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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 3 — Jan. 30, 2012
  • pp: 2015–2024
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Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

Cheng-Ying Chen, Jun-Han Huang, Kun-Yu Lai, Yi-Jun Jen, Chuan-Pu Liu, and Jr-Hau He  »View Author Affiliations


Optics Express, Vol. 20, Issue 3, pp. 2015-2024 (2012)
http://dx.doi.org/10.1364/OE.20.002015


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Abstract

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°–85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.

© 2012 OSA

Due to their large aspect ratio, semiconducting one-dimensional (1D) nanostructures are promising elements for realizing fascinating optoelectronic devices, such as photodiodes [1

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,2

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], photodetectors [3

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7

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], light-harvesting layers [9

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], nanolasers [10

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], and light-emitting diodes (LEDs) [2

2. J. H. He, S. T. Ho, T. B. Wu, L. J. Chen, and Z. L. Wang, “Electrical and Photoelectrical Performances of Nano-Photodiode Based on ZnO Nanowires,” Chem. Phys. Lett. 435(1-3), 119–122 (2007). [CrossRef]

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11. Y. Q. Bie, Z. M. Liao, P. W. Wang, Y. B. Zhou, X. B. Han, Y. Ye, Q. Zhao, X. S. Wu, L. Dai, J. Xu, L. W. Sang, J. J. Deng, K. Laurent, Y. Leprince-Wang, and D. P. Yu, “Single ZnO nanowire/p-type GaN heterojunctions for photovoltaic devices and UV light-emitting diodes,” Adv. Mater. 22(38), 4284–4287 (2010). [CrossRef] [PubMed]

]. Moreover, it was found that the single nanowire/nanorod (NW/NR) with subwavelength diameter, large aspect ratio, and high permittivity exhibits a strong optical anisotropy. For example, the giant polarization anisotropy of light emission/absorption has been reported with individual InP NWs [12

12. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and photodetection from single indium phosphide nanowires,” Science 293(5534), 1455–1457 (2001). [CrossRef] [PubMed]

], Si NWs [13

13. J. Qi, A. M. Belcher, and J. M. White, “Spectroscopy of individual silicon nanowires,” Appl. Phys. Lett. 82(16), 2616–2618 (2003). [CrossRef]

], CdSe NWs [14

14. C. X. Shan, Z. Liu, and S. K. Hark, “Photoluminescence polarization in individual CdSe nanowires,” Phys. Rev. B 74(15), 153402 (2006). [CrossRef]

], GaN NRs [15

15. H. Y. Chen, Y. C. Yang, H. W. Lin, S. C. Chang, and S. Gwo, “Polarized photoluminescence from single GaN nanorods: effects of optical confinement,” Opt. Express 16(17), 13465–13475 (2008). [CrossRef] [PubMed]

], and ZnO NWs [16

16. J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, “Optical cavity effects in ZnO nanowire lasers and waveguides,” J. Phys. Chem. B 107(34), 8816–8828 (2003). [CrossRef]

,17

17. H. Y. Li, S. Rühle, R. Khedoe, A. F. Koenderink, and D. Vanmaekelbergh, “Polarization, microscopic origin, and mode structure of luminescence and lasing from single ZnO nanowires,” Nano Lett. 9(10), 3515–3520 (2009). [CrossRef] [PubMed]

]. Owing to the optical polarization anisotropy, single or ensemble NWs/NRs can function as polarization sensitive photodectectors [12

12. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and photodetection from single indium phosphide nanowires,” Science 293(5534), 1455–1457 (2001). [CrossRef] [PubMed]

,18

18. H. Pettersson, J. Trägårdh, A. I. Persson, L. Landin, D. Hessman, and L. Samuelson, “Infrared photodetectors in heterostructure nanowires,” Nano Lett. 6(2), 229–232 (2006). [CrossRef] [PubMed]

], compact polarization converters in optical communication [19

19. N. Künzner, D. Kovalev, J. Diener, E. Gross, V. Y. Timoshenko, G. Polisski, F. Koch, and M. Fujii, “Giant birefringence in anisotropically nanostructured silicon,” Opt. Lett. 26(16), 1265–1267 (2001). [CrossRef] [PubMed]

,20

20. M. Kotlyar, L. Bolla, M. Midrio, L. O’Faolain, and T. Krauss, “Compact polarization converter in InP-based material,” Opt. Express 13(13), 5040–5045 (2005). [CrossRef] [PubMed]

], phase-matched nonlinear optical components [21

21. M. J. A. de Dood, W. T. M. Irvine, and D. Bouwmeester, “Nonlinear photonic crystals as a source of entangled photons,” Phys. Rev. Lett. 93(4), 040504 (2004). [CrossRef] [PubMed]

,22

22. M. L. Markham, J. J. Baumberg, D. C. Smith, X. Li, T. Gabriel, G. S. Attard, and I. Nandhakumar, “Birefringent cadmium–telluride-based metamaterial,” Appl. Phys. Lett. 86(1), 011912 (2005). [CrossRef]

], interferometer-based optical sensors [23

23. D. Celo, E. Post, M. Summers, T. Smy, M. J. Brett, and J. Albert, “Interferometric sensing platform with dielectric nanostructured thin films,” Opt. Express 17(8), 6655–6664 (2009). [CrossRef] [PubMed]

], and propagation medium for surface waves [24

24. D. Artigas and L. Torner, “Dyakonov surface waves in photonic metamaterials,” Phys. Rev. Lett. 94(1), 013901 (2005). [CrossRef] [PubMed]

]. For the ensembles of aligned 1D nanostructures, polarization anisotropy results in optical birefringence, which has been found in GaP NW arrays (NWAs) [25

25. O. L. Muskens, M. T. Borgström, E. P. A. M. Bakkers, and J. G. Rivasa, “Giant optical birefringence in ensembles of semiconductor nanowires,” Appl. Phys. Lett. 89(23), 233117 (2006). [CrossRef]

,26

26. O. L. Muskens, S. L. Diedenhofen, M. H. M. van Weert, M. T. Borgström, E. P. A. M. Bakkers, and J. G. Rivas, “Epitaxial growth of aligned semiconductor nanowire metamaterials for photonic applications,” Adv. Funct. Mater. 18(7), 1039–1046 (2008). [CrossRef]

] and carbon nanotube arrays [27

27. W. A. deHeer, W. S. Bacsa, A. Châtelain, T. Gerfin, R. Humphrey-Baker, L. Forro, and D. Ugarte, “Aligned carbon nanotube films: production and optical and electronic properties,” Science 268(5212), 845–847 (1995). [CrossRef] [PubMed]

]. The strong artificial birefringence between the in-plane and out-of-plane refractive indices was also observed in the well-aligned GaP NWAs [25

25. O. L. Muskens, M. T. Borgström, E. P. A. M. Bakkers, and J. G. Rivasa, “Giant optical birefringence in ensembles of semiconductor nanowires,” Appl. Phys. Lett. 89(23), 233117 (2006). [CrossRef]

]. However, for practical applications, the in-plane birefringence of nanostructured films is desirable for a wide range of photonic applications [28

28. E. Hecht, Optics, 4th ed., (Addison-Wesley, Boston, 2002; Ch.8).

].

The oblique-angle deposition (OAD) has been used to fabricate thin film materials that exhibit artificial birefringence [29

29. A. C. van Popta, J. Cheng, J. C. Sit, and M. J. Brett, “Birefringence enhancement in annealed TiO2 thin films,” J. Appl. Phys. 102(1), 013517 (2007). [CrossRef]

33

33. H. Qi, X. Xiao, H. He, K. Yi, and Z. Fan, “Optical properties and microstructure of Ta2O5 biaxial film,” Appl. Opt. 48(1), 127–133 (2009). [CrossRef] [PubMed]

]. Several attempts have been made to improve the birefringence of oblique angle-deposited thin films. For example, serial bideposition, a variation of the OAD, was utilized to greatly enhance in-plane birefringence [34

34. I. Hodgkinson and Q. H. Wu, “Serial bideposition of anisotropic thin films with enhanced linear birefringence,” Appl. Opt. 38(16), 3621–3625 (1999). [CrossRef] [PubMed]

]. Post-annealing was demonstrated to improve linear birefringence by the densification of the anisotropic columns [29

29. A. C. van Popta, J. Cheng, J. C. Sit, and M. J. Brett, “Birefringence enhancement in annealed TiO2 thin films,” J. Appl. Phys. 102(1), 013517 (2007). [CrossRef]

]. Xiao et al. reported that the combination of the OAD and the sol–gel techniques can improve the linear birefringence of SiO2 thin films [32

32. X. D. Xiao, G. P. Dong, Z. X. Fan, K. Yi, H. B. He, and J. D. Shao, “A facile process to improve linear birefringence of SiO2 thin films,” J. Phys. D Appl. Phys. 42(16), 165305 (2009). [CrossRef]

]. Moreover, optoelectronic devices would greatly benefit from a thin film material whose in-plane birefringence is as large as possible while remaining single crystalline. However, the OAD nanostructured films show poor crystallinity and thus inefficient light emission ability [29

29. A. C. van Popta, J. Cheng, J. C. Sit, and M. J. Brett, “Birefringence enhancement in annealed TiO2 thin films,” J. Appl. Phys. 102(1), 013517 (2007). [CrossRef]

,30

30. S. M. Wang, G. D. Xia, X. Y. Fu, H. B. He, J. D. Shao, and Z. X. Fan, “Preparation and characterization of nanostructured ZrO2 thin films by glancing angle deposition,” Thin Solid Films 515(7-8), 3352–3355 (2007). [CrossRef]

,35

35. T. Motohiro and Y. Taga, “Thin film retardation plate by oblique deposition,” Appl. Opt. 28(13), 2466–2482 (1989). [CrossRef] [PubMed]

,36

36. Q. H. Wu and I. J. Hodgkinson, “Materials for birefringent coatings,” Opt. Photonics News 5, S9–S10 (1994).

], restricting their applications in passive photonic components, such as waveplates [35

35. T. Motohiro and Y. Taga, “Thin film retardation plate by oblique deposition,” Appl. Opt. 28(13), 2466–2482 (1989). [CrossRef] [PubMed]

,37

37. J. Hodgkinson and Q. H. Wu, Birefringent thin films and polarizing elements, (World Scientific Press, 1997).

]and birefringent/transparent electrical conductors [31

31. K. D. Harris, A. C. van Popta, J. C. Sit, D. J. Broer, and M. J. Brett, “A birefringent and transparent electrical conductor,” Adv. Funct. Mater. 18(15), 2147–2153 (2008). [CrossRef]

]. Additionally, OAD nanostructured films exhibit low in-plane birefringence in the range of 0.01–0.07 [31

31. K. D. Harris, A. C. van Popta, J. C. Sit, D. J. Broer, and M. J. Brett, “A birefringent and transparent electrical conductor,” Adv. Funct. Mater. 18(15), 2147–2153 (2008). [CrossRef]

,35

35. T. Motohiro and Y. Taga, “Thin film retardation plate by oblique deposition,” Appl. Opt. 28(13), 2466–2482 (1989). [CrossRef] [PubMed]

] due to a limited range of the column tilt angles (30°–55° relative to the substrate normal) [30

30. S. M. Wang, G. D. Xia, X. Y. Fu, H. B. He, J. D. Shao, and Z. X. Fan, “Preparation and characterization of nanostructured ZrO2 thin films by glancing angle deposition,” Thin Solid Films 515(7-8), 3352–3355 (2007). [CrossRef]

,33

33. H. Qi, X. Xiao, H. He, K. Yi, and Z. Fan, “Optical properties and microstructure of Ta2O5 biaxial film,” Appl. Opt. 48(1), 127–133 (2009). [CrossRef] [PubMed]

,35

35. T. Motohiro and Y. Taga, “Thin film retardation plate by oblique deposition,” Appl. Opt. 28(13), 2466–2482 (1989). [CrossRef] [PubMed]

].

In this study, a combined method of modified OAD and hydrothermal growth was utilized to obtain a novel photonic metamaterial based on single crystalline ZnO NWAs with oblique angles in the range of 75°–85°, exhibiting large artificial in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of the NWAs layer is almost one order of magnitude higher than that of bulk ZnO (Δn0.015) [38

38. G. E. Jellison Jr and C. M. Rouleau, “Determination of optical birefringence by using off-axis transmission ellipsometry,” Appl. Opt. 44(16), 3153–3159 (2005). [CrossRef] [PubMed]

,39

39. G. E. Jellison Jr and L. A. Boatner, “Optical functions of uniaxial ZnO determined by generalized ellipsometry,” Phys. Rev. B 58(7), 3586–3589 (1998). [CrossRef]

]. The notable polarization degree in the photoluminescence (PL) emission of the NWAs is due to the optical confinement effect. The feasibility of single crystalline oblique-aligned NWA growth offers an excellent opportunity for the application in ensemble polarization-sensitive optical devices.

The oblique-aligned ZnO NWAs were grown on Si(100) substrate with a sputtering process and a subsequent hydrothermal method. Pure ZnO target (99.99%) was utilized as the sputtering source. First, a ZnO buffer layer was prepared by the two-steps sputtering process: (a) the ZnO layer was deposited on a 1-rpm-rotating substrate in argon at 410 °C at the oblique angle of 30° with respect to the surface normal of the sputtering target; (b) the ZnO bent columnar seed was deposited in a reduced atmosphere with 20% hydrogen/argon mixture gas at 265 °C at the oblique angle of 30° without the substrate rotation. For the final hydrothermal process, the oblique-aligned ZnO NWAs were synthesized in a solution mixed by 0.005 M Zinc acetate dehydrate (Zn(Ch3COO)2.2H2O) and 0.005 M Hexamethylenetetramine (HMT C6H12N4) at the ratio of 1:1, heated at 81 °C for 2 hours. More details of oblique-aligned NWA synthesis are described elsewhere [40

40. H. Huang, C. Y. Chen, Y. F. Lai, Y. I. Shih, Y. C. Lin, J. H. He, and C. P. Liu, “Large-area oblique-aligned ZnO nanowires through a continuously bent columnar buffer: growth, microstructure, and antireflection,” Cryst. Growth Des. 10(8), 3297–3301 (2010). [CrossRef]

].

Morphological studies were performed with a JEOL JSM-6500 field emission scanning electron microscopy (SEM) and a JEOL 3000F field emission transmission electron microscopy (TEM). Optical reflectance measurements were performed at the angle of incidence (AOI) of 5° for both s- and p-polarization in the wavelength ranges of 300–1400 nm by a standard UV-VIS-NIR spectrophotometer (JASCO V-670). The reflection of a collimated incident light beam was measured by collecting the specularly reflected cone of light within an acceptance angle of 6°. The PL measurements were performed in air at room temperature using a He-Cd laser (photon energy = ~3.8 eV) as an excitation source with 1.5 mm diameter of beam spot and 35 mW excitation power. The polarized PL measurements were performed by placing respectively an analyzer and a depolarizer into the path of the collected signal beam. The depolarizer was placed between a spectrometer and the analyzer in order to eliminate the polarization dependence of the measurement equipments.

The natural structure of ZnO is wurtzite [space group = P63mc], which lacks of cubic symmetry, resulting in anisotropic optical properties, i.e., the existence of spontaneous polarization along the c-axis [43

43. C. Klingshirn, “ZnO: material, physics and applications,” ChemPhysChem 8(6), 782–803 (2007). [CrossRef] [PubMed]

]. Furthermore, photonic metamaterials consisting of subwavelength-scale structures have shown the effective optical properties significantly different from those of their individual constituent materials [44

44. N. Engheta and R. W. Ziolkowski, Electromagnetic Metamaterials: Physics and Engineering Explorations, 1st ed., (Wiley, 2006).

,45

45. P. Lalanne and M. Hutley, Artificial Media Optical Properties - Subwavelength Scale, in Encyclopedia of Optical Engineering, (Dekker, 2003, pp. 62–71)

]. In order to investigate the optical anisotropy of the oblique-aligned ZnO NWAs, the reflection spectroscopy was performed with the plane of incidence aligning to the orientation of the NWAs. In this measurement, the electric fields of s- and p-polarized light are perpendicular and parallel to the azimuthal direction of the long axis (c-axis) of oblique-aligned NWAs, respectively. The measured reflectivity spectra of the ZnO NWAs/ZnO buffer layer/Si substrate are shown in Fig. 2
Fig. 2 Reflectivity spectra of the oblique-aligned ZnO NWAs for s- and p-polarization at the AOI of 5°. The plane of incidence is aligned to the orientation of NWs. The s- and p-polarization is perpendicular and parallel to the azimuthal direction of the long axis of oblique-aligned NWAs, respectively.
. The nearly vanished reflectivity at the wavelengths shorter than 368 nm is due to the bandgap absorption of ZnO. For the wavelengths longer than 368 nm, strongly modulated reflectivity spectra (i.e., Fabry–Pérot oscillations) can be clearly observed. The modulated spectra are attributed to the multiple reflection at the three optically flat interfaces of air/NWs, NWs/ZnO buffer layer, and ZnO buffer layer/Si substrate. It is worth noting that the reflectivity spectra with the electric field parallel to the azimuthal direction of the long axis of NWAs is significantly different from that perpendicular to the azimuthal direction of the long axis, demonstrating an obvious optical anisotropy of the oblique-aligned ZnO NWAs; i.e., the morphological anisotropy of the oblique-aligned NWA structure results in a reduction of the in-plane symmetry of the optical properties from an isotropy to an in-plane uniaxial symmetry.

In order to quantitatively analyze the optical anisotropy of the NWAs, we performed the calculation of the Fresnel reflection in multiple layers to extract the equivalent reflective indices n(λ) [46

46. J. Hodgkinson, F. Horowitz, H. A. Macleod, M. Sikkens, and J. J. Wharton, “Measurement of the principal refractive indices of thin films deposited at oblique incidence,” J. Opt. Soc. Am. A 2(10), 1693–1697 (1985). [CrossRef]

49

49. H. Y. Chen, H. W. Lin, C. Y. Wu, W. C. Chen, J. S. Chen, and S. Gwo, “Gallium nitride nanorod arrays as low-refractive-index transparent media in the entire visible spectral region,” Opt. Express 16(11), 8106–8116 (2008). [CrossRef] [PubMed]

]. Here the two-layer system is used to simulate the reflectivity spectra. The top layer is the oblique-aligned ZnO NWAs surrounded by air, and the bottom one is the ZnO buffer layer on a semi-infinite Si substrate. To simplify our calculations, we consider the reflectivity spectra in the non-absorbing wavelength range above 700 nm for ZnO. In other words, we only consider the real part of refractive indices and utilize Cauchy equation to describe the dispersion relationship of refractive indices [50

50. W. S. Hu, Z. G. Liu, J. Sun, S. N. Zhu, Q. Q. Xu, D. Feng, and Z. M. Ji, “Optical properties of pulsed laser deposited ZnO thin films,” J. Phys. Chem. Solids 58(6), 853–857 (1997). [CrossRef]

]. As considering wavelength range above 700 nm, the scale of the oblique-aligned ZnO NWAs belongs to the subwavelegnth scale. Therefore, it is reasonable to treat the NWAs as an optical layer. Moreover, the Fabry–Pérot oscillations in the reflectivity spectra of Fig. 2 is an evidence that there is no significant scattering at the interface between air and the ZnO NWAs in the wavelength regions (700–1400 nm) [48

48. S. Heavens, “Optical properties of thin films,” Rep. Prog. Phys. 23(1), 1–65 (1960). [CrossRef]

]. The equivalent refractive index is polarization dependent denoted as n(λ) (s-polarization) n(λ)(p- polarization). The reflection coefficient of the reflected light (r) at normal incidence can be described by the Fresnel equations and obtained by summing up an infinite series of partial reflected waves after multiple reflection from the interfaces of air/NWAs, NWAs/ZnO buffer layer, and ZnO buffer layer/Si substrate. The resulting reflectivity is [47

47. H. Wang, “Reflection/transmission measurements of anisotropic films with one of the principal axes in the direction of columnar growth,” J. Mod. Opt. 42(3), 497–505 (1995). [CrossRef]

,51

51. Y. J. Jen and C. C. Lee, “Reflection and transmission phenomena of waves propagating between an isotropic medium and an arbitrarily oriented anisotropic medium,” Opt. Lett. 26(4), 190–192 (2001). [CrossRef] [PubMed]

,52

52. Y. J. Jen, C. C. Lee, and Y. M. Chang, “Reflection property of anisotropic films: comparison of symmetric and asymmetric theories using attenuated total reflection,” J. Opt. A, Pure Appl. Opt. 4(4), 481–484 (2002). [CrossRef]

]
R=|r|2=|r1+r2'eiφ1r1r2'eiφ|2
(1)
where φ is the phase difference due to the optical path within the NWAs; r1=(1nNWAs)/(1+nNWAs)is the reflection coefficient at the air/NWAs interface; and r2'=(r2+r3eiϕ)/(1r2r3eiϕ), where ϕ is the phase difference due to the optical path within the buffer layer, r2=(nNWAsnbuffer)/(nNWAs+nbuffer)and r3=(nbuffernSi)/(nbuffer+nSi) are the reflection coefficients at the NWAs/ZnO buffer layer and ZnO buffer layer/Si substrate interfaces, respectively [nNWAs(nbuffer) is the equivalent refractive index of the NWAs (the buffer layer) and nSiis the refractive index of Si substrate]. In addition, φ=4πnNWAsdNWAs/λ and ϕ=4πnbufferdbuffer/λ, where dNWAs (dbuffer) is the thickness of the NWAs (the buffer layer). In this case, the reflectivity R is a function of nNWAs, nbuffer, dNWAsand dbuffer, where dNWAsand dbufferwere determined by cross-sectional SEM image (not shown here).

To analyzenNWAs(λ), we have to obtain nbuffer(λ) first. The nbuffer(λ) can be retraced by fitting the reflectivity spectra of the buffer layer/Si substrate sample without the NWAs. The reflectivity R’ of the one-layer system is presented theoretically based on Eq. (1) by substitutingr1=(1nbuffer)/(1+nbuffer),r2'=(nbuffernSi)/(nbuffer+nSi), and φ=4πnbufferdbuffer/λ. The fitting result is shown in the inset of Fig. 3
Fig. 3 Determination of the equivalent reflective index dispersion curve of the ZnO buffer layer. The inset is the comparison of simulated and experimental reflectivity spectra of the ZnO buffer layer on Si substrate.
. Noteworthily, the measured spectra of the buffer layer/Si substrate for s-/p-polarization are almost identical, demonstrating an in-plane optical isotropy on the buffer layer. Moreover, there is good agreement between the calculated and the experimental values. The nbufferexhibits normal dispersion, in which the equivalent refractive index decreases with the wavelength; nbuffer(λ) goes from 1.970 to 1.914 as the wavelength varies from 700 nm to 1400 nm, indicating a perfect consistency with the previously reported results of ZnO thin films [53

53. H. Yoshikawa and S. Adachi, “Optical constants of ZnO,” Jpn. J. Appl. Phys. 36(Part 1), 6237–6243 (1997). [CrossRef]

].

Based on the aforementioned procedure, the reflectivity R of the two-layer system is a function of nNWAs. The nNWAs(λ) is derived by fitting the reflectivity spectra of the NWAs/buffer layer/Si substrate sample for s-/p-polarization, as shown in the insets of Figs. 4(a)
Fig. 4 Determination of the equivalent reflective index dispersion curve of the oblique-aligned ZnO NWAs for the polarization (a) perpendicular and (b) parallel to the azimuthal direction of the long axis of oblique-aligned NWAs. (c) The in-plane birefringence of the oblique-aligned ZnO NWAs. The insets of (a) and (b) are the comparison of simulated and experimental reflectivity spectra of the NWAs on the ZnO buffer layer/Si substrate.
and 4(b). The excellent agreement between the simulations and the measurements demonstrates the validity of the two-layer system. Accordingly, for the polarization perpendicular to the azimuthal direction of the long axis of oblique-aligned NWAs, n(λ) of the NWAs is from 1.271 to 1.266 as the wavelength varies from 700 nm to 1400 nm, as shown in Fig. 4(a); for the polarization parallel to the azimuthal direction of the long axis of oblique-aligned NWAs, n(λ) of the NWAs is from 1.381 to 1.361 in the same wavelength range, as shown in Fig. 4(b). The difference in n(λ) and n(λ) results from a unique geometrical distribution of the structure mixed with air and oblique-aligned ZnO NWAs. As shown in Fig. 4(c), the in-plane birefringence parameter of the oblique-aligned NWAs can be determined by the definition [54

54. G. Rivas, O. L. Muskens, M. T. Borgström, S. L. Diedenhofen, and E. P. A. M. Bakkers, One-Dimensional Nanostructures, Z. M. Wang, ed. (Springer, 2008), Vol. 3, Chap. 6.

]:

Δn=n(λ)n(λ)
(2)

Amazingly, the birefringence of the oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of bulk ZnO (Δn0.015) [38

38. G. E. Jellison Jr and C. M. Rouleau, “Determination of optical birefringence by using off-axis transmission ellipsometry,” Appl. Opt. 44(16), 3153–3159 (2005). [CrossRef] [PubMed]

] and the natural birefringence of quartz (Δn0.01) [28

28. E. Hecht, Optics, 4th ed., (Addison-Wesley, Boston, 2002; Ch.8).

]. Furthermore, our in-plane birefringence is also comparable with that of the anisotropic-nanostructured silicon (Δn0.115) [19

19. N. Künzner, D. Kovalev, J. Diener, E. Gross, V. Y. Timoshenko, G. Polisski, F. Koch, and M. Fujii, “Giant birefringence in anisotropically nanostructured silicon,” Opt. Lett. 26(16), 1265–1267 (2001). [CrossRef] [PubMed]

]. However, our ZnO NWAs can be more easily combined in ZnO or GaN LED applications.

In order to further investigate the in-plane optical anisotropy of the NWAs, the polarization-dependent reñectivity at the AOI of 5° for the NWAs was characterized by rotating the polarization at the wavelength of 528 nm, as shown in Fig. 5
Fig. 5 The reflectivity of the oblique-aligned ZnO NWAs characterized with the polarization angles between 0° and 90° at the wavelength of 528 nm and the AOI of 5°.
. When the polarization is parallel to the azimuthal direction of the long axis of oblique-aligned NWAs (i.e., the angle of polarization was 0°), the minimum reflectivity of ~5.5% is observed. The reflectivity is increased to the maximum value of ~10.5% at the polarization angle of 90°. A huge difference between the reflectivity extrema (3.8%) at 528 nm corroborates the high in-plane birefringence in our oblique-aligned NWAs.

The strong polarization anisotropy of the emission spectra has been observed in the 1D nanostructures [13

13. J. Qi, A. M. Belcher, and J. M. White, “Spectroscopy of individual silicon nanowires,” Appl. Phys. Lett. 82(16), 2616–2618 (2003). [CrossRef]

17

17. H. Y. Li, S. Rühle, R. Khedoe, A. F. Koenderink, and D. Vanmaekelbergh, “Polarization, microscopic origin, and mode structure of luminescence and lasing from single ZnO nanowires,” Nano Lett. 9(10), 3515–3520 (2009). [CrossRef] [PubMed]

]. Due to the geometry anisotropy, the emission intensity of I can be very different from that of I, where I and I are the emission intensities with the polarization parallel and perpendicular to the long axis of 1D nanostructures, respectively. To investigate the anisotropy of the emission in the oblique-aligned ZnO NWAs, we performed the polarized PL measurement at room temperature. The polarized PL spectra of the NWAs with the electric field of the emission parallel (EPLc) and perpendicular (EPLc) to the azimuthal direction of the long axis of oblique-aligned NWAs exhibit the near bandedge emission (NBE) at ~377 nm and the deep level emission (DLE) at ~550 nm, as shown in Fig. 6
Fig. 6 PL spectra of the oblique-aligned ZnO NWAs at different polarization directions in (a) the NBE and (b) the DLE regions.
[55

55. A. B. Djurišić and Y. H. Leung, “Optical properties of ZnO nanostructures,” Small 2(8-9), 944–961 (2006). [CrossRef] [PubMed]

]. Regardless of the polarization of the excitation, the PL has maximum polarized emission in the direction parallel to the azimuthal direction of the long axis of the NWAs. Therefore, we only show the polarized spectra with the polarization of the excitation perpendicular to the azimuthal direction of the long axis of oblique-aligned NWAs. In addition, the strong NBE is attributed to the high crystal quality of the NWAs examined by the HRTEM image in Fig. 1(c). For quantitative analysis, the observed polarization anisotropy is typically defined in terms of the polarization ratio (also known as the degree of polarization) [15

15. H. Y. Chen, Y. C. Yang, H. W. Lin, S. C. Chang, and S. Gwo, “Polarized photoluminescence from single GaN nanorods: effects of optical confinement,” Opt. Express 16(17), 13465–13475 (2008). [CrossRef] [PubMed]

]

ρ=III+I
(3)

In summary, a robust, simple method of fabricating the single crystalline NWA layer with oblique angles ranging from 75° to 85° as an optically anisotropic material using the modified OAD and hydrothermal growth has been demonstrated. The oblique-aligned ZnO NWAs have giant in-plane birefringence (Δn0.11). The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only can be applied to passive photonic components but also open up the possibility of important technological applications in polarized light sensing and emission devices.

Acknowledgment

The authors thank Prof. Shangjr Gwo and Dr. Hung-Ying Chen for fruitful discussion on the anisotropic emission. The work was supported by the National Science Council Grant No. 99- 2112-M-002-024-MY3, 100–2917-I-002–009, 98-2623-E-002-004-ET, and 100-2218-E-008-015.

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OCIS Codes
(000.0000) General : General
(000.2700) General : General science

ToC Category:
Optoelectronics

History
Original Manuscript: November 21, 2011
Revised Manuscript: December 31, 2011
Manuscript Accepted: December 31, 2011
Published: January 13, 2012

Citation
Cheng-Ying Chen, Jun-Han Huang, Kun-Yu Lai, Yi-Jun Jen, Chuan-Pu Liu, and Jr-Hau He, "Giant optical anisotropy of oblique-aligned ZnO nanowire arrays," Opt. Express 20, 2015-2024 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-3-2015


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References

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  18. H. Pettersson, J. Trägårdh, A. I. Persson, L. Landin, D. Hessman, and L. Samuelson, “Infrared photodetectors in heterostructure nanowires,” Nano Lett.6(2), 229–232 (2006). [CrossRef] [PubMed]
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  23. D. Celo, E. Post, M. Summers, T. Smy, M. J. Brett, and J. Albert, “Interferometric sensing platform with dielectric nanostructured thin films,” Opt. Express17(8), 6655–6664 (2009). [CrossRef] [PubMed]
  24. D. Artigas and L. Torner, “Dyakonov surface waves in photonic metamaterials,” Phys. Rev. Lett.94(1), 013901 (2005). [CrossRef] [PubMed]
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