All-optical subcycle microscopy on atomic length scales.

Bringing optical microscopy to the shortest possible length and time scales has been a long-sought goal, connecting nanoscopic elementary dynamics with the macroscopic functionalities of condensed matter. Super-resolution microscopy has circumvented the far-field diffraction limit by harnessing optical nonlinearities1. By exploiting linear interaction with tip-confined evanescent light fields2, near-field microscopy3,4 has reached even higher resolution, prompting a vibrant research field by exploring the nanocosm in motion5-19. Yet the finite radius of the nanometre-sized tip apex has prevented access to atomic resolution20. Here we leverage extreme atomic nonlinearities within tip-confined evanescent fields to push all-optical microscopy to picometric spatial and femtosecond temporal resolution. On these scales, we discover an unprecedented and efficient non-classical near-field response, in phase with the vector potential of light and strictly confined to atomic dimensions. This ultrafast signal is characterized by an optical phase delay of approximately π/2 and facilitates direct monitoring of tunnelling dynamics. We showcase the power of our optical concept by imaging nanometre-sized defects hidden to atomic force microscopy and by subcycle sampling of current transients on a semiconducting van der Waals material. Our results facilitate access to quantum light-matter interaction and electronic dynamics at ultimately short spatio-temporal scales in both conductive and insulating quantum materials.

Periodic Charging of Individual Molecules Coupled to the Motion of an Atomic Force Microscopy Tip.

Individual molecules at the edges of self-assembled islands grown on Ag(111) can be deliberately switched in their charge state with the electric field from a scanning-probe tip. Close to the threshold voltage for a charge state transition, periodic switching of the charge is directly driven by the cantilever motion in frequency-modulated atomic force microscopy (AFM), as can be deduced from the signature in the measured frequency shift. In this regime, the integrated frequency shift yields the tip-sample force that is due to a single additional electron. Further, the signature of the dynamic charging response provides information on the electronic coupling of the molecule to the substrate. In analogy to previous experiments on quantum dots, this may also be used in the future to access excited state properties of single molecules from AFM experiments.

Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy.

Kelvin probe force spectroscopy was used to characterize the charge distribution of individual molecules with polar bonds. Whereas this technique represents the charge distribution with moderate resolution for large tip-molecule separations, it fails for short distances. Here, we introduce a novel local force spectroscopy technique which allows one to better disentangle electrostatic from other contributions in the force signal. It enables one to obtain charge-related maps at even closer tip-sample distances, where the lateral resolution is further enhanced. This enhanced resolution allows one to resolve contrast variations along individual polar bonds.

Manipulation of the charge state of single Au atoms on insulating multilayer films.

We show charge-state manipulation of single Au adatoms on 2-11 monolayer (ML) thick NaCl films on Cu surfaces by attaching or detaching single electrons via the tip of an atomic force microscope (AFM). Tristate charge control (neutral, negatively charged, and positively charged) is achieved. On Cu(100) and Cu(111) supports, charge tristability is achieved independently of the NaCl layer thickness. In contrast, on Cu(311), only Au anions are stable on the thinnest NaCl films, but neutral and positive charge states become sufficiently long lived on films thicker than 4 ML to allow AFM-based charge-state-manipulation experiments.

Controlling the orbital sequence in individual Cu-phthalocyanine molecules.

We report on the controlled change of the energetic ordering of molecular orbitals. Negatively charged copper(II)phthalocyanine on NaCl/Cu(100) undergoes a Jahn-Teller distortion that lifts the degeneracy of two frontier orbitals. The energetic order of the levels can be controlled by Au and Ag atoms in the vicinity of the molecule. As only one of the states is occupied, the control of the energetic order is accompanied by bistable changes of the charge distribution inside the molecule, rendering it a bistable switch.

Confinement and interaction of single indirect excitons in a voltage-controlled trap formed inside double InGaAs quantum Wells.

Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.

Self-organized patterning of an insulator-on-metal system by surface faceting and selective growth: NaCl/Cu(211)

We report experimental results on an insulator-on-metal system which is inherently unstable against lateral pattern formation on the nanometer scale. NaCl deposition on Cu(211) at substrate temperatures >300 K leads to faceting into (311) and (111) facets and selective NaCl growth on (311) facets only, thereby creating alternating stripes of bare Cu and NaCl-covered areas. The mesoscopic restructuring process is brought about by (1) the tendency to form (100)-terminated NaCl layers, (2) epitaxial matching between NaCl(100) and Cu(311), and (3) sufficient mobility of the Cu substrate surface.

The problem of polyspermy in in vitro fertilization.

Polyspermy is a potential complication of attempts at in vitro fertilization. Nine polyspermic oocytes were identified among 169 oocytes obtained from 67 cycles stimulated by human menopausal gonadotropin. Cleavage to the 2-, 3-, and 4-cell stage was observed. Four polyspermic oocytes were identified among 85 oocytes obtained from 47 cycles stimulated by clomiphene citrate. Cleavage was not observed. Careful dispersion of cumulus cells at 15 to 18 hours and examination of the oocytes for polyspermy is essential, because the condition may not be apparent at 40 hours from insemination, when normal-appearing cleavage stages may be observed.

Accumulation, nature, and possible functions of the malachite green affinity material in ejaculated human spermatozoa.

An electron microscopic study was conducted on human sperm over an incubation period of 5 to 6 hours in Tyrode's solution at room temperature. Examination of aliquots of the cells, fixed at timed intervals, with glutaraldehyde, malachite green, and postosmication revealed that malachite green affinity material (MGA-M) was barely discernible at first but did accumulate considerably upon standing. Biochemical analysis of MGA-M, which is extractable by glutaraldehyde, revealed that MGA-M is a mixture of extractable phospholipids and of their lyso-derivatives. Some of these substances have fusogenic properties; i.e., they are able to fuse together the membranes of two different cells. The appearance and accumulation of these fusogens occurred during the incubation period of 5 to 6 hours, which was previously shown to be required to capacitate human sperm in vitro. It is probable, therefore, that human sperm, during their initial period of incubation either in vivo or in vitro, not only become capacitated and undergo the acrosome reaction but also develop the fusogenic substance(s) which are necessary for the imminent fusion of their plasma membrane to the vitelline membrane of the mature oocyte.

Initial evaluation of a convection counter streaming galvanization technique of sex separation of human spermatozoa.

A new method for separating X and Y human spermatozoa called convection counter streaming galvanization was evaluated. The method was independently performed by this semenology laboratory with the use of the special separation equipment and extending media provided by its developer, Dr. Bhairab C. Bhattacharya. The mean number of Y spermatozoa increased from 48% to 77% in the separated fraction predicted to be Y-enriched. The fraction predicted to be X-enriched increased from a mean of 52% to 77%. The one separation process allowed accumulation of both enriched fractions simultaneously. The separated portions of spermatozoa maintained good motility and penetration of cervical mucus but produced a mean recovery concentration in the X- and Y-enriched fractions of only 15% to 16% of the preseparation concentration.

Combined screening laparoscopy and timed follicle aspiration for human in vitro fertilization.

Twenty-seven patients underwent ultrasonographic and clinical monitoring of a spontaneous cycle so that human chorionic gonadotropin administration could be timed in an attempt to schedule the screening laparoscopy at the optimal time for oocyte aspiration. Inadequate ovarian access was encountered in only 2 of 26 laparoscopies. Seven patients had ovulated by the time of scheduled laparoscopy. Nine mature, seven immature, and three postovulatory oocytes were obtained, and seven embryo transfers were accomplished. The number of eggs obtained per laparoscopy was 0.73. The attempt to control the time of ovulation was unpredictable, few mature oocytes were obtained, and the procedure offered little ultimate benefit to the patient.

Direct evidence for the effect of quantum confinement of surface-state electrons on atomic diffusion.

We report on the direct observations of the effect of quantum confinement of surface-state electrons on atomic diffusion. Confined electronic states induced by open nanoscale resonators [consisting of two parallel monatomic Cu chains on Cu(111)] are studied by means of scanning tunneling microscope measurements and first-principles calculations. Strongly anisotropic diffusion of adatoms around and inside resonators is revealed at low temperatures. The formation of diffusion channels and empty zones is demonstrated. We show that it is possible to engineer atomic diffusion by varying the distance between the resonator walls.

Ionic films on vicinal metal surfaces: enhanced binding due to charge modulation.

NaCl films on Cu(311) exhibit a remarkably strong and localized binding between adlayer and substrate. The binding sites of the ions in the NaCl film with respect to the Cu surface are determined from atomically resolved scanning tunneling microscopy images. A new model is proposed in which the binding mechanism is controlled by the charge modulation of a regularly stepped surface due to the Smoluchowski effect. This model can be extended to explain the growth of ionic adlayers on regularly stepped and kinked metal surfaces in general.

Substrate mediated long-range oscillatory interaction between adatoms: Cu /Cu(111).

A quantitative study of the long-range interaction between single copper adatoms on Cu(111) mediated by the electrons in the two-dimensional surface-state band is presented. The interaction potential was determined by evaluating the distance distribution of two adatoms from a series of scanning tunneling microscopy images taken at temperatures of 9-21 K. The long-range interaction is oscillatory with a period of half the Fermi wavelength and decays for larger distances d as 1/d(2). Five potential minima were identified for separations of up to 70 A. The interaction significantly changes the growth of Cu/Cu(111) at low temperatures.