We present a monolithic integrated low-threshold Raman silicon laser based on silicon-on-insulator (SOI) rib
waveguide ring cavity with an integrated p-i-n diode. The laser cavity consists of a race-track shaped ring resonator
connected to a straight bus waveguide via a directional coupler which couples both pump and signal light into and
out of the cavity. Reverse biasing the diode with 25V reduces the free carrier lifetime to below 1 ns, and stable,
single-mode, continuous-wave (CW) Raman lasing is achieved with threshold of 20mW, slope efficiency of 28%,
and output power of 50mW. With zero bias voltage, a lasing threshold of 26mW and laser output power >10mW can
be obtained. The laser emission has high spectral purity with a side-mode suppression of >80dB and laser linewidth
of <100 kHz. The laser wavelength can be tuned continuously over 25 GHz. To demonstrate the performance
capability of the laser for gas sensing application, we perform absorption spectroscopy on methane at 1687 nm using
the CW output of the silicon Raman laser. The measured rotationally-resolved direct absorption IR spectrum agrees
well with theoretical prediction. This ring laser architecture allows for on-chip integration with other silicon
photonics components to provide an integrated and scaleable monolithic device. By proper design of the ring cavity
and the directional coupler, it is possible to achieve higher order cascaded Raman lasing in silicon for extending
laser wavelengths from near IR to mid IR regions.
Recently, low threshold Raman silicon lasers based on ring resonator architecture have been demonstrated. One of the
key elements of the laser cavity is the directional coupler that couples both pump and signal light in and out of the ring
resonator from the bus waveguide. The coupling coefficients are crucial for achieving desired laser performance. In this
paper, we report design, fabrication, and characterization of tunable silicon ring resonators for Raman laser and amplifier
applications. By employing a tunable coupler, the coupling coefficients for both pump and signal wavelength can be
tailored to their optimal values after the fabrication, which significantly increases the processing tolerance and improves
the device performance.
As silicon processes scale toward the 45 nm node using conventional 0.25 magnification, widths of sub-resolution assist feature (SRAF) and printable defects on photomasks drop far below the ArF laser wavelength. Adoption of polarized illumination and higher numerical aperture (NA) could invalidate the scaling relations we used in the past to determine which small mask features or errors will print on wafers. Polarization interaction with small mask features may also plays a role in mask inspection. As mask features shrink below the wavelength, differences between the optical systems used for inspection and printing become more significant, and may affect the rules for disposition of inspection results. The data presented here combines experimental results from high NA imaging of sub-wavelength SRAF and defects, with rigorous calculation of their images based on vector diffraction. The printability of these deep subwavelength mask feature determines the requirements of optical model's rigorousness for SRAF design rule and also mask defect inspection and repair capabilities.
The optical properties of photonic bandgap (PBG) structures are highly sensitive to environmental
variation. PBG structures thus are an attractive platform for biosensing applications. We
experimentally demonstrate a label-free biosensor based on a two-dimensional (2-D) photonic
crystal microcavity slab. The microcavity is fabricated on a silicon-on-insulator substrate and
integrated with tapered ridge waveguides for light coupling. The Finite-Difference Time-Domain
(FDTD) method is used to model the sensor. The resonance of the microcavity is designed to be
around 1.58 μm. In order to capture the target biological materials, the internal surface of the
photonic crystal is first functionalized. Binding of the targets is monitored by observing a red shift of
the transmission resonance. The magnitude of the shift depends on the amount of material captured
by the internal surface. Compared to 1-D PBG biosensors, 2-D devices require a smaller amount of
target material and can accommodate larger targets. Experimental results are compared with the
predictions obtained from the FDTD simulations.
The sensitivity of photonic bandgap (PBG) structures to the environment makes them suitable for sensing applications. In this study, we describe how 1-D and 2-D PBG devices can be used for sensing biological matter, from small DNA segments to larger proteins. Our work focuses on using the tunability of silicon PBGs upon binding of the desired target on the internal surface of the air holes. Modeling of the optical response is performed to identify the material nanostructure and device configuration that lead to optimum performance (e.g., sensitivity).
As photonic bandgap (PBG) technology matures and practical devices are realized, the effects of environmental factors, such as ambient temperature, on PBG device operation must be considered. The position of a PBG is determined by the geometry and refractive index of the constituent materials. Therefore, a thermally induced material expansion or refractive index change will alter the location of the PBG and affect the operation of PBG devices. In order to achieve faster switching times for PBG optical interconnects, enhanced sensitivity for PBG sensors, and smaller channel spacing for PBG-based wavelength division multiplexing, increasingly narrow PBG resonances are required. The drawback for the improved device operation is increased sensitivity to small changes in environmental conditions. A method to control and eliminate thermally induced drifts of silicon-based PBG structures has been developed based on a simple oxidation treatment. Oxide coverage of the silicon matrix provides a counterforce to the effect of the temperature dependent silicon refractive index. Depending on the degree of oxidation achieved, a redshift, no shift, or a blueshift of the PBG resonance results when the silicon-based PBG structure is heated. Control over the effects of thermal fluctuations has been demonstrated for two different PBG structure designs. Extensive reflectance and x-ray diffraction measurements have been performed to understand the mechanism behind this oxidation procedure as it relates to one-dimensional silicon-based PBG microcavities.