We proposed and demonstrated a new modulation scheme of surface plasmon resonance (SPR) sensor for higher sensitivity. We fabricated novel SPR transducers with wedge membrane structure on the Au thin film (Otto configuration) for gas sensing application. The transducers are composed of SiO2 / gas flowing layer / Au trilayers, fabricated by wafer bonding of SiO2 substrate on Au thin film on Si substrate via the ball spacers to define the thickness (t) of the gas flowing layer. There is variation of t within the sample owing to the inhomogeneous bonding force during the adhesion process, leading to variation of t and modulating the SPR within the single transducer. The SPR was measured with attenuated total reflection with a glass prism. We measured SPR for different positions having different t by images detected by a camera with focusing and objective lenses. Asymmetric intensity distributions in the images were fitted with the simulated reflectivity, meaning that the obtained images correspond to the SPR near the cut-off condition in Otto configuration. We observed the shift of the intensity distribution upon the change of the excitation condition through different t. Based on the new modulation scheme, higher sensitivity is expected through analysis of the observed images.
Optical isolators are one of the essential components to protect semiconductor laser diodes (LDs) from backward reflected light in integrated optics. In order to realize optical isolators, nonreciprocal propagation of light is necessary, which can be realized by magnetic materials. Semiconductor optical isolators have been strongly desired on Si and III/V waveguides. We have developed semiconductor optical isolators based on nonreciprocal loss owing to transverse magneto-optic Kerr effect, where the ferromagnetic metals are deposited on semiconductor optical waveguides1). Use of surface plasmon polariton at the interface of ferromagnetic metal and insulator leads to stronger optical confinement and magneto-optic effect. It is possible to modulate the optical confinement by changing the magnetic field direction, thus optical isolator operation is proposed2, 3). We have investigated surface plasmons at the interfaces between ferrimagnetic garnet/gold film, and applications to waveguide optical isolators. We assumed waveguides composed of Au/Si(38.63nm)/Ce:YIG(1700nm)/Si(220nm)/Si , and calculated the coupling lengths between Au/Si(38.63nm)/Ce:YIG plasmonic waveguide and Ce:YIG/Si(220nm)/Si waveguide for transversely magnetized Ce:YIG with forward and backward directions. The coupling length was calculated to 232.1um for backward propagating light. On the other hand, the coupling was not complete, and the length was calculated to 175.5um. The optical isolation by using the nonreciprocal coupling and propagation loss was calculated to be 43.7dB when the length of plasmonic waveguide is 700um.
1) H. Shimizu et al., J. Lightwave Technol. 24, 38 (2006). 2) V. Zayets et al., Materials, 5, 857-871 (2012). 3) J. Montoya, et al, J. Appl. Phys. 106, 023108, (2009).
Semiconductor optical isolators for integrated optics are presented. The requirements and demands for semiconductor optical isolators which can be monolithically integrated with semiconductor lasers and waveguide are discussed. Fundamental theories of magneto-optic effect in optical waveguides including magnetic materials are shown and transverse magneto-optic Kerr effect is reported on ferromagnetic metal Fe, Co, and Fe50Co50 thin films. Based on the fundamental properties, design, fabrication, and characterization of semiconductor optical isolators based on nonreciprocal loss are shown. Transverse electric (TE-) and transverse magnetic (TM-) mode semiconductor optical isolators are reported in telecommunication wavelengths of 1.3 – 1.55 μm with (1) an optical isolation of 14.7 dB/mm in a TE mode semiconductor optical isolator, (2) amplifying characteristics in a TM mode semiconductor optical isolator, and (3) an optical isolation of 18.3 dB by using of nonreciprocal polarization rotation. Furthermore, monolithic integration of a semiconductor optical isolator with distributed feedback laser diode (DFB-LD) is reported. Optical isolator performances are compared with those of previously reported waveguide optical isolators. Finally, future prospect and applications of semiconductor optical isolators are discussed.