Y. Shoji

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.

Ultra-small, self-holding, optical gate switch using Ge 2 Sb 2 Te 5 with a multi-mode Si waveguide

We report a multi-mode interference-based optical gate switch using a Ge(2)Sb(2)Te(5) thin film with a diameter of only 1 µm. The switching operation was demonstrated by laser pulse irradiation. This switch had a very wide operating wavelength range of 100 nm at around 1575 nm, with an average extinction ratio of 12.6 dB. Repetitive switching over 2,000 irradiation cycles was also successfully demonstrated. In addition, self-holding characteristics were confirmed by observing the dynamic responses, and the rise and fall times were 130 ns and 400 ns, respectively.

Compact 2 × 2 polarization-diversity Si-wire switch.

A polarization-independent 2 × 2 switch based on silicon-wire waveguides has been realized with a compact size of 600 × 500 μm². Polarization-independent operation was achieved with a polarization-diversity technique which implements polarization splitters, TE-TM intersections, and Mach-Zehnder switches. The extinction ratios of the 2 × 2 switch for TE, TM, and a mixed polarization at a wavelength of 1550 nm were measured to be larger than 30 dB, 25 dB, and 30 dB, respectively. The measured switching powers for the TE and TM polarizations were 25 and 55 mW, respectively. The measured polarization-dependent loss was lower than 1 dB. The differential group delay (DGD) between the TE and TM modes was also evaluated using the Mueller matrix method, which was in good agreement with the values estimated from the path lengths for each mode. A path-length-compensated switch was fabricated, whose DGDs for all paths were indeed as small as ~2 ps, mainly from the access waveguides. The switch could provide an important route to develop ultra-compact polarization-independent integrated circuits based on silicon-wire waveguides.

Femtosecond index dynamics in silicon wire waveguides

We studied ultrafast optical dynamics in silicon wire waveguides operating at 1.55 μm by employing a heterodyne pump-probe method with 240-fs temporal resolution. From measurement results, nonlinear properties were determined.

Phase change characteristics of Ge2Sb2Te5 thin film for a self-holding optical gate switch

Fast and low power consumption optical switches are required for photonic networks. To this end, we proposed the optical gate switch using phase change material (PCM) and silicon waveguides. This switch had low power consumption because it consumed power only when the state was changed. Furthermore, the chip size is very small due to the large refractive index change of PCM. In this paper, we studied the phase-change characteristics of various kinds of thin GST film on SOI (silicon-oninsulator). A laser diode (LD) with a wavelength of 660 nm was used to irradiate the material. We compared the optical responses by laser pulse irradiation on GST films, and concluded GST-147 was the most suitable material for the optical switch because it had the lowest phase change threshold. The phase-change characteristics of GST-225 films with thickness of 25 nm, 50 nm and 75 nm were also examined. Thicker GST films had lower phase change thresholds. However, thermal simulations showed that the phase of the bottom part of thicker films may not be changed. Therefore, we concluded that GST films with thicknesses between 25 nm to 50 nm are the most suitable for optical switches.

Reversible Switching of an Optical Gate Using Phase-Change Material and Si Waveguide

Optical gate switch that uses Ge2Sb2Te5 phase-change material was fabricated and the reversible switching has been achieved for the first time. The switch is only 5-µm long and laser pulse irradiation was used for switching.

Reversible switching of an optical gate based on Si rib waveguides with a Ge 2 Sb 2 Te 5 thin film

Reversible switching of an optical gate based on Si rib waveguides with a Ge2Sb2Te5 thin film is reported. We demonstrated four cycles of switching by pulsed laser irradiation. The average extinction ratios of each switching event were 9.1, 10.7, 11.6 and 11.7 dB, respectively. The extinction ratio of this optical gate was more than 5.9 dB over the wide wavelength range from 1525 nm to 1600 nm.

Small-sized self-holding optical switch using phase-change material

Optical gate switch that uses phase change material with self-holding characteristics was proposed and fabricated. The length of the switch was only 1 µm and the switching time was less than 100 ns.

25 nm-wide silicon wire fabrication with i-line photolithography for efficient spot size converters

Free from the resolution limit, very narrow inverse taper structure of silicon wire could be fabricated with i-line photolithography. By covering the taper with a polyimide waveguide core, efficient coupling from optical fiber to silicon wire waveguide was possible.

Pump-induced optical nonlinearities in hydrogenated amorphous silicon waveguides

We characterize optical nonlinear properties in hydrogenated amorphous silicon (a-Si:H) wire waveguides at telecom wavelengths using a heterodyne pump-probe measurement. Optical nonlinear coefficients of the phase shift and absorption change are estimated and discussed.