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Fabrication of quasi-phase-matching structure in lithium tetraborate

Kensaku Maeda ,  Kozo Fujiwara ,  Satoshi Uda 

Tohoku University, Institute for Materials Research (IMR), 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan

 The development of an all-solid-state vacuum-UV light source is essential for achieving ultrafine laser processing. Such an all-solid-state light source must be both inexpensive and maintenance free. Devices with QPM (quasi-phase-matching) structure can reduce the incoming wavelength by half. It can be achieved in structures in which the sign of the nonlinear optical coefficient is periodically reversed. This QPM structures can be formed in ferroelectric materials such as LiNbO3 and LiTaO3 by applying an external electric field. However, the converted light with vacuum-UV cannot pass through ferroelectric materials since they are opaque to vacuum-UV (wavelength: < 300 nm).

 Lithium tetraborate (Li2B4O7; LB4) has a high nonlinear optical coefficient (d33 = 0.93 pm/V), and transparent to light with wavelengths down to 160 nm. However, LB4 is not ferroelectric and thus polarization methods cannot apply to the formation of QPM in LB4. Thus, to realize a QPM structure in LB4, we propose to make a crystal with periodically inverted c-axis twin structure.

 We investigate the relationship between crystal growth directions and twin boundary orientations in LB4. When the growth direction was between <100> and <110>, (010) twin boundaries formed. In contrast, when the growth direction was between <110> and <010>, (100) twin boundaries formed. We developed a twin boundary control method based on the above relationship. The intervals of the twin boundaries are dependent on the fineness of heating method. A fabricated crystal includes 100 μm interval periodic twin boundaries and can function as a 5th-ordered QPM structure for a second-harmonic-generation of Nd:YAG laser (1064 nm). A finer periodically twinned crystal would be required to tune in shorter phase-matching wavelengths.

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Related papers

Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Kensaku Maeda
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-25 10:34
Revised:   2013-04-11 14:43