Investigation into mechanically-tunable one-dimensional photonic crystal

Abstract

Recent advances in photonic crystals subject focus mostly on optical properties of rigid structures of two-dimensional or three-dimensional photonic crystals (2-D or 3-D PhCs). While many interesting applications, such as waveguide bends, resonant cavity, add-drop filters, etc., are benefited from the PhCs, tuning optical properties of the PhCs is still a challenging issue. All tuning methods yield small alteration of photonic band gaps (PBGs). An exception from those is the mechanical tuning that promises the largest tuning of the PBG. However, the mechanical tunings for 2-D PhCs and 3-D PhCs have only been conceptualized. Surprisingly, mechanical tunability of one-dimensional photonic crystals (1-D PhCs) has not been investigated much although 1-D PhCs also possess interesting PBG effects. Moreover, simplicity of the 1-D PhC structure encourages easier fabrication and characterization. Therefore, an attempt to demonstrate an application of tunable 1-D PhCs is addressed in this work. A mechanically-tunable PBG polarization splitter was proposed. The device utilized the PBG effect at inclined incidence to separate transverse-electric (TE) mode from transverse-magnetic (TM) mode. Silicon and polydimethylsiloxane (PDMS) were chosen for constructing the tunable 1-D PhC for the device. An improved plane wave expansion method and a transfer matrix method were employed to calculate PBGs of the PhC. The matrix method was further employed to design a finite tunable 1-D PhC at Brewster's angle. Mechanical tuning of the PBG by varying the thickness of PDMS was studied using the method. Transmitted power and polarization degree were calculated. It was found that periodicity of three was appropriate for constructing the tunable 1-D PhC. The designed silicon thickness was 6 μm at operating frequency of 3.5 THz, whereas the range of PDMS thickness was 6-12 μm so that transmittance of TE mode could be varied from 0 to unity. Finitedifference time-domain simulation yielded consistent results. To achieve the mechanical tuning, a thermal microactuator was designed and simulated. Microfabrication processes were developed for the tunable 1-D PhC device. The processes aimed at fabricating together both the 1-D PhC and the for giving the opportunity to work on the topic and for many precious discussions and significant initial supports. The supervisions, the advices, and the encouragements from them were important to the student, and they are much and ever appreciated. Many thanks go to the technicians of Micromachines Lab 1, i.e.