs (7.0at% Nd). According to the emission spectra and measured lifetime, the optimum doping concentration of Nd 3+ ion in Y 2O 3 transparent ceramic might be around 1.0at%.

Cortical neural coding of speech in simple and complex auditory scenes.

The neural representation of continuous speech in human auditory cortex was obtained noninvasively via magnetoencephalography. The neural response was recorded from human subjects listening to a spoken narrative, either in clean or in the presence of interfering speech. The cortical neural response to clean speech is demonstrated to precisely track the slow temporal modulations (<10 Hz) of speech in a broad spectral region between 400 Hz and 2 kHz. The neural code is sufficiently faithful to decode acoustic features of speech. To examine the robustness of, and the role of attention in, this neural code, another spoken narrative was presented simultaneously, either to a different ear (dichotically) or to the same ear (diotically), instructing the subjects to focus on only one of the two speech signals. The cortical representation of the attended speech is found to be substantially stronger than that of the unattended speech. This attentional effect is significant during the subjects’ first exposure to the spoken narratives. These results demonstrate that auditory cortex precisely represents the slow temporal modulations of speech and maintains separate neural representations for concurrent speech signals that can be individually and strongly modulated by attention. [Work supported by the NIDCD Grant No. R01‐DC‐008342.]

Impurity spectroscopy at its ultimate limit: relation between bulk spectrum, inhomogeneous broadening, and local disorder by spectroscopy of (nearly) all individual dopant molecules in solids

We present a technique for the measurement of the low-temperature fluorescence excitation spectra and imaging of a substantial fraction of all single chromophore molecules (hundreds of thousands and even more) embedded in solid bulk samples as nanometre-sized probes. An important feature of our experimental studies is that the full information about the lateral coordinates and spectral parameters of all individual molecules is stored for detailed analysis. This method enables us to study a bulk sample in a broad spectral region with ultimate sensitivity, combining excellent statistical accuracy and the capability of detecting rare events. From the raw data we determined the distributions of several parameters of the chromophore spectra and their variations across the inhomogeneous absorption band, including the frequencies of the electronic zero-phonon lines, their spectral linewidths, and fluorescence count rates. Relationships between these distributions and the disorder of the matrix were established for the examples of two polycrystalline solids with very different properties, n-hexadecane and o-dichlorobenzene, and the amorphous polymer polyisobutylene. We also found spatially inhomogeneous distributions of some parameters.

Coumarins in Phormium (New Zealand Flax) Fibers: Their Role in Fluorescence and Photodegradation

The genus Phormium (New Zealand flax) has fiber possessing a high content of lignin and, like other lignocellulosic materials, it is subject to photodegradation. Photoproducts in the fiber absorbing over a broad spectral region from ∼370 to 600nm are observed as a result of exposure to near-UV radiation from 350 to 400 nm. Irradiation was shown to produce hydrogen peroxide and this can account, at least in part, for the photo-oxidation manifested in changes in the reflectance spectra of the fibers. Unirradiated solid fibers and their aqueous extracts exhibit fluorescence with excitation maxima at 350-360nm and emission maxima at 440 nm. The fluorescence spectra of the fibers change following exposure to near-UV radiation with the major fluorophore being substantially photodegraded, evidenced by a substantial loss of emission between 410 and 480nm, which is largely responsible for the yellow and duller appearance of the fiber. Analysis of the aqueous extracts of the fibers, using electron-spray ionization mass spectroscopy of aqueous extracts, showed the presence of coumarin, hydroxycoumarin and a number of substituted hydroxycoumarins. The spectral distributions of the fluorescence associated with the unirradiated fibers and their aqueous extracts are consistent with them originating from a number of 7-hydroxycoumarins present.

Tailoring the dispersion behavior of silicon nanophotonic slot waveguides

We investigate the chromatic dispersion properties of silicon channel slot waveguides in a broad spectral region centered at ~1.5 μm. The variation of the dispersion profile as a function of the slot fill factor, i.e., the ratio between the slot and waveguide widths, is analyzed. Symmetric as well as asymmetric geometries are considered. In general, two different dispersion regimes are identified. Furthermore, our analysis shows that the zero and/or the peak dispersion wavelengths can be tailored by a careful control of the geometrical waveguide parameters including the cross-sectional area, the slot fill factor, and the slot asymmetry degree.

Multiple spectral window mirrors based on Fibonacci chains of dielectric layers

The unique properties of one-dimensional (1D) Fibonacci chains of dielectric layers are experimentally demonstrated and exploited for the design of new mirrors with multiple reflection spectral windows. The new mirror structures are simple, straightforward to make and enable a wide variety of multiple spectral window device performance to be achieved. By changing the thickness of the layers or the order of the Fibonacci chain, tens or even hundreds of windows can be obtained with the same approximate reflectivity over a very broad spectral region. These mirrors have numerous applications in photonics and optoelectronics.

Spectroscopy and micro-luminescence mapping of Xe-implanted defects in diamond

Ion implantation in diamond creates optically active defects which have emission lines in broad spectral regions and may be used in advanced photonics and optical communication applications. A brief review of the photoluminescence properties of Xe+ ion implanted diamond is presented. The Xe-related center is of particular interest as this center is one of a few centers (Ni, Si, Cr) in diamond having sharp emission lines in the infrared spectral region, specifically at 813 and 794nm. The paper discusses an approach to determine the important and difficult to measure conversion efficiency of implanted ions into emitting optical centers. The method uses micro-luminescence confocal mapping and statistical analysis based on a compound Poisson distribution, accounting for both the implanted centers and the optically excited centers statistics. Results of numerical simulations and experimental measurements are presented.

Optical properties of II-VI colloidal quantum dot doped porous silicon microcavities

In this paper we report on the light emitting properties of mesoporous silicon vertical-cavity optical resonators with II-VI colloidal quantum dots selectively deposited in the cavity layer. Optical resonator structures exhibit reflectivity stop bands of several hundred nanometres and resonant modes with line-widths less than 3.5 nm. The observed modification of spectral and spatial emission properties of the quantum dots and tenfold enhancement at the resonance wavelength is consistent with cavity enhanced spontaneous emission. Using this hybrid fabrication approach we show that narrow band light emitting structures may be fabricated over a broad spectral region in the visible and near-infrared.

N-3 FATTY ACID SUPPLEMENTATION INFLUENCES ASSOCIATIONS BETWEEN MEMBRANE FATTY ACIDS AND PHOSPHOLIPASE A2ACTIVITY IN PATIENTS AT RISK TO DEVELOP PSYCHOSIS

In the present study, p-type conducting polymer of poly [N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) has been explored for nanostructures. A novel approach has been adopted to transform nanorods into nanotubes by altering template-wetting methods. PCDTBT nanorods are fabricated by infiltrating porous alumina template with various solution concentrations of 5, 10 and 15 mg/ml. Upon thermal annealing PCDTBT beyond its melting point, the nanorods are transformed into nanotubes. The morphological and optical investigations reveal that the nanorods prepared with a concentration of 10 mg/ml are longer, denser, well-arranged and red shifted as compared to other nanorods. The PCDTBT nanotubes of the same concentration prepared at 300 °C are found the best among all other nanotubes with improved length, density and alignment as compared to their nanorod counterparts. Furthermore, the optical spectra of the nanotubes demonstrate broad spectral region, augmented absorption intensity and significant red-shift. The changes observed in Raman shift indicate improvement in molecular arrangement of the nanotubes. Optimization of the solution concentration and annealing temperature leads to improvement of PCDTBT nanostructures. PCDTBT nanotubes, with better molecular arrangement and broad optical spectrum, can be exploited in the state-of-the-art photovoltaic devices.

Frequency domain fluorescence microspectrometry: Application to cellular uptake and drug distribution

Time-resolved confocal microspectrofluorometry and fluorescence microimaging were used to monitor how the model antisense oligonucleotide is transported into 3T3 living cells and distributed inside them. Phosphorothioate analog of 15-mer oligothymidylate labeled by ATTO 425 was complexed with Zn(II) 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin as an uptake-mediating agent. Homodyne phase-resolved technique based on a high frequency analog modulation of both exciting diode laser and detector image intensifier was used for time-resolved measurements. Decay-time data obtained within a broad range spectral region have provided unique information about the fate of both fluorophores inside the cell.

Frequency domain fluorescence microspectrometry: Application to cellular uptake and drug distribution

Time-resolved confocal microspectrofluorometry and fluorescence microimaging were used to monitor how the model antisense oligonucleotide is transported into 3T3 living cells and distributed inside them. Phosphorothioate analog of 15-mer oligothymidylate labeled by ATTO 425 was complexed with Zn(II) 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin as an uptake-mediating agent. Homodyne phase-resolved technique based on a high frequency analog modulation of both exciting diode laser and detector image intensifier was used for time-resolved measurements. Decay-time data obtained within a broad range spectral region have provided unique information about the fate of both fluorophores inside the cell.

Tolerances of time-delay-compensated monochromators for extreme-ultraviolet ultrashort pulses

Time-delay-compensated grating monochromators for the spectral selection of extreme-ultraviolet femtosecond pulses in a broad spectral region are designed by adopting both classical diffraction and the off-plane mount. The optical design requires six optical elements: four concave mirrors and two plane gratings. I compare here the two designs in terms of the tolerances in the alignment for time-delay compensation in the femtosecond time scale. The off-plane mount is demonstrated to be far superior to the classical diffraction mount.

Broadband astigmatism-free Czerny-Turner imaging spectrometer using spherical mirrors

We describe the elimination of the astigmatism of a Czerny-Turner imaging spectrometer, built using spherical optics and a plane grating, over a broad spectral region. Starting with the principle of divergent illumination of the grating, which removes astigmatism at one chosen wavelength, we obtain design equations for the distance from the grating to the focusing mirror and the detector angle that remove the astigmatism to first order in wavelength. Experimentally, we demonstrate near diffraction-limited performance from 740 to 860 nm and over a 5 mm transverse spatial extent, while ray-tracing calculations show that barring finite-aperture and detector size limitations, this range extends from 640 to 900 nm and over 10 mm transversely. Our technique requires no additional optics and uses standard off-the-shelf components.

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems. X- and Y-cut LiNbO 3 crystals implanted with 8 MeV Au 3+ ions with a fluence of 1 × 10 17 ions/cm 2 result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold. Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the I 3 - structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of I 3 - and I 5 - aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed. The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10 17 ions/cm 2 during subsequent high temperature annealing is studied by Raman and Brillouin light scattering. Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10 15 to 250 × 10 15 ions/cm −2 induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.

Spectroscopic second-harmonic generation from silicon-on-insulator wafers

Optical second-harmonic generation from a silicon-on-insulator wafer has been investigated over a broad spectral region. The nonlinear signal shows oscillations with wavelengths that depend on the angle of incidence. A number of characteristic structures are caused by linear multiple reflections in the layered structure at the fundamental and second-harmonic wavelengths. Furthermore, an interference between signals generated by second-harmonic sources at different interfaces appears. Possibilities for isolating signals from buried interfaces through variations of pump wavelength and angle of incidence are discussed.

The optical characterization of chromophoric dissolved organic matter using wavelength distribution of absorption spectral slopes

We use a new approach to characterize the absorption characteristics of chromophoric dissolved organic matter (CDOM) in standard solutions and in several tropical lake ecosystems (Lake Victoria, Lake Tanganyika, Laguna Ibera). Compared with the common methods where a single slope of CDOM absorption spectrum is determined over a broad spectral region, we estimate the variations in spectral slope as a function of wavelength from 200 to 700 nm. Hence, our results show a distribution of spectral slope, referred to as the spectral slope curve, Sλ, which was used to identify similarities between lakes and standard solutions as well as to show the wavelength intervals in which photodegradation modifies spectral slope. In contrast to the plethora of wavelength intervals and ratios presented in the CDOM literature, a comparison of spectral slope curves between ecosystems provides a potentially important tool to examine the characteristics of CDOM in a consistent manner.

Ultrafast optofluidic gain switch based on conjugated polymer in femtosecond laser fabricated microchannels

We report on an optofluidic gain switch based on polydioctylfluorene solution in a femtosecond laser fabricated microchannel. The solution provides a stimulated emission cross section of the order of 10−16 cm2. Depending on the good isolation of polymeric chains we obtain ultrafast gain switching with time constants below 150 fs and high on/off ratios (up to 100%) in a broad spectral region (450–500 nm). This enables potential modulation rates well in the terahertz range. The enhanced optofluidic functionalities combined with the flexibility of femtosecond laser micromachining pave the way to an innovative class of optical devices easily integratable in complex systems.

Can geosynchronous imaging Fourier transform spectrometer capture vertical and temporal variability of a convective atmosphere?

Insufficient high vertical and temporal resolution data have limited the precipitation forecast skill of convective initiation. The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS), a new hyperspectral geostationary satellite measurement system, could provide top-of-atmosphere (TOA) radiances across two broad spectral regions with high-resolutions in spectral, horizontal and temporal spaces within a fixed domain. A set of modeled and observed vertical profiles of atmospheric moisture and temperature during a convective initiation (CI) event within the observing period of the International H2O Project (IHOP_2002) are used to assess the potential values of GIFTS measurements to convective precipitation forecast. First, it is shown that the model simulation captures reasonably well the movement of the precipitation bands and the gradient structures of temperature and water vapor of the convectively initiated storm. Second, the observed vertical and temporal variability of water vapor during the CI period is shown to be quite significant in the lower troposphere. The differences between observations and model simulation are also noticed. Using both the observed and the model-predicted profiles as input to GIFTS radiative transfer model (RTM), it is finally shown that the simulated GIFTS radiance could capture the high vertical and temporal variability of the real and modeled atmosphere prior to the CI, as well as the differences between observations and model forecasts. The study suggests the potential for GIFTS to make important contributions to the improvement of the forecast skill of convective precipitation.

Biomimetic Interfaces for High-Performance Optics in the Deep-UV Light Range

We report an innovative approach for the fabrication of highly light transmissive, antireflective optical interfaces. This is possible due to the discovery that metallic nanoparticles may be used as a lithographic mask to etch nonstraightforward structures into fused silica, which results in a quasihexagonal pattern of hollow, pillar-like protuberances. The far reaching optical performance of these structures is demonstrated by reflection and transmission measurements at oblique angles of incidence over a broad spectral region ranging from deep-ultraviolet to infrared light.

Optical properties of As33S67−xSex bulk glasses studied by spectroscopic ellipsometry

Variable angle spectroscopic ellipsometry (VASE) was employed to study the optical properties of As33S67−xSex (x=0, 17, 33.5, 50, and 67at.%) bulk glasses in the UV-vis-NIR (near infrared) spectral region for photon energies from 0.54to4.13eV (photon wavelengths from 2300to300nm). For data analysis, we employed Tauc–Lorentz (TL) dispersion model in the entire measured near bandgap spectral region and standard Cauchy dispersion model in the spectral region below the bandgap. With increasing Se content (x) in the bulk glass, we observed a linear decrease in optical bandgap energy Egopt from 2.52±0.02eV for As33S67 to 1.75±0.01eV for As33Se67 and linear increase in refractive index nTL in the NIR spectral region, e.g., at 0.80eV from 2.327 for As33S67 to 2.758 for As33Se67. The amplitude A decreased with increasing Se content. The peak transition energy E0 and broadening C had a maximum value for x=33.5at.% and systematically decreased for higher S or Se content in glasses. Our study showed that TL model is suitable to describe dielectric functions of studied chalcogenide bulk glasses in the broad spectral region. The bulk glasses had a higher refractive index compared to thin films of corresponding composition. The bulk glasses with high S content had higher value of optical bandgap energy than was previously reported for thin films. The optical bandgap energy of glasses with higher Se content was very similar to the thin films.