Spatially resolved observation of dipole-dipole interaction between Rydberg atoms.

We have observed resonant energy transfer between cold Rydberg atoms in spatially separated cylinders. Resonant dipole-dipole coupling excites the 49s atoms in one cylinder to the 49p state while the 41d atoms in the second cylinder are transferred down to the 42p state. We have measured the production of the 49p state as a function of separation of the cylinders (0-80 microm) and the interaction time (0-25 micros). In addition, we measured the width of the electric field resonances. A full many-body quantum calculation reproduces the main features of the experiments.

Programmable nanolithography with plasmon nanoparticle arrays.

We describe how optical contact lithography based on plasmon particle array masks allows generation of a large number of different subwavelength exposure patterns using a single mask. Within an exact point dipole model, we study the local response of silver particles in small two-dimensional arrays with 50-200 nm spacing. We show how illumination with unfocused light allows optically addressing particles either individually or in controlled configurations; which pattern will be exposed by the mask is programmed by varying the wavelength, incidence angle, and polarization of the incident wave.

Asymmetric response in a line of optically driven metallic nanospheres.

We describe the dipole-dipole interactions between a linear array of optically driven silver metallic nanospheres (MNSs). These model calculations incorporate the full electric field generated by an oscillating dipole and predict several interesting effects due to the retardation of the field. The distribution of the power associated with MNSs along the array shows a strong variation on a scale smaller than the wavelength of the driving light. For a given geometry, there is a small range of frequencies where the relative power in the last MNS compared to the first dramatically changes, suggesting a simple device for wavelength discrimination in this frequency range. Moreover, small changes in the driving frequency can completely change the direction of the scattered light when only a single nanosphere is driven.

Control of atomic ionization by two-color few-cycle pulses.

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.

Carrier phase dependence in the ionization of Rydberg atoms by short radio-frequency pulses: a model system for high order harmonic generation.

We report time-resolved electron emission in experiments on ionization of rubidium Rydberg atoms (n=90) by few-cycle radio-frequency (RF) (1-10 MHz) pulses. The electron emission occurs in multiple bursts and strongly depends on the carrier-envelope phase as well as the duration and amplitude of the RF pulses. Remarkably, ionization is observed during a series of cycles with the same amplitude. Even at the low RF frequencies, ionization is not completed in a single cycle. Remixing of the states at the zero crossing of the field is believed to play an essential role. Similarities with the ionization process leading to high order harmonic generation are discussed.

Asymmetry in the strong-field ionization of Rydberg atoms by few-cycle pulses.

We present measurements of the electron ejection direction in the ionization of high (n=90) Rydberg states of rubidium subjected to few-cycle radio-frequency (RF) pulses. For weak pulses we find a strong asymmetry for even (cosine) pulses and no asymmetry for odd (sine) pulses. This asymmetry disappears for pulses longer than four RF cycles. For strong pulses, very large asymmetry is found for both sine and cosine pulses that persists up to eight RF cycles and is attributed to initial state depletion effects within a cycle.

Rydberg state ionization by half-cycle-pulse excitation: strong kicks create slow electrons.

The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.

Displacing Rydberg electrons: the mono-cycle nature of half-cycle pulses.

A THz "half-cycle" pulse is a fast ( <1 ps) unipolar pulse, followed by a slow unipolar pulse of opposite polarity. We found that the interaction of such THz pulses with very high Rydberg states results in a displacement of the electron within the atom, while the ionization is strongly suppressed. In classical terms: the first fast unipolar feature corresponds to a start kick of the Rydberg electron, while the second unipolar feature acts as a stop kick. A semiclassical model is presented which qualitatively reproduces the ionization suppression and redistribution.

Static and dynamic transport of light close to the Anderson localization transition.

Anderson localization of light refers to an inhibition of wave transport in scattering media due to the interference of multiple scattered waves. We present wavelength dependent midinfrared optical transport measurements in slabs of randomly packed germanium (Ge) micron-sized particles, using a free electron laser as a tunable source of pulsed radiation. Because of their high refractive index and low absorption, Ge and similar semiconductors are excellent systems to study Anderson localization of light. To characterize the samples fully, we have employed several complementary optical techniques: total diffuse transmission, total diffuse reflection, coherent transmission, and time-resolved speckle interferometry. In this way we obtained the scattering (l(s)) and transport (l) mean free paths, the absorption coefficient (alpha), the diffusion constant (D), and the energy transport velocity (v(e)). These measurements have been made as a function of midinfrared wavelength, so that the scattering cross section and absorption coefficients can be varied in the same samples. We found that the Ge samples are close (kl(s) approximately 3) to the localization transition, but still above it. Our measurements of l(s) and l suggest that l is renormalized due to interference at the proximity of the localization transition. We also found that the diffusion constant is significantly reduced in samples thinner than approximately 7l.

Atomic spectral detection of tunable extreme ultraviolet pulses.

A novel method of detecting the spectral width and wavelength of extreme ultraviolet (XUV) pulses with a minimum number of experimental tools is demonstrated. The method relies on the photoionization probability of an atom as a function of the electric field. A tunable laser source in the XUV is used that is based on higher-harmonic generation of the frequency-doubled output of a 50-fs Ti:sapphire laser. The bandwidth and the wavelength of the seventh harmonic (~57nm) are detected with Ne, and the resolving power is lambda/Dlambda=10(5).

Streak camera operating in the mid infrared.

An atomic streak camera has been constructed that operates from the near to the far infrared. The photocathode used in conventional streak cameras for the conversion of photons to electrons has been replaced by gas-phase atoms in a Rydberg state. The low binding energy of the electron in a Rydberg atom combined with the large photoionization cross section of a Rydberg atom makes Rydberg atoms suitable for use in an infrared streak camera. Operation of the streak camera is demonstrated at 2.6 microm, well beyond the spectral range of any conventional streak camera.

Second-harmonic generation of femtosecond pulses: observation of phase-mismatch effects.

We observe new effects in frequency doubling of colliding-pulse, mode-locked dye-amplified pulses (300 fsec, 620 nm, up to 1 mJ of energy) due to phase mismatch. If the second-harmonic generation in the nonlinear crystal (30-mm KDP crystal) is phase matched, the output is a square pulse. In contrast, when a phase mismatch is introduced, the generated pulse contains two peaks. We observe that the time profile is affected by depletion and chirp of the fundamental. The experimental results agree well with our numerical calculations.