The resonant wavelength of long-period fiber gratings (LPFGs) is very sensitive to the ambient refractive index. LPFGs will have many potential applications on biochemistry sensors and environment monitor system. At present, LPFGs chemical sensors can only measure the medium, which has lower refractive index than that of the fiber cladding, however, the detecting range can be greatly enlarged if the LPFGs coated with Langmuir-Blodgett thin film are used. LPFGs will have more extensive applications with the mature of the L-B thin film technology. In this paper, the spectrum performance of the resonant wavelength of LPFGs varying with the changes of the ambient refractive index (1< n < 1.8) is theoretically analyzed. As the ambient index is increased, each resonance wavelength first shifts toward the shorter-wavelength direction and then disappears where the value of n is about 1.45. When the ambient index is larger than that of the cladding (~1.45), the resonance wavelengths reappear at slightly longer wavelength than those measured in ambient air. According to the mode coupling method, the theoretical four-layered fiber model is developed on the relationships among the resonant wavelengths of LPFGs coated with L-B thin film, the refractive index and thickness of the L-B thin film, and ambient refractive index. The shift of the resonant wavelength is calculated through numeric method and is presented graphically.
Random Ag-SiN films consisting of random small Ag particles embedded in a SiN thin film were deposited by radiant-frequency magnetron sputtering. Specimens orderly comprising a random Ag-SiN film and an optical phase change recording layer were exposed to a focused laser beam. It showed that, with a random Ag-SiN layer deposited above the recording layer, the ablation of the recording layer occurred much faster and under much lower power than that of a single recording layer, which verified the local field enhancement of multiple scattering effects of the Ag particles. Finite Difference Time Domain (FDTD) calculation of a sandwiched structure consisting of ZnS-SiO2(130nm)/AgOx(20nm)/ZnS-SiO2(40nm) under a Gaussian beam irradiation has been carried out to simulate the near-field distribution in the structure. Near-field optical data storage adopting a Super-Resolution Near-field Structure (Super-RENS) usually utilizes similar films structure mentioned above to achieve super resolution storage density while getting a high Carrier to Noise Ratio (CNR) at the same time. Many recent works have reported that small Ag particles were formed in the AgOx film after converging laser irradiation onto the sandwiched structure. Here, another FDTD calculation was done to simulate the same model except for that small Ag particles were modeled in the AgOx film in the center region of the incident laser spot. The results showed a huge local near-field enhancement, which indicates that, if the structure full of such small Ag particles are formed in a tiny region beyond the optical diffraction limit, the optical recording and readout out density would be improved as well as a high CNR level achieved due to the multiple scattering of the Ag particles.
Mechanism of Sb-SR (Sb-type super-resoluiton near-field structure) was studied. Experimental results show that the super-resolution performance of amorphous Sb-SR is considerably better than crystal Sb-SR. Considering that Sb is a semi-conductor with small energy gap, a three-order nonlinear response of surface plasmons is deduced to explain the performance of amorphous Sb-SR. Estimation is made and the calculated results are in agreement with the experimental results. Under the new approach, other semiconductors with small energy gap may possibly possess super-resolution performance.
KEYWORDS: Thin films, Plasmons, Data storage, Scanning electron microscopy, Near field scanning optical microscopy, Near field, Transmittance, Optical switching, Germanium antimony tellurium, Near field optics
Nonlinear optical transimittance in the Super-RENS (super-resolution near-field structure) [glass/SiN(20nm)/ Sb(15nm) /SiN(20nm)] was investigated using a static transmittance measuring system with a focused laser illumination. The result shows an optical switching property with a strong nonlinear effect. Some SEM(Sanning Electron Microscope) images ofrecording spots in a PC(Phase Change) GeSbTe layer with and without the Super-RENS layers covering on it are presented. The images demonstrate the field enhancement effect due to the Super-RENS layers. The cause of the enhancement is considered as the local plasmon excitation.
Conference Committee Involvement (1)
19 May 2003 | Maspalomas, Gran Canaria, Canary Islands, Spain