Nanomechanical motion transducers for miniaturized mechanical systems

Reliable operation of a miniaturized mechanical system requires that nanomechanical motion be transduced into electrical signals (and vice versa) with high fidelity and in a robust manner. Progress in transducer technologies is expected to impact numerous emerging and future applications of micro- and, especially, nanoelectromechanical systems (MEMS and NEMS); furthermore, high-precision measurements of nanomechanical motion are broadly used to study fundamental phenomena in physics and biology. Therefore, development of nanomechanical motion transducers with high sensitivity and bandwidth has been a central research thrust in the fields of MEMS and NEMS. Here, we will review recent progress in this rapidly-advancing area. © 2017 by the authors

Hybridly Integrated Diode Lasers for Emerging Applications: Design, Fabrication, and Characterization

The emerging applications of LiDAR, microresonator based frequency comb, and photon pair generation in photonic integrated circuits (PICs) have attracted lots of research interests recently. The single frequency, high power, narrow-linewidth, tunable semiconductor lasers are highly desired for the implementation of these emerging applications in future PICs. In this dissertation, we use the hybrid integration via edge coupling to obtain the integrated diode lasers for future PICs, since the active chip and the passive chip can be fabricated and optimized independently. We demonstrate hybridly integrated narrow-linewidth, tunable diode lasers in the Indium Phosphide/Gallium Arsenide-silicon nitride (InP/GaAs-Si3N4) platform. Silicon nitride photonic integrated circuits, instead of silicon waveguides that suffer from high optical loss near 1 µm, are chosen to build a tunable external cavity for both InP and GaAs gain chips at the same time. Single frequency lasing at 1.55 µm and 1 µm is simultaneously obtained on a single chip with spectral linewidths of 18-kHz and 70-kHz, a side mode suppression ratio of 52 dB and 46 dB, and tuning range of 46 nm and 38 nm, respectively. The resulting dual-band narrow-linewidth diode lasers have potential for use in a variety of novel applications such as integrated difference-frequency generation, quantum photonics, and nonlinear optics. We also demonstrate one potential application of the dual-band diode laser in beam steering. The dual-band diode laser combined with a waveguide surface grating can provide the beam steering by tuning the wavelength of the light signal. However, the output power of the hybridly integrated diode lasers is still limited. Integrated coherent beam combining (CBC) is a promising solution to overcome this limitation. In this dissertation, coherently combined, integrated diode laser systems are experimentally demonstrated through hybrid integration. A chip-scale coherently combined laser system is experimentally demonstrated in the InP-Si3N4 platform through the manipulation of optical feedbacks at different output ports of the coupled laser cavities. Coherent combining of two InP-based reflective semiconductor amplifiers is obtained by use of the cross-coupling provided by an adiabatic 3 dB coupler in silicon nitride, with a combining efficiency of ~92%. The novel system not only realizes the miniaturization of coherent laser beam combining but also provides a chip-scale platform to study the coherent coupling between coupled laser cavities. Besides, the emerging platforms (i.e., gain chips based on semiconductor quantum dots, silicon-carbide-on-insulator and lithium-niobate-on-insulator) have attracted intense interests in recent years. The hybridly integrated diode lasers through edge coupling are demonstrated in these emerging platforms. In addition, we study the Parity-Time (PT) symmetry in the chip-scale hybrid platform. PT symmetric coupled microresonators with judiciously modulated loss and gain have been widely studied to reveal many non-Hermitian features in optical systems. The phase transition at the exceptional points (EPs) is a unique feature of the PT symmetric non-Hermitian systems. In this dissertation, we propose and demonstrate an electrically pumped, hybridly integrated chip-scale non-Hermitian system, where the optical gain, loss and coupling are separately controlled to allow for the PT symmetry breaking and direct access of the EPs. We use the coupled Fabry-Perot resonators through the hybrid integration of two InP active chips with one Si3N4 passive chip to realize the versatile control of the gain and loss. We first demonstrate the PT symmetry breaking and access of the EPs by investigating the spectral and spatial transition processes of the hybrid system induced by the asymmetric gains in the InP active chips. We then control the loss distribution in the Si3N4 passive chip so that the system loss contrast exceeds the coupling coefficient, which leads to the PT symmetry breaking and coherent addition of the two coupled lasers. Our integrated non-Hermitian optical system in the chip-scale hybrid integration platform successfully bridges the non-Hermitian physics and photonic integrated circuits and is able to expand the practical applications of non-Hermitian optical systems to a whole new stage

Optical Frequency Comb Generation in Monolithic Microresonators

This thesis presents an entirely novel approach for frequency comb generation based on nonlinear frequency conversion in micrometer sized optical resonators. Here, the comb generation process can be directly described in frequency domain as energy conserving interactions between four photons (four-photon mixing). This process is a result of extremely high light intensities that build up in microresonators with long photon storage times. The thesis is composed of four main parts that answer fundamental questions in the context of microresonator-based frequency comb generation as well as providing insights in the control and possible applications of this type of comb generators

Control Systems for Silicon Photonic Microring Devices

The continuing growth of microelectronics in speed, scale, and complexity has led to a looming bandwidth bottleneck for traditional electronic interconnects. This has precipitated the penetration of optical interconnects to smaller, more localized scales, in such applications as data centers, supercomputers, and access networks. For this next generation of optical interconnects, the silicon photonic platform has received wide attention for its ability to manifest, more economical, high-performance photonics. The high index contrast and CMOS compatibility of the silicon platform give the potential to intimately integrate small footprint, power-efficient, high-bandwidth photonic interconnects with existing high-performance CMOS microelectronics. Within the silicon photonic platform, traditional photonic elements can be manifested with smaller footprint and higher energy-efficiency. Additionally, the high index contrast allows the successful implementation of silicon microring-based devices, which push the limits on achievable footprint and energy-efficiency metrics. While laboratory demonstrations have testified to their capabilities as powerful modulators, switches, and filters, the commercial implementation of microring-based devices is impeded by their susceptibility to fabrication tolerances and their inherent temperature sensitivity. This work develops and demonstrates methods to resolve the aforementioned sensitivities of microring-based devices. Specifically, the use of integrated heaters to thermally tune and lock microring resonators to laser wavelengths, and the underlying control systems to enable such functionality. The first developed method utilizes power monitoring to show the successful thermal stabilization of a microring modulator under conditions that would normally render it inoperational. In a later demonstration, the photodetector used for power monitoring is co-integrated with the microring modulator, again demonstrating thermal stabilization of a microring modulator and validating the use of defect-enhanced silicon photodiodes for on-chip control systems. Secondly, a generalized method is developed that uses dithering signals to generate anti-symmetric error signals for use in stabilizing microring resonators. A control system utilizing a dithering signal is shown to successfully wavelength lock and thermally stabilize a microring resonator. Characterizations are performed on the robustness and speed of the wavelength locking process when using dithering signals. An FPGA implementation of the control system is used to scale to a WDM microring demultiplexer, demonstrating the simultaneous wavelength locking of multiple microring resonators. Additionally, the dithering technique is adopted to create control systems for microring-based switches, which have traditionally posed a challenging problem due to their multi-state configurations. The aforementioned control systems are rigorously tested for applications with high speed data and analyzed for power efficiency and scalability to show that they can successfully scale to commercial implementations and be the enabling factor in the commercial deployment of microring-based devices