Y.J. Ying

Precision nanoscale domain engineering of lithium niobate via UV laser induced inhibition of poling

Continuous wave ultraviolet (UV) laser irradiation at lambda=244 nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100 nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution micro-structuring approach when followed by chemical etching.

Direct writing of ferroelectric domains on the x- and y-faces of lithium niobate using a continuous wave ultraviolet laser

Ferroelectric domain reversal has been achieved by scanning a tightly focused, strongly absorbed ultraviolet laser beam across the x- and y-faces of lithium niobate crystals. The domains were investigated by piezoresponse force microscopy. The emergence and width of any domain was found to depend on the scanning direction of the irradiating laser beam with respect to the polar z-axis. Full width and half width domains or no domain formation at all could be achieved for scanning along specific directions. We interpret the results by a direct correlation between the local temperature gradient and the resulting polarization direction.

Depth resolution of piezoresponse force microscopy

Given that a ferroelectric domain is generally a three dimensional entity, the determination of its area as well as its depth is mandatory for full characterization. Piezoresponse force microscopy (PFM) is known for its ability to map the lateral dimensions of ferroelectric domains with high accuracy. However, no depth profile information has been readily available so far. Here, we have used ferroelectric domains of known depth profile to determine the dependence of the PFM response on the depth of the domain, and thus effectively the depth resolution of PFM detection.

Spectral and electro-optic response of UV-written waveguides in LiNbO3 single crystals.

An experimental study of the spectral and electro-optic response of direct UV-written waveguides in LiNbO3 is reported. The waveguides were written using c.w. laser radiation at 275, 300.3, 302, and 305 nm wavelengths with various writing powers (35-60 mW) and scan speeds (0.1-1.0 mm/sec). Spectral analysis was used to determine the multimode and single mode wavelength regions and, the cut-off point of the fabricated waveguides. Electro-optic characterization of these waveguides reveals that the electro-optic coefficient (r33) decreases for longer writing wavelengths, with a maximum of 31 pm/V for 275 nm and, is reduced to 14 pm/V for waveguides written with 305 nm.

Waveguide mode filters fabricated using laser-induced forward transfer

Titanium in-diffused lithium niobate index-tapered waveguides have been fabricated using laser-induced forward transfer technique for mode-filtering applications. Details of their fabrication, losses and transmission characterization are presented.

Latent light-assisted poling of LiNbO3

The observation of latent light-assisted poling (LAP) in lithium niobate single crystals is reported. More specifically, the nucleation field is reduced and remains reduced for an extended time period (up to several hours) after irradiation with ultrafast (approximately 150 fs) laser light at a wavelength of 400 nm. The maximum nucleation field reduction measured using latent-LAP (62%) was significantly higher in comparison with regular non-time-delayed LAP (41%) under identical irradiation conditions in undoped congruent lithium niobate crystals. No latent-LAP effect was observed in MgO-doped crystals for the experimental conditions used, despite the strong effect observed using regular LAP. The latent-LAP effect is attributed to the formation of a slowly decaying photo-induced space-charge distribution which assists local ferroelectric domain nucleation. The dynamics of latent-LAP are compared with the dynamics of photorefractive grating decay, recorded in lithium niobate crystals of different doping, confirming the space charge hypothesis.

Ultra-smooth lithium niobate single crystal photonic micro-structures

Thermal treatment of micro-structured lithium niobate, at temperatures close to the Curie point, induces preferential surface melting and re-crystallization. This process yields ultra-smooth single crystal superstructures suitable for fabrication of low scattering loss photonic micro-components.

UV laser-induced poling inhibition in lithium niobate crystals

The development of methods for ferroelectric domain engineering in lithium niobate (LN) is of special importance as it not only enables the fabrication of efficient non-linear devices (PPLN) [1] but also constitutes a very effective path towards micro-structuring of this important ferroelectric crystal [2].

Spectral analysis of UV-written waveguides in LiNbO 3 : Measurement of cut-off wavelengths

Since its inception [1], the UV directly written waveguide procedure in lithium niobate single crystals has held considerable promise because of its single-step process which is well suited for complex micro-optical devices, in addition to the conventional optical integrated circuits. For most applications, single-mode waveguides are essential in order to achieve optimised device performance, and hence the determination of single-mode operating range and cut-off wavelength of the waveguide are fundamental for characterising waveguide optical properties. In this work we report the first results of spectral analysis within the wavelength range from 408 to 1750 nm of waveguides that were UV-written at 275, 300.3, 302, and 305 nm wavelengths, for the determination of the single-mode operating range and cut-off wavelength for different writing conditions.

UV laser radiation inhibits domain inversion in lithium niobate

Continuous wave UV laser (lambda = 244 nm) irradiation of the +z face of lithium niobate single crystals inhibits ferroelectric domain inversion in the volume of the crystal which lies immediately below the UV exposed surface.