Your browser doesn't support javascript.
loading
Show: 20 | 50 | 100
Results 1 - 20 de 41
Filter
Add more filters










Publication year range
2.
Phys Rev Lett ; 109(26): 267402, 2012 Dec 28.
Article in English | MEDLINE | ID: mdl-23368617

ABSTRACT

We report the measurement of field-field and photon-photon correlations of light scattered by two InAs quantum dots separated by ≈40 µm. Near 4 K a large fraction of photons can be scattered coherently by each quantum dot leading to one-photon interference at a beam splitter (visibility ≈20%). Simultaneously, two-photon interference is also observed (visibility ≈40%) due to the indistinguishability of photons scattered by the two different quantum emitters. We show how spectral diffusion accounts for the reduction in interference visibility through variations in photon flux.

4.
Phys Rev Lett ; 99(18): 187402, 2007 Nov 02.
Article in English | MEDLINE | ID: mdl-17995437

ABSTRACT

We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(tau), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3 microeV. Second-order correlation measurements further confirm nonclassical light emission.

5.
Nano Lett ; 6(12): 2920-4, 2006 Dec.
Article in English | MEDLINE | ID: mdl-17163731

ABSTRACT

Using time-resolved photoluminescence spectroscopy, we have studied the Purcell spontaneous emission enhancement provided by a novel type of microcavity that forms a fully buried, all-epitaxial semiconductor heterostructure. The quantum dot containing region and the cavity boundaries are simultaneously defined in a unique way and lead to spatially self-aligned emitters. We demonstrate post-growth control of the quality factor and the capability of directly imaging the spatial field distribution that critically impacts the Purcell effect.

6.
Phys Rev Lett ; 97(20): 206102, 2006 Nov 17.
Article in English | MEDLINE | ID: mdl-17155696

ABSTRACT

By using scanning tunneling spectroscopy to probe a silver thin film that contains both periodic and quasiperiodic modulation, and by using Fourier analysis, we unravel the influences of individual Fourier components of the scattering potential (periodic versus quasiperiodic) on the electronic structure of a one-dimensional quasiperiodically modulated thin Ag film. Along the periodically modulated direction, a Bragg reflection-induced energy gap is observed in k space. On the other hand, the exotic E vs k spectrum with many minigaps was observed along the quasiperiodic direction.

7.
Phys Rev Lett ; 96(2): 027005, 2006 Jan 20.
Article in English | MEDLINE | ID: mdl-16486621

ABSTRACT

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (Tc) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the density of states oscillation at E(F) . The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.

8.
Phys Rev Lett ; 95(18): 187404, 2005 Oct 28.
Article in English | MEDLINE | ID: mdl-16383947

ABSTRACT

In a semiconductor quantum dot, the IIx and IIy transitions to the polarization eigenstates, |x> and |y>, naturally form a three-level V-type system. Using low-temperature polarized photoluminescence spectroscopy, we have investigated the exciton dynamics arising under strong laser excitation. We also explicitly solved the density matrix equations for comparison with the experimental data. The polarization of the exciting field controls the coupling between the otherwise orthogonal states. In particular, when the system is initialized into \Y>, a polarization-tailored pulse can swap the population into |x>, and vice versa, effectively operating on the exciton spin.

9.
Biophys J ; 89(6): 4330-42, 2005 Dec.
Article in English | MEDLINE | ID: mdl-16199496

ABSTRACT

Viscoelastic changes of the lamellipodial actin cytoskeleton are a fundamental element of cell motility. Thus, the correlation between the local viscoelastic properties of the lamellipodium (including the transitional region to the cell body) and the speed of lamellipodial extension is studied for normal and malignantly transformed fibroblasts. Using our atomic force microscopy-based microrheology technique, we found different mechanical properties between the lamellipodia of malignantly transformed fibroblasts (H-ras transformed and SV-T2 fibroblasts) and normal fibroblasts (BALB 3T3 fibroblasts). The average elastic constants, K, in the leading edge of SV-T2 fibroblasts (0.48 +/- 0.51 kPa) and of H-ras transformed fibroblasts (0.42 +/- 0.35 kPa) are significantly lower than that of BALB 3T3 fibroblasts (1.01 +/- 0.40 kPa). The analysis of time-lapse phase contrast images shows that the decrease in the elastic constant, K, for malignantly transformed fibroblasts is correlated with the enhanced motility of the lamellipodium. The measured mean speeds are 6.1 +/- 4.5 microm/h for BALB 3T3 fibroblasts, 13.1 +/- 5.2 microm/h for SV-T2 fibroblasts, and 26.2 +/- 11.5 microm/h for H-ras fibroblasts. Furthermore, the elastic constant, K, increases toward the cell body in many instances which coincide with an increase in actin filament density toward the cell body. The correlation between the enhanced motility and the decrease in viscoelastic moduli supports the Elastic Brownian Ratchet model for driving lamellipodia extension.


Subject(s)
Cell Movement/physiology , Fibroblasts/physiology , Fibroblasts/ultrastructure , Molecular Motor Proteins/physiology , Pseudopodia/physiology , Pseudopodia/ultrastructure , Animals , BALB 3T3 Cells , Elasticity , Mechanotransduction, Cellular/physiology , Mice , Stress, Mechanical , Viscosity
10.
Nano Lett ; 5(10): 1873-7, 2005 Oct.
Article in English | MEDLINE | ID: mdl-16218701

ABSTRACT

Photoluminescence intermittency, or "blinking", was observed in semiconductor InGaAs/GaAs quantum dots (QDs) inside a planar microcavity. Most of the blinking QDs were found around defect sites such as dislocation lines naturally formed in the GaAs barrier layers, and the carrier traps responsible for blinking had an excitation threshold of approximately 1.53 eV. The blinking properties of epitaxial QDs and colloidal nanocrystal QDs were also compared by performing laser intensity dependent measurements and statistics of the "on" and "off" time distributions.

11.
Phys Rev Lett ; 92(10): 106104, 2004 Mar 12.
Article in English | MEDLINE | ID: mdl-15089221

ABSTRACT

We demonstrate a novel scheme for manipulating metallic nanostructures involving a macroscopic number of atoms, yet with precise control in their local structures. The scheme entails a two-step process: (a) a triggering step using a scanning tunneling microscope, followed by (b) self-driven and self-limiting mass-transfer process. By using this scheme, we construct Pb nanomesas on Si(111) substrates whose thickness can be controlled with atomic-layer precision. The kinetic barrier for the mass transfer and the underlying mechanism behind this novel manipulation are determined.

12.
Biophys J ; 86(3): 1777-93, 2004 Mar.
Article in English | MEDLINE | ID: mdl-14990504

ABSTRACT

Viscoelasticity of the leading edge, i.e., the lamellipodium, of a cell is the key property for a deeper understanding of the active extension of a cell's leading edge. The fact that the lamellipodium of a cell is very thin (<1000 nm) imparts special challenges for accurate measurements of its viscoelastic behavior. It requires addressing strong substrate effects and comparatively high stresses (>1 kPa) on thin samples. We present the method for an atomic force microscopy-based microrheology that allows us to fully quantify the viscoelastic constants (elastic storage modulus, viscous loss modulus, and the Poisson ratio) of thin areas of a cell (<1000 nm) as well as those of thick areas. We account for substrate effects by applying two different models-a model for well-adhered regions (Chen model) and a model for nonadhered regions (Tu model). This method also provides detailed information about the adhered regions of a cell. The very thin regions relatively near the edge of NIH 3T3 fibroblasts can be identified by the Chen model as strongly adherent with an elastic strength of approximately 1.6 +/- 0.2 kPa and with an experimentally determined Poisson ratio of approximately 0.4 to 0.5. Further from the edge of these cells, the adherence decreases, and the Tu model is effective in evaluating its elastic strength ( approximately 0.6 +/- 0.1 kPa). Thus, our AFM-based microrheology allows us to correlate two key parameters of cell motility by relating elastic strength and the Poisson ratio to the adhesive state of a cell. This frequency-dependent measurement allows for the decomposition of the elastic modulus into loss and storage modulus. Applying this decomposition and Tu's and Chen's finite depth models allow us to obtain viscoelastic signatures in a frequency range from 50 to 300 Hz, showing a rubber plateau-like behavior.


Subject(s)
Cell Surface Extensions/physiology , Cell Surface Extensions/ultrastructure , Fibroblasts/physiology , Fibroblasts/ultrastructure , Micromanipulation/methods , Microscopy, Atomic Force/methods , Models, Biological , Animals , Computer Simulation , Elasticity , Hardness , Image Interpretation, Computer-Assisted/methods , Mice , NIH 3T3 Cells , Stress, Mechanical , Viscosity
13.
Science ; 303(5659): 816-8, 2004 Feb 06.
Article in English | MEDLINE | ID: mdl-14764872

ABSTRACT

We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.

14.
Phys Rev Lett ; 88(8): 087401, 2002 Feb 25.
Article in English | MEDLINE | ID: mdl-11863973

ABSTRACT

We investigated the manifestation of Rabi oscillation in the coherent dynamics of excitons in self-assembled semiconductor quantum dots. The Rabi oscillation phenomenon was directly observed as a function of the input pulse area. Furthermore, by performing wave packet interferometry in the nonlinear excitation regime, we discover a new type of quantum interference phenomenon, resulting from the interplay between Rabi oscillation and quantum interference.

15.
Phys Rev Lett ; 88(1): 016102, 2002 Jan 07.
Article in English | MEDLINE | ID: mdl-11800970

ABSTRACT

Atomically flat ultrathin Ag films on GaAs(110) can be formed through a kinetic pathway. However, such films are metastable and will transform to 3D islands upon high temperature annealing. Using scanning tunneling microscopy, we have measured quantitatively the layer-resolved metastability of flat Ag overlayers as they evolve toward their stable state, and deduced the corresponding kinetic barrier the system has to overcome in reaching the stable state. These results indicate that the metastability of the Ag overlayer is defined by the quantum nature of the conduction electrons confined within the overlayer.

17.
Phys Rev Lett ; 85(4): 880-3, 2000 Jul 24.
Article in English | MEDLINE | ID: mdl-10991422

ABSTRACT

A new scanning probe-based microrheology approach is used to quantify the frequency-dependent viscoelastic behavior of both fibroblast cells and polymer gels. The scanning probe shape was modified using polystyrene beads for a defined surface area nondestructively deforming the sample. An extended Hertz model is introduced to measure the frequency-dependent storage and loss moduli even for thin cell samples. Control measurements of the polyacrylamide gels compare well with conventional rheological data. The cells show a viscoelastic signature similar to in vitro actin gels.


Subject(s)
Acrylic Resins/chemistry , Cells/chemistry , Polymers/chemistry , Rheology/methods , 3T3 Cells , Actins/chemistry , Animals , Biopolymers/chemistry , Elasticity , Mice , Microscopy, Atomic Force , Microspheres , Polystyrenes , Stress, Mechanical , Viscosity
18.
J Med Chem ; 41(16): 2960-71, 1998 Jul 30.
Article in English | MEDLINE | ID: mdl-9685235

ABSTRACT

Like other nonnucleoside inhibitors of HIV-1 reverse transcriptase, the dipyridodiazepinone nevirapine (Viramune, 1) selects for drug resistant variants of HIV-1, both in cell culture and in patients. In particular, the mutation of residue 181 from tyrosine to cysteine (Y181C) is associated with resistance to most reported nonnucleoside inhibitors. Introduction of an arylethyl substituent at the 8-position of the tricyclic dipyridodiazepinone skeleton confers enhanced potency against Y181C RT. Several analogues of this series display good broad spectrum potency against a panel of mutant enzymes.


Subject(s)
Antiviral Agents/chemical synthesis , Azepines/chemical synthesis , HIV Reverse Transcriptase/antagonists & inhibitors , Mutation , Pyridines/chemical synthesis , Reverse Transcriptase Inhibitors/chemical synthesis , Amino Acid Substitution , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Azepines/chemistry , Azepines/pharmacology , Cell Line, Transformed , Drug Resistance, Microbial , HIV Reverse Transcriptase/genetics , HIV-1/drug effects , HIV-1/enzymology , HIV-1/genetics , Humans , In Vitro Techniques , Microsomes, Liver/drug effects , Microsomes, Liver/metabolism , Nevirapine/chemistry , Nevirapine/pharmacology , Pyridines/chemistry , Pyridines/pharmacology , Reverse Transcriptase Inhibitors/chemistry , Reverse Transcriptase Inhibitors/pharmacology , Structure-Activity Relationship
19.
J Med Chem ; 41(16): 2972-84, 1998 Jul 30.
Article in English | MEDLINE | ID: mdl-9685236

ABSTRACT

Nevirapine (I) is the first human immunodeficiency virus type 1 (HIV-1) nonnucleoside reverse transcriptase (RT) inhibitor to reach regulatory approval. As a result of a second generation program around the tricyclic core system of nevirapine, 2-chloro-5, 11-dihydro-11-ethyl-5-methyl-8-(2-(pyridin-4-yl)ethyl)-6H-dipyrido[3, 2-b:2',3'-e][1,4]diazepin-6-one (II)1a and 2-chloro-5, 11-dihydro-11-ethyl-5-methyl-8-phenylethyl-6H-dipyrido[3,2-b:2', 3'-e][1,4]diazepin-6-one (III)1a were identified as broad spectrum HIV-1 RT inhibitors. A detailed examination of replacing either of the methylenes of the 8-ethyl linker of II or III is presented. It was found that 8-aryloxymethyl and 8-arylthiomethyl are the preferred pattern of substitution for potency against RT. The most potent compounds were further evaluated against a panel of clinically significant mutant RT enzymes (K103N, V106A, G190A, P236L) and in cytotoxicity and in vitro metabolism assays. The most potent compound was 2-chloro-8-phenylthiomethyl analogue 37 which displayed sub-100 nM activity against all HIV-1 RT enzymes tested.


Subject(s)
Antiviral Agents/chemical synthesis , Azepines/chemical synthesis , HIV Reverse Transcriptase/antagonists & inhibitors , Nevirapine/analogs & derivatives , Pyridines/chemical synthesis , Reverse Transcriptase Inhibitors/chemical synthesis , Animals , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Azepines/chemistry , Azepines/pharmacology , Biological Availability , Cell Line, Transformed , Cell Survival/drug effects , Drug Evaluation, Preclinical , Drug Resistance, Microbial , Drug Stability , HIV Reverse Transcriptase/genetics , HIV-1/drug effects , HIV-1/enzymology , HIV-1/physiology , Humans , In Vitro Techniques , Male , Microsomes, Liver/drug effects , Microsomes, Liver/metabolism , Mutation , Nevirapine/chemical synthesis , Nevirapine/chemistry , Nevirapine/pharmacokinetics , Nevirapine/pharmacology , Pyridines/chemistry , Pyridines/pharmacology , Rats , Rats, Sprague-Dawley , Reverse Transcriptase Inhibitors/chemistry , Reverse Transcriptase Inhibitors/pharmacology , Structure-Activity Relationship , Virus Replication/drug effects
20.
J Med Chem ; 40(15): 2430-3, 1997 Jul 18.
Article in English | MEDLINE | ID: mdl-9240358

ABSTRACT

Modification of the non-nucleoside inhibitor of HIV-1 reverse transcriptase nevirapine (Viramune) by incorporation of a 2-indolyl substituent confers activity against several mutant forms of the enzyme.


Subject(s)
HIV Reverse Transcriptase/antagonists & inhibitors , Pyridines/pharmacology , Reverse Transcriptase Inhibitors/pharmacology , Anti-HIV Agents/chemistry , Anti-HIV Agents/pharmacology , Cell Line , Humans , Magnetic Resonance Spectroscopy , Nevirapine , Pyridines/chemistry , Reverse Transcriptase Inhibitors/chemistry
SELECTION OF CITATIONS
SEARCH DETAIL
...