Polarization-insensitive guided-mode resonance (GMR) filters have significant role in applications such as optical communication systems. Here, we report the design and fabrication of two types of simple-structured one-dimensional (1D) GMR gratings with non-polarizing resonance properties under normal incidence. A single-layer rectangular-profile TiO2 grating is fabricated by electron beam lithography and reactive ion etching, which demonstrates, for the first time in experiment, almost perfect non-polarizing filtering effect with 1D grating under normal incidence. Then, a TiO2-coated polycarbonate 1D GMR grating is fabricated by nanoimprinting and atomic layer deposition, which also exhibits good non-polarizing property and the potential of low-cost mass replication of such functional devices.
© 2012 Optical Society of America
Dielectric waveguide gratings with guided mode resonance (GMR) [1, 2] have attracted much attention in the past two decades. Since GMR gratings can generate high-Q resonances, one important application is narrow-band filtering . Such GMR filters have been extensively studied so far by improving their optical performance such as diffraction efficiency, Q factor, passband frequency, and sideband suppression. Meanwhile, various applications based on GMR filters have been proposed, such as those in laser systems and optical communications .
GMR filters based on one-dimensionally (1D) periodic gratings usually have strong polarization dependence, which is useful for polarizing filtering. However, in certain applications such as dense wavelength division multiplexing, polarization insensitive filters are highly desirable due to the unknown polarization state of light emerging from optical fibers . In addition, the GMR gratings can also be employed as biosensors , e.g., to enhance fluorescence  with the advantage of non-polarized light at normal and oblique incidence. To realize non-polarizing GMR filtering, some previous designs have been proposed, which are usually based on either 1D GMR gratings under conical incidence [3, 6] or two-dimensionally periodic GMR gratings [7, 8]. These structures have certain complexity in either their geometries or incident mountings, imposing additional difficulty on fabrication and application.
Recently, we proposed a simple design of single-layer 1D GMR gratings with non-polarizing properties under normal incidence . Preliminary test fabrication has been performed on a sinusoidal-profile non-polarizing 1D GMR grating using photoinduced polymer deformation and atomic layer deposition (ALD), which shows the potential of low-cost fabrication of such devices . However, the measured non-polarizing filtering effect was not satisfactory therein and should be further improved.
In this work, we present the design and experimental realization of a single-layer rectangular-profile non-polarizing 1D GMR grating (Type-I) and a double-layer non-polarizing 1D GMR grating (Type-II), which are both designed to work around a resonance wavelength of 850 nm under normal incidence. High-quality TiO2 Type-I GMR gratings are fabricated with electron-beam lithography (EBL) and reactive ion etching (RIE), exhibiting good non-polarizing effect. By employing nanoimprinting and ALD technique, TiO2-coated Type-II GMR gratings are realized, showing the potential of generating replicable non-polarizing GMR filters with relatively low cost, which are suitable for mass production.
2. Design of the non-polarizing GMR gratings
The investigated two types of GMR gratings are schematically depicted in Fig. 1. The Type-I GMR grating consists of a rectangular-profile TiO2 grating layer on a fused silica substrate. This design aims to verify the non-polarizing filtering effect with the simplest single-layer 1D GMR grating geometry, which has not been realized before. The Type-II grating is a TiO2-coated grating with the polycarbonate substrate replicated from a rectangular-profile master grating stamp by nanoimprinting. It aims to demonstrate the replicability of the 1D non-polarizing GMR grating. Without loss of generality, both gratings are designed to work around a resonance wavelength of 850 nm under normal incidence. The gratings at oblique incidence can also be designed easily by the same method. The electric field vector of the incident field may be parallel (TE) or perpendicular (TM) to the grating lines.
The optical responses of the GMR gratings are simulated rigorously with the Fourier Modal Method . The refractive index of the ALD-prepared TiO2 film, which is sensitive to the deposition method, was measured by ellipsometry and fitted with the Sellmeier formula:
The GMR gratings, for example, Giant reflection to zero order (GIRO) mirrors, are based on the principle of interference of two propagating modes in the grating region for both polarizations TE and TM . The resulting outcoupling wave depends on the phase between the two interfering waves . In the design of non-polarizing GMR gratings, an adjustment of the structural parameters can result in a fine tuning of the dispersion relations of TE and TM excited leaky guided modes in the grating layer. As a result, there exist a situation where both polarizations have the same propagation constant at the cross point of the dispersion curves of TE- and TM-modes at normal incidence . In this design, subwavelength (d < λ) grating structures are employed to allow only the propagation of zeroth transmitted diffraction order at the resonance. Since the resonance wavelength λres is related to the grating period d, one can achieve GMR at any desired wavelength by choosing a suitable period d < λres/nsub . Thus the non-polarizing effect is achieved by optimizing the other structural parameters, by which to engineer the dispersion relations of TE and TM leaky guided modes so that the simultaneous excitation of both can be realized under normal incidence. By applying the optimization procedure as described in Ref.  and taking into account the feasibility of fabrication, we obtained the optimal structural parameters for the designed GMR gratings: for the Type-I grating, d = 540 nm, w = 395 nm, and h = 195 nm; while for the Type-II grating, d = 540 nm, w = 200 nm, h = 145 nm, and t = 60 nm. The simulated transmittance and reflectance spectra of the zeroth orders of the two GMR gratings are shown in Fig. 2. It is seen that non-polarizing resonance effect can be produced at wavelength λ = 850 nm for both gratings.
3. Fabrication of the GMR gratings
We employed ALD, EBL, and RIE techniques to fabricate the Type-I GMR grating. The fabrication procedure is schematically presented in Fig. 3.
First, a 1″ fused silica substrate was cleaned with isopropanol. Then, a layer of amorphous TiO2 of thickness 195 nm was deposited on the substrate by ALD, using Beneq TFS 200–152 reactor with precursor materials TiCl4 and H2O at a deposition temperature of 120 °C. After TiO2 growth, the sample was covered by a thin Cr layer of 30 nm by electron beam evaporation at a vacuum level of 1.5 × 10−6 mbar and a deposition rate of 2 Å/s using Lebold L560 vacuum evaporator from Lebold Heraeus. After the deposition of Cr layer, the sample was spin-coated by positive electron beam resist ZEP 7000 22 at a spinning speed of 3000 rpm for 60 s using Headway PWM101D from Headway research Ltd., followed by soft baking at 180 °C temperature for 180 s on a hot plate. Then, the resist was patterned by an electron beam writer EBPG5000+ES HR from Vistec Lithography on an area of 7×7 mm2 with a dose of 300 μC/cm2. The exposed sample was developed with Ethyl 3-ethoxypropionate (EEP) of 99 % by Aldrich Ltd. for 60 s, followed by isopropanol for 30 s, rinsing with deionized water, and dry nitrogen blow. Afterwards, dry Cr etching by RIE was performed at low pressure (15 mtorr) in Cl2 and O2 atmosphere together with inductively coupled plasma (ICP) at 1500 watt by using Plasmalab 100 from Oxford Plasma Technology. Then, the resist was removed by O2 plasma at RF power 100 watt for 180 s using March CS-1701 from Microtech-Chemitech AB. This process not only removed the resist and its constituent ashes but also cleaned the sample thoroughly for next process. After resist removal, TiO2 etching was performed by SF6 and Ar plasma using Plasmalab 80 from Oxford Plasma Technology. Subsequent to TiO2 etching, the sample was again cleaned by O2 plasma using March CS-1701. The last step was to remove the Cr layer by wet etching, which was performed in a mixture of Ammonium cerium (IV) nitrate from Sigma-Aldrich, acetic acid, and deionized water for sufficient time until the complete removal of the Cr layer. Finally, after rinsing with deionized water and drying with nitrogen blow, the required GMR grating sample was obtained, whose scanning electron microscope (SEM) image is shown in Fig. 4(a).
The fabrication and replication of the Type-II GMR grating were performed with the methods presented in Ref. . In brief, the master stamp grating was fabricated on a Si wafer by EBL with a negative e-beam resist HSQ without any etching process. Then, replication in thermoplastic was realized using an Obducat Eitre imprinter at a temperature of 165 °C which is above the glass transition temperature Tg ∼ 150 °C of polycarbonate. After the nanoimprinting, the polycarbonate substrate was covered by an amorphous TiO2 layer by ALD process at a deposition temperature of 120 °C using commonly known precursors TiCl4 and H2O. The SEM images of the master stamp, the replicated polycarbonate grating, and the final TiO2-coated GMR grating are shown in Fig. 4(b)–(d).
It is seen from Fig. 4 that the fabricated profiles of both types of GMR gratings deviate slightly from the theoretical designs, which are trapezoidal rather than perfect rectangular ones. This is unavoidable in the practical etching (for the Type-I grating) and master grating fabrication (for the Type-II grating) processes. Furthermore, the uncertainty of the refractive indices of materials, especially that of the ALD-prepared TiO2, also sensitively influences the practical resonance properties of the GMR gratings. For these reasons, our fabrication processes, though mainly guided by the theoretical designs, still have to be tuned a little by trial and error so as to get the best structural parameters to demonstrate the non-polarizing GMR effect.
4. Resonance properties of the fabricated GMR gratings
The optical properties of the fabricated GMR grating samples were characterized by a variable angle spectroscopic ellipsometer VASE from J. A. Woollam Co. The ellipsometer was set to measure the spectral transmittance of the gratings under normal incidence of TE and TM polarized light. The collimated beam spot diameter is 3 mm, which is sufficiently smaller than the grating area of 7×7 mm2. The scanning wavelength step is 0.2 nm in measurement in the wavelength range from 700 to 1000 nm. The measured zeroth-order transmittance spectra of the two fabricated grating samples are plotted in Fig. 5, both of which show the non-polarizing GMR effect around a wavelength of 840 nm. The small shift of the resonance wavelength (10 nm from the expected 850 nm) is mainly due to the deviation of the practical grating profiles from the designed ones, as mentioned above. In principle, we can tune the resonance wavelength to 850 nm by further parameter adjustment (for example, by increasing the period of the grating) in the fabrication process. But the current fabrication results already demonstrate well the expected non-polarizing GMR effect at a wavelength very close to the designed one. Furthermore, by comparing Fig. 5 with Fig. 2, we can see very good correspondence between theory and experiment; the main resonance features such as the lineshapes, the resonance linewidths, and the diffraction efficiencies are well reproduced in experiment. These show the reliability of the theoretical design and the fabrication processes. The measurement results are also the first experimental demonstration so far on the realization of polarization-insensitive 1D GMR gratings under normal incidence.
By inspecting the spectra of the Type-I GMR grating in Fig. 5(a), we can see that the TE and TM resonance peaks are almost at the same wavelength, with a small difference of 2.4 nm; the linewidths of the two resonances are also very close to each other, with a full width at half maximum of 30 nm for TE and 38 nm for TM. Therefore, the Type-I grating exhibits the non-polarizing GMR effect almost perfectly. The Type-II grating also demonstrates very good non-polarizing effect (with only 1.4 nm difference between the TE and TM peak wavelengths), but has a larger difference between the TE and TM resonance linewidths and the sidebands are not as well suppressed, as seen in Fig. 5(b). The structure needs to be further optimized to improve the filtering property (for example, by adding underneath layers to suppress the sidebands). Nevertheless, owing to the easier fabrication process and much lower manufacturing cost, the Type-II grating has good perspective for practical applications. We are taking further study to improve its resonance performance while maintaining the relatively simple geometry.
We have presented the design and fabrication of two types of 1D GMR gratings with simple geometries, which demonstrate polarization-insensitive GMRs under normal incidence. The Type-I single-layer rectangular-profile TiO2 grating is fabricated by employing ALD, EBL, and RIE techniques, which shows almost perfect non-polarizing property. The Type-II grating is an TiO2-coated polycarbonate GMR grating manufactured by nanoimprinting and ALD, without any etching process. Both types of 1D non-polarizing GMR gratings are realized for the first time in experiment so far, which show the potential of low-cost mass production of such functional devices for practical applications, for example, in enhancing fluorescence in biosensors.
We acknowledge the support by the National Natural Science Foundation of China (Projects No. 11004119 and No. 61161130005), the Academy of Finland (Projects No. 128420 and No. 250968), the Strategic Funding Initiative TAILOR of the University of Eastern Finland, and the Higher Education Commission, Pakistan.
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