Ordered nano-scale domains in lithium niobate single crystals via phase-mask assisted all-optical poling
Introduction
Fabrication of periodically inverted domain patterns in ferroelectric materials such as lithium niobate and lithium tantalate has been widely researched for the realization of applications as diverse as quasi-phase-matched (QPM) non-linear devices, electro-optic Bragg deflectors, photonic band-gap structures, and piezoelectric devices such as micro-resonators, atom traps and micro-cavities. While several techniques such as Li2O out-diffusion [1], proton-exchange followed by heat treatment [2], Ti-indiffusion [3], scanning force microscopy [4], e-beam [5], [6] and electric field poling [7] have successfully demonstrated domain inversion in lithium niobate crystals over the past few years, even the most routinely used technique of electric-field-induced domain inversion (E-field poling) becomes problematic when periodicities of a few microns and below are required for first-order QPM non-linear processes at blue to near-ultraviolet wavelengths.
To overcome the limitations associated with E-field poling, the technique of light-assisted E-field poling (LAP) which takes advantage of the ultraviolet light-induced transient change in the coercive field of the illuminated ferroelectric material has been developed during the past few years for lithium tantalate [8], [9] and lithium niobate [10], [11], [12] crystals. Similar LAP experiments that use high intensity visible laser light, which has the effect of reducing the coercive field through a light-induced space charge field, have recently demonstrated directly written domain structures of ∼2 μm width in undoped lithium niobate [13] and ∼2 μm overall size in doped lithium niobate [14] samples.
Furthermore, an even simpler method for surface domain inversion has been investigated recently. This method exploits the interaction of intense ultraviolet laser light with ferroelectric lithium niobate to fabricate inverted ferroelectric domains of sub-micron width and few micron separation [15]. The resulting all-optically poled (AOP) ferroelectric domains, as described in reference [15], nucleate randomly within the irradiated laser spot and propagate preferentially along the principal crystal symmetry directions. Of course, for any practical application it is necessary to have control over the nucleation and propagation of the ferroelectric domains using such a method.
In this paper, it is shown that it is indeed possible to impose a degree of control in the alignment of these UV-induced surface domains by illuminating with a spatially modulated UV laser beam. More specifically it is shown that it is possible to obtain ordered and aligned surface domains on the +z face of the crystal via an intensity pattern produced from a phase mask. The characterization of the laser-modified crystals and the domain nature was investigated using hydrofluoric acid etching of the UV exposed surface.
Section snippets
Experimental procedure
The undoped congruent lithium niobate crystal samples were cut from z-cut optically polished, 500 μm thick wafers obtained from Crystal Technology, USA. The +z face was illuminated by two different pulsed UV laser sources to investigate wavelength sensitivity. The first laser was a frequency-quadrupled Nd:YVO4 operating at λ = 266 nm with up to ∼5 mJ pulses of ∼10 ns duration. The second laser system was a frequency-doubled dye laser (Continuum Powerlite 8000) pumped by a frequency-doubled Q-switched
Results and discussion
Initial experiments were conducted to establish the single pulse ablation threshold for the +z face of lithium niobate samples at both laser wavelengths. It is important to note at this point that AOP occurs near the ablation threshold. The single pulse ablation thresholds for 266 nm and 298 nm light were established experimentally to be between 95 mJ/cm2 and 105 mJ/cm2. However, these figures are subject to some degree of uncertainty due to the intrinsic spatial non-uniformity and temporal (pulse
Conclusions
Ordered alignment of AOP domains has been achieved by spatially modulated UV laser radiation using a phase mask. The pulsed UV laser-induced domains nucleate on optical intensity maxima and are encouraged to grow along a specific “y” direction of the crystal specified by the aligned orientation of a periodic optical intensity pattern. However, full replication of the optical periodic pattern was not achieved due to possible electrostatic repulsion between adjacent domains which limits the
Acknowledgements
The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for research funding via grant EP/C515668/1 and to Dr Ian Clark and the Rutherford Appleton Central Laser Facility for the Continuum Powerlite 8000 dye laser loan.
References (17)
- et al.
Opt. Commun.
(1999) - et al.
J. Appl. Phys.
(1991) - et al.
Appl. Phys. Lett.
(1990) J. Appl. Phys.
(1979)- et al.
Appl. Phys. Lett.
(2003) - et al.
Electron. Lett.
(1991) - et al.
Electron. Lett.
(1991) - et al.
Appl. Phys. Lett.
(1993)
Cited by (12)
Quasi-periodic self-assembled sub-micrometer ferroelectric bulk domain gratings in Rb-doped KTiOPO<inf>4</inf>
2013, Applied Physics LettersLight-mediated ferroelectric domain engineering and micro-structuring of lithium niobate crystals
2012, Laser and Photonics ReviewsGeneration of conical and spherical second harmonics in three-dimensional nonlinear photonic crystals with radial symmetry
2011, Journal of the Optical Society of America B: Optical PhysicsThreshold fluence for domain reversal directly induced by femtosecond laser in lithium niobate
2010, Applied Physics A: Materials Science and Processing