| Author: | Xin, Jianzhuo |
| Title: | Tunable photonic devices in ferroelectric-based layered structures |
| Degree: | Ph.D. |
| Year: | 2012 |
| Subject: | Ferroelectric thin films. Thin films -- Optical properties. Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxix, 153 leaves : ill. (some col.) ; 30 cm. |
| Language: | English |
| Abstract: | This thesis presents the studies on the optical properties of perovskite ferroelectric thin films, as well as the preparation and applications of ferroelectric materials in tunable photonic devices, including the computational and experimental results of thermally tunable terahertz (THz) photonic crystal (PC) and surface plasmon polariton (SPP) structures. First of all, BaTiO₃ (BTO)-based ferroelectric thin films are of great interest in the realization of various optical applications. Among them, (Bax,Sr₁-x)TiO₃ (BST) is one of the most studied systems, while Ba(Zrx,Ti₁-x)O₃ (BZT) is reported to show some advantages over BST, such as low leakage current, low dielectric losses, high piezoelectric properties and large microwave tunability. But there is a lack of systematic study on the optical properties of BZT thin films. In this thesis, BZT thin films with x ranging from 0 to 40% were grown on MgO single crystal substrates by pulsed laser deposition, and their optical properties in the visible range were systematically characterized. A linear increase in the out-of-plane lattice constant of BZT unit cell with increasing Zr content was detected by x-ray diffraction. The surface morphology was observed by atomic force microscopy, and the grain size was shown to increase with Zr concentration. Prism coupling and UV-visible transmission spectroscopy techniques were used to analyze relevant optical properties of the films. Refractive indices between 2.15 and 2.3 were observed at 633 and 1547 nm respectively, which decreased with rising Zr content. The BZT films also possessed large optical band gap energy up to 3.92 eV, which increased with rising Zr content. Electro-optic (EO) effect was observed with quadratic EO coefficients between 0.11-0.81x10⁻¹⁸ m²/V², which decreased with rising Zr concentration, and coexistence of linear and quadratic EO effect was also discussed. Optical loss due to surface scattering and absorption were determined, and the absorption coefficient dropped with increasing Zr content near the band gap. The obtained results provided useful information for the design of BZT thin film-based optical devices. Then perovskite ferroelectrics were utilized in tunable photonic devices. One-dimensional PC working in the THz range was studied. To facilitate the design, the transmission properties of SrTiO₃ (STO) crystals were characterized by THz time-domain spectroscopy. Relatively high refractive index (~ 18.9) and transmission ratio (0.08) were observed in STO between 0.2 to 1 THz. A stacked structure (Si tSi/STO tSTO)N/Si tSi was then designed, with the transmission spectra calculated by the transfer matrix method. The effects of filling ratio (tSi/(tSi + tSTO)), periodicity (tSi + tSTO) and the number of repeats N on the transmission of PC were investigated. A high filling ratio at small periodicity (100 μm) was found to yield larger band gaps due to the refractive index contrast between STO and Si (nSi = 3.4). Such a gap enhancement with filling ratio was less prominent for periodicity over 300 μm due to the highly-folded band structure. Besides, while increasing N from 2 to 5 led to widened band gap at small periodicity of 100 μm, little changes in the transmission spectra were observed for N greater than 3 when the periodicity was large (400 μm). Based on these, and realizing the challenges for preparing STO below 100 μm, a stack of (Si 300 μm/STO 100 μm)₃/Si 300 μm was constructed by polishing and stacking. Large band gap with gap-mid gap ratio of 16.7% (0.33-0.39 THz) was predicted for this structure, with the potential of thermal tuning by up to 35% for the temperature decreasing from room temperature to -150 ℃.. Then a defect layer was introduced into this structure to work as a filter. Different defect materials as well as different defect thickness were studied by calculation. Si defects with thickness ranging from 400 to 700 μm can lead to one clearly observed defect mode shifting from 0.335 to 0.375 THz lied in the first band gap. With the increasing defect thickness, the transmittance of defect mode was greatly increased. However, with STO as a high-refractive-index defect, totally different phenomena to that of Si defects were obtained. With STO defect thickness increased from 200 to 500 μm, the number of defect modes appearing in the first band gap increased from 1 to 4. And these modes showed symmetric distribution within the gap. With an increasing STO defect thickness, the transmittance of defect modes greatly decreased. Based on these, Si/STO multilayers with STO defect thickness of 125 μm and 200 μm were experimentally measured. The shift of defect mode was observed and was comparable with calculations. Two-dimensional photonic structures in the optical and infra-red range was then attempted, which requires the patterning of sub-micron scale features on oxide films over large-area. Along this direction, studies of two fabrication methods were conducted. Firstly, a combination of soft ultraviolet nanoimprint lithography and inductively coupled plasma etching techniques were used. Two-dimensional square arrays of pillars were obtained on Ba₀.₇Sr₀.₃TiO₃ thin films. The results were characterized by atomic force microscopy and scanning electron microscopy. The influencing parameters of this process were discussed. Secondly, in order to simplify the nanoimprint process and allow thick metal sacrificial layer deposition for high aspect-ratio etching, a transfer imprint lithography technique was developed. A metal layer was deposited on the PDMS mold, and it was subsequently pressed against an uncured nanoimprint resist layer previously spin coated on the substrate. Subsequent curing of nanoimprint resist by UV exposure naturally provided adhesion between the metal and the substrate. The use of soft PDMS mold generated conformal contact of patterns with target surfaces and greatly reduced the imprinting pressure required for homogenous mask transfer. Finally, SPP tuning via thermally-induced refractive index changes in ferroelectrics was investigated in the visible range. Epitaxial (Ba₀.₇Sr₀.₃)TiO₃ (BST) thin films were deposited on MgO (001) substrates by pulsed laser deposition. The refractive index of BST thin films, as measured by prism coupling technique, was found to increase from 2.3932 (TE)/1.9945 (TM) at room temperature to 2.3949 (TE)/1.9965 (TM) at 66 ℃. Then 30nm-Ag stripes with periodicity of 750 nm and width of 300 nm were fabricated on flat BST films by soft ultraviolet nanoimprint lithography and subsequent lift-off process. Angular-dependent reflection spectra from 500 to 1000 nm were measured at room temperature and 66 ℃, with a collimated and p-polarized light incident perpendicularly to the grating direction. Several modes were observed from the spectra. At 66 ℃, a red shift of the (2) SP mode by 2 nm was observed. Calculations based on a simple analytical model and rigorous coupled wave analysis (RCWA) method qualitatively confirmed that the observed modes belong to the (-1), (2), (-2) and (3) SPP modes from the Ag and BST interfaces, as well as the red shift by thermal tuning. The results indicated the feasibility for active modulation of SPP resonance in solid-state structures. In the near infrared range, Au films on corrugated BST surface with period of 1 μm were designed and fabricated. Thermal tuning of the (-1) SP mode from Au and BST interface was observed and the measurement results was comparable with calculations. |
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| Access: | open access |
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