Nonlinear optics is an essential component of modern laser systems and optoelectronic devices. It has also emerged as an important tool in probing the electronic, vibrational, magnetic, and crystallographic structure of materials ranging from oxides and metals, to polymers and biological samples. This review focuses on the specific technique of optical second harmonic generation (SHG), and its application in probing ferroelectric complex oxide crystals and thin films. As the dominant SHG interaction mechanism exists only in materials that lack inversion symmetry, SHG is a sensitive probe of broken inversion symmetry, and thus also of bulk polar phenomena in materials. By performing in-situSHG polarimetry experiments in different experimental conditions such as sample orientation, applied electric field, and temperature, one can probe ferroelectric hysteresis loops and phase transitions. Careful modeling of the polarimetry data allows for the determination of the point group symmetry of the crystal. In epitaxial thin films with a two-dimensional arrangement of well-defined domain orientations, one can extract information about intrinsic material properties such as nonlinear coefficients, as well as microstructural information such as the local statistics of the different domain variants being probed. This review presents several detailed examples of ferroelectric systems where such measurements and modeling are performed. The use of SHG microscopic imaging is discussed, and its ability to reveal domain structures and phases not normally visible with linear optics is illustrated.

Probing Ferroelectrics Using Optical Second Harmonic Generation

The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. ...

Nano-optical imaging and spectroscopy of order, phases, and domains in complex solids

In recent years ferroelectric domain patterning has become a popular topic of physical research because it enables photonic applications as well as data storage. For generation of tailored domain structures and for further understanding of ferroelectricity, a visualization of the domain patterns is required. A large number of imaging techniques have therefore been developed. This review summarizes these techniques and highlights systematically their strengths and weaknesses.

Visualization of ferroelectric domains in bulk single crystals

We show that focusing a laser light onto the boundary between antiparallel ferroelectric domains leads to the non-collinear generation of two second harmonic (SH) beams. The beams are emitted in a plane normal to the domain boundaries at the angles that satisfy the Cerenkov-type phase matching condition. Moreover, these beam disappear when the laser light is focused on a homogenous part of a single domain. We utilize this effect for 3-dimensional visualization of fine details of the ferroelectric domain pattern with a submicron accuracy.

Three-dimensional ferroelectric domain visualization by Čerenkov-type second harmonic generation

A thin-film periodically poled lithium niobate waveguide was designed and fabricated which generates entangled photon pairs at telecommunications wavelengths with high coincidences-to-accidentals counts ratio $\mathrm{CAR}g67000$, two-photon interference visibility $Vg99%$, and heralded single-photon autocorrelation ${g}_{H}^{(2)}(0)l0.025$. Nondestructive in situ diagnostics were used to determine the poling quality in 3D. Megahertz rates of photon pairs were generated by less than a milliwatt of pump power, simplifying the pump requirements and dissipation compared to traditional spontaneous parametric down-conversion lithium niobate devices.

High Quality Entangled Photon Pair Generation in Periodically Poled Thin-Film Lithium Niobate Waveguides.

Domain walls in periodically poled ferroelectric KTiOPO4 and LiNbO3 crystals are observed by making use of second-harmonic (SH) generation enhancement in the transition regions between neighboring domains. SH images of domain walls obtained with various samples for different polarization configurations are presented. The SH generation enhancement is found especially pronounced for the polarization of the SH radiation being perpendicular to the domain walls. The origin and selection rules for the contrast in SH images of domain walls are discussed. The results obtained suggest that the domain walls produce a deteriorating effect on SH generation by quasiphase matching.

https://orbit.dtu.dk/files/4090407/Fredrik.pdf

Second-harmonic imaging of ferroelectric domain walls

Far- and near-field second-harmonic imaging of ferroelectric domain walls

We report the application of temperature-dependent quasi-phase matching (QPM) for second harmonic generation of green light using periodically field poled LiNbO3. In contrast to the usual QPM devices, here the fundamental and second harmonic waves are polarized orthogonally so that the second harmonic signal corresponds to the extraordinary wave. This requires the utilization of the d31 component of the nonlinear tensor (i.e. the same component as used for ordinary birefringent phase matching). d31 is smaller than the d33 component usually used in QPM devices and therefore yields a lower efficiency. However, the use of QPM in our geometry with orthogonally polarized waves results in a greatly enhanced temperature tunability, which increases the versatility of the devices. Moreover, the domain inversion grating period required in this geometry for first-order QPM at the Nd laser wavelength 1064 nm and room temperature is relatively large (350 µm), and therefore easier to fabricate. Compared with birefringent phase matching, the QPM technique allows for phase matching at any wavelength and eliminates the walk-off effect since the fields propagates along one of the dielectric principal axes. The interaction path can therefore, in principle, be extended over arbitrarily long distances.

https://iopscience.iop.org/article/10.1088/1464-4258/1/1/008/pdf

Michele Belmonte

Papers

Second-harmonic imaging of ferroelectric domain walls

Far- and near-field second-harmonic imaging of ferroelectric domain walls

Frequency doubling in LiNbO3 using temperature-dependent QPM