共查询到20条相似文献，搜索用时 31 毫秒
1.
Faist J Capasso F Sivco DL Sirtori C Hutchinson AL Cho AY 《Science (New York, N.Y.)》1994,264(5158):553-556
A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering. Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2 micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure material. 相似文献
2.
Gust D Moore TA Moore AL Lee SJ Bittersmann E Luttrull DK Rehms AA Degraziano JM Ma XC Gao F Belford RE Trier TT 《Science (New York, N.Y.)》1990,248(4952):199-201
A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (P(Zn)) and the other present as the free base (P). The metailated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (Q(A)-Q(B)). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-P(Zn)-P(.+).-Q(A)(-)-Q(B), with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C(.+)-P(Zn)-P-Q(A)-Q(B)(.-), which is formed with an overall quantum yield of 0.83 and has a lifetime of 55 microseconds. Irradiation of the free-base form of the pentad, C-P-P-Q(A)-Q(B), gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to approximately 340 microseconds. The artificial photosynthetic system preserves a significant fraction ( approximately 1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state. 相似文献
3.
Strukelj M Papadimitrakopoulos F Miller TM Rothberg LJ 《Science (New York, N.Y.)》1995,267(5206):1969-1972
Operating lifetime is the main problem that complicates the use of polymeric light-emitting diodes (LEDs). A class of electron transport (ET) polymers [poly(aryl acrylate) and poly(aryl ether)s] is reported in which moieties with high electron affinities are covalently attached to stable polymer backbones. Devices based on poly(p-phenylenevinylene) (PPV) prepared with these materials exhibited a 30-fold improvement in stability and, in one case, dramatically lower (10 volts versus about 30 volts) operating voltage relative to those having conventional ET layers. The current-carrying capacity of indium tin oxide-PPV-polymeric ET layer-aluminum LEDs was also increased by a factor of 30. These improvements lead to an enhancement in power efficiency of nearly an order of magnitude. Choosing polymers with high glass transition temperatures increases device lifetime. 相似文献
4.
Katcoff S 《Science (New York, N.Y.)》1969,166(3903):382-384
Recoils from alpha-particle decay of naturally occurring radioactive nuclides have energies between 70 and 169 kiloelectron volts. It is shown that these alpha recoils register tracks in mica, observable as etch pits, with an efficiency of about 80 percent. When the recoil energy is degraded to 40 kiloelectron volts the efficiency drops to 50 percent. But, since the decay of each thorium or uranium impurity atom in natural mica is followed by a cascade of six or eight alpha particles, the overall registration efficiency must be very nearly 100 percent. 相似文献
5.
Colombelli R Srinivasan K Troccoli M Painter O Gmachl CF Tennant DM Sergent AM Sivco DL Cho AY Capasso F 《Science (New York, N.Y.)》2003,302(5649):1374-1377
We combine photonic and electronic band structure engineering to create a surface-emitting quantum cascade microcavity laser. A high-index contrast two-dimensional photonic crystal is used to form a micro-resonator that simultaneously provides feedback for laser action and diffracts light vertically from the surface of the semiconductor surface. A top metallic contact allows electrical current injection and provides vertical optical confinement through a bound surface plasmon wave. The miniaturization and tailorable emission properties of this design are potentially important for sensing applications, while electrical pumping can allow new studies of photonic crystal and surface plasmon structures in nonlinear and near-field optics. 相似文献
6.
When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10(9) total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of up to 2 gigaelectron volts per centimeter). Electron acceleration and the appearance of high-frequency modulations in the transmitted light spectrum were both found to have sharp thresholds in laser power and plasma density. A hole in the center of the electron beam may indicate that plasma electrons were expelled radially. 相似文献
7.
Zhang SN Cui W Chen W Yao Y Zhang X Sun X Wu XB Xu H 《Science (New York, N.Y.)》2000,287(5456):1239-1241
Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS 1915+105 and GRO J1655-40 reveals a three-layered atmospheric structure in the inner region of their accretion disks. Above the cold and optically thick disk with a temperature of 0.2 to 0.5 kiloelectron volts, there is a warm layer with a temperature of 1.0 to 1.5 kiloelectron volts and an optical depth around 10. Sometimes there is also a much hotter, optically thin corona above the warm layer, with a temperature of 100 kiloelectron volts or higher and an optical depth around unity. The structural similarity between the accretion disks and the solar atmosphere suggests that similar physical processes may be operating in these different systems. 相似文献
8.
Bayer M Hawrylak P Hinzer K Fafard S Korkusinski M Wasilewski ZR Stern O Forchel A 《Science (New York, N.Y.)》2001,291(5503):451-453
We demonstrate coupling and entangling of quantum states in a pair of vertically aligned, self-assembled quantum dots by studying the emission of an interacting electron-hole pair (exciton) in a single dot molecule as a function of the separation between the dots. An interaction-induced energy splitting of the exciton is observed that exceeds 30 millielectron volts for a dot layer separation of 4 nanometers. The results are interpreted by mapping the tunneling of a particle in a double dot to the problem of a single spin. The electron-hole complex is shown to be equivalent to entangled states of two interacting spins. 相似文献
9.
A glass filter from Surveyor 3 has a surface density of approximately 1 x 10(6) tracks per square centimeter from heavy solar flare particles. The variation with depth is best fitted with a solar particle spectrum dN/dE = 2.42 x 10(6) E(-2) [in particles per square centimeter per year per steradian per (million electron volts per nucleon)], where E is the energy and N is the number of particles, from 2 million electron volts per nucleon to approximately 7 million electron volts per nucleon and dN/dE = 1.17 x 10(7) E(-3) at higher energies. Not much difference is observed between 0.5 and 5 micrometers, an indication that there is a lack of track-registering particles below 0.5 million electron volts per nucleon. The Surveyor data are compatible with track results in lunar rocks, provided an erosion rate of approximately 10(-7) centimeter per year is assumed for the latter. The results also suggest a small-scale erosion process in lunar rocks. 相似文献
10.
Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness. 相似文献
11.
Robert A Sirjean O Browaeys A Poupard J Nowak S Boiron D Westbrook CI Aspect A 《Science (New York, N.Y.)》2001,292(5516):461-464
We report the realization of a Bose-Einstein condensate of metastable atoms (helium in the lowest triplet state). The excitation energy of each atom with respect to the ground state is 20 electron volts, but inelastic processes that would destroy the sample are suppressed strongly enough in a spin-polarized sample to allow condensation. Our detection scheme takes advantage of the metastability to achieve detection of individual atoms as well as of the decay products of inelastic processes. This detection opens the way toward new studies in mesoscopic quantum statistical physics, as well as in atomic quantum optics. 相似文献
12.
Pal SK Itkis ME Tham FS Reed RW Oakley RT Haddon RC 《Science (New York, N.Y.)》2005,309(5732):281-284
An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of approximately 0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson. 相似文献
13.
Schnieder L Seekamp-Rahn K Borkowski J Wrede E Welge KH Aoiz FJ Bañiares L D'Mello MJ Herrero VJ Rábanos VS Wyatt RE 《Science (New York, N.Y.)》1995,269(5221):207-210
The H + H(2) exchange reaction constitutes an excellent benchmark with which to test dynamical theories against experiments. The H + D(2) (vibrational quantum number v = 0, rotational quantum number j = 0) reaction has been studied in crossed molecular beams at a collision energy of 1.28 electron volts, with the use of the technique of Rydberg atom time-of-flight spectroscopy. The experimental resolution achieved permits the determination of fully rovibrational state-resolved differential cross sections. The high-resolution data allow a detailed assessment of the applicability and quality of quasi-classical trajectory (QCT) and quantum mechanical (QM) calculations. The experimental results are in excellent agreement with the QM results and in slightly worse agreement with the QCT results. This theoretical reproduction of the experimental data was achieved without explicit consideration of geometric phase effects. 相似文献
14.
A method is described for increasing luminescence in poly(p-phenylene vinylene) (PPV) light-emitting diodes. Cis linkages were engineered into the PPV chain. These linkages interrupt conjugation and interfere with the packing of the polymer chains, which results in the formation of amorphous PPV. Large-area electroluminescent devices were prepared from this polymer. Devices made of an aluminum electrode, PPV as the luminescent layer, and an electron-transporting layer have internal quantum efficiencies of 2 percent, a turn-on voltage of 20 volts, and can carry current densities of 2000 milliamperes per square centimeter. The current density is at least an order of magnitude higher than previously obtained. 相似文献
15.
The atomic electron orbitals that underlie molecular bonding originate from the central Coulomb potential of the atomic core. We used scanning tunneling microscopy and density functional theory to explore the relation between the nearly spherical shape and unoccupied electronic structure of buckminsterfullerene (C60) molecules adsorbed on copper surfaces. Besides the known pi* antibonding molecular orbitals of the carbon-atom framework, above 3.5 electron volts we found atomlike orbitals bound to the core of the hollow C60 cage. These "superatom" states hybridize like the s and p orbitals of hydrogen and alkali atoms into diatomic molecule-like dimers and free-electron bands of one-dimensional wires and two-dimensional quantum wells in C60 aggregates. We attribute the superatom states to the central potential binding an electron to its screening charge, a property expected for hollow-shell molecules derived from layered materials. 相似文献
16.
Krimigis SM Armstrong TP Axford WI Bostrom CO Gloeckler G Keath EP Lanzerotti LJ Carbary JF Hamilton DC Roelof EC 《Science (New York, N.Y.)》1982,215(4532):571-577
The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies greater, similar 22 and greater, similar 28 kiloelectron volts, respectively) in Saturn's magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( approximately 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend approximately 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( approximately 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies greater, similar 200 kiloelectron volts per nucleon, H(1), H(2), and H(3) (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H(2) and H(3) suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( approximately 22 to approximately 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within approximately 1.1 degrees in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of approximately 1.5 million electron volts and smaller fluxes of electrons of approximately 10 million electron volts and of protons greater, similar 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models. 相似文献
17.
Cesium iodide, a simple ionic salt at low pressures, undergoes a second-order transformation at 40 gigapascals (400 kilobars) from the cubic B2 (cesium chloride-type) structure to the body-centered tetragonal structure. Also, the energy gap between valence and conduction bands decreases from 6.4 electron volts at zero pressure to about 1.7 electron volts at 60 gigapascals, transforming cesium iodide from a highly ionic compound to a semiconductor. The structural transition increases the rate at which the band gap closes, and an extrapolation suggests that cesium iodide becomes metallic near (or somewhat above) 100 gigapascals. Similar changes in bonding character are likely to occur in other alkali halides at pressures above 100 gigapascals. 相似文献
18.
McDonald DG 《Science (New York, N.Y.)》1990,247(4939):177-182
Ideas about quantized energy levels originated in atomic physics, but research in superconductivity has led to unparalleled precision in the measurement of energy levels. A comparison of levels produced by two Josephson junctions shows that they differ by no more than 3 parts in 10(19) at an energy of 0.0003 electron volt. The fact that the myriad of interactions of 10(12) particles in a macroscopic body, a Josephson junction, can produce sharply defined energy levels suggests a dynamical state effectively divorced from the complexities of its environment. The existence of this state, the macroscopic quantum state of superconductors, is well established, but its isolation from intrinsic perturbations has recently been shown to be extraordinary. These new results, with an improved precision of about ten orders of magnitude, are discussed in the context of highly accurate results from quantum electrodynamics, atomic spectroscopy, and the standards of metrology. Further refinements in precision may be achievable at higher energy levels, about 12 electron volts, as they become available from a new series array of 18,992 Josephson junctions. 相似文献
19.
Collier CP Mattersteig G Wong EW Luo Y Beverly K Sampaio J Raymo FM Stoddart JF Heath JR 《Science (New York, N.Y.)》2000,289(5482):1172-1175
A solid state, electronically addressable, bistable [2]catenane-based molecular switching device was fabricated from a single monolayer of the [2]catenane, anchored with phospholipid counterions, and sandwiched between an n-type polycrystalline silicon bottom electrode and a metallic top electrode. The device exhibits hysteretic (bistable) current/voltage characteristics. The switch is opened at +2 volts, closed at -2 volts, and read at approximately 0.1 volt and may be recycled many times under ambient conditions. A mechanochemical mechanism for the action of the switch is presented and shown to be consistent with temperature-dependent measurements of the device operation. 相似文献
20.
The measurement sensitivity of the pointing direction of a laser beam is ultimately limited by the quantum nature of light. To reduce this limit, we have experimentally produced a quantum laser pointer, a beam of light whose direction is measured with a precision greater than that possible for a usual laser beam. The laser pointer is generated by combining three different beams in three orthogonal transverse modes, two of them in a squeezed-vacuum state and one in an intense coherent field. The result provides a demonstration of multichannel spatial squeezing, along with its application to the improvement of beam positioning sensitivity and, more generally, to imaging. 相似文献