Showing papers on "Wavelength published in 2004"

Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon

Abstract: [1] The wavelength dependence of light absorption by aerosols collected on filters is investigated throughout the near-ultraviolet to near-infrared spectral region. Measurements were made using an optical transmission method. Aerosols produced by biomass combustion, including wood and savanna burning, and by motor vehicles, including diesel trucks, are included in the analysis. These aerosol types were distinguished by different wavelength (λ) dependences in light absorption. Light absorption by the motor vehicle aerosols exhibited relatively weak wavelength dependence; absorption varied approximately as λ−1, indicating that black carbon (BC) was the dominant absorbing aerosol component. By contrast, the biomass smoke aerosols had much stronger wavelength dependence, approximately λ−2. The stronger spectral dependence was the result of enhanced light absorption at wavelengths shorter than 600 nm and was largely reduced when much of the sample organic carbon (OC) was extracted by dissolution in acetone. This indicates that OC in addition to BC in the biomass smoke aerosols contributed significantly to measured light absorption in the ultraviolet and visible spectral regions and that OC in biomass burning aerosols may appreciably absorb solar radiation. Estimated absorption efficiencies and imaginary refractive indices are presented for the OC extracted from biomass burning samples and the BC in motor vehicle-dominated aerosol samples. The uncertainty of these constants is discussed. Overall, results of this investigation show that low-temperature, incomplete combustion processes, including biomass burning, can produce light-absorbing aerosols that exhibit much stronger spectral dependence than high-temperature combustion processes, such as diesel combustion.

Dynamical thermal behavior and thermal self-stability of microcavities.

Abstract: As stability and continuous operation are important for almost any use of a microcavity, we demonstrate here experimentally and theoretically a self-stable equilibrium solution for a pump-microcavity system. In this stable equilibrium, intensity- and wavelength-perturbations cause a small thermal resonant-drift that is enough to compensate for the perturbation (noises); consequently the cavity stays warm and loaded as perturbations are self compensated. We also compare here, our theoretical prediction for the thermal line broadening (and for the wavelength hysteretic response) to experimental results.

Geometries and materials for subwavelength surface plasmon modes.

Abstract: Plasmonic waveguides can guide light along metal-dielectric interfaces with propagating wave vectors of greater magnitude than are available in free space and hence with propagating wavelengths shorter than those in vacuum. This is a necessary, rather than sufficient, condition for subwavelength confinement of the optical mode. By use of the reflection pole method, the two-dimensional modal solutions for single planar waveguides as well as adjacent waveguide systems are solved. We demonstrate that, to achieve subwavelength pitches, a metal-insulator-metal geometry is required with higher confinement factors and smaller spatial extent than conventional insulator-metal-insulator structures. The resulting trade-off between propagation and confinement for surface plasmons is discussed, and optimization by materials selection is described.

Direct Measurement of Light Waves

Abstract: The electromagnetic field of visible light performs approximately 10(15) oscillations per second. Although many instruments are sensitive to the amplitude and frequency (or wavelength) of these oscillations, they cannot access the light field itself. We directly observed how the field built up and disappeared in a short, few-cycle pulse of visible laser light by probing the variation of the field strength with a 250-attosecond electron burst. Our apparatus allows complete characterization of few-cycle waves of visible, ultraviolet, and/or infrared light, thereby providing the possibility for controlled and reproducible synthesis of ultrabroadband light waveforms.

Near-field microscopy by elastic light scattering from a tip.

Abstract: We describe ultraresolution microscopy far beyond the classical Abbe diffraction limit of one half wavelength (lambda/2), and also beyond the practical limit (ca. lambda/10) of aperture-based scanning near-field optical microscopy (SNOM). The 'apertureless' SNOM discussed here uses light scattering from a sharp tip (hence scattering-type or s-SNOM) and has no lambda-related resolution limit. Rather, its resolution is approximately equal to the radius a of the probing tip (for commercial tips, a < 20 nm) so that 10 nm is obtained in the visible (lambda/60). A resolution of lambda/500 has been obtained in the mid-infrared at lambda = 10 microm. The advantage of infrared, terahertz and even microwave illumination is that specific excitations can be exploited to yield specific contrast, e.g. the molecular vibration offering a spectroscopic fingerprint to identify chemical composition. S-SNOM can routinely acquire simultaneous amplitude and phase images to obtain information on refractive and absorptive properties. Plasmon- or phonon-resonant materials can be highlighted by their particularly high near-field signal level. Furthermore, s-SNOM can map the characteristic optical eigenfields of small, optically resonant particles. Lastly, we describe theoretical modelling that explains and predicts s-SNOM contrast on the basis of the local dielectric function.

Ultracompact biochemical sensor built with two-dimensional photonic crystal microcavity.

Abstract: We report an experimental demonstration of an ultracompact biochemical sensor based on a two-dimensional photonic crystal microcavity. The microcavity, fabricated on a silicon-on-insulator substrate, is designed to have a resonant wavelength (λ) near 1.5 µm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n=1.0 to n=1.5. From observation of the shift in resonant wavelength, a change in ambient refractive index of Δn=0.002 is readily apparent. The correspondence between absolute refractive index and resonant wavelength agrees with numerical calculation to within 4% accuracy. The evaporation of water in a 5% glycerol mixture is also used to demonstrate the capability for in situ time-resolved sensing.

Absolute values of gravity wave momentum flux derived from satellite data

Abstract: [1] Temperature data obtained by the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) are analyzed for gravity waves (GWs). Amplitude, phase and vertical wavelength are determined from detrended temperature height profiles. The retrieved phases are utilized to estimate the horizontal wavelengths. At 25 km altitude an equatorial maximum of horizontal wavelength with a decrease toward mid and high latitudes is found. Simultaneous estimates of both horizontal and vertical wavelengths and temperature amplitudes allow the direct calculation of GW momentum flux (MF) from satellite observations for the first time. However, histograms of horizontal wavelength distributions indicate severe undersampling which prevents the retrieval of the propagation directions of the waves, and suggests our MF estimates may be too low, particularly at the high latitudes. Therefore an empirical aliasing correction has been applied. A world map of MF at 25 km altitude shows high variability and pronounced source regions and deviates in structure from a map of GW variances at the same altitude. Results from the Warner and McIntyre GW parameterization scheme (three-part model) show better agreement with CRISTA MF estimates than with CRISTA squared GW amplitudes. Best agreement is found for low model launch levels. Large error ranges of the estimated MF values obtained in this paper could be substantially reduced by improved horizontal sampling in future satellite missions.

Fog attenuation prediction for optical and infrared waves

Abstract: The principal disadvantage of using free space optics (FSO) telecommunication systems is the disturbing role played by the atmosphere on light propagation and thus on the channel capacity, availability, and link reliability. The wavelength choice is currently a subject of disagreement among designers and users of FSO equipments. Generally this equipment operates in the visible and the near IR at 690, 780, 850, and 1550 nm. Several authors affirm that equipment working at 1550 nm presents less atmospheric attenuation in the presence of fog and thus better link availability. Others consider that for dense fogs (visibility<500 m), all wavelengths are attenuated in the same way (wavelength independence). Fog attenuation in the visible and IR regions is reviewed from an empirical and theoretical point of view. Laser system performance in the presence of fog (advection and convection) in the 0.4- to 15-µm spectral zone is investigated using FASCOD computation. A transmission gain of 42% for a lasercom system working at 780 nm is observed compared to the same system working at 1550 nm. This gain reaches 48% if the same system works at 690 nm. Finally, we propose a fast transmission relationship based on an exact Mie theory calculation valid in the 0.69- to 1.55-µm spectral bands. It enables us to predict fog attenuation according to visibility without using heavy computer codes.

Sinusoidal phase grating created by a tunably buckled surface

Abstract: We investigate a buckling instability by both small angle light scattering and atomic force microscopy, demonstrating that a tunable phase grating can be created with a mechanical instability. The instability is realized in a prestressed silicone sheet coated with a glassy polymer film. Compression of the sample results in a sinusoidally wrinkled surface where the amplitude is controlled by the degree of compression and the wavelength by film thickness. We model the system with Fourier optics, explaining the positions and relative intensities of the diffraction orders.

Dispersive wave generation by solitons in microstructured optical fibers.

Abstract: We study the nonlinear propagation of femtosecond pulses in the anomalous dispersion region of microstructured fibers, where soliton fission mechanisms play an important role. The experiment shows that the output spectrum contains, besides the infrared supercontinuum, a narrow-band 430-nm peak, carrying about one fourth of the input energy. By combining simulation and experiments, we explore the generation mechanism of the visible peak and describe its properties. The simulation demonstrates that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave. In agreement with simulation, intensity-autocorrelation measurements show that the duration of the blue pulse is in the picosecond time range, and that, by increasing the input intensity, satellite pulses of lower intensity are generated.

Remote wavelength conversion in an illumination device

Abstract: An illumination device uses a wavelength converting element, such as a phosphor layer, that is physically separated from a light source, such as one or more light emitting diodes, a Xenon lamp or a Mercury lamp The wavelength converting element is optically separated from the light source, so that the converted light emitted by the wavelength converting element is prevented from being incident on the light source Accordingly, the temperature limitations of the wavelength converting element are removed, thereby permitting the light source to be driven with an increased current to produce a higher radiance Moreover, by optically separating the wavelength converting element from the light source, the conversion and recycling efficiency of the device is improved, which also increases radiance

Three-dimensional simulations of ultrasonic axial transmission velocity measurement on cortical bone models.

Abstract: The ultrasonic axial transmission technique, used to assess cortical shells of long bones, is investigated using numerical simulations based on a three-dimensional (3D) finite difference code. We focus our interest on the effects of 3D cortical bone geometry (curvature, cortical thickness), anisotropy, and microporosity on speed of sound (SOS) measurements for different frequencies in the MHz range. We first show that SOS values measured on tubular cortical shells are identical to those measured on cortical plates of equal thickness. Anisotropy of cortical bone is then shown to have a major impact on SOS measurement as a function of cortical thickness. The range of SOS values measured on anisotropic bone is half the range found when bone is considered isotropic. Dependence of thickness occurs for cortical shell thinner than 0.5×λbone in anisotropic bone (λbone: wavelength in bone), whereas it occurs for cortical shell thinner than λbone when anisotropy is neglected. Sensitivity of SOS along the bone axis ...

2.9THz quantum cascade lasers operating up to 70K in continuous wave

Abstract: We report the operation of a quantum cascade laser emitting at a 103μm wavelength (2.9THz). The active region is based on a bound-to-continuum design allowing a low parasitic leakage current, and a high upper-to-lower-state lifetime ratio. The latter is demonstrated by a pronounced decrease of the differential resistance at threshold, which is visible up to high temperatures, and by a weak temperature dependence of the slope efficiency. At 4K, we report a threshold current density of only 105A∕cm2 both in pulsed and continuous-wave operation, and an emitted peak power of 15mW independent of the duty cycle. Maximum operating temperatures of 95K and 70K are observed in pulsed and continuous wave modes, respectively.

Femtosecond laser ablation properties of borosilicate glass

Abstract: We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN>1) provides the multishot ablation threshold, FthN>1. The results depict a multishot ablation threshold of ≈1.7J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9μm, and the “effective optical penetration depth,” αeff−1≈238nm in air. The method is systematic, prov...

Stable single-photon source in the near infrared

Abstract: Owing to their unsurpassed photostability, defects in solids may be ideal candidates for single-photon sources. Here we report on generation of single photons by optical excitation of a yet unexplored defect in diamond, the nickel?nitrogen complex (NE8) centre. The most striking feature of the defect is its emission bandwidth of 1.2?nm at room temperature. The emission wavelength of the defect is around 800?nm, which is suitable for telecom fibres. In addition, in this spectral region little background light from the diamond bulk material is detected. Consequently, a high contrast in antibunching measurements is achieved.

Terahertz imaging system based on a backward-wave oscillator

Abstract: We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

Kapitza conductance and phonon scattering at grain boundaries by simulation

Abstract: We use a nonequilibrium molecular-dynamics method to compute the Kapitza resistance of three twist grain boundaries in silicon, which we find to increase significantly with increasing grain boundary energy, i.e., with increasing structural disorder at the grain boundary. The origin of this Kapitza resistance is analyzed directly by studying the scattering of packets of lattice vibrations of well-defined polarization and frequency from the grain boundaries. We find that scattering depends strongly on the wavelength of the incident wave packet. In the case of a high-energy grain boundary, the scattering approaches the prediction of the diffuse mismatch theory at high frequencies, i.e., as the wavelength becomes comparable to the lattice parameter of the bulk crystal. We discuss the implications of our results in terms of developing a general model of scattering probabilities that can be applied to mesoscale models of heat transport in polycrystalline systems.

Observation of bulk HfO2 defects by spectroscopic ellipsometry

Abstract: Spectroscopic ellipsometry was used to investigate the oxidation of pure Hf films on silicon for the formation of HfO2 (hafnium oxide) gate-dielectric films in advanced complementary metal-oxide-semiconductor field-effect transistors Absorption coefficients near the absorption edge were extracted using the data inversion method, in which the optical constants for short wavelengths were calculated using the film thickness determined from long-wavelength data The extracted optical band gap of 57 eV matches well with published data, and a curve shift due to crystallization was detected In addition, an extra absorption peak corresponding to electron transition from the valence band to a defect energy level was observed in the range 45–50 eV above the valence-band edge The 12 eV energy difference between the conduction-band edge and the edge of this extra peak is close to the electron trap energy level reported elsewhere The intensity of the detected peak was clearly correlated with leakage current an

Optimal waveguide dimensions for nonlinear interactions.

Abstract: We investigate strong light confinement in high core-cladding index contrast waveguides with dimensions comparable to and smaller than the wavelength of incident light. We consider oval and rectangular cross sections and demonstrate that an optimal core size exists that maximizes the effective nonlinearity. We also determine that waveguides with asymmetrical cross sections provide the maximum possible nonlinearity, although only a small improvement over the symmetric case. Furthermore, we find that for a specified waveguide shape the largest nonlinearity occurs for nearly the same core area in all cases. Calculations of the dispersion for the optimal-size waveguide at a particular wavelength indicate that the group-velocity dispersion is normal. Ultimately, such designs could be used to develop low-power all-optical devices and to produce waveguides for ultra-low threshold nonlinear frequency generation such as supercontinuum generation.

Phosphor based light sources utilizing total internal reflection

Abstract: A LED package including an LED that emits excitation light at an excitation light wavelength and a layer of phosphor material positioned to receive the excitation light and having a first index of refraction at the excitation light wavelength. The phosphor material emits visible light at a visible light wavelength when illuminated with the excitation light. An interference reflector is positioned adjacent to the layer of phosphor material and a TIR promoting layer is in contact with the layer of phosphor material. The TIR promoting layer has a second index of refraction at the excitation light wavelength that is less than the first index of refraction at the excitation light wavelength.

Highly directional emission from photonic crystal waveguides of subwavelength width.

Abstract: Recently it has been shown that it is possible to achieve directional emission out of a subwavelength aperture in a periodically corrugated metallic thin film. We report on theoretical and experimental studies of a related phenomenon concerning light emitted from photonic crystal waveguides that are less than a wavelength wide. We find that the termination of the photonic crystal end facets and an appropriate choice of the wavelength are instrumental in achieving very low numerical apertures. Our results hold promise for the combination of photonic crystal waveguides with conventional optical systems such as fibers, waveguides, and freely propagating light beams.

Electrodeless Determination of the Trap Density, Decay Kinetics, and Charge Separation Efficiency of Dye-Sensitized Nanocrystalline TiO2

Abstract: We have studied photoinduced charge separation in a bare, 3.4 microm thick layer of nanocrystalline ("nc") anatase TiO(2) and an nc-TiO(2) layer coated with free-base 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (H(2)TPPC) using the electrodeless flash-photolysis time-resolved microwave-conductivity technique (FP-TRMC). Photoconductivity transients, resulting from the formation of mobile, conduction band electrons in the semiconductor have been measured on excitation with 3 ns pulses of UV (300 nm) and visible (410-700 nm) light. The product of the yield of formation of mobile charge carriers, phi, and the sum of their mobilities, Sigmamicro, has been determined from the maximum conductivity for light intensities varying from approximately 10(12) to approximately 10(16) photons/cm(2)/pulse. For the bare nc-TiO(2) layer at 300 nm and the coated layer at all wavelengths, phiSigmamicro initially increased with increasing intensity, reached a maximum, and eventually decreased at high intensities. The initial increase is attributed to the gradual filling of (surface) electron trapping sites. This effect was absent when the samples were continuously illuminated with background irradiation at 300 nm with an intensity of 6 x 10(13) photons/cm(2)/s (40 microW/cm(2)), thereby presaturating the trapping sites prior to the laser pulse. The trap-free mobility of electrons within these 9 nm nanoparticles is estimated to be 0.034 cm(2)/Vs at 9 GHz. The eventual decrease in phiSigmamicro at intensities corresponding to an electron occupancy of more than one electron per particle is unaffected by background illumination, and is attributed to a decrease in micro due to electron-electron interactions within the semiconductor particles. The photoconductivity action spectrum of the coated nc-TiO(2) layer closely followed the photon attenuation spectrum in the visible of the porphyrin, with a charge separation efficiency per absorbed photon of 18% at the Soret band maximum. The after-pulse decay of the photoconductivity showed a power law behavior over a time scale of nanoseconds to several hundreds of microseconds, which is attributed to multiple trapping and detrapping events at chemical or physical defects within the semiconductor matrix.

Wave damping by magnetohydrodynamic turbulence and its effect on cosmic-ray propagation in the interstellar medium

Abstract: Cosmic rays scatter off magnetic irregularities (Alfven waves) with which they are resonant, that is, waves of wavelength comparable to their gyroradii. These waves may be generated either by the cosmic rays themselves, if they stream faster than the Alfven speed, or by sources of MHD turbulence. Waves excited by streaming cosmic rays are ideally shaped for scattering, whereas the scattering efficiency of MHD turbulence is severely diminished by its anisotropy. We show that MHD turbulence has an indirect effect on cosmic-ray propagation by acting as a damping mechanism for cosmic-ray-generated waves. The hot ("coronal") phase of the interstellar medium is the best candidate location for cosmic-ray confinement by scattering from self-generated waves. We relate the streaming velocity of cosmic rays to the rate of turbulent dissipation in this medium for the case in which turbulent damping is the dominant damping mechanism. We conclude that cosmic rays with up to 10^2 GeV could not stream much faster than the Alfven speed but 10^6 GeV cosmic rays would stream unimpeded by self-generated waves, unless the coronal gas were remarkably turbulence-free.

A multiwavelength study of solar flare waves. I. Observations and basic properties

Abstract: Propagating wave-like disturbances associated with solar flares - commonly observed in the chromosphere as Moreton waves - have been known for several decades. Recently, the phenomenon has come back into focus prompted by the observation of coronal waves with the SOHO/EIT instrument ("EIT waves"). It has been suggested that they represent the anticipated coronal counterpart to Moreton waves, but due to some pronounced differences, this interpretation is still being debated. We study 12 flare wave events in order to determine their physical nature, using Hα, EUV, He I 10 830 A SXR and radioheliographic data. The flare wave signatures in the various spectral bands are found to lie on closely associated kinematical curves, implying that they are signatures of the same physical disturbance. In all events, and at all wavelengths, the flare waves are decelerating, which explains the apparent "velocity discrepancy" between Moreton and EIT waves which has been reported by various authors. In this paper, the focus of the study is on the morphology, the spatial characteristics and the kinematics of the waves. The characteristics of the common perturbation which causes the wave signatures, as well as the associated type II radio bursts, will be studied in companion Paper II, and a consistent physical interpretation of flare waves will be given.

Enhanced millimeter-wave transmission through subwavelength hole arrays.

Abstract: We explore, both experimentally and theoretically, the existence in the millimeter-wave range of the phenomenon of extraordinary light transmission through arrays of subwavelength holes We have measured the transmission spectra of several samples made on aluminum wafers by use of an AB Millimetre quasi-optical vector network analyzer in the wavelength range 42–65 mm Clear signals of the existence of resonant light transmission at wavelengths close to the period of the array appear in the spectra

Auroral ion acceleration in dispersive Alfvén waves

Abstract: [1] Observations from the FAST satellite are used to create a model for dispersive Alfven waves above the auroral oval. Using this model, it is shown how these waves may accelerate ionospheric ions transverse to the geomagnetic field and cause ion outflow. The model waves grow from ionospheric conductivity variations due to auroral electron precipitation and resonate in the cavity between the ionosphere and the peak in the Alfven speed that occurs at altitudes of ∼1 Earth radius (Re). By tracing ions in the model wave field, it is demonstrated that for transverse wave amplitudes (E⊥) satisfying E⊥/Bo Ωi/k⊥ the ion motion may become stochastic allowing acceleration to energies exceeding that prescribed by λ⊥. The transversely accelerated ions in both the coherent and stochastic cases flow upward from the ionosphere under the influence of the mirror force to altitudes of 1 Earth radii over timescales as small as a few seconds to minutes with energies in the keV range. Ions accelerated by these means may account for the intense outflowing ion fluxes observed in Alfven waves above the auroral oval.

Extended-cavity semiconductor wavelength-swept laser for biomedical imaging

Abstract: We demonstrate a compact high-power rapidly swept wavelength tunable laser source based on a semiconductor optical amplifier and an extended-cavity grating filter. The laser produces excellent output characteristics for biomedical imaging, exhibiting >4-mW average output power, 80-dB noise extinction with its center wavelength swept over 100 nm at 1310 nm at variable repetition rates up to 500 Hz.

Differential effects of light wavelength in phase advancing the melatonin rhythm

Abstract: Shorter wavelength light has been shown to be more effective than longer wavelengths in suppressing nocturnal melatonin and phase delaying the melatonin rhythm. In the present study, different wavelengths of light were evaluated for their capacity to phase advance the saliva melatonin rhythm. Two long wavelengths, 595 nm (amber) and 660 nm (red) and three shorter wavelengths, 470 nm (blue), 497 nm (blue/green), and 525 nm (green) were compared with a no-light control condition. Light was administered via a portable light source comprising two light-emitting diodes per eye, with the irradiance of each diode set at 65 lW/cm 2 . Forty-two volunteers participated in up to six conditions resulting in 15 per condition. For the active light conditions, a 2-hr light pulse was administered from 06:00 hr on two consecutive mornings. Half-hourly saliva samples were collected on the evening prior to the first light pulse and the evening following the second light pulse. The time of melatonin onset was calculated for each night and the difference was calculated as a measure of phase advance. The shorter wavelengths of 470, 495 and 525 nm showed the greatest melatonin onset advances ranging from approximately 40-65 min while the longer wavelengths produced no significant phase advance. These results strengthen earlier findings that the human circadian system is more sensitive to the short wavelengths of light than the longer wavelengths.