Ionization of Rydberg atoms by subpicosecond half-cycle electromagnetic pulses

Abstract

We have ionized Rydberg atoms using subpicosecond half-cycle electromagnetic pulses. The threshold electric field required to ionize a Rydberg state with effective quantum number AZ* is found to scale as n*~2 for states with n* > 13 in contradistinction to the n*~4 threshold scaling for static field ionization and high order multiphoton ionization. This novel result is explained using a classical model. PACS numbers: 32.80.RmThe ionization of atoms by pulsed electromagnetic radiation has been studied using pulsed lasers [1], microwaves [2,3], and ramped dc electric fields [4]. However, until very recently, the temporal width of available pulses was quite large compared to the internal time scales of the atoms under study. In the last decade, however, ultrashort (< 100 fs) laser pulses have been produced, which have durations of only tens of optical cycles [5,6]. Ionization in these laser fields can differ drastically from ionization with longer pulses because the large laser bandwidth may coherently excite several atomic states [71.We have extended the study of ionization by coherent broadband laser pulses to its logical limit by employing ultrashort, half-cycle electromagnetic pulses. Half "optical-cycle" pulses with widths T < 200 fs have been created in photoconducting semiconductor switches illuminated by 100 fs laser pulses [8]. These freely propagating electromagnetic pulses have central frequencies around 1 THz (33 cm -1 ). We have succeeded in increasing the available pulse energy by more than an order of magnitude, and have demonstrated that peak fields in excess of 100 kV/cm can be produced in a nearly unipolar 500 fs electric field pulse [9]. While these fields are still insufficient to ionize the ground state of atoms, they are quite capable of ionizing high-lying Rydberg states.We have studied the ionization of Na Rydberg atoms using freely propagating, 500 fs single-polarity pulses. Complete ionization of Rydberg states with principle quantum number n > 13 is observed. We show that Rydberg states subjected to these short pulses begin to ionize when the peak electric field is proportional to their binding energies. In contrast, ionization of atoms by relatively long (JAS) microwave pulses and ramped dc electric fields requires peak fields which scale as n ~5 and n ~4, respectively [3,4]. The novel threshold field scaling can be explained by classical mechanics.In the experiment, two tunable dye lasers excite ground state Na atoms to a Rydberg state. The half-cycle pulse (HCP) is then weakly focused on the Rydberg atoms. Any ions formed by the HCP are collected using a microchannel plate (MCP) detector. We record the number of ions (or electrons) which are produced as a function of the peak electric field in the HCP.The source of the HCP [10] is a thin (0.5 mm) GaAs semiconductor wafer with a surface area of -3 cm 2 . An electric field (F<10 kV/cm) is applied parallel to the surface of the GaAs. The electric field is then shorted across the semiconductor surface by illuminating one side of the wafer w...

Transitions Driven by a Missing Frequency

“…On second thought, the interaction of strong ultrashort half-cycle pulses with Rydberg atoms, which are highly susceptible to electric fields, is almost always strongly driven and poorly described by a single-photon picture. In fact, this interaction is frequently described as a semiclassical ''momentum kick'' p e RẼ HCP t dt to a free electron with charge e due to the electric field E HCP t of the half-cycle pulse [3]. The energy lost or gained in the impulsive approximation is then E 1 2m 2p p p 2 , wherep is the initial momentum and m is the mass of the electron.…”

“…If the intensity of the light becomes very high, multiphoton transitions can become important, ultimately reaching the situation that the electric field of the light suppresses the binding Coulomb forces as happens, e.g., in above-threshold ionization [1,2] or in the interaction of Rydberg atoms with strong half-cycle electric field pulses [3][4][5][6]. In another class of light-matter interactions, for example, in stimulated Raman adiabatic passage, the coherent properties of light are employed [7].…”

Fractional Revival of Rydberg Wave Packets in Twice-Kicked One-Dimensional Atoms

“…But since the pioneering experiment of Jones et al [3], unipolar electric field pulses of tetrahertz spectrum, often called half-cycle pulses (HCPs), have been found to provide a very convenient tool to produce such packets. The duration of an HCP is much shorter than the Kepler period of the Rydberg electron such that during interaction the electron effectively receives an impulsive kick from the former.…”

Quantum interference in the field ionization of Rydberg atoms

“…In the terahertz regime the impulsive energy transfer of a fast half cycle pulse has been used to study wave-packet dynamics [5]. Ionization of low-lying Rydberg states with single cycle terahertz pulses has revealed an unexpected n −3 scaling of the field ionization threshold [6].…”