Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Selection of stimulated Raman scattering signal by entangled photons Purevdorj Munkhbaatar a,b , Kim Myung-Whun b,c, ⁎ a Department of Physics and Electronics, National University of Mongolia, Ulaanbaatar 210646, Mongolia b Institute of Photonics and Information Technology, Chonbuk National University, Jeon-ju 54896, Republic of Korea c Department of Physics, Chonbuk National University, Jeon-ju 54896, Republic of Korea ARTICLE INFO Keywords: Excitation-probe measurement Entangled photon pulses Stimulated Raman scattering ABSTRACT We propose an excitation-probe measurement method utilizing entangled photon pulses. The excitation-probe signal is dominated by stimulated Raman scattering as well as two-photon absorption when the time delay between the excitation pulse and the probe pulse is shorter than the pulse duration. We demonstrate that the two-photon-absorption signal can be suppressed when the photons of the pulses are entangled. The stimulated Raman scattering signal can be composed of many peaks distributed over broad photon energies owing to the transitions between numerous quantum states in complex materials. We show that the desired peaks among the many peaks can be selected by controlling the thickness of the nonlinear crystal, the pump pulse center frequency, and the polarization of the excitation pulse and probe pulse. 1. Introduction Excitation-probe (or pump-probe) technique is useful to investigate the optical excitations between quantum states of matter [1,2]. Light pulse can excite electric charges, thereby creating an excited state in the sample material. The excited state decays spontaneously to the ground state or to a quasi-ground state by emitting photon fields. In the presence of strong radiation such as in the excitation-probe measure- ment, stimulated emission becomes more dominant while spontaneous emission becomes negligible. During the excitation-probe measure- ment, a part of probe pulse interferes with the emitted photon field, producing measurable change in the intensity of probe pulse itself, which often appears as sinusoidal function of pulse delay time [3]. The sinusoidal modulation is the signature of a stimulated Raman scatter- ing (SRS) process in the sample material [4–6]. In complex materials, various interactions participate in the formation of Raman active modes. Simple harmonic lattice interaction is most prevailing, but spin-exchange or orbital-exchange interaction between neighboring charges also constitute Raman active modes [1]. The interactions are often strongly coupled to each other, so indepen- dent modes can be merged into one composite mode soon after the excitation. Early time-delay signal of the excitation-probe measure- ment is essential to understand the intrinsic properties of the Raman modes. However, the conventional excitation-probe signal suffers from the strong nonlinear background when excitation pulse and probe pulse are overlapped. Fortunately, stimulated Raman scattering (SRS) and two-photon absorption (TPA) are dominant in simple excitation- probe processes [3]. If it is possible to separate the TPA signal from the SRS signal in typical excitation-probe measurement geometry, the early time-delay SRS signal can be investigated by the excitation-probe measurement. Some techniques have been invented to discriminate the SRS and TPA signals. Some of the techniques utilized the combinations of classical optical devices including electro-optical or acousto-optical devices [7,8]. Recently, quantum mechanically entangled light becomes used to improve the performance of nonlinear spectroscopic techni- ques. A theoretical approach showed that the intensity of TPA can be manipulated in the excitation-probe measurement if entangled photons were used [9]. A quantum SRS technique was proposed theoretically, which suggested the combination of entangled light source produced via type-II parametric down conversion and Hanburry-Brown-Twiss type interferometry to enhance the resolution and selectivity of Raman signals [10]. The study theoretically demonstrated that SRS signals obtained by the sixth-order perturbation of entangled photon pulse fields can eliminate the off-resonant background and can select Raman gain and Raman loss processes. However, so far it has been rarely shown the way to suppress the intensity of TPA in the excitation-probe measurement by utilizing the entanglement of photon pulse excitation- probe measurement. Conventional excitation-probe measurements in many laboratories are performed with two optical modes with wave vectors k 1 and k 2 and frequencies ω 1 and ω 2 in the simplest description. We have thought about a way of suppressing TPA signal in two-mode geometry. Suppose (k 1 , ω 1 ) is the excitation mode and (k 2 , ω 2 ) is the probe mode. Typical http://dx.doi.org/10.1016/j.optcom.2016.09.061 Received 2 April 2016; Received in revised form 9 September 2016; Accepted 29 September 2016 ⁎ Corresponding author at: Institute of Photonics and Information Technology, Chonbuk National University, Jeon-ju 54896, Republic of Korea. E-mail address: mwkim@chonbuk.ac.kr (K. Myung-Whun). Optics Communications 383 (2017) 581–585 0030-4018/ © 2016 Elsevier B.V. All rights reserved. Available online 14 October 2016 crossmark