Fringe Spacing Research Articles

Dynamically ordered thin film nanoclusters

Experimental evidence for dynamic modification of a growing thin film into ordered nanoclusters is presented. Co nanoclusters of approximately 50nm diameter were assembled into one-dimensionally ordered arrays spaced 400nm apart on Si(100) substrates during film growth. This ordered arrangement was achieved under e-beam evaporation of Co with simultaneous two-beam laser interference irradiation of the substrate. The ordering length scale was consistent with the two-beam fringe spacing of 400nm. In comparison, the unirradiated film shows a random distribution of Co clusters with average diameter of 17nm. The mechanism for assembly is tentatively attributed to thermal effects arising from the spatially periodic laser interference heating of the substrate and∕or film. This one step process, without the need of any pre- or postpatterning of the substrate or film, is promising as an economical and simple approach to assemble ordered nanostructured films.

Enhancement of the measurement sensitivity in small lateral shearing holographic interferometry

To increase the measurement sensitivity in lateral shearing holographic interferometry, it is proposed to record, without the use of nonlinear effects, a pair of interference patterns tuned to closely spaced fringes and to specify small lateral shears between interfering beams having the same magnitude but opposite directions. The measurement sensitivity is enhanced at the stage of optical processing of such a pair of holographic interference patterns upon their exact coincidence or when they are located in optically conjugate planes. According to the results of experimental verification of the method, the sensitivity of the interference patterns increased twofold and fourfold.

Monochromatic heterodyne fiber-optic profile sensor for spatially resolved velocity measurements with frequency division multiplexing

Investigating shear flows is important in technical applications as well as in fundamental research. Velocity measurements with high spatial resolution are necessary. Laser Doppler anemometry allows nonintrusive precise measurements, but the spatial resolution is limited by the size of the measurement volume to approximately 50 microm. A new laser Doppler profile sensor is proposed, enabling determination of the velocity profile inside the measurement volume. Two fringe systems with contrary fringe spacing gradients are generated to determine the position as well as the velocity of passing tracer particles. Physically discriminating between the two measuring channels is done by a frequency-division-multiplexing technique with acousto-optic modulators. A frequency-doubled Nd:YAG laser and a fiber-optic measuring head were employed, resulting in a portable and flexible sensor. In the center of the measurement volume of approximately 1-mm length, a spatial resolution of approximately 5 microm was obtained. Spatially resolved measurements of the Blasius velocity profile are presented. Small velocities as low as 3 cm/s are measured. The sensor is applied in a wind tunnel to determine the wall shear stress of a boundary layer flow. All measurement results show good agreement with the theoretical prediction.

Applications of Wide-Band-Gap Materials for Optoelectronic Functional Devices Fabricated by a Pair of Interfering Femtosecond Laser Pulses

A pair of interfering near-IR femtosecond pulses from a mode-locked Ti:sapphire laser with a regenerative amplifier have been applied to wide-band-gap materials such a lithium fluoride (LiF) to create laser-active F2 and F3+ color centers and to encode nonerasable periodic gratings with fine fringe spacings of submicrometer size simultaneously. Using such a photon-written microstructure, a distributed-feedback LiF laser oscillation based on F2 color centers has been realized at room temperature. A lasing output performance with a linewidth of less than 1 nm, slope efficiency of ∼10%, and beam divergence of ∼20 mrad was obtained at 710 nm.

Optical system for double-biprism electron holography

A novel system of electron interferometry and holography using two electron biprisms has been developed. The first biprism is installed in the image plane of the objective lens and the second one is set behind the first magnifying lens, inside the shadow area of the first biprism. The system can independently control two important parameters for interferograms and holograms, the fringe spacing and interference width. Thus, it gives us more flexibility on performing electron interferometry and holography. The good performance of the system was demonstrated using a 1MV field-emission electron microscope. We introduce a variety of optical set-ups for the system and explain the advantages of each set-up in detail, with experimental results.

Distributed‐feedback color center lasers in wide‐band gap materials fabricated by a pair of chirped femtosecond pulses

A room-temperature-operated, pulsed distributed-feedback (DFB) LiF:F2 color center laser was achieved. A distributed-feedback structure, based on non-erasable periodic gratings of laser-active color centers with fringe spacings as fine as ∼510 nm, was fabricated by a pair of chirped near-IR femtosecond laser pulses. The DFB LiF:F2 laser outputs at 710 nm were obtained with a linewidth of less than ∼1 nm, the slope efficiency ∼10% and beam divergence ∼20 mrad. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cosmic Microwave Background Observations with a Compact Heterogeneous 150 GHz Interferometer in Chile

We report on the design, first observing season, and analysis of data from a new prototype millimeter-wave interferometer, MINT. MINT consists of four 145 GHz SIS mixers operating in double-sideband mode in a compact heterogeneous configuration. The signal band is subdivided by a monolithic channelizer, after which the correlations between antennas are performed digitally. The typical receiver sensitivity in a 2 GHz band is 1.4 mK s1/2. The primary beams are 045 and 030 FWHM, with fringe spacing as small as 01. MINT observed the cosmic microwave background (CMB) from Cerro Toco, in the Chilean Altiplano. The site quality at 145 GHz is good, with median nighttime atmospheric temperature of 9 K at zenith (exclusive of the CMB). Repeated observations of Mars, Jupiter, and a telescope-mounted calibration source establish the phase and magnitude stability of the system. MINT is the first interferometer dedicated to CMB studies to operate above 50 GHz. The same type of system can be used to probe the Sunyaev-Zel'dovich effect in galaxy clusters near the SZ null at 217 GHz. We give the essential features of MINT and present an analysis of sideband-separated, digitally sampled data recorded by the array. Based on 215 hours of data taken in late 2001, we set an upper limit on the CMB anisotropy in a band of width ?? = 700 around ? = 1540 of ?T < 105??K (95% confidence). Increased sensitivity can be achieved with more integration time, greater bandwidth, and more elements.

Direct observation of electrostatic microfields by four-electron-wave interference using two electron biprisms

A new method has been developed for directly visualizing electromagnetic microfields in real time by the interference of four-electron waves obtained using a transmission electron microscope equipped with a field-emission gun and two electron biprisms. When one object wave and three reference waves interfere, equal-phase lines of the object wave are displayed as the intensity modulation of the interference fringes, in the manner same as in the three-electron-wave interference. The advantage of the four-electron-wave interference is that equal-phase lines are observed without precisely adjusting the angle between the two biprisms or rigorously controlling the fringe spacings of the two biprisms.

High-resolution observation by double-biprism electron holography

High-resolution electron holography has been achieved by using a double-biprism interferometer implemented on a 1MV field emission electron microscope. The interferometer was installed behind the first magnifying lens to narrow carrier fringes and thus enabled complete separation of sideband Fourier spectrum from center band in reconstruction process. Holograms of Au fine particles and single-crystalline thin films with the finest fringe spacing of 4.2pm were recorded and reconstructed. The overall holography system including the reconstruction process performed well for holograms in which carrier fringes had a spacing of around 10pm. High-resolution lattice images of the amplitude and phase were clearly reconstructed without mixing of the center band and sideband information. Additionally, entire holograms were recorded without Fresnel fringes normally generated by the filament electrode of the biprism, and the holograms were thus reconstructed without the artifacts caused by Fresnel fringes.

Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer

An asymmetric surface brightness distribution of the rapidly rotating A7IV-V star, Altair, has been measured by the Navy Prototype Optical Interferometer (NPOI). The observations were recorded simultaneously using a triangle of three long baselines of 30m, 37m, and 64m, on 19 spectral channels, covering the wavelength range of 520nm to 850nm. The outstanding characteristics of these observations are (a) high resolution with the minimum fringe spacing of 1.7mas, easily resolving the 3-milliarcsecond (mas) stellar disk, and (b) the measurement of closure phase which is a sensitive indicator to the asymmetry of the brightness distribution of the source. Uniform disk diameters fit to the measured squared visibility amplitudes confirms the Altair's oblate shape due to its rapid rotation. The measured observables of Altair showed two features which are inconsistent with both the uniform-disk and limb-darkened disk models, while the measured observable of the comparison star, Vega, are consistent with the limb-darkened disk model. The first feature is that measured squared visibility amplitudes at the first minimum do not reach 0.0 but rather remain at about 0.02, indicating the existence of a small bright region on the stellar disk. The other is that the measured closure phases show non-zero/180 degrees at all spectral channels, which requires an asymmetric surface brightness distribution. We fitted the measured observables to a model with a bright spot on a limb-darkened disk and found the observations are well reproduced by a bright spot, which has relative intensity of 4.7%, on a 3.38 mas limb-darkened stellar disk. Rapid rotation of Altair indicates that this bright region is a pole, which is brighter than other part of the star owing to gravity darkening.

Off-axis electron holography with a dual-lens imaging system and its usefulness in 2-D potential mapping of semiconductor devices

A variable magnification electron holography, applicable for two-dimensional (2-D) potential mapping of semiconductor devices, employing a dual-lens imaging system is described. Imaging operation consists of a virtual image formed by the objective lens (OL) and a real image formed in a fixed imaging plane by the objective minilens. Wide variations in field of view (100–900nm) and fringe spacing (0.7–6nm) were obtained using a fixed biprism voltage by varying the total magnification of the dual OL system. The dual-lens system allows fringe width and spacing relative to the object to be varied roughly independently from the fringe contrast, resulting in enhanced resolution and sensitivity. The achievable fringe width and spacing cover the targets needed for devices in the semiconductor technology road map from the 350 to 45nm node. Two-D potential maps for CMOS devices with 220 and 70nm gate lengths were obtained.

Method of characterization of effective shrinkage in reflection holograms

A method is described to evaluate plane-wave volume hologram fringe inclination and spacing, as well as the recording material average refraction index. This evaluation allows us to estimate changes in the hologram parameters due to recording material shrinkage. The method is based on measurements of the incident and diffracted beam Bragg angles for two different wavelengths. The conditions for minimizing measurement errors are obtained. The experimental data are in good agreement with the calculated data.

Double-biprism electron interferometry

Electron holography based on two electron biprisms was developed. The upper biprism was installed just on the image plane of the objective lens, and the lower one was set between the crossover point and image plane of the magnifying lens. This system was able to control two important parameters of the hologram—fringe space and width of interference region—independently. The system enabled us to perform electron holography and interferometry more flexibly. We confirmed the good performance of the system and did preliminary applications using a 1-MV field-emission electron microscope.

Relaxation and diffusion of photoexcited carriers in ZnO epitaxial thin films

Relaxation and diffusion of carriers in an electron-hole plasma (EHP) in ZnO epitaxial thin films were studied by femtosecond transient grating spectroscopy. By performing several experiments of different fringe spacing, the lifetime and diffusion coefficient of the EHP state are determined to be 23.8 ps and 10.8 cm2/s, respectively. The observed lifetime is in good agreement with the previous values obtained by pump-probe and time-resolved optical Kerr gate luminescence measurements. Based on a simple diffusion equation, we conclude that the carrier diffusion in the EHP state does not contribute to reduction of the carrier population within such a short carrier lifetime of a few tens picosecond. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of local deformations in heterostructures containing short period superlattices by high-resolution transmission electron microscopy

This work describes the application of the Lattice Fringe Spacing Measurement (LFSM) method to the study of complex multiquantum well heterostructures containing both low-misfit and strain compensated short period superlattices in barriers and wells, respectively. 90°-wedge cross-sectional samples have been used. The adequate choice of both experimental conditions and digitized sampling allows the whole heterostructure to be visualized and studied in a single High Resolution Transmission Electron Microscopy (HRTEM) image. Sample preparation and image processing technique are simple and inexpensive, resulting a fast procedure particularly suited for the analysis of large areas. By this way, in a single HRTEM image we have measured, in the growth direction, the lattice spacings at either side of the multiple grown interfaces as well as the period variations of both types of superlattices; in addition, we have measure on the same image the lattice strain in a direction perpendicular to the growth direction by using the LFSM and the Cumulative Sum methods. We have observed local lateral variations within the wells, with regions tensile or compressively strained, while a vestige of the grown SL remains, indicating the occurrence of a strain induced lateral composition modulation process spontaneously produced during the growth of strain compensated short-period superlattices. This is further confirmed in cross-section prepared by the tripod mechanical polisher method.

Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings.

We demonstrate implementation of an all-fiber Mach-Zehnder interferometer formed in a photonic crystal fiber (PCF). We formed the all-PCF Mach-Zehnder interferometer by mechanically inducing two identical long-period fiber gratings (LPGs) in the PCF. The spectral properties of a LPG and a LPG pair were investigated. The interference fringe formed within the stop band of the LPG pair varied with the period and the strength of the gratings, and the fringe spacing was decreased with increasing grating separation. From the fringe spacing measurement the differential effective group index of the PCF was calculated to be deltam approximately equal to 2.8 x 10(-3).

Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses

We report a new technique to fabricate both laser-active F2 and F3+ colour centres in lithium fluoride and permanent periodic gratings with fringe spacings as fine as sub-micron size simultaneously by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties of such colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as X-rays. Moreover, the present technique is applied to the first production of three-dimensional active channel waveguide and a pulsed distributed-feedback (DFB) laser at around 700nm in LiF containing F2 colour centres with fine-pitched micro-grating structures.

Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates

Petrographic, mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from ‘ hot’ deserts and Antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments. Terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of Fe,Ni metal, producing Fe-oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins. Troilite weathers readily in ‘ hot’ desert environments but undergoes very little alteration under Antarctic conditions. Most of the primary porosity of ordinary chondrites has been occluded by the time that ∼ 15 to 25% of the initial Fe 0 and Fe 2+ has been oxidised to Fe 3+ in both environments. Results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products. Dilation of primary pores and brecciation, which has been observed in parts of some meteorites, will only occur if the meteorite is especially metal-rich, or has a low primary porosity. These weathering products are absent from recent falls but have formed in a fall after ∼ 100 yr of museum storage. Cl-bearing akaganéite and hibbingite are common weathering products in Antarctic finds but occur in abundance in only one ‘ hot’ desert meteorite, Daraj 014. The majority of Fe-rich weathering products in meteorites from both environments contain low, but variable concentrations of Si, Mg and Ca. In most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a ∼ 1.0-nm lattice fringe spacing; where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene. In the heavily-weathered Antarctic finds ALHA 78045 and 77002, Si is concentrated in cronstedtite, a Fe-rich phyllosilicate. An unidentified hydrous Si-Fe-Ni-Mg mineral or gel has also partially replaced taenite in ALHA 78045. In addition to Fe-rich weathering products, ‘ hot’ desert meteorites contain sulphates, Ca-carbonate and silica, whereas such minerals are largely absent from Antarctic finds. The abundance of silicate weathering products in Antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments. As preterrestrial cronstedtite is abundant in CM2 carbonaceous chondrites, the Antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites.

Dynamic spatial ordering of thin film nanostructures by rapid spatio-temporal surface modulations

AbstractWe present novel results showing that the application of a rapid spatio-temporal surface modulation in-situ with film deposition directs the assembly of ordered nanostructures. Co nanoclusters of approximately 50 nm in size were assembled into one-dimensionally ordered arrays spaced 400 nm apart on Si (100) substrates during film growth of a 0.5 nm thick film. This ordered arrangement was achieved under e-beam evaporation of Co with simultaneous two-beam pulsed laser interference irradiation. The ordering length scale was consistent with the theoretical two-beam fringe spacing. For a thicker Co film, the particles were irregularly shaped, like that of a rapidly solidified liquid-like structure. From this evidence, the mechanism for ordering is partly attributed to thermal effects due to the spatially periodic laser interference heating of the cobalt nanoparticles. This dynamic in-situ process, without the need of any pre-or post-patterning of the substrate or film, is promising as an economical and simple approach to assemble ordered nanostructured films. The author can be reached via e-mail at ramkik@wuphys.wustl.edu

Studying Star Formation with the Keck Interferometer

The Keck Interferometer utilizes the two 10-meter Keck telescopes as a direct detection interferometer in the infrared. The 85-meter baseline produces a fringe spacing of 5 milliarcseconds at 2 microns, or 0.7 AU for sources in the Taurus star forming region. The first observations with the Keck Interferometer of T Tauri and Herbig Ae/Be sources are presented and the implications for circumstellar disk models are discussed.