#
Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO_{4} via Raman Imaging

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## Abstract

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## 1. Introduction

## 2. Experimental Design

## 3. Raman Analysis on KTP

## 4. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

AFM | Atomic force microscopy |

CCD | Charged-coupled device |

DW | Domain wall |

FWHM | Full width at half maximum |

IR | Infrared |

KTP | Potassium titanyl phosphate |

LN | Lithium niobate |

LO | Longitudinal optical |

NA | Numerical aperture |

PDC | Parametric down conversion |

PFM | Piezoresponse force microscopy |

QPM | Quasi phase-matching |

TO | Transversal optical |

UV | Ultraviolet |

## References

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**Figure 1.**Principle sketch of: (

**a**) Poling and monitoring geometry used in this study. (

**b**) Poling start: With application of the first pulse over the coercive field, inverted domains will nucleate under the electrically contacted area. (

**c**) Poling stop: After application of multiple pulses, the domains will have grown deeper and have merged until an approximately 50/50 duty cycle in the monitored regions is achieved. (

**d**) Geometries for the presented Raman analysis. Figure is not to scale.

**Figure 2.**Raman spectra of a DW compared to bulk spectra for the scattering geometry z(y,y)$\overline{\mathrm{z}}$. For better visibility of the changes, a difference spectrum is given.

**Figure 3.**Raman images of the -z-surface of a periodically poled KTP sample based on the variation of intensity (

**a**), FWHM (

**b**) and peak wave numbers (

**c**) of the 760 ${}^{-1}$ phonon mode highlighted in Figure 2.

**Figure 4.**Raman spectra of a DW compared to bulk spectra for the scattering geometry y(x,z)$\overline{\mathrm{y}}$. For better visibility of the changes a difference spectrum is given.

**Figure 5.**Raman images of the non-polar y-surface of a periodically poled KTP sample based on the variation of intensity (

**a**), FWHM (

**b**) and peak wave numbers (

**c**) of the 783 ${}^{-1}$ phonon mode highlighted in Figure 4.

**Figure 6.**Detailed images of domain structures on y-cut (y(x,z)$\overline{\mathrm{y}}$) measured in three areas of interest, (

**a**) close to the -z surface, (

**b**) in the region, where irregular domain structures appear and (

**c**) close to the back side evaluated from the 783 cm${}^{-1}$ mode. A sketch of the measurement geometry is given in (

**d**). A scale for the peak frequency with respect to the acquired Raman images (

**a**–

**c**) is to be found in (

**e**).

**Table 1.**Comparison of the intensity, FWHM and Raman shift of the peak frequency of the measured spectra in y(x,z)$\overline{\mathrm{y}}$ scattering geometry of bulk KTP and a KTP DW. Positive change indicates, respectively, a gained intensity, FWHM or shift to higher frequencies, whereas a negative value indicates a lowered intensity, FWHM or a shift to lower frequencies.

Mode y(x,z)$\overline{\mathbf{y}}$ (cm${}^{-1}$) | Intensity Bulk (a.u.) | Intensity DW (a.u.) | ${\Delta}_{\mathrm{Int}.}$ (%) | FWHM Bulk (cm${}^{-1}$) | FWHM DW (cm${}^{-1}$) | ${\Delta}_{\mathrm{FWHM}}$ (cm${}^{-1}$) | Shift Bulk (cm${}^{-1}$) | Shift DW (cm${}^{-1}$) | ${\Delta}_{\mathrm{Shift}}$ (cm${}^{-1}$) |
---|---|---|---|---|---|---|---|---|---|

175 | 5026 | 5198 | 3 | 8.2 | 8.3 | 0.1 | 174.6 | 174.7 | 0.1 |

190 | 1712 | 1680 | −2 | 4.7 | 4.7 | −0.1 | 190.3 | 190.3 | 0.0 |

210 | 1246 | 1660 | 33 | 5.1 | 6.3 | 1.3 | 209.7 | 209.8 | 0.1 |

265 | 8015 | 8172 | 2 | 8.4 | 8.3 | −0.1 | 265.4 | 265.4 | 0.0 |

287 | 8325 | 7656 | −8 | 7.4 | 7.9 | 0.5 | 287.2 | 287.2 | 0.1 |

313 | 11,860 | 10,815 | −9 | 10.1 | 9.8 | −0.3 | 313.0 | 313.2 | 0.2 |

360 | 10,980 | 11,004 | 0 | 14.7 | 15.6 | 0.9 | 359.8 | 360.2 | 0.4 |

400 | 4235 | 3636 | −14 | 19.8 | 18.7 | −1.1 | 400.0 | 400.1 | 0.1 |

423 | 2571 | 2521 | −2 | 10.1 | 10.6 | 0.6 | 423.1 | 423.3 | 0.1 |

514 | 3356 | 3115 | −7 | 9.1 | 8.5 | −0.6 | 514.4 | 514.4 | 0.0 |

544 | 2150 | 1579 | −27 | 15.0 | 13.4 | −1.7 | 554.1 | 554.4 | 0.3 |

691 | 8095 | 8214 | 1 | 17.7 | 18.1 | 0.4 | 691.5 | 691.6 | 0.1 |

737 | 1117 | 2488 | 123 | 11.6 | 18.9 | 7.2 | 737.7 | 737.3 | −0.4 |

783 | 19,738 | 20,040 | 2 | 15.3 | 16.7 | 1.3 | 783.2 | 784.0 | 0.9 |

991 | 9999 | 8495 | −15 | 28.8 | 27.0 | −1.7 | 991.1 | 990.4 | −0.6 |

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**MDPI and ACS Style**

Brockmeier, J.; Mackwitz, P.W.M.; Rüsing, M.; Eigner, C.; Padberg, L.; Santandrea, M.; Silberhorn, C.; Zrenner, A.; Berth, G.
Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO_{4} via Raman Imaging. *Crystals* **2021**, *11*, 1086.
https://doi.org/10.3390/cryst11091086

**AMA Style**

Brockmeier J, Mackwitz PWM, Rüsing M, Eigner C, Padberg L, Santandrea M, Silberhorn C, Zrenner A, Berth G.
Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO_{4} via Raman Imaging. *Crystals*. 2021; 11(9):1086.
https://doi.org/10.3390/cryst11091086

**Chicago/Turabian Style**

Brockmeier, Julian, Peter Walter Martin Mackwitz, Michael Rüsing, Christof Eigner, Laura Padberg, Matteo Santandrea, Christine Silberhorn, Artur Zrenner, and Gerhard Berth.
2021. "Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO_{4} via Raman Imaging" *Crystals* 11, no. 9: 1086.
https://doi.org/10.3390/cryst11091086