DOI 10.1007/s10946-016-9568-6

Journal of Russian Laser Research, Volume 37, Number 3, May, 2016

SECOND HARMONIC GENERATION IN MICROSTRUCTURED BARIUM TITANATE V. S. Gorelik,1 ∗ K. I. Zaitsev,2 V. A. Lazarev,2 S. O. Leonov,2 S. O. Yurchenko,2 Yu. P. Voinov,1 L. I. Zlobina,1 and P. P. Sverbil1 1 Lebedev

Physical Institute, Russian Academy of Sciences Leninskii Prospect 53, Moscow 119991, Russia 2 Bauman

Moscow State Technical University Vtoraya Baumanskaya Street, Moscow 107005, Russia ∗ Corresponding

author e-mail:

gorelik @ sci.lebedev.ru

Abstract We investigate the second harmonic generation under femtosecond pulse-periodic laser radiation in barium titanate in the form of ceramics, in pores of a globular photonic crystal, and in a water colloidal suspension. We measure the dependence of the second harmonic radiation intensity on the incident laser power. Excitation of the second harmonic was carried out by powerful (108 W) pulses of a solid-state Yb:KGW laser (wavelength, 1,026 nm) operating at 200 kHz. We estimate the efficiency of the second harmonic generation in various microstructured phases of barium titanate and show that the threshold of plasma formation in a suspension of barium titanate microparticles in water is substantially higher than in ceramics and in the ferroelectric photonic crystal. The second-harmonicgeneration power can be significantly increased in a water suspension of barium titanate microparticles.

Keywords: second-harmonic generation, barium titanate, microparticles, suspension, ferroelectric, photonic crystal.

1.

Introduction

Nonlinear optical materials, which provide generation of laser-radiation optical harmonics, are used in various applications. These enable conversion of laser radiation from near infrared to ultraviolet [1– 5]. Among known nonlinear optical materials, a special place is occupied by barium titanate, which is characterized by high values of the second-order nonlinear optical susceptibility and electro-optical coefficients [3]. A number of studies on the second harmonic generation (SHG) in BaTiO3 [4–6] have been carried out to date. However, the small size of single-crystal barium titanate and the lack of conditions necessary for the implementation of phase synchronism restrict the potential of barium titanate for efficient generation of optical harmonics. Attempts to increase the SHG efficiency were made in [7] where barium titanate thin films were investigated under excitation by 1,064 nm Nd:YAG, 10 ns laser pulses with a 50 Hz repetition rate and peak power up to 5·107 W/cm2 . In [8,9], the second harmonic generation in barium titanate nanoparticles was investigated at various levels of excitation (up to 2·1011 W/cm2 and 3·1015 W/cm2 , respectively). Similar measurements were carried out with other materials. In particular, in [10] the second harmonic generation was investigated in KTP and LiNbO3 nanoparticles excited by a 10–25 kHz repetition-rate Nd:YAG laser, as well as a 76 MHz Ti:sapphire laser. In the 1.06 μm Nd:YAG laser, pulse duration varied Translated from manuscript submitted on October 20, 2015. c 2016 Springer Science+Business Media New York 1071-2836/16/3703-0254  254

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Journal of Russian Laser Research

from 320 to 950 ns for 25 kHz operation, and laser power varied from 50 to 300 mW. The Ti:sapphire laser generated 100 fs pulses at 795 nm with a 76 MHz repetition rate and 5–30 mW laser power. An increase in exciting pulsed radiation power was found to lead to degradation of samples, preventing generation of larger-power SHG. Owing to the development of femtosecond laser equipment, the pattern of nonlinear optical experiments was considerably changed, and the possibility to study processes at extremely high-power laser radiation exceeding 1012 W/cm2 appeared. There were publications in the literature on the increase in SHG efficiency in nonlinear optical materials affected by superstrong light fields [11,12]. Still, the limiting factor of this increase is the plasma-formation process [13] observed at the action of superstrong light fields. The plasma that forms under the influence of high-intensity radiation in a sharp focusing mode modifies the linear and nonlinear susceptibilities of the substance and decreases the SHG efficiency. An increase in the plasma formation threshold (in particular, by embedding barium titanate into the structure of a photonic crystal [14], or producing a colloidal suspension in water) will enable an increase in the intensity limit of the incident laser radiation and an improvement in the SHG efficiency. The purpose of this work was to study features of the SHG process in barium titanate in three various structural modifications, namely, ceramics, photonic crystals, and colloidal suspensions in distilled water under high-intensity radiation of a femtosecond laser. We also investigate the plasma-formation threshold and assessed the SHG efficiency.

2.

Experimental

The experimental setup is shown in Fig. 1. As a source of radiation, use was made of a Yb:KGW laser consisting of the main oscillator and the regenerative amplifier. The duration of laser radiation pulses at a wavelength of 1,026 nm was 250 fs, the repetition frequency was 200 kHz, and the peak power reached 70 MW at an average power of 3.6 W. The maximum energy in a single laser pulse was 10 μJ.

Fig. 1. Experimental setup for SHG studies in BaTiO3 . Here, Yb:KGW laser 1, focusing lens (f  = 30 mm) 2, investigated samples 3, optical fiber holder 4, optical fiber 5, and minispec- Fig. 2. The second harmonic generation in a water suspension of barium titanate. trometer 6.

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Various structural modifications of barium titanate were used as samples under investigation, namely, ceramics, a globular photonic crystal with BaTiO3 embedded into pores, and a colloidal suspension of barium titanate in distilled water. The photonic crystal had a regularly ordered structure constructed from opal spheres (globules) 250 nm in diameter. The suspension of barium titanate in water was made from powder of milled ceramics of BaTiO3 with an average particle diameter of ∼50 μm. The suspension was placed into a cylindrical quartz cuvette with plane–parallel windows. A series of experiments with three various structural modifications of barium titanate was carried out. Radiation of a femtosecond laser was focused at an end face of each sample under study. The SHG radiation and a part of the exciting radiation reflecting from the sample was registered by means of a fiber light guide and a minispectrometer (CCS200, Thorlabs, spectral resolution 2 nm, spectral range 200–1000 nm). A snapshot of the experimental setup at the second harmonic generation in a water suspension of barium titanate is shown in Fig. 2.

a)

c)

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b)

Fig. 3. Second-harmonic and secondary-radiation spectra at pumping powers ranging from 0.07 up to 3.6 W for three different structural modifications of barium titanate: in the ceramics (a), where curves 1–10 correspond to 3.6, 2.8, 2.2, 1.3, 1.0, 0.75, 0.54, 0.36, 0.18, and 0.07 W pumping powers, respectively, in the opal-based photonic crystal (b), where curves 1–8 correspond to 3.4, 3.0, 2.2, 1.6, 1.3, 1.0, 0.5, and 0.07 W pumping powers, respectively, and curve 9 illustrates the spectrum of the plasma in air without a sample, and in the suspension of barium titanate microparticles in distilled water (c), where curves 1–5 correspond to 3.6, 2.9, 2.5, 2.2, and 1.6 W pumping powers, respectively.

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3.

Journal of Russian Laser Research

Results and Discussion

In the experiments, the second harmonic spectra, the reflected pumping, and the secondary radiation were registered at different average pumping powers ranging from 0.07 up to 3.6 W. Figure 3 shows the spectra of the second harmonic and secondary radiation for three investigated structural modifications of barium titanate, namely, in the ceramics (Fig. 3 a), in an opal-based photonic crystal (Fig. 3 b), and in a suspension of barium titanate microparticles in distilled water (Fig. 3 c). In Fig. 3 a and b, we present additional spectral components caused by formation of plasma at large pumping power. Curve 9 in Fig. 3 b illustrates the spectrum of the plasma in air without sample. In Fig. 3 a, we see that the intensity of the second harmonic generation in BaTiO3 ceramics drops at a pumping power greater than 0.75 W. In the case of a BaTiO3 suspension, the SHG intensity gradually increases as the pumping power goes up, and no SHG limitation is observed. Comparing the spectra in Fig. 3 a–c, we see that in switching from the photonic crystal and ceramics to the water suspension of barium titanate, the intensity of the plasma spectrum sharply falls. This indicates that the plasma formation threshold in the water suspension increases as compared to the ceramic and photonic-crystal samples. The data obtained in the experiments were used to plot dependences of the second harmonic radiation intensity on the pumping power for three structural modifications of barium titanate (see Fig. 4). The decrease in the intensity of radiation of the second harmonic at pumping power exceeding 0.75 W for the case of ceramic BaTiO3 (curve 1) is explained by the process of plasma formation. The efficiency of conversion to the second harmonic for ceramic BaTiO3 at a power of 0.75 W was 1.5%. The SHG efficiency in the case of BaTiO3 embedded into the photonic crystal and in the suspension of BaTiO3 microparticles in water at the maximum pumping power reached 0.3%. No limitation of SHG Fig. 4. The second harmonic intensity versus power was observed, and the possibility of a further the pumping power for BaTiO in the form of ce3 increase in the generation efficiency and power of the ramics (curve 1), in the opal-based photonic cryssecond harmonic with increase in exciting radiation tal (curve 2), and in the suspension of BaTiO3 microparticles in distilled water (curve 3). power was preserved.

4.

Summary

We investigated the features of the SHG processes at a high-power femtosecond laser pumping in barium titanate in its three various structural modifications: in ceramics, in a photonic crystal, and in a fine suspension in distilled water. We established an increase in the most achievable power of SHG for a suspension of barium titanate microparticles in distilled water and showed the possibility of a substantial increase in the plasma-formation threshold for the case of a fine suspension of barium titanate powder in distilled water. A further rise in the efficiency of second- and higher-harmonics generation [15, 16] is possible with increase in exciting radiation power.

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Acknowledgements The authors acknowledge the financial support provided by the Russian Foundation for Basic Research under Projects Nos. 14-02-00190, 16-38-60147 mol a dk, and 16-52-00026

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