We investigate the stabilization of a hydrogen atom in circularly polarized laser fields. We use a time-dependent, fully three dimensional approach to study the quantum dynamics of the hydrogen atom subject to high intensity, short wavelength laser pulses. We find enhanced survival probability as the field is increased under fixed envelope conditions. We also confirm wavepacket dynamics seen in prior time-dependent computations restricted to two dimensions.PACS numbers: 32.80. Fb,32.80.Wr,42.50.Hz The advent of lasers producing electric fields at or above inter-atomic electric fields has led to the discovery of many new and highly nonlinear phenomena [1]. As nonlinear laser-atom physics has matured, computational approaches employed by researchers in the field have naturally been applied to scenarios not easily realized in experiments. One such scenario is the interaction of an intense, high frequency laser with the single-electron hydrogen atom. Commercially available pulsed laser systems readily produce intensities at or above the atomic unit I = 3.51 × 10 16 W/cm 2 , but available photon energieshω are well below the atomic unit frequency, which is given by the ground state ionization energy |E o |. In the case where the driving frequency is near or above the ground state ionization energy,hω ≥ |E o |, a remarkable phenomena known as "stabilization" may occur. Stabilization is characterized by a decrease in the ionization probability as the laser intensity increases. For constant driving field, stabilization is manifest as an increase in dressed state lifetime. However, due to the initial increase in ionization rate for increased fields as the interaction turns on, increased steady-state lifetimes do not necessarily translate into increased end-of-pulse survival probabilities. This poses a significant challenge to experimental observation of stabilization and is commonly known as the "death-valley" problem.Nevertheless, indications of stabilization have been seen by tailoring the initial state to act as an effective ground state [2][3][4][5]. In particular, the high frequency condition has been realized using dipole forbidden circular Rydberg states (n = 4, 5) of neon (|E o | < 1eV ), and a driving laser ofhω = 2eV [2]. In this experiment, a decrease in total ionization yield was observed with increasing peak laser intensity, while fluence was held constant [3]. Fermi's golden rule predicts the total ionization depends only on the fluence, so this result clearly indicates non-perturbative stabilization [4,5]. With this notable exception, however, stabilization remains experimentally unconfirmed, and detailed studies have been confined to the realm of simulation.The vast majority of these studies have concentrated on the case of linearly polarized (LP) fields, where much has been learned about the dynamics of stabilization [6]. Studies considering the case of circularly polarized (CP) fields have also noted comparable or enhanced stabilization [7][8][9][10]. However, the physical mechanism for stabilization...

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We investigate the interaction of a two-dimensional model atom with an intense, high-frequency circularly polarized laser pulse. As the laser intensity is increased, the ionization rate initially increases, then decreases dramatically, with the electron wave function developing an asymmetric ring form which rotates with the electric field. We provide evidence that this wave form is due to localization of the electron onto nonlinear classical structures.

In this paper we study the radiation spectrum generated by the quantum dynamics of a double resonance model and a driven square well system. We use Floquet theory to analyze the radiation generated by these systems. We present the results of numerical simulations that indicate a connection between high harmonic generation and underlying classical chaos in these models. Our results provide a means of predicting the radiative characteristics of multilevel quantum systems subject to a strong periodic driving force.

Photoreflectance ͑PR͒ provides an optical means for rapid and precise measurement of near-surface electric fields in semiconductor materials. This article details the use of PR to characterize dopant activation in ultrashallow junction ͑USJ͒ structures formed using millisecond annealing processes. USJ structures were formed in silicon using 500 eV B implantation with a dose of 10 15 / cm 2 , followed by flash anneals at 1250-1350°C. Reference metrology was performed using secondary ion mass spectrometry and various sheet resistance ͑R s ͒ methods. Methods to calibrate PR signals to active carrier concentration in USJ structures, including halo-doped samples, are described. PR is shown to be highly sensitive to active dopant concentrations in USJ structures formed by millisecond annealing.