H. Steigerwald

Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators.

Whispering-gallery resonators (WGRs), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04  μm pump wave, the signal and idler waves are tuned from 1.78 to 2.5  μm--including the point of degeneracy--by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.

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.

Determination of Refractive Indices From the Mode Profiles of UV-Written Channel Waveguides in

We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in LiNbO3 for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of ~0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.

{\hbox {LiNbO}}_{3}

We report ferroelectric domain inversion in strontium barium niobate (SBN) single crystals by irradiating the surface locally with a strongly focused ultraviolet (UV) laser beam. The generated domains are investigated using piezoresponse force microscopy. We propose a simple model that allows predicting the domain width as a function of the irradiation intensity, which indeed applies for both SBN and LiNbO3. Evidently, though fundamentally different, the domain structure of both SBN and LiNbO3 can be engineered through similar UV irradiation.

-Crystals for Optimization of Writing Conditions

Ferroelectric domain reversal by application of electric-fields to near-stoichiometric lithium niobate crystals containing 1.3 mol % magnesium is investigated. The +z-face of the crystal is coated with a patterned photoresist layer that yields both a structured ultraviolet light absorber and a structured electric insulator. Periodic ferroelectric domain inversion in a z-cut crystal is then achieved by combination of illumination with light of the wavelength of 334 nm that reduces the coercive field strength inside the crystal by up to 50% and application of an external electrical field. Domain patterns with a period length of 10 μm are fabricated in a 500 μm thick crystal.

Direct writing of ferroelectric domains on strontium barium niobate crystals using focused ultraviolet laser light

Non-contact laser osteotomy offers new opportunities in various surgical fields, since it allows very precise pre-programmed incisions with completely free geometry. However laser osteotomy is a demanding task, because bone is a tough composite material, which is at the same time a living tissue and sensitive to temperature increases. Besides thermal side effects, practical laser applicability was limited until now because of very low cutting rates and limited incision depths. We discuss how to overcome these disadvantages by means of an optimal arrangement of thermo-mechanical ablation with a pulsed CO2 laser and with a water-spray as an assisting media. To this arrangement belong optimal pulse duration, irradiance and radiant exposure of the laser pulses, as well as multi-pass cutting procedures. Effective ablation of hard bone tissue with minimal thermal damage is possible with relatively long CO2 laser pulses of 80 µs duration and an average laser power of up to 40 W. To overcome the depth limitation special scanning techniques, which allow deep incisions even in thick multi-layer bones in feasible irradiation times, were developed in our group.

Ultraviolet light assisted periodic poling of near-stoichiometric, magnesium-doped lithium niobate crystals

The impact of UV laser irradiation on the distribution of lithium ions in ferroelectric lithium niobate single crystals has been numerically modelled. Strongly absorbed UV radiation at wavelengths of 244–305 nm produces steep temperature gradients which cause lithium ions to migrate and result in a local variation of the lithium concentration. In addition to the diffusion, here the pyroelectric effect is also taken into account which predicts a complex distribution of lithium concentration along the c-axis of the crystal: two separated lithium deficient regions on the surface and in depth. The modelling on the local lithium concentration and the subsequent variation of the coercive field are used to explain experimental results on the domain inversion of such UV treated lithium niobate crystals.

Laser Osteotomy with Pulsed CO2 Lasers

Direct UV laser writing on chromium coated lithium niobate (LiNbO3) crystals is found to produce spontaneous domain inversion associated with the exposed UV laser tracks. Experimental evidence suggests that this effect is attributed to local out-diffusion of oxygen, reducing the LiNbO3 crystal surface due to the presence of chromium. The thin chromium film becomes hot and reactive after absorbing the UV laser radiation thus acting as an oxygen getter. This very efficient process enables the inversion of domains at lower intensities as compared to other direct laser based poling methods practically eliminating the deleterious surface damage induced by the direct absorption of the UV laser radiation by the crystal. Furthermore, the versatility of this domain fabrication method, is demonstrated by the production of inverted domain structures on Z-, Y- and 128°YX-cut substrates.

Pyroelectric field assisted ion migration induced by ultraviolet laser irradiation and its impact on ferroelectric domain inversion in lithium niobate crystals

We report the realization of high-resolution bulk domains achieved using a shallow, structured, domain inverted surface template obtained by UV laser-induced poling inhibition in MgO-doped lithium niobate. The quality of the obtained bulk domains is compared to those of the template and their application for second harmonic generation is demonstrated. The present method enables domain structures with a period length as small as 3 μm to be achieved. Furthermore, we propose a potential physical mechanism that leads to the transformation of the surface template into bulk domains.

Ultraviolet laser induced domain inversion on chromium coated lithium niobate crystals

The advantages of laser osteotomy are free cut geometry and minimal thermal damage. Due to the lack of haptic feedback there is need for an alternate feedback method for accurate Laser Osteotomy. Based on the frequency analysis of the acoustic signal, generated by the ablation process, we are developing a feedback system to obtain in situ information on the ablation and for differentiation between different sorts of biological tissue. We used a pulsed slab CO2-laser (wavelength 10.6 µm, pulse length 80 µs) and piezoelectric sensors for sound detection. We studied the correlation of the ablation signal of different kinds of tissue in the frequency domain.