In this paper, we report enhanced performance of quantum dot infrared photodetector (QDIP) by means of surface plasmonic structure. A 10-layer InGaAs quantum dot (QD) structure with AlGaAs barriers were grown on GaAs(001) substrate by molecular beam epitaxy. A periodic gold pillar array was fabricated on the surface of the wafer by i-line lithography and ICP dry etching. The wafer was processed to form a circular mesa of 300 μm diameter. The pillar pitch was varied from 2.0 to 2.5 μm and the pillar diameter was varied from 1.1 to 1.6 μm. The detectors were illuminated from substrate side to evaluate spectral responsivity and detectivity D* at temperature T = 78 K. The pillar array was found to enhance the detector performance at particular wavelength which depends on the pillar property. We found the optimal property of the pillar array whose enhancement peak matches the QDIP's response peak at 7.7 μm. At that wavelength, the responsivity and detectivity were almost doubled compared to the detector without the pillar array.
We report on a channel drop filter with a mode gap in the propagating mode of a photonic crystal slab that was
fabricated on silicon on an insulator wafer. The results, simulated with the 3-dimensional finite-difference time-domain
and plane-wave methods, demonstrated that an index-guiding mode for the line defect waveguide of a photonic crystal
slab has a band gap at wave vector k = 0.5 for a mainly TM-like light-wave. The mode gap works as a distributed Bragg
grating reflector that propagates the light-wave through the line defect waveguide, and can be used as an optical filter.
The filter bandwidth was varied from 1-8 nm with an r/a (r: hole radius, a: lattice constant) variation around the
wavelength range of 1550-1600 nm. We fabricated a Bragg reflector with a photonic crystal line-defect waveguide and
Si-channel waveguides and by measuring the transmittance spectrum found that the Bragg reflector caused abrupt dips
in transmittance. These experimental results are consistent with the results of the theoretical analysis described above.
Utilizing the Bragg reflector, we fabricated channel dropping filters with photonic crystal slabs connected between
channel waveguides and demonstrated their transmittance characteristics. They were highly drop efficient, with a flattop
drop-out spectrum at a wavelength of 1.56 μm and a drop bandwidth of 5.8 nm. Results showed that an optical adddrop
multiplexer with a 2-D photonic crystal will be available for application in WDM devices for photonic networks
and for LSIs in the near future.
We fabricated various microscopic optical devices using photonic crystal slab and Si-wire waveguides and demonstrated their fundamental characteristics. We demonstrated a channel-dropping filter with a photonic crystal slab point-defect optical cavity. Wavelength resolution of less than 1.5 nm and signal dropping efficiency of more than 90 % were obtained for a 20-μm-square device. We also demonstrated an optical add/drop multiplexer with Bragg grating reflectors made from Si-wire waveguides. Its dropping wavelength bandwidth was less than 2 nm, and the center wavelength of the dropped optical signal could be tuned by thermo-optic control using a microheater formed on the Bragg reflector. Using Si-wire waveguide, we also demonstrated thermo-optic switches by forming a micro heater on a branch of a Mach-Zehnder interferometer constructed from the waveguides. In this switching operation, we observed an extinction ratio of more than 30 dB, switching power of less than 100 mW, and switching response speed of less than 100 μs using a 1 × 2 optical switch with an 85 × 30 μm2 footprint. Using the 1 × 2 optical switch elements, we also fabricated a compact 1 × 4 optical switch and demonstrated its fundamental operation.
The tuning of the light propagation and localization properties in photonic crystal (PhC) slabs by using microactuators was demonstrated numerically and experimentally. A micromechanical actuator controls the position of the exterior structural element, which is located close to the PhC slab, and modulates the PhC properties through the change of the evanescent interaction of light confined in the PhC slab with the exterior element. When the exterior structural element approaches to a line-defect PhC waveguide, intensity and phase modulations occur. In the preliminary experiment using a line-defect PhC waveguide, we demonstrated the optical switching operation with an extinction ratio of ~10 dB at a wavelength of 1.55 μm. The localized state of light in a point-defect cavity can also be controlled. The tuning of the resonant wavelength over the spectral range of ~60nm at around the wavelength of 1.55 μm was numerically demonstrated by combining two PhC slabs. The approach discussed here can be widely employed for realizing functional and tunable PhC slab devices.
Micro-lightwave circuit technologies based on photonic crystal slabs were studied to realize integrated photonic node circuits for use in photonic networks. First, a unique optical multi-exposure technique, which is suitable for drawing large-area, two-dimensional, photonic crystal lattice patterns, is introduced here. The relationship between the resolved pattern size and the light-beam wavelength used for exposure is also discussed. Next, a high-density optical interconnection technique with photonic crystal line-defect waveguides and Si channel waveguides is introduced. A low-loss connection structure for both waveguides and their low bending loss characteristics are also discussed. Furthermore, slab-type, photonic crystal-based optical devices, such as channel-drop filters and optical switches, for constructing the photonic node circuits were proposed and their characteristics investigated by FDTD simulations. A high wavelength resolution for the filters and extremely small switching power for the optical switches were predicted. Before fabricating the optical switches, directional couplers based on photonic-crystal slabs were fabricated and the basic properties of complementary power splitting to two output ports were demonstrated. These results strongly support the possibility of realizing integrated photonic node circuits with photonic crystals.
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