We demonstrate a wideband circular polarization reflector fabricated as cascades of helical films with different pitch thickness by using glancing angle deposition (GLAD) technique. The full-width-at-half-maximum bandwidth of this reflector is measured from the reflectance spectra and is found about 200 nm indicating the feasibility of wideband reflector. A helical TiO2 film with three sections, each of different pitch thickness, is also studied. It shows three Bragg peaks at different wavelengths. To select appropriate material for this circular reflector, the optical properties of 5-turns TiO2, ZrO2, and Ta2O5 helical films and the porosity effect on the TiO2 helical film are investigated.
© 2008 Optical Society of America
Although Young and Kowal  in 1959 first reported inorganic chiral sculptured thin film (CSTFs) deposited by physical vapor deposition onto rotating substrates, further developments evidently did not occur for more than three decades. Now, it is being rapidly matured as an optical nanotechnology. Chiral sculptured thin films (CSTFs) also known as helical thin films are a new class of nanoengineered optical materials that have become more valuable as a platform of optical devices [2, 3] to the optics community in the recent years. These films consist of helical nanowires oriented perpendicular to the surface of the substrate [4–7] and the fabrication method is glancing angle deposition (GLAD), a new technique for fabricating designed microstructures at the nanometer scale . Glancing angle deposition (GLAD) technique is based on physical vapor deposition and employs oblique angle deposition conditions as well as substrate rotation to control the microstructure by an atomic shadowing effect [9, 10]. The microstructure of an obliquely deposited thin film can be controlled by varying the orientation of rotating substrate during deposition and it is utilized for specific applications such as three-dimensional photonic crystals , birefringent omnidirectional reflectors [12, 13], graded index optical filters [14, 15], broadband antireflection coatings , linear polarizer , and fluid concentration sensing applications  among others.
The chief optical signature of a helical film is the circular Bragg phenomenon displayed by it on axial excitation , 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 helical film can be used for circular polarization elements including sources, reflectors, filters, and detectors [7, 20]. The center-wavelength (λBr 0) of the Bragg regime is proportional to both the average refractive index, naν≈(nc+nd)/2 as well as film half-pitch thickness, Ω and is calculated by using equation, λBr 0=2Ωnaν, while the FWHM bandwidth of that regime is given by Δλ 0=2Ω|nc-nd| [21,13]. Here nd=[n 2 a n 2 b/(n 2 acos2 β+n 2 bsin2 β)]1/2 is composite refractive index and na,nb, and nc are principal refractive indices of the film with column angle β. It is clear that a very high degree of anisotropy is required to get a large bandwidth reflector at reasonable values of Ω. But the maximum value of birefringence |nc-nd| reported from our group is 0.06 for TiO2 films . It is also seen that the bandwidth of the Bragg regime is proportional to the half-pitch for fixed birefringence. This suggests growing not one but a cascade of helical films in order to fabricate color separator and wideband circular polarization reflector . Hence, this feasibility has motivated the study presented in this work.
In this paper, we represent a wideband circular polarization reflector and color separator realized as cascades of helical films with different pitch thickness. These helical films were fabricated by using electron-beam evaporation via glancing angle deposition (GLAD) technique. Deposition angles, rates, and substrate rotation speed were employed to control the columnar microstructures of the films. The helical TiO2, ZrO2, and Ta2O5 thin films of five turns were also studied to find the appropriate material and deposition conditions for the wideband circular polarization reflector and color separator.
All helical films were prepared by electron-beam evaporation using the GLAD technique. The simplified schematic diagram of the GLAD technique is shown in Fig. 1. The deposition was performed in a vacuum chamber with a base pressure of ~5×10-6 Torr. The electron-beam evaporator with a 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. Various glancing angles (45°, 60° and 70°) and substrate rotation speed (0.09 rpm) were employed to control the columnar microstructure of the films. The range of deposition rates of the films were 0.3 to 0.5 nm/s. Helical TiO2, ZrO2, and Ta2O5 films deposited on glass substrates were used for optical analysis. The wideband circular polarization reflector and color separator deposited on glass substrates and silicon wafers using TiO2 material were used for optical and structural analysis.
Optical transmittance and hence reflectance of the films and the devices were measured by a spectrophotometer (Cary 500, Varian) in the wavelength range of 400 to 1200 nm. The beam size of the spectrophotometer was 13.35 mm×5.1 mm. The porosity of 5 turns TiO2 helical films for both structural handednesses was investigated by controlling the humidity in the chamber of spectrophotometer. A linear polarizer, achromatic quarter-wave plate, sample, second achromatic quarter-wave plate and a second linear polarizer 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 helical films and the devices were investigated using a scanning electron microscope (SEM).
3. Results and discussion
3.1 Optical characterizations
Helical films exhibit the Bragg phenomenon due to their periodic nonhomogeneity along the thickness direction. This means a structurally left/right-handed film reflects normally incident left/right circularly polarized (LCP/RCP) light with wavelength lying in the so-called Bragg regime; while the reflection of normally incident RCP/LCP light in the same region is very small. The reflectance spectra of helical TiO2, ZrO2, and Ta2O5 films deposited at different oblique incident angles with each of 5-turns as shown in Fig. 2. In Fig. 2(a), the reflectance spectra of TiO2 helical film shift toward the short wavelength with the increase of deposition angles and the maximum reflectance is 21.6 % at an angle of 60°. A similar behavior is found for helical ZrO2 and Ta2O5 films at different deposition angles as shown in Fig. 2(b) and 2(c), respectively. However, in this case, the maximum reflectance is 12.3 % for the ZrO2 film and 20.2 % for the Ta2O5 film at an angle of 70°. This may happen due to the decrease of pitch thickness with increase of deposition angles . The FWHM bandwidths of the films are also measured from their reflectance spectra and it is found that the bandwidth of the helical TiO2 film is higher than the others.
Figure 3 gives the effect of porosity on the both structural handednesses helical TiO2 film of 5 turns. The reflectance spectra shift toward the longer wavelength from vacuum to air for both cases in that the voids of the film infiltrated with adsorbed moisture in air cause the increase of effective refractive index. The spectra of the films carry the signature of the circular Bragg phenomenon, which occurs due to unidirectionally periodic nonhomogeneity and helical morphology. This phenomenon can also be explained by using grating theory . A circular plane wave of same handedness effectively encounters Bragg grating, while that of the other handedness does not. The Bragg phenomenon is employed to design optical filters, laser mirrors, and polarization-discriminatory handedness inverter etc. To enhance the optical efficiency of a LCD projection system, a broad-band circular polarizer covering the entire visible range is needed in order to avoid color shift at oblique angles . Therefore, as an application for display system, we deposited a wideband circular polarization reflector of seven sections with different pitch thickness and a color separator of three sections with different pitch thickness using TiO2 materials. The different pitch thickness was controlled by varying deposition rates.
Figure 4 illustrates the transmittance and reflectance spectra of the wideband circular reflector. It is observed that the bandwidth of the circular reflector is about 200 nm from 465 nm to 665 nm in the wavelength region indicating the feasibility of wideband circular reflector. The seven peaks in the range of bandwidth of the reflectance spectra occur due to the different pitch thickness of each section.
The transmittance and reflectance spectra of the color separator are shown in Fig. 5. The three circular Bragg peaks are observed at 465, 582, and 748 nm in the spectra which gives the feasibility of application in the display system. The two Bragg peaks of 582 and 748 nm are more prominent than that of 465 nm. The ripples observed at the left side of 465 nm may happen due to the variation of the pitch thickness throughout the film during deposition.
3.2 Cross-section SEM images
Scanning electron microscopic cross-section images for the wideband circular polarization reflector, the color separator and 5 turns TiO2 helical films of both structural handednesses are shown in Fig. 6 that ensure the helical structure of the films. In Fig. 6(a), the variation of pitch thickness along the thickness direction for the wideband circular reflector is not clear due to small change of pitch thickness among the sections of the reflector. But, in case of color separator, this variation is clear as shown in Fig. 6(b). The zigzag symbols marked in Figs. 6(c) and 6(d) confirm the opposite structural handednesses. It is also observed that the center axis of the helical structure is normal to the substrate indicating atomic-shadowing only occurs in the deposition plane. As a result, obliquely deposited thin films tend to consist of inclined columns which fan out and chain together perpendicular to the deposition plane . Moreover, the SEM micrographs reveals that the helical TiO2 films deposited at 60° are relatively close packed helical columns, indicating a minimal contribution from scattering in this regime.
Helical TiO2, ZrO2 and Ta2O5 films were prepared by glancing angle deposition (GLAD) and substrate rotations were employed to control the microstructure of the films. The optical properties of these films were investigated. These films exhibit a circular Bragg response, preferentially reflecting circular polarized light of same handedness and the reflectance peak shifts toward the short wavelength with the increase of deposition angles. Moreover, helical TiO2 film shows maximum reflectance peak and high anisotropy at an angle of 60° than others give it at an angle of 70°. Examination of the SEM micrographs reveals that the helical TiO2 films deposited at 60° are relatively close packed helical columns, indicating a minimal contribution from scattering in this regime.
The wideband circular reflector, deposited as cascades of helical films with seven different pitch thicknesses, separates the circular polarized light with a range of wavelength, 200 nm, suggesting the high feasibility of wideband circular polarization reflector. The color separator with three sections, each of different pitch thickness, shows three Bragg peaks which indicate the potential application in display system.
This work was supported by the Korea Science and Engineering Foundation through the Quantum Photonic Science Research Center at Hanyang University.
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