35 research outputs found
Complete elimination of nonlinear light-matter interactions with broadband ultrafast laser pulses
The absorption of a single photon that excites a quantum system from a low to
a high energy level is an elementary process of light-matter interaction, and a
route towards realizing pure single-photon absorption has both fundamental and
practical implications in quantum technology. Due to nonlinear optical effects,
however, the probability of pure single-photon absorption is usually very low,
which is particularly pertinent in the case of strong ultrafast laser pulses
with broad bandwidth. Here we demonstrate theoretically a counterintuitive
coherent single-photon absorption scheme by eliminating nonlinear interactions
of ultrafast laser pulses with quantum systems. That is, a completely linear
response of the system with respect to the spectral energy density of the
incident light at the transition frequency can be obtained for all transition
probabilities between 0 and 100% in a multi-level quantum systems. To that end,
a new multi-objective optimization algorithm is developed to find an optimal
spectral phase of an ultrafast laser pulse, which is capable of eliminating all
possible nonlinear optical responses while maximizing the probability of
single-photon absorption between quantum states. This work not only deepens our
understanding of light-matter interactions, but also offers a new way to study
photophysical and photochemical processes in the "absence" of nonlinear optical
effects.Comment: 11 pages, 5 figure
Two-photon induced ultrafast coherence decay of highly excited states in single molecules
Coherence is a key aspect of a large variety of processes, ranging from the coherent delocalisation of excitation energy, which is important for energy transfer in supramolecular nanostructures, to coherence between electronic states of a single quantum system, which is essential for quantum optical applications. Coherent control schemes exploit this quantum mechanical property by actively manipulating the outcome of dynamical processes. Moreover, this technique allows measuring dynamical processes under the influence of dephasing. However, going beyond the ensemble averaged situation, i.e. working on the level of single quantum systems, is highly challenging for quantum systems embedded in a solid matrix at elevated temperature. Since interactions between the quantum system and its specific local environment are a priori unknown, this requires a reliable approach to retrieve the relevant parameters governing the ultrafast coherent dynamics. Here, we present measurements of the ultrafast coherence decay of two-photon accessible excited states in single organic molecules embedded in a disordered environment at room temperature. Wecombine this experimental approach with a quantum dynamics identification procedure, which yields a minimum three-level model to describe the obtained data with very good agreement. In particular, we are able to retrieve the ultrafast (coherent) excited state dynamics in single molecules and demonstrate its sensitivity to the local nanoenvironment from molecule to molecule. This work provides a robust approach to measure and analyse ultrafast quantum dynamics in complex nanosystems
Orientational quantum revivals induced by a single-cycle terahertz pulse
The phenomenon of quantum revivals resulting from the self-interference of
wave packets has been observed in several quantum systems and utilized widely
in spectroscopic applications. Here, we present a combined analytical and
numerical study on the generation of orientational quantum revivals (OQRs)
exclusively using a single-cycle THz pulse. As a proof of principle, we examine
the scheme in the linear polar molecule HCN with experimentally accessible
pulse parameters and obtain strong field-free OQR without requiring the
condition of the sudden-impact limit. To visualize the involved quantum
mechanism, we derive a three-state model using the Magnus expansion of the
time-evolution operator. Interestingly, the THz pulse interaction with the
electric-dipole moment can activate direct multiphoton processes, leading to
OQR enhancements beyond that induced by a rotational ladder-climbing mechanism
from the rotational ground state. This work provides an explicit and feasible
approach toward quantum control of molecular rotation, which is at the core of
current research endeavors with potential applications in atomic and molecular
physics, photochemistry, and quantum information science.Comment: 17 pages, 6 Figure