Showing papers by "Herbert Walther published in 1995"

Classical rescattering effects in two-color above-threshold ionization

Abstract: Rescattering effects in above-threshold ionization such that the electron is driven back to the ionic core by the laser field and rescatters are considered in a completely classical framework, for a laser field consisting of a linearly polarized monochromatic fundamental and its second harmonic, as a function of their relative phase. Several features appear, owing to the two-color field: the energies of the rescattered electrons extend up to 21${\mathit{U}}_{\mathit{p}}$, with ${\mathit{U}}_{\mathit{p}}$ the ponderomotive potential of the entire field; the angular distributions of the rescattered electrons are no longer symmetric with respect to an interchange of the backward and forward direction and more than one ring or side lobe may appear in either direction; the importance of rescattering can be deduced from the violation of a certain symmetry of the total above-threshold ionization rates for fixed energy with respect to the phase between the two fields; the spectra of the rescattered electrons change dramatically when the field intensity is raised from the multiphoton into the tunneling regime.

Injection locking of a diode-pumped Nd:YAG laser at 946 nm.

Abstract: Injection locking in the quasi-three-level laser system Nd:YAG (4)F(3/2)-(4)I(9/2) at 946 nm is reported. The master and slave oscillators are pumped by laser diodes. The master oscillator is frequency stabilized to a high-finesse cavity, resulting in a laser linewidth of less than 10 Hz. Using intracavity frequency doubling of the slave oscillator, we achieve a single-mode output power of 60 mW at 473 nm. The laser radiation was frequency quadrupled, resulting in an UV power of 0.55 mW at 236.5 nm. The laser system was used to excite a strongly forbidden In(+) transition, proposed as a new optical frequency standard.

Optical mixing of CO2‐laser radiation in a scanning tunneling microscope

Abstract: Two infrared laser beams coupled into the tunneling junction of a scanning tunneling microscope lead to the generation of a signal at the difference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One part of the signal is generated through a mixing process owing to the nonlinearity in the static current‐voltage characteristic. Another part has its origin in a nonlinear susceptibility at the surface; it therefore corresponds to frequency mixing in nonlinear optics. It will be shown that the difference‐frequency signals generated by the two processes can be separated owing to their different dependence on the tip‐sample distance.

Experiments with Single Atoms in Cavities and Traps

Abstract: In this paper experiments performed with the one-atom maser and with a few ions stored in a Paul trap are reviewed. Special emphasis is placed on experiments for the test of quantum physics.

Double resonance spectroscopy of a single trapped indium ion

Abstract: The wavelength of the highly forbidden 5s2 1S0 → 5s5p 3P0 transition of the In+ ion, a promising candidate for an optical frequency standard, was measured in an optical-optical double resonance experiment using a single laser-cooled ion in a radiofrequency trap.

Cavity-induced decay of Floquet states in a bichromatic driving field

Abstract: A theoretical study of the dynamics of Rydberg atoms in a microwave cavity driven by a strong bichromatic field is presented. The resonator is assumed to operate in the low-Q regime. As a consequence, photons emitted by the atoms are dissipated in the cavity walls during the interaction time of the atoms inside the resonator. In this situation the cavity field follows the atomic dynamics adiabatically. The transient behavior of the system is analyzed in terms of Floquet states, and cavity-induced transition rates between these states are calculated for a large range of parameters of the bichromatic field. Narrow resonances are found in the transition rates, in agreement with recent experimental investigations of cavity Rydberg atoms subjected to strong bichromatic driving. We explain in detail the structure of the resonances, which is determined by the frequency-dependent cavity-mode density as well as the Rabi frequencies of the applied fields. The intensity-dependent shifts of the resonance frequencies are also calculated and found to be largely insensitive to inhomogeneous broadening. Finally, the numerical results are compared with experimental observations.

Quantum theory of the one-mode Lambda -type micromaser and laser.

Abstract: We derive the master equations for the fields in the one-mode {Lambda}-type micromaser and laser with injected atoms in a superposition of their states. In terms of effective parameters, these equations are equivalent to those for a two-level micromaser and laser. The interference terms in the master equations may cancel the absorption terms independent of statistical properties of the field. Due to the atomic coherence between the two degenerate lower levels, the generation of field states with sub-Poissonian photon statistics is possible in the micromaser without the need for a population inversion. If an equivalent level system is used in a laser operating without population inversion it can produce field states with Poisson distributions of the photon number.

A New Mechanism for Laser-Frequency Mixing in a Scanning Tunneling Microscope

Abstract: Two infrared laser frequencies coupled into the tunneling junction of a scanning tunneling microscope (STM) lead to the generation of a difference-frequency signal. In this article two different mechanisms of difference-frequency generation (DFG) in the tunneling junction are presented. Besides DFG due to the nonlinearity of the current-voltage characteristic known from previous experiments, nonlinear optical DFG arising from the breakdown of the inversion symmetry at the metal-vacuum interface is demonstrated. This nonlinear optical DFG may be a step further towards laser spectroscopy with an STM.

From micromaser to microlasers

Abstract: In this paper the work on the one-atom-maser or micromaser is reviewed. This maser system is the most fundamental onewhere a single atom interacts with a single mode of a cavity. With this setup the generation process of non-classical light and other quantum phenomena can be investigated in detail. Furthermore a brief review of the microlaser work will be given where the phenomena of cavity quantum electrodynamics lead to a very low threshold of the laser system.Keywords: one-atom-maser, micromaser, cavity quantum electrodynamics, microlasers 1. OVERVIEW The simplest and most fundamental system for studying radiation-matter coupling is a single two-level atom interactingwith a single mode of an electromagnetic field in a cavity. It received a great deal of attention shortly after the maser wasinvented, but at that time, the problem was of purely academic interest since the matrix elements describing the r'diation-atom interaction are so small that the field ofa single photon is not sufficient to lead to an atom field evolution time shorterthan the other characteristic times of the system, such as the excited state lifetime, the time of flight of the atom through thecavity and the cavity mode damping time. It was therefore not possible to test experimentally the fundamental theories ofradiation-matter interaction, which predict amongst other effects:(a) a modification of the spontaneous emission rate ofa single atom in a resonant cavity,(b) oscillatory energy exchange between a single atom and the cavity mode, and(c) the disappearance and quantum revival of Rabi nutation induced in a single atom by a resonant field.

Images of Surface Plasmons at 1064 nm Obtained with an STM

Abstract: The application of the scanning tunneling microscope (STM) to obtain the two-dimensional intensity distribution of surface plasmons (SPs) is described. The different detection mechanisms of the plasmon field with the STM are discussed. The images are obtained by using the thermal-expansion signal. The values for the decay lengths determined by the STM method agree well with those obtained by other techniques and with the theoretical predictions.