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Abstract
We experimentally investigate ferroelectric domains close to the phase transition in a nonlinear photonic crystal exhibiting disordered domain distribution implementing Čerenkov 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.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
In recent years, much attention has been devoted to investigate frequency conversion in ferroelectrics with a random distribution of ferroelectric domains, i.e. with a random distribution of the χ(2) nonlinearity [1–4]. This class of nonlinear photonic crystals offers a possibility to realize nonlinear optical processes over a wide spectral range [5, 6]. One of the most interesting phase-matching schemes to achieve second-harmonic emission in such media is the so-called Čerenkov second harmonic generation (ČSHG). Its phase-matching condition is noncollinearily fulfilled when the longitudinal phase mismatch is zero and the transverse phase mismatch is partially compensated by the nonlinear structure. It is automatically satisfied at χ(2)-boundaries as for example at ferroelectric domain walls [6–10]. Due to its sensitivity to spatial disparities of the second-order nonlinearity, ČSHG has been recently developed as a versatile tool for ferroelectric domain diagnostics.
In two dimensional nonlinear photonic structures, a ČSH signal is emitted on a cone at an angle Θ = arccos(nω/n2ω) determined by the refractive indices of the fundamental and second-harmonic waves, respectively (see Fig. 1). The conical emission reduces itself to two bright second harmonic spots when a single domain wall is illuminated [11] (Fig. 1). Several studies have approved the direct relation of the spatial properties of ČSHG and ferroelectric domain structures. For instance, the intensity distribution of the second-harmonic cone can be attributed to the individual domain shape and the crystallographic symmetry, e.g. the four-fold symmetry in strontium barium niobate (SBN) [12, 13], or the hexagonal symmetry in LiNbO3 and LiTaO3, respectively [14–17]. The nonlinear second-order tensor properties also have an impact on the azimuthal intensity distribution of the second harmonic ring [18]. Moreover, ČSHG has been employed to characterize the ferroelectric domain size distribution in random nonlinear photonic structures [19]. A further progress has been made by embedding Čerenkov SHG in a microscopic system to profile the domain structures in all three dimensions [20]. The developed technique recently allowed us profiling the ferroelectric domain kinetics during electrical switching in SBN [21].
Fig. 1 Illustration of the Čerenkov-type phase-matching condition. The ring-shaped emission in a disordered 2D nonlinear structure, and the point-shaped emission on a domain wall.
As-grown SBN crystals typically have needle-like ferroelectric domains with a cross-section of a square shape with rounded corners [13, 22] (see Figs. 2(a)–2(c)). The domains are randomly distributed in size and space. The average size ranges between a few nanometers and a few micrometers [7, 23–25]. Due to this random domain distribution SBN is best suited for broadband quasi-phase matching processes [6, 7, 26, 27]. The distribution of domains can be influenced by electric-field poling and by heating. SBN shows a relaxor-type phase transition from the ferroelectric to the paraelectric phase, i.e., the Curie temperature Tc is not sharply defined [28, 29]. The Curie temperature of SBN crystals with the congruently melting composition Sr0.61Ba0.39Nb2O6 is relatively low, about 70 °C–80 °C.
Fig. 2 (a) and (b): Čerenkov-type SHG microscopic images of ferroelectric domains in SBN perpendicular to the optical axis with different averaged domain sizes, respectively; (c) a three-dimensional visualization of needle-like ferroelectric domains. Insets show the corresponding far-field Čerenkov SHG. Schematic of the experimental setup (d): The fundamental wave λFW = 1200 nm propagates along the optical axis of the crystal, and the Čerenkov SH ring is emitted at λSH = 600 nm. The crystal is sandwiched between thermo electrical coolers (TEC).
The evolution of ferroelectric domains with increasing temperature up to the phase transition is still an open question and under active debate. Specially, how the individual domains locally transform in shape near the relaxor phase transition.
In this article, we analyze Čerenkov SHG in random SBN in dependence of the temperature from the ferroelectric to the paraelectric phase. SBN is the ideal platform here because of its low Curie temperature. This makes SBN more suitable technically to investigate microscopically. In addition the domains are naturally distributed and spatially separated. The advantage here is that the domains can be traced individually during changing the temperature.
In the first part, we measure the overall second-harmonic intensity of Čerenkov SHG in the far-field while heating up a strontium barium niobate crystal over the Curie temperature Tc. This allows tracing the order parameter, i.e., the spontaneous polarization Ps, which approaches zero at Tc. In the second part, we use Čerenkov-type second-harmonic microscopy to image the cross section of ferroelectric domains in dependence of temperature. Finally, by comparing far-field and microscopic Čerenkov SHG we gain new insights in the domain dynamics near the phase transition.
2. ČSHG near the phase transition
2.1. Far-field investigations
Our Sr0.61Ba0.39Nb2O6 crystal has the dimensions of 6.6 × 6.6 × 1.6 mm3 and is doped with Cr2O3. The large surfaces perpendicular to the c-axis are polished to optical quality. We use femtosecond laser pulses with a wavelength of λ = 1200 nm, a pulse duration of about 80 fs, pulse energies up to 100 µJ, and a repetition rate of 1 kHz. The laser pulses are collimated, having a beam diameter of about 1.5 mm. The beam is propagating along the c-axis of the crystal illuminating a large number of domains. The second harmonic signal is emitted in this configuration on a Čerenkov cone that is projected on a color CCD camera. A simplified schematic is depicted in Fig. 2(2).
In order to ensure the same electrical and thermal history, the samples were initially heated up to 200 °C for 2 hours to erase any spurious polarization. This process is necessary since the phase transition in SBN strongly depends on the poling history [30]. In this as-grown case, the domains are smaller than 1 µm, and the intensity of the second harmonic on the ring is homogeneous (see Fig. 2(a) inset). Before recording the dynamics of the second harmonic on the ring, the crystal is partially repoled several times at room temperature leading to micro-scaled squircle-shaped domains (see Fig. 2(b)). The intensity of the second harmonic on the ring shows a four-fold modulation attributed to the domain shape (inset in Fig. 2(b)). To measure Čerenkov SHG on a cone in dependence of temperature, the SBN sample is sandwiched between two controlled Peltier elements. A thermistor at the sample face controls the temperature (Fig. 2(d)). The sample faces along the optical axis are kept free for the propagation of the fundamental beam. Čerenkov SHG is recorded in situ while increasing the temperature of the sample above Curie temperature and cooling it down. The temperature is increased in steps of 1 °C/s starting from room temperature regularly up to 100 °C. The temperature is then decreased to room temperature with the same velocity. The exposure time of the CCD camera is kept fixed during the experiment. The results are depicted in Fig. 3.
Fig. 3 Experimental images of Čerenkov SH emission in the far-field recorded for an SBN of micro-scaled squircle-shaped domain, heated above Curie temperature and then cooled down at the fundamental wavelength 1200 nm. (see Visualization 1)
Figure 3 shows the evolution of the modulated Čerenkov SH cone while increasing and decreasing the sample temperature. At room temperature (Fig. 3(a)), the modulation of the azimuthal second harmonic intensity can clearly be seen. The total intensity becomes weaker with increasing sample temperature. At about 75 °C, the second harmonic signal vanishes completely (Figs. 3(e)–3(f)).
Subsequently, the sample is cooled down. The second harmonic signal appears again at about 70 °C and increases with decreasing sample temperature (see Figs. 3(g)–3(h)). The intensity of Čerenkov SHG on the ring is clearly weaker than in the repoled case at the beginning, and appears to be homogeneous, i.e., the four-fold modulation is no longer visible.
2.2. Microscopic view
To combine the far-field analysis with the corresponding microscopic measurements of the domain structures in dependence of temperature we use a Čerenkov SHG microscope that is described in details elsewhere [21]. The fundamental beam of a Ti-Sapphire laser (800 nm, 80 MHz repetition rate, about 60 fs pulse duration, and up to 3.5 nJ pulse energy) is coupled into a commercial laser scanning confocal microscope (Nikon eclipse, Ti-U) and tightly focused by the microscope objective with a numerical aperture of 0.8 to a near diffraction limited spot in the sample to generate the second-harmonic signal at the domain walls individually. The position of the focus is raster-scanned in the xy-plane by a piezoelectric table (P-545, PI nano). The intensity of the second-harmonic signal is collected by a condenser lens (NA 0.9) and measured by a photomultiplier as a function of the focus position.
To control the temperature, a copper plate is mounted on the top of the sample. This plate is equipped with two thermal resistors and a temperature sensor (PT100), and has a hole allowing to pass the second-harmonic signal to the photomultiplier. For this experiment an SBN sample with the dimensions 6.0 × 6.0 × 1.1 mm3 is employed. The large surfaces perpendicular to the c-axis are polished to optical quality. The crystal is prepared to have micro-scaled domains by several poling cycles in the same manner as before (cfg. Fig. 2(b) and Fig. 4(a)).
Fig. 4 Images of ferroelectric domain structures in SBN using Čerenkov SHG microscopy. Green signals represent domain walls. The temperature is increased from 22 °C (a) up to 85 °C (i). First, the crystal has undergone some poling cycles to start with micro-scaled domains (a). In (i) only background noise is detected.
An area of 40 µm × 40 µm is raster-scanned, while heating up the sample to reach the paraelectric phase. To avoid loosing the observed structures in the scanned area due to the temperature-dependent drift while heating, the crystal is readjusted between the scans in the xy-plane to compensate for this shift.
The results are shown in Fig. 4. Starting from room temperature (Fig. 4(a)) the crystal has random square micro-scaled domains. With increasing temperature the domains become smaller. The mean diameter shrinks from <d> ≈ 4.9 µm down to about 3 µm at 55 °C (Fig. 4(e)) and further down to about 1.8 µm at 75 °C (Fig. 4(f)). At 80 °C, the intensity of the ČSHG signal has decreased to such a degree that the background noise becomes apparent (Fig. 4(h)). The ČSHG completely vanishes as the Curie temperature is exceeded at ≈ 85 °C (Fig. 4(i)).
3. Analysis of the dynamics
The intensity distribution of far-field ČSHG is directly related to the averaged domain size and shape of the ferroelectric domains in SBN. This SHG intensity in the undepleted pump approximation can be described in the following way [12]:
Here, IFW is the intensity of the fundamental wave propagating along the z-axis, deff is the effective nonlinear coefficient, Lz is the average domain length, and S(Δkxy) is the Fourier spectrum of the domain structure with the transverse phase mismatch vectors Δkxy in the x- and y-direction, respectively. At the Čerenkov angle Θ, the longitudinal phase mismatch Δkz is 0. The four-fold modulation on the SHG ring (cfg. Fig. 3(a)) originates from the shape of the domains (cfg. Fig. 4(a)) which is included in the Fourier spectrum S(Δkxy)(T). At the fundamental wavelength 1200 nm, the external Čerenkov angle Θ amounts to about 37.5° and the corresponding wave vectors of the fundamental and second-harmonic are 24.3 µm−1 and 11.73 µm−1, respectively. In general, at room temperature, a strong ČSHG signal is expected for strong Fourier coefficients, i.e. when S(Δkxy) is maximum. The transverse mismatch Δkxy = k(2ω) sin θinternal = 6.4 µm−1 would be best compensated for by an inverted domain distribution with an average domain diameter of about 0.5 µm and a duty cycle of 50%. However, the length Lz of the nanoscaled domains in the unpoled sample is considerably smaller and leads to a weaker SHG signal than for the microscaled domains in the repoled case.When the temperature is increased, the domain size decreases. Simultaneously, the order parameter, i.e. the spontaneous polarization Ps, and therefore the nonlinear coefficient deff decreases. Above the Curie temperature, the crystal is in the paraelectric phase (point group 4/mmm), and the nonlinear susceptibility tensor, i.e., deff is zero, hence no second harmonic is emitted anymore. When the temperature is subsequently decreased, the domain distribution in the ferroelectric phase is different compared to the original repoled distribution. After this heating treatment, the domains are much smaller, and on the order of a few hundred nanometers. The SHG ring is not modulated anymore due to the changed domain shape, i.e., the Fourier spectrum S(Δkxy)(T) cannot be completely resolved for such small domains. Also, the total ČSHG intensity is smaller for nanodomains than for microdomains as can be seen in Fig. 5 where the results of the far-field and the microscopic measurements are compared. The ČSHG intensity of the complete ring is plotted for the heating and cooling process as well as the mean domain diameters determined from the microscope images.
Fig. 5 Far-field ČSHG intensities (cfg. Fig. 3) and mean domain diameters determined from the microscope images (cfg. Fig. 4) in dependence of the crystal temperature.
The phase transition can be described by the temperature dependence of the spontaneous polarization P(T) = P0[1 − (T + 273)/(Tc + 273)]β with the parameter β, which describes the type of the phase transition. The second-harmonic intensity is in turn proportional to the square of the nonlinearity: I(2ω) (T) = I(2ω),0[1 − (T + 273)/(Tc + 273)]2β. Applying this relation to our measured data from Fig. 4, we find the best fit for β = 0.16±0.03 and Tc = 75 °C, which is very close to its value in [31]. The minimal deviation may be attributed to using different SBN crystals with different pre-history. Nonetheless, this supports the hypothesis that the relaxor-type phase transition of SBN can be described by a 3D Ising model. Above the Curie temperature the ferroelectric looses its polarization information with further heating up. This makes the cooling down process of a different prehistory from that of the repoling state at the beginning. Thus, the comparison of the heating and cooling processes does not necessarily obey the same fitting parameters.
Figure 5 also shows that the raising of the temperature is associated with decreasing not only the nonlinearity represented by the signal to noise ratio (Fig. 4(i)), but also with decreasing the local spontaneous polarization represented by shrinking of the individual domains, and therefore its spatial distribution in the far-field too. This non-separable process explains the switch-off of the four-fold modulation on the ring in the far-field and its total intensity.
4. Conclusion
In conclusion, we have analyzed optically the ferroelectric domains near the phase-transition region in a relaxor strontium barium niobate using the combination of two methods based on Čerenkov SHG. From the SHG intensity in the far-field, we could determine the Curie temperature and the critical exponent β. The phase transition in SBN can be characterized by a 3D random-field Ising model in accordance with literature. We also found that the decreasing of the nonlinearity strength at the domain walls is accompanied with the shrinkage of the domains locally at the microscopic level when increasing the temperature towards phase transition, i.e. with changing the Fourier spectrum of the domain distribution.
Funding
Open Access Publication Fund of University of Muenster.
References and links
1. S. Kawai, T. Ogawa, H. S. Lee, R. C. DeMattei, and R. S. Feigelson, “Second-harmonic generation from needlelike ferroelectric domains in Sr0.6Ba0.4Nd2O6 single crystals,” Appl. Phys. Lett. 73, 768–770 (1998). [CrossRef]
2. E. Y. Morozov, A. A. Kaminskii, A. S. Chirkin, and D. B. Yusupov, “Second optical harmonic generation in nonlinear crystals with a disordered domain structure,” JETP Lett. 73, 647–650 (2001). [CrossRef]
3. M. Baudrier-Raybaut, R. Haidar, P. Kupecek, P. Lemasson, and E. Rosencher, “Random quasi-phase-matching in bulk polycrystalline isotropic nonlinear materials,” Nature 432, 374–376 (2004). [CrossRef] [PubMed]
4. K. A. Kuznetsov, G. K. Kitaeva, A. V. Shevlyuga, L. I. Ivleva, and T. R. Volk, “Second harmonic generation in a strontium barium niobate crystal with a random domain structure,” JETP Letters 87, 98–102 (2008). [CrossRef]
5. X. Vidal and J. Martorell, “Generation of light in media with a random distribution of nonlinear domains,” Phys. Rev. Lett. 97, 013902 (2006). [CrossRef] [PubMed]
6. A. R. Tunyagi, M. Ulex, and K. Betzler, “Noncollinear optical frequency doubling in strontium barium niobate,” Phys. Rev. Lett. 90, 243901 (2003). [CrossRef] [PubMed]
7. P. Molina, M. de la O Ramírez, and L. E. Bausá, “Strontium barium niobate as a multifunctional two-dimensional nonlinear “photonic glass”,” Adv. Funct. Mater. 18, 709–715 (2008). [CrossRef]
8. Y. Zhang, X. P. Hu, G. Zhao, S. N. Zhu, and M. Xiao, “Čerenkov second-harmonic arc from a hexagonally poled LiTaO3 planar waveguide,” J. Phys. D 42, 215103 (2009). [CrossRef]
9. V. Roppo, K. Kalinowski, Y. Sheng, W. Krolikowski, C. Cojocaru, and J. Trull, “Unified approach to Čerenkov second harmonic generation,” Opt. Express 21, 25715–25726 (2013). [CrossRef] [PubMed]
10. X. Zhao, Y. Zheng, H. Ren, N. An, X. Deng, and X. Chen, “Nonlinear Cherenkov radiation at the interface of two different nonlinear media,” Opt. Express 24, 12825–12830 (2016). [CrossRef] [PubMed]
11. A. M. Vyunishev, A. S. Aleksandrovsky, A. I. Zaitsev, and V. V. Slabko, “Čerenkov nonlinear diffraction in random nonlinear photonic crystal of strontium tetraborate,” Appl. Phys. Lett. 101, 211114 (2012). [CrossRef]
12. M. Ayoub, P. Roedig, J. Imbrock, and C. Denz, “Domain-shape-based modulation of Čerenkov second-harmonic generation in multidomain strontium barium niobate,” Opt. Lett. 36, 4371–4373 (2011). [CrossRef] [PubMed]
13. M. Ayoub, P. Roedig, J. Imbrock, and C. Denz, “Cascaded Čerenkov third-harmonic generation in random quadratic media,” Appl. Phys. Lett. 99, 241109 (2011). [CrossRef]
14. S. M. Saltiel, Y. Sheng, N. Voloch-Bloch, D. N. Neshev, W. Krolikowski, A. Arie, K. Koynov, and Y. S. Kivshar, “Cerenkov-type second-harmonic generation in two-dimensional nonlinear photonic structures,” IEEE J. Quantum. Electron. 45, 1465–1472 (2009). [CrossRef]
15. A. Arie and N. Voloch, “Periodic, quasi-periodic, and random quadratic nonlinear photonic crystals,” Laser and Photon. Rev. 4, 355–373 (2010). [CrossRef]
16. Y. Sheng, S. M. Saltiel, W. Krolikowski, A. Arie, K. Koynov, and Y. S. Kivshar, “Čerenkov-type second-harmonic generation with fundamental beams of different polarizations,” Opt. Lett. 35, 1317–1319 (2010). [CrossRef] [PubMed]
17. Y. Zhang, F. Wang, K. Geren, S. N. Zhu, and M. Xiao, “Second-harmonic imaging from a modulated domain structure,” Opt. Lett. 35, 178–180 (2010). [CrossRef] [PubMed]
18. P. Molina, M. O. Ramírez, B. J. García, and L. E. Bausá, “Directional dependence of the second harmonic response in two-dimensional nonlinear photonic crystals,” Appl. Phys. Lett. 96, 261111 (2010). [CrossRef]
19. M. Ayoub, P. Roedig, K. Koynov, J. Imbrock, and C. Denz, “Čerenkov-type second-harmonic spectroscopy in random nonlinear photonic structures,” Opt. Express 21, 8220–8230 (2013). [CrossRef] [PubMed]
20. Y. Sheng, A. Best, H.-J. Butt, W. Krolikowski, A. Arie, and K. Koynov, “Three-dimensional ferroelectric domain visualization by Čerenkov-type second harmonic generation,” Opt. Express 18, 16539–16545 (2010). [CrossRef] [PubMed]
21. M. Ayoub, H. Futterlieb, J. Imbrock, and C. Denz, “3d imaging of ferroelectric kinetics during electrically driven switching,” Adv. Mater. 29, 1603325 (2017). [CrossRef]
22. L. Tian, D. A. Scrymgeour, and V. Gopalan, “Real-time study of domain dynamics in ferroelectric Sr0.61Ba0.39Nb2O6,” Journal of Applied Physics 97, 114111 (2005). [CrossRef]
23. K. Terabe, S. Takekawa, M. Nakamura, K. Kitamura, S. Higuchi, Y. Gotoh, and A. Gruverman, “Imaging and engineering the nanoscale-domain structure of a Sr0.61Ba0.39Nb2O6 crystal using a scanning force microscope,” Appl. Phys. Lett. 81, 2044–2046 (2002). [CrossRef]
24. V. V. Shvartsman, W. Kleemann, T. Łukasiewicz, and J. Dec, “Nanopolar structure in Srx Ba1−x Nb2O6 single crystals tuned by Sr/Ba ratio and investigated by piezoelectric force microscopy,” Phys. Rev. B. 77, 054105 (2008). [CrossRef]
25. M. Ayoub, J. Imbrock, and C. Denz, “Second harmonic generation in multi-domain χ(2) media: from disorder to order,” Opt. Express 19, 11340–11354 (2011). [CrossRef] [PubMed]
26. M. Horowitz, A. Bekker, and B. Fischer, “Broadband second-harmonic generation in Srx Ba1−x Nb2O6 by spread spectrum phase matching with controllable domain gratings,” Appl. Phys. Lett. 62, 2619–2621 (1993). [CrossRef]
27. W. Wang, V. Roppo, K. Kalinowski, Y. Kong, D. N. Neshev, C. Cojocaru, J. Trull, R. Vilaseca, K. Staliunas, W. Krolikowski, S. M. Saltiel, and Y. Kivshar, “Third-harmonic generation via broadband cascading in disordered quadratic nonlinear media,” Opt. Express 17, 20117 (2009). [CrossRef] [PubMed]
28. L. E. Cross, “Relaxor ferroelectrics,” Ferroelectrics 76, 241–267 (1987). [CrossRef]
29. J. R. Oliver, R. R. Neurgaonkar, and L. E. Cross, “A thermodynamic phenomenology for ferroelectric tungsten bronze Sr0.6Ba0.4Nb2O6 (SBN:60),” Journal of Applied Physics 64, 37–47 (1988). [CrossRef]
30. T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, and W. Kleemann, “Local electric-field-driven repoling reflected in the ferroelectric polarization of ce-doped Sr0.61Ba0.39Nb2O6,” Appl. Phys. Lett. 80, 470–472 (2002). [CrossRef]
31. U. Voelker and K. Betzler, “Domain morphology from k-space spectroscopy of ferroelectric crystals,” Phys. Rev. B 74, 132104 (2006). [CrossRef]
