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IEEE Journal of Quantum Electronics

Optical Response of Atomic Gases to Ultrafast Pump-Probe Pulses

Jonathan Andreasen, Ewan M. Wright, Miroslav Kolesik

Published Nov 11, 2013• 2 citations
Publisher

Abstract

We present a computational study of the pump-probe response of a single atom to assess any microscopic anisotropy induced by strong, non-resonant optical fields. Using simulations of the Schrödinger equation for an atom exposed to a linearly polarized ultrafast pump pulse, we calculate the induced dipole moment along the probe polarization directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.7-0.9 on the timescale of tens of femtoseconds following the excitation pulse. In the regime studied, we conclude that excited bound states prepared by the pump are primarily responsible for the birefringence observed, but that the precise response reflects the probe pulse properties.

DOI
10.1109/jqe.2013.2288526

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We present a computational study of the pump-probe response of a single atom to assess any microscopic anisotropy induced by strong, non-resonant optical fields. Using simulations of the Schrödinger equation for an atom exposed to a linearly polarized ultrafast pump pulse, we calculate the induced dipole moment along the probe polarization directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.7-0.9 on the timescale of tens of femtoseconds following the excitation pulse. In the regime studied, we conclude that excited bound states prepared by the pump are primarily responsible for the birefringence observed, but that the precise response reflects the probe pulse properties.
We present a computational study of the pump-probe response of a single atom to assess any microscopic nonlinear optical anisotropy due to freed electrons for non-resonant optical excitation. Using simulations of the Schrödinger equation for an atom exposed to a strong, short-duration linearly polarized pump, we calculate the induced dipole moment along probe directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.78-0.9 on the timescale of tens of femtoseconds following the excitation pulse.
In this experiment, Li2 is excited from ground electronic state to the first excited state by 1 μJ, 75 s pulses from an amplified CPM laser at about 620 nm. The birefringence created by the pump pulses is measured at a delayed time by using a white-light-continuum probe (10 nm width, center at 770 nm and 800 nm). Unlike similar experiments, we are pumping in the resonance regime (the Rabi period here is 1-10 ps). This experiment is laying the groundwork for the use of shaped ultrafast pulses to develop a birefringence time history by design. We have computed a birefringence and dichroism signal based upon spectroscopy parameters. Most of the features have been confirmed experimentally. Experimentally, a birefringence step is observed after the initial pump pulse; this is due to a population transfer that results from the pumping process. Peaks that appear later are believed to be due to rotational coherences in the ground and first excited states.
Observation of pulsed superradiance from two-level systems under direct excitation is challenging because the much stronger excitation pulse overlaps with the emitted radiation in time, space, and spectrum. Previous demonstrations have therefore mostly relied on excitation via auxiliary levels or multiphoton excitation at different wavelengths, limiting applicability. Here we present an interferometric pump-probe technique that enables observation of picosecond superradiance from a moderately-dense-but-optically-thin ensemble of two-level atoms resonantly excited under substantial Doppler and collisional broadening. Sequential pump and probe pulses tuned to the 555.6-nm E1 transition of 174 Yb allowed us to reconstruct the temporal waveform of the superradiant field from time-averaged transverse fluorescence. The fluorescence displayed interference fringes as a function of the pump-probe delay, whose envelope corresponds to the superradiant polarization P ( t ); taking its time derivative yields the radiated electric field. Pulse widths extracted from the reconstructed field follow a power-law scaling of 1/N00.441/{N}_{0}^{0.44}1/N00.44​ , where N 0 is the number of atoms in the interaction volume. The measured temporal dynamics and scaling behavior are in good agreement with simulations based on the coupled Maxwell-Schrödinger equations. This technique provides direct access to ultrafast superradiant dynamics without requiring auxiliary transitions, multiphoton excitation, or ultrafast detectors.
We present a theoretical study of the induced transient birefringence of a low density homogeneous molecular gas in a resonant pump–probe experiment. The molecular coherent state induced by the resonant pump field is described by second-order perturbation theory. The induced birefringence can be detected by a delayed probe pulse propagating through the molecular medium after illumination by the pump pulse. In the case of a nonresonant probe, the birefringence is linearly proportional to the mean value of the electronic polarizability of the molecular gas. The birefringence signal is composed of distinct components due to population change and those of rotational, vibrational, and mixed vibrational–rotational origins. This is demonstrated by numerical simulations on Li2 gas. Moreover, the quantum beats contained in the birefringence, as a function of the time delay between the pump and probe pulses, is dominated by the pure rotational motion. Finally, the birefringence is sensitive to the shape of the applied pump pulse and dependent on the spectral phase of the pump pulse.
Abilities of the method of accumulation of coherent unstable response of a medium for amplification of the rotated component of the probe wave’s polarization in the atomic medium of three-level atoms are studied. Probe wave is examined in frames of the perturbation theory and its frequency is considered near to one photon resonance from the ground to the first excited level. The intense monochromatic pump field connects two excited energy levels and propagates through the medium without a change. The broadening of spectral lines caused by the relaxation processes and the Doppler broadening are included into the calculations. It is shown that by multiple consistent instantaneous change of input probe field’s phase from 0 to π and back, it is possible to obtain, in the tail part of the rotated component of the probe field, a separate narrowed light pulse along with the increase in intensity by more than an order of magnitude with respect to the input field.
We present a method for generating a regular train of ultrashort optical pulses in a prepared two-level medium. The train develops from incident monochromatic probe radiation traveling in a medium of atoms, which are in a quantum mechanical superposition of dressed internal states. In the frame of linear theory for the probe radiation, the energy of individual pulses is an exponentially growing function of atom density and of interaction cross section. Pulse repetition rate is determined by the pump field's generalized Rabi frequency and can be around 1 THz and greater. We also show that the terms, extra to the dipole approximation, endow the gas by a new property: nonsaturating dependence of refractive index on dressing monochromatic field intensity. Contribution of these nonsaturating terms can be compatible with the main dipole approximation term contribution in the wavelength region of about ten micrometers (the range of CO2{\text{CO}}_{2}CO2​ laser) or larger.