Alexei Bogdanov

Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides

A monolithically integrated asymmetric graded index (GRIN) or step-index (GRIN) mode converters for microphotonic waveguides are proposed and described. The design parameters and tolerances are calculated for amorphous silicon (a-Si) couplers integrated with silicon-on-insulator waveguides. The GRIN and step-index couplers operate over a wide wavelength range with low polarization dependence, and the lithographic resolution needed is only +/-1 microm. Finally, experimental results are presented for a single layer 3 microm thick step-index a-Si coupler integrated on a 0.8 microm thick SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 microm for this device.

A novel silicon patch‐clamp chip permits high‐fidelity recording of ion channel activity from functionally defined neurons

We report on a simple and high‐yield manufacturing process for silicon planar patch‐clamp chips, which allow low capacitance and series resistance from individually identified cultured neurons. Apertures are etched in a high‐quality silicon nitride film on a silicon wafer; wells are opened on the backside of the wafer by wet etching and passivated by a thick deposited silicon dioxide film to reduce the capacitance of the chip and to facilitate the formation of a high‐impedance cell to aperture seal. The chip surface is suitable for culture of neurons over a small orifice in the substrate with minimal leak current. Collectively, these features enable high‐fidelity electrophysiological recording of transmembrane currents resulting from ion channel activity in cultured neurons. Using cultured Lymnaea neurons we demonstrate whole‐cell current recordings obtained from a voltage‐clamp stimulation protocol, and in current‐clamp mode we report action potentials stimulated by membrane depolarization steps. Despite the relatively large size of these neurons, good temporal and spatial control of cell membrane voltage was evident. To our knowledge this is the first report of recording of ion channel activity and action potentials from neurons cultured directly on a planar patch‐clamp chip. This interrogation platform has enormous potential as a novel tool to readily provide high‐information content during pharmaceutical assays to investigate in vitro models of disease, as well as neuronal physiology and synaptic plasticity. Biotechnol. Bioeng. 2010;107:593–600.

Fabrication and optical characterization of hexagonal photonic crystal microcavities in InP-based membranes containing InAs∕InP quantum dots

Hexagonal photonic crystal microcavities with missing-hole defects were fabricated in suspended InP membranes. Embedded InAs quantum dots were utilized as broadband emitters to characterize the modes of the cavities. Photoluminescence emission consists of two orthogonally polarized peaks corresponding to the two dipole modes of the hexagonal defect cavity of reduced symmetry. The emission wavelength ranges from 745 to 840 meV, depending on the crystal structure, and quality factors are up to 850. Finite-difference time-domain simulations reproduce the cavity mode energies and the quality factor dependence on the crystal structure, but predict quality factors systematically lower. The experimental quality factors and mode splittings are associated with a slight ellipticity of the lattice holes.

A 100-channel near-infrared SOI waveguide microspectrometer: design and fabrication challenges

We present recent advances in the development of a waveguide microspectrometer chip with high spectral resolution in the near-infrared part of the spectrum. The microspectrometer is designed for a high index contrast silicon-on-insulator (SOI) platform, where high number of spectral channels can be obtained with high spectral resolution in a compact device. We present a 100-channel SOI microspectrometer with designed spectral resolution of ~0.08 nm at 1.5 μm wavelength and about 8x8 mm2 in size. A number of critical design issues are discussed, including design of deep-etched tapers near the Rowland circle required for high-resolution performance as well as coupling between closely spaced waveguides. Device fabrication process is discussed in detail, including two-step e-beam patterning and two-level ICP etches, the focus being on achieving deep and smooth vertical etches in silicon with controllable sub-micron waveguide widths and gaps. The potentially available spectral range of the microspectrometer is limited by the transparency of silicon, extending from the band edge of Si at 1.107 μm to the onset of lattice phonon absorption band near 5 μm. Such compact high-resolution multi-channel integrated microspectrometers are promising for a variety of applications, including spectroscopy, telecommunications, optical interconnects, environmental and bio-sensing.

Graded-index coupler for microphotonic SOI waveguides

Coupling light into and out of small high index contrast waveguides is a fundamental challenge to implementing practical microphotonic waveguide and photonic crystal devices. Previous approaches include three-dimensional tapers, inverse taper waveguides, and grating based couplers. We propose and describe a much simpler coupler based on a short length of graded index (GRIN) material deposited on top of a silicon-on-insulator (SOI) microphotonic waveguide. The GRIN coupler has a refractive index that decreases from the index of silicon at the waveguide-coupler interface, to an optimized value at the coupler surface. Beam propagation method calculations are used to evaluate the coupling efficiency from a 4 μm thick coupler section to the fundamental mode of a 0.5 μm thick SOI waveguide. Coupling efficiencies are compared for couplers with smoothly varying quadratic index profiles and with one, two and three index steps. Coupling efficiencies of 75% (1.3 dB) or better are predicted using a three step GRIN structure with indices ranging from n=3.30 to 3.41 (Si). This index range is easily accessed using a-Si layers deposited by PECVD at varying deposition conditions, or by using composite digital alloys of high and low index films. With this method, microphotonic waveguide couplers can be designed and fabricated using only PECVD deposition and one patterning etch step with very modest tolerances. Efficiency increases to 87% (0.6 dB) when the index range of the 3-step coupler is extended to 3.0.

Single layer a-Si GRIN waveguide coupler with lithographically defined facets

The fabrication of a monolithically integrated single layer graded index (GRIN) layer waveguide coupler, including a process for creating lithographically defined optical facets, is described. The coupling efficiency of GRIN couplers of varying lengths is measured, yielding an optimal coupling length of 15 /spl mu/m for a single 3 /spl mu/m thick a-Si layer on a 0.8 /spl mu/m Si waveguide.

Silicon-based Integrated Optics: Waveguide Technology to Microphotonics

Light emission in aperiodic thue-morse dielectrics p. 15 Photoluminescent coupled multiple microcavity structures made of porous silicon p. 21 Raman gain in silicon using highly confined waveguide structure p. 27 Computer simulation of charging and erasing transients of a Ge/Si hetero-nanocrystal-based flash memory p. 33 Threshold voltage shift in hetero-nanocrystal floating gate flash memory p. 39 Structural characterization and coulomb blockade of a-SiN[subscript x]/Nanocrystalline Si/a-SiN[subscript x] asymmetric double-barrier structures p. 45 Evolution of the luminescence spectrum during the dry and steam oxidation of SiGe films p. 51 Towards a Si/SiGe quantum cascade laser for terahertz applications p. 59 Recent results on the road to a SiGe quantum cascade laser p. 69 The emission of terahertz radiation from doped silicon devices p. 81 Progress in the growth and characterization of Ge quantum dots and islands p. 93 Growth and overgrowth of Ge/Si quantum dots : an observation by atomic resolution HAADF-STEM imaging p. 105 Si[subscript 1-x]Ge[subscript x] nanocrystals observed by EFTEM : influence of the dry and wet oxidation process p. 111 Uniform dome-shaped self-assembled Ge islands by UHV/CVD after boron pre-deposition p. 117 Classical versus Ab initio structural relaxation : electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix p. 121 Origin of the multi-exponential decay dynamics in light-emitting silicon nanocrystals p. 129 Improved optoelectronic characteristics of nanocrystalline porous silicon by high-pressure water vapor annealing p. 141 Theory of large-scale electronic structure calculation and nanostructures formed in silicon cleavage simulation : surface reconstruction, step and bending p. 147 Precipitation of highly luminescent phases from PECVD Si suboxides p. 153 Pump-probe experiments on low loss silica waveguides containing Si nanocrystals p. 159 Multi-color luminescence from nanocrystalline silicon p. 165 Improvement of operating voltage and luminescent properties in nanocrystalline silicon electroluminescent device p. 171 Electric force microscopy of individually charged silicon nanoparticles p. 179 Quasiballistic electron emission from planarized nanocrystalline-Si cold cathode p. 189 Enhancing the sound pressure of thermally induced ultrasonic emitter based on nanocrystalline porous silicon p. 195 Charge transport in silicon nanocrystal arrays p. 201 Possible operation of periodically layered nanocrystalline porous silicon as an acoustic band crystal device p. 207 Thermal conductivity of porous silicon evaluated from phase characteristics of photoacoustic spectroscopy p. 213 Structural modifications of nc-Si/SiO[subscript 2] superlattices by localized photo-induced heating p. 219 Anisotropically nanostructured silicon : a first-principle approach p. 225

Design of polarization-insensitive components using geometrical and stress-induced birefringence in SOI waveguides

We review the use of the oxide cladding stress induced photoelastic effect to eliminate the modal birefringence in silicon-on-insulator (SOI) ridge waveguide components, and highlight characteristics particular to high index contrast (HIC) systems. The birefringence in planar waveguides has its origin in the electromagnetic boundary conditions at the waveguide boundaries, and can be further modified by the presence of stress in the materials. It is shown that geometrical constraints imposed by different design and fabrication considerations become increasingly difficult to satisfy with decreasing core sizes. On the other hand, with typical stress levels of -100 MPa to -400 MPa (compressive) in SiO2 used as the upper cladding, the effective indices are altered up to the order of 10-3 for ridges with heights ranging from 1 μm to 5 μm. We demonstrate that the stress can be effectively used to balance the geometrical birefringence. Birefringence-free operation is achieved for waveguides with otherwise large birefringence by using properly chosen thickness and stress of the upper cladding layer. This allows the waveguide cross-section profiles to be optimized for design criteria other than zero-birefringence. Since the index changes induced by the stress are orders of magnitude smaller than the waveguide core/cladding index contrast, changes in the mode profiles are insignificant and the associated mode mismatch loss is negligible. We study the stress-induced effects in two parallel waveguides of varying spacing, to emulate the condition in directional couplers and ring-resonators. In the arrayed waveguide grating (AWG) demultiplexers fabricated in the SOI platform, we demonstrated the reduction of the birefringence from 1.3x10-3 (without the upper cladding) to below 1x10-4 across the spectral band by using a 0.6 μm oxide upper cladding with a stress of -320 MPa (compressive). Design options for relaxed dimensional tolerance and improved coupler performance made available by using stress engineering are discussed.

SOI Waveguide Fabrication Process Development Using Star Coupler Scattering Loss Measurements

We show that integrated optical star couplers can be useful characterization devices to measure the sidewall roughness-induced scattering losses of planar waveguides. We describe the detailed fabrication processes of these star couplers on the silicon-on-insulator (SOI) platform and the process improvements implemented to reduce the waveguide sidewall roughness and scattering loss. We report the main process challenges, particularly to assure a clear gap between any adjacent waveguides of the dense and closely spaced output waveguide array. These challenges are addressed by optimizing the exposure dose of the resist and adding an oxygen ashing treatment to eliminate waveguide footings. We demonstrate further improvement on the waveguide profile and sidewall roughness through the use of a thin Cr hardmask for the dry plasma etching. This optimized fabrication process is capable of producing approximately a 3 nm root-mean-square sidewall roughness, measured using both scanning electron microscopy (SEM) and atomic force microscopy (AFM). Using the fabricated star couplers, we manage to measure the relative scattering losses of various waveguides with the width varying from 0.2 to 2.0 μm in a single measurement, and show that the measured losses agree with the measured sidewall roughness.

Monolithically integrated graded-index waveguide input couplers for silicon-photonics

A monolithically integrated asymmetric graded index (GRIN) waveguide structure for coupling light into high index contrast waveguides is described. When analyzed in terms of its waveguide modes, the GRIN coupler is shown to be a multimode interference (MMI) device. The design parameters and tolerances are calculated for quadratic index profile and uniform index amorphous silicon (a-Si) GRIN couplers optimized for coupling light into silicon-on-insulator waveguides. Calculations of coupling efficiencies into 0.5 μm SOI waveguides show that asymmetric GRIN couplers operate over a very wide wavelength range with low polarization dependence, and fabrication requires lithographic resolution of only ±1 μm. Experimental results are presented for a 3 μm thick single layer a-Si coupler integrated with a 0.8 μm SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 μm.