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Annual Meeting Optical Society of America

Coherent amplification without inversion of femtosecond optical pulses

John N. Sweetser, Thomas J. Dunn, Ian A. Walmsley

Published Jan 1, 1993• 0 citations
Publisher

Abstract

Recent progress has been made in the theoretical and experimental study of the amplification of optical signals without population inversion (AWI) of the participating electronic levels. Induced coherence among appropriate levels in the active medium is the necessary condition for obtaining AWI. Similarly, the generation of wave packet states in atoms and molecules also relies on the coherent superposition of several states. It is feasible, therefore, that AWI is possible in a system which has been excited into a wave packet state. We have thus demonstrated experimentally such amplification in an extended sample of sodium dimers and we term this process wave packet amplification (WPA). An important characteristic of WPA is that the gain is doubly phase-sensitive—it is modulated at both the wave packet oscillation period and the optical period. The former time-dependence is a result of the periodic revivals of the pump-induced electronic polarization and the latter optical phase-sensitivity is a manifestation of the coherent nature of the molecular wave packet. The experiments to be described use a time-resolved upconversion method to detect the change in energy of a 50 fs probe pulse after it passes through a sodium heat pipe that has been pumped with another 50 fs pulse. The maximum saturated energy gain in this system is four and corresponds to all of the available energy being transferred from the wave packet to the probe pulse.

DOI
10.1364/oam.1993.tha.3

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Recent progress has been made in the theoretical and experimental study of the amplification of optical signals without population inversion (AWI) of the participating electronic levels. Induced coherence among appropriate levels in the active medium is the necessary condition for obtaining AWI. Similarly, the generation of wave packet states in atoms and molecules also relies on the coherent superposition of several states. It is feasible, therefore, that AWI is possible in a system which has been excited into a wave packet state. We have thus demonstrated experimentally such amplification in an extended sample of sodium dimers and we term this process wave packet amplification (WPA). An important characteristic of WPA is that the gain is doubly phase-sensitive—it is modulated at both the wave packet oscillation period and the optical period. The former time-dependence is a result of the periodic revivals of the pump-induced electronic polarization and the latter optical phase-sensitivity is a manifestation of the coherent nature of the molecular wave packet. The experiments to be described use a time-resolved upconversion method to detect the change in energy of a 50 fs probe pulse after it passes through a sodium heat pipe that has been pumped with another 50 fs pulse. The maximum saturated energy gain in this system is four and corresponds to all of the available energy being transferred from the wave packet to the probe pulse.
Four-wave optical parametric amplification (FWOPA) in a Raman-active medium is experimentally investigated by use of an air-filled hollow fiber. A femtosecond pump pulse shorter than the period of molecular motion excites the coherent molecular motion of the Raman-active molecules during the parametric amplification of a signal pulse. The excited coherent motion modulates the frequency of the signal pulse during the parametric amplification, and shifts it to lower frequencies. The magnitude of the frequency redshift depends on the pump intensity, resulting in intensity-dependent spectral characteristics that are different from those in the FWOPA induced in a noble-gas-filled hollow fiber.
This thesis presents original work contributing to the understanding of coherent control of wave packet dynamics in atomic and molecular systems excited by sequences of ultrafast laser pulses. An introduction to coherent control and a review of the literature is presented in chapter 1. Different approaches to coherent control are considered from the perspective of the phase complexity of the light fields used to achieve them, starting with a single transform-limited pulse, advancing through two-colour and multi-pulse schemes to chirped pulses, and finally arbitrarily shaped pulses. The purpose of this literature review is to position the work described in this thesis within the context of contemporary coherent control. In particular, our approach is seen to be in line with the current trend towards understanding the physics underlying coherent control processes. In chapter 2, development of a sodium atom and dimer beam source for use in optical coherent control experiments is described. The concepts of molecular beams are introduced along with a brief review of the literature on sodium atom and dimer beams. The beam source is considered in two parts. Firstly the sodium oven and nozzle are described in detail, along with the vacuum chamber that houses them. This is followed by a description of the experimental chamber, which contains a time-of-flight mass spectrometer. Finally a number of potential improvements to the system are discussed. Experiments to control the angular momentum composition of Rydberg electronic wave packets in atomic sodium are reported in chapter 3. The control scheme uses phase-locked pairs of transform-limited picosecond laser pulses to create pairs of identical radial wave packets in the Rydberg states. Quantum interference between the wave packets, or the excitation pathways to them, is manipulated to control the total angular composition of the resultant wave packet, which is detected either by the optical Ramsey method or by state-selective field ionisation. The results are interpreted both from the perspective of time evolution of the wave packet and the frequency content of the laser fields. The control strategy is then extended theoretically by the inclusion of linear chirp in one of the laser pulses. Chapter 4 describes progress towards a novel experiment for optical coherent control of vibrational wave packet dynamics in the sodium dimer. A coherent superposition of high-lying Rydberg states of the Na2 is excited by the broad bandwidth of a femtosecond laser pulse. The Rydberg states converge to several vibrational ionisation limits of the dimer, so the excitation effectively forms a vibrational wave packet in the X+ 22g+ ionic potential. Excitation of several wave packets by a series of phase-locked transform-limited pulses allows control over the composition and dynamics of the wave packet, and its detection using a combination of the optical Ramsey method and pulsed-field ionisation techniques adapted from ZEKE spectroscopy. A simple genetic algorithm is presented in chapter 5. It has been implemented to investigate the suitably of this type of optimisation procedure in simulations of Rydberg electronic wave packet coherent control in atomic sodium. The operation of the algorithm is described in detail. It is then tested against known solutions for two experimental scenarios: coherent control based on pairs of transform-limited laser pulses and transform-limited first pulse followed by a chirped second pulse. The behaviour of the algorithm is discussed in detail for each of these cases. Finally, the potential for application of the algorithm to unknown problems is briefly discussed.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.
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.
Introduction We were able to show in Chapter 3 that a medium in which we can obtain a population inversion (i.e. a situation in which the population density in the excited state is greater than that in the fundamental level) allows for optical gain of an electromagnetic wave having a frequency near to the resonant frequency of the system. By introducing feedback of the amplified signal into the medium, the system can be made to oscillate naturally, resulting in laser oscillations . To obtain this population inversion, we must introduce at least a third (and perhaps even a fourth) energy level into the system. (We saw how a two-level system under the influence of an intense pump beam will saturate with no resulting population inversion.) The aim then of this chapter is to introduce the concepts necessary to extend our two-level system into a working model capable of illustrating the phenomenon of laser oscillation. We will not spend too much time discussing atomic transition lasers as they do not figure readily in our treatment of quantum electronic properties of semiconductors. An exception will be made, however; we brush upon the particular topics of a diode pumped laser in Complement 4.E and a quantum cascade laser in Complement 13.H. Population inversion and optical amplification Population inversion We will show how population inversion can be achieved by carrier transfer from higher lying levels to the upper level of a two-level subsystem of interest.