Mousa Ayoub

Second harmonic generation in multi-domain χ 2 media: from disorder to order

We investigate systematically the evolution of second harmonic generation in strontium barium niobate with different degrees of disorder of its χ2 nonlinearity. These different degrees of disordered domain structures are achieved through electrically switching of the polarization at room temperature. The size and distribution of the domains change during the poling process and this in turn strongly affects the spatial distribution of the second harmonic signal. The degree of disorder can be determined by analyzing the angular distribution and wavelength dependence of the second harmonic emission patterns combined with measurements of the spontaneous polarization. We demonstrate evidence of the control of the second harmonic emission pattern by creating defined states of order, and successfully reproduce the resulting patterns theoretically.

Domain-shape-based modulation of Čerenkov second-harmonic generation in multidomain strontium barium niobate

We study experimentally and numerically the second-harmonic Čerenkov emission with two different characteristic azimuthal intensity distributions in strontium barium niobate with a random structure of χ2 nonlinearity. We monitor in situ the Čerenkov emission during domain switching and show that a change of domain size and shape results in a fourfold azimuthal modulation of the Čerenkov cone.

Cascaded Čerenkov third-harmonic generation in random quadratic media

We investigate experimentally and theoretically the conical emission of Cerenkov-type third-harmonic generation in strontium barium niobate of random 2D-χ(2) distribution. The azimuthal intensity distribution is explained by the polarization properties of the fundamental and Cerenkov second-harmonic waves, depending on the cascaded origin of the generation process. Moreover, we show the role of the individual domain shape in an additional modulation on the conical emission, controlled by the electrical switching of the spontaneous polarization of the ferroelectric medium.

Čerenkov-type second-harmonic spectroscopy in random nonlinear photonic structures.

We report on experimental and theoretical studies of Čerenkov-type second-harmonic generation in random nonlinear photonic structures with different two-dimensional distributions of the ferroelectric domains. We utilize Čerenkov-type second-harmonic generation spectroscopy to estimate the average ferroelectric domain size by analyzing the spatial Fourier spectrum of the domain patterns. This is measured by scanning the Čerenkov second-harmonic signal for different angles of incidence and light polarizations of the fundamental wave. By comparing the experimental results with numerically simulated Fourier spectra, the corresponding domain patterns are retrieved, which are in a good correspondence with images obtained by Čerenkov-type second-harmonic generation microscopy.

3D Imaging of Ferroelectric Kinetics during Electrically Driven Switching

Čerenkov-type second-harmonic generation microscopy is used for in situ 3D imaging of the switching process of ferroelectric domains in a relaxor-type ferroelectric crystal. The lateral and longitudinal domain wall motion is optically measured and compared with the hysteresis loop to reveal the connection between the domain kinetics and the ferroelectric aging effect in strontium barium niobate.

Controlling the effective second-order susceptibility in random quadratic media

We study systematically the effect of spatial disorder of ferroelectric domains in nonlinear media on the polarization properties of optical frequency conversion. Experimentally, different statistics of domain sizes are created using electric field poling at room temperature. We analyze the evolution of polarization properties of the second- and third-harmonic signals for each created statistic by determining the corresponding relative strength of non-zero components of the second-order susceptibility tensor, d24, d32 and d33. The relative strengths are labeled by means of the control parameter E on the characteristic P-E loop of the studied ferroelectric medium.

Ferroelectric domain diagnostics near the phase transition by Čerenkov second-harmonic generation

We experimentally investigate ferroelectric domains close to the phase transition in a nonlinear photonic crystal exhibiting disordered domain distribution implementing Cerenkov second-harmonic generation. The interplay of domain structures and the second-harmonic signal is studied exemplarily in strontium barium niobate measuring the far-field distribution as well as imaging domain structures microscopically.

From disorder to order: Second harmonic generation in a multi-domain χ (2) nonlinearity

Nonlinear parametric processes are known to depend critically on phase matching between the phase velocities of the interacting waves. Phase matching is mostly achieved by using conventional methods like crystal birefringence or quasi-phase matching (QPM) techniques [1]. Recently, it was pointed out that unpoled ferroelectric crystals can also be used for frequency conversion in nonlinear optics being considered as randomly structured domain media [2,3,4]. This wide distribution, representing reciprocal wave vectors, allows e.g. for tunable phase-matched second-harmonic (SH) generation practically in the whole transparency range of the crystal [5]. All experiments shown here rely on the propagation of the fundamental beam in the plane perpendicular to the ferroelectric domain walls. The SH emission in this configuration is distributed in a plane normal to the optical axis of the ferroelectric crystal. This distribution, referring the ferroelectric domain size variety, is quantitatively studied at different degrees of disorder [6]. The ability to obtain different degrees of disorder, starting from disorder to order, is achieved, utilizing the possibility of switching the electrical alignment of the domains at room temperature. The resulting SH intensity distributions are monitored with directly scanning. As order parameters, the spontaneous polarization, corresponding to the poling status, the mean value, and the variance of the domain sizes are considered. Moreover, the wavelength dependency of this complex behavior for different poling status is also studied in detail [6].

Direct writing of order in naturally disordered nonlinear photonic crystals

We directly write with fs laser pulses differently ordered patterns of permanent seeds into a disordered nonlinear photonic crystal to effectively influence the distribution of ferroelectric domains. The new order is nonlinearly revealed by Cerenkov-type second- harmonic microscopy (CSHM).

Three-dimensional visualizing of ferroelectric domain growth and switching using Čerenkov second-harmonic generation

In this contribution we demonstrate for the first time to our knowledge a three-dimensional monitoring of the evolution of the spontaneous polarization (Ps) during electrical poling and temperature annealing in multi-domain strontium barium niobate using Cerenkov-type second-harmonic generation.