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A miniaturized photoreactor based on a hollow-core metal cladding waveguide
Optics Communications 333 (2014) 67–70
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A miniaturized photoreactor based on a hollow-core metal cladding waveguide Tian Xu a, Liming Huang a,1, Danzhu Wei a, Yonglong Jin a, Jinhuai Fang a, Guoqiu Yuan a,n, Zhuangqi Cao b, Honggen Li b a b
Physics Department, Nantong University, Jiangsu, No. 999 Tong Jin Road, Nantong 226007, PR China Department of Physics, Shanghai Jiao Tong University, No. 800 Dong Chuan Road, Shanghai 200240, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2014 Received in revised form 21 July 2014 Accepted 22 July 2014 Available online 2 August 2014
We present a novel reactor based on the ultrahigh order modes in a hollow-core metal cladding waveguide (HCMW). Aqueous Fe2 þ =Fe3 þ salt solutions in the hollow-core act as the HCMW guiding layer. Light power is induced into the guiding layer using the free-space coupling technique. A series of enhanced power points, considered to be optical trapping points, are formed because of the excitation of the ultrahigh-order modes. It is demonstrated that, at these optical trapping points, crystalline magnetite γFe2 O3 is synthesized after the deionized water is photolyzed into OH . The magnetooptical effect of this γFe2 O3 is explored for potential applications. & 2014 Elsevier B.V. All rights reserved.
Keywords: Photoreactor Magnetite Waveguide Magneto-optical effect
Recently, many new photoreactors have been developed for photocatalysis, liquid waste treatment, and other applications [1,2]. Using a reactor to facilitate electrosteric stabilization of colloidal polymer particles can greatly shorten the reaction time or improve the reproducibility of the reaction [3]. The TiO2-coated optical fiber photoreactor (OFR), which involves a strict evanescent wave mechanism for light propagation, has been used for in situ remediation of contaminated subsurface environments [4]. Use of a reactor coated waveguide in an attenuated total reflection (ATR) mode has been proposed to use light energy more efficiently or enhance quantum efficiency for the photocatalytic oxidation of formic acid in water [5]. In an ATR mode of propagation, incident light upon the silica/TiO2/water interface is totally reflected back into the silica. At each reflection, the film absorbs a portion of the UV light (310–380 nm) that has greater energy. Here, we present a novel reactor based on the ultrahigh order modes in a miniaturized hollow core metal-cladding waveguide (HCMW) stirred by a low-power laser. In contrast to the usual photo-reactor, three major benefits can be summarized as follows. First, the HCMW structure can be triggered by a 100 mW laser. Secondly, the HCMW structure provides an extremely large reaction area, since the whole guiding layer region will have a similar enhanced
n
Corresponding author. The author contributed equally to this work. E-mail address: [email protected] (G. Yuan). 1 The author contributed equally to this work.
http://dx.doi.org/10.1016/j.optcom.2014.07.059 0030-4018/& 2014 Elsevier B.V. All rights reserved.
intensity due to the excitation of several oscillating modes instead of evanescent waves. Third, the HCMW structure enables excitation of guided modes with orthogonal polarizations (TE or TM). The HCMW structure in Fig. 1 is composed of three parts: (i) A sample cell about 5 mm in radius and 0.7 mm in depth fabricated from the precise placement of two identical C-shaped glass gaskets; (ii) a thin silver film (about 30 nm) coated on the top side of a thin glass slab for better coupling; (iii) a relatively thick gold film (200 nm) deposited on a thick glass slab to prevent light leakage. To ensure parallelism, the three parts of the HCMW structure are attached by optical cement. The top silver film serves as a coupling layer as well as a metal cladding, the glass slab (0.5 mm) and the solution injected from the inlet of the sample cell (0.7 mm) work as a guiding layer, and the base gold film acts as a substrate of the waveguide. In this paper, the subscripts 1, 2, 3, and 4 represent the free space, silver layer, guiding layer, and gold substrate, respectively. h2, h3, and h4 are the thickness of each film, respectively. Here, the thickness of the guiding layer h3 ¼ hglass þ hsolution . In our experiment, the solution (about 250 μL aqueous Fe2 þ =Fe3 þ salt solution) was made by dissolving 34 mg FeCl3 6H2 O and 29 mg FeCl2 7H2 O (purchased from Slnopharm Group Co. Ltd.) in 100 mL deionized water, and was injected into the sample cell of HCMW by a micro-injector. The HCMW was settled on a goniometer as shown in Fig. 2. A 100 mW collimated light beam emitted from a diode laser at wavelength of 780 nm (AUTFSL-780-100T, Shanghai Haoliang Optoelectronic Equipment Co., Ltd.) was incident upon the upper silver layer after passing
68
T. Xu et al. / Optics Communications 333 (2014) 67–70
through one polarizer and two apertures (the diameter of the apertures was 2 mm, and the distance between them was 0.2 m). The reflected light was detected by a photodiode (PD). Homemade software allowed us to carry out angular scanning. The intensity of the reflected beam varies with the incident angle and forms a series of sharp dips in the reflection spectrum at the extremely small phase-matching resonance angles, as shown in the inset of Fig. 3, indicating that the energy of the light source has been coupled to the ultrahigh-order modes in the HCMW structure [6–10]. Subsequently, one reflection dip was selected as shown in Fig. 3, and the incident angle was adjusted at the minimum of the dip, where maximum energy can be coupled. After the laser had been coupled into the HCMW for two hours, the solution in the sample cell was rinsed and cleaned with 10 mL deionized water twice to scour off Cl and SO24 . High resolution transmission electron microscopy (HRTEM, Tecani G2 F20 S-TWIN) demonstrated that the product in the sample cell was composed of Fe and O, and the iron oxides were a nano-scale mixture of crystalline and amorphous form as displayed in Fig. 4. The measured lattice spacing of the iron oxide was 0.2692 nm (Fig. 4(b)), which is in accordance with the distance of 0.27 nm between two {310} crystal planes in maghemite (γFe2 O3 ). [11] The mixture of crystalline position and amorphous forms (Fig. 4(a)) illustrates that the higher the light intensity, the better the crystallization of γFe2 O3 . This result further confirms the existence of optical trapping [9,12,13] at the crystalline position. An experiment without the HCMW structure (by removing the upper silver film of the HCMW) was also carried out, and the magnetic product was not found. The most conventional method for obtaining Fe3 O4 or γFe2 O3 is by co-precipitation. This method involves mixing Fe3 þ and Fe2 þ ions in a 1:2 molar ratio in highly basic solution at room temperature or at elevated temperature. Highly basic solutions or OH is necessary for synthesizing Fe3 O4 or γFe2 O3 . Why does the HCMW reactor allow the synthesis of magnetic fluid in aqueous Fe3 þ =Fe2 þ salt solutions without alkaline conditions, and why does the optical trapping effect occur in the HCMW structure? Owing to the advantage of the free-space coupling technique [14], HCMW with a relatively thick (submillimeter scale) optical waveguide can support a large amount of reflection dips, and so called ultrahigh-order guided modes can be excited at the
Fig. 1. Schematic structure of the symmetrical metal-cladding waveguide (HCMW).
extremely small phase-matching resonance angles [6–10]. Those modulus coefficients are m 4 1000. The optical energy is mainly confined in the guiding layer (including the glass slab and the solution), where appears the oscillating field of guided modes. Compared with the power confined in the guiding layer, the power confined in the metal claddings approaches zero. Therefore, the sample in a HCMW is different from that in conventional waveguides and the surface plasmon resonance (SPR) structure, where an evanescent field exists. The light (blue line) coupled in the guiding layer of the HCMW propagated along the “Z” path shown in Fig. 5 because of total reflection between the guiding layer and cladding. The traveling wave is therefore along the z-axis. Moreover, a standing wave exists along the x-axis due to the overlaying of incident and reflected waves in Fig. 5. The red points of periodic and strong power, which are considered to be the optical trapping points, appear in the sample cell as shown in Fig. 5. Maghemite crystallized in these trapping points, while out of them was FeOðOHÞx with amorphous form. The amorphous material was unstable intermediate of iron oxides. Thus, the optical field in the HCMW is anisotropic and the energy of each optical trapping point in the HCMW structure is sufficient for promoting the photolysis of H2 O. Therefore, the mechanism of the reaction should be H2 O
photoionization
⟶
O2 þ 4e þ 4H þ ;
ð1Þ
O2 þ 2H2 O þ4e ⟶4OH ;
ð2Þ
Fe2 þ þ 2Fe3 þ þ 8OH ⟶Fe3 O4 þ 4H2 O;
ð3Þ
4Fe3 O4 þO2 ⟶6γ Fe2 O3 :
ð4Þ
Fig. 3. Reflection spectrum of the ultra-high modes in a HCMW structure with the following parameters: λ ¼ 780 nm, ε2 ¼ 26:3 þ 1:4693i, εglass ¼ 2:25, εsolution ¼ 1:8225, ε4 ¼ 24:1þ 1:7230i, h3 ¼ 1:2 mm, d2 ¼ 30 nm. The inset is the reflectivity for incident angles of 0–6 degrees.
Fig. 2. Schematic diagram of the experimental setup.
T. Xu et al. / Optics Communications 333 (2014) 67–70
69
Fig. 4. (a) High resolution transmission electron micrographs (TEM) of the obtained magnetite, (b) the lattice spacing is 0.2692 nm, and (c) elemental analysis of the magnetite by X-ray energy dispersive spectroscopy (EDX).
The threshold behavior of magnetic fluid (Fe3 O4 ) has been discussed by Yuan [9] and Horng [15]. In this paper, the magnetooptical effect of γFe2 O3 was investigated. After the sample had been irradiated by the laser for two hours, a magnetic field, H, was introduced perpendicular to the sample cell (along the x-axis as displayed in Fig. 1) and measured by a HT100G Gauss meter (Shanghai Hengtong Magneto Electric Technology Co. Ltd.). Here, TE-polarization was considered. The selected reflection dip in Fig. 3 shifted to the left after the introduction of the magnetic field. The operation incident angle was fixed at the middle of the falling edge in a reflection dip to achieve good linearity and high sensitivity. We experimentally measured the reflectivity variation as a function of magnetic field and plotted the relation between the normalized reflectivity (R=R0 ) and the field strength as shown in Fig. 6(a). R is the reflectivity at operation incident angle with different H from 0 Oe to 150 Oe, while R0 is that of H¼ 0. In such a liquid-core waveguide structure, the property of having a normalized reflectivity sensitive to a magnetic field as displayed in Fig. 6(a) can be applied to switching, modulation, and detection through the tuning of light intensity in a magnetic field. Based on a dispersion equation, a rough estimation of the variation in refractive index Δn3 of the guiding layer in the HCMW can be written as [7]
Δn 3
ΔR W θ dn3 ; 1 Rmin dθ
where
" 2
Rmin ¼ jr 12 j
4Imðβ ÞImðΔβ 0
1
ðImðβ Þ þ Imðβ 0
rad
rad
Þ
ÞÞ2
# ;
ð6Þ
2½Imðβ Þ þ Imðβ Þ ; pffiffiffiffiffi k0 ε1 cos θres 0
Wθ ¼
ð5Þ
Δβrad ¼
rad
ð7Þ
κ 3 r 12 ½r 23 þr 34 expð2iκ 3 h3 Þexpð2iκ 2 h2 Þ ; 0 2β ðκ12 þ κ14 ih3 Þ
dθ 1 ¼ pffiffiffiffiffi ; dN ε1 cos θ
r ij ¼
κi κj ; κi þ κj
ð8Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
κ i ¼ k0 εi N 2 :
ð9Þ
and θ is the angle of the incident beam from the air space (ε1 ¼ 1), pffiffiffiffiffi k0 is the wavenumber in vacuum, N ¼ ε1 cos θ is the effective index of the selected modes, and rij denotes the Fresnel reflection coefficient. W θ is the full width at half maximum (FWHM) of the
Fig. 5. Plot of the optical trapping points in the sample cell. Light is the blue line, the standing waves are the green and black lines, and the red points are the optical trapping points. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
reflection dip determined by the intrinsic dampings Imðβ Þ and rad radiative dampings ImðΔβ Þ. Fig. 6(b) is plotted with the following parameters: h2 ¼ 30 nm, h3 ¼ 1.2 mm, h4 ¼ 200 nm, ε1 ¼ 1, ε2 ¼ 26:3 þ1:4693i, ε3 ¼ 1:85; 1:95; 2:0; 2:2, ε4 ¼ 24:1 þ 1:7230i, λ ¼ 780 nm. When the magnetic field is perpendicular to the HCMW structure, the TE mode refractive index changes with the reflectivity as shown in Fig. 6(b). When ε3 increases, n3 changes greatly. The main reason that n3 changed with H is that the magnetic particles were dispersed in the water without a magnetic field, whereas magnetic columns, agglomerated by the originally dispersed particles, formed as the magnetic field was raised [9,15–17]. Fig. 6(b) shows that the variation of n3 was on the order of 10 5, which illustrates that the HCMW structure was highly sensitive to the refraction index of the guiding layer [6–9]. In conclusion, a novel HCMW photoreactor triggered by a lowpower diode laser was proposed. Because the aqueous Fe2 þ =Fe3 þ salt solutions are located in the guiding layer in which the oscillating wave propagates rather than the evanescent wave, a great portion of the light energy is concentrated. Magnetic γFe2 O3 is produced in the sample cell as a result. The ultrahigh-order modes in the HCMW structure significantly improve its light use efficiency and its magneto-optical effect sensitivity. Therefore, the present novel photoreactor may be beneficial for practical 0
70
T. Xu et al. / Optics Communications 333 (2014) 67–70
Fig. 6. (a) Normalized reflectivity versus magnetic field for TE mode. R0 is the reflectivity of H¼ 0 at operation incident angle; (b) variation in refractive index Δn3 for TEpolarization versus magnetic field H for different values of ε3 ¼ 1:85; 1:95; 2:0; 2:2. The error bars (red lines) are the standard deviation of 10 individual experiments. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
applications such as switching, modulation, and detection, by tuning light intensity in a magnetic field. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 61171015), National Special Fund for the Development of MAJOR Research Equipment and Instruments (No. 2011YQ03013403), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 13KJB140014), Science and technology project (BK2012049), Startup Fund for PhD of Nantong University (Natural Science), and Research Fund of Nantong University (03080122, 13040055). References [1] M.H. Beak, J.W. Yoon, J.S. Hong, J.K. Suh, Appl. Catal. A Gen. 450 (2013) 222. [2] M. Gomez, M.D. Murcia, J.L. Gomez, E. Gomez, M.F. Maximo, A. Garcia, Appl. Catal. B Environ. 117–118 (2012) 194.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
M. Schrinner, B. Haupt, A. Wittemann, Chem. Eng. J. 144 (2008) 138. T.V. Nguyen, J.C.S. Wu, Appl. Catal. A Gen. 335 (2008) 112. L.W. Miller, M.I. Tejedor, M.A. Anderson, Environ. Sci. Technol. 33 (1999) 2070. H. Lu, Z. Cao, H. Li, Q. Shen, Appl. Phys. Lett. 85 (2004) 4579. J. Sun, C. Yin, C. Zhu, X. Wang, W. Yuan, P. Xiao, X. Chen, Z. Cao, J. Opt. Soc. Am. B. 29 (2012) 769. G. Chen, Z. Cao, J. Gu, Q. Shen, Appl. Phys. Lett. 89 (2006) 081120. W. Yuan, C. Yin, P. Xiao, X. Wang, J. Sun, S. Huang, X. Chen, Z. Cao, Microfluid. Nanofluid. 11 (2011) 781. Y. Wang, H. Li, Z. Cao, T. Yu, Q. Shen, Y. He, Appl. Phys. Lett. 92 (2008) 061113. V. Barrón, J. Torrent, E.D. Grave, Am. Mineral. 88 (2003) 1679. A. Ashkin, J.M. Dziedzic, T. Yamane, Nature 330 (1987) 769. Y.Y. Sun, J. Bu, L.S. Ong, X.C. Yuan, Appl. Phys. Lett. 91 (2007) 051101. H. Li, Z. Cao, H. Lu, Q. Shen, Appl. Phys. Lett. 83 (2003) 2757. H.E. Horng, C.S. Chen, K.L. Fang, S.Y. Yang, J.J. Chieh, C.Y. Hong, H.C. Yang, Appl. Phys. Lett. 85 (2004) 5592. H.E. Horng, C.Y. Hong, S.Y. Yang, H.C. Yang, Appl. Phys. Lett. 82 (2003) 2434. S.Y. Yang, J.J. Chieh, H.E. Horng, C.Y. Hong, H.C. Yang, Appl. Phys. Lett. 84 (2004) 5204.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A miniaturized photoreactor based on a hollow-core metal cladding waveguide Tian Xu a, Liming Huang a,1, Danzhu Wei a, Yonglong Jin a, Jinhuai Fang a, Guoqiu Yuan a,n, Zhuangqi Cao b, Honggen Li b a b
Physics Department, Nantong University, Jiangsu, No. 999 Tong Jin Road, Nantong 226007, PR China Department of Physics, Shanghai Jiao Tong University, No. 800 Dong Chuan Road, Shanghai 200240, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2014 Received in revised form 21 July 2014 Accepted 22 July 2014 Available online 2 August 2014
We present a novel reactor based on the ultrahigh order modes in a hollow-core metal cladding waveguide (HCMW). Aqueous Fe2 þ =Fe3 þ salt solutions in the hollow-core act as the HCMW guiding layer. Light power is induced into the guiding layer using the free-space coupling technique. A series of enhanced power points, considered to be optical trapping points, are formed because of the excitation of the ultrahigh-order modes. It is demonstrated that, at these optical trapping points, crystalline magnetite γFe2 O3 is synthesized after the deionized water is photolyzed into OH . The magnetooptical effect of this γFe2 O3 is explored for potential applications. & 2014 Elsevier B.V. All rights reserved.
Keywords: Photoreactor Magnetite Waveguide Magneto-optical effect
Recently, many new photoreactors have been developed for photocatalysis, liquid waste treatment, and other applications [1,2]. Using a reactor to facilitate electrosteric stabilization of colloidal polymer particles can greatly shorten the reaction time or improve the reproducibility of the reaction [3]. The TiO2-coated optical fiber photoreactor (OFR), which involves a strict evanescent wave mechanism for light propagation, has been used for in situ remediation of contaminated subsurface environments [4]. Use of a reactor coated waveguide in an attenuated total reflection (ATR) mode has been proposed to use light energy more efficiently or enhance quantum efficiency for the photocatalytic oxidation of formic acid in water [5]. In an ATR mode of propagation, incident light upon the silica/TiO2/water interface is totally reflected back into the silica. At each reflection, the film absorbs a portion of the UV light (310–380 nm) that has greater energy. Here, we present a novel reactor based on the ultrahigh order modes in a miniaturized hollow core metal-cladding waveguide (HCMW) stirred by a low-power laser. In contrast to the usual photo-reactor, three major benefits can be summarized as follows. First, the HCMW structure can be triggered by a 100 mW laser. Secondly, the HCMW structure provides an extremely large reaction area, since the whole guiding layer region will have a similar enhanced
n
Corresponding author. The author contributed equally to this work. E-mail address: [email protected] (G. Yuan). 1 The author contributed equally to this work.
http://dx.doi.org/10.1016/j.optcom.2014.07.059 0030-4018/& 2014 Elsevier B.V. All rights reserved.
intensity due to the excitation of several oscillating modes instead of evanescent waves. Third, the HCMW structure enables excitation of guided modes with orthogonal polarizations (TE or TM). The HCMW structure in Fig. 1 is composed of three parts: (i) A sample cell about 5 mm in radius and 0.7 mm in depth fabricated from the precise placement of two identical C-shaped glass gaskets; (ii) a thin silver film (about 30 nm) coated on the top side of a thin glass slab for better coupling; (iii) a relatively thick gold film (200 nm) deposited on a thick glass slab to prevent light leakage. To ensure parallelism, the three parts of the HCMW structure are attached by optical cement. The top silver film serves as a coupling layer as well as a metal cladding, the glass slab (0.5 mm) and the solution injected from the inlet of the sample cell (0.7 mm) work as a guiding layer, and the base gold film acts as a substrate of the waveguide. In this paper, the subscripts 1, 2, 3, and 4 represent the free space, silver layer, guiding layer, and gold substrate, respectively. h2, h3, and h4 are the thickness of each film, respectively. Here, the thickness of the guiding layer h3 ¼ hglass þ hsolution . In our experiment, the solution (about 250 μL aqueous Fe2 þ =Fe3 þ salt solution) was made by dissolving 34 mg FeCl3 6H2 O and 29 mg FeCl2 7H2 O (purchased from Slnopharm Group Co. Ltd.) in 100 mL deionized water, and was injected into the sample cell of HCMW by a micro-injector. The HCMW was settled on a goniometer as shown in Fig. 2. A 100 mW collimated light beam emitted from a diode laser at wavelength of 780 nm (AUTFSL-780-100T, Shanghai Haoliang Optoelectronic Equipment Co., Ltd.) was incident upon the upper silver layer after passing
68
T. Xu et al. / Optics Communications 333 (2014) 67–70
through one polarizer and two apertures (the diameter of the apertures was 2 mm, and the distance between them was 0.2 m). The reflected light was detected by a photodiode (PD). Homemade software allowed us to carry out angular scanning. The intensity of the reflected beam varies with the incident angle and forms a series of sharp dips in the reflection spectrum at the extremely small phase-matching resonance angles, as shown in the inset of Fig. 3, indicating that the energy of the light source has been coupled to the ultrahigh-order modes in the HCMW structure [6–10]. Subsequently, one reflection dip was selected as shown in Fig. 3, and the incident angle was adjusted at the minimum of the dip, where maximum energy can be coupled. After the laser had been coupled into the HCMW for two hours, the solution in the sample cell was rinsed and cleaned with 10 mL deionized water twice to scour off Cl and SO24 . High resolution transmission electron microscopy (HRTEM, Tecani G2 F20 S-TWIN) demonstrated that the product in the sample cell was composed of Fe and O, and the iron oxides were a nano-scale mixture of crystalline and amorphous form as displayed in Fig. 4. The measured lattice spacing of the iron oxide was 0.2692 nm (Fig. 4(b)), which is in accordance with the distance of 0.27 nm between two {310} crystal planes in maghemite (γFe2 O3 ). [11] The mixture of crystalline position and amorphous forms (Fig. 4(a)) illustrates that the higher the light intensity, the better the crystallization of γFe2 O3 . This result further confirms the existence of optical trapping [9,12,13] at the crystalline position. An experiment without the HCMW structure (by removing the upper silver film of the HCMW) was also carried out, and the magnetic product was not found. The most conventional method for obtaining Fe3 O4 or γFe2 O3 is by co-precipitation. This method involves mixing Fe3 þ and Fe2 þ ions in a 1:2 molar ratio in highly basic solution at room temperature or at elevated temperature. Highly basic solutions or OH is necessary for synthesizing Fe3 O4 or γFe2 O3 . Why does the HCMW reactor allow the synthesis of magnetic fluid in aqueous Fe3 þ =Fe2 þ salt solutions without alkaline conditions, and why does the optical trapping effect occur in the HCMW structure? Owing to the advantage of the free-space coupling technique [14], HCMW with a relatively thick (submillimeter scale) optical waveguide can support a large amount of reflection dips, and so called ultrahigh-order guided modes can be excited at the
Fig. 1. Schematic structure of the symmetrical metal-cladding waveguide (HCMW).
extremely small phase-matching resonance angles [6–10]. Those modulus coefficients are m 4 1000. The optical energy is mainly confined in the guiding layer (including the glass slab and the solution), where appears the oscillating field of guided modes. Compared with the power confined in the guiding layer, the power confined in the metal claddings approaches zero. Therefore, the sample in a HCMW is different from that in conventional waveguides and the surface plasmon resonance (SPR) structure, where an evanescent field exists. The light (blue line) coupled in the guiding layer of the HCMW propagated along the “Z” path shown in Fig. 5 because of total reflection between the guiding layer and cladding. The traveling wave is therefore along the z-axis. Moreover, a standing wave exists along the x-axis due to the overlaying of incident and reflected waves in Fig. 5. The red points of periodic and strong power, which are considered to be the optical trapping points, appear in the sample cell as shown in Fig. 5. Maghemite crystallized in these trapping points, while out of them was FeOðOHÞx with amorphous form. The amorphous material was unstable intermediate of iron oxides. Thus, the optical field in the HCMW is anisotropic and the energy of each optical trapping point in the HCMW structure is sufficient for promoting the photolysis of H2 O. Therefore, the mechanism of the reaction should be H2 O
photoionization
⟶
O2 þ 4e þ 4H þ ;
ð1Þ
O2 þ 2H2 O þ4e ⟶4OH ;
ð2Þ
Fe2 þ þ 2Fe3 þ þ 8OH ⟶Fe3 O4 þ 4H2 O;
ð3Þ
4Fe3 O4 þO2 ⟶6γ Fe2 O3 :
ð4Þ
Fig. 3. Reflection spectrum of the ultra-high modes in a HCMW structure with the following parameters: λ ¼ 780 nm, ε2 ¼ 26:3 þ 1:4693i, εglass ¼ 2:25, εsolution ¼ 1:8225, ε4 ¼ 24:1þ 1:7230i, h3 ¼ 1:2 mm, d2 ¼ 30 nm. The inset is the reflectivity for incident angles of 0–6 degrees.
Fig. 2. Schematic diagram of the experimental setup.
T. Xu et al. / Optics Communications 333 (2014) 67–70
69
Fig. 4. (a) High resolution transmission electron micrographs (TEM) of the obtained magnetite, (b) the lattice spacing is 0.2692 nm, and (c) elemental analysis of the magnetite by X-ray energy dispersive spectroscopy (EDX).
The threshold behavior of magnetic fluid (Fe3 O4 ) has been discussed by Yuan [9] and Horng [15]. In this paper, the magnetooptical effect of γFe2 O3 was investigated. After the sample had been irradiated by the laser for two hours, a magnetic field, H, was introduced perpendicular to the sample cell (along the x-axis as displayed in Fig. 1) and measured by a HT100G Gauss meter (Shanghai Hengtong Magneto Electric Technology Co. Ltd.). Here, TE-polarization was considered. The selected reflection dip in Fig. 3 shifted to the left after the introduction of the magnetic field. The operation incident angle was fixed at the middle of the falling edge in a reflection dip to achieve good linearity and high sensitivity. We experimentally measured the reflectivity variation as a function of magnetic field and plotted the relation between the normalized reflectivity (R=R0 ) and the field strength as shown in Fig. 6(a). R is the reflectivity at operation incident angle with different H from 0 Oe to 150 Oe, while R0 is that of H¼ 0. In such a liquid-core waveguide structure, the property of having a normalized reflectivity sensitive to a magnetic field as displayed in Fig. 6(a) can be applied to switching, modulation, and detection through the tuning of light intensity in a magnetic field. Based on a dispersion equation, a rough estimation of the variation in refractive index Δn3 of the guiding layer in the HCMW can be written as [7]
Δn 3
ΔR W θ dn3 ; 1 Rmin dθ
where
" 2
Rmin ¼ jr 12 j
4Imðβ ÞImðΔβ 0
1
ðImðβ Þ þ Imðβ 0
rad
rad
Þ
ÞÞ2
# ;
ð6Þ
2½Imðβ Þ þ Imðβ Þ ; pffiffiffiffiffi k0 ε1 cos θres 0
Wθ ¼
ð5Þ
Δβrad ¼
rad
ð7Þ
κ 3 r 12 ½r 23 þr 34 expð2iκ 3 h3 Þexpð2iκ 2 h2 Þ ; 0 2β ðκ12 þ κ14 ih3 Þ
dθ 1 ¼ pffiffiffiffiffi ; dN ε1 cos θ
r ij ¼
κi κj ; κi þ κj
ð8Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
κ i ¼ k0 εi N 2 :
ð9Þ
and θ is the angle of the incident beam from the air space (ε1 ¼ 1), pffiffiffiffiffi k0 is the wavenumber in vacuum, N ¼ ε1 cos θ is the effective index of the selected modes, and rij denotes the Fresnel reflection coefficient. W θ is the full width at half maximum (FWHM) of the
Fig. 5. Plot of the optical trapping points in the sample cell. Light is the blue line, the standing waves are the green and black lines, and the red points are the optical trapping points. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
reflection dip determined by the intrinsic dampings Imðβ Þ and rad radiative dampings ImðΔβ Þ. Fig. 6(b) is plotted with the following parameters: h2 ¼ 30 nm, h3 ¼ 1.2 mm, h4 ¼ 200 nm, ε1 ¼ 1, ε2 ¼ 26:3 þ1:4693i, ε3 ¼ 1:85; 1:95; 2:0; 2:2, ε4 ¼ 24:1 þ 1:7230i, λ ¼ 780 nm. When the magnetic field is perpendicular to the HCMW structure, the TE mode refractive index changes with the reflectivity as shown in Fig. 6(b). When ε3 increases, n3 changes greatly. The main reason that n3 changed with H is that the magnetic particles were dispersed in the water without a magnetic field, whereas magnetic columns, agglomerated by the originally dispersed particles, formed as the magnetic field was raised [9,15–17]. Fig. 6(b) shows that the variation of n3 was on the order of 10 5, which illustrates that the HCMW structure was highly sensitive to the refraction index of the guiding layer [6–9]. In conclusion, a novel HCMW photoreactor triggered by a lowpower diode laser was proposed. Because the aqueous Fe2 þ =Fe3 þ salt solutions are located in the guiding layer in which the oscillating wave propagates rather than the evanescent wave, a great portion of the light energy is concentrated. Magnetic γFe2 O3 is produced in the sample cell as a result. The ultrahigh-order modes in the HCMW structure significantly improve its light use efficiency and its magneto-optical effect sensitivity. Therefore, the present novel photoreactor may be beneficial for practical 0
70
T. Xu et al. / Optics Communications 333 (2014) 67–70
Fig. 6. (a) Normalized reflectivity versus magnetic field for TE mode. R0 is the reflectivity of H¼ 0 at operation incident angle; (b) variation in refractive index Δn3 for TEpolarization versus magnetic field H for different values of ε3 ¼ 1:85; 1:95; 2:0; 2:2. The error bars (red lines) are the standard deviation of 10 individual experiments. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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