A novel bandwidth-tunable notch filter is proposed based on the guided-mode resonance effect. The notch is created due to the superposition spectra response of two guided-mode resonant filters. The compact, bandwidth tuning capability is realized by taking advantage the effect of spectra-to-polarization sensitivity in one-dimensional classical guided-mode resonance filter, and using a liquid crystal polarization rotator for precise and simple polarization control. The operation principle and the design of the device are presented, and we demonstrate it experimentally. The central wavelength is fixed at 766.4 nm with a relatively symmetric profile. The full width at half maximum bandwidth could be tuned from 8.6 nm to 18.2 nm by controlling the applied voltage in electrically-driving polarization rotator.
© 2015 Optical Society of America
Dielectric waveguide gratings with guided-mode resonance (GMR) have recently attracted much attention [1, 2 ]. GMR filters are based on the principle of the GMR effect, which occurs in periodic waveguide structures where an incident wave is coupled to a leaky waveguide mode and yields a resonant peak [3, 4 ]. The resonant wavelength of such filters can be easily tuned by changing the structural and material properties. This characteristic has been used in optical tunable filters [5–7 ], high-resolution bio-sensing [8, 9 ], and anti-counterfeit technology , among others. In addition, the resonant effects of dielectric one-dimensional (1D) GMR filters are sensitive to the polarization of incident light . The spectral-response properties of GMR filters, such as bandwidth symmetry, sideband reflection, and full width at half maximum (FWHM), can be affected by the incident polarization states. However, few applications based on the polarization sensitivity of resonant effects have been proposed , and this characteristic is worthy of further investigation and development.
Owing to the importance of optical filters in applications such as spectrometers and optical communications, various types of optical filters have been developed to date [13, 14 ]. Among them, the tunable optical filter is an essential component of many devices. Inaddition to wavelength-tunable filters, bandwidth-tunable filters are also very useful in dynamic bandwidth allocation for optimal spectral efficiency and in optical performance monitoring . Compared to bandwidth-tunable filters, which need mechanical control, non-mechanically driven devices with simple structures are more desirable for realizing high-stability systems . In our recent work, a bandwidth-tunable filter based on a 1D non-polarizing GMR structure was proposed . In our design, a non-polarizing GMR filter was used, and the filter bandwidth was controlled using the polarization of incident light. In the design of non-polarizing GMR gratings, the adjustment of structural parameters can result in the fine-tuning of the dispersion relations of leaky guided modes excited by either transverse-electric (TE) or transverse-magnetic (TM) polarization in the grating layer. TE and TM polarization are the electric-field vectors of the incident field that are parallel and perpendicular, respectively, to the grating lines . Consequently, different incident polarizations have the same propagation constant when the TE- and TM-modes have the same dispersion. However, structural parameters have to be precisely controlled, and the trade-offs between the parameters lead to an inflexible situation for the filter design. In general, non-polarizing GMR structures are either complex in their geometries or need complicated incident mountings, and they impose additional difficulties in practical applications .
In the present study, an electrically driven bandwidth-tunable optical notch filter was proposed and realized. The tunable device involves two nano-structured spectral filters, each of which is based on a 1D sub-wavelength GMR grating integrated with a dielectric waveguide stacked onto a polarization rotator. The polarization-sensitive spectral features of 1D GMR are exploited to construct the integrated device. Polarization-sensitive bandwidth-tunable GMR device operating under polarization-sensitive 1D GMR structure have not been reported to date. We think such device is integrating and flexible, meanwhile, presents a pioneering study about the bandwidth-tunable GMR filters.
2. Proposed device and principle of operation
The proposed bandwidth-tunable device is composed of a twisted nematic liquid crystal (TN-LC) cell loaded with two 1D GMR filters, as illustrated in Fig. 1 . Each single-layer GMR filter consist of a high-refractive-index waveguide layer (Ta2O5 in the present study) and a low-refractive-index substrate (BK7 glass). The device is operated under normal illumination with a linearly polarized beam. The input light is first coupled to GMRF1, and it passes through the TN-LC cell, which changes the polarization angle of the incident light in accordance with the voltage applied to a pair of indium tin oxide (ITO) electrodes. Subsequently, the light with the modified polarization state is coupled to GMRF2. Finally, a notch is observed in the transmission spectrum. By dynamically adjusting the applied voltage in the TN-LC cell, the notch-filter device effectively acts as a bandwidth-tunable device.
The operation principle of the device is shown in Fig. 2 . First, linearly polarized light is vertically incident on the device, and it is subsequently diffracted by the surface grating and resonantly coupled to the TM guided mode of the waveguide in GMRF1. The TM polarization corresponds to a polarization angle of φ = 90°. When the phase-matching condition is satisfied, a high reflection peak arises because of the GMR effect at a certain wavelength λ1, which corresponds here to the first-order spectrum (FOS). A corresponding notch-filter response appears in the transmission spectrum. The FOS transmits to the TN-LC cell with a dip bandwidth of d0 in the spectra (pink solid line). In the TN-LC cell, according to the voltage applied to the ITO electrodes (i.e., according to the optical rotational power), the polarization of the FOS is varied , leading to a change in the polarization while the spectral profile remains unchanged. Briefly, if no voltage is applied, the polarization angle φ is initialized to 0° by the rotator. In the present design, the two GMR filters are placed with their grating grooves perpendicular to each other. Therefore, the transmitted light still couples to the TM mode in GMRF2. In addition, the notch (resonant) wavelength λ2, which is attributed to the TM polarization in GMRF2 (green dashed line), deviates from λ1. Hence, two notches exist, corresponding to the coupling of the TM mode in GMRF1 and GMRF2. Consequently, the output spectrum is the superposition of the TM-polarized spectrum (red solid line) from both GMRF1 and GMRF2. The narrowest bandwidth d1 for notch filtering is achieved when no voltage is applied. When a voltage greater than a certain value Vth is applied to the device, the polarization rotatory power in the TN-LC cell is deactivated, and the FOS reaches GMRF2 experiencing no polarization rotation, i.e. φ = 90°. It is noted that the FOS has a narrow bandwidth and the TE-polarized resonance in GMRF2 has a larger line-width. In this case, the input light with an FOS couples to the GMRF2, and a notch arises at a wavelength λ3, which is attributed to the TE guided mode. The value of λ3 is equal to λ1. Therefore, at the designed wavelength λ1, we obtain the widest bandwidth d2. Finally, when increasing Va from 0 to Vth, the bandwidth of the spectrum increases from the narrowest bandwidth d1, which is the same as that for the FOS, to the widest bandwidth d2, which results from the superposition of the two notches at the same central wavelength λ1. The increase in notch bandwidth is due to the changing transmission of the induced notch in GMRF2 resulting from the changing polarization in the TN-LC cell. However, as the input light impinging GMRF2 exhibits a notch with zero transmission at the central wavelength, the superposition spectrum shows a bandwidth-tuning feature without efficiency changing (null transmission) at the central wavelength.
The two GMR filters can also be arranged such that the grating grooves are parallel, rather than perpendicular, to each other, as mentioned above. In this manner, without a driving voltage applied in the TN-LC cell, the transmission light from the rotator induces an orthometric polarization mode in GMRF2 with respect to that in GMRF1. The superimposed notch results in the widest bandwidth only if the two notches resulting from the coupling of the TM mode in GMRF1 and TE mode GMRF2 are located at the same wavelength. In contrast, as the applied voltage increases from 0 to Vth, the bandwidth of the notch decreases from the widest, d2, to the narrowest, d1.
3. Numerical demonstration and characterization analysis
One-dimensional polarization-sensitive GMR filters are designed using the rigorous coupled wave analysis (RCWA) method. The filtering mechanism is based on the fact that the spectral response can be manipulated by changing the incident polarization to the GMR structure. The calculated spectra of the filters are shown in Fig. 3 . The grating structure consists of a material with a high refractive index nh = 2.1 (Ta2O5), and a BK7 substrate (ns = 1.51). The grating-layer filling factor f is 0.5, and the grating thicknesses of the two gratings GMRF1 and GMRF2 are 60 and 80 nm, respectively. The waveguide-layer thickness of both structures is120 nm, and the grating period Λ is 450.3 nm and 498.1 nm for GMRF1 and GMRF2, respectively. The spectral profile of GMRF1 remains unchanged during the operation of the device.
The spectral notch of GMRF1 is located at 770 nm with a bandwidth of 5.4 nm, as shown in Fig. 3(a). In Fig. 3(b), the spectral response of GMRF2 is polarization-sensitive: for TE-polarized (φ = 0°) input light, the output notch is located at 770 nm, while for TM-polarized (φ = 90°) input light, the spectral notch is at 713.3 nm. It is observed that the notch transmission at 770 nm presents a contrary changing trend to the notch at 713.3 nm. The intensity of spectral transmission monotonically decreases at the wavelength of 713.3 nm as the polarization angle increases from 0° to 90°. As the polarization angle decreases to 0°, the spectral transmittance monotonically decreases at 770 nm, which could be attributed to the boundary conditions of the electromagnetic field described in our previous work . Figure 3(c) shows these variation characteristics and the spectral transmittance for GMRF2 as the incident polarization angle increases from 0° to 90°. The combined response of GMRF2, when superimposed with GMRF1, is presented in Fig. 3(d), wherein the bandwidth tuning of the notch filter is realized. The widest bandwidth is 17.7 nm, when the polarization angle is 0°. Accordingly, we can define this as the bandwidth upper limit for the design sample. The lower limit of the tuning bandwidth corresponding to a polarization angle of 90° is 5.2 nm, which is the same as that observed in the FOS. In addition, one noteworthy feature at the design notch is that the bandwidth upper limit in Fig. 3(d) is greater than the highest bandwidth in Fig. 3(c)because the final spectral-line characteristics result from two transmittances. For each wavelength, the transmission intensity decreases after multiplication, and the bandwidth is slightly amplified in the macroscopic superimposed spectrum.
In different applications, there will likely be a desire to change the bandwidth-tuning characteristics; we present a brief discussion on how this can be achieved in what follows. It is readily appreciated that some of the device parameters will change the leaky waveguide mode supported by the waveguide, which, in turn, will affect the resonance wavelengths and coupling strength between the input light and leaky waveguide modes . Generally, a greater coupling strength yields an increased spectral bandwidth. For the device described in the present paper, one can alter the modulation intensity in the grating layer of GMRF2 in order to change the upper limit of the bandwidth-tuning range because increasing the modulation intensity can increase the bandwidth of the spectral notch. For example, if nh is increased to 2.2 and dg is increased to 100 nm, the bandwidth upper limit is increased to 25.6 nm, as shown in Fig. 4(a) (in comparison with Fig. 3(d)), and because the spectral profile of GMRF1 is kept unchanged, the lower limit remains at 5.2 nm, as shown in Fig. 4(b). Alternatively, the grating depth dh can also be altered to adjust the bandwidth, but this effect is weaker than the effect of altering the grating-layer modulation intensity. For example, if the grating depth is increased to 80 nm in GMRF1, the bandwidth lower limit is increased to 8.3 nm, as shown in Fig. 4(c) and Table 1 , while the upper limit is only slightly increased by approximately 1 nm. Therefore, we can conclude that altering the bandwidth in GMRF2 can change the upper limit of bandwidth tuning without affecting the lower limit of bandwidth tuning. Alternatively, if the GMRF1 bandwidth is varied, both the upper limit and lower limit will be affected after superposition; however, the variation mainly influences the lower limit in the tuning process.
4. Device fabrication
In order to fabricate the GMR filters, a Ta2O5 thin film is deposited onto a cleaned BK7 substrate through evaporation coating. Here, the structural parameters for the experiment are the same as those for the device shown in Fig. 3. As two different GMR filters are required, substrates coated with a high-refractive-index material with a thickness of 180 nm and 200 nm are prepared. Subsequently, a 200-nm-thick photoresist film was spin coated upon the Ta2O5 layer, and a photoresist grating (PRG) mask pattern was formed through two-beam interference lithography. A PRG mask was obtained with a period Λ = 490 nm and dg = 120 nm for GMRF1. To etch the Ta2O5 film, ion etching involving Ar gas was employed. A similar fabrication step was adopted to form GMRF2 but with different structural parameters (Λ = 440 nm and dg = 110 nm). Some deviations from the target parameters are inevitable in the production. AFM images of the final fabricated GMR gratings are shown in Fig. 5 , which correspond to GMRF1 and GMRF2 in the device described above. After fabricating the GMR filters, ITO was coated on the other side of the substrates of these two GMR filters for fabricating the TN-LC cell. Two pieces of ITO-coated glass were separated using insulating SiO2 micro-ball layers, which set the spacing between the upper and lower substrates. Wires were connected to the ITO films using conductive epoxy, which facilitated the application of voltage. The two pieces of ITO glass were coated with polyimide (PI) and subsequently baked for 1 h. The two PI layers were rubbed in orthogonal directions to yield an LC with a homogeneous alignment, resulting in perfectly planar textures. The alignment layer, which is adjoined to the GMR filter, was rubbed perpendicular to the direction of the grating lines. In order to place LC molecules regularly along the helical axis, a chiral dopant was introduced at an appropriate concentration into the LC mixture. The empty cell was filled with the nematic-LC mixture through capillary action.
5. Results and discussion
Before integration with the TN-LC cell, the transmission spectra of the fabricated GMRFs were measured as a function of wavelength by using a spectrum analyzer, handy-type spectrophotometer (USB-4000, wavelength range: 200–1100 nm), and tungsten halogen (LS-1, wavelength range: 360–2000 nm), which serves as the light source in the experiments. A polarizer was mounted in front of the light source to select a specific incident polarization state. The spectral response of the GMR filters was measured for different polarization modes, as shown in Fig. 6 . The deviation of the spectral response from the design response is ascribed to the deviation of the device parameters from the target values during the fabrication process. As the spectral line profile is directly related to the incident polarization in GMR and each wavelength has a different rotational power in the LC rotator, a bandwidth difference will exist between the simulation and experimental results. Two 1D GMRFs were fabricated in our device. The spectra under TM polarization for GMRF1 are shown in Fig. 6(a), in which the notch was observed to be centered at 766.49 nm with a bandwidth of 8.6 nm; the spectra for GMRF2 are shown in Fig. 6(b), in which the notch was observed to be centered at 711 nm and 766.43 nm for TM-polarized and TE-polarized incident light, respectively.
As shown in Figs. 7 and 8 , after integration with an LC polarization rotator, the manufactured devices were observed to produce tunable bandwidths under various applied voltages, and the spectral line profile showed good symmetry after the superposition of the spectral responses of the two filters GMRF1 and GMRF2. Without an applied voltage, the lower limit of the tuning bandwidth was observed to be 8.6 nm, which was the same as that for the transmission spectrum of GMRF1 alone. As the voltage increases, the bandwidth of the notch increases until the TN-LC cell losses its rotatory power, such that only the TE mode in GMRF2 existed at the upper limit. In our experiment, after superposition, the observed upper limit for the bandwidth was approximately 18.2 nm, which is approximately twice the tuning lower limit. In the measurement, the losses caused by the sheet polarizer are ignored, and it is assumed that the incident light was already polarized along the direction of the sheet polarizer. In addition, low-loss substrates were introduced. A total optical loss amounting to approximately 16% was produced through Fresnel reflection and dielectric absorption in the integrated structure. Because an insulation spacer is used and the LC molecule has its own driving voltage (Vdr), the polarization rotation will be difficult to observe when Va< Vdr. For our device, Vdr is treated as the start voltage as soon as the bandwidth begins to deviate from the original value. Here, Vdr is 1.2 V for a SiO2 micro-ball diameter of 15 μm in the cell. However, the rate of change of bandwidth of this device is low at low voltages, even when Va > Vdr, implying that the TN-LC cell possesses optical rotatory power but with weak changes. As Va increases to 3.5 V, the highest bandwidth of 18.2 nm is obtained; the bandwidth remains unchanged as Va increases further, implying that the optical rotator has lost its rotation ability. Relatively large change ratios are observed between voltages of approximately 1.5 V to 2.2 V. However, some solutions can be considered to improve voltage sensitivity. For example, a larger spacer leads to lower voltage sensitivity, which may lead to easier control of our device in some cases. Alternatively, high voltage sensitivity can be achieved with a smaller spacer, which can be used in components requiring a quick response. Additionally, an appropriate dopant is effective for obtaining components with a controllable response time.
We have demonstrated the design and fabrication of a bandwidth-tunable notch filter which takes advantage of the polarization-selective characteristics of 1D GMR filter. In conjunction with a TN-LC polarization rotator, bandwidth tuning is realized by changing the applied voltage. The notch wavelength is fixed, and a bandwidth variation from 8.6 nm to 18.2 nm has been realized by tuning the voltage from 1.2 V to 3.5 V in the LC cell. The positions of the notch can be tailored by adjusting the periodicity of the GMRF period, and the bandwidth can be also controlled by changing some structure parameters. Compared to a GMR bandwidth device with non-polarizing GMR structure, the device demonstrated here is practical and flexible. By improving the deposition recording and etching process, it should be possible to further reduce the device losses and thereby increase the finesse of the device. These types of tunable filters broaden the applications of the GMR effect, and have potential applications in optical performance monitoring and signal processing.
We acknowledge the support by National Natural Science Foundation of China (61378060, 61205156), National Basic Research Program of China (2015CB352001), Pujiang Project of Shanghai Science and Technology Commission (14PJ1406900), and the Innovation Fund Project For Graduate Student of Shanghai (JWCXSL1401)
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