Mathew J. Zablocki

Nanomembrane transfer process for intricate photonic device applications

We demonstrate a process for the fabrication and transfer of silicon nanomembranes (Si-NMs) that have been released from their host substrates and redeposited on foreign flexible or flat substrates. The transfer process developed allows intricate photonic devices to be transferred via NMs to a variety of new substrate materials. This allows the transferred devices to benefit from the material properties of both substrate and NM. Our process is designed to transfer and stack large-area photonic devices without compromising their optical performance. The process has been used to transfer large-area unpatterned silicon NMs, in excess of 2.5 cm(2), and photonic devices with intricate device designs containing various fill factors. We have also demonstrated transferred photonic crystal devices that have maintained structural integrity and functionality.

Electro-optically switched compact coupled photonic crystal waveguide directional coupler

In this paper, we present a compact photonic crystal directional coupler in a silicon on insulator platform electro-optically switched at 150 kHz with a switching time of 620 ns under a low voltage operation of 2.9 V. The switch design utilizes a coupled photonic crystal structure designed to operate in the slow light regime. Switching is attained by modulating the coupling coefficient of the coupled photonic crystal waveguide system by using a p-i-n diode to modulate the carrier concentration with a density of ∼104 A/cm2 across the plane of the photonic crystal.

Nanomembrane Enabled Nanophotonic Devices

Nanomembranes (NM) are crystalline semiconductor materials (Si, GaAs, SiGe, etc) that have been released from their substrates and redeposited on foreign, flexible or flat substrates enabling the best features of both materials. Although they are in fact crystalline in nature and possess the electronic/photonic properties of bulk material, they are flexible, deformable, and conformable. An obvious choice is silicon-on-insulator (SOI). SOI provides, beyond its application in the Si industry, the ultimate platform for exploring novel science and technological advancements in this class of nanomaterial. In SOI, a SiO2 layer is interspersed between a thin crystalline top Si layer and the bottom Si wafer; the ability to etch this buried oxide selectively creates the nanomembranes. When released from the oxide, this layer can form extremely flexible strain-engineered thin nanomembranes with thicknesses from several hundred nanometers to less than 10 nm, and in various shapes. Photonic devices originally structured in an SOI substrate can now be transferred and stacked on new substrates, rigid and flexible, to increase optical interconnect densities.

Micromachining of a fiber-to-waveguide coupler using grayscale lithography and through-wafer etch

For some time, the micro-optics and photonics fields have relied on fabrication processes and technology borrowed from the well-established silicon integrated circuit industry. However, new fabrication methodologies must be developed for greater flexibility in the machining of micro-optic devices. To this end, we have explored grayscale lithography as an enabler for the realization of such devices. This process delivers the ability to sculpt materials arbitrarily in three dimensions, thus providing the flexibility to realize optical surfaces to shape, transform, and redirect the propagation of light efficiently. This has opened the door for new classes of optical devices. As such, we present a fiber-to-waveguide coupling structure utilizing a smoothly contoured lensing surface in the device layer of a silicon-on insulator (SOI) wafer, fabricated using grayscale lithography. The structure collects light incident normally to the wafer from a singlemode optical fiber plugged through the back surface and turns the light into the plane of the device layer, focusing it into a single-mode waveguide. The basis of operation is total internal reflection, and the device therefore has the potential advantages of providing a large bandwidth, low polarization sensitivity, high efficiency, and small footprint. The structure was optimized with a simulated annealing algorithm in conjunction with two-dimensional finite-difference time-domain (FDTD) simulation accelerated on the graphics processing unit (GPU), and achieves a theoretical efficiency of approximately seventy percent, including losses due to Fresnel reflection from the oxide/silicon interface. Initial fabrication results validate the principle of operation. We discuss the grayscale fabrication process as well as the through-wafer etch for mechanical stabilization and alignment of the optical fiber to the coupling structure. Refinement of the through-wafer etch process for high etch rate and appropriate sidewall taper are addressed.

Chip-scale photonic routing fabrics for avionic and satellite applications

We presented a reconfigurable bi-directional optical routing fabric that utilizes slow-light photonic crystals to reduce SWaP for avionic and satellite platforms. The optical router function uses the electro-optic effect in conjunction with a p-i-n diode to provide active switching and reconfiguration, while using slow-light to reduce the overall footprint of router and operate with low power consumption.

Processing and modeling optimization for grayscale lithography

Grayscale lithography is an extension of the conventional binary lithographic process for realization of arbitrary three-dimensional features in photoresist materials, with applications especially in micro-optics fabrication. The grayscale photomask possesses a spatially varying transmission that modulates the exposure dose received in the photoresist. By using a low contrast photoresist, such as those based on diazonaphthoquinone (DNQ), the material is only partially removed during development in proportion to the local exposure dose received. In this way, an arbitrary surface topography can be sculpted in the photoresist material. It is common practice in grayscale lithography to encode the transmission levels of the photomask by using the photoresist contrast curve to determine the exposure dose required for a given photoresist thickness at each lateral point in the pattern. This technique is adequate when the surface topography is slowly varying and the photoresist film is thin. However, it is inaccurate when these conditions are not met, because the technique essentially represents a one-dimensional approximation to the lithographic process where the isotropy of the development and the diffractive imaging of the photomask are neglected. Currently we are applying grayscale lithography to the fabrication of a fiber-to-waveguide coupler based on the parabolic reflector, where the efficiency of the device is quite sensitive to fabrication errors in the coupler geometry. In this case the thin photoresist and slowly varying topography conditions are not met, and we turn to more comprehensive process models to determine the appropriate transmission levels to encode in the photomask. We demonstrate that the photomask can be optimized, based on simulation of the lithography process, to produce the required three-dimensional photoresist pattern.

Reconfigurable bi-directional optical routing in photonic crystals enabled by silicon nanomembrane modules

Wavelength-division multiplexing (WDM) is the transmission of many signals through a single communication channel using different wavelengths, each of which carries a separate, independent signal. We present and discuss a reconfigurable WDM based on slow-light, functioning as a bi-directional optical routing and processing network, consisting of photonic crystals designed as drop/add filters. The photonic crystal based routing elements consist of two waveguides coupled through a resonant cavity. Photonic crystals offer the ability to achieve separation of many channels on a much smaller scale than their predecessors. Photonic crystals have led a challenging frontier of miniaturization and large scale integration of high-density optical interconnects, and with the aid of nanomembranes, optical routing networks can set a new standard for high-density optical interconnects.

Progress towards dual vertical slot modulator for millimeter wave photonics

Dual vertical slot modulators leverage the field enhancement provided by the continuity of the normal electric flux density across a boundary between two dielectrics to increase modal confinement and overlap for the propagating optical and RF waves. This effect is achieved by aligning a conventional silicon-based optical slot waveguide with a titanium dioxide RF slot. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. The dual slot design confines both the optical and RF modes to the same void between the silicon ribs of the optical slot waveguide. To obtain modulation of the optical signal, the void is filled with an organic electro optic material (OEOM), which offers a high optical non-linearity. The optical and RF refractive index of the OEOM is lower than silicon and can be deposited through spin processing. This design causes an extremely large mode overlap between the optical field and the RF field within the non-linear OEOM material which can result in a device with a low Vπ and a high operational bandwidth. We present work towards achieving various prototypes of the proposed device, and we discuss the fabrication challenges inherent to its design.

Chip-scale photonic interconnects for reconfigurable computing

We present and discuss several of the benefits associated with using chip-scale optical interconnects in reconfigurable computing systems. As is well known, by removing metallic traces in high-speed systems, many signal integrity issues are reduced, or eliminated, e.g., parasitic capacitance and inductance associated self-induced affects and trace overlay. In addition, photonic systems can require less power and offer higher efficiency, thereby, giving rise to reduced thermal energy dissipation. However, at least in the case of reconfigurable processors there are several additional advantages. A case in point is that of field programmable gate arrays (FPGAs), which is a technology that has been plagued by interconnect limitations. To address this, we have developed an interconnect network that will enable fully reconfigurable processors, or FPGAs. Our approach is based on a photonic crystal cross-bar switch that enables complete interconnectivity over large computational-block arrays. Perhaps one of the most attractive benefits of our approach is that it alleviates the need to perform place and route during processor layout. As such, our approach may allow for reconfigurable processors consisting of a higher density of computing-blocks along with a faster interconnect medium. Accordingly, this talk will present numerical studies, design and fabrication of various implementations of candidate photonic crystal devices for reconfigurable optically interconnected chip-scale networks.

Dual slot modulator for millimeter wave photonics

Silicon slot waveguides leverage the field enhancement provided by the continuity of normal electric flux density across a dielectric boundary to confine an optical mode to a void between two proximal silicon strips. Silicon-organic hybrid slot modulators make use of this mode profile by infiltrating the slot region with a non-linear organic electro-optic material (OEOM) for modulation. The dual slot modulator takes this idea a step further by similarly confining a propagating RF mode to the same slot region to increase modal overlap for improved modulation efficiency. This effect is achieved by aligning a titanium dioxide RF slot along a conventional silicon slot waveguide. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. As a result of the large modal overlap and high electro-optic activity of the OEOM this design can produce measured phase modulated VπL of less than 1.40 V•cm. Furthermore, as the modulator operates without the introduction of a doping scheme it can potentially realize high operational bandwidth and low loss. We present work towards achieving various working prototypes of the proposed device and progress towards high frequency operation.