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In this paper, we report a linear polarization-discriminatory state inverter made of three-layer sculpture thin film fabricated by oblique angle deposition technique. The first and third layers are quarter-wave plates of zigzag structure and the middle of them is a circular Bragg reflector of left-handed helical structure. It is found that the normal incidence of P-polarized light on this polarization-discriminatory state inverter becomes the S-polarized light at output, while the incident S-polarized light of wavelength lying in the Bragg regime is reflected. The microstructure of the linear polarization-discriminatory state inverter is also investigated by using a scanning electron microscope.
©2009 Optical Society of America
1. Introduction
Young and Kowal [1] in 1959 first conceptualized and fabricated inorganic thin film onto rotating substrate about an axis passing normally through it, and in 1966, Nieuwenhuizen and Haanstra [2] deliberately altered the substrate tilt with respect to the average direction of the incident vapor flux once during deposition. The products of these experiments were the precursors of the sculptured thin films (STFs) developed during the last two decades [3, 4], chiefly for optical but also other applications [5]. Sculptured thin films (STFs) are nanoengineered inorganic materials with anisotropic and unidirectionally varying properties that can be designed and realized in a controllable manner using variants of physical vapor deposition. In the recent years, STFs have become more valuable as a platform of optical devices [6, 7] to the optics community. Oblique angle deposition (OAD), a sophisticated physical evaporation technique for fabricating designed microstructures at nanometer scale, is used to prepare the STFs. Basically, it combines a typical deposition system with a tilted and rotating substrate to form a structure composed of nanometer scale columns of designed shape [8].
Sculptured thin films (STFs) fall into two canonical classes. One class of STF, the so-called sculptured nematic thin films (SNTFs), is characterized by zigzag, chevron, S-shape and C-shape columns. SNTFs have widely been used in optics as polarizers, retarders and filters [9, 10]. The other class, the so-called chiral STFs, is characterized by helicoidal columns. The chief optical signature of a chiral STF is the circular Bragg phenomenon displayed by it on axial excitation [11], i.e., it will preferentially reflect circularly polarized light of same handedness, while transmitting circularly polarized light of opposite handedness. Due to this ability to discriminate between left and right circularly polarized lights, any chiral STF can be used for circular polarization elements including sources, reflectors, filters, and detectors [12-14]. Other applications of the STFs are three-dimensional photonic crystals [15], broadband antireflection coatings [16], and sensors of gas or fluid concentrations [17, 18]. The properties of the SNTFs and the chiral STFs suggest growing not one but a combination of them in order to fabricate a linear and a circular polarization inverters. Hence, this feasibility has motivated to study this work.
The aim of this paper is to report our experimental findings on a linear polarization-discriminatory state inverter of three-layer STF. The first and third layers of the linear polarization-discriminatory state inverter are zigzag structures that serve approximately as quarter-wave plates and the middle layer of them is a circular Bragg reflector of left-handed helical structure that transmits right circularly polarized light but reflects left circularly polarized light in narrow wavelength regime. These devices were fabricated by using electron-beam evaporation via oblique angle deposition (OAD) technique. Deposition angles, rates, and substrate rotation speed were employed to control the columnar microstructures of the devices. The optical properties and the microstructures were investigated using a spectrophotometer and a scanning electron microscope (SEM).
2. Experimental
The electron-beam evaporation with OAD technique was used to fabricate the films of zigzag and helical structures. The simplified schematic diagram of the OAD technique is shown in Fig. 1. High purity (99.8%) TiO2 in the form of tablet was used as an evaporation source material. Pre-evaporation of this material was performed for 5 minutes prior to the main evaporation. The deposition was performed in a vacuum chamber with a base pressure of ~5×10-6 Torr. The evaporation source of 3-cm crucible pocket was located at 45 cm, directly beneath the substrate. The deposition rate and thickness of the growing films were measured by a quartz-crystal sensor, which was placed near substrate. Glasses (B270, 70 mm×50 mm×1 mm) and polished Si (100) wafers were used as substrates. Vapor flux incident angle, 60° and substrate rotation speed, 0.09 rpm were employed to control the columnar microstructures of the films. The deposition rate of the films was kept at 0.5 nm/s by using a quartz-crystal sensor. The circular Bragg reflector, the quarter-wave plate, and the linear polarization-discriminatory state inverter deposited on glass substrates and silicon wafers were used for optical and structural analysis.
Optical transmittance spectra of these devices were measured by a spectrophotometer (Cary 500, Varian) in the wavelength range of 400 to 800 nm. A linear polarizer, achromatic quarter-wave plate, sample, second achromatic quarter-wave plate and an analyzer were placed in the beam path of a spectrophotometer.
Left circular polarized (LCP) and right circular polarized (RCP) lights were generated by passing unpolarized light through a linear polarizer followed by an achromatic quarter-wave plate with its fast axis oriented ±45° relative to the transmission axis of the linear polarizer. The cross-section of the devices was investigated using a scanning electron microscope.
3. Results and discussion
3.1 Circular Bragg reflector
Cross-sectional SEM image of a circular Bragg reflector with 5 left-handed structural periods is shown in Fig. 2(a). Examination of the SEM micrographs reveals that the circular Bragg reflector deposited at 60 ° with rotation speed 0.09 rpm is relatively close packed helical columns and the center axis of the helical structure is normal to the substrate. Because atomic-shadowing only occurs in the deposition plane, obliquely deposited thin films tend to consist of inclined columns which fan out and chain together perpendicular to the deposition plane.
Any chiral STFs, simply referred to as helical films, exhibit the circular Bragg phenomenon upon axial excitation by a plane wave [19] in accordance with their periodic nonhomogeneity along the thickness direction, i.e., it will preferentially reflect circularly polarized light of same handedness, while transmitting circularly polarized light of opposite handedness with wavelength lying in the so-called Bragg regime. As an extensive theoretical analysis about the chiral STFs has been discussed elsewhere [6], we content ourselves here by mentioning only the significant quantities. Each one of the chiral STF layers possesses a structural period, 2Ω (320 nm) and is of thickness L=2NΩ, where the integer N (5) is the number of structural periods. The wavelength at which the maximum circular Bragg reflectance occurs is proportional to both the average refractive index, nav=(nc+nd)/2 as well as structural period, 2Ω and is calculated by using equation, λBr0=2Ωnav. Here nd=[n 2 an 2 b/(n 2 acos2 β+n 2 bsin2β)]1/2 is composite refractive index and na, nb and nc are principal refractive indices of the film with column angle β.
The good agreement of the measured and simulated transmittance spectra of helical TiO2 film with 5 period left-handed structures is illustrated in Fig. 2(b). These spectra clearly show the circular Bragg phenomenon with peak wavelength, 545 nm in the Bragg regime for both cases, which occurs due to unidirectionally periodic nonhomogeneity and helical morphology. This phenomenon can also be explained by using grating theory [20]: a circular plane wave of same handedness effectively encounters Bragg grating, while that of the other handedness does not. The magnitude of the circular response can be controlled by the number of periods in the film.
Fig. 2. Circular Bragg reflector of 5 left-handed structural periods: (a) Cross-section SEM image and (b) transmittance spectra (TRCP, TLCP, and TRCP – TLCP).
3.2 Quarter-wave plates
In our previous work [9], the optical anisotropic nature of TiO2 films deposited by OAD technique was reported and the results suggest that the TiO2 films deposited at an angle of 60° are potential candidates for a retardation plate. Based on these results, the quarter-wave plate (QWP) at 545 nm with zigzag structure is fabricated and tested. The zigzag structure is selected for the uniform thickness distribution and the optical anisotropy which is close to the tilted structure. The cross-sectional SEM image of the quarter-wave plate in Fig. 3(a) ensures the zigzag structure and its total thickness is 2.0 µm.
The transmitted intensity of a thin film phase retardation plate between a polarizer and an analyzer, the transmission axes of which are perpendicular to each other, is given by [21]
where θ is the rotation angle of the retardation plate with respect to the polarizer transmission axis and Δφ is its phase retardation of π/2. The normalized intensity of the thin film QWP placed between the polarizer and the analyzer in the spectrophotometer is measured as a function of the rotation angle, as illustrated in Fig. 3(b). It is found that the intensity is maximum at θ=π/4, 3π/4, 5π/4, and 7π/4 and minimum at θ=0, π/2, π, 3π/2, and 2π. Therefore, the QWP shows the same properties as that of conventional QWP made from natural crystal of inorganic oxides such as calcite, mica, and quartz.
Fig. 3. Quarter-wave plate of zigzag structure: (a) Cross-section SEM image and (b) normalized intensity as a function of rotation angle.
3.3 Linear polarization-discriminatory state inverter
Exploiting the characteristics of the QWP and the circular Bragg reflector, a linear polarization-discriminatory state inverter is realized by two QWPs of zigzag structure and a circular Bragg reflector of helical left-handed structure. The Bragg reflector is sandwiched between the quarter-wave plates. In Fig. 4(a), the cross-sectional SEM image ensures the structures and positions of the QWPs as well as the Bragg reflector in the linear polarization-discriminatory state inverter i.e., [air|QWP-Bragg reflector-QWP|glass]. The total thickness of each QWP is 2.0 µm and that of the Bragg reflector is 1.6 µm with 5 left-handed structural periods. It is clear that the zigzag structure is oblique to the substrate while the center axis of the helical structure is normal to the substrate and the structures are closely packed.
Figure 4(b) illustrates the four measured transmittance spectra of the linear polarization-discriminatory state inverter of three-layer structure. The subscript SP indicates S-polarized light transmitted when the incident is P-polarized light; and the other three transmittances are similarly defined. It is found that the cross-transmittance TSP is higher than the remaining three transmittances within the Bragg regime. The values of TSP, TPS, TSS, and TPP at the wavelength of 545 nm in the Bragg regime are respectively 47.5%, 27.8%, 5.6%, and 7.5%. In fact, the value of TSP should be very high and the remaining three transmittances should be very low in the Bragg regime, but the transmittances in our measurement do not show the exact desired values due to the circular Bragg reflector of a few periods. However, the cross-transmittance, TSP is much greater than the cross-transmittance, TPS and the selective transmittance (TSP – TPS) at 545 nm in Fig. 4(c) is approximately 20%. These results show that the inversion of the incident P-polarized light to S-polarized is more than the inversion of the incident S-polarized light to P-polarized light, indicating the conformation of a linear polarization-discriminatory state inverter. Our observations also show that anisotropic titanium oxide films absorb in the short-wavelength range because of proximity to absorption bands [22].
In Fig. 4(d), we have plotted the transmittance spectra for a simulation of the device. A specialized book [6] and an article [23] with full of information about the STFs have made possible to perform this simulation. The articles [22, 23] are for polarization-discriminatory handedness inverter in which half-wave plate (HWP) is used to invert the handedness of circularly polarized plane wave, because its phase retardation is π. In our study, the QWP, whose phase retardation is half of the HWP, is used to convert the linearly polarized plane wave to the circularly polarized plane wave and vice versa. The state of the emerging polarized plane wave depends on the azimuth angle of the plate and the state of the incident polarized plane wave on it. From Fig. 4(d), it is clear that the experimental results show a good agreement with the simulation but not exactly. One contributing reason is the dielectric absorption as well as dispersion effects, and another relates to index matching on either side of the fabricated device. These are not considered in our simulation. The calculated selective transmittance (TSP – TPS) is also approximately 20% at a wavelength of 545 nm as shown in Fig. 4(c).
From the above discussions, it is clear that the top QWP layer converts the incident S-polarized light on it into LCP plane wave, which is reflected by the left-handed Bragg reflector layer. On the other hand, an incident P-polarized light is first transformed into a RCP plane wave by the top QWP, and is then transmitted easily through the left-handed Bragg reflector layer. This transmitted RCP plane wave is again converted into an S-polarized light by the bottom QWP. The linear polarizer is placed after the sample in the beam path of the spectrophotometer to ensure the P- or S-polarized light at the output of the sample. This is actually preliminary study which gives the feasibility of fabricating the linear polarization-discriminatory state inverter. The performance of the device can be improved by increasing the number of periods in the Bragg reflector.
Fig. 4. Linear polarization-discriminatory state inverter of three-layer structure: (a) Cross-section SEM image, (b) measured transmittance spectra (TPS, TSP, TSS, and TPP), (c) measured and simulated selective transmittance spectra (TSP - TPS), and (d) simulated transmittance spectra (TPS, TSP, TSS, and TPP).
4. Conclusions
The OAD technique was used to fabricate the circular Bragg reflector, the quarter-wave plate, and the linear polarization-discriminatory state inverter of three-layer structure. It is found that the circular Bragg reflector of helical structure is a polarization-selective transmitter of circularly polarized lights, while the quarter-wave plate (QWP) of zigzag structure changes the polarization state of linearly polarized lights. The linear polarization-discriminatory state inverter is a structure of QWP-Bragg reflector-QWP and is combined them into an all-thin-film device. The incident linear P-polarized light on it results the linear S-polarized light at output, while it reflects the incident S-polarized light with wavelength lying in the Bragg regime. Therefore, it can be used as a linear polarization-discriminatory state inverter.
Acknowledgements
This work was supported in part by (1) the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (Quntum Photonic Science Research Center) and the Ministry of Knowledge Economy (MKE) and (2) the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology.
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