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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Interrupted virtual single-photon transition

J. Herrmann, M. Weger, Reto Locher, Mazyar Sabbar, P. Rivière, Ulf Saalmann, Jan Michael Rost, L. Gallmann, Ursula Keller

Published May 1, 2013• 0 citations
Publisher

Abstract

The temporal evolution of the dipole response of a system excited by an electromagnetic field usually is not accessible with traditional spectroscopy. Only the time-integrated dipole response (TIDR) is detected. Here, we investigate the case of the off-resonant excitation of a quantum-mechanical two-level system (TLS) where the TIDR is expected to be zero. Our time-frequency representation of the dipole response reveals that even in this case positive and negative contributions are present during its temporal evolution. The zero TIDR is a result of the exact balance of positive and negative contributions, which cancel out. We present a way to create and control optical gain and loss by interrupting the evolution of the dipole in time. In this way, we make these nonzero contributions accessible for spectroscopy.

DOI
10.1109/cleoe-iqec.2013.6801160

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The temporal evolution of the dipole response of a system excited by an electromagnetic field usually is not accessible with traditional spectroscopy. Only the time-integrated dipole response (TIDR) is detected. Here, we investigate the case of the off-resonant excitation of a quantum-mechanical two-level system (TLS) where the TIDR is expected to be zero. Our time-frequency representation of the dipole response reveals that even in this case positive and negative contributions are present during its temporal evolution. The zero TIDR is a result of the exact balance of positive and negative contributions, which cancel out. We present a way to create and control optical gain and loss by interrupting the evolution of the dipole in time. In this way, we make these nonzero contributions accessible for spectroscopy.
We examine the optical response of a virtual dipole transition of a quantum mechanical two-level system (TLS). In the case of off-resonant excitation the time-integrated dipole response (TIDR) is expected to be zero, which corresponds to transparency of the system with respect to the exciting pulse. Our new time-frequency representation reveals that even for a zero TIDR there are positive and negative contributions included in the response. Furthermore, we present a way to access these contributions by using a second electromagnetic field, which interrupts the temporal evolution of the dipole response. The theoretical results are confirmed by attosecond transient absorption spectroscopy in helium (He).
The response of matter to an optical excitation consists essentially of absorption and emission. Traditional spectroscopy accesses the frequency-resolved and time-integrated response, while the temporal evolution stays concealed. However, we will demonstrate here that the temporal evolution of a virtual single-photon transition can be mapped out by a second pulsed electromagnetic field. The resulting optical signal shows optical gain and loss, which can be gated and controlled via the relative delay of the electromagnetic fields.
Abstract This work is dedicated to the generalization of the spectroscopic correspondence principle to the time dependence of the evolution of a dipole-allowed quantum transition in a two-level system (TLS) under the action of an electromagnetic pulse (EMP). The formulas for the time dependence of the absorbed EMP energy for excitation of a classical oscillator and an oscillator of a transition in a TLS are derived within the framework of the first order of perturbation theory. It is shown that in the case of zero width of a transition line (or in case of zero damping constant γ for a classical oscillator) classical and quantum approaches give the same results for arbitrary EMP parameters (duration and carrier frequency). A noticeable distinction between the time dependences of the absorbed energy for a classical oscillator and a quantum TLS with a Lorentz spectral profile is observed for sufficiently long pulses.
The reconstruction of the full temporal dipole response of a strongly driven time-dependent system from a single absorption spectrum is demonstrated, only requiring that a sufficiently short pulse is employed to initialize the coherent excitation of the system. We apply this finding to the time-domain observation of Rabi cycling between doubly excited atomic states in the few-femtosecond regime. This allows us to pinpoint the breakdown of few-level quantum dynamics at the critical laser intensity near 2 TW/cm^{2} in doubly excited helium. The present approach unlocks single-shot real-time-resolved signal reconstruction across timescales down to attoseconds for nonequilibrium states of matter. In contrast to conventional pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The potential future applications of this technique range from testing fundamental quantum dynamics in strong fields to measuring and controlling ultrafast chemical and biological reaction processes when applied to traditional transient-absorption spectroscopy.
The absorption of a single photon by a single two-level system (TLS) in free space is a fundamental physical process. In this contribution, we present the experimental realization of a model system to simulate the dynamics of such an absorption experiment: Comparing the absorption process of a single photon by a single TLS and the incoupling dynamics of an optical resonator, one finds an analogy between the energy stored inside the resonator and the probability of the photon to be absorbed by the TLS [1]. Both systems, resonators and TLS, respond in an equivalent way to the temporal profile of the incident light pulse. The energy storage in a resonator as well as the absorption of a single photon by a TLS can reach an unit efficiency under idealized conditions. Such an optimized process is achieved by an exponentially rising incident light field with a time constant which matches the lifetime of the system [2]. For such a perfect coupling, the end mirror of the resonator needs to have a reflectivity of unity. This corresponds to the excitation of a TLS from the complete solid angle. For a smaller reflectivity of the end mirror, the cavity simulates the absorption dynamics in the case of excitation from only a part of the solid angle. However, using real mirrors, a good analogy with excitation from a large solid angle can still be obtained by using a strongly asymmetric resonator, i.e. an incoupling mirror which has a significantly lower reflectivity than the end mirror of the cavity.