Researches On Spatial Filtering Properties Of Periodically Artificial Microstructure Materials

Spatial filters have very important applications in image enhancement, information processing, laser technology and other fields. A traditional spatial filter that is a4f (f is the lens focal length) focusing type system formed by two lens has several deficiencies, such as a relatively large size, impossibility of near-field filtering, and so on. Aiming at these deficiencies of traditional spatial filters, we carry out exploration and research of a new flat plug-and-play (non-focusing type) spatial filter, and mainly focus on the spatial filtering properties of periodically artificial microstructure materials. A series of schemes are proposed, including polarization independence, tuning operating frequency, omnidirectional filtering and so on. It is expected to be helpful theoretically for the design and manufacture of the novel spatial filters. The mainly achievements are as follows:Firstly, based on the spatial properties of the bandgap of Rugate structures, a new application of Rugate structures is proposed as low-pass spatial filters, and numerical simulation proves that the spatial filters can achieve beam smoothing. The spatial filters have both an almost ideal flat bandpass and a rather steep switching between pass-and stop-bands, and the angle-domain bandwidth of the spatial filters can be tuned by chang-ing the parameters of Rugate structures. The near-field simulations carried out by using the finite-difference time-domain technique confirm the possibility of an efficient light smoothing.Secondly, in order to overcome the polarization-dependent deficiency of conven-tional photonic crystals spatial filters, we propose a polarization-independent spatial fil-ter made from one-dimensional photonic crystals containing negative-index materials. Through optimizing the parameters of defect layer, a series of polarization-independent defect modes in the zero-average-index gap of the photonic crystals are obtained with the increase of the incident angle. Based on these defect modes, low-, high-and band pass spatial filters are designed. The spatial-frequency bandwidth of the spatial filters can be adjusted by changing the period number of the defective photonic crystal structures.Thirdly, generic conditions of polarization-independent transmissions in one-dimensional magnetic photonic crystals are derived, and the strict polarization-independent low-pass spatial filters of photonic crystals and wider omnidirectional photonic band gap are achieved. It is found that polarization-independent transmission in complete photonic crystals is ob-tained when the refractive index and optical thickness of the two layers are the same as well as the wave impedances are the mutually reciprocal. When the two complete pho-tonic crystals satisfying the above relations constitute the photonic quantum well struc-ture, the polarization-independent transmission can be achieved. When a defective layer whose wave impedance equals to1is introduced, the defective photonic crystals also has the polarization-independent transmission properties.Finally, the filtering properties of one-dimensional quantum-well structures are re-searched, and the symmetrical confined states are found in photonic quantum-well struc-tures with single-negative materials. The number and frequencies of the symmetrical con-fined states can be tuned by varying the period number and thicknesses of the well pho-tonic crystal, respectively. Compared with previous confined states of photonic quantum-well structures based on the Bragg or zero-average-index gap, the confined states of the structures are less sensitive to the incident angle and polarization. The structures open a promising way to fabricate symmetrically tunable and omnidirectional multichannel fil-ters for future dense wavelength division multiplexing applications.