Enhancement of second harmonic generation by wavefront shaping in the scratch of BaMgF4 nanocrystal powder film HAOYING WU, ZHUO WANG, HAIGANG LIU, YANQI QIAO, YUANLIN ZHENG, AND XIANFENG CHEN* The State Key Laboratory on Fiber Optic Local Area Communication Networks and Advanced Optical Communication Systems, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China *xfchen@sjtu.edu.cn

Abstract: During the exploration of nonlinear crystal, such as BaMgF4 crystal for all-solid-state lasers in the vacuum ultraviolet region, many problems emerged in the scattering centers. These were caused by immature crystal-growth technology, which may bring difficulties to the periodic poling process for quasi-phase-matching. In previous studies, research has shown that nonlinear random materials can also be used for frequency conversion. In this paper, a random nonlinear process was observed when the fundamental wave is illuminated onto the scratch of the BaMgF4 nanocrystal powder film. Then, the second-harmonic waves scattered from the nonlinear turbid media are re-collected to a forward direction using feedback wavefront shaping. The method shows a repeated way to improve the conversion efficiency, which may be viable to improve the second order conversion efficiency of BaMgF4 at other wavelengths, especially in the VUV regime. Additionally, more interesting applications in random nonlinear material, such as nanocrystal ceramics, can be expected in the future.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction All-solid-state lasers (ASSLs) have a lot of advantages, such as narrow bandwidth, high beam quality and compact setup compared with currently used excimer laser [1]. Due to these advantages, ASSLs in the vacuum ultraviolet (VUV) have been required for optical lithography, photoelectron spectroscopy and medical uses [2,3]. In 2007, C. Chen et.al. obtained watt-level average-power from 185 nm to 200 nm by using KBe2BO3F2 (KBBF) crystal [3]. Compared with KBBF, a new ferroelectric crystal BaMgF4 (BMF) was found to be another candidate for ASSLs in the VUV and mid-IR region because of its transparency from 125 nm to 13 μm [4,5]. The second-order nonlinear optical coefficient of BMF was reported to be d31 = 0.15 pm/V, d32 = 0.36 pm/V, d33 = 0.12 pm/V [2]. Several years ago, E.G. Villora et al. obtained the shortest frequency-doubling emission at 368 nm by quasi phase matching in BMF crystal [5,6]. Nonetheless, scattering centers are always present in the BMF crystal grown by the temperature gradient technique or the Czochralski method [7,8]. These scattering centers may bring difficulties to the periodic poling process for quasi-phase-matching, and restrict device performance [9]. Besides, the birefringence in these materials was too small to birefringently phase match any interactions to generate VUV [10].

The random nonlinear system has emerged as a new research interest because it does not have strict requirement of crystals. It was demonstrated that the purposeful focusing of a second harmonic generated and scattered from superfine lithium niobate nanocrystal powder via feedback wavefront shaping method [11].

In this paper, the second harmonic generation (SHG) scattered from BMF nanocrystal powders was enhanced by feedback wave-front shaping, and the efficiency can be enhanced

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#345315 Journal © 2018

https://doi.org/10.1364/OME.8.003593 Received 7 Sep 2018; revised 20 Oct 2018; accepted 22 Oct 2018; published 29 Oct 2018

by 30 times. obviously whreason is that also played an

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Vol. 8, No. 11 | 1 Nov 2018 | OPTICAL MATERIALS EXPRESS 3594

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Vol. 8, No. 11 | 1 Nov 2018 | OPTICAL MATERIALS EXPRESS 3595

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Vol. 8, No. 11 | 1 Nov 2018 | OPTICAL MATERIALS EXPRESS 3596

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Vol. 8, No. 11 | 1 Nov 2018 | OPTICAL MATERIALS EXPRESS 3597

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Funding National KeyFoundation oTechnology o

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