Showing papers on "Wavelength published in 2005"

Magnetic Metamaterials at Telecommunication and Visible Frequencies

Abstract: Arrays of gold split rings with a 50-nm minimum feature size and with an LC resonance at 200 THz frequency (1.5 microm wavelength) are fabricated. For normal-incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800 nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.

Ultimate low loss of hollow-core photonic crystal fibres.

Abstract: Hollow-core photonic crystal fibres have excited interest as potential ultra-low loss telecommunications fibres because light propagates mainly in air instead of solid glass. We propose that the ultimate limit to the attenuation of such fibres is determined by surface roughness due to frozenin capillary waves. This is confirmed by measurements of the surface roughness in a HC-PCF, the angular distribution of the power scattered out of the core, and the wavelength dependence of the minimum loss of fibres drawn to different scales.

Dependence of the enhanced optical scattering efficiency relative to that of absorption for gold metal nanorods on aspect ratio, size, end-cap shape,and medium refractive index

Abstract: The current intense interest in the properties of plasmonic nanostructures for their applications in chemical and biochemical sensors, medical diagnostics and therapeutics, and biological imaging is fundamentally based on their enhanced optical absorption and scattering properties. In this study, the optical extinction, absorption, and scattering efficiencies were calculated as a function of shape definition, aspect ratio, surrounding medium, and material selection. The discrete dipole approximation method was used, which is known to be a very useful and versatile computational tool for particles with any arbitrary shape. Relative contribution of scattering to the total extinction for the longitudinal mode was found to be significantly dependent on the aspect ratio of the nanorod in a somewhat complex manner, different from a typical linear relationship for the resonance wavelength. A slight elongation of Au nanosphere gives rise to a drastic increase in the relative scattering efficiency, which eventually reaches a maximum and begins to decrease with further increase in the aspect ratio. This is ascribed to the increasing absorptive contribution from the larger imaginary dielectric function of the metal particle in the longer wavelength region where the red-shifted excitation of the longitudinal resonance mode occurs. For transverse mode exhibiting the blue-shift in the resonance peak, on the contrary, the absorption efficiency is relatively enhanced compared to the scattering efficiency with increasing aspect ratio. This is thought to result from the dominant effect of the interband transition present in this wavelength region. Besides the dependence of plasmonic characteristics on the aspect ratio of nanorod, the DDA results for a small change of the end-cap shape and the index of the surrounding medium lead us to conclude that there exist two competing key factors: a weighting factor assigned to the shape parameter and the dielectric function of the metal particle, which control the relative enhancement in the scattering and absorption as well as the linearity of resonance wavelength with regard to the aspect ratio.

Nanoscale resolution in the focal plane of an optical microscope

Abstract: Utilizing single fluorescent molecules as probes, we prove the ability of a far-field microscope to attain spatial resolution down to 16 nm in the focal plane, corresponding to about 1/50 of the employed wavelength. The optical bandwidth expansion by nearly an order of magnitude is realized by a saturated depletion through stimulated emission of the molecular fluorescent state. We demonstrate that en route to the molecular scale, the resolving power increases with the square root of the saturation level, which constitutes a new law regarding the resolution of an emerging class of far-field light microscopes that are not limited by diffraction.

Wavelength-converted semiconductor light-emitting device

Abstract: In a wavelength converted semiconductor light emitting device with at least two wavelength converting materials, the wavelength converting materials in the device are arranged relative to the light emitting device and relative to each other to tailor interaction between the different wavelength converting materials in order to maximize one or more of the luminous equivalent, color rendering index, and color gamut of the combined visible light emitted by the device.

Guided subwavelength plasmonic mode supported by a slot in a thin metal film

Abstract: We demonstrate the existence of a bound optical mode supported by a slot in a thin metallic film deposited on a substrate, with slot dimensions much smaller than the wavelength. The modal size is almost completely dominated by the near field of the slot. Consequently, the size is very small compared with the wavelength, even when the dispersion relation of the mode approaches the light line of the surrounding media. In addition, the group velocity of this mode is close to the speed of light in the substrate, and its propagation length is tens of micrometers at the optical communication wavelength.

Plasmon-assisted two-slit transmission: Young's experiment revisited.

Abstract: We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other.

Increased cut-off wavelength for a subwavelength hole in a real metal

Abstract: A waveguide mode of a subwavelength rectangular hole in a real metal is analyzed. Due to coupling between surface plasmons on the long edges of the hole, the cut-off wavelength increases as the hole-width is reduced. The cut-off wavelength is found to be much larger than Rayleigh's criterion for perfect metals - 2.3 times as large for a 15 nm wide hole. The analytical results are verified by finite-difference calculations. The finite difference calculations also show the influence of including material loss.

Limits of adaptive optics for high-contrast imaging

Abstract: The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10-4 to 10-5 in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10-6 and 10-7 can be achieved within 1'' on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.

Formation of nanogratings on the surface of a ZnSe crystal irradiated by femtosecond laser pulses

Abstract: Two collinear femtosecond laser pulses, one at wavelength of 800 nm and the other at 400 nm (double frequency), simultaneously irradiated the surface of ZnSe crystal, which resulted in regular nanograting with period of 180 nm on the whole ablation area. We attribute the formation of the nanograting to be due to the interference between the surface scattered wave of 800 nm lasers and the 400 nm light. The period of the nanograting Lambda is about lambda/2n, where n is refractive index of the sample, and lambda, the laser wavelength. This mechanism is supported by observation of rotation of the nanograting with the polarization of 400 nm light, and by the dependence of Lambda similar to lambda of the nanoripples on the surface of semiconductors and dielectrics.

115?kHz tuning repetition rate ultrahigh-speed wavelength-swept semiconductor laser

Abstract: We demonstrate an ultrahigh-speed wavelength-swept semiconductor laser using a polygon-based wavelength scanning filter. With a polygon rotational speed of 900 revolutions per second, a continuous wavelength tuning rate of 9200 nm/ms and a tuning repetition rate of 115 kHz were achieved. The wavelength tuning range of the laser was 80 nm centered at 1325 nm, and the average polarized output power was 23 mW.

Lossless interface modes at the boundary between two periodic dielectric structures

Abstract: Tamm states of light are lossless interface modes decaying exponentially in the surrounding media. We show that they can be formed at the boundary between two periodical dielectric structures, one having a period close to the wavelength of light and another one having a period close to the double of the wavelength. The order of layers at the interface has a crucial effect on the Tamm states. The in-plane dispersion of these states is parabolic with effective masses slightly different for TE and TM polarizations, both of the order of ${10}^{\ensuremath{-}5}$ of the free electron mass.

Thermospheric responses to gravity waves: Influences of increasing viscosity and thermal diffusivity

Abstract: [1] A gravity wave anelastic dispersion relation is derived that includes molecular viscosity and thermal diffusivity to explore the damping of high-frequency gravity waves in the thermosphere The time dependence of the wave amplitudes and general ray trace equations are also derived In the special case that the thermal structure is isothermal and the Prandtl number (Pr) equals 1, exact linear solutions are obtained For high-frequency gravity waves with ωIr/N ≪ 1 an upward propagating gravity wave dissipates at an altitude given by ≃ z1 + H ln(ωIr/2H∣m∣3ν1), where H is the density scale height, N is the buoyancy frequency, ν1 is the viscosity at z = z1, and ωIr and m are the gravity wave intrinsic frequency and vertical wave number, respectively Thus high-frequency gravity waves with large vertical wavelengths dissipate at the highest altitudes, resulting in momentum and energy inputs extending to very high altitudes We find that the vertical wavelength of a gravity wave with an initially large vertical wavelength decreases significantly by the time it dissipates just below where it begins to reflect The effect of diffusion on a gravity wave is similar to the effect of shear in the sense that as the molecular viscosity and thermal diffusivity increase due to decreasing background density, the intrinsic frequency plus mν/H decreases and the vertical wave number increases in order to satisfy the dispersion relation for Pr = 1 We also briefly explore the results with different Prandtl numbers using numerical ray tracing Gravity waves in a Pr = 07 environment dissipate just a few kilometers below those in a Pr = 1 environment when H = 7 km, showing the utility of the analytic, Pr = 1 solutions

Probability distributions of surface gravity waves during spectral changes

Abstract: Simulations have been performed with a fairly narrow band numerical gravity wave model (higher-order NLS type) and a computational domain of dimensions 128 x 128 typical wavelengths. The simulations are initiated with ∼ 6 x 10 4 Fourier modes corresponding to truncated JONSWAP spectra and different angular distributions giving both short- and long-crested waves. A development of the spectra on the so-called Benjamin-Feir timescale is seen, similar to the one reported by Dysthe et al. (J. Fluid Mech. vol. 478, 2003, p. 1). The probability distributions of surface elevation and crest height are found to fit theoretical distributions found by Tayfun (J. Geophys. Res. vol. 85, 1980, p. 1548) very well for elevations up to four standard deviations (for realistic angular spectral distributions). Moreover, in this range of the distributions, the influence of the spectral evolution seems insignificant. For the extreme parts of the distributions a significant correlation with the spectral change can be seen for very long-crested waves. For this case we find that the density of large waves increases during spectral change, in agreement with a recent experimental study by Onorato et al. (J. Fluid Mech. 2004 submitted).

Diffuse optical tomography with spectral constraints and wavelength optimization

Abstract: We present an algorithm that explicitly utilizes the wavelength dependence of tissue optical properties for diffuse optical tomography. We have previously shown that the method gives superior separation of absorption and scattering. Here the technique is described and tested in detail, and optimum wavelength sets for a broad range of chromophore combinations are discovered and analyzed.

Method and apparatus for angular-resolved spectroscopic lithography characterisation

Abstract: An apparatus and method to determine a property of a substrate by measuring, in the pupil plane of a high numerical aperture lens, an angle-resolved spectrum as a result of radiation being reflected off the substrate. The property may be angle and wavelength dependent and may include the intensity of TM- and TE-polarized light and their relative phase difference.

Wavelength-scanning digital interference holography for tomographic three-dimensional imaging by use of the angular spectrum method

Abstract: A tomographic imaging system based on wavelength-scanning digital interference holography is developed by applying the angular spectrum method. Compared to the well-known Fresnel diffraction formula, which is subject to a minimum distance requirement in reconstruction, the angular spectrum method can reconstruct the wave field at any distance from the hologram plane. The new system allows three-dimensional tomographic images to be extracted with an improved signal-to-noise ratio, a more flexible scanning range, and an easier specimen size selection. Experiments are performed to demonstrate the effectiveness of the method.

Three-photon absorption in ZnO and ZnS crystals.

Abstract: We report a systematic investigation of both three-photon absorption (3PA) spectra and wavelength dispersions of Kerr-type nonlinear refraction in wide-gap semiconductors. The Z-scan measurements are recorded for both ZnO and ZnS with femtosecond laser pulses. While the wavelength dispersions of the Kerr nonlinearity are in agreement with a two-band model, the wavelength dependences of the 3PA are found to be given by (3Ephoton/Eg -1)5/2(3Ephoton/Eg )-9. We also evaluate higher-order nonlinear optical effects including the fifth-order instantaneous nonlinear refraction associated with virtual three-photon transitions, and effectively seventh-order nonlinear processes induced by three-photon-excited free charge carriers. These higher-order nonlinear effects are insignificant with laser excitation irradiances up to 40 GW/cm2. Both pump-probe measurements and three-photon figures of merits demonstrate that ZnO and ZnS should be a promising candidate for optical switching applications at telecommunication wavelengths.

Method and apparatus for angular-resolved spectroscopic lithography characterization

Abstract: According to the present invention, an apparatus and method are disclosed for determining the properties of a substrate by measuring the angle-resolved spectrum as a result of the radiation being reflected off the substrate in the pupil plane of the high numerical aperture lens. The properties may be dependent on angle and wavelength and may include the intensity of TM- and TE-polarized light and their associated phase differences.

Verification of a plasma photonic crystal for microwaves of millimeter wavelength range using two-dimensional array of columnar microplasmas

Abstract: We experimentally verified that a microplasma assembly can create a functional dielectric layer for the propagation of electromagnetic waves as a “plasma photonic crystal.” A two-dimensional array in a square lattice was composed of columnar plasmas of about 2mm in diameter, and the transmitted microwaves at 70–75GHz showed a change of energy flow direction. This result is attributed to the fact that periodical structure is composed of individual plasma columns with a different dispersion than the ambient part and the experimental frequency range lies in the vicinity of the lowest band gap of the photonic crystal calculated theoretically.

Forward and backward leaky wave radiation with large effective aperture from an electronically tunable textured surface

Abstract: A resonant texture allows the impedance of a metal surface to be changed from an electric conductor to a magnetic conductor, or any boundary condition in between. Varactor diodes incorporated into the structure allow electronic control the reflection phase and the surface wave properties. This tunable textured surface is used as an electronically steerable leaky wave antenna, by coupling energy into a leaky wave band, and tuning the surface to change the radiation angle. With a simple optimization algorithm, the beam can be electronically scanned over a wide range in both the forward and backward directions. Because the surface geometry provides multiple degrees of freedom per half wavelength, it allows independent control of the magnitude and phase of the surface wave radiation, so the antenna can be programmed to have a large effective aperture over the entire scan range. Radiation in the backward direction can also be understood in terms of a backward band, which can be measured directly from the surface reflection properties.

Subwavelength atom localization via amplitude and phase control of the absorption spectrum-II

Abstract: We propose a scheme for subwavelength localization of an atom conditioned upon the absorption of a weak probe field at a particular frequency. Manipulating atom-field interaction on a certain transition by applying drive fields on nearby coupled transitions leads to interesting effects in the absorption spectrum of the weak probe field. We exploit this fact and employ a four-level system with three driving fields and a weak probe field, where one of the drive fields is a standing-wave field of a cavity. We show that the position of an atom along this standing wave is determined when probe-field absorption is measured. We find that absorption of the weak probe field at a certain frequency leads to subwavelength localization of the atom in either of the two half-wavelength regions of the cavity field by appropriate choice of the system parameters. We term this result as sub-half-wavelength localization to contrast it with the usual atom localization result of four peaks spread over one wavelength of the standing wave. We observe two localization peaks in either of the two half-wavelength regions along the cavity axis.

All-optical, wavelength and bandwidth preserving, pulse delay based on parametric wavelength conversion and dispersion.

Abstract: We demonstrate an all-optical tunable delay in fiber based on wavelength conversion, group-velocity dispersion, and wavelength reconversion. The device operates near 1550 nm and generates delays greater than 800 ps. Our delay technique has the combined advantages of continuous control of a wide range of delays from picoseconds to nanoseconds, for a wide range of signal pulse durations (ps to 10 ns), and an output signal wavelength and bandwidth that are the same as that of the input. The scheme can potentially produce fractional delays of 1000 and is applicable to both amplitude- and phase-shift keyed data.

Development of a fast temperature sensor for combustion gases using a single tunable diode laser

Abstract: The 12 best NIR water transition line pairs for temperature measurements with a single DFB laser in flames are determined by systematic analysis of the HITRAN simulation of the water spectra in the 1–2 μm spectral region. A specific line pair near 1.4 μm was targeted for non-intrusive measurements of gas temperature in combustion systems using a scanned-wavelength technique with wavelength modulation and 2f detection. This sensor uses a single diode laser (distributed-feedback), operating near 1.4 μm and is wavelength scanned over a pair of H2O absorption transitions (7154.354 cm-1 & 7153.748 cm-1) at a 2 kHz repetition rate. The wavelength is modulated (f=500 kHz) with modulation amplitude a=0.056 cm-1. Gas temperature is inferred from the ratio of the second harmonic signals of the two selected H2O transitions. The fiber-coupled-single-laser design makes the system compact, rugged, low cost and simple to assemble. As part of the sensor development effort, design rules were applied to optimize the line selection, and fundamental spectroscopic parameters of the selected transitions were determined via laboratory measurements including the temperature-dependent line strength, self-broadening coefficients, and air-broadening coefficients. The new sensor design includes considerations of hardware and software to enable fast data acquisition and analysis; a temperature readout rate of 2 kHz was demonstrated for measurements in a laboratory flame at atmospheric pressure. The combination of scanned-wavelength and wavelength-modulation minimizes interference from emission and beam steering, resulting in a robust temperature sensor that is promising for combustion control applications.

Near-infrared diode laser absorption diagnostic for temperature and water vapor in a scramjet combustor

Abstract: Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34–1.47 μm spectral region (2v1 and v1 + v3 overtone bands). Ratio thermometry was performed using direct-absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy in such situations.

High brightness single mode source of correlated photon pairs using a photonic crystal fiber

Abstract: We demonstrate a picosecond source of correlated photon pairs using a micro-structured fibre with zero dispersion around 715 nm wavelength. The fibre is pumped in the normal dispersion regime at ~708 nm and phase matching is satisfied for widely spaced parametric wavelengths. Here we generate up to 10;7 photon pairs per second in the fibre at wavelengths of 587 nm and 897 nm, while on collecting this light in single-mode-fibre-coupled Silicon avalanche diode photon counting detectors, we detect ~3.2x10;5 coincidences per second at pump power 0.5 mW.

Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan

Abstract: [1] The subducting Pacific plate acts an efficient waveguide for high-frequency signals and often produces anomalously large intensity on the eastern seaboard of northern Japan during deep earthquakes. The waveform records in the region of high intensity show a low-frequency (f 2 Hz) later arrivals with a long coda. This behavior is not explained by a simple subduction zone model comprising a high-velocity plate with low attenuation. From the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the Pacific subduction zone we demonstrate that the high-frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure. Our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von Karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0.5 km in thickness. The standard deviation of wave speed fluctuations from the averaged background model is about 2%. This new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness, but low-frequency waves, with long wavelengths, can easy tunnel through such lamina structure. The result is frequency-selective propagation characteristics with a faster low-frequency phase followed by large and high-frequency signals with very long coda. A low-wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high-frequency coda. Three-dimensional simulations, using the Earth simulator supercomputer for modeling of high-frequency seismic wave propagation in the Pacific subduction zone including plate heterogeneity, clearly demonstrate the scattering waveguide effects for high-frequency seismic waves traveling in the plate. The region of large intensity for the heterogeneous model migrates away from the hypocenter into northern Japan with an elongated zone along the Pacific coast, almost comparable to the observations from deep events in the Pacific plate.

Fe-implanted InGaAs terahertz emitters for 1.56μm wavelength excitation

Abstract: We have measured the terahertz (THz) radiations from unimplanted and Fe-implanted In0.53Ga0.47As photoconductive switches excited by the femtosecond laser pulse of 1.56μm wavelength. It has been also observed that the amplitude of the radiated wave form from these photoswitches deviates from linear behavior, and saturates with increase in the power of excitation pulse. Fe implantation to the samples leads to about 1.2 times decrease in the pulse width of radiated THz wave form and 6.5 times reduction in the carrier mobility compared to the unimplanted sample.

Kelvin wave variability near the equatorial tropopause observed in GPS radio occultation measurements

Abstract: [1] Temperature fields in the equatorial upper troposphere and lower stratosphere, derived from GPS radio occultation measurements for 2001–2002, show evidence for planetary-scale Kelvin waves. These waves have a characteristic eastward phase tilt with height and typical vertical wavelengths of ∼4–8 km. The Kelvin waves exhibit coherent vertical structure over ∼12–25 km, with maximum amplitudes near the tropical tropopause (∼17 km). The waves are often quasi-stationary near the tropopause but exhibit regular eastward propagation in the lower stratosphere (with periods near 20 days). The quasi-stationary waves modulate the climatological cold tropopause over Indonesia. The transient lower stratospheric waves show enhanced amplitudes coincident with the descending westerly shear phase of the quasi-biennial oscillation (QBO). Correlations with outgoing long-wave radiation (OLR) data show that global temperature patterns over ∼12–17 km (with characteristic Kelvin wave structure) vary coherently with transient deep convection over Indonesia.

Optical Imaging in Projection Microlithography

Abstract: From the corpuscular theory in which light propagates in the form of minute particles, to the wave theory that elucidates diffraction phenomena, to the quantum theory in which both the wave and corpuscular theories are simultaneously valid, humankind's concept of light has evolved much over the last two hundred years. The principles of optical projection lithography with which we are concerned were substantially formulated before the twentieth century, prior to the general theory of relativity, which stipulates the bending of light rays by gravitational fields. By that time, Augustin Jean Fresnel (1788-1827) had laid the wave theory of light on a firm foundation, and James Clerk Maxwell's (1831-1879) conjecture that light waves are electromagnetic had been verified by Heinrich Hertz (1857-1894). In the first three chapters of this text, we review properties of light that are relevant for analysis of image formation in photolithography. Starting with Maxwell's equations, we deduce the characteristics of light in this chapter. We shall learn that light is a transverse wave, with the electric and magnetic field vectors vibrating in a plane that is normal to its direction of propagation. When light interacts with objects whose physical dimensions are large compared with its wavelength, we can neglect the field vectors under many circumstances, and approximate Maxwell's equations by laws formulated in the language of geometry. This topic of geometrical optics is treated in Chapter 2. To describe light transmission through apertures whose dimensions are comparable to or smaller than the wavelength, however, we need to resort to diffraction theory, a subject we discuss in Chapter 3.