Integrated Silicon Photonics for High-Speed Quantum Key Distribution

Integrated photonics offers great potential for quantum communication devices in terms of complexity, robustness and scalability. Silicon photonics in particular is a leading platform for quantum photonic technologies, with further benefits of miniaturisation, cost-effective device manufacture and compatibility with CMOS microelectronics. However, effective techniques for high-speed modulation of quantum states in standard silicon photonic platforms have been limited. Here we overcome this limitation and demonstrate high-speed low-error quantum key distribution modulation with silicon photonic devices combining slow thermo-optic DC biases and fast (10~GHz bandwidth) carrier-depletion modulation. The ability to scale up these integrated circuits and incorporate microelectronics opens the way to new and advanced integrated quantum communication technologies and larger adoption of quantum-secured communications

The integration of InP /InGaAsP ridge waveguide structures with dielectric waveguides on silicon

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2007.Includes bibliographical references (p. 261-271).Chip-to-chip optical interconnect technology, which is being explored as a potential replacement for copper chip-to-chip interconnects at data transmission rates exceeding 10 Gb/s, is one of several technologies that could be enabled by the monolithic integration of III-V optoelectronic devices on a silicon integrated circuit. Two significant capabilities required to achieve this monolithic integration were addressed: the assembly of III-V device structures on silicon and the fabrication of the waveguides that perform the intra-chip routing of the optical signal to and from these integrated device structures. These waveguides, consisting of a silicon oxynitride core (n = 1.6) and a silicon dioxide cladding (n = 1.45) were deposited via plasma-enhanced chemical vapor deposition (PECVD). The integrated InP/InGaAsP structures were fabricated using an existing novel technique for preparing very thin (on the order of 5 pm thick) substrate free rectangular structures (approximately 145 pm wide by 300 pm long) with cleaved facets. Using a pick-and-place method, the InP/InGaAsP structures were assembled in 6 pm deep rectangular wells formed by etching through the waveguide stack. The resulting configuration of the integrated devices in the wells facilitated end-fire coupling with the silicon oxynitride waveguides.(cont.) Transmission spectrum measurements for this configuration verified the desired end-fire optical coupling through the integrated InP/InGaAsP device structures with a total coupling loss of 17.75 dB. This loss was shown through measurements and finite difference time domain (FDTD) simulations to be a function of integrated device misalignment, silicon oxynitride waveguide design, length of the gaps between the etched well edges and the device facets, and the well etch properties. Based on FDTD simulations and device misalignment statistics, it was shown that realistic, feasible improvements in the device alignment coupled with the use of higher index contrast waveguides could lower the coupling loss to 3.25 dB.by Edward R. Barkley.Ph.D

On the feasibility of integrated optical waveguide-based in situ monitoring of microelectromechanical systems (MEMS)

This dissertation explores the feasibility of using integrated optical waveguides to measure the motion of microelectromechanical structures (MEMS). MEMS are a class of silicon devices which are being developed as sensors and actuators. Because these free moving structures are fabricated using processes similar to microfabrication, MEMS devices and traditional electronics can be integrated on the same substrate. This merging of the technologies will allow the miniaturization of large scale mechanical systems. A difficulty with MEMS devices is determining the submicron motion. One method of noninvasive measurement is optical measurement. Research focused on the characterization of one particular MEMS device, a linear comb resonator. Linear comb resonators displace linearly along a single axis when drive with a sinusoidal voltage signal. This research presents how single mode and multimode guided waves have potential to yield significant positional information. Using optical fibers to create a bulk optical metrology probe, the displacement and operating frequency of this device was characterized. Integration of this an optical probe structure with the MEMS devices can create integrated optical metrology (IOM), which is an in-situ method of device characterization and can represent an enabling technology for MEMS. Co-integration of the two technologies can be achieved through either processing or post processing of integrated waveguides with the MEMS devices. The fabrication process for co-integration of polymer optical waveguides has been experimentally defined in this dissertation, however final results indicate guides wave IOM would best be explored through process interruption or hybrid techniques given existing polymer materials. Analysis yields that the co-integration of inorganic waveguide structures first requires optimization of the design of the microprobe layout

Implantable Neural Probes for Electrical Recording and Optical Stimulation of Cellular Level Neural Circuitry in Behaving Animals.

In order to advance the understanding of brain function, it is critical to monitor how neural circuits work together and perform computational processing. For the past few decades, a wide variety of neural probes have been developed to study the electrophysiology of the brain. This work is focused on two important objectives to improve the brain-computer interface: 1) to enhance the reliability of recording electrodes by optimizing the shank structure; 2) to incorporate optical stimulation capability in addition to electrical recording for applications involving optogenetics. For the first objective, a flexible 64-channel parylene probe was designed with unique geometries for reduced tissue reactions. In order to provide the mechanical stiffness necessary to penetrate the brain, the miniaturized, flexible probes were coated with a lithographically patterned silk fibroin, which served as a biodegradable insertion shuttle. Because the penetration strength is independent from the properties of the probe itself, the material and geometry of the probe structure can be optimally designed without constraints. These probes were successfully implanted into the layer-V of motor cortex in 6 rats and recorded neural activities in vivo for 6 weeks. For the second objective, either optical waveguides or μLEDs were monolithically integrated on the probe shanks for optogenetic applications. Compared to existing methods, this work can offer high spatial-temporal resolution to record and stimulate from even subcellular neural structures. In the experiments using wild type animals, despite optimized recording of spontaneous neural activities, the cells never responded to illumination. In contrast, for the ChR2 expressed animals, light activation of neural activities was extremely robust and local, which phase-locked to the light waveform whenever the cell was close to the light source. In particular, the probes integrated with μLEDs were capable of driving different neural circuit behaviors using two adjacent μLEDs separated only by a 60-μm-pitch. With 3 μLEDs integrated at the tip of each of the 4 probe shanks, this novel optogenetic probe can provide more than 480 million (12!) different spiking sequences at the sub-cellular resolution, which is ideal to manipulate high density neural network with versatility and precision.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111604/1/wufan_1.pd

Amorphous Silicon Photonics

This chapter introduces our research on amorphous silicon photonics. By exploring our high-quality silicon thin-film technology, we have demonstrated hydrogenated amorphous silicon (a-Si:H) waveguides with ultra-low-loss, vertical interlayer transition (VIT) devices for cross coupling between vertically stacked optical circuits. These device technologies are promising for three-dimensional photonic integrated circuits integrated in microelectronics chips. A record low loss of 0.6 dB cm−1 was achieved for a submicron-scale single-mode waveguide, and the VIT devices allow low-loss, broadband, and polarization-insensitive operation

Technological challenges in the development of optogenetic closed-loop therapy approaches in epilepsy and related network disorders of the brain

Epilepsy is a chronic, neurological disorder affecting millions of people every year. The current available pharmacological and surgical treatments are lacking in overall efficacy and cause side-effects like cognitive impairment, depression, tremor, abnormal liver and kidney function. In recent years, the application of optogenetic implants have shown promise to target aberrant neuronal circuits in epilepsy with the advantage of both high spatial and temporal resolution and high cell-specificity, a feature that could tackle both the efficacy and side-effect problems in epilepsy treatment. Optrodes consist of electrodes to record local field potentials and an optical component to modulate neurons via activation of opsin expressed by these neurons. The goal of optogenetics in epilepsy is to interrupt seizure activity in its earliest state, providing a so-called closed-loop therapeutic intervention. The chronic implantation in vivo poses specific demands for the engineering of therapeutic optrodes. Enzymatic degradation and glial encapsulation of implants may compromise long-term recording and sufficient illumination of the opsin-expressing neural tissue. Engineering efforts for optimal optrode design have to be directed towards limitation of the foreign body reaction by reducing the implant’s elastic modulus and overall size, while still providing stable long-term recording and large-area illumination, and guaranteeing successful intracerebral implantation. This paper presents an overview of the challenges and recent advances in the field of electrode design, neural-tissue illumination, and neural-probe implantation, with the goal of identifying a suitable candidate to be incorporated in a therapeutic approach for long-term treatment of epilepsy patients