Search

C60 molecules absorbed on a monoatomic Cu step have been reversibly repositioned at room temperature with the tip of a scanning tunneling microscope by performing controlled displacements along the step direction. We demonstrate the feasibility of building an abacus on the nanometer scale using single molecules as ‘‘counters,’’ Cu monoatomic steps as ‘‘rods’’ that constrain the molecular motion to one dimension, and the scanning tunneling microscope as an ‘‘actuator’’ for counting operations. © 1996 American Institute of Physics [ABSTRACT FROM AUTHOR]

Investigates the effects of electron beam irradiation on the luminescence and microstructure of n-type porous silicon (PS) by photoelectrochemical etching. Use of cathodoluminescence (CL) in the scanning electron microscope; Decrease of CL with irradiation; Performance of the CL of PS in scanning electron microscopy.

Develops an approach for the examination of chemically etched (CE) porous silicon layer by transmission electron microscopy (TEM). Fabrication of the CE layer following plan-view TEM sample preparation; Existence of crystalline, polycrystalline and amorphous phases; Transition of microstructure from crystalline to amorphous during chemical etching.

Describes the design and performance of a planar microwave radio frequency-superconducting quantum interference devices (SQUID) by a cryogenic preamplifier. Composition of the SQUID-sensor; Employment of a microwave high electron mobility transistor by the cryogenic preamplifier; Measurement of flux noise and fractional step rise.

The properties of rhodium (Rh) as a deep-level dopant in InGaAs lattice matched to InP grown by molecular beam epitaxy are investigated. When InGaAs:Rh is used as an ultrafast photoconductor, carrier lifetimes as short as 100 fs for optically excited electrons are measured. Rh doping compensates free carriers so that a near intrinsic carrier concentration can be achieved. At the same time, InGaAs:Rh exhibits a large electron mobility of 1000 cm2/V s. Therefore, this material is a very promising candidate for application as a semi-insulating layer, THz antenna, or semiconductor saturable absorber mirror. [ABSTRACT FROM AUTHOR]

Surface acoustic waves (SAWs) generated in a piezoelectric substrate couple with a liquid according to Snell's law such that a compressional acoustic wave propagates obliquely at a Rayleigh angle ( θ t ) inside the microchannel to form a region devoid of a direct acoustic field, which is termed a microchannel anechoic corner (MAC). In the present study, we used microchannels with various heights and widths to characterize the width of the MAC region formed by a single travelling SAW. The attenuation of high-frequency SAWs produced a strong acoustic streaming flow that moved the particles in and out of the MAC region, whereas reflections of the acoustic waves within the microchannel resulted in standing acoustic waves that trapped particles at acoustic pressure nodes located within or outside of the MAC region. A range of actuation frequencies and particle diameters were used to investigate the effects of the acoustic streaming flow and the direct acoustic radiation forces by the travelling as well as standing waves on the particle motion with respect to the MAC region. The width of the MAC ( w c ), measured experimentally by tracing the particles, increased with the height of the microchannel ( h m ) according to a simple trigonometric equation w c = h m × tan ( θ t ). [ABSTRACT FROM AUTHOR]

Continuous tuning of a room-temperature-operated distributed feedback quantum cascade laser in the range of 0.6 cm-1 (20 GHz) was achieved using 1.3 μm telecom diode laser as a pumping source. We demonstrate wavelength modulation of the quantum cascade laser at frequencies up to 300 MHz. [ABSTRACT FROM AUTHOR]

Through isotopic self-diffusion measurements, the present work resolves a discrepancy in the literature about the primary oxygen-related point defect in rutile TiO2 by showing that suitably prepared surfaces can controllably inject large numbers of an exceptionally mobile defect. Results strongly suggest that this defect is the oxygen interstitial, whose existence in TiO2 has been predicted computationally but never experimentally confirmed. The surface pathway offers an approach for replacing donor oxygen vacancies with acceptor oxygen interstitials facilitating manipulation of near-surface electronic bands. [ABSTRACT FROM AUTHOR]

The forming-free (FF) and forming-required (FR) resistive switching behaviors of Pt/TaOx/Pt structures were analyzed. Changes in the length and shape of the conducting filaments (CFs) suggested different evolution processes in the FF and FR cells when the CFs grew from weaker filaments to stronger ones. In the FR cells, the filament changed from a conical shape to a cylindrical shape with no change in length. In the FF cells, the filament maintained a cylindrical shape while increasing its diameter and decreasing its length. [ABSTRACT FROM AUTHOR]

The output modal content of the oxide-confined vertical-cavity surface-emitting lasers (VCSELs) crucially depends upon the thickness of the low-index oxide aperture, its position with respect to the standing waves of the transverse-longitudinal modes and the separation from the cavity. Three-dimensional cold-cavity optical modes of typical AlGaAs/GaAs VCSELs at 850 nm were simulated to study these dependencies quantitatively taking into account the field diffraction and the material dispersion. Modification of one or two periods of the distributed Bragg reflector by positioning the thin oxidized aperture layers in the mode nodes allows single-mode regime to extend to the aperture diameters as large as 10 μm. [ABSTRACT FROM AUTHOR]

Well-ordered striped structures are developed from alkanethiol self-assembled monolayers on an Au(111) surface following well-controlled annealing processes. We demonstrate here that such regular concave–convex molecular structures can be used as the templates for growing one-dimensional molecular and atomic chains. By depositing C[sub 60] molecules onto the striped surface, C[sub 60] bimolecular chains are self-assembled. Due to the breaking of C–S bonds under certain conditions, residual S atoms can form a S monoatomic chain between two adjacent stripe pairs of thoroughly lying-down molecules. Possible growth mechanisms are discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present very compact Nd:YVO[sub 4] lasers which are passively mode-locked with a semiconductor saturable absorber mirror at very high repetition rates between 39 and 59 GHz. We achieved between 30 and 80 mW of average output power and pulse durations of 4.8-5.5 ps. As the pulse-to-pulse spacing is only 17 ps at 59 GHz, we are approaching the limit in repetition rate which is set by the pulse duration. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultrabroadband semiconductor saturable absorber mirrors (SESAMs) are required to support self-starting sub-10-fs-pulse generation with Ti:sapphire lasers. Conventional Al[sub x]Ga[sub 1-x]As/AlAs SESAMs are limited by the reflection bandwidth of about 60 nm of the bottom Bragg mirror. In this letter, we demonstrate a GaAs saturable absorber which is epitaxially grown on CaF[sub 2] using molecular-beam epitaxy. Even though the difference of the thermal expansion coefficient is very large, we were able to demonstrate good modulation depth with small nonsaturable losses. This is interesting for ultrabroadband SESAMs because the large refractive-index difference between CaF[sub 2] and Al[sub x]Ga[sub 1-x]As results in very broadband Al[sub x]Ga[sub 1-x]As/CaF[sub 2] Bragg mirrors extending over about a 400-nm-wide reflection bandwidth for a center wavelength of 850 nm. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated optical nonlinearity in low-temperature (LT) molecular-beam-epitaxy-grown GaAs/AlAs multiple quantum wells (MQWs). Minimum saturation intensity, that is, maximum optical nonlinearity, is observed at around the excitonic absorption peak. The saturation intensity of the LT MQW is smaller by an order of magnitude than that of LT bulk GaAs. The response time of the LT MQW is 1/4-1/2 of the LT GaAs, and becomes less than 1 ps, corresponding to ∼1/400 of the standard-temperature-grown MQW. These results demonstrate a clear advantage of the room-temperature excitons in the LT MQW having large optical nonlinearity as well as fast response time. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Joule heating is one of the dominant mechanisms determining the transverse mode formation in vertical-cavity surface-emitting lasers at high injection currents. We give experimental evidence that in this operation regime, strong heating results in local gain suppression in the center of the laser, which overbalances the confining effect of thermal lensing, and thus favors the formation of high order modes. From our investigations of small aperture devices, we conclude that efficient heat removal is crucial for achieving single mode emission at high injection currents. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A two-step metalorganic chemical vapor deposition growth process is used to fabricate antiguided vertical-cavity surface-emitting lasers (VCSELs) incorporating a simplified-antiresonant reflecting optical waveguide (S-ARROW) design. Preliminary results show single-mode cw operation up to 1 mW output power from a 12 μm-diam (λ=930 nm) S-ARROW VCSEL with a large lateral index step (Δn=0.1). Modal discrimination in the S-ARROW-VCSEL is calculated using a fiber-mode approximation and device optimization for high-single-mode powers is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have templated Cu(100) surfaces with self-assembled arrays of atomic nitrogen islands and then used these islands as masks for Co growth. This method of nanolithography enables the creation of Co dot arrays with dot densities 4x10[sup 12] cm[sup -2] (27 teradots/in.[sup 2]). Adjusting the nitrogen coverage also enabled the creation of arrays of Co lines spaced 10 nm (0.01 μm) apart. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The thickness and position of an oxide layer inside a vertical-cavity surface-emitting laser structure have been optimized for minimum optical scattering loss. In the resulting structure, the index guiding provided by the oxide aperture is very small. Consequently, for an oxide aperture radius <2 μm, the optical mode is only weakly confined. In devices using such small apertures, the formation of a thermal lens has a strong influence on optical guiding. The thermal lens leads to lower threshold currents and increased differential efficiency with continuous wave as compared to pulsed injection operation in devices with small apertures. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate that the differential transmission (DT) response of bulk semiconductors excited well above the band edge can be manipulated by chirping of the broadband excitation and readout pulses. In particular, the maximum transmission change in spectrally integrated DT experiments can be modified on the 20 fs time scale. Spectrally resolved DT studies explain this chirp dependence. Depending on the sign of the chirp, positive or negative DT contributions at low or high photon energies are probed with varying efficiency around zero time delay. These results demonstrate that chirp can become an additional degree of freedom for the optimization of device performance in ultrafast all-optical switching. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An injection distributed-feedback vertical-cavity surface-emitting laser (VCSEL) with tunnel junctions served as quasi-Ohmic intercontacts (tunnel-junction-connected distributed-feedback VCSEL) is proposed. A periodic structure of vertically stacked double-heterostructure laser diodes connected by low-resistance tunnel junctions forms a vertical distributed-feedback (DFB) laser medium. To minimize the threshold, the DFB structure is placed in a Fabry–Perot cavity designed to match gain layers with the maximums of the optical mode, and the tunnel junctions—with its minimums. The passive regions with tunnel junctions provide effective vertical injection into each active region of this multiple-active-region laser. This DFB VCSEL is expected to have an improved performance, specifically, reduced threshold current and heightened output power. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We passively Q-switched a diode-pumped Er/Yb:glass microchip laser at a 1.535 μm wavelength using semiconductor saturable absorber mirrors and demonstrated pulses as short as 1.2 ns. By varying the design parameters of the saturable absorber, the pump power, and the pump spot size, we achieved repetition rates from 300 Hz to 100 kHz with pulse energies up to 4 μJ. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Manipulation of Ag nanocrystals derived from up to 200 000 atoms was performed using the tip of a scanning tunneling microscope. By varying the scanning conditions, it was possible to move them laterally on contamination-free Si(111)-7×7 surfaces or to remove them. In both cases, thin Ag tracks were left behind. This demonstrates the concepts of nano patterning and nano painting with metals on clean semiconductor surfaces. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vertical cavity surface emitting lasers (VCSELs) are demonstrated with high-index-contrast native-oxide-based (Al[sub x]O[sub y]) distributed Bragg reflectors (DBRs) on both sides of a “2λ” cavity, thus creating a compact (thin, ∼2.8 μm) laser structure. Selective oxidation of high Al composition Al[sub x]Ga[sub 1-x]As layers yields a structure with a four period upper Al[sub x]O[sub y]/GaAs DBR, a 5.5 period lower Al[sub x]O[sub y]/GaAs DBR, and a buried oxide current aperture. A reverse-biased tunnel contact junction provides hole injection via lateral electron current between the upper DBR and the oxide aperture layer. These VCSELs operate with submilliampere thresholds, high spontaneous efficiencies, and excellent polarization control. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Highly regular arrays of steps are produced on vicinal Si(111)7×7. The step edges are atomically straight for up to 2×10[sup 4] lattice sites. The terraces are single domain, which produces a minimum kink width of 2.3 nm (half a 7×7 unit cell) and thus a high barrier for creating kinks. Criteria for obtaining optimum step arrays are established, such as the miscut [approx. 1° towards (1¯1¯2)] and an annealing sequence which passes through step bunching regions quickly. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The resonant wavelengths and mode radii of the dominant and certain higher order TEM[sub mp] modes supported by an oxide apertured vertical cavity surface emitting lasers are determined using a scalar variational formula. The reflectivity of the Bragg mirrors and the photon lifetime are calculated for each mode and both are found to decrease with decreasing oxide aperture diameter and increasing mode number. The aperture diameter, thickness, and axial position in the cavity, together with the refractive index contrast ratio used in the Bragg mirrors, are shown to be important in controlling the suppression of higher order modes. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]