Chapter 3
Enhanced and Selective Two-Photon
Excitation of Molecular Vibronic States
Using Entangled Photons
Hisaki Oka
Abstract We introduce molecular two-photon absorption and two-step excitation
using quantum-mechanically entangled-photon pairs. The entangled photons possess
the inherent simultaneity of photons originating from quantum correlation with
energy anticorrelation, which is not shared by conventional laser light. This simultaneity enables entangled photons to achieve highly efficient two-photon absorption
and two-step excitation under the condition of low light intensity. We show that
ultrabroadband frequency-entangled photons can enhance the transition rates of twophoton absorption and two-step excitation more than 1000 times, compared to uncorrelated photons like laser light, and can also selectively excite a single vibrational
mode in the molecular two-photon processes.
Keywords Two-photon absorption · Two-step excitation · Entangled photons
3.1 Introduction
Two-photon process is an optical nonlinear process in which two photons contribute
to excitation of quantum states in materials. Generally, this process is classified
into two types, namely two-photon absorption and two-step (stepwise) excitation as
depicted in Fig. 3.1. In the two-photon absorption (Fig. 3.1a), the energy ω m of intermediate state is far-off-resonant to the energy ω of an incident photon, and hence the
intermediate state is not really but virtually excited only during the photon–molecule
interaction. This short-lived intermediate state is therefore called “virtual state.” On
the other hand, in the two-step excitation (Fig. 3.1b), one photon is absorbed by
the intermediate state and then another photon is absorbed by the excited state stepwisely. Generally, the energy configuration of 2ω ≈ ω e is required for the two-photon
absorption, whereas ω ≈ ω m and ω ≈ ω e − ω m are sufficient for the two-step excitation. As a result, the two-photon absorption is unfavorable compared to the two-step
excitation because simultaneous absorption of two photons is required.
H. Oka (B)
Division of Engineering, Faculty of Engineering, Niigata University, Niigata 950-2102, Japan
e-mail: h-oka@eng.niigata-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
H. Miyasaka et al. (eds.), Photosynergetic Responses in Molecules
and Molecular Aggregates, https://doi.org/10.1007/978-981-15-5451-3_3
43
Enhanced and Selective Two-Photon
Excitation of Molecular Vibronic States
Using Entangled Photons
Hisaki Oka
Abstract We introduce molecular two-photon absorption and two-step excitation
using quantum-mechanically entangled-photon pairs. The entangled photons possess
the inherent simultaneity of photons originating from quantum correlation with
energy anticorrelation, which is not shared by conventional laser light. This simultaneity enables entangled photons to achieve highly efficient two-photon absorption
and two-step excitation under the condition of low light intensity. We show that
ultrabroadband frequency-entangled photons can enhance the transition rates of twophoton absorption and two-step excitation more than 1000 times, compared to uncorrelated photons like laser light, and can also selectively excite a single vibrational
mode in the molecular two-photon processes.
Keywords Two-photon absorption · Two-step excitation · Entangled photons
3.1 Introduction
Two-photon process is an optical nonlinear process in which two photons contribute
to excitation of quantum states in materials. Generally, this process is classified
into two types, namely two-photon absorption and two-step (stepwise) excitation as
depicted in Fig. 3.1. In the two-photon absorption (Fig. 3.1a), the energy ω m of intermediate state is far-off-resonant to the energy ω of an incident photon, and hence the
intermediate state is not really but virtually excited only during the photon–molecule
interaction. This short-lived intermediate state is therefore called “virtual state.” On
the other hand, in the two-step excitation (Fig. 3.1b), one photon is absorbed by
the intermediate state and then another photon is absorbed by the excited state stepwisely. Generally, the energy configuration of 2ω ≈ ω e is required for the two-photon
absorption, whereas ω ≈ ω m and ω ≈ ω e − ω m are sufficient for the two-step excitation. As a result, the two-photon absorption is unfavorable compared to the two-step
excitation because simultaneous absorption of two photons is required.
H. Oka (B)
Division of Engineering, Faculty of Engineering, Niigata University, Niigata 950-2102, Japan
e-mail: h-oka@eng.niigata-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
H. Miyasaka et al. (eds.), Photosynergetic Responses in Molecules
and Molecular Aggregates, https://doi.org/10.1007/978-981-15-5451-3_3
43