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One-Hundred Eighty Degree Domain Detection by Surface Phase Sensitive Second Harmonic Generation Microscopy of Polar Materials H. Aboulfadl and J. Hulliger* Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, CH-3012 Berne, Germany ABSTRACT: Phase sensitive second harmonic microscopy (PS-SHM) is applied to different nonlinear optical crystals such as N,N-dimethyl-2-acetamido-4-nitroaniline (DAN), 2-cyclooctylamino-5-nitropyridine (COANP), a channel-type inclusion compound of perhydrotriphenylene (PHTP)/N,N-dimethyl-3-nitroaniline (DMNA), potassium dihydrogen phosphate (KH2PO4) crystals stained by amaranth, and the zeolite AlPO45 loaded by 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). Present experiments show that phase sensitive experiments can be performed using a sufficiently flat surface of samples irrespective of their thickness. The new application of PS-SHM reveals a bipolar state in stained KH2PO4 and dye-loaded AlPO4-5.
1. INTRODUCTION Materials featuring an inhomogeneous bulk distribution of polar order may result from ferroelectric twinning or be grown in orientational disorder of dipolar entities.1,2 Analysis of the real structure with respect to bulk polarization may lead to an understanding of attachment processes during growth and allow the verification of unipolar alignment in systems providing unidirectional growth. For analysis, three different physical methods have been developed and applied to various materials: Scanning pyroelectric microscopy (SPEM), allowing for a surface analysis at micrometer scale of lateral resolution;3,4 scanning piezoelectric microscopy (SPM)5,6 to study effects of polarization at surfaces down to nanometer lateral resolution; and phase sensitive second harmonic generation microscopy (PS-SHM) operating in transmission, providing an average across a nearly micrometer-thick spot.7,8 SPEM and SPM thus require a pyroelectric or a piezoelectric effect, respectively, which may be generated by cooperative effects (ferroelectricity) or alignment of dipolar components in materials. For a recent review on pyro- and piezoelectric microscopy techniques, see ref 9. A fourth method bearing a great potential is confocal phase sensitive second harmonic generation microscopy, a technique still under developement.10 Because of conditions of phase matching, in transmission PS-SHM samples have to show a thickness in the range of the coherence length lc or better (below). Application of this technique was thus limited to the preparation of micrometer-thin samples of appropriate quality. Here we show that phase sensitive studies can be performed using sufficiently flat areas of samples irrespective of their thickness. This is offering most feasible conditions to investigate various grown materials and especially those that could not be obtained in micrometer-thick layers. The basic principle of PS-SHM is to interfere a reference 2ω wave with that of a sample area to be analyzed. Areas featuring opposite orientation of polarity (180° domains) will create a different response signal: In case the intensity, of the reference r 2011 American Chemical Society
beam is tuned to maximum interference by use of the lowest required intensity of 2ω light in one domain; the opposite domain will undergo destructive interference and thus produces a minimum of second harmonic light (see refs 7 and 8 for more details). In the case of a material providing a flat surface but being much thicker than the lc, SHG light emerges essentially from the lower and upper surface layer. Within this responding layers, effects of interference with a reference 2ω beam can occur. Consequently, a qualitative analysis of the polarization distribution, that is, 180° domain formation, may not require micrometer-thick samples. Experimental demonstration is here provided for materials that have been characterized before by other means: (i) Plate like single crystals of N,N-dimethyl-2-acetamido-4-nitroaniline (DAN),11,12 (ii) millimeter-thick single crystals of 2-cyclo-octylamino-5-nitropyridine (COANP),13 (iii) hundreds of micrometers-thick needles of a perhydrotriphenylene (PHTP) /N,N-dimethyl-3-nitroaniline (DMNA) inclusion crystals,7 (iv) thick stained potassium dihydrogen phosphate (KH2PO4) crystals dyed by amaranth14 and (v) 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)4H-pyran (DCM) doped in AlPO4-5.15 For all of these representative cases, we have demonstrated either a mono- or bipolar state using the PS-SHM technique under nonphase matching conditions of samples much thicker than the coherence length.
2. EXPERIMENTAL SECTION Sample Preparation. DAN. Dissolved in ethanol, droplets were placed on glass slides for evaporation. Four-hundred micrometer-thick and flat crystals were obtained. From earlier analyses,16 we know the morphology of solution grown DAN (point group P21) featuring a maximum nonlinearity in the plane of obtained crystals. Received: March 14, 2011 Revised: April 21, 2011 Published: May 04, 2011 3045
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Crystal Growth & Design COANP. Centimeter-cubed large crystals were obtained from supercooled melt growth.13 The maximum of optical nonlinearity is along the axis 2 of mm2. The morphology of these crystals growing only in one direction of the polar axis is known and was used to align crystals. PHTP/DMNA. Needle-shaped ∼40 80 μm thin crystals were produced from a solution of paraldehyde containing a 7:1 molar of PHTP/ DMNA. Such inclusion crystals show growth induced polarity formation along the channel (needle) direction, resulting in a bipolar state.7 KH2PO4/Amaranth. Following the work of Kahr et al.,17 amaranth is selectively staining the {101} sectors of KH2PO4, when grown from evaporating water solutions containing a molar ratio of about 2 10 4:1 (amaranth/KH2PO4). Amaranth shows nonlinear optical properties, because of its dipolar structure. Polarized optical microscopy has demonstrated that chromophores exceed alignment, when entering {101} faces.
Scheme 1. Chemical Structures of Molecules That Have Been Used in This Study
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AlPO4-5/DCM. Needle shaped transparent and well developed crystals were obtained from (Weiss et al., Max-Planck Institute for Coal Research) in situ inclusion of the DCM in AlPO4-5 crystals.15 In the zeolite channels, the nonlinear optical dye DCM is nearly aligned along the channel axis. Here, polar order results from in-diffusion from both sides of open channels, resulting in a bipolar state. For molecular structures (i) (v), see Scheme 1. Second Harmonic Generation (SHG) Measurements. A Leica polarizing microscope (DM RXP, Leitz) used in transmission mode, coupled to a dynamic photon counting camera (DynaMight 2000 Camera System, La Vision GmbH) and a Q-switched Nd:YAG laser (Surelite I-10, Continuum) providing a repetition rate of 10 Hz with a pulse width of 20 25 ns and a pulse energy of about 25 mJ (1064 nm, pulse intensity 10 MW cm 2, beam diameter of 4 mm) served for SHG measurements. The Leica microscope was additionally joined to a 3CCD color video camera (DXC-950P Sony) to take color pictures. Objectives of 5x and 10x magnifications (LMPLFL Olympus) were used. Phase-Sensitive SHG Measurements. The SH reference beam was generated by an angle-tuned KH2PO4 single crystal (KDP) placed into the fundamental beam. The phase delay between the fundamental and the SH waves from the reference beam was adjusted by rotation of a glass plate placed between the KDP crystal and samples. For details of the experimental set up, see refs 7 and 8.
3. RESULTS AND DISCUSSION The PS-SHM technique was developed to map 180° domains of polarization. Domain contrast is achieved by using the interference effect between SHG responses of a sample and a homogeneous reference material. In our previous work, we were exclusively using micrometer-thin crystalline layers, featuring a thickness close to the coherence length lc of the SHG effect. Present results take advantage of the fact that in the case of a sample being much thicker than lc, nevertheless SHG light is generated in transmission near surfaces. As observed, this response undergoes constructive or destructive interference with the reference beam 2ω. The five examples were selected to demonstrate that for different materials, thicknesses and surface quality, a phase contrast can be found. To read Figures 1 5 properly, we have to note, that in (c) or (d) the sample was rotated by 180°. However, the frame is presented in line with (a) and (b). DAN is known to grow into monopolar crystals in point group P21. Consequently, an individual crystal taken for a PS-SHM experiment is expected to show either a SHG response or none. Here, two different domains were seen, meaning that because of rather fast growth by evaporation from droplets, two individuals with opposite main NLO direction were developed. This
Figure 1. NLO responses for DAN at a thickness of 400 μm. (a) SHG response for a polarization of the fundamental wave 1064 nm, along the main axis of growth. (b) Effect of phase contrast for the lower halve (the SH intensity of one domain is enhanced, while it is attenuated in the other). (c) Effect of phase contrast for the upper part (contrast inversion occurs when the sample is rotated by 180°). Here, PS-SHM reveals 180° twinning of a monopolar material (see text). The polarization of the ω wave is indicated by arrows. 3046
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Figure 2. Nonlinear optical microscopy for a 80 μm thick single crystal of COANP. (a) SH response polarized along 2 (polar axis). (b) Effect of phase contrast (monopolar state). (c) After rotation by 180°, no SHG effect was observed showing that this material is a monopolar crystal.
Figure 3. (a) SHG experiment for PHTP/DMNA crystals grown from solution (thickness 40 80 μm). (b) Domain contrast was tuned at 0°. (c) Rotating the sample by 180° lead to the opposite contrast.
Figure 4. Optical and nonlinear optical images for KH2PO4/ amaranth. (a) Illuminated by daylight. (b) SH response. (c) Phase sensitive experiment showing an SHG effect only for one sector of the crystal. (d) Same as c but sample turned around 180°.
technique allows thus the detection of, for example, 180° twinning of crystal aggregates formed by polar point groups (Figure 1b, 1c). Present COANP crystals were obtained from slow growth (supercooled melt) and therefore the sample is monopolar as previously demonstrated by bulk nonlinear optical measurements.18 This is featured by the PS-SHM experiment as well: Figure 2 shows a
crystal of COANP with a thickness of 80 μm. The direction of the polarization was identified by a maximum of the SH response, while no SHG effect was observed at 90°. For PHTP inclusion (μm thick) crystals, PS-SHM and SPEM (thick crystals) have clearly demonstrated that growth along the channel (needle) axis leads to a bipolar state. Here, we confirm by rather thick needles that a bipolar state can nevertheless be observed (Figure 3). KH2PO4/Amaranth. Previous work has shown that stained inorganic or organic materials can yield SHG active sectors,19 meaning that acentric or even polar order is established by orientational selectivity when entering a crystal face. However, here we can demonstrate for the first time, that polar order in individual sectors features opposite alignment of dipoles. This can be understood by the molecular structure of amaranth interacting most likely by sulfonate groups to the surface (thickness: 400 μm). Because of the symmetry relation of opposing capping face, a bipolar staining will result (Figure 4). Here, no visible SH light was observed from KH2PO4 as the incident wave was clearly not under phase matching conditions, and 2ω from the KH2PO4 surface is too weak. Thus, SH light is predominantly emerging from aligned amaranth molecules. AlPO4-5/DCM. So far only SPEM had revealed the bipolar state of dye-loaded AlPO4-5.20,21 Second harmonic generation by polarized laser light was also demonstrated for such materials.22 25 Although we use crystals of AlPO4-5/DCM much thicker than 3047
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Figure 5. NLO response for AlPO4-5/DCM. (a) Second harmonic generation (SHG) microscopy in transmission. (b) PS-SHM for constructive interference in one sector. (c) PS-SHM after turning the crystal by 180°: SH response appears in the opposite sector.
lc, we can observe the bipolar state of loading (Figure 5). This results from orientational selectivity when polar molecules enter preexisting channels of a zeolite.
4. CONCLUSIONS In essence, the present analysis provides clear evidence that crystalline materials which do not fit basic requirements for proper phase sensitive SHG experiments, neverthless can qualitatively be analyzed to identify a (i) monopolar or a (ii) bipolar state. For the first time we provide a nonlinear optical analysis showing that both KH2PO4/amaranth (staining) and AlPO4-5/DCM undergo polar ordening by building a bipolar state, related by a rotation of 180°. For the benefit of avoiding artifacts with such samples, it is important to measure allways a number of different crystals out of a synthetic attempt. Because of naturally grown or cleaved surfaces, as well the observation of effects from the lower and the upper surface, it can not be expected to find the same homogeneity for SHG responses than obtained from samples with a thickness matching lc.7 Problems of scattering may be reduced here by using proper immersion liquids. Further development is exploring the possibility to perform PS-SHM under reflection of the incident ω wave. ’ AUTHOR INFORMATION Corresponding Author
*
[email protected].
’ ACKNOWLEDGMENT € We thank Dr. Ozlem Weiss and Prof. Ferdi Sch€uth (MaxPlanck Institute for Coal Research) for the preparation of DCM/ AlPO4-5 crystals. We thank also Prof. Bart Kahr for drawing our attention to stained crystals. This work was supported by the Swiss National Science Foundation, project no. 200021129472/1.
(6) Kalinin, S. V.; Rodriguez, B. J.; Shin, J.; Jesse, S.; Grichko, V.; Thundat, T.; Baddorf, A. P.; Gruverman, A. Ultramicroscopy 2006, 106, 334–340. (7) Rechsteiner, P.; Hulliger, J.; Fl€orsheimer, M. Chem. Mater. 2000, 12, 3296–3300. (8) Kluge, S.; Budde, F.; Dohnke, I. Appl. Phys. Lett. 2002, 81 247–249. (9) Batagiannis, A.; W€ubbenhorst, M.; Hulliger, J. Curr. Opin. Solid State Mater. Sci. 2010, 14, 107–115. (10) Project in collaboration with Dr. Sophie Brasselet, FRESNEL Institute, Marseille, France. (11) Kerkoc, P.; Zgonik, M.; Sutter, K.; Bosshard, Ch.; G€unter, P. J. Opt. Soc. Am. B 1990, 7, 313–319. (12) Kerkoc, P.; Hulliger, J. J. Cryst. Growth 1990, 99, 1023–1027. (13) Hulliger, J.; Brezina, B.; Ehrensperger, M. J. Cryst. Growth 1990, 106, 605–610. (14) Kahr, B.; Vasquez, L. CrystEngComm 2002, 4, 514–516. € Loerke, J.; W€ustefeld, U.; Marlow, F.; Sch€uth, F. (15) Weiss, O.; J. Solid State Chem. 2002, 167, 302–309. (16) Baumert, J. -C; Twieg, R. J.; Bjorklund, G. C.; Logan, J. A.; Dirk, C. W. Appl. Phys. Lett. 1987, 51, 1484–1486. (17) Kahr, B.; Gurney, R. W. Chem. Rev. 2001, 101, 893–951. (18) G€unter, P.; Bosshard, Ch.; Sutter, K.; Arend, H.; Chapuis, G.; Twieg, R. J.; Dobrowolski, D. Appl. Phys. Lett. 1987, 50, 486–488. (19) Kumaresan, P.; Moorthy Babu, S.; Anbarasan, P. M. J. Cryst. Growth 2008, 310, 1999–2004. (20) Klap, G. J.; van Klooster, S. M.; W€ubbenhorst, M.; Jansen, J. C.; van Bekkum, H.; van Turnhout, J. J. Phys. Chem. B 1998, 102, 9518–9524. (21) Klap, G. J.; W€ubbenhorst, M.; Jansen, J. C.; van Koningsveld, H.; van Bekkum, H.; van Turnhout, J. Chem. Mater. 1999, 11 3497–3503. (22) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609–619. (23) Werner, L.; Caro, J.; Finger, G.; Kornatowski, J. Zeolites 1992, 12, 658–663. (24) Caro, J.; Marlow, F.; W€ubbenhorst, M. Adv. Mater. 1994, 6, 413–416. (25) Herance, J. R.; Das, D.; Marquet, J.; Bourdelande, J. L.; García, H. Chem. Phys. Lett. 2004, 395, 186–189.
’ REFERENCES (1) Hulliger, J.; Bebie, H.; Kluge, S.; Quintel, A. Chem. Mater. 2002, 14, 1523–1529. (2) Hulliger, J. Chem.—Eur. J. 2002, 8, 4579–4586. (3) Quintel, A.; Hulliger, J.; W€ubbenhorst, M. J. Phys. Chem. B 1998, 102, 4277–4283. (4) Behrnd, N. -R.; Couderc, G.; W€ubbenhorst, M.; Hulliger, J. Phys. Chem. Chem. Phys. 2006, 8, 4132–4137. (5) Flores Suarez, R.; Mellinger, A.; Wegener, M.; Wirges, W.; Gerhard-Multhaupt, R.; Singh, R. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 1030–1035. 3048
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