Transient transmission oscillations in doped and undoped lithium niobate induced by near-infrared femtosecond pulses
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ARTICLE Transient transmission oscillations in doped and undoped lithium niobate induced by near-infrared femtosecond pulses Bryan J. Crossman and Gregory J. Tafta) Department of Physics, College of St. Benedict/St. John’s University, Collegeville, Minnesota 56321, USA (Received 15 August 2018; accepted 12 October 2018)
Transient transmission oscillations in X-cut and Z-cut congruent, iron-doped, and magnesium-doped lithium niobate samples were measured using 50 fs, 800 nm, 0.5 nJ pulses from a self-mode-locked Ti:sapphire laser in an optical pump–probe system. Several Raman-active oscillation modes excited by these pulses were observed as changes in the transmitted probe intensity versus time delay between the pump and probe pulses. The samples were rotated to determine how the incident polarization of the pump pulses affects the mode excitations. The observed Raman-active oscillations correspond to previously reported symmetry modes measured with traditional, continuous-wave, Raman spectroscopy using the same scattering geometry. In addition, a polariton mode and other, previously unreported, lower-frequency modes were observed in each of the samples. The transmission intensity data for each sample were fit successfully to a superposition of sinusoidal functions with exponentially decaying amplitudes.
I. INTRODUCTION
Lithium niobate (LiNbO3) is a ferroelectric material with interesting nonlinear optical, electro-optic, and piezoelectric properties that allow for applications in holographic memory storage, fast optical switching, and waveguide writing.1–4 Because of the interest in utilizing lithium niobate in optical devices, it has been studied extensively with traditional continuous-wave (CW) Raman spectroscopy for different crystal cut orientations and incident light polarizations.5–7 With the development of lasers capable of producing femtosecond pulses, suitably high electric field intensities and short pulse durations allowed for the possibility of making timedomain observations of coherent phonon oscillations in crystal lattices.8–10 The coherent phonon oscillation modes are Raman active, which lead to easy comparison with Raman spectra. Impulsive stimulated Raman scattering (ISRS) is a process in which a sufficiently highenergy pump pulse stimulates these vibrational modes, which are then observed by sending a lower-energy probe pulse into the excited sample at a controllable time delay after the pump pulse.11 The pump-induced excitations in the sample can cause changes in the amount of probe pulse energy transmitted through the sample or small rotations to the polarization of the transmitted probe pulse.12 Both processes can lead to changes in the amount of detected probe energy. In general, multiple vibrational modes of different frequencies are excited simultaneously, causing a beating pattern in the time-resolved a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.414 J. Mater. Res., Vol. 33, No. 24, Dec 28, 2018
scans on the sub-picosecond timescale. By comp
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