Silicon has been the dominant material in electronics since the invention of the integrated transistor. In contrast, silicon's indirect bandgap and vanishing second-order optical nonlinearity limit its applications in optoelectronics 1. Although all-optical components such as Raman lasers 2 , parametric amplifiers 3 and electro-optic modulators 4,5 have recently been reported, control over charge motion in silicon has only ever been achieved electronically. Here, we report all-optical generation of ultrafast ballistic electrical currents in clean, unbiased, bulk silicon at room temperature. This current injection, which provides new insights into optical processes in silicon, results from quantum interference between one-and twophoton absorption pathways across the indirect bandgap despite phonon participation and the multi-valley conduction band. The transient currents induced by 150 fs pulses are detected via the emitted THz radiation. The efficiency of this third-order optical process is surprisingly large for fundamental wavelengths in the 1,420-1,800 nm range. The use of phase-related optical beams has advanced the application of light in a variety of processes such as controlling photochemical reactions 6 , forming attosecond pulses 7 and producing high-intensity THz pulses in air 8. In directbandgap semiconductors, both charge 9,10 and pure spin currents 11,12 have been generated through quantum interference of singleand two-photon interband absorption of light at frequencies 2ω and ω with the current dependent on the phase parameter φ = 2φ ω − φ 2ω ; here φ ω,2ω is the phase of the optical beam. For electrical-current generation in Si, we use 150 fs pulses with 0.69
We report on the first creation of ultracold bosonic heteronuclear molecules of two fermionic species, 6 Li and 40 K, by a magnetic field sweep across an interspecies s-wave Feshbach resonance. This allows us to associate up to 4 × 10 4 molecules with high efficiencies of up to 50%. Using direct imaging of the molecules, we measure increased lifetimes of the molecules close to resonance of more than 100 ms in the molecule-atom mixture stored in a harmonic trap.PACS numbers: 03.75. Ss, 37.10.Pq Two-component mixtures of fermionic quantum gases have attracted much interest over the past years. In these systems, long-lived, weakly bound molecules were produced made up of two atoms of the same species in different internal states [1]. The long lifetimes observed for these molecular gases are a consequence of the Pauli principle, which suppresses three-body collisions, and hence vibrational quenching, in a system of not more than two distinguishable components [2]. More recently, the interest has shifted towards ultracold heteronuclear diatomic molecules, which can have a large electric dipole moment [3]. So far, Bose-Bose [4] and Bose-Fermi [5] dimers have been produced. However, among the ultracold heteronuclear dimers the Fermi-Fermi molecules are of special interest since they are expected to exhibit long lifetimes for the same reasons as in the homonuclear case [6]. Long-lived polar molecules open the door to the creation of a molecular Bose-Einstein condensate (BEC) [7] with anisotropic, electric dipolar interaction and show potential for precision measurements [8] and novel quantum information experiments [9]. Furthermore, the two-species Fermi-Fermi mixture may allow the realization of novel quantum phases [10,11,12] and offers the possibility to tune interactions and to conveniently apply componentselective experimental methods.In this Letter, we present the first production of ultracold diatomic molecules composed of two different fermionic atomic species. We study the creation process of the molecules, their lifetime in a molecule-atom mixture and give an upper bound for their magnetic moment.We initially create an ultracold two-species FermiFermi mixture by sympathetic cooling of the fermionic species 6 Li and 40 K with an evaporatively cooled bosonic species, 87 Rb, as described previously [13,14]. During the cooling process, the three species are confined in a magnetic trap in their most strongly confined and collisional stable states 87 Rb |F = 2, m F = 2 , 40 K |9/2, 9/2 , and 6 Li |3/2, 3/2 . For the exploitation and study of Feshbach resonances (FR), the apparatus was extended by an optical dipole trap (ODT) and a setup that allows us to apply a stable homogeneous magnetic field (FB field) of up to 1 kG in the horizontal plane. The ODT is realized by two perpendicular laser beams with the two foci coinciding at the center of the magnetic trap. The first (second) beam points along the horizontal (vertical) axis and has a 1/e 2 -radius of 55 µm (50 µm). The two beams originate from a single-mode, s...
The absorption of phase-related near-infrared fundamental ͑ , 0.7 eVഛប ഛ 0.9 eV͒ and second harmonic ͑2͒ pulses of 150 fs duration results in ballistic electrical currents in clean bulk germanium and silicon at room temperature. The ultrafast charge motion is directly monitored via a time-resolved analysis of the emitted bursts of terahertz radiation. The current generation process relies on a third-order optical nonlinearity with a current injection efficiency only slightly reduced compared to the established current injection in direct-gap semiconductors such as GaAs. In the present case, current injection takes place across the direct band gap of germanium, whereas it involves indirect optical transitions in silicon. The vector direction of the current is defined by the polarization of the two-color pump field and the relative phase ⌬⌽ =2⌽ − ⌽ 2. Microscopically, current injection can be understood as arising from the quantum interference of one-and two-photon absorption processes. In the case of silicon, these indirect optical transitions may involve different types of phonons and can occur via numerous pathways. We therefore propose a model based on third-order perturbation theory which qualitatively explains why a current injection can occur across an indirect band gap.
The manipulation of individual colloidal particles using optical tweezers has allowed vacancies to be created in two-dimensional (2d) colloidal crystals, with unprecedented possibility of real-time monitoring the dynamics of such defects (Nature 413, 147 (2001)). In this Letter, we employ molecular dynamics (MD) simulations to calculate the formation energy of single defects and the binding energy between pairs of defects in a 2d colloidal crystal. In the light of our results, experimental observations of vacancies could be explained and then compared to simulation results for the interstitial defects. We see a remarkable similarity between our results for a 2d colloidal crystal and the 2d Wigner crystal (Phys. Rev. Lett. 86, 492 (2001)). The results show that the formation energy to create a single interstitial is 12% − 28% lower than that of the vacancy. Because the pair binding energies of the defects are strongly attractive for short distances, the ground state should correspond to bound pairs with the interstitial bound pairs being the most probable.
We investigate s-wave interactions in a two-species Fermi-Fermi mixture of 6Li and 40K. We develop for this case the method of cross-dimensional relaxation and find from a kinetic model, Monte Carlo simulations, and measurements that the individual relaxation rates differ due to the mass difference. The method is applied to measure the elastic cross section at the Feshbach resonance that we previously used for the production of heteronuclear molecules. Location (B0=155.09(5) G) and width are determined for this resonance. This reveals that molecules are being produced on the atomic side of the resonance within a range related to the Fermi energies, therefore establishing the first observation of a many body effect in the crossover regime of a narrow Feshbach resonance.
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