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Light transmission spectrum of a multilayer photonic crystal with a central liquid-crystal defect layer placed between crossed polarizers has been studied. Transmittance was varied due to the magnetically induced reorientation of the nematic director from homeotropic to planar alignment. Two notable effects were observed for this scheme: the spectral shift of defect modes corresponding to the extraordinary light wave and its superposition with the ordinary one. As a result, the optical cell allows controlling the intensity of interfering defect modes by applied magnetic field.
© Optical Society of America
1. Introduction
Multilayer photonic crystal (PC) structures are widely applied in optics such as dielectric mirrors and antireflection coatings [1]. A new approach [2,3] to the study of these structures has initiated the development of PC materials with tunable spectral properties [4]. One of such materials is the PC’s with liquid-crystal (LC) structural units [5–14]. A high sensitivity of liquid crystals to the external stimuli allows designing various optical devices based on PC/LC structures. For example, multilayer photonic crystals with a nematic defect layer can be used to create narrow-band spectral filters with defect modes (i.e., spectral windows) removable on wavelength by applied voltage [8].
As a matter of fact, a high-contrast light modulation in a multilayer PC/LC cell can be achieved by placing the cell between two crossed linear polarizers. In this scheme the spectral overlap of the extraordinary and ordinary components of defect modes with different serial numbers can result in the interference addition of their intensities as well as in the mutual quenching of both components projected to the transmission axis of the analyzer. Here we propose an experimental method to control the intensities of defect modes within the transmission spectrum of a PC/LC cell based on the aforementioned interference effect. The realization of such a tunable device regulated by magnetic field is demonstrated.
2. Experimental
The PC/LC cell (Fig. 1 ) consisted of two identical dielectric mirrors, the gap between which was filled with the nematic liquid crystal 4-methoxybenzylidene-4’-butylaniline (MBBA). In the initial (i.e., unperturbed) state the nematic director n was aligned homeotropically, n || z. The thickness of the LC defect layer was L = 13.8 μm. The refractive indices of MBBA were n || = 1.765 and n ⊥ = 1.552 at wavelength λ = 0.589 μm and temperature T = 25°C for the light polarized parallel and perpendicular to n, respectively. The multilayer film of each mirror comprised six layers of a high-index substance, zirconium dioxide (ZrO2), with the refractive index n 1 = 2.04 and thickness l 1 = 0.052 μm and five layers of a low-index dielectric, silicon dioxide (SiO2), with the refractive index n 2 = 1.45 and thickness l 2 = 0.102 μm. These layers were deposited in alternating sequence onto fused quartz substrates.
Fig. 1 Schematic of experimental setup used to study transmission spectra of a PC/LC cell with a magnetically controlled nematic as a tunable defect layer. (P) and (A) are crossed linear polarizers.
The PC/LC cell was placed in a steady magnetic field H || x, which enabled to reorient the nematic director smoothly through 90° within the xz-plane. Consequently, for the normally incident light the refractive index ne of extraordinary (e) wave changed from n ⊥ at n || z to n || at n || x as the strength of the field increased, whereas the refractive index of ordinary (o) wave no = n ⊥ remained constant. The transmission spectra of the PC/LC cell were recorded by a high-resolution spectrometer (Ocean Optics HR4000) at fixed temperature T = 25°C. Both input light to the cell and output signal to the spectrometer were transmitted via the fiber waveguides. In the experimental setup for optical transmission measurement, two Glan prisms as polarizer P and analyzer A were oriented in the way such that their transmission axes lay within the xy-plane at the angle β = ± 45° to the х-axis, respectively. Note that only polarizer P oriented along either the x- or y-axis was employed to measure the polarized extraordinary (ordinary) component of the transmission spectrum.
3. Results and discussion
The magnetic field dependence of spectral positions of the defect-mode maxima for the polarizations P || x and P || y of light normally incident on the sample are presented in Fig. 2 . The spectra do not change up to field strength H с = 6.3 kOe due to the threshold character of the reorientation of the nematic layer [15]. Above the threshold H с, the spectrum of defect modes is divided into two independent, orthogonally polarized components with the wavelengths corresponding to the maxima of defect modes
where the integers me and mo denote the serial numbers of defect modes for e- and o-waves, respectively. For example, the serial number mo = 73 for о-mode corresponds to a spectral peak at λo = 584.4 nm. The number mo (me) rises when λo (λe) decreases and vice-versa. Spectral positions of о-modes remain invariable while the e-modes smoothly shift to the long-wavelength region. Each e-mode crosses sequentially several o-modes with various serial numbers. When an e-mode overlaps an o-mode in the crossed-polarizer scheme, their intensities are added if me − mo = 2k + 1 (k = 0, 1, 2…) and the modes quench each other if me − mo = 2k. Such behavior of polarized components of defect modes gives rise to the complex resulting transmittance as a function of the field (see Fig. 3 ) when the PC/LC cell is arranged between crossed polarizers. All data presented further were acquired with this scheme.Fig. 2 Spectral positions of the maxima of polarized defect modes versus applied magnetic field. Experimental data for e-wave are denoted by symbols and approximated by the solid lines. Dashed lines show the invariable positions of maxima for o-wave. Threshold magnetic field H c = 6.3 kOe.
Fig. 3 Typical transmission spectrum of the PC/LC cell placed between crossed polarizers as a function of the reduced magnetic field.
Figure 3 depicts only a fragment of transmission spectrum comprising four о-modes within the long-wave range of the PC band gap. The spectrum exhibits several features. The transmittance vanishes at all λ below H с. Above the threshold field, the light transmission is modulated, passing through some maxima and minima, as the field is increased. The most dramatic modulations (see Fig. 4 ) can be observed at the wavelengths λo corresponding to the spectral positions of о-modes. In this case the transmittance curves take no monotonically sinusoidal waveform; instead, there appear two plateaus approximately at one-third maximal light transmission as shown in Fig. 4. This character is revealed most clearly in the middle of the band gap. The curves become smoother near the edges of the band gap.
Fig. 4 Magnetically induced modulation of transmittance of the PC/LC cell at various wavelengths corresponding to three distinct o-mode spectral positions.
An especially promising fact is that the first maxima of transmittance (me − mo = 1) at various wavelengths are reached practically for the same value of applied field (see Figs. 4 and 5 ). It means that one can switch all spectral windows simultaneously between optically on and off states by varying the field within a small range (merely (1–1.06)H c in this study) above the threshold. Figure 6 shows how the simulated spectrum changes from 6.1 to 20 kOe.
Fig. 5 Magnetic-field-induced switching of the spectrum. Solid line represents the transmittance at H ≤ H c. Dashed line displays the first maximum of defect mode intensity at Н/Hc = 1.06. Dotted line is a second maximum at Н/Hc = 1.21. Dashed-dotted line is a second minimum at Н/Hc = 1.32. All curves correspond to the extrema at λo = 584.4 nm.
Fig. 6 The variation of simulated transmission spectrum depending on the applied magnetic field between 6.1 and 20 kOe (Media 1). H = 6.68 kOe corresponds to Н/Hc = 1.06.
The values of applied field required to obtain a second and following maxima and minima of light transmission at various λo differ considerably (see Figs. 4 and 5). The more a serial number of a maximum (minimum) the more this distinction. This characteristic can be used to produce the optical states of a PC/LC device with different color gamuts.
4. Concluding remarks
In conclusion, we have demonstrated that the configuration with a one-dimensional PC/LC cell placed between two crossed polarizers can yield interesting spectral profile because of the vector sum of two orthogonal components belonging to different serial numbers. The defect modes and the consequent transmittance through the two-polarizer scheme can be modulated by an external field applied upon the LC infiltrated as the central defect layer in the multilayer structure. Knowing that it could be of less use in a practical device, we have employed magnetic field instead of electric field. To our best knowledge, this work is the first study on a PC/LC controlled by a magnetic field. One can certainly expect similar results by means of the application of an electric field for reorientation of the nematic director in the LC bulk.
Acknowledgments
This work was partially supported by the Russian Federal Grant No. 02.740.11.0220 and SB RAS Grant Nos. 5, 27.1 and 144. W. Lee gratefully acknowledges financial support from the National Science Council of the Republic of China (Taiwan) under Grant No. NSC 98-2923-M-033-001-MY3 dedicated to an internationally joint effort between Russia and Taiwan.
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