Dark blue Cerenkov second harmonic generation in the two-layer-stacked hexagonal periodically poled MgO: LiNbO3s

By bonding two identical hexagon polarized lithium niobate crystals with a 30 degree intersection angle, the diffraction patterns at the second harmonic frequency for (left) a single PPMgOLN structure and (middle) a two-layer-stacked hexagonal PPMgOLN structure are recorded. (right) The magnified luminous spots on the inner and the outer ring from (middle).

The researchers, led by Prof. Wanhua Zheng, from Institute of Semiconductors, Chinese Academy of Sciences, use two identical hexagon polarized lithium niobate crystals to bond with a 30° intersection angle along Z axis to form a laminated structure. When a fundamental frequency laser with a pulse duration of picosecond or femtosecond perpendicularly on the bonded crystals, 12 diffraction points uniformly distributed on a ring are clearly observed due to the ÄŒerenkov frequency-doubled effect. As is known, there are only six diffraction points in the case of single layer hexagon polarized lithium niobate crystal. Thus, by bonding two identical hexagon polarized lithium niobate crystals with a 30 degree intersection angle, the diffraction points can be doubled. The third-order and even higher-order ÄŒerenkov effect are also observed experimentally. It is reported in Chinese Optics Letters Vol. 12, No. 3, 2014 (/col/abstract.cfm?uri=col-12-3-030501).

Given that a high-speed charged ion is propagating in transparent medium, if the ion speed is larger than the phase velocity of light in the medium, electromagnetic waves will be excited and transmit in the direction whose cosine is the ratio of phase velocity to ion speed. The intersection angle between the propagation direction of ion and electromagnetic waves is the ÄŒerenkov angle. This phenomenon is so called ÄŒerenkov effect. Similarly, in nonlinear optics, if the phase velocity of fundamental frequency light is larger than that of the second-harmonic light, a doubling-frequency light will be excited and propagates in the direction of Cerenkov angle.

A new phenomenon of Cerenkov frequency-doubled effect has been revealed in the past few years. Namely, when a beam of laser is transmitting in ferroelectric crystal such as lithium niobate, the nonlinear polarization excited at the ferroelectric domain wall will lead to the emission of second harmonics in particular direction that is called Cerenkov angle. Ever since then, this effect has been widely used in investigating the spatial distribution of the ferroelectric domain, the properties of the domain wall, and the nonlinear Talbot effect. The obtained achievements enrich the nonlinear optics significantly.

The authors report the dark blue nonlinear ÄŒerenkov radiation by second harmonic generation (SHG) in a two-layer-stacked hexagonal periodically-poled-MgO: LiNbO3s (PPMgOLNs). Based on the direct wafer bonding of two rotating PPMgOLNs, twelve bright spots as twice of those in a single PPMgOLN are observed at each second-harmonic ÄŒerenkov ring. This result suggests that the ferroelectric domain in the crystal not only can be used to generate short-wavelength laser, but also can be used to control the shape and the spatial distribution of the generated laser dot. Therefore, it can be widely used in the laser display, high-density storage, high-resolution printing, biological tissue imaging and so on.

Like the ÄŒerenkov diffraction pattern from two-layer-stacked structure, complex frequency-multipled patterns such as uniform diffraction rings would be obtained by stacking different polarized lithium niobate wafers. Moreover, if the relative displacement and rotation can made for the laminations, a dynamic or rotating diffraction pattern could be obtained, which can be used in developing special lasers, information coding, and dynamic laser display.

两层键合的六角极化掺镁铌酸锂晶体的切伦科夫蓝光倍频效应

图片说明：利用键合技术观察到的（左）单片铌酸锂晶体和（中）两片键合结构铌酸锂晶体的衍射点；（右）为（中）图中方框内放大后的光斑。

中国科学院半导体研究所集成光电子学国家重点实验室郑婉华研究员课题组利用键合技术将两片相同六角极化的铌酸锂晶体沿Z向旋转30°键合在一起形成叠层结构，当皮秒或飞秒基频激光垂直入射(即沿着Z轴)到键合晶片上, 成功地观测到切伦科夫倍频产生的12个o光衍射点, 呈环状均匀分布。通常单片六角极化的铌酸锂晶体的切伦科夫倍频有6个衍射点，叠片能使倍频衍射点正比成倍增加。实验中还观察到三次或高次谐波切伦科夫衍射。相关研究成果发表在Chinese Optics Letters 2014年第3期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-3-030501）。

高速带电粒子在透明介质中穿行，当粒子速度大于光在这种介质中的相速度（即单一频率的光波在介质中的传播速度）时，会激发出电磁波，沿着与入射粒子夹角成切伦科夫角度(即其余弦为相速度与粒子速度之比)方向传输, 这种现象称为切伦科夫辐射。类似于切伦科夫辐射，在非线性光学过程中, 如果基频光的相速大于倍频光的相速, 在特定条件下(如本研究中的畴壁处)会产生频率加倍的光波, 并沿着切伦科夫角度(即其余弦为倍频与基频相速之比)方向传输。

近年来的研究发现, 当激光在铁电晶体如铌酸锂晶体中传输时, 在铁电畴壁处所激发的非线性极化能在满足切伦科夫角的空间特定方向辐射激光的二次谐波, 这一现象被称之为切仑科夫倍频效应。切仑科夫倍频效应发现后被用来研究铁电晶体中铁电畴的空间分布、畴壁的性质、以及非线性Talbot现象等, 得到了许多有意义结果, 丰富了非线性光学的研究内容。

该研究演示了两片具有六角对称的周期极化Z切掺镁铌酸锂晶片被键合在一起后，在光束垂直入射时产生的十二重切伦科夫蓝光倍频衍射效应。这一结果显示，铁电晶体中的电畴不但可以用来产生短波长激光, 如本研究中的蓝光, 也可以用来控制所产生的激光光斑的形状和在空间的分布, 因而在激光显示、高密度存储、高分辨率打印和生物组织成像等诸多领域具有广泛应用。

类似于两层叠层结构的切伦科夫倍频衍射图形，更多不同极化图型的铌酸锂晶片的叠合可得到更多、更复杂的倍频图案, 甚至呈环状均匀分布。如果叠片之间能相对位移或旋转, 可使衍射图型产生动态变化或旋转, 这可用于研制特种激光器、信息编码和动态激光显示等。