Junichi Takahashi

Microphotonics devices based on silicon microfabrication technology

This work presents our recent progress in the development of an Si wire waveguiding system for microphotonics devices. The Si wire waveguide promises size reduction and high-density integration of optical circuits due to its strong light confinement. However, large connection and propagation losses had been serious problems. We solved these problems by using a spot-size converter and improving the microfabrication technology. As a result, propagation losses as low as 2.8 dB/cm for a 400/spl times/200 nm waveguide and a coupling loss of 0.5 dB per connection were obtained. As we have the technologies for the fabrication of complex, practical optical devices using Si wire waveguides, we used them to make microphotonics devices, such as a ring resonator and lattice filter. The devices we made exhibit excellent characteristics because of the microfabrication with the precision of a few nanometers. We have also demonstrated that Si wire waveguides have great potential for use in nonlinear optical devices.

Four-wave mixing in silicon wire waveguides.

We report the observation of four-wave mixing phenomenon in a simple silicon wire waveguide at the optical powers normally employed in communications systems. The maximum conversion efficiency is about -35 dB in the case of a 1.58-cm-long silicon wire waveguide. The nonlinear refractive index coefficient is found to be 9x10-18 m2/W. This value is not negligible for dense wavelength division multiplexing components, because it predicts the possibility of large crosstalk. On the other hand, with longer waveguide lengths with smaller propagation loss, it would be possible to utilize just a simple silicon wire for practical wavelength conversion. We demonstrate the wavelength conversion for data rate of 10-Gbps using a 5.8-cm-long silicon wire. These characteristics are attributed to the extremely small core of silicon wire waveguides.

Silicon-wire-based ultrasmall lattice filters with wide free spectral ranges.

Using silicon-on-insulator-based silicon-wire waveguides with submicrometer cross sections, we constructed ultrasmall channel-dropping lattice filters for 1.5-microm infrared systems. The waveguides low-loss bends with 2.5-microm radius reduce the total length of the filter to less than 100 microm and enlarge the free spectral range to more than 80 nm. The measured spectra show fine channel-dropping characteristics, and the results agree well with numerical predictions. Moreover, we have succeeded in tuning the dropping wavelength by adjusting the lengths of the delay lines.

High-density speed optical near-field recording reading with a pyramidal silicon probe on a contact slider.

We demonstrate high-density-speed phase-change recording-reading by use of a pyramidal silicon structure. The contact slider, which has a pyramidal silicon probe array with height dispersion of less than 10 nm, is fabricated by use of a silicon-on-insulator wafer. By illumination with a laser beam (lambda = 830 nm) of one element of the probe array, we find the shortest phase-change mark length and the carrier-to-noise ratio to be 110 nm and 10 dB, respectively, corresponding to a data transmission rate of 2.0 MHz. This rate can be increased to 200 MHz by use of all elements of the probe array.

Oxidation-induced improvement in the sidewall morphology and cross-sectional profile of silicon wire waveguides

Progress in sidewall morphology smoothing of Si wire waveguides by thermal oxidization was observed using a focused-ion-beam (FIB) transmission-electron-microscopy technique. The roughness of Si∕SiO2 interface was drastically reduced with increasing oxidation time and temperature. Deformation of the cross-sectional profile of the waveguides was observed with secondary electron images of scanning FIB irradiation. The initial rectangular profile is transformed due to stress concentration at the rectangle corners at oxidation temperatures of 900 and 1000°C. In contrast, at 1100°C, the profile maintains the original rectangular profile due to the stress release by the viscous flowing of SiO2. These results indicate that the optimum oxidation condition for the Si wire waveguide has been found, which provides an extremely smooth sidewall without deformation of the cross-sectional profile.

Si-based photonic crystals and photonic bandgap waveguides

We experimentally demonstrate the structural tuning of the waveguiding modes of line defects in photonic crystal slabs. By tuning the defect widths, we realized efficient single-mode waveguides that operate within photonic band gap frequencies in SOI photonic crystal slabs. The observed waveguiding characteristics agree very well with 3D finite- difference time-domain calculations. The propagation loss of the line defect waveguides is experimentally determined to be 6 dB/mm. In addition, we measure group velocity dispersion of line defects by using Fabry-Perot resonance of the sample. Extremely large group dispersion is observed, and the traveling speed of light is reduced down to 1/100 of the light velocity in air.

SOI-based photonic crystal line-defect waveguides

We have experimentally demonstrated single-mode light-wave transmission and tunable waveguiding characteristics in photonic crystal (PC) waveguides constructed on a silicon-on-insulator (SOI) substrate as is most likely to be used for the a large scale integration of photonic circuits. Although off-plane diffractive leakage has been a serious problem in SOI-PC waveguides, we have overcome this problem in our narrow line-defect and phase-shifted-hole line-defect waveguide structures. These devices were developed through intensive theoretical studies on PC line-defect waveguieds. We have also demonstrated low-loss mode profile converter that will enable efficient connection between conventional silica-based waveguides and PC line-defect waveguides. The converter features an inversely-tapered silicon wire waveguide with an ultra-thin tip constructed on an SOI substrate. In our experiments, this converter proved capable of coupling loss as low as 0.5dB per conversion. These SOI-based devices represent an important step towards practical large-scale integrated photonic crystal circuits.

Subwavelength-sized phase-change recording with a silicon planar apertured probe

We propose and demonstrate a new optical near-field slider with a planar apertured probe array for optical memory. The slider was fabricated by utilizing anisotropical etching of a silicon membrane and anodic bonding of a silicon membrane and glass substrate. We also present for the first time a subwavelength-sized phase-change recording/reading by using the planar apertured probe array. Apertures were fabricated at the bottom end of the pyramidal grooves. A SiO2/AgInTe2/glass substrate was used as the recording medium. By scanning the planar apertured probe array, we obtained resolved images with line width of 250 nm.

Silicon wire waveguides and their applications for microphotonics devices

We report our recent progress in a Si wire waveguides, which promises size reduction and high-density integration of optical circuits. The application to functional devices and their nonlinear optical effects are also discussed.

Parametric amplification of Raman-inactive lattice oscillations induced by two-color cross-beam excitation

The second-order Raman bands of SrTiO(3) are excited under two-color cross-beam configuration using femto-second laser pulses. Raman-inactive one-phonon waves are generated by the coherently excited large amplitude two-phonon wave. The one-phonon waves are observed as a train of visible light spots, the frequency steps of which are coincident with the frequencies of the one-phonon modes. In order to understand the mechanism, a model of three-wave interaction among one two-phonon wave and two one-phonon waves is proposed.