We have introduced a novel plasmonics planar slit lens, consisting of five slits with gradually decreasing widths and interspace distances. The cascading constant k to correlate the geometrical parameters with each other is proposed. At specific k values, light propagating through the slits is amplified and shows extraordinary transmission and focusing features beneath the metallic layer that we name it as the cascading effect. The focusing is due to the additional phase imposed on traveling light through the slits and their constructive interference. Numerical simulation Results show that the slit width should not be larger than half wavelength, and the cascading constant should be between 0.3 and 0.4.
A circular slit-groove surface plasmon polaritons (SPPs) launcher surrounding a photodetector is employed theoretically
to enhance the photocurrent of a typical Si-Ge photodetectors operating at telecommunication frequency regime. The slit
and grooves are designed such that the SPPs are focused at the center of the absorption layer of the photodetector to
result in additional electric current. Phase difference approach is applied to lead constructive interference between the
incident light impinging from the top and the SPPs propagating toward the photodetector. Simulation result proves the
interference and periodic behavior is observed. Finally the period of the groove, slit-groove distance, and slitphotodetector
distance is determined via simulation. Furthermore it was shown that photocurrent increases by
approximately 13-fold when the SPPs are introduced.
Three types of Si-based photodetectors (PD) operating at long wavelength were introduced: the
strained SiGe/Si multi-quantum-wells PD and Ge/Si islands PD with resonant cavity enhanced (RCE)
structure, Ge p-i-n PD on silicon and SOI, Ge/Si avalanche photodetectors (APDs) with separate
absorption, charge and multiplication (SACM) structure. The strained SiGe/Si MQW RCE PD and
Ge/Si islands RCE PD has a threefold enhanced responsivity compared with the conventional PD
without a resonant cavity. The Ge p-i-n PD on SOI has a responsivity of 0.65 A/W at 1.31μm and 0.32
A/W at 1.55μm. The 3dB bandwidth is 13.3GHz at reverse bias of 3 V. The Ge/Si SACM APD
operating at 1310 nm have a responsivity of 4.4A/W (with a gain of 8.8) biased at 90% of break
SOI (Silicon on Insulator) based photonic devices, including stimulated emission from Si diode, RCE (Resonant Cavity Enhanced) photodiode with quantum structure, MOS (Metal Oxide Semiconductor) optical modulator with high frequency, SOI optical matrix switch and wavelength tunable filter are reviewed in the paper. The emphasis will be played on our recent results of SOI-based thermo-optic waveguide matrix switch with low insertion loss and fast response. A folding re-arrangeable non-blocking 4×4 matrix switch with total internal reflection (TIR) mirrors and a first blocking 16×16 matrix were fabricated on SOI wafer. The extinction ratio and the crosstalk are better. The insertion loss and the polarization dependent loss (PDL) at 1.55 μm increase slightly with longer device length and more bend and intersecting waveguides. The insertion losses are expected to decrease 2~3 dB when anti-reflection films are added in the ends of the devices. The rise and fall times of the devices are 2.1 μs and 2.3 μs, respectively.
The high quality Ge islands material with 1.55μm photo-response grown on SOI substrate is reported. Due to the modulation of the cavity formed by the mirrors at the surface and the buried SiO<sub>2</sub> interface, seven sharp and strong peaks with narrow linewidth are found. And a 1.55 μm Ge islands resonant-cavity-enhanced (RCE) detector with narrowband was fabricated by a simple method. The bottom mirror was deposited in the hole formed by anisotropically etching in a basic solution from the backside of the sample with the buried SiO<sub>2</sub> layer in silicon-on-insulator substrate as the etch-stop layer. Reflectivity spectrum indicates that the mirror deposited in the hole has a reflectivity as high as 99% in the range of 1.2~1.65 μm. The peak responsivity of the RCE detector at 1543.8 nm is 0.028 mA/W and a full width at half maximum of 5 nm is obtained. Compared with the conventional p-i-n photodetector, the responsivity of RCE detector has a nearly threefold enhancement.