Zn nanoparticles (NPs) embedded in a silica matrix subjected to irradiation with swift heavy ions of 200 MeV Xe14+ have been found to undergo shape elongation from spheres to prolate-spheroids while maintaining the major axes of the NPs in parallel alignment. The directionally-aligned Zn spheroids enable acquisition of optical properties, such as linear dichroism and birefringence. In this paper, the birefringence of the Zn spheroids was evaluated by the crossed-Nicols (XN) transmittance, where a sample was inserted between a pair of optical polarizers that were set in an orthogonal configuration. Linearly-polarized light aligned by the first polarizer was transformed to an elliptic polarization by the birefringence of the Zn spheroids. The existence of the birefringence was confirmed by the non-zero transmittance of the second polarizer in the orthogonal configuration. The sample irradiated with a fluence of 5.0 × 1013 ions/cm2 exhibited a maximum XN transmittance of 2.1% at a photon energy of ~4 eV. The XN transmission was observed down to a fluence of 1.0 × 1012 ions/cm2, but reduced below the detection limit at a fluence of 1.0 × 1011 ions/cm2. The possible application of the elongated Zn NPs as a polarizer with nanometric thickness working in the near- and mid-ultraviolet region is discussed.
© 2014 Optical Society of America
Metal nanoparticles (NPs) have received much attention because of their various fascinating optical, electronic, and magnetic properties that are not observed in their bulk counterparts: ultra-fast optical response  and nanometric electric field enhancement  of the surface plasmon resonance (SPR), single electron transport , super-paramagnetism [4, 5], the finite-size effect of phase transitions in NPs , etc. These properties can be tuned by controlling the size, size distribution, and shape of the NPs.
Regarding shape control of metal NPs, a new approach was discovered in 2003 by D’Orleans et al., who irradiated cobalt NPs embedded in SiO2 with swift heavy ions (SHIs) of 200 MeV iodine . They observed the deformation of the NPs from a spherical shape to a lemon-shape, and finally to nano-rods with increasing fluence. One of the fascinating properties of this phenomenon is that the elongation is induced parallel to the direction of the SHI beam, i.e., the majority of the NPs are elongated in the same direction which is controlled by the SHI beam. Anisotropic properties can be expected for even a single elongated NP. However, if the elongation directions of the NPs as a group are not aligned, the anisotropy of an individual NP is canceled out relative to neighboring NPs. Ensembles of misaligned elongated NPs therefore exhibit little anisotropy. Thus, the aligned nature of the elongated NPs fabricated by this method is very attractive.
Since its discovery, this phenomenon has been extensively studied by many groups [8–25]. However, most of the observations were obtained from transmission electron microscopy (TEM), which requires highly distinguishable deformation of NPs, resulting from high fluence on the order of 1014 ions/cm2 involving tens to thousands of overlapping ion-tracks. A fundamental question that arises when considering the elongation process is whether a single SHI impact yields NP elongation or if multiple impacts are necessary.
To evaluate this question, we applied a more sensitive method than TEM, i.e., the optical linear dichroism (OLD) spectroscopy reported in our previous paper . As the elongation of each NP can be very small at low fluences, and because some NPs may not be completely spherical even in a non-irradiated state, the detection of the induced elongation of each NP by TEM is difficult. However, while the deformation of each NP may be small, the elongation direction is always parallel to the SHI beam. As such, the macroscopic properties, which derive from the statistical summation of the microscopic properties of each NP, should exhibit definite differences between the parallel and the perpendicular directions relative to the SHI beam. In fact, the linearly-polarized-light absorption spectra taken at the polarization planes of 0° and 90°, which include the major and minor axes of the NPs, respectively, are found to be respectively shifted towards the high and low energy sides relative to the spectrum obtained from the non-irradiated state. From the different spectra between the 0° and 90° polarization planes, i.e., OLD spectroscopy, the elongation of the NPs was evaluated .
As schematically shown in Fig. 1(a), however, OLD spectroscopy requires, for each single spectrum, measurements of an independent pair of absorption spectra with the polarization parallel (α = 0°) and perpendicular (α = 90°) to the elongation axes. However, the two independent scans of the spectra inevitably sum up experimental fluctuation of each scan, which limits the detection of very small elongations. In our previous paper , we concluded that an OLD signal was observed down to a fluence of 5.0 × 1011 ions/cm2, but the signal was almost undetectable below that fluence.
In this paper, we applied another property, i.e., the birefringence. From this property, the elongation is evaluated using a single scan of the spectrum under the crossed-Nicols (XN) configuration, as illustrated in Fig. 1(a), which is free from the scan-to-scan fluctuation. Furthermore, this is a null-method wherein spherical NPs exhibit a null signal and only a deviation from the spherical shape is detected. This paper describes the birefringence of Zn NPs elongated by 200 MeV Xe14+ irradiation. Unfortunately, it turns out that another limitation prevents the determination of the possible elongation below the fluence of 5.0 × 1011 ions/cm2. However, it is possible to overcome this limitation using a more sophisticated detection system than that presently employed.
Zn NPs were fabricated in a matrix composed of silica glass of the KU-1 type (OH ~1000 ppm) by implantation of 60 keV Zn ions with a fluence of 1.0 × 1017 ions/cm2. The detailed fabrication conditions and the fundamental properties of the NPs are described in our previous papers [26–28]. The silica samples containing Zn NPs were subsequently irradiated with 200 MeV Xe14+ ions at room temperature (RT) using the tandem accelerator at the Japan Atomic Energy Agency, Tokai Research and Development Center (JAEA-Tokai). The fluence ranged from 1.0 × 1011 to 5.0 × 1013 ions/cm2. The electronic and nuclear energy losses, and the projected range of the 200 MeV Xe14+ ions in SiO2, were estimated from SRIM2008 code  as Se = 14.5 keV/nm, Sn = 0.0496 keV/nm, and RP = 21.7 μm, respectively.
The samples were irradiated at an incident angle of 45° from the surface normal because this configuration is appropriate for the detection of NP elongation via optical anisotropy such as OLD or birefringence. If the sample was irradiated at an incident angle of 0° from the surface normal, the major axes of the NPs would be aligned with the surface normal. Such elongated NPs would appear isotropic in the plane of the sample surface, inhibiting detection of the NP elongation. However, for off-normal incidence irradiation, the anisotropy can be observed in the surface plane.
A standard dual-beam spectrophotometer was used for conducting birefringence spectroscopy at RT in the wavelength region of 215–800 nm with a resolution of 1 nm, and a pair of optical polarizers, P and A, each with an extinction ratio < 5 × 10−5, were employed as shown in Fig. 1(b). Hereafter, the polarization angle of polarizer P is defined as 0°, and all other angles in the system were defined relative to that. The angle of the second polarizer A was defined as θ. A sample was set between the two polarizers P and A in the XN configuration (θ = 90°), and the angle of the major axes of the elongated NPs was defined as α. The sample was illuminated by monochromatic light from the spectrophotometer passing first through polarizer P. Due to the birefringence function of the elongated NPs, a non-zero portion of the incident light passed through the second polarizer A even in the XN configuration. The XN transmittance was defined as the ratio of the transmitted intensity in the XN configuration to the intensity in the configuration where θ = 0° in the absence of a sample.
Figure 2 shows cross-sectional TEM images of Zn NPs embedded in silica irradiated with 200 MeV Xe14+ ions to a fluence of 2 × 1014 ions/cm2. While the NPs were spherical shapes before the irradiation (Fig. 2(b)), the NPs exhibited further growth and shape elongation to the same direction after the irradiation (Fig. 2(a)).
The Zn NP sample irradiated with SHIs was inserted between polarizers P and A. The major axes of the NPs were set at angles (a) α = 0° and (b) α = 45°, and the optical transmittance spectra were recorded while adjusting the angle θ from 0° to 90°. As shown in Fig. 3, the transmittance gradually decreases with increasing θ. A curve for θ = 90°, i.e., XN configuration, is not shown in the case of α = 0° (Fig. 3(a)), because the transmittance was almost zero. This fact indicates the conservation of the incident linear polarization through the sample. Contrary, in the case of α = 45°, non-zero transmittance was observed even at the XN configuration as shown in Fig. 3(b), indicating the modification of the polarization state through the sample.
The elongated Zn NPs exhibit birefringence, i.e., having two different refractive indices for polarizations, respectively, parallel and perpendicular to the major axes of the NPs. Linearly polarized light incident with an arbitrary polarization angle except parallel (α = 0°) and perpendicular (α = 90°) to the major axes propagates through the sample as two different modes with different velocities. The linearly polarized light transforms to elliptically-polarized light, which results in non-zero transmission in the XN configuration. Contrary, in the cases of α = 0° and 90°, only one of the eigen-modes can be excited with keeping the linear polarization.
Since the transmittance spectra shown in Figs. 3(a) and 3(b) include effects of the optical absorption of Zn NPs, which also depends on the angle α, the spectra were divided according to the spectrum taken at θ = 0°. The ratio T(θ)/T(θ = 0) is plotted in Figs. 4(a) and 4(b). In the absence of a sample, the transmission intensity through the first and the second polarizers relative to the angle of θ follows the squared cosine law :
If the sample is inserted but does not change the polarization state, the squared cosine law continues to hold. Figure 4(a) generally exhibits nearly photon-energy independent spectra that are well fit by horizontal broken lines in the figure which denote the values given by Eq. (1). While relatively large deviations from the broken lines are visible with increasing θ, they are artifacts due to a log-plot. It should be noted, however, that a weak peak around 4 eV could be ascribed to the birefringence. The good agreement between Eq. (1) and the data shown in Fig. 4(a) indicates that the elongated Zn NPs mostly do not alter the polarization state when linearly polarized light is incident at α = 0°, i.e., with the polarization parallel to the major axes of the elongated NPs.
In contrast, the ratio T(θ)/T(0) shows significant photon energy dependence in the case of α = 45° (Fig. 4(b)) and largely deviates from the values given by Eq. (1). In this case, the polarization state was modified, and the linearly-polarized incident light was converted to elliptically-polarized light by the elongated Zn NPs. Furthermore, the modification of the polarization is also observed to depend on the photon energy. This phenomenon is due to the birefringence of the elongated Zn NPs, indicative of a polarization-dependent refractive index, which corresponds to the imaginary part of the dichroism, i.e., the polarization-dependent absorption.
The XN transmittance, where a pair of polarizers is sequentially set to an orthogonal configuration (θ = 90°), is an appropriate detection method for birefringence . This configuration ensures that unless the polarization of the incident light is modified by the sample, no transmission of light is observed through the second polarizer set orthogonal to the first. The results are shown in Fig. 5(a). If no sample was inserted between the polarizers, no transmitted light was observed. In addition, if the elongated Zn NP sample was inserted at α = 0° or 90°, no transmitted light was observed. For a uniaxially-anisotropic composite, such as Zn prolate spheroid NPs embedded in a silica matrix, two different refractive indices exist with different polarizations. Because of the different propagation velocities of light with different polarizations, linearly polarized light is transformed to elliptically-polarized light in general cases. However, the cases with α = 0° and 90° are exceptions because only one of the eigen-polarizations is excited, and consequently, linear polarization is conserved.
In the process of rotating the angle α from 0° while maintaining the angle θ = 90°, light transmission gradually appears at ~4 eV and the transmitted intensity exhibits a maximum around α = 45°. At values of α above 45°, the intensity again decreases and becomes zero around α = 90°. The α-dependence of the transmitted intensity at a photon energy of 4 eV is plotted in Fig. 5(b). The α-dependence of the transmittance is well fitted by the following relation:18]. The best fitting was obtained with αo = 1.6° and To = 2.1%. An αo value of 1.6° could be due to misalignment of the sample relative to the SHI beam and the polarized light.
The fluence dependence of the XN transmission is shown in Figs. 6(a) and 6(b). The spectral shape at a fluence of 1.0 × 1013 ions/cm2 is similar to that obtained at a fluence of 5.0 × 1013 ions/cm2, but the peak intensity of the former is ~1/3 of the latter. A very weak signal was detected at 1.0 × 1012 ions/cm2. Finally the signal became below the noise level at 1.0 × 1011 ions/cm2. However, this limitation is simply due to the very low value of TXN obtained. If a light source with a much higher intensity, such as a continuous-mode mid-ultraviolet (UV) laser, was applied, the evaluation of TXN at lower fluences could be possible. The XN transmittance at a photon energy of 4 eV is plotted in Fig. 6(b) with respect to the fluence, which is observed to increase in a roughly linear fashion. However, according to preliminary measurements, saturation was observed at higher fluences.
4.1 Spectrum of birefringence and of the XN transmission
Because of the spheroidal shapes of the elongated NPs, where the lengths of two of the three principle axes are equivalent, the irradiated samples exhibited a birefringence of the uniaxial symmetry which was observed as the XN transmittance shown in Fig. 5. A fundamental quantity of birefringence is the difference in the refractive indices Δn between the non-equivalent principle axes. Here, we derive the difference in the refractive indices between the principle axes, Δnmax, from our experimental results following Ref . We set a Cartesian coordinate system whose z-axis is normal to the sample surface and whose x-axis is parallel to the projected major axes of the elongated NPs . Polarized light incident normal to the sample surface has an electric field vector Ein ∝ T(cos α, sin α, 0), where α is again defined as that shown in Fig. 1(b), and the superscript T represents the transpose of the vector. In this coordinate system, the directional vectors of the s-polarization and p-polarization are given as s = T(0 1 0) and p = T(1 0 0), respectively. The electric fields of both the polarizations transmitted through the sample are given as:Fig. 7 with respect to the photon energy, and a comparison with the spectrum of the XN transmittance TXN is also shown. A value of ~75 nm was used for the thickness of the NP layer L, which was determined from TEM observation . While TXN exhibits a maximum around 4.0 eV, Δnmax exhibits a maximum around 4.4 eV. An interesting feature of Δnmax is that a nearly constant non-zero component is observed below 3.5 eV down to 1.5 eV, whereas TXN decreases monotonically below ~4 eV. This indicates that anisotropic refractive indices, which do not contribute to the XN transmittance, exist below 3.5 eV.
4.2 Possible application to a UV polarizer of nanometric thickness
One of the possible applications of this system is as an optical polarizer of the absorption type working from the near to deep-UV region . The same application has already been proposed for elongated Ag NPs . However, the UV SPR absorption band of Zn NPs is wider than that of Ag NPs, resulting in a wider working photon-energy range. The performance of the polarizer is characterized by the extinction ratio, i.e., the ratio of transmittance of one polarization to that of the orthogonal polarization. In this application, a higher extinction ratio is desirable which produces strong absorption for one polarization but simultaneously weak absorption for the orthogonal polarization at the same photon energy.
An extinction ratio of ~5 dB was observed around 4 eV in the sample irradiated with a fluence of 5.0 × 1013 ions/cm2. While an extinction ratio of ~5 dB would seem small, this was attained from an NP layer of just ~75 nm thick in the present sample, which was confirmed by TEM observation . Table 1 compares the extinction ratio and layer thickness of the present sample with some literature data, which have been referred to as exhibiting “giant birefringence” . While the extinction ratio of our sample is lower than those of the other samples listed in Table 1, the extinction ratio of our sample was attained by a layer that is 2−4 orders of the magnitude thinner than the others. If the thickness of the NP layer could be increased to 250 nm, e.g., using higher energy ion implantation or some other fabrication method, the extinction ratio would be expected to increase to 17 dB. According to Ref , an extinction ratio of 20 dB is a typical commercial value used in the 1.55 μm band. We consider that our sample has much higher potential usefulness because a comparable extinction ratio is attainable with a sample thickness a few orders of magnitude thinner.
Regarding to the methods to form thicker NP layer, various candidates are expected: A simple extension of the present method is higher energy implantation with, e.g., 250 keV Zn ions, which introduces Zn ions up to the depth of 250 nm. Simultaneous multi-energy implantation is better recommended to form a layer with a homogenous concentration along the depth. Other methods, e.g., sputtering co-deposition, could be applicable. A limitation which comes from SHI irradiation is weak tolerance against vacuum degradation, while the SHI irradiation may strongly enhance degasifying from a sample. This can be disadvantageous for nano-composites formed by wet chemical processes and so on. Since the projectile range of 200 MeV Xe ions is 20 μm long, the elongation can be induced whole the NP layer up to, say, 18 μm thick or less. Also the elongation is induced only NPs larger than the track size of the SHI beam used . In the case of 200 MeV Xe ions, only NPs larger than 9 nm in diameter can be elongated. The upper limit could be several tens nm. The elongation of NPs by SHI irradiation strongly depends on the NP sizes, but not on the NP concentration. This method is applicable to much more dilute NPs.
Spherical Zn NPs were fabricated in a silica matrix by implantation of 60 keV Zn+ ions with a fluence of 1.0 × 1017 ions/cm2. The NPs were subsequently irradiated with SHIs of 200 MeV Xe14+ ions at an incident angle of 45°. The fluence of the Xe14+ ions ranged from 1.0 × 1011 to 5.0 × 1013 ions/cm2. The SHI irradiation induced shape deformation of the NPs leading from spheres to prolate spheroids while maintaining the major axes of the NPs parallel to each other. The directionally-aligned Zn spheroids enable the acquisition of macroscopic optical anisotropies such as linear dichroism and birefringence. In a previous paper , the elongation of Zn NPs were studied mainly using TEM and OLD spectroscopy. In this paper, the birefringence of the Zn spheroids were evaluated in the XN configuration, where the sample was inserted between a pair of polarizers that were set in an orthogonal configuration. Linearly polarized light aligned through the first polarizer was transformed to elliptically-polarized light by the birefringence of the Zn spheroid NPs. The existence of the birefringence was confirmed by non-zero transmittance of the second polarizer in the orthogonal configuration. In the sample irradiated with a fluence of 5.0 × 1013 ions/cm2, a maximum XN transmittance of 2.1% at a photon energy of 4 eV and the major-axis angle α dependence typical to birefringence were observed. XN transmission was confirmed with a fluence of 1.0 × 1012 ions/cm2, but the intensity was below the detection limit at a fluence of 1.0 × 1011 ions/cm2. A possible application of the elongated Zn NPs embedded in silica as an absorption-type polarizer film of nanometric thickness working in the near- and mid-UV regions was discussed. Our sample was found to demonstrate high potential usefulness because an extinction ratio comparable to presently available commercial products is expected with a sample thickness a few orders of magnitude thinner.
This work was partly supported by JSPS-KAKENHI Grant Number 26390032. The SHI irradiations were performed under the Common-Use Facility Program of JAEA. The authors are grateful to the technical staff of the accelerator facilities at JAEA-Tokai for their kind help. The authors also thank Prof. S. Mantl, and Mr. A. Dahmen (FZ Juelich) for Zn ion implantation.
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